Radiocairbon,65

An International Journal of Cosmogenic Isotope Research

AUSTIN LONG

Managing Editor RENEE S. KRA

Assistant Editors JAMES M. DEVINE R. TRUED

31 }

Department of Geosciences The University of Arizona 4717 East Ft. Lowell Road Tucson, Arizona 85712 USA ISSN: 0033-8222 RADIOCARBON An International Journal of Cosmogenic Isotope Research

Editor: AUSTIN LONG Managing Editor: RENEE S. KRA Assistant Editors: JAMES M. DEVINE, JONETTA R. TRUED Published by Department of Geosciences The University of Arizona

Published three times a year at The University of Arizona, Tucson, AZ 85712 USA. ® 1993 by the Department of Geosciences, The University of Arizona.

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Citations. A number of radiocarbon dates appear in publications without laboratory citation or reference to published date lists. We ask that laboratories remind submitters and users of radiocarbon dates to include proper citation (laboratory number and date-list citation) in all publications in which radiocarbon dates appear.

Radiocarbon Measurements: Comprehensive Index, 1950-1965. This index covers all published 14C measurements through Volume 7 of RADIOCARBON, and incorporates revisions made by all laboratories. It is available at $25.00 per copy.

List of laboratories. Our comprehensive list of laboratories is available upon request. We are expanding the list to include additional laboratories and scientific agencies with whom we have established contacts. The editors welcome information on these or other scientific organizations. We ask all laboratory directors to provide their current telephone, telex, fax numbers and E-mail addresses. Changes in names or addresses, additions or deletions should be reported to the Managing Editor. VOL. 35, No. 2 RADIOCARBON 1993

CONTENTS

FROM THE EDITOR - Journal Cost Crisis Austin Long ...... iii

OBITUARY - Elizabeth K. Ralph v Henry N. Michael ......

Late Pleistocene-Recent Atmospheric S"C Record in C4 Grasses L. J. Toolin and C. J. Eastoe ...... 263 Carbon Isotopic Composition of Deep Carbon Gases in an Ombrogenous Peatland, Northwestern Ontario, Canada R. Aravena, B. G. Warner, D. J. Charman, L. R. Belyea, S. P. Mathur and H. Dinel ...... 271 Isotopic Analysis of Groundwater and Carbonate System in the Surdulica Geothermal Aquifer M. Had5.ifehovi4, N. Miljevic, V. Sipka, D. Golobo.,anin and R. Popovii ...... 277 Radiocarbon Dating of Paleoseismocity Along an Earthquake in Southern Italy 287 G. Calderoni and V. Petrone ...... A Batch Preparation Method of Graphite Targets With Low Background for AMS 14C Measurements H. Kitagawa, T. Masazawa, T. Nakamura and E. Matsumoto ...... 295 AMS-Graphite Target Production Methods at the Woods Hole Oceanographic Institution During 1986-1991 A. R. Gagnon and G. A. Jones ...... 301 Radiocarbon to Calendar Date Conversion: Calendrical Bandwidths as a Function of Radiocarbon Precision 311 F. G. McCormac and M. G. L. Baillie ...... 14C A Simplified Approach to Calibrating Dates 317 A. S. Talma and J. C. Vogel ...... Radiocarbon Dates from American Samoa 323 J. T. Clark ......

NOTES AND COMMENTS "C Dating of Laser-Oxidized Organics A. L. Watchman, R. A. Lessard, A. J. T. Jull, L. J. Toolin and W. Blake, Jr...... 331

An Assessment of the Radiocarbon Dating of the Dead Sea Scrolls 335 G. A. Rodley ......

BOOK REVIEWS ...... 339

343 RADIOCARBON UPDATES ...... 345 LETTER TO THE EDITOR ......

r . F [RADIOCARBON, VOL. 35, No. 2, 1993, P. ii]

ASSOCIATE EDITORS

For Accelerator Physics

DAVID ELMORE West Lafayette, Indiana, USA ROBERT E. M. HEDGES Oxford, England D. ERLE NELSON Burnaby, British Columbia, Canada

For Archaeology

ANDREW M. T. MOORE New Haven, Connecticut, USA MICHAEL B. SCHIFFER Tucson, Arizona, USA

For Atmospheric Sciences

GEORGE A. DAWSON Auckland, New Zealand Tucson, Arizona, USA KUNIHIKO KIGOSHI Tokyo, Japan DAVID C. LOWE Lower Hutt, New Zealand

For Geochemistry

PAVEL POVINEC Bratislava, Slovakia MINZE STUIVER Seattle, Washington, USA

For Geophysics

G. E. KOCHAROV St. Petersburg, Russia WILLEM G. MOOK Groningen, The Netherlands

For Hydrology

JEAN-CHARLES FONTES Orsay, France

For Ice Studies

HAROLD W. BORNS, JR. Orono, Maine, USA ULRICH SIEGENTHALER Berne, Switzerland

For Oceanography

EDOUARD BARD Gif-sur-Yvette, France Palisades, New York, USA Marseille, France ELLEN R. M. DRUFFEL Woods Hole, Massachusetts, USA

For Paleobotany

CALVIN J. HEUSSER Tuxedo, New York, USA

ii [RADIOCARBON, VOL. 35, No. 2, 1993, P. iii]

FROM THE EDITOR

JOURNAL COST CRISIS

Seemingly, every week a new crisis pushes itself into the headlines, or at least into the interde- partmental memos that deluge us in the academic realm. Most universities inthe US are undergoing budgetary re-evaluation, downsizing, consolidation, or simply trimming. (Administrators are very good at creative euphemization.) One recent memo concerned the fact that our Science and Engineering Library is considering solving its budgetary problems by cancelling subscriptions to selected journals. In our case, fortunately, the library will consult with users before making these decisions. Nevertheless, they have supplied a list of journals that, if deleted, would ease their budgetary problems. On this list were subscription costs, trends in subscription costs in the past two years, cost per page and a number called the "ISI Impact Index", which attempts to gauge the value of the journal in terms of citations of its articles in subsequent publications. The librarians will rely on several criteria of journal retention, including testimony from individuals and departments verifying value of and dependence on specific journals.

It is instructive to peruse this "hit list" and learn how much many geologically oriented journals have increased their prices, without significantly increasing their number of pages. I especially noted a few journals that most of us in the geological sciences would consider mainline, high- profile and essential for the library to retain. One outlier increased subscription rates 487% in two years, another 261%, a third, 147%. Then the curve started leveling off with several clustered around 100%, and a large group in the 66 to 33% range. The physics journals showed a similar curve. Roughly one-half of the journals on the complete list of geology and physics serials to which our library subscribes have increased their subscription prices more than 20% in the past two years, well ahead of inflation. No doubt some of these will be dropped. I suspect that other science libraries are going through the same processes as ours.

One does not need remarkable prescience to construct a plausible scenario for the outcome of this. Based on the principle that libraries are multi-user entities with institutional support, most journals charge higher prices for library subscriptions than for individual subscriptions. Consequently, many journals, including this one, depend strongly on library subscriptions for support. As libraries drop subscriptions, some journals will cease to exist. The librarians' rationale that "if we drop this journal, you can still get it on interlibrary loan" may not hold for some journals in the future. We may experience a pruning of some of the financially weak journals. "We have too many specialty journals, anyway", some say. Authors in need of publications may not suffer, however, but they may not be able to publish in the journal that targets the most appropriate readership. Another change that we observe is the digitization of scientific publications. Some journals offer CD-ROMs of one year's worth of journal issues. At present, only the largest (in size and subscriptions) can afford this, and the CD-ROM is available only at the end of the year. These are no cheaper than the paper version, but have the advantages of compact storage and remote computer access. The extreme case of digitization is represented by the few journals already published entirely electronically - no paper ... unless you make a copy from your PC or work station.

As yet, RADIOCARBON has no firm plans for either a CD-ROM version or for electronic publishing. Like most journals, we depend greatly on library subscriptions. Our subscription prices have increased only 16% over the past two years. In 1990, our page area increased 30%, when we iv From the Editor enlarged our book size. We hope that you will discourage your local library from pushing the "delete" key on this journal.

ANOTHER CALIBRATION DISK As all who have written computer codes know, errors in software occur. Some are obvious; others are insidiously elusive, appearing only under certain circumstances and manifesting themselves subtly. The latter was the case for CALIB v. 3.0. Thus, the insert in the current issue of RADIOCARBON. CALIB v. 3.0.3 is the updated version; please erase the previous version and replace it with this one. The authors regret the inconvenience.

Austin Long E. K. Ralph and combustion tube for converting "C samples to CO2 (possibly 1969). Courtesy of MASCA.

DR. ELIZABETH K. RALPH (5 FEBRUARY 1921 - 23 MARCH 1993)

The scientific life of Elizabeth Ralph was marked by a spirit of pioneering. This spirit was reflected in her work in the new field of radiocarbon dating and eventually led to the establishment, at the University of Pennsylvania, of the second radiocarbon laboratory in the world (after Nobel laureate Willard Libby's laboratory in Chicago). Problems encountered with the inaccuracies of radiocarbon dating led Beth to another pioneering task - the establishment of correction factors for radiocarbon dates based on precisely dated tree rings. After "intermediate" publication of correction curves resulting from presentations at the 1969 Uppsala, Sweden, and 1972 Wellington, New Zealand meetings, a definitive and much appreciated form was published in 1973. For many years, it remained an important tool for those archaeologists who accepted radiocarbon dating. Another innovative work that Beth undertook was that of archaeological surveying with magnetometers. She collaborated with manufacturers of magnetometers to improve the sensitivity of the instruments to measure variations in magnetic intensity. The results were the rubidium and cesium magnetometers she used to find the "sunken" city of Sybaris in the very south of Italy. Her explorations there over an 8-yr period became legendary. This work required walking hundreds of miles over archaeological territory, and Beth had the stamina to do it. Her endurance was no doubt reinforced by her membership in the U.S. land hockey team, which often met with tough international competition. During the 1960s, Beth surveyed some 50 archaeological sites in eight countries. vi Obituary

For her contributions to archaeological research through the use of chronometric techniques, Dr. Ralph received the Pomerance Award of the Archaeological Institute of America. With all the research projects, administrative duties as Associate Director for the Museum Applied Science Center for Archaeology (MASCA), as well as Director of the Radiocarbon Laboratory, Beth was indeed a busy person. Yet she always found time to discuss problems of archaeological techniques with both colleagues and graduate students. Her advice to the latter was often instrumental in shaping their subsequent careers.

Beth Ralph graduated from Wellesley College with a major in chemistry. Her graduate degrees in physics and geology were earned at the University of Pennsylvania. Her life's work and major interests are reflected in her publications. A few of her notable publications are listed below.

Henry N. Michael Museum Applied Science Center for Archaeology (MASCA) University Museum, University of Pennsylvania Philadelphia, Pennsylvania 19104-6324 USA SELECTED BIBLIOGRAPHY

1955 University of Pennsylvania radiocarbon dates I. 1970 Correction factors applied to Egyptian radiocar- Science 121(3136): 149-151. bon dates from the BC era. In Olsson, I. U., ed., 1955 Radiocarbon dates for KaraKamar, Afghanistan, Radiocarbon Variations and Absolute Chronology, University of Pennsylvania II. Science 122(3176): Proceedings of the 12th Nobel Symposium. New 921-922 (with C. S. Coon). York, John Wiley & Sons: 109-120 (with H. N. 1956 14C dating. Pennsylvania Archaeologist XXVI(1): Michael). 27-31. 1971 Dating Techniques for the Archaeologist, edited 1959 Radiocarbon dating in Arctic. American Antiquity by H. N. Michael and E. K. Ralph. Cambridge, 24(4): 365-374 (with F. G. Rainey). Massachusetts, M.I.T. Press. 14C 1960 Carbon-14 measurements of known age samples. 1971 dating. Chapter 1. In Michael, H. N. and Nature 188(4746): 185-187 (with R. Stuckenrath). Ralph, E. K., eds., Dating Techniques for the Archae- 1960 New radiocarbon dates and the Maya correlation ologist. Cambridge, Massachusetts, M.I.T. Press. problem. American Antiquity 26(2): 165-184 (with L. 1972 A cyclic solution for the relationship between Satterthwaite). magnetic and atmospheric carbon-14 changes. Pre- 1962 New instrument techniques in archaeology. Pro- print. In Rafter, T. A. and Grant-Taylor, T., eds., ceedings of the Symposium on Detection of Under- Proceedings of the 8th International Conference on ground Objects, Materials and Properties, 19-20 Radiocarbon Dating. Wellington, New Zealand, March 1962. U.S. Army Engineer Research and Royal Society of New Zealand: A76-A84. Development Laboratories, Fort Belvoir, Virginia: 1973 Radiocarbon dates and reality. MASCA Newsletter 151-155 (with F. G. Rainey). 9(1): 1-20 (with H. N. Michael and M. C. Han). 1965 Review of radiocarbon dates for samples from 1974 Twenty-five years of radiocarbon dating: Retro- Tikal related to the Maya calendar correlation prob- spect and prospect. American Scientist 62(5): 553- lem. American Antiquity 30(4): 421-427. 560 (with H. N. Michael). 1965 Carbon-14 date for the Antikythera Shipwreck. 1978 The chronometric gap from Early Jomen in south- Transactions of the American Philosophical Society, ern Hokkaido: A radiocarbon and thermoluminescence N. S. 55(3): 48 p. View. Asian Perspective XIX(1): 116-144 (with W. 1966 Dating of pottery by thermoluminescence. Nature M. Hurley, M. C. Han and N. Yoshizaki). 210: 245-247 (with M. C. Han). 1979 Composite computer plots of 14C dates for tree- 1966 Archaeology and its new technology. Science ring dated bristlecone pine and sequoia. In Berger, R. 153(3743): 1481-1491 (with G. F. Rainey). and Suess, H. E., eds., Radiocarbon Dating. Proceed- 1967 Instrument surveys. The Search for Sybaris, ings of the 9th International 14C Conference. Berke- 1960-65 (with F. G. Rainey and C. M. Lerici) Rome, ley/Los Angeles, University of California Press: Lerici Editori: 53-134. 545-553 (with J. Klein). 1967 Problems of the radiocarbon calendar. Archaeo- 1980 Radiocarbon dates from Akrotiri: Problems and a metry 10: 3-11 (with H. N. Michael). strategy. In Thera and the Aegean World, Vol. IT 1968 Archaeological surveying utilizing a high sensi- Papers and Proceedings of the Second International tivity difference magnetometer. Geoexploration 6: Scientific Congress, Santorini, Greece, August 1978 109-122 (with F. Morrison and D. P. O'Brien). (with M. Biddle). [RADIOCARBON, VOL. 35, No. 2, 1993, P. 245-251] Radiocarbon

1993

INTRA-ANNUAL VARIABILITY OF THE RADIOCARBON CONTENT OF CORALS FROM THE GALAPAGOS ISLANDSI

T. A. BROWN2 3 G. W. FARWELL2 P. M. GROOTES 2,4, F. H. SCHMIDT 2,5 and MINZE STUIVER4

ABSTRACT. We report AMS 14C measurements on subannual samples of coral from the Galapagos Islands that span the period, 1970-1973. Both the major 1972 El Nino/Southern Oscillation event and intra-annual changes in regional upwelling of 14C-depleted waters associated with alternation of surface-ocean current patterns are evident in the record. Our data show that the corals preserve a detailed record of past intra-annual variations of the 14C content of surface ocean water.

INTRODUCTION

Radiocarbon concentrations in aragonite skeletons of hermatypic (reef-building) corals record the 14C concentrations of dissolved inorganic carbon (DIC) in local sea water at the time of skeletal accretion. Annual density bands in corals from the Galapagos Islands (and many other locations) enable the independent determination of the growth year of particular bands. Thus, we can reconstruct the past variations of 14C concentration in local sea water by measuring 14C concentrations in annual bands of coral skeletons. In previous studies, several researchers (Druffel & Linick 1978; Druffel 1981,1987,1989; Druffel & Suess 1983; Nozaki et al. 1978) have used "C measurements on annual bands in corals from tropical and temperate locations to investigate the incorporation of 14C-depleted "Suess-effect" carbon and bomb-produced 14C into the surface layers of the Atlantic and Pacific Oceans, and variations of surface ocean currents and mixing processes.

Druffel (1981) measured the 14C content of Galapagos corals using gas proportional 3-counting systems and, for the most part, measured complete annual band couplets. The results showed the incorporation of bomb-produced 14C into surface ocean waters from 1961 to 1977, and possible effects of incorporation of 14C-depleted "Suess-effect" carbon into surface ocean waters between 1929 and 1954. Druffel also suggested that annual average 014C values from coral samples from the years of El Nino/Southern Oscillation events (ENSO) were significantly higher than the values obtained for adjacent non-ENSO years due to diminished upwelling of 14C-depleted waters during these events. To investigate this effect further, Druffel measured the 14C content of pairs of six-month-average coral samples from a weak ENSO year, 1969, a strong ENSO year, 1972, and a non-ENSO year, 1974. Whereas the values obtained from the 1969 "seasonal" samples were compatible with decreased upwelling during the first part of the year, the values for the 1972 "seasonal" samples were not significantly different from each other, and their average was incompatible with the annual average value obtained for that same year. The values obtained for

'This paper was presented at the 14th International Radiocarbon Conference, 20-24 May 1991, Tucson, Arizona. 2Nuclear Physics Laboratory GL-10 and Department of Physics, University of Washington, Seattle, Washington 98195 USA 3Geophysics Program AK-50, University of Washington 4Quaternary Isotope Laboratory AK-60 and Department of Geological Sciences, University of Washington SWe regret the passing of our friend and colleague, Professor Emeritus Fred H. Schmidt, who died on 17 January 1991.

245 246 T. A. Brown et al. the "seasonal" samples from the non-ENSO year, 1974, did not differ significantly. Druffel concluded that the 14C levels of the summer and winter waters in the Galapagos Islands during non-ENSO years were not significantly different.

In this study, we have used the small-sample capability of 14C accelerator mass spectrometry (AMS) to measure subannual samples of coral from the Galapagos Islands. The goal of these measurements was to use the increased time resolution attainable through these smaller samples to search for intra-annual variations in 14C levels of surface waters of the region. The details of variations in the 14C levels of the surface waters of this region in the years preceding, during and following an ENSO event were of particular interest to us. To ensure that we had sufficient sensitivity to detect the variations of interest in the 14C content of the corals, we made repeated measurements of a secondary standard during the course of this study; these measurements show that our 14C AMS system is capable of precise and accurate (±4%o) measurements. Our coral data show sharp intra-annual variations in the 14C concentration of the local surface waters of the Galapagos region and a distinct ENSO signal.

We present below a brief, simplified description of the surface ocean currents in the Galapagos Islands region and the possible influences of these currents on the 14C content of Galapagos waters.

GALAPAGOS ISLANDS The Galapagos Islands lie on the equator off the coast of Peru at 90°W longitude. The influence of the major surface ocean currents on the waters of the Galapagos Islands has been described by Glynn and Wellington (1983) and by Enfield (1989) in an extensive review of ENSO phenomena.

Fig. 1. Map of Galapagos Islands area that indicates the major surface ocean currents influencing the 14C content of the Galapagos Islands waters, i.e., the Peru, Panama and South Equatorial Currents

Briefly, during non-ENSO years, two surface currents alternate as the major source of surface ocean waters in the Galapagos Islands region. From about April to December, the Peru Current is the dominant source of the waters that flow through the Galapagos region as the South Equatorial Current (Fig. 1). The waters of the Peru Current are relatively cool (18°-22°C) due to the upwelling and mixing of subsurface waters during the northward, offshore movement of the current along the coast of Chile and Peru. From January to about March, southeast tradewinds weaken, and relatively warm (25°-28°C) tropical surface waters of the Panama Current flow from the north and become a major source of the waters in the Galapagos Islands region. This annual intrusion of warm northern waters is evident in the sea-surface-temperature (SST) record that has been obtained at Academy Bay in the Galapagos Islands (Fig. 2). Because the upwelled waters in the Peru Current Intra Annual Variability of 14C Content of Corals 247

60 28

50 26

C) 24 30 F- U U) 20 Cl) r 22 d 10

20 0

-10 68 69 70 71 72 73 74 75

Time (year;I9XX)

Fig. 2. A14C data obtained on Galapagos Island corals and Galapagos SST, 1969-1974, o = our preliminary i14C data 014C obtained on "three-month" samples of coral from Punta Pitt, San Crist6bal Island (1970-1973); annual average data (Druffe11981) on coral taken from Gardner Bay, Espanola (Hood) Island; - = SST recorded at Academy Bay, Santa Cruz Island (data from Heinrich Sievers as presented by McConnaughey (1986)). The horizontal error bars on the i14C data show the approximate time span each sample represents; the vertical error bars show the 1 Q uncertainty in the determination of the 14C content of each coral sample. are depleted in 14C relative to the surface waters of the Panama Current, the intra-annual variation between these two sources should be evident in variations of the 14C content of the corals.

In addition to the other oceanographic and atmospheric effects associated with ENSO events, the alternation between these two sources of surface waters is disturbed during ENSO years. Surface waters that have been influenced by strong upwelling of relatively cool subsurface waters do not return in the April-to-December period following the onset of an event. SST in the Galapagos Islands stays anomalously high during this period, and the diminished influence of 14C-depleted 14C upwelled waters should result in the suppression of the annual return to lower content for coral 14C growth during this April-to-December period. Thus, a sustained period of high content, characteristic of surface waters from the Panama Current, should be evident in the coral growth during the ENSO periods.

METHODS We obtained coral samples from a coral core collected by T. A. McConnaughey (Quaternary Isotope Laboratory, University of Washington) from Punta Pitt, San Cristobal Island in the Galapagos (89.5°W, 1°S). McConnaughey (1986) described the core collection and initial processing methods. For this study, we divided each annual band for the years 1970-1973 into four equal sections to produce a total of 16 samples for measurement, each representing about a three-month growth period. Variations in coral growth rate in response to changes in local water 248 T. A. Brown et al. temperature and light levels complicate the establishment of accurate subannual time scales for corals (Glynn & Wellington 1983; McConnaughey 1986). In light of this, it is clear that the actual time periods represented by our "three-month" coral samples probably vary somewhat. In this initial report, we have treated each of our coral samples as representative of an approximately three-month period, but we recognize the uncertainties introduced by growth-rate variations.

We extracted the carbon in each of the coral samples as CO2 by carbonate dissolution in 100% phosphoric acid at ca. 75°C. Typically, the coral samples weighed 8-25 mg and produced 1-3 mg of carbon as CO2 (chemical yields for this preparation step were >95%). The CO2 was then converted to graphite by iron-catalyzed hydrogen reduction (Vogel et al. 1984) and prepared for 14C measurement as described previously (Balsley et al. 1987; Brown et al. 1990). Typical ion-source targets prepared for measurement by our Ta encapsulation method contain 200-300 ,ug of carbon; hence, we were able to prepare several ion-source targets from each coral sample. 14C We measured the content of the coral samples using the AMS 14C system that we developed at the University of Washington Nuclear Physics Laboratory (Grootes et al. 1986; Brown et al. 1990; Schmidt et al. 1990). We discuss below the current precision and accuracy of our 14C AMS measurements.

PRECISION AND ACCURACY

We demonstrated previously (Brown et a1.1990) that the distribution of our measurements having a counting statistics precision of ca. 1% is consistent with a Gaussian distribution with a 1 o variance of 1% (i.e., for measurements with 1% counting statistics, uncertainties in counting statistics are the only significant source of scatter in our data). In this analysis, we also showed, through the measurement of approximately contemporary standards, that our measurement system can attain an accuracy of at least 4%o (i.e., any systematic bias introduced into our data by our measurement system is <4%o).

As part of our continuing evaluations of the precision and accuracy of our measurement system, we have measured an approximately one-half-life-old secondary standard previously measured by high-precision Figure 3 shows the 14C/13C data we obtained on this sample from June to November, 1990. These data are normalized with respect to our Chinese Sucrose 14C laboratory standard. The 1 o scatter of the measurements is essentially equal to the typically 1.7% counting statistics uncertainties of the individual measurements. From the weighted mean of our normalized 14C/13C 14C data, we calculated a age for the sample of 6150 ± 30 BP (this calculation includes an S13C estimated uncertainty of ± 1%o in the of the AMS sample due to fractionation during graphitization); this agrees, at the 1 o level, with the high-precision f3-counting age for the sample of 6120 ± 35 BP (QL-11658) (based on the uncertainties of these dates, the 1 o uncertainty in the expected difference between the dates is ± 46 yr; the difference between the dates is only 30 yr). These results confirm that, for measurements with counting statistics precisions of 1-2%, counting statistics uncertainties are the only significant source of scatter in our measurements, and that our measurement system introduces no systematic bias into our data >4%0.

RESULTS AND DISCUSSION

Figure 2 shows the preliminary &4C results of our measurements of the "three-month" coral samples for the years 1970-1973 (the data are also given in Table 1). Each data point represents an average of 14C/13C at least four determinations, each having a counting statistics uncertainty of ca. ± 1% (except for the 4th sample of 1972, which was measured only twice). These measurements were made on at least two separately prepared ion-source targets (except for the 4th sample of 1972 and the 3rd and 4th sam- Intra Annual Variability of 14C Content of Corals 249

0.38 T U TM 0.37 U 0.36 r 0.35 w r w r rrr r r w r r r w r

as 0.34 L. w r w r rr r r r N 0.33 as 0.32 0.31 0 5 10 15 20 25 Run 14C/13C 14C/13C Fig. 3. Normalized data obtained on the secondary standard, QL-11658. o = measured ratios for runs between June and November 1990, on targets prepared from QL-11658 (these measurements are normalized with respect to our Chinese 14C 14C/13C Sucrose laboratory standard). The error bars represent 1 o uncertainties in the ratios derived from counting statistics; - = the weighted mean of all of the data; - - - = ± 1 standard deviation calculated from the data. This distribution is important as it clearly indicates the method used to calculate the quoted values, i.e., as opposed to error estimate propagation. (The values from runs 3 and 4, while included in the calculation of the weighted mean, have been excluded from the standard deviation calculation because the format of this calculation does not allow these values, which are relatively far from the weighted mean and have unusually large as, to be given appropriately lesser weights than the other, more precise values.)

TABLE 1. Measured i14C values for "three-month" coral samples from Punta Pitt, San Cristobal Island, Galapagos Islands Months represented Year by sample (approx.) (%o)

1970 January-March +31 ± 7 April-June -1 ± 4 July-September +29 ± 7 October-December +37 ± 7 1971 January-March +22 ± 6 April-June +21 ± 6 July-September +30 ± 6 October-December +24 ± 6 1972 January-March +15 ± 6 April-June +46 ± 6 July-September +53 ± 6 October-December +34 ± 9 1973 January-March +54 ± 5 April-June +2 ± 5 July-September +27 ± 6 October-December +51 ± 6 250 T. A. Brown et al. pies of 1973). We calculated these preliminary age-corrected e14C values following the conventions of Stuiver and Polach (1977), and using S13C values estimated for the samples from previously published 813C data (McConnaughey 1986). Analysis of the scatter of individual measurements of each sample about the mean for that sample confirms that the 1 Q error bars (derived from counting statistics uncertainties and typically ca. ± 6%0) accurately represent the variance in our data. These data clearly show significant variations in the 14C content of the coral on time scales shorter than one year.

Figure 2 also shows the SST record obtained at Academy Bay over the same period. Notwithstanding the uncertainty in the actual time period that each of the "three-month" coral samples represents, the similarity between our &4C results and the SST variations is striking. The correlation coefficient, r, for these two data sets as shown is 0.39 (n, the number of data points, =16, and P, the probability that this correlation coefficient value could be obtained from two uncorrelated data sets, = 0.2). In light of evidence that the growth rates of Galapagos corals in the cooler waters of April to December may drop to less than half the rates in the warmer waters of January to March (Glynn & Wellington 1983), we made appropriate adjustments to the length of time that each sample represents in a preliminary attempt to compensate for coral growth-rate variations due to changes in water temperature. Keeping in mind the limitations of the data presented by Glynn & Wellington (1983) and the uncertainties in these preliminary adjustments, we found that the correlation between the SST and adjusted L14C data sets is significant (r = 0.74, n =16, P < 0.002). We believe that this correlation clearly shows the effects of variations in the surface-ocean currents that flow into the Galapagos Islands on both the temperature and 14C content of the surface waters of the region.

Figure 2 also shows the annual average &4C values obtained by Druffel (1981) on corals collected from Gardner Bay, Espanola (Hood) Island. In general, these annual average data are consistent with annual averages of the "three-month" average E14C values that we obtained. Note that the locations within the annual bands chosen to mark the beginning of each year may have been slightly different in these two studies. Because of the significant intra-annual L14C variations evident in our data, such differences could be the source of small disagreements between the annual average i14C values obtained by Druffel and those calculated from our data. Some differences may also exist in local upwelling that could cause the 14C content of the waters at our Punta Pitt collection site to differ slightly from that at Druffel's Gardner t14C Bay site. The values that Druffel obtained for the "six-month" average coral samples from 1972 (28 ± 3%o and 32 ± 4%o) are clearly incompatible with Druffel's annual average and our "three-month" average &4C values obtained for that same year.

CONCLUSIONS

At this initial stage of our study of the intra-annual variability of the 14C content of Galapagos Islands corals, we emphasize two points. First, it is evident from the preliminary "three-month" average values that intra-annual variations exist that are significant both statistically and oceanographically. These intra-annual variations reflect subannual changes in the sources of the surface waters that flow through the Galapagos region. Second, the 1972 ENSO event is reflected in the 14C content of the surface waters in the Galapagos Islands, and the ENSO signal preserved in the coral is consistent with expected changes in the surface waters due to variations in surface-ocean currents during such events.

Clearly, the magnitude of the subannual variations (35-50%o during the period that we studied) is such that one must use caution in attempting to identify the origin of upwelled waters in the Galapagos region using average annual 14C content data. We believe that further subannual measurements on Galapagos and other corals will help us understand the current structure of the tropical Pacific, and help to constrain ocean-circulation models of the region. The ENSO signal in our 0140 data suggests that more detailed measurements of the coral record of the 1972 and other ENSO events will provide information on the Intra Annual Variability of 14C Content of Corals 251 variations of the surface-ocean currents in the Galapagos region during similar events throughout the available coral record.

ACKNOWLEDGMENTS We thank the following individuals at the University of Washington: Joseph A. Caggiano and Nancy Mar, for their efforts in the development and utilization of the 0.7-mm sample Ta encapsulation technique; Travis Saling, Quaternary Isotope Laboratory, for graphitization of samples; Glen Shen, Oceanography, for sampling the coral core; the staff of the Nuclear Physics Laboratory at the University of Washington (NPL), for their assistance; and William Weitkamp, Technical Director, NPL and Derek Storm, Director, NPL, for their encouragement and support.

REFERENCES

Balsley, D. R., Farwell, G. W., Grootes, P. M. and Grootes, P. M., Stuiver, M., Farwell, G. W., Leach, D. Schmidt, F. H. 1987 Ion source sample preparation D. and Schmidt, F. H. 1986 Radiocarbon dating with techniques for carbonl4 AMS measurements. In the University of Washington Accelerator Mass Gove, H. E., Litherland, A. E. and Elmore, D., eds., Spectrometry System. In Stuiver M. and Kra, R. S., Proceedings of the 4th International Symposium on eds., Proceedings of the 12th International 14C AMS. Nuclear Instruments and Methods in Physics Conference. Radiocarbon 28(2A): 237-245. Research B29: 37-40. Glynn, P. W. and Wellington, G. M. 1983 Corals and Brown, T. A., Farwell, G. W., Grootes, P. M., Quay, P. Coral Reefs of the Galapagos Islands. Berkeley, D. and Schmidt, F. H.199014C AMS at the Univer- University of California Press: 330 p. sity of Washington: Measurements in a shared facility McConnaughey, T. A. (ms) 1986 Oxygen and carbon at the 1% level on 0.4 mg samples. In Yiou, F. and isotope disequilibria in Galapagos corals: Isotopic Raisbeck, G. M., eds., Proceedings of the 5th Interna- thermometry and calcification physiology. Ph.D, dis- tional Conference on AMS. Nuclear Instruments and sertation, University of Washington, Seattle: 340 p. Methods in Physics Research B52: 351-356. Nozaki, Y., Rye, D. M., Turekian, K. K. and Dodge, R. Druffel, E. R. M. 1981 Radiocarbon in annual coral E. 1978 A 200 year record of carbon-13 and carbon- rings from the eastern tropical Pacific Ocean. Geo- 14 variations in a Bermuda coral. Geophysical physical Research Letters 8(1): 59-62. Research Letters 5(10): 825-828. 1987 Bomb radiocarbon in the Pacific: Annual Schmidt, F. H., Brown, T. A., Farwell, G. W. and and seasonal timescale variations. Journal of Marine Grootes, P. M. 1990 The University of Washington Research 45: 667-698. AMS system: A six-year technical update, In Yiou, 1989 Decade time scale variability of ventilation F. and Raisbeck, G. M., eds., Proceedings of the 5th in the North Atlantic: High-precision measurements International Conference on AMS. Nuclear Instru- of bomb radiocarbon in banded corals. Journal of ments and Methods in Physics Research B52: 229- Geophysical Research 94(C3): 3271-3285. 232. Druffel, E. M. and Linick, T. W. 1978 Radiocarbon in Stuiver, M. and Polach, H. A. 1977 Discussion: Report- annual coral rings of Florida. Geophysical Research ing of 14C data. Radiocarbon 19(3): 355-363. Letters 5(11): 913-916. Vogel, J. S., Southon, J. R., Nelson, D. E. and Brown, Druffel, E. M. and Suess, H. E. 1983 On the radiocar- T. A. 1984 Performance of catalytically condensed bon record in banded corals: Exchange parameters carbon for use in accelerator mass spectrometry. In and net transport of 14C02 between atmosphere and WOlfli, W., Polach, H. A. and Andersen, H. H., eds., surface ocean. Journal of Geophysical Research Proceedings of the 3rd International Symposium on 88(C2): 1271-1280. AMS. Nuclear Instruments and Methods in Physics Enfield, D. B. 1989 El Nino, past and present. Reviews Research B5: 289-293. of Geophysics 27: 159-187.

[RADIOCARBON, VOL. 35, No. 2, 1993, P. 253-262]

RADIOCARBON AGES OF LACUSTRINE DEPOSITS IN VOLCANIC SEQUENCES OF THE LOMAS COLORADAS AREA, SOCORRO ISLAND, MEXICO

JACK D. FARMERI, MARIA C. FARMER2 and RAINER BERGER3

ABSTRACT. Extensive eruptions of alkalic basalt from low-elevation fissures and vents on the southern flank of the dormant volcano, Cerro Evermann, accompanied the most recent phase of volcanic activity on Socorro Island, and created 14C the Lomas Coloradas, a broad, gently sloping terrain comprising the southern part of the island. We obtained ages of 4690 ± 270 BP (5000-5700 cal BP) and 5040 ± 460 BP (5300-6300 cal BP) from lacustrine deposits that occur within volcanic sequences of the lower Lomas Coloradas. Apparently, the sediments accumulated within a topographic depression between two scoria cones shortly after they formed. The lacustrine environment was destroyed when the cones were breached by headward erosion of adjacent stream drainages. This was followed by the eruption of a thin basaltic flow from fissures near the base of the northernmost cone. The flow moved downslope for a short distance and into the drainages that presently bound the study area on the east and west. The flow postdates development of the present drainage system and may be very recent. Our 14C data, along with historical accounts of volcanic activity over the last century, including submarine eruptions that occurred a few km west of Socorro in early 1993, underscore the high risk for explosive volcanism in this region and the need for a detailed volcanic hazards plan and seismic monitoring.

INTRODUCTION Socorro Island, located 460 km southwest of Cabo San Lucas and 650 km west of Manzanillo, Mexico, lies at the northern end of the Mathematician Ridge near its intersection with the Clarion Fracture Zone (Fig. 1). The Revillagigedo Archipelago comprises Socorro, Clarion, San Benedicto and Roca Partida Islands, which form the emergent part of a chain of seamounts that make up the north-trending Mathematician Ridge. The topography and paleomagnetic record of the sea floor in this region (Anderson & Davis 1973; Klitgord & Mammerickx 1982) provide evidence for the most recent plate-tectonic reorganization in the eastern Pacific (Handschumacher 1976). The Mathematician Ridge lies at a much shallower depth than expected, based on the average sea-floor age of the region (Parsons & Sclater 1977), and is interpreted to be a failed spreading center (Mammerickx & Klitgord 1982). Ridge abandonment was completed by ca. 3.15 Ma BP, with a shift in the principal locus of sea-floor spreading to its present position on the East Pacific Rise (Mammerickx, Naar & Tyce 1988).

Socorro, the largest of the Revillagigedo Islands (18°47'N, 110°57'W), is broadly oval in outline and covers ca. 140 km2. It comprises the emergent portion of a large basaltic shield volcano that rises from a sea-floor depth of about 3000 m. The highest point on the island is the dormant volcanic peak, Cerro Evermann (1130 m asl). The island landscape is dominated by lava flows, cinder cones and volcanic domes. The lavas of Socorro Island are predominantly trachyte, ranging to pantellerite; evidently, Socorro is the only volcanic island in the Pacific exhibiting siliceous, highly peralkaline compositions (Bryan 1976; Batiza & Vanko 1985).

Early expeditions to the Revillagigedos focused primarily on the biology of the islands (Richards & Brattstrom 1959). However, the unexpected explosive eruption of Volcan Barcena on San Benedicto in 1952 led to several studies that addressed the stratigraphy, geochemistry, petrology and eruptive history of the islands (Bryan 1959, 1964,1966,1967,1976; Richards 1957, 1959,

1NASA-Ames Research Center, MS-239-4, Moffett Field, California 94035 USA 2Departments of Geography and Anthropology, Unversity of California, Los Angeles, California 90024 USA 3Departments of Geography and Anthropology and Institute of Geophysics and Planetary Sciences, University of California, Los Angeles, California 90024 USA

253 254 J. D. Farmer, M. C. Farmer and R. Berger

Bahia Cornwallis 0 5 L KM

Rule Braithwaite

ISLA SOCORRO 30°N CE Cerro Evermann Fm.

Lomas Coloradas Fm.

Pre-Caldera Eruptives 25°N X Location of samples discussed in text

(Based on Bryan, 1976)

20°N

115°W 110°W 105°W

Fig. 1. Generalized geological map of Socorro Island, showing important place names, sample location and major lithostratigraphic units (after Bryan 1976) with inset of regional map of the Eastern Pacific showing the location of major sea floor features and Revillagigedo Islands. (EPR= East Pacific Rise; MR= Mathematician Ridge; RFZ= Rivera Fracture Zone; CFZ= Clarion Fracture Zone; SB= San Benedicto Island; S= Socorro Island; C= Clarion Island; RP= Roca Partida Island). Radiocarbon Ages from Volcanic Sequences, Socorro Island, Mexico 255

1964, 1966). Continuing interest in the tectonics of rift propagation and abandonment (Hey & Wilson 1982; Mammerickx, Naar & Tyce 1988) and the petrogenesis of alkalic magmas in failed rift settings (Batiza & Vanko 1985; Castillo et al. 1988; Bohrson, Reid and Grunder 1991) underscore the significance of present geologic studies. The Revillagigedo Islands exhibit high faunal and floral endemism (Brattstrom 1990; Walter 1989 a,b), suggesting the existence of viable island habitats for at least several hundred thousand years (H. Walter, personal communication 1990). However, previous attempts to date the volcanic sequences on Socorro Island by potassium-argon (K-Ar) and argon-argon (Ar-Ar) methods have met difficulties, perhaps from inherited argon, or because the island is geologically very young.

We conducted reconnaissance fieldwork in the summer of 1990. Given the apparently young age of rocks on Socorro, our approach emphasized the search for sedimentary facies (e.g., lacustrine, fluvial, paleosol horizons) that could be 14C dated. We believe that this approach, coupled with palynologic or other biostratigraphic techniques, will eventually provide a reliable basis for developing a chronostratigraphic framework for Socorro Island. Here, we report 14C ages for lacustrine/paludal deposits that we discovered within volcanic sequences of the Lomas Coloradas area. We hope that our preliminary work will heighten awareness of the potential volcanic hazard to the resident population. The major population center on Socorro Island is located in the southern Lomas Coloradas at Cabo Rule (Fig. 2). Explosive eruptions devastated San Benedicto Island in 1952 (Richards 1959, 1966; Brattstrom 1963); this, along with the young age of volcanism indicated by our study, serves to direct attention to the regional volcanic hazard. This concern was amplified in early 1993 when explosive eruptions occurred ca. 2.4 km west of Socorro Island (personal communication, RIDGE Office, February 1993).

ERUPTIVE HISTORY OF SOCORRO ISLAND

Despite abandonment of the Mathematician Ridge as an active spreading center by 3.15 Ma BP (Mammerickx, Naar and Tyce 1988), the Revillagigedo Islands have remained volcanically active. Richards and Dietz (1956) discussed in detail the recent eruptive history of Socorro Island. Explosive eruptions apparently occurred on Socorro Island in 1848, 1896 and 1953, although the activity was poorly documented and reportedly very localized. Present activity on the island seems to be restricted to fumaroles near the summit of Cerro Evermann, and steam emissions from several lava tunnels on the southwest side of the island.

Three primary eruptive phases are recognized in the stratigraphic record of Socorro Island (Bryan 1959, 1966, 1976). The earliest period was a shield-building phase that created the submerged portion of the island; Bryan (1966) suggested that the volcanoes emerged as islands in the late Tertiary. Based on sea-floor ages in the area, the Mathematician Ridge was active as a spreading center between ca. 6.5 and 3.0 Ma BP (Mammerickx & Klitgord 1982; Mammerickx, Naar & Tyce 1988). The stratigraphically oldest rocks on the island are thin flows of alkalic basalt (transitional to tholeiite), which are exposed in the sea cliffs at Cabo Pearce (Fig. 1). This early phase apparently preceded an explosive period when peralkaline silicic pyroclastics were erupted, mostly as ash flows, accompanying caldera collapse (Bohrson, Reed & Grunder 1991). This was followed by a period of quiescence, weathering and paleosol development (Bryan 1966). The most recent eruptive period was initiated by downfaulting of the western half of the island, accompanied by pyroclastic eruptions and pumice/obsidian flows that built Cerro Evermann, filling and largely obliterating the caldera. Late in this period, viscous rhyolite and trachyte flows issued from high- level vents on Cerro Evermann (Bryan 1966). 256 J. D. Farmer, M. C. Farmer and R. Berger

200 300 300 300 200 / 'I \ 100 \

HiD000ta a

0 05km C.l.: 100'

Qvb: Vesicular basalt [_J 01: Lascustrine Ss./Slt. Op: Basaltic pyroclastic cones Olct: Trachyte flows/domes Lomas Coloradas Qpb: Porphyritic Basalt Formation Meters Pre-Caldera Eruptives Qvb: Vesicular Basalt X Sample site — rD_? Unconformity 01: grainedia!cuirin: sandstone - - 5 Horizontal and Vertical 6 rhizomorphs Qp: Coarse lapilli of . 7 black, vesicular basalt ,_. - Carbonized plant fossils

Weathered surface Qpb: Porphyritic basalt

Pre-Caldera Unexposed ? Eruptives

A' Description of Dated Samples: 01: Light, greenish-grey fine-grained sandstones and siltstones containing —' carbonized roots and 100 100 root traces.

1— Interval 0—15cm ' from top of unit 4690 ± 270 years 0 2— Interval26—40cm from top of unit Oj4 5040 ± 460 years

Fig. 2. Composite topographic-geologic map of the Lomas Coloradas area of south Socorro Island (after Bryan 1959), with a generalized cross-section (A-A') and stratigraphic column for sample location (X), located just north of Bahia Braithwaite. The older units of the Lomas Coloradas are porphyritic alkalic basalt flows (Qpb) that erupted on the lower slopes of Cerro Evermann. Dated samples were obtained from lacustrine deposits (QI) associated with the pyroclastic cones (Qp) north of Bahia Braithwaite. The youngest unit is a thin, vesicular basalt flow (Qvb) that erupted from a small fissure at the base of the northernmost pyroclastic cone (Qp) near the sample location (X). The pyroclastic cones and trachyte domes postdate alkalic fissure basalts (Qpb) that underlie the Lomas Coloradas. The relative ages of the trachyte domes and pyroclastic cones (Qp) remain unresolved; they may actually overlap. (Reprinted with permission of author) Radiocarbon Ages from Volcanic Sequences, Socorro Island, Mexico 257

In the southeastern portion of the island, flows of alkalic basalt erupted extensively from lower elevation fissures and vents located near a line of pyroclastic cones that bound the upper slopes of the Lomas Coloradas. These flows range from porphyritic alkalic basalt to aphyric hawaiite (Bohrson, Reid & Grunder 1991), and cover ca. 15 km2 at the southeastern end of the island. Although areally extensive, the basalts are volumetrically insignificant, averaging <30 m thick (Bryan 1966).

Northeast of Caleta Binner (Fig. 2), the alkalic basalts of the Lomas Coloradas are overlain by cones and domal extrusions of peralkaline trachyte. These stratigraphic relations are exposed in the creek along the northwest side of the small trachyte dome designated "Cupula Hipopotamo" in Figure 2 (see also Bryan 1959). In this area, a bulbous mass of trachyte was extruded onto basalt flows of the Lomas Coloradas. The trachyte dome immediately west of Bahia Braithwaite exhibits a similar relation, where a slab of Lomas Coloradas basalt (the "trap door" structure of Bryan 1959) was uplifted to an elevation 150-200 m above the surrounding flow surface by intrusion of the trachyte dome immediately west of Bahia Braithwaite (Fig. 2). As dome growth proceeded, trachyte flows eventually broke through and were extruded onto the uplifted surface of the basalt flow (Bryan 1959).

SAMPLE LOCATION The sample locality is in the southern portion of the Lomas Coloradas, in "a peculiar area of flat- lying, tuffaceous sediments between two scoria cones near Bahia Braithwaite. . ." (Bryan 1959: 61). Stratigraphic relations are best exposed in the saddle area between the two cinder cones (Figs. 2, 3A) and within the southerly flowing drainage that bounds the scoria cones to the east (Fig. 3B). The cones differ compositionally, the northern one being peralkaline trachyte, the southern one, hawaiite (Wendy Bohrson, personal communication 1991). Interfingering of the basal pyroclastic units of the two cones suggests temporal overlap, although the southern cone appears to have persisted longer. The two scoria cones north of Bahia Braithwaite sit upon the gently, seaward-sloping surface formed by alkalic basalt flows of the lower Lomas Coloradas. Along the eastern margin of the southern cone, the creek has exposed the upper surface of the flows, which are mantled by 20-30 cm of buff-colored, fine-grained tuffaceous sandstone. At the base of this unit is a thin, irregular deposit of matrix-supported, reddish-brown conglomerate containing weathered boulders derived from the underlying basalt. This sequence is overlain by a massively bedded pyroclastic deposit of coarse, black, vesicular lapilli, which forms the base of the southern cone. The cinder deposit is interstratified with fine-grained, moderately indurated, tuffaceous sandstone and siltstone, which is exposed along the creek to the southeast (Fig. 3B), and further west, between the two cones.

In the saddle between the cones (Fig. 3B, arrow) the lower contact of the sandstone-siltstone sequence is gradational with the underlying cinder deposit, which has been extensively weathered, locally, to a rusty yellow color. The lower 5 cm of the sequence contains a dense zone of vertically oriented rhizoliths (root traces) that are preserved as casts in a friable, iron oxide-cemented sandstone. Above this zone is a thinly bedded, light greenish-gray tuffaceous sandstone and siltstone that weathers tan to rusty brown. Extensive, horizontal networks of branching root traces are preserved as casts on several of the exposed bedding plane surfaces (Fig. 4B, arrows). In addition, silicified roots and branches (up to 1 cm in diameter), small fragments of carbonized plant material, and rare, back-filled burrows, were found at several horizons within the deposit. 258 J. D. Farmer, M. C. Farmer and R. Berger

Fig. 3A. View eastward across Bahia Braithwaite from the trachyte dome just west of the "trap door" structure in Fig. 2. Arrow indicates "saddle area" between scoria cones, where dated samples were collected.

Fig. 3B. View looking northeast from within the saddle area indicated by arrow in Fig. 3A. Rock hammer to right of arrow in center of photo rests upon lacustrine sandstones (Ql) near position of oldest dated horizon (Sample 1, UCLA-2618, Fig. 2). The lacustrine deposit is overlain locally by a black vesicular basalt, Qvb, (upper left at base of cone), the stratigraphically youngest unit in the map area. This flow issued from fissures near the base of the northern cone, and flowed downslope, entering the upper part of the stream drainages that breached the cones from the east and west.

Retallack (1988) points out that fossil root traces are best preserved in formerly waterlogged sedimentary profiles. The location of the deposit in the low area between the scoria cones, the well- sorted nature of the sediment, absence of marine indicators and abundance of plant fossils, suggest deposition in a terrestrial aquatic (lacustrine) or perhaps paludal (swampy) environment, similar to the ephemeral lakes that are found within the craters of some cinder cones on the west side of Socorro Island (e.g., Lago Luna, Fig. 3 of Bryan 1959). The lacustrine units exhibit a broadly lenticular geometry and low-angle cross-bedding that dips gently to the north, toward the saddle area (Fig. 4A). The extensively developed, in-situ root networks, fine-grained and well-sorted texture of the sediments and abundance of organic material argue strongly against a pyroclastic surge origin for the sampled units.

A black, fine-grained, vesicular basalt overlies the lacustrine deposit in the saddle area between the two cones (Fig. 3B). The flow, which attains a maximum thickness of ca. 3 m, moved south a short distance from an east-west oriented fissure at the base of the northern cone. Apparently, the present drainage system existed prior to this eruption, because lobes of this flow moved downslope Radiocarbon Ages fromn Volcanic Sequences, Socorro Island, Mexico 259

Fig. 4A. View looking southwest from creek bound- ing the study area on the east. Outcrop in fore- ground exposes well-stratified lacustrine sandstones (01, arrow) underlain by a cinder deposit containing coarse lapilli of fine-grained, vesicular basalt, which makes up the base of the southernmost cone (hill in background). Note the northward-dipping, low-angle cross-stratification of the lower beds of the sandstone sequence, which thicken toward the saddle area between the two cones.

Fig. 4B. Bedding plane exposure of lower part of lacustrine sequence (Ql) at the location of the arrow shown in Fig. 3B. The bedding plane surface is covered by a dense, branching network of horizontal root casts (rhizoliths) and permineralized wood. Scale bar is 2.0 cm. and into the stream valleys that incise the saddle area from the east and west. The eastern lobe of the flow evidently moved beyond the lateral extent of the lacustrine sequence and far enough downslope so that, in places, the distal edges actually lie at a lower elevation than the stratigraphically older lacustrine sequence. This relation is most visible in the southerly flowing stream that bounds the area to the east. At that location, erosion has stripped back the edge of the younger flow, exposing its basal contact, an irregular unconformable surface lying just above the weathered surface of the older alkalic basalts of the Lomas Coloradas Formation.

METHODS AND RESULTS We collected bulk sediment samples for 14C dating from two intervals within the fossiliferous lacustrine sequence described above (see Fig. 2; Locality 83901C, Samples UCLA-2617 and -2618. The samples were collected from fine-grained, greenish-gray sandstones and siltstones containing permineralized roots, root traces and carbonized woody material (Fig. 4B). To minimize contamination from younger sources (e.g., the grass-covered slopes above the outcrops), we collected samples by excavating back about 1 m from the surface of the outcrop. Samples were 260 J. D. Farmer, M. C. Farmer and R. Berger wrapped in aluminum foil, or dropped into sterile glass vials and sealed with screw caps over aluminum foil, placed in zip-lock bags and double-bagged in plastic-coated cloth bags. Samples were pretreated first by mechanically removing root hairs, etc., to free them from gross contamination. Thereafter, samples were treated with dilute HCl to remove carbonate (found to be minimal). After repeated washing with distilled water, samples were placed in dilute, carbonate-free NaOH to eliminate any humic acids introduced from hypothetical younger sources. Samples were then washed with distilled water, and oven-dried at 110°C for several days. Standard lab procedures were used to obtain analytically pure CO2 from the samples, which were stored for more than a month in pressure cylinders to allow radon decay. They were then proportionally counted in a 200-m1 anticoincidence system with the following results: Sample interval: UCLA-2617 (0-15 cm below top of bed): 4690 ± 270 BP (5000-5700 cal BP) Sample interval: UCLA-2618 (26-40 cm below top of bed): 5040 ± 460 BP (5300-6300 cal BP).

DISCUSSION Although the results obtained for the two samples are consistent with the expected relative age relationships based on stratigraphic order, the 14C ages overlap for the sampled intervals and have a 50% probability of being contemporaneous (Austin Long, personal communication). Averaging the results for the two sample intervals gives an age estimate of 5575 cal BP. Combining the ages plus errors for the two intervals suggests a maximum age range of 5000-6300 cal BP.

Given the proposed geomorphic setting for the dated deposits, namely, a crater basin with internal drainage, it seems reasonable that a lacustrine/paludal environment would have developed there rather quickly, probably within tens to hundreds of years after the formation of the scoria cones. Sedimentologic evidence suggests that lacustrine deposition was fairly continuous over the time interval sampled and was apparently uninterrupted by pyroclastic eruptions. Thus, we believe that the age of the lacustrine deposits approximates the end of the pyroclastic activity that created the cones.

It is likely that the lacustrine environment that existed at this site was short-lived and that breaching and capture of the internal drainage by existing streams terminated deposition. The saddle area then became a site of erosion. Some time after erosion began, a thin vesicular basalt flow erupted from fissures at the base of the northernmost cone. Lobes of this young flow extend downslope to the east and west, entering the existing stream drainages. This indicates that the eruption postdates establishment of the present drainage system and, therefore, may be very young.

Without exception, the scoria cones and trachyte domes of the Lomas Coloradas are superimposed on the more widespread flood basalts, indicating that this activity postdates the fissure eruptions that created the southern portion of the island. The cones and domes do not overlap spatially, and their relative ages cannot be determined using field stratigraphic methods. Therefore, the relative timing of dome emplacement and the pyroclastic eruptions that produced these younger units, is presently unresolved. However, weathering profiles suggest that they may be similar in age.

Wave-cut terraces and coral platforms have been recognized on many islands (Stearns 1945) and coastal areas (Upson 1951) around the Pacific, lying at ca. 2 m asl. Bryan (1959) documented a well-developed coral bench near this elevation on Clarion Island. The age of the "2-m" terrace in the Pacific is not well-defined, although Stearns (1945) suggested an age of ca. 5000 BP. A comparable terrace has not yet been identified on Socorro Island, and Bryan (1959) suggested that it may have been buried by younger flows (e.g., the flows that created the Lomas Coloradas). If Radiocarbon Ages from Volcanic Sequences, Socorro Island, Mexico 261 we accept an age of ca. 5000 BP for the terrace on Clarion, and attribute its absence on Socorro to burial by younger flows, it follows that the basalts of the Lomas Coloradas must be younger than this estimate. In this study, we establish that the lacustrine deposits overlying Lomas Coloradas basalts are between 5000 and 6300 cal BP. Adopting this scenario significantly compresses the time frame available for creation of the Lomas Coloradas to an interval of perhaps a few hundred years, separating the development of the 2-m terrace and the cessation of lacustrine deposition at the study site. A corollary of this hypothesis is that the most recent phase of volcanism on Socorro Island is younger than that on Clarion. We emphasize, however, that the age of the terrace on Clarion Island is presently unknown. Given the controversy surrounding late Holocene eustatic sea-level change (Goudie 1983; Newman, Pardi & Fairbridge 1989), it is not possible to place any confidence in the implied age of 5000 BP for the 2 m terrace on Clarion. Further evaluation of the above hypotheses will depend on 1) obtaining a radiometric age for the Clarion terrace, and 2) demonstrating the presence or absence of a comparable terrace on Socorro Island.

Geochemical and isotopic studies suggest a complex magmatic history for Socorro Island (Bohrson, Reid & Grunder 1991). Data from such studies, in combination with palynologic methods, may eventually provide the most reliable basis for correlating stratigraphic units and constructing a relative time scale of events. However, establishment of an absolute chronology, particularly for the younger sequences on Socorro Island, may require the targeting of non-volcanic facies (e.g., lacustrine deposits, paleosols), which can be 14C dated.

The suggestion that the Lomas Coloradas has a very recent origin is important for assessing volcanic risk. Given the remote location of Socorro Island, it seems prudent at this time to develop a volcanic hazards plan that will afford some measure of safety to the resident population. This is evident when we recall the devastation that resulted from the 1952 eruption of Volcan Barcena on San Benedicto Island. Shortly after this eruption, a small seismic station was installed on Socorro. In recent years, the seismic station has not been maintained. During our field season in 1990, we experienced several small-magnitude earthquakes that suggested the possibility of subsurface magma movement. In addition, eruptions were reported a few kilometers west of Socorro in early 1993 (RIDGE office, personal communication, February, 1993). Accordingly, we recommend that seismic monitoring on Socorro Island be resumed immediately to assist in developing an ongoing program of volcanic risk assessment.

ACKNOWLEDGMENTS The authors gratefully acknowledge the help of the following: Drs. Hartmut Walter (UCLA), Kathy Marsaglia (University of Texas, El Paso) and James Bauer (Florida State University, Tallahassee) provided valuable discussion during initial development of the project. Enrique Carballido Sanchez (Tulane University), Adriana Guzman (University of New Orleans) and Wendy Bohrson (UCLA) provided valuable support in the field. We are also grateful to the following persons who arranged logistical support for the project: Dr. Hilberto Lopez Lira (Director, Institute of Oceanography, Mexico City), Almirante Salvador Gomez Bernal (Manzanillo), Contralmirante Ricardo Mendoza Anzo (Manzanillo), Capitan Gustavo Calderon Riveroll (Director, Institute of Oceanography, Manzanillo), and Corporal Jose Angel Hinjosa who acted as our personal attache in Manzanillo. Wendy Bohrson (UCLA), M. F. Sheridan (SUNY, Buffalo) and Austin Long (University of Arizona, Tucson) provided valuable comments on the manuscript. The project was funded by a grant from the Program on Mexico, University of California, Los Angeles. 262 J. D. Farmer, M. C. Farmer and R. Berger

REFERENCES

Anderson, R. and Davis, E. 1973 A topographic inter- ence Letters 58: 167-188. pretation of the Mathematician Ridge, Clipperton Klitgord, K. D. and Mammerickx, J. 1982 Northern East Ridge, East Pacific Rise System. Nature 241: 191- Pacific Rise: Magnetic anomaly and bathymetric 193. framework. Journal of Geophysical Research 87: Batiza, R. and Vanko, D. 1985 Petrologic evolution of 6725-6750. large failed rifts in the eastern Pacific: Petrology of Mammerickx, J. and Klitgord, K. D. 1982 Northern East volcanic and plutonic rocks from the Mathematician Pacific Rise: Evolution from 25 my. B.P. to the Ridge area and the Guadelupe Trough. Journal of present. Journal of Geophysical Research 87: 6751- Petrology 26: 564-602. 6759. Bohrson, W., Reid, M. and Grunder, A. 1991 The Mammerickx, J., Naar, D. and Tyce, R. 1988 The evolution of a predominantly peralkaline volcano in Mathematician paleoplate. Journal of Geophysical the eastern Pacific: Socorro Island, Mexico. Geologi- Research 93: 3025-3040. cal Society of America, Abstracts with Programs 23: Newman, W., Pardi, R. and Fairbridge, R. 1989 Some 331-332. considerations of the compilation of late Quaternary Brattstrom, B. 1963 Barcena Volcano, 1952. Its effect sea-level curves: A North American perspective. In on the fauna and flora of San Benedicto Island, Scott, D., Pirazzoli, P. and Honig, C., eds., Late Mexico. In Gressitt, J., ed., Pacific Basin Biogeogra- Quaternary Sea-Level Correlation and Applications. phy: A Symposium. Tenth Pacific Science Congress, Boston, Kluwer Academic Publishers: 207-228. Honolulu, Hawaii, 1961: 499-525. Parsons, B. and Sclater, J. 1977 An analysis of the 1990 Biogeography of the Islas Revillagigedo, variations of ocean floor bathymetry and heat flow Mexico. Journal of Biogeography 17: 177-183. with age. Journal of Geophysical Research 82: 803- Bryan, W. (ms.) 1959 High-silica alkaline lavas of 827. Clarion and Socorro Islands Mexico - their genesis Retallack, G. 1988 Field recognition of paleosols. and regional significance. Ph.D, dissertation, Univer- Geological Society of America, Special Paper 216: sity of Wisconsin: 164 p. 1-20. 1964 Relative abundance of intermediate members Richards, A. (ms.) 1957 Geology, Volcanology, and of the oceanic basalt-trachyte associations: Evidence Bathymetry of Isla San Benedicto, Mexico. Ph.D. from Clarion and Socorro Islands, Revillagigedo Dissertation, University of California, Los Angeles: Islands, Mexico. Journal of Geophysical Research 225 p. 69: 3047-3049. 1959 Geology of the Islas Revillagigedo, Mexico 1966 History and mechanism of eruption of soda 1. Birth and development of Volcan Barcena, Isla - rhyolite and alkali basalt, Socorro Island, Mexico. San Benedicto. Bulletin Volcanologique 22: 73-123. Bulletin Volcanologique 29: 453-479. 1964 Geology of the Islas Revillagigedo, Mexico 1967 Geology and petrology of Clarion Island, 4. Geology and petrography of Isla Roca Partida. Mexico. Geological Society of America Bulletin 78: Geological Society of America Bulletin 75: 1157- 1461-1476. 1164. 1976 A basalt-pantellerite association from Isla 1966 Geology of the Islas Revillagigedo, Mexico 2. Socorro, Islas Revillagigedo, Mexico. In Aoki, H. Geology and petrography of Isla San Benedicto. Pro- and Iizuka, S., eds., Volcanoes and Tectonosphere. ceedings of the California Academy of Sciences 33: Tokai University Press: 75-91. 361-414. Castillo, P., Batiza, R., Vanko, D., Malavassi, E., Richards, A. and Brattstrom, B. 1959 Bibliography, Barquero, J. and Fernandez, E. 1988 Anomalously cartography, discovery and exploration of the Islas young volcanoes on old hot-spot traces: I. Geology Revillagigedo. Proceedings of the California Academy and petrology of Cocos Island. Geological Society of of Sciences 29: 315-360. America Bulletin 100: 1400-1414. Richards, A. and Dietz, R. 1956 Eruption of Barcena Goudie, A. 1983 Environmental Change: Contemporary Volcano, San Benedicto Island, Mexico. Proceedings of Problems in Geography. Oxford, Clarendon Press: the Eighth Pacific Science Congress 2: 157-176. 258 p. Steams, H. 1945 Shorelines in the Pacific. Geological Handschumacher, D. 1976 Post-Eocene plate tectonics Society of America Bulletin 56: 1071-1078. of the eastern Pacific. In Sutton, G., Manghnani, M., Upson, J. 1951 Former shorelines of the Gaviota Quad- Moberly, R. and McAfee, E., eds., The Geophysics of rangle, Santa Barbara County, California. Journal of the Pacific Ocean Basin and Its Margin. American Geology 59: 415-446. Geophysical Union, Geophysical Monographs 19: Walter, H. 1989a Re-establishing the Socorro Dove: A 177-202. challenge for the Californias. Western Tanager 55: Hey, R. and Wilson, D. 1982 Propagating rift explana- 1-3. tion for the tectonic evolution of the northeast 1989b Small viable population: The red-tailed hawk Pacific, The pseudomovie. Earth and Planetary Sci- of Socorro Island. Conservation Biology 4: 441-443. [RADIOCARBON, VOL. 35, No. 2, 1993, P. 263-269]

b13C LATE PLEISTOCENE-RECENT ATMOSPHERIC RECORD IN C4 GRASSES1

L4 URENCE J. TOOLIN NSF Accelerator Facility for Radioisotope Analysis, The University of Arizona, Tucson, Arizona 85721 USA and CHRISTOPHER J. EASTOE Department of Geosciences, The University of Arizona, Tucson, Arizona 85721 USA

ABSTRACT. Samples of Setaria species from packrat middens, herbarium specimens and modern plants preserve a record of b13C of atmospheric CO2 from 12,600 BP to the present. No secular trend is detected between 12,600 and 1800 BP, when the mean value of 813C during that period was -6.5 ± 0.1%o (the error is the standard deviation of the mean). Our value agrees with S13C averages of pre-industrial CO2 from polar ice cores, and differs significantly from modern regional (-8.2 ± 0.1%o) and global (-7.7%c) values, which are higher because of fossil fuel burning.

INTRODUCTION

Recently, Marino and McElroy (1991) showed that carbon isotope ratios in a C4 grass, Zea mays (cultivated corn) tracked changes in the carbon isotope ratios of atmospheric CO2 from the years S13C 14C 1948-1987. Here, we present and age data for species of the C4 grass genus, Setaria. C4 studies. Briefly, plants have advantages over C3 plants as proxies for atmospheric carbon isotope photosynthesis is affected by in C3 plants, the fractionation of C isotopes that occurs during environmental factors, such as temperature and atmospheric CO2 concentrations. Fractionation in 1988). Tiezen and C4 plants is little influenced by such factors and is more uniform (O'Leary Boutton (1989) confirm reduced variation in C4 grasses. Henderson, von Caemmerer and Farquhar (1992) explore details of photosynthetic fractionation in C4 dicots and monocots.

Two Setaria species, S. macrostachya and S. leucopila, are common today across the southwest USA and into central Mexico (Rominger 1962). Both respond to summer rains, growing and flowering from July to October. These species are commonly preserved in middens constructed by packrats (Neotoma). Packrat middens are well-known reservoirs of paleoecological information and have been the subject of intensive study (Betancourt, Van Devender & Martin 1990). Packrats forage up to 50 m from their dens for plants for food and den-building, sampling the local vegetation. In dry shelters in arid climates, middens of plant matter and fecal pellets at the den may become indurated with crystallized urine, and persist for tens of thousands of years.

One of us (L. J. T.) has studied grasses preserved in dozens of packrat middens from sites across the American Southwest (e.g., Betancourt 1984; Van Devender & Toolin 1983; Van Devender, Toolin & Burgess 1990). Floral parts of Setaria macrostachya have been found in many midden assemblages from Arizona to Mexico (Fig. 1). The fragments studied for identification include the distinctive, indurated floral bracts (lemma and palea) that enclose the reproductive organs.

METHODS AND RESULTS All samples were given standard acid-base-acid treatment to remove carbonate and humic b13C contaminants, and combusted to CO2. Values of of the CO2 were measured with a 1a analy-

14C 'This paper was presented at the 14th International Conference, 20-24 May 1991, Tucson, Arizona.

263 264 L. J. Toolin and C. J. Eastoe

114° 111° 108° 105° 103° Phoenix (Sacramento Mts.) BB 33*. ARIZONA (Waterman an Manuel (AjoMts.) Mts.) NEW MEXICO AC, WM . MH Tucson x Kilt (Hueco Mts.) Peak EI Paso TT' TEXAS Noga les MEXICO Gulf I of Alpine California (Moravillas Can ) SONORA MC Bend I CHIHUAHUA

t Chihuahua /

.1 / 1 1 1 COAHUILA

l PV L 0 IOOMILES ((Puerto de , ` r Ventanrllas) OOKILOMETERS L. 0

Fig, 1. Sample location map. Locations of Holocene packrat middens are indicated by letters (e.g., BB, Big Boy). Locations of modem (1990) specimens are within the cross-hatched area near Tucson. tical precision of 0.13%o, based on repeated measurements of a laboratory standard calibrated to PDB. The CO2 was then reduced to graphite (Slota et al. 1987) for 14C dating by accelerator mass spectrometry (AMS) (Linick et al. 1986; Donahue, Jull & Toolin 1990). Although it is preferable b13C to measure on plant cellulose rather than on a mix of tissues, the small mass of Setaria fragments found in most samples (<1mg-5mg) precludes further loss of datable material that would occur on cellulose extraction. Direct dating of the Setaria was imperative because all plant species in a given midden may not be contemporaneous (Van Devender et al. 1985). In fact, in the course 14C of this study, we found that the age of the Setaria differed greatly from previous measurements on other plants from the same middens. For example, Waterman Mountains #2 was originally dated on juniper twigs at 21,510 BP, whereas the Setaria from this midden dated to 9895 BP. The lack of contemporaneity of different plant species in a single midden calls into question the value of results in which the age of one plant, or of bulk materials, is assumed to be the same as for other plants. This situation appears to have been overlooked by Marino et al. (1992).

Five midden samples were large enough to compare cellulose (method modified after Green S13C (1963)) and mixed-tissue values for (Table 1). These results can be compared to those from modern (AD 1990 ) Setaria samples. Modern samples were collected in eastern Pima and adjacent Santa Cruz counties, Arizona, and from The University of Arizona (ARIZ) herbarium. Cellulose versus mixed-tissue values for midden fragments differed less in the older specimens than in modern material (Table 1). We believe this is because the modern material contained more varied tissues (anthers, pistils, etc.) than the midden samples, where only the indurated, higher-cellulose- content lemmas and paleas remain. This effect is consistent with variations in different tissues of maize kernels (Tiezen & Fagre 1993). Atmospheric W 3C Record in C4 Grasses 265

TABLE 1. Mixed-Tissue versus Cellulose &3C (PDB)

Sample no. Mixed-tissue (%o) Cellulose (%o) (%o)

Modern Samples AA-6630 -11.8 AA-6631 -11.9 -11.4 AA-6646a -11.7 -10.9 AA-6646b -11.7 -10.9 AA-6342a -11.8 -11.1 AA-6342b -11.6 -11.1 AA-6864 -12.4 -11.5 AA-6644 -11.6 -10.9 AA-6343 -11.4 -10.8 0.7

Holocene Samples AA-6634 -9.9 0.3 AA-7056 -9.7 -9.5 AA-7058 -9.5 -9.1 AA-7059 -10.3 -10.1 AA-7061 -10.3 -10.0

Mean = 0.3

TABLE 2. Herbarium and Modern Samples Herbarium no. b13C Arizona county Year (ARIZ) PDB)

Pima 1939 23344 Pima 1949 77865 Pima 1957 123359 Pima 1968 169494 Pima 1978 211886 Pima 1986 285825 Pima 1990 290001 Pima 1990 289153 Pima 1990 --- Pima 1990 --- Pima 1990 290010 Santa Cruz 1990 289593 Santa Cruz 1990 289593 Santa Cruz 1990 289594 Santa Cruz 1990 289594

*Sample no, suffixes; L = S. leucopila; M = S. macrostachya; a, b indicate different plants collected under the same field number. 266 L. J. Toolin and C. J. Eastoe

TABLE 3. Specimens from Packrat Middens

Midden Locality AA-no. age (BP)

Waterman Mts lA Pima, AZ 7058 55 Waterman Mts 1C Pima, AZ 7313 70 Waterman Mts 1E Pima, AZ 7059 55 Waterman Mts 2 Pima, AZ 6862 160 Waterman Mts 9A2 Pima, AZ 7049 75 Waterman Mts 9B Pima, AZ 7050 70 Waterman Mts 9C Pima, AZ 7033 120 Waterman Mts 9D Pima, AZ 7051 10 Pima, AZ 7034 105 Waterman Mts 12A Pima, AZ 7035 75 Waterman Mts 12A Pima, AZ 7060 70 Waterman Mts 13A1 Pima, AZ 6863 90 Ajo Mts AC1B Pima, AZ 6951 175 Ajo Mts MH1D Pima, AZ 6953 115 Big Boy 3 Otero, NM 6952 255 Tank Trap 2 El Paso, TX 6860 175 Navar Ranch 1C1 E1 Paso, TX 7056 65 Navar Ranch 4B El Paso, TX 7061 85 Navar Ranch 11 El Paso, TX 7047 75 Navar Ranch 12 El Paso, TX 7048 85 Navar Ranch 14D E1 Paso, TX 7037 90 Navar Ranch 18C El Paso, TX 7038 60 Navar Ranch 19C El Paso, TX 7036 60 Maravillas Canyon 13 Brewster, TX 7046 50 Maravillas Canyon 16 Brewster, TX 7055 60 Puerto de Ventanillas Coahuila, Mex. 7314 190

*AZ = Arizona; NM = New Mexico; TX = Texas: Mex. = Mexico. No county given for Mexican sample.

S13C Table 2 presents mixed-tissue values of for all recent samples. S13C measurements for modern S. inacrostachya and S. leucopila (indicated by M or L in the table) were essentially the same, ranging from -11.6 to -12.4%o and -11.5 to -12.8%0, respectively. For nine modern (1990) plants, oC averages -11.90 ± 0.14%0.

Table 3 presents the results of our measurements on Setaria specimens from fossil packrat middens. The two species cannot be distinguished in this material. We show the values of S13C of florets (b1P). The data for 26 midden samples of Setaria (12,600-1800 BP) average -9.67 ± 0.10%o, and the data from the herbarium samples (AD 1939-1986) range between the data for ancient and 1990 samples (Table 2).

Farquhar (1983) and Henderson, von Caemmerer and Farquhar (1992) have established a function b13C b13C for relating the of C4 plants (81P) to b , the of the atmospheric CO2 fixed by the plant. For our Setaria, the relation between SlP of florets and Sla (Farquhar, personal communication) is

b1a = 2.9 + blP (1) 513C Atmospheric Record in C4 Grasses 267

To account for the offset between mixed-tissue and cellulose Slp values, we added the mean difference of the Holocene samples, 0.3%o from Table 1, to Equation (1); thus the relation for midden florets becomes Sla=3.2+S1p (2)

For the AD 1939-1990 samples, we added the mean differences for 1990 samples (0.7, from Table 3) to Equation (1) and applied the relation

513 - 3.6 + 513 a P (3)

We applied these corrections (Eq. 2, 3) to relate all of our S1P measurements to a common datum, independent of variability in the character of the mixed tissues, following Marino and McElroy (1991). Figure 2 presents the resulting Sla data for all samples, derived with the above equations.

-5 Long o

7 o Leuenberger et al. (1992) 7 7 ) This study -6

7 y

-7

8 -8

I I I I l i I I 13 II 9 7 5 3 I (940 (950 (960 (970 (980 (990 2000 YEARS BP(x)03) YEARS AD Fig. values Holocene . 2.6°C of atmospheric CO2. = Setaria data; _ = mean ± 1 o of pre-1939 samples. --- = scatter of individual measurements about the mean; ; = mean of data for samples collected in 1990; the error bar = 1Q. Direct measurements: e, o; o = the mean of Arctic and Antarctic ice core pre-industrial 6°C measurements.

The mean Sla from nine 1990 Setaria samples is -8.2 ± 0.14%0, lower than the 1986 global mean of -7.7%o (Keeling et al. 1989), but in line with differences between 513C values from clean-air and inland sites (Keeling 1961). Our value is empirically supported by comparison with an N20- corrected, seasonally averaged, mean atmospheric 513C value of -8.2%c, measured in 1983-1984 at Kitt Peak, southern Arizona (Leavitt & Long 1989).

From Equation (2), the mean value of Sla for all 26 midden samples is -6.5 ± 0.1%o, which differs significantly from the modern and global values. Our Holocene mean agrees with 513C data for CO2 in pre-industrial ice from Antarctica (Siple, -6.5 ± 0.07%o; South Pole -6.70 ± 0.13%o, Byrd, -6.49 ± 0.05%o, and Dye 3, Greenland, -6.41 ± 0.09%o) (Leuenberger, Siegenthaler & Langway 1992). The scatter of each of our Holocene measurements (0.5%o) is close to that of the modern samples 268 L. J. Toolin and C. J. Eastoe samples (0.42%o). This suggests that long-term changes in 8'a from 12,600-1800 BP could not have exceeded a few tenths of 1%o. Although our data indicate no secular trend in b'a during the Holocene, events of short duration or small amplitude might remain undetected.

The nearly step-wise drop of 1.8%o between 1957 and 1968 may be real, the few data notwithstanding. This interval of time corresponds approximately with a surge in the burning of fossil fuels in the southwestern USA, as a result of urban growth and ore smelting, and to a global increase in fossil fuel CO2 production (Marland 1990). We plan to make more measurements on 20th century specimens to determine this segment of the Sla vs. time curve with better precision. Recently, Martinelli et al. (1991) showed that large outputs of biogenically fractionated CO2 from the Amazon River produced a gradient in bthat could mimic secular changes produced, for example, by changes in atmospheric CO2 concentration. In our region, there is no such large biogenic effect today, and none appears to have operated in the past. If our Holocene samples are grouped by region, there is no distinction between the western group (Arizona) with mean 8'P = -9.6 ± 0.15%o and the eastern group (Texas and New Mexico) with mean 8'P = -9.7 ± 0.13%o.

CONCLUSION Our results provide the first measurements of C-isotope values of Holocene atmospheric CO2, constrained by AMS 14C dating of the carbon used for the stable-isotope measurements. Reconstructions of b'a such as ours may help clarify the interpretation of 8'P changes in C3 plant tissues (e.g., tree rings) in which C-isotope fractionation is strongly influenced by environmental factors (Leavitt & Danzer 1991; Krishnamurthy & Epstein 1990).

ACKNOWLEDGMENTS We are greatly indebted to our colleagues at the University of Arizona. D. J. Donahue and A. J. T. Jull reviewed the manuscript and made important suggestions. T. Lange provided support for the laboratory work. C. T. Mason, Jr., gave access to Herbarium specimens. T. R. Van Devender of the Arizona-Sonora Desert Museum, Tucson, provided all midden Setaria. We also thank B. Marino, U. Siegenthaler and G. Farquhar for helpful reviews of an earlier manuscript. S. Leavitt provided further review and much valuable discussion.

REFERENCES

Betancourt, J. L. 1984 Late Quaternary plant zonation G. D. 1992 Short-term measurements of carbon and climate in southeastern Utah. Great Basin isotope discrimination in several C4 species. Austra- Naturalist 44: 1-35. lian Journal of Plant Physiology 19: 263-285. Betancourt, J. L., Van Devender, T. R. and Martin, Keeling, C. D. 1961 The concentration and isotopic P. S., eds., 1990 Packrat Middens - The Last 40,000 abundances of carbon dioxide in rural and marine air. Years of Biotic Change. Tucson, The University of Geochimica et Cosmochimica Acta 24: 227-298. Arizona Press: 467 p. Keeling, C. D., Bacastow, R. B., Carter, A. F., Piper, S. Donahue, D. J., Jull, A. J. T. and Toolin, L. J. 1990 C., Whorf, T. F. Heimann, M., Mook, W.G. and Radiocarbon measurements at the University of Roeloffzer, H. 1989 A three dimensional model of Arizona AMS Facility. Nuclear Instrumentation and atmospheric CO2 transport based on observed winds: Methods in Physics Research B52: 224-228. 1. Analysis of observational data. American Geo- Farquhar, G. D. 1983 On the nature of carbon isotope physical Monograph 55: 165-236. discrimination in C4 species. Australian Journal of Krishnamurthy, R. V. and Epstein, S. 1990 Glacial- Plant Physiolology 10: 205-226. interglacial excursion in the concentration of atmo- 13C/12C Green, J. W. 1963 Wood cellulose. In Whistler, R. L., spheric C02: Effect in the ratio in wood ed., Methods of Carbohydrate Chemistry III. New cellulose. Tellus 42B: 423-434. b13C York, Academic Press: 9-21. Leavitt, S. W. and Danzer, S. R. 1992 variations Henderson, F. A., Von Caemmerer, G. D. and Farquhar, in C3 plants over the past 50,000 years. In Long, A. S13C Atmospheric Record in C4 Grasses 269

and Kra, R. S., eds., Proceedings of the 14th Interna- O'Leary, M. H. 1988 Carbon isotopes in photosynthesis. 14C tional Conference. Radiocarbon 34(3): 783-791. Bioscience 38: 328-336. Leavitt, S. W. and Long, A. 1989 Variation of concen- Rominger, J. 1962 Taxonomy of Setaria (Gramineae) in 13C/12C tration,14C activity, and ratios of CO2 in air North America. Illinois Biological Monograph 29. samples from Kitt Peak, Arizona. In Long, A. and Urbana, University of Illinois Press: 1-132. Kra, R. S., eds., Proceedings of the 13th International Slota, P. J., Jull, A. J. T., Linick, T. and Toolin, L. J. 14C Conference. Radiocarbon 31(3): 464-468. 1987 Preparation of small samples for 14C accelerator Leuenberger, M., Siegenthaler, U. and Langway, C. targets by catalytic reduction of CO. Radiocarbon 1992 Carbon isotope composition of atmospheric CO2 29(2): 303-306. during the last ice age from an Antarctic ice core. Tiezen, L. L. and Boutton, T. W. 1989 Stable carbon Nature 357: 488-490. isotopes in terrestrial ecosystem research. In Rundel, Linick, T. W., Jull, A. J. T., Toolin, L. J. and Donahue, P. W., Ehleringer, J.R, and Nagy, K. A., eds., Stable D. J. 1986 Operation of the NSF-Arizona accelerator Carbon Isotopes in Ecological Research, Ecological facility for radioisotope analysis, and results from Studies Series. New York, Springer Verlag:167-195. selected collaborative research projects. In Stuiver, Van Devender, T. R,, Martin, P. S., Thompson, R. S., M. and Kra, R. S., eds., Proceedings of the 12th Jull, A. J. T., Long, A., Toolin, L. J. and Donahue, 14C International Conference. Radiocarbon 28(2A): D. J. 1985 Fossil packrat middens and the tandem 522-533. accelerator mass spectrometer. Nature 317: 610-613. Marino, B. D. and McElroy, M. B. 1991 Isotope com- Van Devender, T. R. and Toolin, L. J. 1983 Late position of atmospheric CO2 inferred from carbon in Quaternary vegetation of the San Andres mountains, C4 plant cellulose. Nature 349: 127-131. Sierra County, New Mexico. In Eidenbach, P. L., ed., Marino, B. D., McElroy, M. B., Salawitch, R. J. and Prehistory ofRhodes Canyon, New Mexico. Tularosa, Spaulding, W. G. 1992 Glacial-to-interglacial varia- New Mexico, Human Systems Research: 33-54. tions in the carbon isotopic composition of atmo- Van Devender, T. R., Toolin, L. J. and Burgess, T. L. spheric CO2. Nature 357: 461-466. 1990 The ecology and paleoecology of grasses in Marland, G. 1990 Global CO2 emmisions. In Boden, T. selected Sonoran Desert plant communities. In A., Danciruk, P. and Farrell, M. P., eds., Trends '90: Betancourt, J. L., Van Devender, T. R. and Martin, A Compendium of Data on Global Change. Oak P. S., eds., Packrat Middens - The Last 40,000 Years Ridge National Laboratory/CDIAC-36: 92 p. of Biotic Change. Tucson, The University of Arizona Martinelli, L. A., Devol, A. H., Victoria, R. L. and Press: 326-349. Rickey, J. E. 1991 Stable carbon isotope variation in C3 and C4 plants along the Amazon River. Nature 353: 57-59.

[RADIOCARBON, VOL. 35, No. 2, 1993, P. 271-276]

CARBON ISOTOPIC COMPOSITION OF DEEP CARBON GASES IN AN OMBROGENOUS PEATLAND, NORTHWESTERN ONTARIO, CANADA

RAMON ARAVENA1, B. G. WARNER2, D. J. CHARMAN3, L. R. BELYEA4 S. P. MATHURS and HENRI DINEL6

ABSTRACT. Radiocarbon dating and carbon isotope analyses of deep peat and gases in a small ombrogenous peatland in northwestern Ontario reveals the presence of old gases at depth that are 1000-2000 yr younger than the enclosing peat. We suggest that the most likely explanation to account for this age discrepancy is the downward movement by advection of younger dissolved organic carbon for use by fermentation and methanogens bacteria. This study identifies a potentially large supply of old carbon gases in peatlands that should be considered in global carbon models of the terrestrial biosphere.

INTRODUCTION

Northern peatlands play an important role in the cycling of atmospheric trace gases. Recent estimates suggest that about 65% of all methane (CH4) emissions from natural peatlands originate in northern regions (Harriss et al. 1985; Matthews & Fung 1987; Crill et al. 1988; Gorham 1991). These estimates are based on measurements of surface fluxes, but little is known about the internal carbon dynamics of peatlands. Recent studies using theoretical models have shown that decomposition of organic matter is a continuous long-term process in these systems (Clymo 1984, 1991; Warner, Clymo & Tolonen 1993). High concentrations of CH4 and CO2 at depth in peatlands (Dinel et al. 1988; Brown, Mathur & Kushner 1989; Claricoates 1990; Mathur, Dinel & Levesque 1991; Buttler et al. 1991; Romanowics, Siegel & Glaser 1991) support these findings. The fate of these gases is virtually unknown, and specifically, their export by groundwater has not been considered. Thus, accurate estimates of future contributions to atmospheric CO2 and CH4 pools by peatlands will depend on our understanding of the natural carbon dynamics of these systems. Of particular importance are the pathways for production and efflux of the gaseous components, and coupling of these pathways with those in the surrounding environment.

In this paper, we present preliminary findings on the use of carbon isotopes to investigate carbon sources for gases and carbon transport in peatlands. This work is part of ongoing research employing an integrated approach of examining the hydrological, geochemical and ecological characteristics of northern peatlands to understand carbon-cycling processes.

STUDY SITE

The study site is a small ombrogeneous peatland about 180 hat, situated 6 km north of the town of Rainy River on the Minnesota-Ontario border in northwestern Ontario, Canada (48°47' N; 94°33' W; Fig. 1). The peatland has developed in a small, shallow basin lacking any surface outlets. The basin became available for peatland development after deglaciation and withdrawal of proglacial Lake Agassiz by ca. 9.5 ka BP, but it remained dry until ca. 4.6 ka BP (Belyea 1991; B. G. Warner,

'Centre for Groundwater Research and Wetlands Research Centre, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 2Wetlands Research Centre and Department of Geography, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 3Department of Geographical Sciences, University of Plymouth, Plymouth, Devon PL4 8AA, United Kingdom °School of Biological Sciences, Queen Mary and Westfield College, London El 4NS, United Kingdom 575 Foxleigh Crescent, Kanata, Ontario, Canada K2M 1V6 6Centre for Land and Biological Resources Research, Research Branch, Agriculture Canada, Ottawa, Ontario, Canada KIA 0C6

271 272 Ramon Aravena et al.

Fig. 1. Location of study area

unpublished data). Hydrological conditions after this time promoted peatland inception through primary peat formation. The modern vegetation consists of an open shrub peatland community with well-developed microtopography of Sphagnum fuscum, S. angustifolium and Cladina lichens. Closed Picea mariana forest occurs in some areas of the peatland.

MATERIAL AND METHODS Seven bulk peat samples were collected from a 208-cm-deep sequence from the western (oldest) part of the peatland for 14C analysis. Peatland gases and water samples were collected at three depths in the profile, 65-85,102-122 and 150-170 cm from the surface, at the end of the summer. The water table was positioned at 60 cm during sampling. Gases were collected with a stainless steel probe described by Dinel et al. (1988).

For analyses of 14C and 13C, CO2 and CH4 were separated and purified cryogenically. CH4 was converted to CO2 by passing the gas with a helium oxygen mixture through copper oxide heated to 850°C. A background of <0.5% modern carbon was obtained using commercial 14C-free methane for the whole conversion process. 14C analyses on gases were done by accelerator mass 14C spectrometry (AMS) at the IsoTrace Radiocarbon Facility, University of Toronto, Canada. Peat dates were obtained by liquid scintillation counting (LSC) at the Environmental Isotopes 14C b13C Laboratory, University of Waterloo. All dates were normalized to = -25 %o. All ages are reported to ± 1 a. Tritium (3H) was analyzed on water samples by LSC with a counting error of ± 8 TU (direct counting) and ± 0.8 TU (electrolytic enrichment), respectively. Carbon Isotopes in Peat Gases in Ontario 273

RESULTS AND DISCUSSION Carbon Isotope Data 14C dates obtained on peat samples indicated that the peat carbon began accumulating at least 4.6 ka BP in the peatland basin. The application of a model of peat growth (Clymo 1984) to the record of peat accumulation at the site suggests that decay occurs throughout the anoxic layer of the peat mass (Belyea 1991).

"C dates on both species of gas at each depth were of comparable age, but about 1-2 ka younger than the enclosing peat (Table 1). Gas and peat 14C ages should have agreed if the sole carbon source for gas production was the enclosing peat, without input of younger carbon from the overlying strata. The ombrogeneous character of the peatland suggests two alternative explanations for the discrepancies between gas and peat ages. First, CO2 and CH4 that had been produced closer to the surface from younger carbon may have been transported downward in peatland waters and mixed with older gases produced at depth. Alternatively, and more likely, dissolved organic carbon (DOC) originating from younger peat may have infiltrated deeper strata and provided fermentation and methanogen bacteria with a carbon source younger than the enclosing peat. Recent studies do not support the first hypothesis. They show deeper peat layers more highly saturated with methane than peat layers closer to the surface (Dinel et al. 1988; Brown, Mathur & Kushner 1989; Claricoates 1990; Mathur, Dinel & Levesque 1991; Buttler et a1.1991). In either case, the gas ages represent an average estimate of the ages of carbon sources used in gas production. 14C TABLE 1. Ages for Bulk Peat and CO2 and CH4 Gases from the Rainy River Peatland, Northwestern Ontario

Water Peat Gases

813C 14C 3H %o age %o age Depth (cm) (TU) (PDB)

CO2 CH4 CO2

50-51 -27.4 1520 70 71-73 -25.6 1780 70 65-85 34 ± 8.0 70 100-101 -26.9 2460 80 102-122 -5.9 -63 70 70 70 70 150-170 46 ± 3.0 70 181-182 -27.4 3440 70 208-209 -29.0 4600 70

3H analyses on water accompanying the gases (Table 1) indicate the presence of 3H of thermonuclear origin in the peat pore water at a depth of at least 150-170 cm. Thermonuclear 3H in Canadian precipitation reached peak values of 3400 TU in 1963, and values have steadily decreased to around 40 TU since 1985. Precipitation before the start of thermonuclear tests in 1950 have 3H concentrations less than 5 TU (Robertson & Cherry 1988). Gorham and Hofstetter (1978) have also reported the presence of thermonuclear 3H at a depth >2.0 m in a raised peatland in nearby Minnesota. We have found high concentrations of thermonuclear 3H at 140 cm depth in 4500-yr-old peat in northeastern Ontario (Charman, Aravena & Warner 1992). 3H at depth in the Rainy River peatland further confirms the possibility of transport of young carbon from the shallow 274 Ramon Aravena et al.

14C part of the peatland. Similarly, atmospheric carbon enriched in thermonuclear (McNeely 1988) has accumulated as peatland organic matter since 1950, thereby providing a carbon source enriched in 14C for transport downwards and gas production deep within the peatland. This is confirmed by '4C data on near-surface peat at Rainy River that range from 101 to 111 pMC (Belyea & Warner 1993). The other possibility that could explain the apparent age discrepancy between gases and peat carbon is transport of younger carbon from shallow depths induced by the sampling apparatus. Gas collection is carried out by a small vacuum created in the sampling chamber. This pressure differential could induce gas movement through the outer part of the gas probe. This possibility can be evaluated using the b13C results obtained on the CO2 and CH4 gases. S13C These data show a trend toward depleted values for CO2 with values of -1.5%o at 150-170 cm depth, changing to values of -12.4%o at the shallowest sampling depth (Table 1). We observed a similar trend, decreasing from -60%o to -74%o in the CH4 samples. These isotopic changes have been reported in peat (Landsdown, Quay & King 1992) and in marine sediments (Claypool & Kaplan 1974), and can be attributed to isotopic effects occurring during CH4 production. One parameter used to estimate the importance of acetate fermentation and CO2 reduction for CH4 production is the relation

(813Ccx +1000) a- (1) (b +1000) 13Cco1

Previous estimates suggest that values <0.95 are typical of marine environments where CO2 reduction is the main carbon pathway, as opposed to freshwater environments characterized by c >0.95 where acetate fermentation is the main carbon pathway (Whiticar, Faber & Schell 1986). However, values <0.95 have been reported recently for wetland gases (Landsdown, Quay & King 1992). Their results show that CO2 reduction can also be an important mechanism in wetland environments. values for Rainy River gases range from 0.937 to 0.942 in agreement with data reported by Landsdown, Quay and King (1992). Thus, the 13C gas depth profiles are clearly controlled by methanogenesis and do not support a mixing scenario between shallow and deeper gases. Transport of younger shallow carbon by the sampler seems unlikely, and should not cause the 14C age discrepancies between gases and peat at this site. Definitive information about transport of carbon and carbon sources for gases in peatlands requires a thorough evaluation of the internal carbon dynamics of whole peatland systems.

Carbon Dynamics in the Peatland The role of advection and dispersion as transport mechanisms for carbon in peatlands is not well known; however, recent hydrological studies in northern Minnesota have shown that advection plays a significant role in the groundwater flow regime of peatlands (Chason & Siegel 1986; Siegel & Glaser 1987). Carbon transport by diffusion could also occur in peatlands because DOC concentration profiles in peatland pore water show high concentrations near the surface, decreasing with depth (McKnight, Thurman & Wershaw 1985).

Significant quantitites of CO2 and CH4 at depth in peatlands have been documented previously (Dine! et al. 1988; Brown, Mathur & Kushner 1989; Claricoates 1990; Mathur, Dine! & Levesque 1991; Buttler et al. 1991; Romanowics, Siegel & Glaser 1991), but the ultimate fate of these deep gases remains a mystery. 14C dates on CH4 emitted from peatland surfaces suggest that recent carbon is not the only source involved in the production of these gases (Wahlen et al. 1989). Carbon Isotopes in Peat Gases in Ontario 275

Clearly, at least some old gas leaves the deep peat reservoir via surface evolution, but another fraction may remain trapped in void spaces, and eventually will be transported out of the peatland by water. The efflux of dissolved CO2 and CH4 via surface and groundwater regimes has been effectively ignored in previous considerations of peatland carbon dynamics; so too, the nature of the carbon source (peat or DOC) and the translocation of carbon sources dissolved in peatland waters. These processes are linked intimately to the dynamics of water movement in peatland systems, especially in the deeper anaerobic zone. Recent studies have shown that active groundwater movement occurs in peatland systems, and these systems can behave as recharge or discharge areas (Chason & Siegel 1986; Siegel & Glaser 1987). The origins and dynamics of peatland gases are evidently far more complex than previously recognized. These complexities have important implications for predicting future concentrations of atmospheric trace gases. Carbon isotope mass balance models of CH4 generation currently consider only two carbon sources: a modem carbon pool derived from recent biogenic sources, and a fossil (14C-free) pool derived from coal and natural gas (Wahlen et al. 1989; Lowe et al. 1988; Manning et al. 1990). We have shown that old peat of northern peatlands is another potentially large carbon source whose isotopic signal is masked by that of the modem carbon pool. Our preliminary results suggest that peatland waters may play an important role in translocation and cycling of carbon gases and their organic precursors. Although we have come a long way from the early view that peatlands function indefinitely as carbon sinks, much remains to be learned about the carbon dynamics of these systems.

ACKNOWLEDGMENTS This work was funded by the Natural Sciences and Engineering Research Council of Canada. This is Centre for Land and Biological Resources Research Contribution 92-60.

REFERENCES

Belyea, L. R. (tins.) 1991 Dynamics and implications of College, London. peat accumulation in a northwestern Ontario peatland. Claypool, G. E. and Kaplan, I. R. 1974 The origin and M. Sc. thesis, University of Waterloo. distribution of methane in marine sediments. In Belyea, L. R. and Warner, B. G. (ms.) 1993 Dating of Kaplan, I. R., ed., Natural Gases in Marine Sedi- the near-surface layer of a peat deposit from North- ments. New York, Plenum Publishing Co.: 99-103. western Ontario, Canada. Submitted to Oikos. Clymo, R. S. 1984 The limits to peat bog growth. Brown, A., Mathur, S. P. and Kushner, D. J. 1989 An Philosophical Transactions of the Royal Society, ombrotrophic bog as a methane reservoir. Global London B303: 605-654. Biogeochemical Cycles 3: 205-213. 1991 Peat growth. In Shane, L. C. K. and Buttler, A. J. Dinel, H., Levesque, M. and Mathur, S. P. Cushing E. J., eds., Quaternary Landscapes. Minne- 1991 The relation between movement of surface apolis, University of Minnesota Press: 76-112. water and gaseous methane in a basin bog with a Crill, P. M., Bartlett, K. B., Sebacher, D. I., Harris, R. novel instrument. Canadian Journal of Soil Science C., Verry, E. S., Gorham, E., Madzar, L. and Sanner, 71: 427-438. J. 1988 Global Biogeochemical Cycles 2:371-384. Charman, D. J., Aravena, R. and Warner, B. G. 1992 Dinel, H., Mathur, S. P., Brown, A. and Levesque, M. Carbon dynamics and accumulation in a forested 1988 A field study on the effect of depth on methane peatland in northeastern Ontario, Canada. Abstract. production in peatland waters: Equipment and pre- Program, Carbon and Peatlands Symposium, Nott- liminary results. Journal of Ecology 76: 1083-1091. ingham, England, British Ecological Society. Gorham, E. 1991 Northern peatlands: Role in the carbon Chason, D. and Siegel, D. I.1986 Hydraulic conductivi- cycle and probable responses to climatic warming. ty and related physical properties of peat, Lost River Ecological Applications 1: 182-195. peatland, northern Minnesota. Soil Science 142: Gorham, E. and Hofstetter, R. H. 1978 Penetration of 91-99. bog peats and lake sediments by tritium from atmo- Claricoates, J. (ms.) 1990 Gas production during peat spheric fallout. Ecology 52: 898-902. decay. Ph.D. thesis, Queen Mary and Westfield Harriss, R. C., Gorham, E., Sebacher, D. I., Bartlett, 276 Ramon Aravena et ad.

K. B. and Febble, P. A. 1985 Methane flux from McNeely, R. 1988 Radiocarbon dating laboratory. northern peatlands. Nature 315: 652-653. GEOS 17: 10-12. Lansdown, J. M., Quay, P. D. and King, S. L. 1992 CH4 Robertson, W. D. and Cherry, J. A. 1988 Tritium as an production via CO2 reduction in a temperate bog: A indicator of recharge and dispersion in a groundwater source of b13C-depleted CH4. Geochimica et Cosmoc- system in central Ontario. Water Resources Research hi mica Acta 56: 3493-3503. 25: 1097-1109. Lowe, D. C., Brenninkmeijer, A. M., Manning, M. R., Romanowics, E. A., Siegel, D. I. and Glaser, P. H. 1991 Sparks, R. and Wallace, G. 1988 Radiocarbon Deep methane in the Glacial Lake Agassiz peatland. determination of atmospheric methane at Baring Abstract. American Geophysical Union Spring Head, New Zealand. Nature 332: 522-525. Meeting. EOS 72: 80. Manning, M. R., Lowe, D. C., Melhuish, W. H., Sparks, Siegel, D. I. and Glaser, P. H. 1987 Groundwater flow R. J., Wallace, G., Brenninkmeijer, C. A. M. and in a bog-fen complex, Lost River peatland, northern McGill, R. C. 1990 The use of radiocarbon measure- Minnesota. Journal of Ecology 75: 743-754. ments in atmospheric studies. Radiocarbon 32(1): Wahlen, M., Tanaka, N., Henry, R., Deck, B., Zeglen, 37-58. J., Vogel, J. S., Southon, J., Shemesh, A., Fairbanks, Mathur, S. P., Dinel, H. and Levesque, M. 1991 The R. and Broecker, W. 1989 Carbon-14 in methane role of methane gas in peatland hydrology: A new sources and in atmospheric methane: The contribu- concept. In Jeglum, J. K. and Oberend, R. P., eds., tion from fossil carbon. Science 245: 286-290. Proceedings, Symposium '89, Peat and Peatlands: Warner, B. G., Clymo, R. S. and Tolonen, K. 1993 Diversification and Innovation 1. Quebec City, Can- Implications of peat accumulation at Point Escumin- ada: 153-157. ac, New Brunswick. Quaternary Research 39: Matthews, E. and Fung, I. 1987 Methane emmission 245-248. from natural wetlands: Global distribution, area, and Whiticar, M. J., Faber, E. and Schoell, M. 1986 Biogen- environmental characteristics of sources. Global ic methane formation in marine and freshwater Biogeochemical Cycles 1: 61-86. environments: CO2 reduction vs. acetate fermentation- McKnight, D., Thurman, M. E. and Wershaw, R. L. isotope evidence. Geochimica et Cosmochimica Acta 1985 Biogeochemistry of aquatic humic substances 50: 693-709. inThoreau's Bog, Concord, Massachussets. Ecology 66: 1339-1352. [RADIOCARBON, VOL. 35, No. 2, 1993, P. 277-286]

ISOTOPIC ANALYSIS OF GROUNDWATER AND CARBONATE SYSTEM IN THE SURDULICA GEOTHERMAL AQUIFERI

MUNEVERA HADZISEHOVIC, NADA MIME VIC, VOJISLAVA SIPKA DUSAN GOLOBOCANIN and RADULE POPOVIC2

Boris Kidric Institute of Nuclear Sciences, Vinca, P. 0. Box 522, 11001 Belgrade, Yugoslavia

ABSTRACT. We present here results of our investigation of the isotopic chemical composition of groundwater and carbonates in the Surdulica geothermal aquifer, Serbia. We considered the effects of carbonate dissolution and measured 13C, 14C, D,180, 3H, field pH, temperature, Na`, Ca2+, Mg2`, HC03 and other aqueous species from 30 springs and boreholes. Geothermal waters are supersaturated with calcite. Carbon isotope compositions vary with carbonate mineral dissolution. The 8D and S'80 of groundwater samples fit the meteoric water line, and indicate that groundwater is recharged mainly from higher altitudes and the cold season. Different groundwater residence times point out two mechanisms for their formation; fissure flow for young waters and standard diffusion processes for old ones.

INTRODUCTION

The Surdulica geothermal system is a major segment of the Serbia-Macedonian massif. The -250 km2 area is a potential source of geothermal energy and drinking water (-250 liter s, 45-126°C). The largest hot springs are concentrated near the Bujanovac and Vranjska Spas (ca. 30 springs); this group of springs, which seems to discharge at locations of the main aquifer system reaching ca. 3000 m deep, has not yet been described. Also, relations between groundwater flow patterns and the physical structure of the aquifer have been poorly understood. Deep drilling has been limited to a few exploratory boreholes (up to 2000 m). Faults, particularly along the periphery of the South Morava valley, appear to be significant factors controlling the discharge from many of the parts of the Surdulica-Vranj e-Buj anovac area. Several researchers have investigated geophysical and hydrological conditions (Zujovic 1893; Dimitrijevic et a1.1965; Babovic et a1.1970; Anderson 1985; Popovic 1990). Since 1986, we (Milovanovic et al. 1989) have investigated the isotopic chemistry of the aquifer carbonate and groundwater circulation systems, range of residence times and the origin of the geothermal water. This study surveys carbon isotopic compositions of the system and groundwater 14C ages and variations, and attempts to understand the processes upon which the isotopic values and their spatial distributions are based.

BASIC CHARACTERISTICS OF THE SURDULICA AQUIFER

The Vranje-Surdulica geothermal system is a narrow catchment area of the South Morava valley between Grdelica and Bujanovac (Fig. 1) in the Besna Kobila, Kukavica and Kozjak Mountains. The system is situated mostly in the Morava massif, consisting of Paleozoic gneiss, mica schists, amphibolites and greenstone (metagabbros, metadiabases, calc-schists, quartz and other chlorite schists) complexes. The Tertiary Surdulica system contains granitoids and dacite-andesite volcanics.

A large amount of CO2 was introduced to the system during Tertiary magmatism, which could explain the strong carbonization of older rocks. This is apparent in metamorphic rocks from Boreholes 10 and 10.1 (drilled to 1603 m). Abundant sulfates in thermal waters (2-5 mmol liter 1) and little sulfide suggest an anaerobic environment; thus, anaerobic decay with sulfate reduction

1This paper was presented at the 14th International 14C Conference, 20-24 May 1991, Tucson, Arizona. 2Geoinstitute, Rovinjska 12,11000 Belgrade, Yugoslavia

277 278 Munevera Hadzisehovic et al.

V + 1. T-Q 2. 3. 4.

X A 5. 6. 7 G 8. G1

0 10 20 30 km 9. 10.

Fig. 1. Geological map of the study area with sampling points; 1. Tertiary and Quaternary sediments; 2. Tertiary volcanic rocks; 3. Surdulica granitoid; 4. Bujanovac granitoid; 5. Kukavica granitoid; 6. green complex; 7, gneiss and mica schist (Morava massif); 8. gneiss (Pelagonia-Rhodope massif); 9. contact of these two massifs; 10. contact of gneiss and the green complex in the Morava massif

as a source of HC03 can be ignored. However, the possibility that some modern CO2 in thermo- mineral waters of this system has a mantle origin should not be excluded. The study region lies between moderate continental and Mediterranean climates, with mean annual air temperature around 10.5°C. Runoff from Besna Kobila (1923 m asl) drains into the Aegean and Black Seas. The average annual precipitation ranges from 915 mm in the high areas to 655 mm Isotopic Analysis in the Surdulica Aquifer 279

in the lower elevations, where most of the precipitation occurs from October to March. Stream flow is also highest during these months.

The system discharges to wells and springs about 150 liter sof cold and nearly 100 liter s'1 of thermal waters (45-126°C). Relatively impermeable layers in the divided aquifer formation direct the subsurface flow to large springs (up to 80 liter s'1), which discharge along many faults. The configuration and geological structure of the aquifer and D, 180, 13C, 14C isotopic data indicate that the Besna Kobila Mountain is a major recharge area. Natural thermal springs appear along the foothills of a schist massif at about 400 m as!, mostly near the Vranjska and the Bujanovac Spas.

The regional geothermal gradient is 1°C/25 m (Anderson 1985) with a local high gradient of up to 1°C/8 m. A plot of temperature vs, depth (Fig. 2) shows that even higher temperatures are found at some loci (here 2, 3, 4, 10). From this gradient, the maximal depth of circulation is ca. 2500 m. We also estimated the depth from which hot water discharges, based on silica and cation-ratio geothermometers (Henley et al. 1985), and concluded that two temperature regimes exist: for the Bujanovac Spa (sampling point 30, Table 1), with <700 m and temperature of 100°C, and for the Vranjska Spa, with up to 4000 m depth and temperature up to 200°C.

E a. 0w

500 . 10

Fig. 2. Relation of water temperature to depth of discharge (all points represent boreholes, see 10 20 30 40 50 60 70 80 90 100 110 120 Table 1) TEMPERATURE ('C)

SAMPLING AND ANALYSES

From 1986 to 1990, we measured 13C, 14C, 3H, D and 180 of groundwater, CO2 gas and rocks at 30 sites. To define modern isotope input parameters, we collected samples from soil CO2 and vegetation, and determined pH values, alkalinity and major ion chemistry in water samples.

We analyzed the 13C and 180 content in the CO2 gas prepared from samples according to IAEA (1980), using a Micromass 602 mass spectrometer. We obtained deuterium data by reducing water with zinc at 460°C (Coleman et al. 1982). We report stable isotope data in S values (%o) related 180 b13C to the V-SMOW standard for D, and for to PDB with 1 a error of 2, 0.2 and 0.4%0, respectively. The b13C value refers to the total dissolved carbon (TDC). TASK 1. Chemical Analyses of Waters from the Surdulica Aquifer (concentrations in mmol liter1 unless otherwise indicated) Sampling Collection Sample point date (m) °C pH Na' K' Ca" M? Sr' Mn Zn HCO; COQ SO F CI" (mg/liter) Freshwater

Spring 14 10/20/86 1475 8.0 7.7 0.25 0.02 0.44 0.2 0.02 0.02 0.01 0.8 0.05 0.01 0.10 3 Capt. spring" 21 10/10/86 1300 7.5 7.5 0.15 0.01 0.45 0.3 0.02 0.01 0.01 1.0 0.05 0.05 5 Fountain 22 07/14/88 912 13.1 7.7 0.23 0.02 0.60 0.3 0.02 0.02 0.01 13 0.10 0.10 Capt. spring 5 07/12188 700 17.6 6.8 0.24 0.05 0.60 0.4 0.01 12 0.10 0.05 3 River 6 1288/88 687 4.4 7.3 0.13 0.02 0.60 02 0.01 0.01 1.0 0.10 0.01 0.10 9 River 28 10i01B7 430 63 7.6 0.46 0.05 0.74 0.4 0.02 0.01 1.4 0.20 0.20 5 Transition and !ow-temperature thernial water Mine water, dt .600 16 06/18/86 1310 112 7.3 0.20 0.01 0.30 02 0.01 0.01 OS 0.10 0.02 0.10 7 B=, d .112 25 07/12188 1354 10.6 7.3 0.19 0.01 0.40 0.1 0.01 0.01 12 0.10 0.05 0.10 10 Tunnel, d =300 27 09/14/88 693 205 7.6 0.90 0.10 1.20 0.7 0.02 0.01 1.8 0.80 0.20 0.10 20 Capt. thermal spring 29 06p5/86 393 23.0 8.4 12.10 0.10 0.10 0.1 0.005 10.6 038 0.60 0.07 030 72 Capt. thermal spring 29 05/25/89 393 22.8 8.4 11.80 0.10 0.10 0.1 102 0.39 0.60 0.10 0.70 15 Capt. thermal spring 9 06/18/86 430 17.1 7.0 5.60 0.20 1.20 0.1 53 0.90 0.10 1.10 14 Capt. thermal spring 8 05/25/89 415 16.8 73 7.70 030 0.80 OS 53 3.10 0.10 130 25 Hot thermal water Capt. thermal spring i 0505/86 azs 782 7.6 izso 0.30 0.40 0.2 72 4.00 0.60 1.40 az B, d 120 = 3 05i06/86 294 805 73 1150 0.20 0.70 0.1 6.4 4.20 0.60 130 90 B, d .520 10 01/25/88 -105 79.4 82 1350 0.30 030 0.4 9.8 320 020 130 39 B, d 1063 = 10 04/14/65 -648 111 8.3 10.70 0.60 0.40 02 8.0 3.00 0.20 1.30 30 B, d .450 10.1 0525/89 -33 67 73 12.80 0.40 0.30 0.2 81 3.10 030 1.70 88 B, d 535 = 30 07/12/88 -125 43 7.1 58.90 1.60 1.20 OS 0.02 58.7 2.00 1.60 41 B, d .535 30 05,26/89 -125 421 7.1 5730 130 1.02 0.6 0.02 565 0.38 1.70 0.13 1.70 Sl

*Alt = altitude of water horizon 'Capt. = captured td = depthd ()m ;B = borehole s"' = measured temperature at 1063 m depth =126°C, flow = 27 liter Isotopic Analysis in the Surdulica Aquifer 281

We measured 14C activities using a Packard 3380 liquid scintillation spectrometer or gas proportional counting system (Srdoc, Breyer & Sliepcevic 1971).14C values are expressed in percent modem carbon (pMC), with a measurement error of ± 3-25% (for hot waters); we measured tritium activity after electrolytic enrichment (Hut 1986).

RESULTS AND DISCUSSION Water Chemistry and Isotopic Composition of Groundwater Surdulica groundwaters are of Na(Ca)-HCO3(S04) type, with pH values ranging from 7.0-8.4 (Table 1). The partial pressure of CO2 is within 10"°to 10-29 atm (Table 2) and increases downgradient from cold to geothermal water (GTW). The cold springs and most low-temperature thermal waters are unsaturated with calcite. Ionic concentrations (especially those of Na+, HC03 and S0) and groundwater temperature increase with depth. This is associated with considerable carbonization and pyritization of older rocks. 8180 A plot of SD vs. of groundwater samples fits a local meteoric water line of SD = (7.3 ± 0.2) 818O + (7 ± 2). Similarity between isotopic content of precipitation and GTW demonstrate their

TABLE 2. Isotopic Analyses and Chemical Data of Studied Waters S13CPDB 14C Sampling Log PCO:* 3H 3H, TU Sample point (atm) (%o) (pMC) (TU) intervalt Freshwater Spring Capt. spring 21 -2.8 -1.2 -12.9 58 32-69 Fountain 22 -2.9 -0.7 -13.5 67 37 28-46 Capt. spring 5 -1.9 -1.0 -13.4 71.1 19 17-37 Capt. spring 5 -2.3 -1.4 -16.3 94.1 23 17-37 Capt. spring 11 -2.8 -0.2 -10.2 61.3 18 15-35 Transition and low-temperature thermal water Mine water 16 -2.9 Bt, d§ = 212 25 -2.5 Tunnel water 27 -2.7 7 7 Thermal spring 29 -2.7 Thermal spring 29 -2.5 5 Thermal spring 9 -1.9 Thermal spring 8 -2.0 Hot thermal water

Thermal spring 1 B, d = 120 3 -1.7 B, CO2 gas 3 -4.5 B, d = 520 10 -2.2 B, d = 867 10 -2.5 B, d = 450 10.1 -1.6 B, d = 535 30 -0.3 B, d = 535 30 -0.4

[HZC031; ion activity prouct *Log Pca = log log equilibrium constat Kco tStudy period -1986-1990; B = borehole; depth (m) 282 Munevera Hadiiehovic et al. meteoric origin. Distribution and seasonal variations of D and 180 isotopes in the water indicate that GTW have been recharged predominantly from higher elevations (>500 m asl) and during colder seasons.

All of the water samples, except for a few old geothermal samples, contain tritium from 5 to 59 1-1) tritium units (TU; 1 TU = 0.118 Bq (Table 2); this large interval is a result of different drainage rates throughout the system due to variable rock porosities. In rivers and shallow 14C groundwater (freshwater, Table 2), concentrations are between 61.3 and 94.1 pMC, whereas b13C values range from -16.3 to -10.2%o (Table 2). Transition- and low-temperature thermal water from deeper portions of the aquifer have lower 14C (16.4-77.5 pMC) and higher 13C (-13.8 to 14C -3.1%o). The samples from hot thermal springs and boreholes had the lowest (3.3-6.9 pMC) and the highest b13C values (-6.6 to -0.4%o). The b13C value of the CO2 gas analyzed from Borehole 3 is -4.5%o. The 13C content of core samples from Boreholes 10, 10.1 and 26 (-8.4 to -1.1%o) are very similar to those in GTW from deep circulations. Samples taken from marble rocks S13C ranged from = +0.41 to +2.7%o. Isotopic composition of dissolved carbonates formed during water interaction with rock matrix is S13C plotted vs. the calcite saturation (Fig. 3). and generally increase with calcite saturation, while 14C decreases. To determine conditions of calcite mineral dissolution, we calculated the relation between HCO3 and 13C content of water for systems open and closed to a CO2 gas reservoir (Fig. 4). Under open-system conditions (Fig. 4A), cold waters gather between calcite saturation 13C b13Cs curves corresponding to initial values of content of soil gas, = -24 to -20%o at initial pH values, pH; = 5-6. The isotopic composition of GTW is out of this range. This could be explained by significant dissolution of rocks and isotopic exchange between carbonate species in the aquifer. In Figure 4, we give the saturation curves for the supposed systems, assuming their initial para- S13Cs meters, = -13%o (mean value for freshwater), -8%o (low thermal water, point 29) and pH; = 5-7. Similar analysis derived for closed-system conditions (Fig. 4B) shows that cold groundwater 813Cs recharge should have pH; = 5-6 and = -27 to -24%o, and Ps may range from 10-1'5 to 813Cr, 10-25. The 13C content of the dissolved carbonate from rocks, could be between -3.4%o S13Cs (mean for Borehole 10, Table 3) and +1.4 (mean for marble). Taking into account = -13 and 13C -8%o, and p1-I1 = 5-6, as initial parameters, we show that most measured values for warm waters are still out of the range of the saturation curves. These values are greater than for open- system conditions.

Age of Groundwater

As shown in Table 2, freshwaters contain considerable tritium, and to a certain degree, reduced concentration of 14C. Thus, for an estimate of mean residence time (MRT), we used the finite-state, mixing-cells model (Yurtsever 1983). For most samples, this model gives 2 or 3 solutions for MRT 14C and an adjustment factor Q (a parameter for non-decay loss induced by chemical interaction with carbonate rock). There may be two reasons for this: l) the sampling was performed at the end of tritium-response curves of the groundwater system (1986-1990); 2) water takes fast and slow paths through the system. The analyzed water is a mixture of these two components. This assumption is validated by the fact that most of the studied waters cluster around Q = 0.7-0.75, which corresponds to a MRT of 100-700 yr (Table 4). On the other hand, tritium was detected in the same samples, which leads to 10-80-yr-old water, i.e., young water. If waters with two very different MRTs exist simultaneously, we must assume two different mechanisms for their formation. In the case of fast water, it passes through fissures that conform to the geological structure of the aquifer, whereas a standard diffusion process is responsible for old water. We estimate the MRT at less than 80 yr for the fissured process and to 200-500 yr for the older waters. Isotopic Isotopic Analysis in the Surdulica Aquifer 283

120 VEGETATION,SOIL 1'C (pMC) 100

m COLD WATERS (RECENT WATER)

25 60 X16 TRANSITIONAL LOW-TEMPERATURE THERMAL WATERS 29

20 29 OLD THERMAL WATERS i 30,0 10 10

1 1 j13 2 C watsrs MARBLE (7..) 0

ROCKS -6

-8 29 -10 11

-12 16

-14 5 22

-16

-18

-27 VEGETATION, SOIL

-3

109 PCO 2 (atm) 10

-2

-0,5 SIC

-2 0 1 2 3

14C, 13C Fig, 3, Dependence of water and Pin, on the saturation index of calcite 284 Munevera Hadiisehovic et al.

TABLE 3. b13C Data for Soil, Vegetation and Cores Alt. Sampling Collection Sample (m as!) point Plant 700-1340 5-7, oak evergreen, 20, 23 05/15/90 -27.6 grass Soil 700-1340 5-7, 20, cm depth with roots Peat 1200 23

Depth (m)- Borehole 415 10 with py- 438-447 -4.0 rite encrustation 997-999 -2.9 Borehole 417 10.1 schist 435-446 -1.3 447-450 -1.1 Borehole 895 26 granodiorite 275-284 -8.4 with pyrite encrustation 354-355 -4.9 374-375 -6.6 Marble 1300 21 sample mine +2.7 Dark sample +1.1 Random sample

*Undisturbed core samples from borehole

TABLE 4. Calculated Mean Residence Times (yr) of Surdulica Groundwaters Using A Finite-State Mixing-Cells Model

14C Q (Adjustment parameter for non-decay loss or dilution in the aquifer)

Sample 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 slow

14 1-15 30-45 60-95 150-250 330-550 700-1000

21 7-20 30-40 57-76 100-150 200-600 1-20 25-75

22 1-35 58-87 130-220 350-560 750-990 1-10 35-85

5 7-20 35-50 70-100 160-270 400-640 800-1000 1-10 50-150

5 5-30 40-50 65-90 100-350 1-10 50-150

11 0-95 150-270 420-720 900-1000 1-5 60-180

9 0-25 35-50 70-100 140-250 330-520 670-1000 <2 160-660

16 180-260

29 150-700

29 150-700 Isotopic Analysis in the Surdulica Aquifer 285

logHC03 (ppm)

c A. Open system

1 pH1:5 . / ------pHi:6 0

-21 -20 -18 -16 -16 -12 -10 -8 -6 -' -2 0 2

logHc03

(ppm) B.Closed system 3000 30

-22 -20 -18 -16 -14 -12 -10 -8 -6 -1. -2 0 2 (y..l

Fig. 4, Relation between &3C and HC03 for open and closed systems for various initial conditions (10, 30°C). A. Open-system solution paths: 813C, g13C, (1) &3C, = -24; (2) &3C, = -20%o; (3) = -13%c; (4) = -8% B. Closed-system solution paths: 8130 g13C, 813Cr b13C, 813Cr (1)b13C, = -27%0, = -3.4%o, pH; = 5; (2) = -27%o, = +1,4%0, pH; = 5; (3) = -24%o, = -3.4%, 813C, 813Cr S13C, 813Cr 813C, pH, = 5; (4) = -24%0, = +1,4%0, pH; = 6; (5) = -13%0, = -3.4 %o, pH; 5; 6) g13C,=-13%o, =+1.4%o,pH;=5;(7)813C,=-13%o,8'3Cr=+1.4%o,pH;=6;(8)813C,=-8%o,813Cr=-3.4%c,pH;=6;(9)813C,= 8'3Cr 813C3 S13Cr -8%0, = +1.4%o, pH; = 5; (10) = -8%o, = +1.496o, pH; = 6. Subscripts, s and r, stand for soil and rock, respectively, .. .- = calcite saturation curves; -.- - = calcite solution paths for various Pcoz atm, 0 sample. analyses indicate a complex carbon origin (biogenic CO2 from the soil zone, carbonate species from carbonate mineral dissolution, CO2 from the mantle); now, the question of age for these waters is 14C open. Tritium-free waters suggest ages older than 100 yr. The low content (ca. 5 pMC) would infer waters ca. 20,000 yr old (hydrochemical age, Fontes 1979). Measured TDIC &3C, > -6.6%o, indicates dilution by carbon of unknown origin. Other models (Fontes 1979) give unrealistic, even negative ages, and are not applicable to the Surdulica system. We believe that these waters are considerably younger than 20,000 yr because of high bicarbonate concentrations (up to 60 mmol 286 Munevera HadiRehovic et al. lite(1) in the basin, which cannot be explained only by calcite and dolomite dissolution (mca + mMg = 0.5-1.5 mmol lite(1).

CONCLUSION Geothermal waters are of meteoric origin, formed in recharge areas at higher elevations and during colder seasons. The results of tritium and 14C measurements indicate fast (fissure) and slow (inter- granular) flow paths through the system. Water composition is characterized by bicarbonate sodium (calcium) sulfates. The concentration of the main cations, anions and silica increases by a factor of almost ten, descending from the recharge area to the deep waters. Calcite saturation increases with depth, and GTWs are almost supersaturated. 613C, It appears that cold water recharges under closed-system conditions, with initial parameters, b13C5 = -27+3%o, pH; = 5-6, reaching a value of -13%o for at the discharge. The concentrations of 13C in GTW are significantly higher (up to 0%o), which indicate processes of intensive interaction between water and rocks, isotopic exchange, and a possible interior CO2 source. The MRT for mixed waters calculated by finite-state mixing models is less than 80 yr for the fast component, and 200-500 yr for the older one. The age of geothermal waters based on isotopic analyses ranged from 100 to 20,000 yr.

ACKNOWLEDGMENTS Partial financial support of this study was provided by the IAEA (Research Contract No. 4852/R1/R2) and Scientific Funds of Serbia.

REFERENCES

Anderson, E. 1985 Review of Vranjska Banja Geother- Economic Geology 1: 31-43. mal Prospect, Scientific and Engineering Aspects. Hut, G. 1986 Intercomparison of Low-Level Tritium Wellington, New Zealand, GENZ-Geothermal Energy Measurements in Water. Vienna, IAEA. New Zealand, Ltd.: 12 p. IAEA (ms.) 1980 Preparation of carbon dioxide for C- Babovid, M., Vukanovid, M., Krajadh, V., Raki6, M. 14 measurements. Technical Procedure Note: 25 p. and Dimitrijevid, M. 1970 Basic Geological Map, Milovanovid, B., Stankovid, S., Komatina, M., Had1i e- Chart Vranje 1:1000 000. Geozavod, Federal Geolog- hovid, M., Zupandid, M., Miljevid, N., Stepid, R. and ical Institute, Belgrade. Obelid, B. 1989 Isotopic investigation of the Surdul- Coleman, M. L., Shepherd, T. J., Durham, J. J., Rouse, ica geothermal system. In Long, A. and Kra, R. S., 14C J. B. and Moore, G. R. 1982 Reduction of water with eds., Proceeding of the 13th International Confer- zinc for hydrogen isotope analysis. Analytical Chem- ence, Radiocarbon 31(3): 893-901. istry 54: 993-995. Popovid, R. 1990 The Serbian-Macedonian mass - yes Dimitrijevid, M., Petrovid, B., Cikin, M., Mozina, A., or no? Records of the Serbia Geological Society, Vikanovid, M. and Karajidid, L. J. 1965 Interpreter 1990, Belgrade. of the Basic Geological Map of the SFRJ, Chart Srdo6, D., Breyer, B. and A. 1971 Rudjer Leskovac 1:100000. Federal Geological Institute, Bel- Bokovid Institute radiocarbon measurements I. grade. Radiocarbon 13(1): 35-140. Fontes, J. Ch, and Gamier, J. M. 1979 Determination of Yurtsever, Y. 1983 Models for tracer data analysis. In the initial 14C activity of the total dissolved carbon: IAEA/UNESCO Guidebook in Nuclear Techniques in A review of the existing models and a new approach. Hydrology. IAEA Technical Reports Series 91: Water Resources Research 15: 399-413. 381-400. Henley, R. W., Truesdell, A. H., Barton, P. B., Whitney, ujovid, J. M. 1893 Thermal springs of Vranjska Spa. 1. A. and Robertson, J. M., eds., 1985 Fluid-mineral In Geology of Serbia Vol. II-Granite. Belgrade, equilibrium in hydrothermal systems. Review of Serbian Royal Academy: 54-73. [RADIOCARBON, VOL. 35, No. 2, 1993, P. 287-293]

RADIOCARBON DATING OF PALEOSEISMICITY ALONG AN EARTHQUAKE FAULT IN SOUTHERN ITALY

GILBERTO CALDERONI and VINCENZO FETRONE

Department of Earth Sciences, University of Rome I "La Sapienza" Piazzale Aldo Moro, 5, 00100 Rome Italy

ABSTRACT. On 23 November 1980, a major earthquake (M, = 6.9) struck a large area of the southern Apennines (Campania and Lucania regions, southern Italy). This seismic event, the largest in Italy over the last 80 years, almost completely destroyed 15 villages and caused extensive damage to other towns, including Naples. The quake produced the first well-documented example in Italy of surface dislocation, represented by a fault scarp 38 km long. We undertook a study that included 14C dating of organic materials from layers displaced by paleoseismic events to assess the seismologic hazard for the area. We collected peat and charred wood samples from the walls of two trenches excavated across the 1980 fault at Piano di Pecore di Colliano, Salerno, where the sedimentary suite is faulted and warped by five quakes (including that of 1980). This produced comparable vertical throw and deformation patterns. Chronological data for pre-1980 events, coupled with detailed stratigraphic analysis, yielded a dip-slip rate and a recurrence interval of 0.4 mm/yr and 1700 yr, respectively.

INTRODUCTION The study of earthquakes is difficult in that, unlike many other naturally occurring geologic processes, seismic events, including those of higher magnitude, produce effects that only occasionally are preserved in the geologic records, and often, just within a narrow strip along the main fault systems. Terranes that undergo seismic shocks may develop a complex array of structural features, most of which are "seismites" (Seilacher 1969; Vittori, Sylos-Labini & Serva 1988), which include faults, sandblows, folds and fissures. Regardless of pattern, seismites share a common origin that reflects the mechanical characteristics of the involved terranes, intensity of shock(s) and slipping displacement on a fault plane. Thus, survey of geologic structures attributable to seismic events (viz., seismite identification) is the primary fieldwork in paleoseismologic analysis. Seismite identification implies selecting the sites most favorable for their formation and preservation through time and differentiating them from non-seismic analogs. The latter task relies on geologists' experience, and the former is very difficult to accomplish, even in well-known seismic areas, owing to the rarity of obvious surficial faulting. This diagnostic geologic feature usually results only from quakes of magnitudes 6.0-6.5 with hypocenters from <15 to 20 km deep (Bonilla 1979; Bonilla, Mark & Lienkaemper 1984). Surface geologic features produced occasionally by quakes of lower magnitude on unstable terranes are considered too unreliable for seismologic analysis. Thus, seismites are recognized mostly in suitable terranes close to active fault scarps.

It is reasonable to infer that, for a given fault system, earthquakes are non-random events displaying comparable patterns. Thus, in earthquake research, we need to understand paleoevents over a long time span to formulate consistent hypotheses and predictions. Studying the geologic records of seismites through the recent past is a viable approach to evaluating long-term behavior of active faults, which is essential in areas lacking long historical records of seismicity (Allen 1975). During the last two decades, geologists have used seismites preserved in young sediments for dating and for evaluating recurrence intervals and magnitudes of paleoevents (Sims 1973; Sieh 1978, 1984; Berger & Kaufman 1980).

We discuss here the 14C chronology of Holocene sediments close to the fault scarp produced by the 23 November 1980 Campania-Lucania earthquake, especially in terms of the late Holocene

287 288 G. Calderoni and V. Petrone

LIONI

1000 P 6 0 1980QU1AKE EPICENTER

MT. MARZANO 'a MT. PICENTINI Q A PIANO' DI ` PECORE D MT. CAR PINETA

W N O

ST. GREGORIO

0 5 10 km FAULT SCARP OF 1980 QUAKE

Fig, 1. Map of the study area showing a significant reach of the 1980 earthquake fault scarp, trending NW-SE.

seismic history along this active fault. In particular, we attempt to establish a time scale for the major prehistoric events and understand their pattern and recurrence interval.

SAMPLING SITE AND MATERIALS We collected samples at Piano di Pecore di Colliano (1220 m as!, Salerno, southern Italy (44°44'N, 15°22'E) (Fig.1). The site is a small intermountain basin lying along the Mt. Marzano-Mt. Carpineta ridge, crossed by a southern segment of the 23 November 1980 fault scarp that shows ca. 80 cm of downthrow. We excavated two trenches with a backhoe in neighboring locations (Fig. 2) across the 1980 active fault. The trench walls expose a 4-m-thick sequence of interbedded lacustrine and colluvial sediments along with air-borne volcanic materials. The profile, following detailed paleoseismologic analysis, revealed seismites (e.g., faultings and warpings) related to quakes before 1980. Figure 3 shows the stratigraphic column for Trench 1. Although the thickness of lacustrine and colluvial deposits changes according to trench location (e.g., colluvium and alluvium deposits maximize toward the rim and the center of the basin, respectively) all the exposures exhibit a common sedimentation pattern. The occurrence of volcanic ash and pumice is noteworthy, in that volcanites are lacking throughout the whole catchment area of the basin. Based on historical accounts (the topmost volcanic ash relates to the 1944 Mt. Vesuvius eruption), we infer that tephra at Piano di Pecore was ejected by Vesuvius and/or Phlegrean Fields from vents ca. 90 km to the northwest. Figure 3 gives the stratigraphic position for the analyzed samples, along with the identified sedimentary units. 14C dated samples included charcoal, both scattered and in macrofragments, organic clay and a fibrous vegetal mat resembling peat, likely resulting from deposition and compaction of subaerial and lacustrine plant debris. Following routine pretreatment, the samples were converted to benzene and measured for 14C activity by liquid scintillation counting with a (3 spectrometer. Calderoni & Petrone (1991) reported procedural details. Dating Earthquakes in Southern Italy 289

Mt. MARZANO

UAKE EPICENTER

EPHEMERAL STREAM lam- PIANO di PECORE

DRAINING PIANO DI PECORE 84 -N Fig. 2. Sketch of the trenching site depicting the morphologic changes following the 1980 event. The downthrow experienced by Piano di Pecore affected the former drainage toward Sele Valley, resulting in pond formation.

RESULTS AND DISCUSSION

The detailed paleoseismologic analysis of the geologic record in the trench walls at Piano di Pecore allows the depiction of the recent history of the surface fault produced by the 1980 earthquake. Principal findings include the identification of seismites resulting from 5 distinct events (including that of 1980) throughout the 8 sedimentary units occurring in the filling basin.

At Piano di Pecore, conditions for preserving the seismic imprinting were favorable over thousands of years. On the one hand, seismicity has ruptured the sediments repeatedly and, on the other, their rapid accumulation resulted in stratigraphic separation of the faulting events. Further, there have not been long sedimentation hiatuses or significant scour. Sedimentation rate, lack of erosion and preservation of carbonaceous materials were greatly enhanced by the acting tectonic mechanism, as well as by humidity. In fact, fault displacement associated with paleoseismic events recurrently ponded the basin by damming its ephemeral outpouring stream (Fig. 2), thus establishing a low-energy, reducing depositional environment where the solid input from the catchment area accumulated.

Analysis of the exposures in both trenches shows that subsurface sediments are significantly more displaced than the topographic surface produced by the 1980 earthquake, and that the amount of vertical displacement (= downthrow) increases with depth (= age). We argued that such a trend could be explained by the occurrence of pre-1980 events along the same seismic fault, which, in this respect, are supposedly comparable in tectonic style, genetic mechanisms and effects. The exposure in Trench 1 provided a better opportunity to characterize the paleoevents, in that we were able to make reliable comparisons, on both fault walls, among analogous packages (= sedimentary units) of sediment. Detailed analysis of the exposure revealed that the vertical separation of the stratigraphic units on the fault plane, rather than being constant, display a discontinuous character featuring four sudden increases. Each increment in throw, generally of an amount comparable to that of the 1980 surficial fault, involves one or more stratigraphic units. On this basis, the sharp throw increases were interpreted as records of seismic events and designated, top to bottom, from 290 G. Calderoni and V. Petrone

V y WN, yWZ C W C) T

MAT 2B3 nv PEBBLE

I 1A1O 2

1A1 2B6 SILT 2B2 1 2B9 2A2 2B5 be, 01 PUMICE z

PEAT 4 2 -ORGANIC CLAY 1A1

IV 1A9 at COLLUVIUM

7 3

V 1A5

l E u W

Fig, 3. Stratigraphy of the sampled exposure in Trench 1 showing both walls of the fault. The sedimentary suite consists mainly of peat-like organic debris, sandy, silty and clayey layers with scattered wood and charcoal fragments and pebbles. Location of samples and recognized seismic events, along with stratigraphic units, are reported to the left and right of the column, respectively. The recognized sharp throw increases, each referred to a seismic event, are represented by thicker segments on the trace of the main fault.

I (modern) through V (Fig. 3). We infer that each significant earthquake following Event V (the older event recognized) superimposed further vertical separation on the entire sedimentary suite existing at the time of its occurrence. To calculate the downthrow caused by individual paleoevents Dating Earthquakes in Southern Italy 291 from the apparent vertical offsets, we iteratively subtracted the amount of displacement produced by the above-recorded earthquakes from those of Events II-V. The first subtraction (80 cm) was that of the surface fault scarp. The resulting throws, 50, 50, 85 and 60 cm for Events II-V, respectively, apply only to the study site, as the magnitude of local displacement may change significantly along a seismic fault.

Among the samples collected during summer 1989, only 11 from Trench 1 were suitable, both in amount and reliability, for 14C dating. The ages obtained, listed in Table 1, provide a basic chronology for the identified paleoevents.

TABLE 1. Field, analytical and calculated data for the measured samples

S13C Calendar Sample Lab no. Material age (BP) (BP)

MAT Rome-124 Surface debris 2B3 Rome-120 Charcoal t 50 1A10 Rome-116 Charcoal 45 1A1 Rome-113 Humified charcoal** 40 2B6 Rome-122 Peat 50 2B2 Rome-119 Humified charcoal** 45 2B9 Rome-123 Charcoal 45 2A2 Rome-118 Charcoal 80 2B5 Rome-121 Organic clay 45 1A1 Rome-117 Peaty-clay 55 1A9 Rome-115 Peat 50 lA5 Rome-114 Charcoal 50

*Calibration according to Stuiver and Reimer (1993) **NaOH-soluble charcoal

DATING THE PALEOEVENTS

We summarize the data used in assigning each event a chronological constraint in Figure 3 and Tables 1 and 2.

Unit 1 - Soil. The topmost deposit is a coarse immature soil (sample MAT, altered surficial litter, yielded percent modern (pMC) = 102.2 ± 0.5) high in organic matter content. Descend- ing, abundant pebbles occur, suggesting sedimentation under high-energy conditions. Sample 2B3, tree roots in living position from the middle of the unit, and Sample 1A10, from the bottom, yielded 345 ± 50 BP and 480 ± 45 BP, respectively. Unit 1 experienced only Event I (1980 earthquake, which produced an 80-cm-high surficial fault scarp).

Unit 2 - Colluvium. An ocher-colored deposit composed of reworked slopewash and volcanic material. Samples 1A1 and 2B6, from the upper and middle part of the unit, were dated at 1220 ± 40 BP and 1550 ± 50 BP, respectively. Descending, the erosion of the fault scarp (50 cm high) associated with Event II resulted in the formation of a typical colluvial wedge overlying Samples 2B2 and 2B9, dated at 2620 ± 45 BP and 2730 ± 45 BP, respectively (mean: 2675 ± 30 BP). Samples 2A2 and 2B5, just below the colluvial wedge, yielded 3100 ± 80 BP and 3300 ± 45 BP, respectively (mean: 3250 ± 40 BP). Thus, Event II is bracketed between 2675 ± 30 BP and 3250 ± 40 BP.

Unit 3 - Upper Lacustrine. Both mineralogy and stratigraphy of the abundant volcanic tephra occurring here suggest that the Avellino eruption of Mt. Vesuvius (estimated at 3800 BP) was the 292 G. Calderoni and V. Petrone

TABLE 2. Ages of Seismic Events Along the Studied Seismogenetic System Upper limiting date (BP) Calendar age range* Event Lower limiting date (BP) (BP) I 23 November 1980 II 2675 ± 30 3250 ± 40 III 3250 ± 40 3850 ± 55 IV 3920 ± 50 5900 ± 50 V >5900 ± 50 *Calibration with software after Stuiver and Reimer (1993) most likely source. Although no dates are available for the unit, the age of Event III, recorded here by 50 cm of vertical displacement, is constrained between 3250 ± 40 BP (mean of the ages for samples 2A2 and 2B5) and 3850 ± 55 BP (sample 1A11).

Unit 4 - Colluvium. Abundant weathered pumice and dark peaty layers occur through the deposit. Sample 1A11, at the top of the unit, was dated at 3850 ± 55 BP.

Unit 5 - Lower Lacustrine. The unit includes graded sandy layers intercalated with thin clay beds. It exhibits the seismic imprinting of Event IV, represented here by faulting with an overall throw of 85 cm. Sample 1A9, from the topmost clay level, was dated at 3920 ± 50 BP. This age, along with that measured for Sample 1A5 (5900 ± 50 BP), provide chronological constraints on the occurrence of Event IV.

Unit 6 - Colluvium. A reddish, coarse colluvial deposit, rich in weathered pumice and containing scattered charcoal fragments. Descending, a thin lacustrine layer shows the Event V structural imprinting (ca. 60 cm of vertical displacement) just overlying airborne tephra probably derived from an explosive eruption of Mt. Vesuvius (estimated at 7900 BP) responsible for the eastward spread of a pumice fall. Thus, according to stratigraphy, Event V is bracketed between 5900 ± 45 and 7900 BP. Sample 1A5 provides the former age and overlies the seismite-bearing lacustrine layer. The latter date is a reasonable estimate for the age of the underlying tephra.

CONCLUSION Paleoseismologic analysis coupled with 14C dating of the geologic record exposed on the walls of two trenches excavated across an active seismic fault provided interesting data for earthquake research. We identified four seismite assemblages, each representing the imprinting of a paleoevent occurring along the same fault. As the displacements produced by the five quakes (including that of 1980) in the study area are comparable in both tectonic style and downthrow, we infer that the 1980 quake is "typical" for the investigated fault. In terms of seismic hazard, this means that quakes of comparable magnitude could recur.

For the first time in Italy, 14C dating provided sound chronologic constraints on paleoevents, allowing the evaluation of quake recurrence intervals and slip rate for a representative site along the major Italian seismogenetic structure. Events I-IV took place over the last 6700 yr and, based on stratigraphy, <2000 yr elapsed between Events IV and V. On this basis, calculations yielded a Dating Earthquakes in Southern Italy 293 surficial slip rate of 0.4 mm y(1 for the fault (which, at depth, can increase dramatically) and ca. 1700 yr as the mean recurrence interval.

ACKNOWLEDGMENTS We are indebted to the National Institute of Geophysics (ING) for field data and sampling campaign. Funds were provided by Italian CNR, National Committee no. 5. The usual, friendly attitude of Dr. Meyer Rubin, USGS (Reston, Virginia) in improving the manuscript has been greatly appreciated.

REFERENCES

Allen, C. R. 1975 Geologic criteria for evaluating dates I. Radiocarbon 34(1): 105-113. seismicity. Geological Society of America Bulletin Seilacher, A. 1969 Fault-graded beds interpreted as 86:1041.1057. seismites. Sedimentology 3: 155-159. Berger, R. and Kaufman, T. S. 1980 Radiocarbon dating Sieh, K. E. 1978 Prehistoric large earthquakes produced of earthquakes. In Stuiver, M. and Kra, R. S., eds., by slip on the San Andrea fault at Pallet Creek, Proceedings of the 10th International 14C Conference. California. Journal of Geophysical Research 83: Radiocarbon 22(3): 746-756. 3907-3939. Bonilla, M. G. 1979 Historic surface faulting, map 1984 Lateral offsets and revised dates of large patterns, relation to subsurface faulting and relation prehistoric earthquakes at Pallet Creek, southern to preexisting faults. In Proceedings of the 8th California. Journal of Geophysical Research 89: Conference on National Earthquake Hazard Reduc- 7641-7670. tion Program. USGS Open-File Report 79-1239: Sims, J. D. 1973 Earthquake-induced structures in 36-65. sediments of Van Norman Lake, San Fernando, Bonilla, M. G., Mark, R. K. and Lienkaemper, J. J. California. Science 182: 161-163. 1984 Statistical relations among earthquake magni- Stuiver, M. and Reimer, P. J. 1993 Extended 14C data tude, surface rupture length and surface fault dis- base and revised Calib 3.0 14C age calibration pro- placement. Bulletin of the Seismolological Society of gram. Radiocarbon 35(1): 215-230. America 74: 2379-2411. Vittori, E., Sylos-Labini, S. and Serva, L. 1991 Paleo- Calderoni, G. and Petrone, V. 1992 Department of Earth seismology: review of the state of the art. Tectono- Sciences at the University of Rome I radiocarbon physics 193: 9-32.

[RADIOCARBON, VOL. 35, No. 2, 1993, P. 295-300J

A BATCH PREPARATION METHOD FOR GRAPHITE TARGETS WITH LOW BACKGROUND FOR AMS 14C MEASUREMENTS

HIROYUKI KITAGAWAI'2, TOSHIYUKI MASUZAWAI, TOSHIO MAKAMURA3 and EIJI MATSUMOTOI

14C ABSTRACT. We have developed a method of graphitization from CO2 samples for accurate measurements by accelerator mass spectrometry. Our batch method, using a sealed Vycor tube, reduces the risk of contamination during graphitization and makes it possible to prepare many samples in a short time (typically 20 samples per day).

INTRODUCTION For accurate 14C age determination by accelerator mass spectrometry (AMS), initial materials are usually transformed to solid-state carbon, such as graphite, which delivers an intense, long-lasting ion beam (Lowe 1984; Andree et al.1984; Vogel et al. 1984). The contamination of modern carbon during the procedure must be minimized and reproducible.

In our target preparation method, we reduced the CO2 samples catalytically to graphite on Fe powder in the presence of H2 gas by modifying the method of Vogel et al. (1984). The most important modification involves graphite production by a batch method using a sealed Vycor tube. This reduces the risk of modern carbon contamination and memory effect during graphitization (Vogel, Nelson & Southon 1987; Gurfinkel 1987), and also allows preparation of many samples in a short time (typically 20 samples per day). We also describe details of the target-preparation method involving carbon isotopic fractionation during graphitization, yield of graphite from C02, ion-beam intensity of the target, and background (or blank) level estimated using bituminous coal.

METHODS AND MATERIALS The CO2 sample gas is reduced to graphite on Fe powder in a sealed Vycor tube. The reaction is expressed as Fe catalyst C02 + 2H2 C + 2H2O (1) 650°C

The catalyst, 99.9% spherical Fe powder <325 mesh (44 mm), was weighted in an inner Vycor cup (4 mm I. D. and 6 mm 0. D., 10 mm long). The cup with Fe powder was inserted into an outer Vycor tube (7 mm I. D. and 9 mm 0. D., length depending on sample size) with a sealed end as a reaction vessel. The inner cup and outer tube were first heated at 1000°C for 2 h to remove possible volatile contamination. The outer Vycor tube, used as the reaction vessel, was connected to a vacuum line via an 0-ring stopcock (Young Co., Ltd.) and two Ultra-torr unions (Cajon Co., Ltd.) and then evacuated (Fig. 1). After a high vacuum was attained, 0.5 atm (500 kPa) of pure the H2 was introduced into the tube and the stopcock was closed. The tube was removed from preparation line and set in a hand-made heater with holes (12 mm I. D. and 50 mm deep). The bot-

1Water Research Institute, Nagoya University, Nagoya 464-01, Japan 2Present address: International Research Center for Japanese Studies, Oeyama-cho, Goryo, Nishikyo-ku, Kyoto 610-11, Japan 3Dating and Materials Research Center, Nagoya University, Nagoya 464-01, Japan

295 296 H. Kitagawa et al.

TO PREPARATION LINE

JOINT

JOINT SEALED

Fe --- Fig,1. Apparatus design ELECTRIC FURNACE

tom (ca. 2 cm) of the tube was heated at 450°C for 1 h to remove carbon contamination and to reduce the oxidized surface of spherical Fe powder to pure metal. The apparatus was remounted to the vacuum line and evacuated to <103 torr.

A measured volume of purified CO2 from sample material was cryogenically trapped in the reaction tube. About twice as much pure H2 gas as sample CO2 was added to the volume, and the tube was sealed off with a torch. The bottom (ca. 2 cm) of the sealed tube with the CO2 sample, H2 and Fe powder was heated at 650°C for 4-6 h. In our laboratory, three reaction tubes can be heated at the same time. As the reaction progresses, liquid H2O appears at the upper end of the reaction tube, and CO2 is graphitized onto the Fe powder. The mixture of graphite and Fe powder was pressed directly into a 1.5-mm-diameter hole in an aluminum target holder. The graphite 14C targets were used for measurements with a Tandetron accelerator mass spectrometer (TAMS) at the Dating and Materials Research Center, Nagoya University. Nakai et al. (1984) and Nakamura, Nakai and Ohishi (1987) described our procedure for TAMS 14C measurements.

We investigated isotopic fractionation during graphitization by measuring S13C of CO2 prepared by combustion of the resultant graphite targets using a Finnigan MAT 251 gas ion-source mass spectrometer at the Water Research Institute, Nagoya University. We assessed the level of 14C contamination using different amounts of bituminous coal collected from Yubari Mine, Hokkaido, Japan. The coal, treated by acid-base washing and heated in vacuo to eliminate possible contamination of modern carbon, was combusted to CO2 at 850°C in a sealed Vycor tube with CuO, and the CO2 gas was then graphitized as described above.

RESULTS AND DISCUSSION Isotopic Fractionation and Yield of Graphite During Graphitization b13C We determined the of the initial CO2 sample and resultant graphite, and the yield of graphite from CO2. Graphitization at low yields (<60%) was performed by stopping the reaction within 1 h to measure fractionation. Isotopic fractionation depended strongly on the reaction's progress, and decreased with the increasing yield of graphite from CO2 (Fig. 2). This tendency can be explained by the Rayleigh condensation model of a solid forming from the gas phase in a closed system Batch Preparation of Graphite Targets 297

Fig. 2. Isotopic fractionation, A = 813C(graphite)-813C(initial C02), as a function of the yield, f, of graphite from sample CO2. A least-squares fit of the results for the Rayleigh condensation model is depicted by the solid line.

0 0.2 0.4 f 0.6 0.8 1.0 0, given by (Hoefs 1987). At a yield, f, of graphite from C02, the isotopic fractionation, is (2) E = 8s-Sg = (1-f)/fEln(1-f)

b13C respectively, where 8s and bg are values of the initial CO2 sample and resultant graphite, analysis of 10 data points using Eq. (2) and E is the carbon isotopic enrichment factor. Regression fractionation (Fig. 2) gives an t of -5.0 ± 1.3 (1Q). The solid line of Figure 2 shows the isotopic vs. f.

We estimated graphite yield from a CO2 sample for different conditions using the experimental The extent correlation between the yield of graphite from a CO2 sample and isotopic fractionation. the initial of isotopic fractionation between a CO2 sample and resultant graphite depended on graphite with a high pressure of CO2 in the reaction vessel (Fig. 3). We expected a high yield of of reaction initial pressure of CO2 in the reaction vessel. Therefore, we changed the tube length should vessel depending on sample size; to prepare ultra-small samples (<200 µg), graphitization (4 mm I. D. and be performed at a high initial pressure of CO2 using an extremely small vessel 10 cm long).

"C Background (or Blank) Levels 14C from 14C-free material Figure 4 shows the results of measurements on graphite targets prepared during the (Miocene bituminous coal from the Yubari Mine). These targets were prepared 14C 14C level for preparation of other samples with various activities. The average background 14C measurements, graphitized targets with >200 carbon, including the background from TAMS which combustion and graphitization, was 0.112 ± 0.057 percent modern carbon (pMC), 14C higher corresponds to an equivalent age of ca. 55 ka BP. This background value is somewhat Nakai 1992; than that of the TAMS system background (0.03 pMC or ca. 65 ka BP; Nakamura & might be Nakamura, Oka & Sakamoto 1992) and does not vary with sample size. This difference procedure. Our due to a slight residual contamination of the bituminous coal through the cleaning coal. On the pretreatment procedure before combustion may not be adequate for the bituminous higher 14C other hand two measurements of targets with <200 g carbon showed somewhat 14C size, as reported concentrations. The background level tends to increase with decreasing sample 14C with de- by Vogel, Nelson and Southon (1987). It seems to be that the increase of background 298 H. Kitagawa Ct al.

A 2F 0r__nt rg 3

1 0

1 0 -D Pt

-10 -g -g _ -2 A6(°loo) Fig, 3. Isotopic fractionation with different pressures of initial CO2. A. <100 mm Hg; B. 100-150 mm Hg; C. 150-200 mm Hg; D. >200 mm Hg

100

, f

H 10 20 7

00 50 1 .1 Q :Q7 ... , 60 ------AMS system backgroud------,

1 1111111 1 l 1 1111,, 1 _- 11 1111 .01 .1 1 10 SAMPLE SIZE(mgC)

Fig, 4, Results of "C measurements on graphite prepared from Miocene bituminous coal from the Yubari Mine Batch Preparation of Graphite Targets 299 creasing sample size is due to the addition of a constant contamination during the reduction or combustion process, which was not determined precisely.

1303+ Ion-Beam Intensity from Graphitized Target

The 13C3+ ion-beam intensity from a graphitized target is expressed as the average intensity ratio, Is/Ig, of the 13C3+ ion-beam intensity of a graphitized target vs. that from spectroscopic pure graphite because the 13C3+ ion-beam intensity from targets was changed by the geometry and ion source condition of the TAMS system. Is/Ig depends strongly on the C/Fe weight ratio (Fig. 5).

1 a a 1.5 a

0 0 0 0 0 0 e8 ° o $ 0 0 0 ° 0 0 0 0 0 00 0 0 °°& 0 0 0 Fig. 5. The 1363+ ion-beam intensity 0 ratio from graphitized targets to the 0 spectroscopic graphite (Is/Ig) as a a 8° function of C/Fe weight ratio

0 0.5 1.0 1.5 C/Fe (weight)

Fig. 6. The expected 1303+ ion-beam intensity ratio from graphitized targets to the spectroscopic pure graphite, using 1 mg Fe powder, deduced from a formula obtained by the multiple regression analysis of 78 14C TAMS 1 measurements. For example, the Is/Ig ratio from the target containing 200 µg carbon prepared using 1 ml volume reaction vessel is 0.5. 10 50 100 5001000 Carbon (pg) 300 H. Kitagawa et al.

The 13C3+ ion-beam intensity from targets with a high C/Fe weight ratio was higher than that from 13C3+ the spectroscopic graphite. ion-beam intensity of the target with a high C/Fe weight ratio 14C (C/Fe = 1.0 ) was typically 100 nA, and counting ratio of up to 15 cps was achieved for a graphitized target from a modern sample. On the other hand, a target with a low C/Fe weight ratio showed a low ion-beam intensity; thus, measurement precision worsens. In our TAMS system, we need a minimum of 1 mg of Fe powder for target preparation so that the measurement precision depends on sample size.

We obtained a formula by the multiple regression analysis of 78 TAMS 14C measurements of graphitized targets. We deduced the expected Is/Ig ratios for these graphitized targets, using 1 mg Fe powder, from graphitization conditions, their carbon contents, and volumes of the reaction vessels, which correlate with the initial pressure of CO2. Figure 6 shows that when the graphitization is performed in a reaction vessel of 1-ml volume, the ion-beam intensity of a target with >200 ug carbon is >1/z the beam intensity of spectroscopic graphite. This also indicates that samples with 200 ug carbon are sufficient to date Holocene materials with a precision 200 yr in routine analysis. However, ultra-small samples containing <200,ug carbon have significantly lower ion-beam intensities than those with >200,ug carbon. The precision of the determined age is limited by the ion-beam intensity of targets.

CONCLUSIONS

We were able to determine the 14C ages of ultra-small samples and extremely old samples by the batch graphitization method in a sealed Vycor tube. The transformation of CO2 samples to graphite targets is rapid and simple and does not require special and expensive apparatus.

REFERENCES

Andree, M., Beer, J., Oeschger, H., Bonani, G., Hafma- niques of tandem accelerator mass spectrometry and 14C nn, H. J., Morenzoni, E., Nessi, M., Suter, M. and their applications to measurements. In Gove, H. Wolfi, W. 1984 Target preparation for milligram-size E., Litherland, A. E. and Elmore, D., eds., Proceed- 14C samples and data evaluation for AMS measure- ings of the 4th International Symposium on Accelera- ments. In Wolfli, W., Polach, H. A. and Anderson, H. tor Mass Spectrometry. Nuclear Instruments and H., eds., Proceedings of the 3rd International Sympo- Methods in Physics Research B29: 335-360. sium on Accelerator Mass Spectrometry. Nuclear Nakamura, T., Oka, S. and Sakamoto, T. 1992 Radio- Instruments and Methods in Physics Research 233 carbon ages of charred wood from the Tokyo pumice (B5): 274-279. flow deposit measured with the Tandetron accelerator Gurfinkel, D. M. 1987 An assessment of laboratory mass spectrometer. Journal of Geological Science, contamination at the Isotrace Radiocarbon Facility. Japan 98(9): 908-908 (in Japanese). Radiocarbon 29(3): 335-346. Nakamura, T. and Nakai, N. 1992 A Study on older Hoefs, J. 1987 Stable Isotope Geochemistry. Berlin, radiocarbon age measurable with accelerator mass Spring-er-Verlag: 241 p. spectrometer. Proceedings of the 29th International Lowe, D. C. 1984 Preparation of graphite targets for Geological Congress, Kyoto, Japan: 632. radiocarbon dating by Tandetron accelerator mass Vogel, J. S., Southon, J. R., Nelson, D. E. and Brown T. spectrometer (TAMS). International Journal of A. 1984 Performance of catalytically condensed Applied Radiation and Isotopes 35: 349-359. carbon for use in accelerator mass spectrometry. In Nakai, N., Nakamura, T., Kimura, M., Sakase, T., Sato, Wolfli, W., Polach, H. A. and Anderson, H. H., eds., S. and Sakai, A. 1984 Accelerator mass spectrometry Proceedings of the 3rd International Symposium on 14C of at Nagoya University, In Wolfli, W., Polach, Accelerator Mass Spectrometry. Nuclear Instruments H. A. and Anderson, H. H., eds., Proceedings of the and Methods in Physics Research 233 (B5): 289- 3rd International Symposium on Accelerator Mass 293. Spectrometry. Nuclear Instruments and Methods in Vogel, J. S., Nelson, D. E. and Southon, J. R.198714C Physics Research 233(B5): 171-174. background levels in an accelerator mass spectrome- Nakamura, T., Nakai, N. and Ohishi, S. 1987 Tech- try system. Radiocarbon 29(3): 323-333. (RADIOCARBON, VOL. 35, No. 2, 1993, P. 301-3101

AMS-GRAPHITE TARGET PRODUCTION METHODS AT THE WOODS HOLE OCEANOGRAPHIC INSTITUTION DURING 1986-19911

ALAN R. GAGNON and GLENN A. JONES

Geology and Geophysics Department, Woods Hole Oceanographic Institution Woods Hole, Massachusetts 02543 USA

ABSTRACT. In July 1986, an AMS radiocarbon target preparation laboratory was established at the Woods Hole Oceanographic Institution to produce graphite to be analyzed at the NSF-Accelerator Facility for Radioisotope Analysis at the University of Arizona (Tucson). By June 1991, 923 graphite targets had been prepared and 847 analyzed. Our lab procedures during this time included the careful documentation of weights of all starting samples, catalysts and final graphite yields, as well as the volume of CO2 gas evolved during CaCO3 hydrolysis or closed-tube organic carbon combustions. From these data, we evaluate the methods used in general and in our lab.

INTRODUCTION We present here the sample preparation and graphitization procedures used at the Woods Hole Oceanographic Institution (WHOI) AMS-Graphite Target Preparation Laboratory from July 1986 through June 1991. The methods are based on those developed by Jull et al. (1986) and Slota et al. (1987) and modified slightly by Jones et al. (1989). We describe additional refinements or changes to these methods. All graphite produced during this period was analyzed at the NSF- Accelerator Facility for Radioisotope Analysis at the University of Arizona (Linick et al. 1986). L. Toolin from the Arizona TAMS facility assisted with the initial design and setup of the glass vacuum lines for this lab. The initial line was designed to produce and quantify CO2 generated from either CaCO3 hydrolysis or closed-tube organic carbon combustion reactions and reduce the CO2 gas to graphite in two reactor ports (Fig. 1). By June 1988, we had expanded our capabilities to include extraction of dissolved inorganic carbon (DIC) from seawater and increased the number of graphite reactor ports to six. A typical work day consisted of graphitizing five carbonate or organic carbon samples and either an NBS oxalic acid I (OX I) or oxalic acid II (OX II) gas standard. We generated vacuum line blanks (14C-"dead" carbonate) at the beginning, middle and end of each graphite target preparation batch run of ca. 100-120 targets. Of the 923 targets produced from July 1986 to June 1991, 847 were AMS-dated at the Arizona facility. All targets produced after June 1991 were analyzed at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility (Jones et al. 1990) located at the Woods Hole Oceanographic Institution. In July 1991, the lab was moved and integrated with the newly completed NOSAMS graphite preparation lab. We present summary results on 528 CaCO3 hydrolysis reactions (497 AMS-dated), 144 closed-tube organic carbon combustion reactions (129 AMS-dated), and 38 DIC extractions from seawater (32 AMS-dated). We also present summary results for 170 NBS OX I and OX II standards (156 AMS-dated), 36 CaCO3 hydrolysis blanks (27 AMS-dated) and 7 closed-tube organic carbon combustion blanks (6 AMS-dated). Laboratory procedures during this time consisted of documenting and recording into a database the starting sample and catalyst weights, reactor port utilized, CO2 reaction pressures, final graphite yields and the volume of CO2 gas evolved by CaCO3 hydrolysis or closed-tube organic carbon combustion of the samples. Also included in the database are fields such as the date the sample

'This paper was presented at the 14th International Radiocarbon Conference, 20-24 May 1991, Tucson, Arizona.

301 302 A. R. Gagnon and G. A. Jones

VARIAN THERMOCOUPLE GAUGE TO DIFFUSION AND ROUGH PUMPS

'` 1n TUBE & OVEN OMEGA GRAPHITE PRESSURE REACTOR TRANSDUCER 9 . VALVE (not to scale)

Fig. 1. Schematic of the initial WHOI graphite preparation vacuum line. Shown is the typical CO2 transfer setup for a CaCO3 hydrolysis reaction prior to acidification of the sample.

14C was AMS-analyzed, the age and precision, fraction modern and precision and the corresponding Arizona AMS target accession numbers. Using this database, we can sort and calculate a wide range of parameters that allow us to evaluate the methods used in general and in our lab.

METHODS NBS Oxalic Acid Standards

We made NBS oxalic acid standards using the wet digestion methods presented in Valastro, Land and Varela (1979). Our standards were made at the WHOI Radiocarbon Laboratory using the techniques outlined in Griffin and Druffel (1985). We synthesized 5 liters each of clean CO2 gas from NBS OX I and OX II. We used an approximate 3.05 cc aliquot of CO2 gas, resulting in a 1.5 mg graphite target, for each oxalic acid graphite target produced. We used the same 5-liter bulbs of OX I and OX II CO2 standards during the entire period, 1986-1991.

CaCO3 Hydrolysis

The primary carbonate sample types processed were open-ocean microfossil species of foraminifera (both benthonic and planktonic), miscellaneous macrofossil bivalve shell species and total carbonate from ocean sediments. These samples were weighed (8-15 mg CaCO3) and transferred to a glass reaction vessel (Fig. 1). We added 4 ml of 85% phosphoric acid to the side arm of the reaction vessel, and connected a glass-plug vacuum valve to the vessel via a Cajon Ultra-Tory union. We connected the vessel to the vacuum line and applied a dewar containing a dry ice/isopropanol slush AMS-Graphite Methods at Woods Hole 303 trap to the acid arm to contain water residing in the acid. The valve was opened slowly until vacuum was achieved, as indicated on a Varian 801 thermocouple gauge. The valve was closed, the vessel removed from the vacuum line and the acid warmed to ambient temperature. The vessel was tilted manually to allow the acid to flow from the side arm until it made contact with the carbonate. We added the acid slowly to prevent the sample from "splashing up" into the upper part of the reaction vessel. We then heated the acid/sample mixture to near boiling with a Bunsen burner every 30 min until we observed no reaction. Reaction time was usually ca. 2 h. Once the reaction was completed, the vessel was re-attached to the vacuum line, a dry ice/isopropanol slush trap was attached to the lower finger of the reaction vessel, a second dry ice/isopropanol slush trap was attached to the vacuum line, the reaction vessel was opened and the gas was transferred cryogenically for 3-4 min to a region of known volume. The CO2 was expanded in the known-volume region, warmed to ambient temperature, measured with an MKS Baratron 222BA pressure transducer and transferred directly to the graphite reactor (Fig. 1).

All carbonate blanks were made from a Pennsylvanian limestone obtained from the Arizona lab. From September 1986 to June 1989, CaCO3 blanks were made without pretreatment (acid etching) of the sample. A 40-50 mg sample was crushed, ca. 10-15 mg were acidified and the evolved CO2 was graphitized. The remaining 30-40 mg of crushed carbonate were saved for the two remaining blanks associated with each batch run of 100 target preparations. This approach resulted in each successive blank giving increasingly higher backgrounds due to adsorption of atmospheric CO2 onto the carbonate. After May 1989, we implemented the following procedure for each blank produced. About 15-20 mg limestone were placed in a precleaned beaker and acidified for 1-3 min with 10% organic-free HCl (Froelich 1990). The limestone was rinsed thoroughly with double-distilled water (DDH2O) (Froelich 1990) to remove the acid and placed in a 50°C oven until dry. A precleaned (10% HCI, DDH2O rinse) agate mortar and pestle were used to crush the sample to a fine powder. The powder was weighed (10-15 mg), transferred to a reaction vessel and hydrolyzed according to the procedures outlined above. Any remaining CaCO3 powder was discarded.

Closed-Tube Organic Carbon Combustion The primary organic carbon sample types were derived from total organic carbon and miscellaneous organic fragments in ocean sediments, wood fragments, insect parts and egg cases, and particulate organic matter from deep-ocean sediment traps. CO2 was generated from each sample using the methods outlined in LeFeuvre and Jones (1988), with slight adjustments by the authors. This combustion method was chosen from review of studies conducted by Leventhal (1976). These modifications follow closely the study performed by Swerhone et al. (1991), which showed this method to be scientifically viable, economical and simple to perform. All materials used for these analyses (i.e., beakers, syringes, tweezers and aluminum foil) were precleaned with 10% HC1 and baked at 550°C in a muffle furnace for 2 h. All sediments for 14C organic carbon analysis were weighed and placed in a precleaned Pyrex beaker, acidified with 10% organic-free HCI, filtered onto a prebaked (600°C) quartz fiber filter (5 x 5 mm) and rinsed with DDH2O. The filter was wrapped lightly in precleaned aluminum foil and dried in a 50°C oven. Sample combustion tubes (9 mm o.d. x 200-mm-long Pyrex glass) were soaked overnight in Chromerge, rinsed with distilled water, then dried and baked at 550°C for 2 h in a muffle furnace. Two grams CuO (prebaked at 850°C) and a silver foil strip (4.0 x 5.0 mm, cleaned in 10% HCI, rinsed with methanol and baked at 550°C) were added to the tube. The dried, filtered sample was rolled tightly using clean stainless-steel tweezers, and fitted into the combustion tube. The tube was re-attached to the vacuum line and flame-sealed in vacuo. Samples were 304 A. R. Gagnon and G. A. Jones

combusted at 550°C for 5 h in a muffle furnace. After cooling slowly, the tubes were cracked under vacuum, evolved water vapor was trapped with a dry ice/isopropanol slush trap, the CO2 was cryogenically transferred to a region of known volume, and the frozen sample was opened to vacuum for 10-30 s to remove incondensibles. The CO2 was then measured manometrically with an MKS Baratron 222BA pressure transducer and transferred to a graphite reactor. Experiments performed on plant material by Swerhone et al. (1991) showed no significant fractionation in b13C values utilizing this method.

All other material selected for organic carbon analysis was placed in a clean Pyrex beaker, acidified with 10% organic-free HCl and rinsed with DDH2O. A 2% NaOH solution was added, and the beaker placed in a 60°C constant-temperature water bath for 1 h. To remove all humic acids, the NaOH step was repeated (discarding the supernatant, usually 2-4 times, but occasionally left overnight) until the solution remained clear. The sample was rinsed with DDH2O, re-acidified with 10% organic-free HCI, and rinsed again with DDH2O during filtering (as previously described). CO2 was generated by closed-tube combustion as outlined above. To test the background of the closed-tube organic carbon combustion procedure, we generated -1.5 cc of CO2 from the Pennsylvanian limestone in the same manner as outlined for carbonate blanks in the CaCO3 hydrolysis section. The CO2 evolved was transferred cryogenically to a Pyrex glass combustion tube containing the following precleaned ingredients; 2 g CuO, a strip of silver foil and a quartz fiber filter that had been rinsed with DDH2O and dried. The tube with CO2 was flame- sealed in vacuo and combusted according to the procedures outlined above.

DIC from Seawater

We adapted methods described in Bard et al. (1987) to strip CO2 from seawater. Each sample for 14C DIC analysis was drawn from a Niskin bottle, stored in a precleaned 1-liter glass bottle and poisoned with a saturated HgC12 solution. We transferred 0.5 liter of the 1-liter sample to a stripping vessel. Ultra-pure helium was used as the carrier gas. We added 4 ml of 85% phosphoric acid to the water, and the sample was bubbled in a helium atmosphere for 1 h at a flow rate of 0.10 cfh. The vacuum-line trapping sequence consisted of 1 dry-ice/isopropanol slush trap followed by 3 liquid nitrogen traps. After 1 h, the helium was pumped away, and the LN2 traps were replaced by 3 dry-ice/isopropanol slush traps (to dry the CO2 thoroughly). The CO2 was transferred cryogenically to a region of known volume and measured manometrically with an MKS Baratron 13C 222BA pressure transducer. Splits for analysis and an archive were saved in flame-sealed glass 14C tubes. The split was transferred directly to a graphite reactor. Tests performed on samples of known >CO2 concentrations, measured coulometrically, showed that our technique recovers >99.0% of the CO2.

Graphite Target Preparation

The methods used follow closely those detailed in Slota et al. (1987). Glass tubing used in the reaction was precleaned with the following procedure: Vycor (quartz) and Pyrex tubing (6 mm o.d. x 130 mm) were soaked overnight in a Chromerge solution, and then baked for 3 h at 850°C for Vycor, and 550°C for Pyrex, in a muffle furnace. The Vycor tube was preweighed,1-2 mg of 200 mesh Fe powder added, and the tube reweighed. About 50-80 mg of 300 mesh Zn powder was added to the Pyrex tube. The tubes were connected to the reactor via a 6.5 mm Cajon Ultra-Torr tee fitting with an attached Omega PX176 pressure transducer (Fig. 1). The reactor ovens were placed on the tubes and the catalyst and reagent roasted in vacuo for 20 min. Reactor-oven temperatures were 650°C for Fe and 435°C for Zn (controlled by variable transformers). We AMS-Graphite Methods at Woods Hole 305 removed the ovens, isolated the reactor from the vacuum pump and monitored the reactor pressure for 1-2 h to guarantee no leaks to atmosphere. A known amount of CO2 gas was transferred cryogenically to the reactor, brought to ambient room temperature and the pressure recorded. The 435°C oven was placed over the Pyrex tube containing the Zn, so that the tip of the tube was located at the "hot spot" (± 5°C) of the oven. We monitored the pressure until stable (ca. 30 min). Residual gas analysis (RGA) performed on several reactions showed that most CO2 had dissociated to CO by this time (McNichol et al. 1992). The 650°C oven was then placed over the Vycor tube containing the Fe, so that the tip of the tube was within the "hot spot" (± 5°C). The reaction proceeded overnight, and was complete (typically, 7-10 h) when the pressure in the reactor returned to zero (Fig. 2), indicating a manometrically determined 100% yield. The ovens were removed from the tubes, and the reactor cooled to room temperature. Vacuum was broken slowly at the Zn tube Ultra-Torr connector, and the Zn tube discarded. The Fe tube (with graphite) was removed immediately and weighed. A graphite yield was determined gravimetrically. The tube containing graphite was then labeled, capped with parafilm and stored in a desiccator.

RESULTS Having stored all of the graphite reaction values in a database file, we were able to determine a range of relations. As a check on the CO2-to-graphite reaction, several graphitizations were manometrically monitored overnight with the Omega PX176 pressure transducer connected to a HP9000 UNIX workstation via an HP3852 data acquisition control unit. A final pressure of zero would indicate a 100% yield. The reaction time for three different initial CO2 pressures of OX II

NBS Oxalic Acid II

1.6

Reaction Time (hours)

Fig. 2. Reaction times for catalytic reduction of CO2 to graphite using Fe-Zn. With a starting CO2 pressure of 1.1 atm (6.5 cc), the reaction was complete in 7.0 h. The reaction time increased to 10.5 h by halving the CO2 pressure (0.58 atm = 3.25 cc). Reducing the CO2 to 0.25 (0.33 atm = 1.8 cc) further increases the reaction time to 11.6 h. A manometrically determined 100% graphite yield was obtained from all three reactions. 306 A. R. Gagnon and G. A. Jones

Woods Hole Graphites 045-969

% Yield

Fig. 3. Histogram of all Woods Hole graphite yields of samples produced between 1987 and 1991. Manometric measurements of the initial and final pressures for all samples indicate that all CO2 is converted to graphite; however, gravimetric measurements of the graphite suggest an average yield of 93.5%. We believe the slightly lower yields calculated gravimetrically are due to preroasting and vacuum pumping on the iron catalyst and tube, which occurs after the iron has been weighed into the tube. Much of the scatter in the data is due to the precision limitations of the gravimetric measurements. Mean weight of graphites produced was 1.3 ± 0.45 mg where the precision of the analytical balance used was ± 0.05 mg or 4% of the mean. Percent yields are calculated as actual weighed graphite divided by the theoretically calculated graphite as determined manometrically. increased from ca. 7 h for initial pressures of -1.2 atm (STP) to ca. 12 h for initial pressures of --0.4 atm (STP) (Fig. 2). The average CO2 pressure of samples analyzed at this lab from 1987-1991 was 0.6 atm (STP). Monitoring the reaction with CO2 evolved from closed-tube organic carbon combustion yielded a reaction time of 8-10 h for a nominal pressure of 0.8 atm (STP).

A comparison of the gravimetrically measured graphite vs. that calculated manometrically from the C02 pressure gave a mean yield of 93.5 ± 9.2% (Fig. 3). Monitoring the initial and final gas pressure within the reactors via the pressure transducer indicated a 100% conversion of CO2 to graphite. Samples were weighed immediately after vacuum was broken at the reactor to reduce moisture absorption from the atmosphere. Much of the scatter (± 9.2%) can probably be attributed to weighing the increase of 1-2 mg (graphite yield) in a tube that initially weighed 4-5 g (typical weight of tube + Fe) on an analytical balance that has an internal reproducibility of ± 0.05 mg. The difference between the mean manometric (100%) and mean gravimetric (93.5%) yields may be, in part, a result of preroasting and pumping on the iron catalyst after initial weighing.

On each target wheel run at the Arizona facility, there were 10 positions consisting of 8 samples, an OX I (position 1), and an OX II (position 6) standard. An OX II/OX I ratio was calculated for each wheel, and the mean value obtained from 75 pairs of OX II/OX I graphite targets produced in our lab between 1987 and 1991 was 1.292 ± 0.012 (Fig. 4). The OX II/OX I value used by the Arizona facility was 1.2909 (Donahue, Linick & Jull 1990). Our standard method of interaction AMS-Graphite Methods at Woods Hole 307

WHOI NBS Oxalic Acid Ratios

12 WHO! Mean=1.292±0.012

Total=75 pairs Accepted Mean=1.2909 1

OxII/OxI

Fig. 4. Results of all Woods Hole NBS oxalic acid standard ratios obtained from 1987-1991. On each wheel of 10 WHOI targets analyzed at the Arizona TAMS facility, there is 1 OX I and 1 OX II standard. The WHOI ox II value is divided by the value of OXI (=1.046 fraction modern), yielding a mean of 1.292 ± 0.012. The ox II/OX I ratio used by the Arizona facility was 1.2909 (Donahue, Linick & Ju111990). The differences in the ratios determined by the two labs are statistically insignificant. with the Arizona facility was to produce 100 targets (10 wheels = 1 WHOI batch run) consisting of 75-76 unknown samples, 10 OX I standards, 10 OX II standards, 3 CaCO3 hydrolysis blanks, and 1-2 closed-tube organic carbon combustion blanks. Figures 5A and 5B show the precision vs, the 14C age (in ka BP) for all Woods Hole graphite targets run at Arizona. Over 80% of the graphite targets analyzed were younger than 15 ka BP (Fig. 5B). The precision obtained for all Woods Hole graphite samples of near modern age, based largely on counting statistics and random laboratory error, is 0.6-0.7%. Figures 6A and 6B show histograms of uncorrected percent modern carbon (pMC) values for carbonate blanks analyzed before June 1989 and from June 1989 to June 1991, respectively. Before we began pretreating (acid-etching) our carbonate blank material in June 1989, as described in the methods section, our mean blank value was 0.556 ± 0.37 pMC, with higher values obtained for increased time between the crushing of the blank material and graphitization (Fig. 6A). This increase probably results from adsorption of atmospheric CO2 on the crushed CaCO3 powder (i.e., large surface-to-volume ratio). After May 1989, the pre-etching of the carbonate blank material with 10% organic-free HCl for 1-3 min before crushing decreased our CaCO3 blank value to 0.256 ± 0.12 pMC (Fig. 6B). The results suggest that atmospheric CO2 can contribute to contamination 14C of old CaCO3 samples, which, in turn, can result in erroneous values. Several tests of pre- etching vs. no treatment on foraminifera recovered from marine sediments show that pre-etching results in 2-3 ka older ages for samples near 30 ka BP (2.4 pMC). Our standard procedure is to pre-etch, with 10% organic-free HCI, all CaCO3 samples estimated to be >25 ka BP (<4.4 pMC). Our closed-tube organic carbon combustion blanks show an uncorrected value of 0.762 ± 0.12 308 A. R. Gagnon and G. A. Jones

14 C Ale vs. Precision % 40

30 638 AMS-run Targets >0 kaBP 25

0 LLJ 0 5 10 15 20 25 30 35 40 45 50 14C Age (kaBP)

14C Fig. 5A. age vs. precision % for all targets produced at WHOI and run at the Arizona TAMS facility with ages >0 ka BP (excluding blanks). Precision % is calculated by dividing the value of fraction modern error by the actual fraction modern multiplied by 100.

14 C Age vs. Precision % 5 c

M 4 505 AMS-run Targets >0 kaBP and <15 kaBP

r r r '

r ti 1 ii tiI r t i f 1 2,4

, I i 1, 1 1, 1 i 1 i 0 I, I, I, 1,1 1, 1 1 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 "C Age (kaBP)

14C Fig. 5B. age (<15 ka BP) vs. precision % showing only those samples in Figure 5A that are younger than 15 ka BP. Note that sample precision approaches ± 0.6-0.7%. AMS-Graphite Methods at Woods Hole 309

All WHO! CaCO3 Blanks (Sept 1987-May 1989)

Fig. 6A. Uncorrected 14C/12C values for 14 WHOI CaCO3 blanks run before June 1989. Samples were not subjected to the pretreatment procedures outlined in the CaCO3 hydrolysis methods section. Our mean 0.0 0.2 0.4 0.6 0.8 1.0 1.1 1.4 1.6 pMC during the period was 0.556 ± 0.37. % Modern (uncorrected)

All WHO! CaCO3 Blanks (June 1989-June 1991)

'4C/12C Fig. 6B. Uncorrected values for 13 WHO! CaCO3 blanks and 5 organic carbon blanks (hatched). All blanks made after May 1989 were subjected to the pretreatment procedures outlined in the CaCO3 hydrolysis methods section. Our mean pMC during 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 the period was 0.256 ± 0.12 for CaCO3 and 0.762 ± % Modern (uncorrected) 0.12 for organic carbon blanks. pMC. Our laboratory CaCO3 hydrolysis and closed-tube organic carbon combustion blank values are indistinguishable from similar results obtained in Arizona (Dull, personal communication).

CONCLUSIONS Between July 1986 and June 1991, we produced 923 graphite targets at the Woods Hole Ocean- ographic AMS Graphite Preparation Lab using the methods of Slota et al. (1987). Our results are consistent with those obtained at the NSF-Arizona Accelerator Facility for Radioisotope Analysis, using the same methodology. We have also shown that the graphite yields obtained with these methods have a mean yield of ca. 94%, with a reaction time of 7.0-12.0 h. Analysis of 75 pairs of OX I and OX II standards reveal an OX II/OX I ratio of 1.292 ± 0.012, which is in agreement with the value used by Arizona (Donahue, Linick & Jull 1990). Our background correction is 0.25 pMC (48 ka BP) for CaCO3 hydrolysis samples and 0.76 pMC (39 ka BP) for closed-tube organic carbon combustion samples.

Samples estimated to be older than 25 ka BP should be pre-etched in dilute acid to remove the surface layer of CaCO3. Although this pre-etching should be performed on all samples, the very small samples often available for AMS dating preclude this as a routine procedure, and, in fact, result in an insignificant improvement in accuracy for samples younger than 25 ka BP at the levels 310 A. R. Gagnon and G. A. Jones of precision obtained routinely. Limestone used for carbonate blanks should be pre-etched in dilute acid and converted to CO2 on the same day to reduce the effect of atmospheric CO2 contamination.

ACKNOWLEDGMENTS We would like to thank the members of the Arizona TAMS facility for all their efforts and patience in producing the 14C results presented in this paper; Tim Jull, Doug Donahue, Larry Toolin, Tim Linick (deceased), Art Hatheway and Todd Lange. We would especially like to thank Larry Toolin and Tim Jull for helping us in the initial setup and design of our graphite lines, and for answering our many questions over the years. Special thanks to many members of the Woods Hole Oceanographic Institution; Kathryn Elder, Sheila Griffin, Ellen Druffel, Ann McNichol, Bob Schneider, Amy Witter, Dan McCorkle, John Lee, Gregory Cohen and Carol Ann Kauffman. We are indebted to the glassblowing talents of Bob Anderson. This facility was funded by grants from The National Science Foundation, Office of Naval Research and the Mellon Foundation. This is contribution no. 8136 from the Woods Hole Oceanographic Institution.

REFERENCES

Bard, E., Arnold, M., Maurice, P. and Duplessy, J. C. of biological samples sealed in glass tubes as a 1987 Measurements of bomb radiocarbon in the preparation for S13C determination. Analyst 113: ocean by means of accelerator mass spectrometry: 817-823. Technical aspect. Nuclear Instruments and Methods Leventhal, J. S. 1976 Stepwise pyrolysis-Gas chroma- in Physics Research B29: 297-301. tography of kerogen in sedimentary rocks. Chemical Donahue, D. J., Linick, T W. and Jull, A. J. T. 1990 Geology 18: 5-20. Isotope-ratio and background corrections For acceler- Linick, T. W., Jull, A. J. T., Toolin, L. J. and Donahue, ator mass spectrometry radiocarbon measurements. D. J. 1986 Operation of the NSF-Arizona Accelerator Radiocarbon 32(2): 135-142. Facility for radioisotope analysis and results from Froelich, P. N. 1980 Analysis of organic carbon in selected collaborative research projects. In Stuiver, marine sediments. Limnology and Oceanography 25 M. and Kra, R. S., eds., Proceedings of the 12th (3): 564-572. International 14C Conference. Radiocarbon 28(2A): Griffin, S. and Druffel, E. R. M. 1985 Woods Hole 522-533. Oceanographic Institution Radiocarbon Laboratory: McNichol, A. P., Gagnon, A. R., Jones, G. A. and Sample treatment and gas preparation. Radiocarbon Osborne, E. A. 1992 Illumination of a black box: Gas 27(1): 43-51. composition changes during graphite target prepara- Jones, G. A., Jull, A. J. T., Linick, T. W. and Donahue, tion for AMS. In Long, A. and Kra, R. S., eds., D. J. 1989 Dating of deep-sea sediments: A compari- Proceedings of the 14th International 14C Conference. son of accelerator mass spectrometer and beta-decay Radiocarbon 34(3): 321-329. methods. Radiocarbon 31(2): 105-116. Slota, P. J., Jr., Jull, A. J. T., Linick, T. W. and Toolin, Jones, G. A., McNichol, A. P., von Reden, K. F. and L. J. 1987 Preparation of small samples for 14C Schneider, R. J. 1990 The National Ocean Sciences accelerator targets by catalytic reduction of CO. AMS Facility at Woods Hole Oceanographic Institu- Radiocarbon 29(2): 303-306. tion, In Yiou, F. and Raisbeck, G. M., eds., Proceed- Swerhone, G. D., Hobson, K. A., van Kessel, C. and ings of the 5th International Conference on Accelera- Boutton, T.W. 1991 An economical method for the tor Mass Spectrometry. Nuclear Instruments and preparation of plant and animal tissue for 813C Methods in Physics Research B52: 278-284. Analysis. Communications in Soil Science and Plant Jull, A. J. T., Donahue, D. J., Hatheway, A. L., Linick, Analysis 22(3 & 4): 177-190. T. W. and Toolin, L. J. 1986 Production of graphite Valastro, S., Jr., Land, L. S. and Varela, A. G. 1979 An targets by deposition from CO/HZ for precision improved procedure for wet oxidation of the 14C NBS accelerator 14C measurements. In Stuiver, M. and oxalic acid standard. In Berger, R. and Suess, H. E, Kra, R. S., eds., Proceedings of the 12th International eds., Radiocarbon Dating, Proceedings of the 9th 14C Conference. Radiocarbon 28(2A): 191-197. International Radiocarbon Conference. Berkeley/Los LeFeuvre, R. P. and Jones R. J. 1988 Static combustion Angeles, University of California Press: 125-134. [RADIOCARBON, VOL. 35, No. 2, 1993, P. 311-316]

RADIOCARBON TO CALENDAR DATE CONVERSION: CALENDRICAL BAND WIDTHS AS A FUNCTION OF RADIOCARBON PRECISION

F. G. McCORMAC and M. G. L. BAILLIE

The Queen's University of Belfast, School of Geosciences, Palaeoecology Centre Belfast BT7 1NN, Northern Ireland

14C ABSTRACT. Accurate high-precision dating (i.e., ± 20 yr precision or less on the 14C date) provides the narrowest calendrical band width and, hence, the best age range determination possible. However, because of the structure in the 14C 14C calibration curve, the calendar age range for a given precision is not constant throughout the calibration range. In this study, we quantify the calendar band widths for a range of 14C previsions throughout the calibration range. We show that an 14C estimate of the likely calendar band width in years can be obtained from the expression: Band width (yr) = 2.12 x precision (1 Q) + 54.6. We also show that calendar band widths are widest around 4000 BP at the start of the Bronze Age, and become narrow through the later Bronze Age and Iron Age and back into the Neolithic.

INTRODUCTION 14C dates are reported as BP values with an associated precision (Stuiver & Polach 1977). Typically, the quoted error represents ± one standard deviation (1 o), as determined by the total number of accumulated counts for that sample. Most laboratories also include an error multiplier that reflects the uncertainty associated with repeated dating of identical sample material. The 14C date and total associated error are then converted to a calendar date (Stuiver 1989), using the internationally agreed 14C calibration curve determined by Stuiver and Pearson (1986) and Pearson and Stuiver (1986) using a computer program developed by Stuiver and Reimer (1986). Proba- bilistic calibration techniques have also been developed by van der Plicht and Mook (1989) and Stuiver and Reimer (1987). Calibration introduces two additional sources of uncertainty in the final calendar date. First, because the calibration curve, itself, is a set of 14C measurements made at decadal/bidecadal resolution on dendrochronologically dated wood, each point on the curve has an associated 14C error and error multiplier. Second, because the curve is not smooth, but contains considerable structure, the calendrical band width for a 14C date of a given precision varies at different points on the calibration curve. Stuiver and Pearson (1986), Pearson and Stuiver (1986) and Stuiver and Becker (1986) published 14C high-precision calibration data in the Calibration Issue of RADIOCARBON (Stuiver & Kra 1986). In their papers, they included tables of calibrated age ranges, for a set of 14C precisions, at dates in the 80-4020 BP range in bidecadal intervals. These tables provide a useful guide to the calendar band width that can be expected for a 14C date of a given precision, but are restricted in range (80-4020 BP), and do not readily inform the user of the variations in band width as a function of iaC age. Here, we quantify the calendar age ranges for a set of 14C precisions at all 14C dates within the calibration range. This will be of use to archaeologists, who should be able to obtain an estimate of laboratory 14C precision based on sample size and expected age, and thereby determine whether or not the probable calendrical band width is useful. We also show that calibration band widths are greatest for all precisions around 4000 BP, at the start of the Bronze Age, and decrease through the later Bronze Age and Iron Age and into the Neolithic. This is attributed to the greatest rate of change of OI4C being near 4000 BP.

311 312 F. G. McCormac and M. G. L. Baillie

METHOD Stuiver and Reimer (1986) compiled a data set, ATM20, for use in their calibration computer program. This file consists of calendar dates derived by dendrochronology and the associated 14C dates and errors at decadal/bidecadal intervals. In this study, we used ATM20 to determine the calendrical band widths of 14C dates at a series of precisions. The method involved fitting a cubic spline curve to the data set and interpolating at intervals of one year. Figure 1 shows the cubic spline interpolation superimposed on a linear interpolation (Stuiver & Reimer 1986) of a section of the original data. Checks at a range of 14C dates showed that both techniques yielded calendrical band widths that were within a few years of each other. We chose the spline because we agree with Suess and Linick (1990) that unknown parts of a function in nature can best be approximated 14C 14C by a spline. Having obtained the spline we then stepped through age (1 step every 5 yr) from 8100-150 BP, and recorded the calendar ages at the intercepts of the values

+ ((14C precision)2 + (calibration curve Q)2) with the calibration curve. The difference between the calendar ages at the intercepts gives the calendrical band width for a given 14C date and associated precision. The band widths, as a function of 14C age (BP), were plotted for four 14C precisions in Figure 2 (A-D).

T-- L I ------T r 6200 6100 6000 5900 5800 Calendar date (yr BP)

14C Fig. 1, Section of the calibration curve. _ = linear interpolations between data points, ----- = cubic spline interpolations; C = interpolated measurements; A and B = ± 1 a errors on the measurements raC to Calendar Date Conversion 313

400 B

300

200

100 H

200 -

2000 4000 6000 8000 0 2000 4000 6000 8000 14C date (yr BP)

14C Fig. 2. Calendrical band widths in years as a function of age, in increments of 5 yr from 8100-150 BP. A. 14C 14C '4C 14C precision of ± 20 yr; B. precision of ± 50 yr; C. precision of ± 100 yr; D. precision of ± 200 yr

RESULTS

Figures 2(A-D) allow users of 14C dates to ascertain what level of 14C precision is required to obtain a given calendar band width at any time during the past nine millennia. The figures also show that even at the highest precisions (Fig. 2A), short intervals (e.g., 2400-2500 BP and 7900-8000 BP) exist when the advantages of high-precision dating are minimal. The period, 4000-5300 BP (i.e., the Neolithic to the beginning of the Bronze Age) gives the largest calendrical band widths. As the dating precision decreases from "high" to "routine" (i.e., ± 50 yr) the calendrical band widths widen (Fig. 2B), but the trends in band width remain similar, with the 4000-5300 BP period giving the largest uncertainties in calendar dates. The mean calendrical band widths for precisions of ± 20 and ± 50 yr, found by averaging all band widths over 8100-150 BP, is 101 yr and 158 yr, respectively. Figure 2(C, D) shows that, as the dating precision decreases, the graphs representing calendrical band width as a function of 14C precision take on a sinusoidal form, with an amplitude of ca. 250 yr and a period of 12,000 yr (assumed by extrapolation). The peak of the curve is at ca. 4000 BP which is the point of greatest rate of change of i14C in the radiocarbon record (Fig. 3). Thus, as the precision decreases, the band width reflects more closely the rate of change of L14C. To illustrate this graphically, we fitted a 4th-order Legendre polynomial to the L14C data derived from the ATM20 data set. We differentiated the resultant polynomial, plotted the calendrical band widths 314 F. G. McCormac and M. G. L. Baillie

k 40 P 4 20

-20

8000 BC 6000 4000 2000 D 2000 AD Calendar date (yr BC/AD)

Fig. 3.0"C (%o) as a function of calendar year. Solid line is a 4th-order polynomial fit to the data.

900 0.035

800- 0.030

100 f- 0.025

0.020 . a , D n

300 0.005

200-x------r T T 8000 BC 6000 4000 2000 0 Calendar date (yr BC/AD)

14C Fig. 4. Calendar band width as a function of calendar age for a precision of ± 200 yr with the differentiated 4th-order polynomial shown in Figure 3 superimposed. This illustrates the good correspondence between the rate of change of A14C and calendrical band width at lower previsions. IaC to Calendar Date Conversion 315

Fig, S. Mean calendrical band width as a function of "C precision 14C Counting Precision t 10 as a function of calendar year, and superimposed the differentiated curve to show the good correspondence between peaks and troughs (Fig. 4). This illustrates that subtle changes in slope of the calibration curve are mimicked in the calendrical band widths at the lower precisions. 14C As the dating precision decreases, the calendrical band width increases. Figure 5 shows the 14C mean calendrical band width over 795014C yr vs. the precision of ± 20, ± 50, ± 100, ± 150 and ± 200 yr. A linear regression fit to these data gives

14C Mean band width (yr) = 2.12 x precision (1 a) + 54.56. (1)

CONCLUSIONS

14C 14C The band width of calendar dates vs. age (for dates with 1 Q > 100 yr) can be approximated by a sinusoid with an amplitude of ca. 250 yr and a period of ca. 12,000 yr with a peak at 4000 BP. For low-precision dates, this result illustrates the response of calibrated age range to 14C, long-term variations of atmospheric and specifically, to the long-term rate of change of O14C.

Although evidence for a similar cyclical variation exists in the band widths obtained at higher precisions (i.e.,1 Q < 50 yr), the calendar age range is dominated by a series of isolated periods with very large band widths, even for precisions as low as ± 20 yr. These correspond to periods of rapid fluctuations or slow change in the atmospheric 14C content. A linear relationship exists between 14C dating precision and the mean calendrical band width, found by averaging all band widths over 8100-150 BP 14C Band width (yr) = 2.12 x precision (1 Q) + 54.6. (2) Thus, for high-precision dates (i.e., ± 20 yr), the calendar age range is typically 100 yr, whereas for routine dates ( ± 50 yr), a 160-yr band width can be expected. Of course, these dates will be modulated by the variability noted above and can only be used as a guide. 316 F. G. McCormac and M. G. L. Baillie

ACKNOWLEDGMENTS F. G. McCormac would like to thank the Royal Society for a grant that helped initiate this study.

REFERENCES

Pearson, G. W. and Stuiver, M. 1986 High-precision carbon 28(2B): 805-838. calibration of the radiocarbon time scale, 500-2500 Stuiver, M and Polach, H. A. 1977 Discussion: Report- 14C BC. In Stuiver, M. and Kra, R. S., eds., Proceedings ing of data. Radiocarbon 19(3): 355-363. of the 12th International 14C Conference. Radiocar- Stuiver, M. and Reimer, P. J. 1986 A computer program and bon 28(2B): 839-862. for radiocarbon age calibration. In Stuiver, M. Stuiver, M. 1989 Dating proxy data. In Berger, A., Kra, R. S., eds., Proceedings of the 12th International 14C Schneider, S. and Duplessy, J., eds., Climate and Conference. Radiocarbon 28(2B): 1022-1030. Geosciences. Dordrecht, The Netherlands, Kluwer __1987 User's guide to the programs CALIB and Academic Publishers: 39-45. DISPLAY Rev. 2.1: Quaternary Isotope Laboratory, M. and Becker, B. 1986 High-precision decadal University of Washington. Stuiver, 14C calibration of the rad iocarbon time scale, AD 1950- Suess, H. E. and Linick, T. W. 1990 The record in based 2500 BC. In Stuiver, M. and Kra, R. S., eds., Pro- bristlecone pine wood of the past 8000 years ceedings of the 12th International 14C Conference. on the dendrochronology of the late C. W. Ferguson. Radiocarbon 28(2B): 863-910. In Proceedings ofA Royal Society and Academic des Stuiver, M. and Kra, R. S., eds. 1986 Calibration Issue. Sciences Discussion Meeting. London, The Royal Proceedings of the 12th International 14C Conference. Society: 5-14. Radiocarbon 28(2B): 805-1030. Van der Plicht, J. and Mook, W. G. 1989 Calibration of and Pearson, G. W. 1986 High-precision calibration radiocarbon ages by computer, In Long, A. and Kra, Stuiver, M. 14C of the rad iocarbon time scale, AD 1950- R. S., eds., Proceedings of the 13th International 500 BC. In Stuiver, M. and Kra, R. S., eds., Proceed- Conference. Radiocarbon 31(3): 805-816. ings of the 12th International 14C Conference. Radio- [RADIOCARBON, VOL. 35, No. 2, 1993, P. 317-322]

A SIMPLIFIED APPROACH TO CALIBRATING 14C DATES

A. S. TALMA and J. C. VOGEL

Quaternary Dating Research Unit, CSIR, P. 0. Box 395, Pretoria 0001, South Africa

ABSTRACT. We propose a simplified approach to the calibration of radiocarbon dates. We use splines through the tree-ring data as calibration curves, thereby eliminating a large part of the statistical scatter of the actual data points. To express the age range, we transform the ± 1 a and ± 2 a values of the BP age to calendar dates and interpret them as the 68% and 95% confidence intervals. This approach bypasses the conceptual problems of the transfer of individual probability values from the radiocarbon to the calendar age. We have adapted software to make this calibration possible.

INTRODUCTION

The tree-ring calibration data now available have prompted the user community to insist increasingly on working with the calibrated ages of their 14C dates. Different computer programs 14C have been developed to accomplish conversion of (BP) ages to calendar dates (e.g., Stuiver & Reimer 1986; van der Plicht & Mook 1989). Experience shows that considerable confusion can arise in the interpretation of values obtained from available programs (see review by Aitchison et al. 1989). For this reason, we propose a somewhat simplified calibration procedure without actually sacrificing accuracy. First, we suggest that the calibration curve be smoothed to a certain extent without seriously harming the real "wiggles" of the 14C levels. Second, we propose that the counting error, expressed as ± 1 a, should be transformed directly to a corresponding 68% confidence interval of calendar dates on the X-axis.

SMOOTHING OF THE CALIBRATION CURVE 14C analyses of dated tree rings have provided data sets (xi, y,, a,), where x;= AD/BC date of the ring 14C yi= BP age of the ring ai= standard deviation of the BP age measurement.

The aim is to use such a data set to derive the most acceptable relation between y and x, y = g(x), that can be used for interpolation of the measured BP ages (y) to obtain calibrated dates of samples (x). In effect, one seeks the best estimate of past atmospheric 14C levels.

Although it can be argued that the most probable value of the 14C level at a certain point in time (sic!) is represented by the actually measured 14C age, it is also true that the real calibration curve should not pass through all of the measured points. In fact, counting statistics demand that, on average, 68.2% of the measured points should be within ± 1 a of the actual curve, 27.2% of the data points in the 1-2 a interval and the remaining 4.6% > 2 a from the calibration curve. A curve produced by connecting all the measured points by straight lines or by a flowing curve will thus contain numerous small-scale fluctuations that realistically do not represent the actual variations 14C of the past atmospheric level. The slow decay of the post-bomb atmospheric 14C level indicates the sluggishness of the atmosphere/ocean system in this respect, and places an upper limit on the rate of short-term decrease of atmospheric 14C levels.

Our work with a calibration data set of 1-3-year tree-ring samples (Vogel et al. 1993) has convinced us that smoothing techniques, such as multiple-point running averages, are unsatisfacto- ry, and a more statistically justified approach is required.

317 318 A. S. Talma and J. C. Vogel

In this situation, the most appropriate mathematical technique to apply is the spline curve (Reinsch 1967) with a "stiffness" that corresponds to the statistical uncertainty of the individual measured 14C values. This approach also meets the geophysical requirement that the level in the atmosphere does not change too abruptly.

Splines are cubic interpolation curves, g(x;), specified separately for each interval between two from the adjacent measuring points: (x1, y;) and (x1, yi+1) (Reinsch 1967). They are calculated entire data set based on the conditions that - The curve be continuous at the data points - The integral slope change rate be a minimum, compatible with required precision constraints - The variability, or smoothness, of the curve can be specified to match the precision of the analytical data. The precision of the spline curve, s, is defined as

s n

This is a parameter of the closeness of fit or, inversely, the stiffness of the curve. Calculating g(x1) with s = 0 implies that the curve thus produced connects each of the data points. This cannot be a valid representation of the real-world situation, because it suggests that each data point be true, an unrealistic requirement in view of the counting statistics. Setting s to a large number allows maximum deviation between the line and the data points and will produce a near-straight line. Stipulating s = 1 is the most realistic requirement, because it produces a curve, such that the root-mean-square distance of the sample points is 1 Q away from the curve. Thus, it is the best compromise between the slowest varying calibration function and the counting statistics.

To illustrate, the example in Figure 1 shows a particularly rough part of the 2500 BC-AD 1940 calibration curve (Stuiver & Pearson 1986; Pearson & Stuiver 1986). The spline curve eliminates the "zig-zags" of the calibration curve while retaining the essential structure of the wiggles (e.g., at 1400-1300 BC). Dates calibrated near a "zig-zag" in the curve produced by connecting all the points will have three or more possible calendar dates, whereas the spline curve will produce only one. This particular spline curve closely fits the requirements laid down by the counting statistics, viz., 66.8% of the tree-ring measurements are within ± 1 Q of the spline curve, 28.3% in the 1-2 Q interval, and 4.9% beyond 2 Q. These percentages indicate that the spline is a good approxima- tion of the past 14C level and that deviations of the data points from the spline closely obey Gaussian statistics. Other smoothing techniques, such as running averages, rely on arbitrary choices of width or weighting that are selected on the basis of the final result for a given data set. Only splines can produce smoothing based on statistical requirements. A useful and practical property of spline-curve calculation is that the x values of the data points need not be evenly spaced (Reinsch 1967), a considerable advantage when using irregularly spaced short-interval data (e.g., Vogel et al. 1993).

TRANSFORMATION OF PROBABILITIES The purely Gaussian distribution of probability around the measurement of a 14C sample (BP) is quantified by the 1 Q error attached to the age. The transformation of this probability distribution to the calendar date (or x-axis) is a complicated matter, and the result depends, to some extent, on the statistical approach taken (Dehling & van der Plicht 1993). Simplified Approach to 14C Calibration 319

3100

a. 3050

3000

2950 1

2900 - 1 -1400 -1300 -1200 -1100 HISTORICAL DATE (BC)

Fig.1. A selection of tree-ring calibration data from Pearson & Stuiver (1986) with the calculated spline curve. Note the manner in which the wiggle valley at 1350 BC is preserved by the spline curve. In the range, 3000-2900 BP, the spline curve averages between varying measurement points and removes the multiple intercepts that would have occurred if all the sample points were connected.

The textbook approach proceeds from a sample analysis as a measurement with a Gaussian probability distribution around the BP age. Projection of the probability density via the calibration curve onto the calendar date (X) axis is then required. This should produce a probability distribution around the calendar date. The alternative approach assumes that some distribution of calendar dates (prior knowledge) exists, and that the counting statistics reflect the (transformed) probability of such a distribution of occurrence. Its practical application is that probabilities are directly transformed from the Y- to X-axis. Differences between these approaches are evident where the calibration curve exhibits sharp changes in slope (Stuiver & Reimer 1989; Dehling & van der Plicht 1993). In our view, prior knowledge of an age distribution cannot be assumed when calibrating results of a 14C analysis.

Whatever the merits of the various approaches, users of 14C dates are accustomed to the concept of a 14C age and of age ranges with 68% (or 95%) confidence levels. These can be converted legitimately into calibrated dates and their associated 68% (or 95%) confidence ranges on the calendrical scale. This implies projecting the BP ages and their ± 1 (or ± 2 Q) ranges from the Y-axis onto the X-axis. The 5 BP values are thereby transformed to 5 (or more) calendar dates, and the same probability statements concerning the date ranges can be made as for the original BP ages. 320 A. S. Talma and J. C. Vogel

PRETORIA CALIBRATION PROCEDURE

Test Sample: Pta-xxxx + 2 Q AD 1416 14C age: 440 ± 40 BP + 1 a AD 1429 Calibration file: cal2.cal cal-age AD 1443 Stuiver & Pearson (1986), - 1 a AD 1461 northern hemisphere - 2 Q AD 1492 440 +/- 40 BP TEST SAMPLE Pta-xxxx 600 Stuiver & Pearson (1986); N. NeMisph.

sao

300 - -`-`--1400 1500 1600 cal AD

Fig. 2. Test example of calibration on a simple part of the calibration curve. Due to the change of slope, the calendar date ranges are distributed asymmetrically around the calibrated date.

This concept can be illustrated in a simple singular calibration case at a BP age of 440±40 (Fig. 2). At this point, only a single possible calendar date exists, but the 1 and 2 Q ranges are slightly asymmetrical due to the change in slope. In a more complex situation, where a single BP age corresponds to more than one calibrated date, the 1 and 2 Q ranges can be split, and more than one age range is possible (Fig. 3).

We have modified the Groningen calibration program, CAL4 (van der Plicht & Mook 1989), to accomplish the above calibration procedure (Figs. 2, 3). Different calibration data sets can be implemented to accommodate the reservoir effects of atmospheric 14C in the northern and southern hemispheres (Stuiver & Pearson 1986; Pearson & Stuiver 1986; Vogel et al. 1993) and in the ocean (Stuiver, Pearson & Braziunas 1986). Our laboratory routinely provides these calibrated dates to sample submitters.

SETS AND SERIES OF 14C DATES Various 14C laboratories have proposed procedures for dealing with groups of 14C dates, and computer programs have been constructed for their evaluation (see review by Aitchison et al. 1989). The lack of a standard approach is bound to cause confusion in the user community; thus, further discussion is needed.

If a set of 14C dates for a single event or a specific occupation level is available, the weighted mean of the 14C results obviously can be transformed to a calendar date. The problem is that the Simplified Approach to 14C Calibration 321

PRETORIA CALIBRATION PROCEDURE

Test Sample: Pta-xxxx + 2 Q 1482 14C age: 330 ± 20 BP + 1 Q 1502 Calibration file: cal2.cal cal-age 1516 Stuiver & Pearson (1986), - 1 Q AD 1530 northern hemisphere - 2 Q AD 1556 cal-age AD 1584 cal-age AD 1624 -1Q AD1633 -2o AD 1640 330 +/- 20 BP TEST SAMPLE Pta-xxxx

400

cal AD

Fig. 3. Test example of a calibration at a multiple interception point on the calibration curve. Three calibrated ages are possible. Adding ± 1 o to the BP age yields two separate age ranges, while ± 2 o increases the age to a single wide range. true contemporaneity of the samples is seldom certain. If it were, separate analyses would not be justified, and samples would be combined for a more accurate or longer measurement. In fact, the reason for producing more than one analysis on a specific level is that the sample need not be exactly the same age as the event with which it is associated. The most common problem (in the relevant time range) is that the sample, e.g., charcoal, can contain older wood. For example, a set of four dates from a specific level should be enough to reveal such outliers, which can then be rejected. However, it is clear that such a set should not be averaged on any time scale. Waterbolk (1971) discussed this problem extensively and proposed procedures for interpreting both sets and series of dates. This approach should be applied to the individual dates after calibration.

Further, the "old wood" problem implies that 14C dates are most useful for estimating the end of a depositional phase. The duration should be determined by dating the end of the preceding phase as well. A better estimate could be made in this way than, for example, in rejecting the outer quartiles, which would be rejecting half the measurements. Thus, we believe that, in interpreting groups of 14C dates, mathematical procedures are no longer very helpful, but insight and experience become more important. 322 A. S. Talma and J. C. Vogel

CONCLUSIONS

We have attempted to show that the calibration data set should be considered with some degree of uncertainty, because it represents a set of measurements (with inherent analytical uncertainty) of past atmospheric 14C levels. As such, some smoothing is justified, for which we have found the spline curve very useful. The spline calibration enables adjustment of the average curve by a quantified closeness-of-fit parameter to the measured data points, and reduces the number of situations where more than one calendar date is obtained by calibration.

In view of the complexity of transforming probabilities from the BP to the AD/BC axis, and the resulting confusion this creates among the 14C user community, a simpler statement of 68% and 95% confidence intervals along the AD/BC axis seems to be the most practical current application of calibration programs.

ACKNOWLEDGMENTS

We thank the following colleagues with whom we have communicated while formulating our concepts: J. van der Plicht in Groningen, C. Elphinstone in Pretoria and P. Reimer in Seattle. We thank M. Stuiver and J. van der Plicht for supplying us with their programs and calibration data sets.

REFERENCES

Aitchison, T. C., Leese, M., Michczynska, D. J., Mook, Stuiver, M., Pearson, G. W. and Braziunas, T. F. 1986 W. G., Otlet, R. L., Ottoway, B.S., Pazdur, M. F., Radiocarbon age calibration of marine samples back van der Plicht, J., Reimer, P. J., Robinson, S. W., to 9000 cal yr BP. In Stuiver, M. and Kra, R. S., eds., Scott, E. M., Stuiver, M. and Weninger, B. 1989 A Proceedings of the 12th International 14C Conference. comparison of methods used for the calibration of Radiocarbon 28(2B): 980-1021. radiocarbon dates: In Long, A. and Kra, R.S., eds., Stuiver, M. and Reimer, P. J. 1986 A computer program 14C Proceedings of the 13th International Conference. for radiocarbon age calibration. In Stuiver, M. and 14C 31(3): 846-864. Kra, R. S., eds., Proceedings of the 12th International Dehling, H. and van der Plicht, J. 1993 Statistical 14C Conference. Radiocarbon 28(2B): 1022-1030. problems in calibrating radiocarbon dates. In Stuiver, 1989 Histograms obtained from computerized M., Long, A. and Kra, R. S., eds., Calibration 1993. radiocarbon age calibration. In Long, A. and Kra, R. Radiocarbon 35(1): 239-244. S., eds., Proceedings of 12th International 14C Confer- Pearson, G. W. and Stuiver, M. 1986 High-precision ence. Radiocarbon 31(3): 817-823. calibration of the radiocarbon time scale, 500 BC-25- van der Plicht, J. and Mook, W. G. 1989 Calibration of 00 BC. In Stuiver, M. and Kra, R. S., eds., Proceed- radiocarbon dates by computer, In Long, A. and Kra, 14C ings of the 12th International Conference. Radio- R. S., eds., Proceedings of the 13th International 14C carbon 28(2B): 839-862. Conference. Radiocarbon 31(3): 805-816. Reinsch, C. H. 1967 Smoothing by spline functions. Vogel, J. C., Fuls, A., Visser, E. and Becker, B. 1993 Numerische Mathematik 10: 177-183. Pretoria calibration curve for short-lived samples, Stuiver, M. and Pearson, G. W. 1986 High-precision 1930-3350 BC. In Stuiver, M., Long, A. and Kra, R. 14C calibration of the time scale, AD 1950-500 BC. In S., eds., Calibration 1993. Radiocarbon 35(1): 73-85. Stuiver, M. and Kra, R. S., eds., Proceedings of the Waterbolk, H. T. 1971 Working with radiocarbon dates. 14C 12th International Conference. Radiocarbon Prehistory Society Proceedings 37: 15-33. 28(2B): 808-838. [RADIOCARBON, VOL. 35, No. 2, 1993, P. 323-330]

RADIOCARBON DATES FROM AMERICAN SAMOA

JEFFREY T. CLARK

Department of Sociology-Anthropology, North Dakota State University P.O. Box 5075, Fargo, North Dakota 58105 USA

ABSTRACT. Between 1988 and 1991, I directed five archaeological research projects in American Samoa. The goal of of the that research was to reveal changes in the prehistoric settlement system of Samoa, from initial colonization facet archipelago to the time of significant European contact. The chronological placement of key sites was an essential ceramics, and of the research. A secondary goal was to locate sites with ceramic components, particularly sites with Lapita These investigations relate the ceramic assemblages typologically and chronologically to those known for Western Samoa. 14C samples, generated 1614C dates from archaeological contexts. I present here the previously unpublished data from those and briefly summarize their importance for understanding Samoan prehistory.

INTRODUCTION The Samoan Archipelago lies in the central South Pacific Ocean (168°-173°W, 13°-15°S), and with the neighboring Tonga Archipelago, constituted the prehistoric western "gateway" to Polyne- sia. Consequently, both archipelagoes are extremely important for understanding the prehistory of central and eastern Oceania. The large western islands of 'Upolu and Savai'i, along with the small islands of Manono and Apolima, constitute the independent nation of Western Samoa (Fig. 1). The eastern islands of the archipelago compose American Samoa, a USA territory. Tutuila, the largest island of the group, and the small neighboring island of Aunu'u form a western subgroup, whereas the islands of Ta'u, Ofu and Olosega form an eastern subgroup, Manu'a.

In the late 1960s and early 1970s, Green and Davidson (1969, 1974) conducted extensive and highly informative archaeological research in Western Samoa. In the mid-to-late 1970s, Jennings et al. (1976) and Jennings and Holmer (1980) directed additional investigations in Western Samoa. Little research has been done on the island nation since that time. In American Samoa, Kikuchi (1963) compiled a site inventory based on extensive interviews and some survey, Frost (1978) conducted limited test excavations at seven sites, and others carried out a few small surveys (Clark 1980,1981; Ladd & Morris 1970; Kikuchi et al. 1975; Silva & Palama 1975; McCoy 1977). Since 1985, several archaeological projects have been conducted in the territory, substantially expanding our knowledge of the area (e.g., Ayres & Eisler 1987; Best, Leach & Witter 1989; Clark & Herdrich 1988, 1993; Clark 1989, 1990, 1992; Gould, Honor & Reinhardt 1985; Hunt & Kirch 1988; Kirch et al. 1990; Leach & Witter 1987, 1990).

As a result of these projects, numerous ceramic sites have been found in the archipelago. Only Mulifanua on the west coast of 'Upolu Island yielded pottery of the distinctive Lapita type. The 14C Mulifanua site is submerged and was discovered by dredging for a ferry harbor. A date (NZ- 1958) from shell in a coralline crust containing and overlying potsherds provided a calibrated age, based on Pearson and Stuiver (1986), of 3399-2779 cal BP at one sigma (1Q) (Leach & Green 1989:319). All other sites have yielded only Plain Ware and have been interpreted as dating to later than the Lapita site. Based on the early work in Western Samoa, Green (1974a, b) summarized data on the portable artifact sequence for Samoan prehistory. In that proposed sequence, which is now widely accepted, the islands were settled over 3000 years ago by people making Lapita pottery.

323 324 J. T. Clark

Fig, 1. The major islands of the Samoan Archipelago with enlargements of Tutuila and Ta'u Islands

Eventually, Samoan Plain Ware developed from Lapita and went through two stages: an early thin, fine-tempered ware and a later thick, coarse-tempered ware. Pottery-making ended in Samoa between ca. AD 200 and 600, as it did in nearby Tonga. Though no new Lapita sites were discovered in subsequent research, an assumption remains that such sites are present but are yet to be found. Some of the dates presented here, however, alter this picture of Samoan prehistory.

DATA AND DISCUSSION 14C The ages reported here were determined by Beta Analytic, Inc. (Miami, Florida, USA) from samples of charcoal collected from four areas of American Samoa. The samples were air-dried in Samoa; most underwent preliminary cleaning at the Archaeology Lab at North Dakota State University, Fargo. At Beta Analytic, samples were given standard pretreatment. They were examined for rootlets, and then underwent acid-base-acid treatment. This was followed by benzene syntheses and counting, but, in several cases, the small carbon content necessitated extended b13C counting time, was measured for only ten samples. The 14C ages were calibrated according to ATM20.14C (Pearson & Stuiver 1986; Stuiver & Pearson 1986). The calibrated dates are presented at 2 Q, as this provides the most reliable calculation. Dates are corrected for fractionation where b13C measurements are reported below.

Excavations at three valleys on Tutuila generated 15 of the dates below. Most of these (8) came from a single site, AS-21-5 Locality 2, in 'Aoa Valley on the northeast coast (Fig. 1). Additional dates came from limited excavations at Alega Valley on the south coast of west-central Tutuila, and from Leone on the south coast of western Tutuila. A final 14C age is from a site at Faga on the northeast coast of Ta'u Island. 14C Dates from American Samoa 325

The excavations at 'Aoa Valley were conducted in three phases; the last and most extensive of these has not yet been reported (Clark & Herdrich 1988,1993; Clark 1989). Site AS-21-5, Locality 2, is in the eastern portion of the valley, about 220 m from the shoreline, near the taluvial slope that backs the valley. The investigations revealed two ceramic components, both buried under alluvial and colluvial deposits. The early component is pottery-rich, obsidian-rich, and basalt-poor, and is contemporaneous with the Lapita site of Mulifanua, starting at ca. 3000 BP. However, no sherds show any decoration. The late component dates to ca. 500-300 BP, which is ca. 1000 yr later than the conventionally accepted abandonment of ceramics in Samoa. This artifact assemblage is pottery-poor, obsidian-poor, and basalt-rich. Again, no sherds are decorated.

The 14C-dated samples from Leone came from a series of test pits distributed through Leone Valley. Surprisingly, all are comparatively late, after 1000 BP. Each of the dated sites falls into the extended period of ceramic use shown at 'Aoa, yet no ceramics were found in any excavations. A few sherds were found in the valley and surrounding ridges, but not in securely dated contexts (Best, Leach & Witter 1989; Clark 1980, 1981). Both Alega Valley dates are from a terrace constructed at the rear of the small valley to create a surface for manufacturing stone tools. The dates mark the beginning of terrace use, probably ca. 600 BP (Clark 1992). This probably corresponds generally to the onset of exploitation of one or more of the three basalt quarries on the ridge slopes a short distance above the terrace. I recovered no ceramics from the limited test excavations, but found a few surface sherds at the nearest quarry. The single date from Ta'u Island reflects occupation beginning ca. 800-900 BP on a portion of the large coastal flat of Faga (Clark 1990). This area developed through localized coastal progradation during the last millennium, and the site area on the surrounding alluvial and taluvial land is probably older, perhaps significantly. No ceramics were recovered from the single excavation unit at the site.

CONCLUSIONS Five conclusions can be drawn from the archaeological sites and dates discussed here: 1. Site AS-21-5 at 'Aoa is the oldest reported site in American Samoa. As artifacts were recovered from beneath the dated charcoal sample, the date of initial occupation at the site is sometime prior to the age assessment of Beta-48049 (3389 cal BP (3006) 2749 at 2 Q).

2. Lapita occupation in the Samoan archipelago, as identified by the presence of the distinctive dentate-stamped and incised pottery, was quite limited. Given the number of archaeological investigations that have now been carried out in the archipelago, several with the intent of finding Lapita sites, the absence of such sites beyond Mulifanua is striking and probably meaningful. Of the possible explanations for this situation, only two will be mentioned here. First, this easternmost extension of Lapita may be represented by ceramic assemblages in which the distinctive decorations were rarely applied, and were abandoned soon after island settlement in favor of a derived Samoan Plain Ware. Decorated sherds at Lapita sites elsewhere typically compose only a small percentage of the total assemblage, often under 10%. Therefore, a complete abandonment of decorations would not be a decidedly dramatic change. Some time ago, Green (1974b) proposed such a shift from decorated Lapita to Samoan Plain Ware, but the data discussed here suggest that the shift was sooner than previously suspected. 326 J. T. Clark

Second, it may be that the Mulifanua site was not occupied by "Lapita people," and the small percentage of decorated Lapita ceramics at the site represent trade ware from outside the archipelago (see Terrel11989). This would suggest a contemporaneous non-Lapita occupation of Samoa, perhaps reflecting a widespread Plain Ware tradition and associated culture(s). Plain Ware sites have been reported from throughout the central Pacific, although the precise relation of Plain Ware to the Lapita tradition has not been firmly established.

3. The late component at 'Aoa suggests that pottery was used in Samoa for ca. 1000 yr after the supposed abandonment of ceramics. Other sites, e.g., Alega, with sherds at an associated late quarry, provide some support for this suggestion, but none so clearly as 'Aoa. This late component is represented by Layers II-V (rapid sediment deposition due to landscape instability), where five reliable dates (a sixth date is probably contaminated) indicate the 15th century AD.

4. The absence of pottery from other sites contemporary with the late ceramic component, e.g., at Leone and Faga, suggests that pottery was not abandoned uniformly and completely throughout the archipelago, but differentially, with some sites or areas manufacturing ceramics substantially longer than others, and perhaps some sites securing small quantities of ceramics through trade. The fact that the Leone area was a center of basalt production for trade suggests, however weakly, the possibility of regional specialization on Tutuila. S13C 5. The ten measurements listed below range between -24.4%o and -28.8%0, and average b13C -27.32%o. When 18 measurements are added, all from charcoal samples from other studies, the range shifts to -24.4%o to -29.68%o and the mean shifts slightly to -27.45%o (Ayres, personal communication; Kirch, Hunt & Tyler 1989; Leach & Witter 1990; DSIR Institute of Nuclear Sciences, New Zealand, correspondence, 1992). While it is possible that some dated charcoal samples may be from plants with photosynthetic pathways that would yield a very different 13C value, none of the dated charcoal samples from Samoa with b13C measured showed widely divergent values. Thus, it seems reasonable to assume a S13C of -27%o for adjusting ages on charcoal which were not S13C measured, or to estimate an adjustment of -30 yr for such ages. Such a correction should improve the age assessments of most, if not all, of the samples lacking b13C values. Regardless of the precise adjustment 13C, 14C for the ages shift comparatively little and the broad picture of Samoan prehistory is unaltered.

14C The 16 dates presented here demonstrate that previous interpretations of Samoan prehistory may be flawed due to limited data. Additional research is needed in all parts of Samoa, and a re- evaluation of previously reported chronological data is needed to clarify critical issues in the prehistory of central Oceania.

ACKNOWLEDGMENTS

The archaeological projects that produced the dates reported here were funded by the National Science Foundation, Grant No. 9111566; the Historic Preservation Office, Department of Parks and Recreation, American Samoa Government, Pago Pago; G. M. Meredith and Associates, Apia, Western Samoa; McConnell Dowel! Constructors, Ltd., Sydney; and North Dakota State University, Fargo. I extend my appreciation to all those people, far too numerous to list, who assisted me in these projects with funding, administration, fieldwork and lab analyses. Special thanks, however, are extended to Michael Michlovic, Erik Pearthree, David Herdrich, Scott Dundelberger and Todd Clark for their assistance in collecting and processing these samples. 14C Dates from American Samoa 327

ARCHAEOLOGICAL SAMPLES 'Aoa Valley, Tutuila Island

Beta-28210. 'Aoa, Tutuila 330 ± 40 Charcoal, Site AS-21-5, Locality 2, Unit 4, stream bank, Layer II, Feature 1, dark charcoal- stained basin, 113-123 cm below surface, (field sample 88RC1). Cal AD 1453 (1519, 1587, 1623) 1651 at 2 a; 497 cal BP (431, 363, 327) 299 at 2 Q. Comment: This layer is part of the late ceramic component.

Beta-28211. 'Aoa, Tutuila 350 ± 50 Charcoal pieces, Site AS-21-5, Locality 2, Unit 4, stream bank, Layer V, 150-160 cm below surface, from a small band in upper portion of layer, charcoal pieces abundant and large sample submitted (field sample 88RC2). Cal AD 1440 (1506) 1650 at 2 Q; 510 cal BP (444) 300 at 2 Q. Comment: This layer is part of the late ceramic component.

Beta-28212. 'Aoa, Tutuila 170 ± 40 Charcoal pieces, Site AS-21-5, Locality 2, Unit 4, stream bank, Layer V, Feature 9, 160-165 cm below surface, collected from soil in upper area of fireplace (field sample 88RC3). Cal AD 1650 (1676, 1747, 1799, 1942, 1955) 1955 at 2 a; 300 cal BP (274, 203, 151, 8, 0) 0 at 2 Q. Comment: An area of apparent disturbance, possibly due to land crabs, is within 20 cm of the feature. The disturbance conceivably extended to the fireplace area, though not recognized during excavation, which could account for the unusually young age of the sample. This date should be rejected due to probable contamination. This layer is part of the late ceramic component.

400 ± 80 b13C Beta-48047. 'Aoa, Tutuila = -2.70%o Charcoal, Site AS-21-5, Locality 2, stream bank profile, Layer V, 140-156 cm below surface, collected from small area of concentration, ceramic layer (field sample A-8). Cal AD 1400 (1460) 1650 at 2 Q; 550 cal BP (490) 300 at 2 Q. Comment: This layer is part of the late ceramic component.

470 ± 60 813C Beta-48048. 'Aoa, Tutuila = -28.0%o Charcoal pieces, Site AS-21-5, Locality 2, Unit 5, Layer V, 84-94 cm below surface, scattered through 10-cm-thick level, ceramic layer (field sample A-9). Cal AD 1321 (1434) 1611 at 2 Q; 629 cal BP (516) 339 at 2 Q. Comment: This layer is part of the late ceramic component.

2890 ± 140 b13C Beta-48049. 'Aoa, Tutuila = -28.2%o Charcoal, Site AS-21-5, Locality 2, Unit 7, Layer VII, ca. 170 cm below surface, scattered pieces, small sample (0.22 g) given extended counting time (four times normal amount), ceramic 328 J. T. Clark

layer, deepest sample collected from site (field sample A-10). Cal BC 1440 (1057) 800 at 2 o; 3389 cal BP (3006) 2749 at 2 a. Comment: This layer represents the early ceramic component. Artifacts were recovered in comparatively small numbers from below this dated sample.

510 t 70 S13C Beta-48910. 'Aoa, Tutuila = -26.9%o Charcoal pieces, Site AS-21-5, Locality 2, Stream Bank Profile, Layer II, scattered through layer, small sample (0.46 g) given extended counting time (four times normal amount) (field sample A-7). Cal AD 1290 (1418) 1443 at 2 Q; 660 cal BP (532) 470 at 2 Q. Comment: This layer is part of the late ceramic component.

2460 ± 110 S13C Beta-48911. 'Aoa, Tutuila = -24.4%o Charcoal pieces, Site AS-21-5, Locality 2, Unit 8, Layer VII, 128-148 cm below surface, scattered in small area of unit, small sample (0.32 g) given extended counting time (four times normal amount) (field sample A-12). Cal BC 830 (755, 698, 537) 370 at 2 Q; 2779 cal BP (2704, 2647, 2486) 2319 at 2 Q. Comment: This layer represents the early ceramic component. The date agrees with the older and more deeply buried sample, Beta-48049, above.

Leone Valley, Tutuila Island

520 ± 60 S13C Beta-48051. Leone, Tutuila = -26.5%o Charcoal, Site AS-34-38, Test Pit 5, Layer IX, 200-220 cm below surface, scattered pieces collected throughout 20-cm-thick layer (field sample L-3). Cal AD 1290 (1414) 1460 at 2 Q; 660 cal BP (536) 490 at 2 0.

Comment: This is a lower valley site and the excavation unit is located 37 m from the coastline. Beta-48913, below, from 100-110 cm, is younger than this sample.

780 ± 70 Beta-48052. Leone, Tutuila W 3C = -28.1%o Charcoal pieces, Site AS-34-44, Test Pit 9, Layer I, 90-100 cm below surface, scattered through 10-cm-thick level (field sample L-4). Cal AD 1047 (1259) 1290 at 2 Q; 903 cal BP (691) 660 at 2 Q.

Comment: This is an upper valley site; the excavation unit is ca. 661 m from the coastline.

930 ± 80 S13C Beta-48912. Leone, Tutuila0 = -28.8%o Charcoal, Site AS-34-45, Test Pit 6, Layer V, level 9, 85-90 cm below surface, small concentration of charcoal, small sample (0.61 g) given extended counting time (four times normal amount) (field sample L-1). Cal AD 970 (1043, 1105, 1112, 1150) 1210 at 2 Q; 980 cal BP (907, 845, 838, 800) 690 at 2 Q. I4C Dates from American Samoa 329

Comment: This is a middle valley site; the excavation unit is ca. 302 m from the coastline.

340 ± 80 813C Beta-48913. Leone, Tutuila = -26.5%o Charcoal, Site AS-34-38, Test Pit 5, Layer VII, level 11,100-110 cm below surface, scattered pieces (field sample L-2). Cal AD 1420 (1514, 1600, 1616) 1955 at 2 Q; 530 cal BP (436, 350, 334) 0at2a Comment: This age determination, which is from a unit in the lower valley some 37 m from the coastline, agrees with the deeper sample, Beta-48051, above.

280 ± 60 Beta-48915. Leone, Tutuila W3C = -28.8%o Charcoal, Site AS-34-40, Test Pit 3, Layer II, level 4, 30-40 cm below surface, small scatter of charcoal, small sample (0.70 g) given extended counting time (four times normal amount) (field sample L-6). Cal AD 1460 (1642) 1660 at 2 Q; 490 cal BP (308) 0 at 2 a Comment: This site is on a small raised area in the midst of a mangrove swamp of the inner bay at Leone.

Alega Valley, Tutuila Island Beta-38438. Alega, Tutuila 1040 ± 230 Charcoal, Site AS-23-21, Unit 1, base Layer I/top Layer II, small pieces of scattered charcoal, small sample (0.16 g) given extended counting time (two times normal amount) (field sample RC Al-1). Cal AD 560 (999) 1395 at 2 Q; 1390 cal BP (951) 555 at 2 a Comment: The sample was collected at the interface of Layers I and II, at ca. 30-40 cm below surface. Layer I represents terrace fill over subsoil of Layer II; thus, the date marks the start of terrace use. The unit is ca. 153 m from the coastline.

Beta-38753. Alega, Tutuila 590 ± 70

Charcoal, Site AS-23-21, Units 2 & 4, top Layer II/base Layer I, scattered pieces collected from band ca. 6 cm thick at layer interface, ca. 30-40 cm below surface (field sample RC Al-2). Cal AD 1270 (1322, 1340, 1392) 1440 at 2 Q; 680 cal BP (628, 610, 558) 510 at 2 Q. Comment: This date also marks initial terrace use. The unit is ca. 152 m from the coastline.

Faga Coastal Flat, Ta'u Island Beta-38752. Faga, Ta'u 910 ± 80 Charcoal, Site AS-11-1, Unit 1, Layer VII, 130-136 cm below surface, from a layer of sand underlying a house floor (field sample RC T-1). Cal AD 980 (1058, 1078, 1125, 1136, 1156) 1270 at 2 Q; 970 cal BP (892, 872, 825, 814, 794) 680 at 2 Q. Comment: The unit is ca. 50 m from the coastline. This date probably reflects the approximate time of the earliest use of this portion of the coastal flat, but slightly higher ground immediately surrounding this low area may have been occupied earlier. 330 J. T. Clark

REFERENCES

Ayres, W. S. and Eisler, D. (ms.) 1987 Archaeological survey Hunt, T. L and Kirch, P. V.1988 An archaeological survey of in western Tutuila: A report on archaeological site survey the Manu'a Islands, American Samoa. Jaunal of the and excavations (85-2). Report on file, Historic Preserva- Polynesian Society 97(2):153-183. tion Office, American Samoa Government, Pago Pago. Jennings, J. D. and Holmer, R N. 1980 Archaeological Best, S., Leach, H. M. and Witter, D. C. 1989 Report on the excavations in Western Samoa. Pacific Anthropological second phase of fieldwork at the Tataga-matau site, Records 32, B.P. Bishop Museum, Honolulu. American Samoa, July-August 1988. Working Papers in Jennings, J. D, Holmer, R N, Janetski, J. and Smith, H. L Anthropology, Archaeology, Linguistics, Maori Studies 83. 1976 Excavations on Upolu, Western Samoa. Pacific Department of Anthropology, University of Auckland, Anthropological Records 25, B.P. Bishop Museum, New Zealand. Honolulu. Clark, J. T. (ms.) 1980 Historic preservation in American Kikuchi, W. K (ma.) 1963 Archaeological Surface Ruins in Samoa: Program evaluation and archaeological site American Samoa. Unpublished M.A. thesis, Department of inventory. Report on file, Historic Preservation Office Anthropology, University of Hawaii, Honolulu. American Samoa Govemment, Pago Pago. Kikuchi, W. K, Pajama, S. L and Silva, T. E. (ms.) 1975 1981 Archaeology in American Samoa. In Atlas of Archaeological reconnaissance survey proposed Ta'u American Samoa. U.S. Office of Coastal Zone Man- Harbor at Fusi and quarry site between Fusi and Fagamoto agement, the American Samoa Government, and the Ta'u Island, Manu'a Group, American Samoa. Report on Department of Geography, University of Hawaii, Hono- file, Department of Anthropology, B.P. Bishop Museum, lulu. Honolulu. _(ms.) 1989 The eastern Tutuila archaeological project: Kirch, P. V., Hunt, T. L, Nagaoka, L and Tyler, J. 1990 An 1988 final report. Report on file, Historic Preservation ancestral Polynesian occupation site at To'aga, Ofu Island, Office, American Samoa Government, Pago Pago. American Samoa Archaeology in Oceania 25(1):1-15. _(ms.) 1990 The Ta'u Road archaeological project. Report Kirch, P. V, Hunt, T. L and Tyler, J. 1989 A radiocarbon on file, Historic Preservation Office, American Samoa sequence from the To'aga site, Ofu Island, American Government, Pago Pago. Samoa. Radiocarbon 31(1): 7-13. _(ins.) 1992 The archaeology of Alega Valley: Residence Iadd, E. J. and Moms, D. K (ma.) 1970 Archaeological and and small industry in prehistoric Samoa Report on file, ecological survey of 'Olovalu Crater, Island of Tutuila, Historic Preservation Office, American Samoa Govern- American Samoa Report on file, National Park Service, ment, Pago Pago. Washington, D.C. Clark, J. T. and Herdrich, D. J. (ins.) 1988 The eastern Tutuila Leach, H. M. and Green, R G 1989 New information for the Archaeological project: 1986 final report. Report on file, Feny Berth site, Mulifanua, Western Samoa. Journal of Historic Preservation Office, American Samoa Govern- the Polynesian Society 98: 319-329. ment, Pago Pago. Leach, H. M. and Witter, D. C. 1987 Tataga-matau `redisco- 1993 Prehistoric settlement system in eastern Tutuila, vered'. New Zealand Journal of Archaeology 9:33-54. American Samoa. Journal of the Polynesian Society 102, 1990 Further investigations at the Tataga-matau site, in press. American Samoa. New Zealand Journal of Ardnaeology Frost, J. (ms.) 1978 Archaeological Investigations on Tutuila 12: 51-83. Island, American Samoa. PhD dissertation, Department of McCoy, P. (ms.) 1977 Cultural reconnaissance survey 'Au'asi Anthropology, University of Oregon, Eugene. harbor project 'Au'asi, Tutuila Island, American Samoa. Gould, R A, Honor, K E. and Reinhardt, K J. (ms.) 1985 Report on file, Department of Anthropology, B.P. Bishop Final project report for Tulauta and Fagatele Bay prehistor- Museum, Honolulu. ic villages and Leone Bay petroglyphs. Report on file, Pearson, G. W. and Stuiver, M. 1986 High-precision calibration Historic Preservation Office, American Samoa Govern- of the radiocarbon time scale, 500-2500 BC. In Stuiver, M. ment, Pago Pago. and Kra, R. S., eds., Proceedings of the 12th International Green, R C. 1974a Excavations of the prehistoric occupations 14C Conference. Radiocarbon 28(2B): 839-862. of SU-Sa-3. In Green, R C. and Davidson, J. M., eds, Silva, T. E. and Pajama, S. L (ms.) 1975 Archaeological Archaeology in Western Samoa, Vol, 1. Bulletin of the reconnaissance survey proposed shoreline and highway Auckland Institute and Museum 7:108-154. improvements, Tutuila Island, and Aunu'u Boat Harbor, 1974b A review of portable artifacts from Western Aunu'u Island, American Samoa. Report on file, U.S. Samoa. In Green, R C. and Davidson, J. M., eds, Archae- Army Corps of Engineers, Honolulu. ology in Western Samoa, Vol.11. Bulletin of the Auckland Stuiver, M. and Pearson, G. W. 1986 High-precision calibration Institute and Museum 7: 245-275. of the radiocarbon time scale, AD 1950-500 BC. In Stuiver, Green, R C. and Davidson, J. M, eds. 1969 Archaeology in M. and Kra, R. S., eds., Proceedings of the 12th Interna- Western Samoa, Vol. I. Bulletin of the Auckland Institute tional 14C Conference. Radiocarbon 28(2B): 805-838. and Museum 6. Terrell, J. 1989 Commentary: What Lapita is and what Lapita 1974 Archaeology in Western Samoa, Vol. ll. Bulletin of isn't. Antiquity 63: 623-626. the Auckland Institute and Museum 7. [RADIOCARBON, VOL. 35, No. 2, 1992, P. 331-333]

NOTES AND COMMENTS

14C DATING OF LASER-OXIDIZED ORGANICS

A. L. WATCHMAN', R. A. LESSARD2 A. J. T. JULL3 L. J. TOOLIN3 and WESTON BLAKE, JR.4

ABSTRACT. We used a continuous krypton ion laser to rapidly oxidize milligram-sized fragments of coniferous driftwood of known ages, and dated the resulting carbon dioxide by accelerator mass spectrometry (AMS). AMS 14C ages of non-pretreated young wood from different parts of two logs were within 10% of the ages of conventionally determined alkaline insoluble fractions. The age of the oldest whole wood measured after laser oxidation was within the error ranges of conventional values.

INTRODUCTION Organic matter encapsulated in silica skins (Watchman 1990, 1992), oxalate crusts (Watchman 1991), rock varnish (Dorn et at. 1992) and other media (Loy et al. 1990) is generally scraped, dissolved or selectively oxidized (Russ et al. 1990; Russ, Hyman & Rowe 1992) from the underlying rock, and chemically processed into graphite for AMS 14C dating (Jell et a1.1986). Loss of microstratigraphic context and contamination are two significant problems that we hope to overcome in dating laminae in rock-surface accretions by oxidizing organic matter in cross-sections with a focused, low-energy, continuous laser.

METHODOLOGY

We mounted each sample on wire supports in a vacuum-tight, micro-combustion chamber (3 cm3) fitted with glass windows. Atmospheric CO2 was evacuated from the chamber under high vacuum and pure oxygen was injected at 5 k Pa. Using a diffraction-reduced convergent lens (f=153 mm, 0.003 mm spot diameter), we focused onto the sample a continuous Kr-ion laser (Coherent Innova 2000, 2.5 W light power, 56.4 A tube current, 413 nm). CO2 formed by laser oxidization was collected in a liquid nitrogen "trap" and S13C was measured by mass spectrometry prior to making graphite (Dull et al. 1986).

We prepared an oxalate wafer from a precipitate of NBS oxalic acid (OX 1) and calcium chloride at room temperature because we required an oxalate standard for our future research goal of dating finely laminated whewellite (CaC2O4.H2O) deposits on rock surfaces. We collected samples of Larix sp. (WB-182-66) and Picea sp. (WB-100-66) on Nordaustlandet, Svalbard (80°N 20°E) from driftwood logs embedded in raised shingle-beach deposits; we also collected from glacial till Larix sp. (WB-18-66) (Blake 1980,1986; Lowdon & Blake 1980).

RESULTS 14C (14C Our preliminary, uncalibrated AMS age results yr before AD 1950) from the Arizona AMS Lab (AA) are for laser-oxidized untreated whole wood (Table 1). The Geological Survey of Canada

'Centre for Australian Regolith Studies, Australian National University, G.P.O. Box 4, Canberra, ACT 2601, Australia Present address: 192 St.-Omer, L6vis, Quebec, Canada G6V 5C7 Centre d'Optique, Photonique et Laser, Pavilion A.-Vachon, University Laval, Quebec, Canada G1K 7P4 3NSF-Arizona Accelerator Facility for Radioisotope Analysis, Physics Department, The University of Arizona, Tucson, Arizona 85721 USA 4Terrain Sciences Division, Geological Survey of Canada, Ottawa, Canada KIA 0E8

331 332 A. L. Watchman et al.

14C TABLE 1. Comparison between laser-AMS and conventional carbon isotopic and uncorrected age data for oxalate and coniferous driftwood

alk. insoluble untreated S13C%0 GSC-nos. Sample WT (mg) 813C%o (2 a) wood (1 v)

Oxalate 10.0 -3500 WB18-66 12.6 t 60 85 -2441 WB100-66 3.6 60 60 -2441 WB100-66 5.8 60 75 -1728 WB182-66 9.8 140 135

(GSC)14C measurements are for alkaline-insoluble fractions (cellulose and lignin) determined by 813C conventional methods. We found that, by using the laser-AMS method, both the (_ [(13C/12C) sample/(13C/12C) standard] -1) and the fraction of modern 14C from a standard oxalate wafer were about 4.7% lower (equivalent to ca. 30014C yr) than the accepted values for NBS OX I (-19.0%o and 1.046%, respectively). We suspect the difference is caused by non-linear isotope exchange during formation of mono- and bi-hydrated oxalate salts rather than laser-induced processes, but further tests are needed to rule out laser fractionation. The 813C values for whole wood are higher in our laser-generated CO2 than for the alkaline-insoluble fractions because different components were measured, and because slight isotopic fractionation may occur during incomplete laser oxidation (Powell & Kyser 1991). The laser-oxidative method combined with 14C AMS gives an age for the oldest wood within error ranges identical to the conventionally determined value (Table 1). Two laser-AMS 14C ages for specimen WB100-66 only differ by about 1%, and indicate acceptable reproducibility of results. However, the ages for the young wood samples determined by laser-AMS are beyond the error ranges of their previously determined values, possibly because the two laser-AMS samples (WB-18-66, WB-100-66) were not sub-samples of fragments used in conventional dating. More important, the whole wood specimens are not homogeneous because they contain soluble fractions not in isotopic equilibrium with cellulose.

CONCLUSIONS Our preliminary results demonstrate convincingly the feasibility of focusing a low-energy continuous laser to oxidize rapidly small samples of organic matter in geological, geomorphological and archaeological contexts for AMS 14C dating. We are now collecting more data from our driftwood samples and developing the laser-oxidative method for routine dating of molluscan growth bands, subvarnish organic matter, siliceous coatings containing algal remains and oxalate-rich laminations.

ACKNOWLEDGMENTS This work was supported by the Australian Institute of Aboriginal and Torres Strait Islander Studies, the Natural Sciences and Engineering Research Council of Canada and the U.S. National Science Foundation. D. Lessard, M. Denis and L. Turgeon provided administrative and technical support. 14C Dating of Laser-Oxidized Organics 333

REFERENCES

Blake, W., Jr. 1980 Radiocarbon dating of driftwood: gel, J., Southon, J. and Cosgrove, R. 1990 Accel- Inter-laboratory checks on samples from Nordaustlan- erator radiocarbon dating of human blood proteins in det, Svalbard. Geological Survey of Canada Paper pigments from Late Pleistocene art sites in Australia. 80-1C: 149-151. Antiquity 64: 110-116. 1986 Geological Survey of Canada radiocarbon Powell, M. D. and Kyser, T. K. 1991 Analysis of b13C dates XXVI. Geological Survey of Canada Paper and 5180 in calcite, dolomite, rhodochrosite and 86-7: 1-60. siderite using a laser extraction system. Chemical Dom, R. I. 1983 Cation-ratio dating: A new rock Geology 94: 55-66. varnish age determination technique. Quaternary Russ, J., Hyman, M. and Rowe, M. 1992 Direct radio- Research 20: 49-73. carbon dating of rock art, In Long, A. and Kra, R. S., Dom, R. I., Clarkson, P. B., Nobbs, M. F., Loendorf, L. eds., Proceedings of the 14th International 14C L. and Whitley, D. S. 1992 New approach to the Conference. Radiocarbon 34(3): 867-872. radiocarbon dating of rock varnish, with examples Russ, J., Hyman, M., Shafer, H. J. and Rowe M. W. from drylands. AnnalsAssociation American Geogra- 1990 Radiocarbon dating of prehistoric rock paintings phers 82(1): 136-151. by selective oxidation of organic matter. Nature 348: Jull, A. J. T., Donahue, D. J., Hatheway, A. L., Linick, 710-711. T. W. and Toolin, L. J. 1986 Production of graphite Watchman, A. 1990 What are silica skins and how are targets by deposition from CO/H2 for precision they important in rock art conservation? Australian 14C accelerator measurements, In Long, A. and Kra, Aboriginal Studies 90(1): 21-29. 14C R. S., eds., Proceedings of the 12th International _1991 Age and composition of oxalate-rich crusts Conference. Radiocarbon 28(2A): 191-197. in the Northern Territory, Australia. Studies in Lowdon, J. A and Blake, W. Jr. 1980 Geological Survey Conservation 36: 24-32. of Canada radiocarbon dates XX. Geological Survey _1992 Composition, formation and age of some of Canada Paper 80-7: 1-28. Australian silica skins. Australian Aboriginal Studies Loy, T. H., Jones, R., Nelson, D. E., Meehan, B., Vo- 92(1): 61-66.

[RADIOCARBON, VOL. 35, No. 2, 1993, P. 335-338]

AN ASSESSMENT OF THE RADIOCARBON DATING OF THE DEAD SEA SCROLLS

G. A. RODLEY

Pharmacy Department, University of Sydney, Sydney NSW 2006 Australia

ABSTRACT. I suggest, on the basis of a statistical analysis, that recently determined "conventional radiocarbon ages" of Dead Sea Scroll documents are offset systematically by about +40 yr, leading to a similar overestimate of the ages of these documents. Much closer agreement with paleographic and specific dates is obtained when a correction of this magnitude '4C" 14C is made to the "conventional values. This indicates that dates may convey more precise information about the ages of these documents than initially recognized.

INTRODUCTION 14C Bonani et al. (1991, 1992) recently published details of a dating study of Dead Sea Scroll 14C/12C 13C1'2C documents. This was based on the quasi-simultaneous determination of and ratios relative to standard NBS (PDB) values (Stuiver & Pearson 1986). The authors made corrections 14C for natural fractionation (Stuiver & Polach 1977) and presented the results as conventional ages. Each value corresponded to the weighted mean date of several independent measurements of differently prepared samples of each document. Error ranges were quoted as either the statistical error (one standard deviation (1 a)) or the variance, whichever was the higher. 14C Conventional 14C ages were converted to "calibrated 1 Q age ranges" using the high-precision calibration curve of Stuiver and Pearson (1986) and Wolfli (1987). These values were compared with specific dates and paleographically determined age ranges (Bonani et al. 1991).

In their initial paper, Bonani et al. (1991: 29, 31) stated that: 1) agreement with the four "date- bearing" scrolls indicates "no methodical offset, either in the radiocarbon method or in the calibration curve ..."; and 2) "our research put to test both the radiocarbon method and paleography: seemingly, both disciplines have fared well." However, in a subsequent paper, Bonani " et al. (1992: 847) commented that ... a slight systematic shift between the calibrated radiocarbon ages and the estimates of the paleographers might be inferred from the data. The calibrated radiocarbon ages are, on average, 35 years older. The statistical significance of this offset remains to be proven." This offset may be appreciated by reference to the plot of the data in 14C Bonani et al. (1991: Fig. 2), which shows that most of the estimates are older than paleographic dates.

ANALYSIS 14C To evaluate possible systematic displacement of the results, I performed a statistical analysis of these data (excluding Sample 2, which involves a major discrepancy of unknown origin (Bonani et al. 1991)). First, I determined sets of specific age values from Table 1: 14C 14C 1. I obtained specific calibrated ages (Table 1) from the set of specific conventional age values given in Bonani et al. (1991, Table 1), derived using Table 3 of Stuiver and Pearson 14C (1986). In the case of Sample 1, I used the higher of the two possible values and the younger value for Sample 5. 2. I obtained a set of paleographic/specified dates by taking the midpoints of the ranges given in the last column of Table 1 (Bonani et al. 1991), together with specified dates for documents 1, 12, 13 and 14. I list these values in Table 1 under the heading "Paleographic or specified (PIS) age".

335 336 G. A. Rodley

14C Linear least-squares regression analysis of the two sets of values yielded the resulting age = 41.6 + 0.992(P/S), at a R2(adj) value of 97.9%. A T-test and a Wilcoxon-test of (14C - P/S) values gave similar "offset" values of 41.8 and 44.5 yr (with P values of 0.001 and 0.006). These tests indicate strongly a systematic difference of ca. 40 yr between the 14C measurements and paleographic estimates. Using the first two estimates, I chose to decrease the conventional 14C ages reported in Table 1 of Bonani et al. (1991) by 42 yr. A new set of "adjusted 14C ages" was then determined using Table 3 of Stuiver and Pearson (1986). An exception was Sample 14, where the value was determined from the higher-precision calibration curve (Stuiver & Becker 1986) by assuming that the experimental value actually corresponded to the dip in the curve at about AD 730, rather than the alternative AD 770 region. The adjusted value of 1289 - 42 =1247 BP would have to be increased only by a few years to intercept the AD 730 region; this is reasonable in terms of the estimated error in the experimental value. Table 1 gives the new set of calibrated ages, under 14C "Adjusted age". While specific age values, rather than age ranges, were used for the statistical 14C analysis, an adjusted set of age ranges may be determined from the ages (yr BP) of Bonani et al. (1991) by subtracting the 42 yr from each entry and using the error values listed.

DISCUSSION

The statistical analysis, coupled with the offset of specific calibrated 14C ages from the paleographic or specified ages strongly suggest a systematic displacement of the 14C values. It is possible that the discrepancy arises from varying ages of the materials on which the documents were written. However, it is unlikely that the materials would be consistently older (by Ca. 40 yr) than the times of writing.

What is more likely is that, with the particular procedures involved in obtaining "conventional" 14C values (Stuiver & Polach 1977), a systematic offset of Ca. 40 yr. resulted. Bonani et al. (1991) followed the recommended procedure of reporting conventional 14C ages without adjustment (Stuiver & Polach 1977), and correctly noted the general agreement of the derived calibrated age ranges with the paleographic/specified ages. It is possible that the offset could be due to either 1) small errors in age, arising in the use of the equations of Stuiver and Polach (1977), when the 13C/12C isotope ratio is not adequately determined (although an instrumental error in this determination would result in scatter of the points) or 2) a small calculation error in the isotope correction.

If it is accepted that an offset exists, the reported 14C ages may be decreased by a fixed amount; I chose 42 yr based on the statistical analysis. The new set of adjusted 14C ages (Table 1) shows good overall agreement with the paleographic and specified ages (especially for Samples 1,12,13 and 14 that have specified dates). This indicates that the 14C dating study may be more significant than initially indicated (Bonani et al. 1991). I suggest that the paleographic estimate for Sample 8, which shows a marked discrepancy with the corresponding "adjusted" 14C value, may require revision, if the possibility that the document studied is a later copy of an earlier original can be eliminated. The result for Sample 11 may be a good indication of the age of a document for which the paleographic estimate covers a relatively wide age range. The residual difference of 44 yr for Sample 7 also may be significant.

Samples 12 and 13 provide evidence for the merit of the analysis presented here. The values reported by Bonani et al. (1991) of 1917 BP (Sample 12) and 1892 BP (Sample 13) fall in a shallow region of the calibration curve that shows a significant difference of 20 yr in the corres- Radiocarbon Dating of the Dead Sea Scrolls 337

TABLE 1. Specific Age Values for Scroll Documents

Scroll no. Description Cal 14C age* age* * 14C age*

1 Daliyeh 390(39) BC BC(S) BC

2 Testament of Qahat (not included in analysis - refer to text)

3 Pentateuchal paraphrase 186(75) BC BC BC

4 Book of Isaiah 176(65) BC BC BC

5 Testament of Levi 173(73) BC BC BC

6 Book of Samuel 114(28) BC BC BC

7 Masada - Joshua 109(94) BC BC BC 8 Masada - Sectarian AD 22(-37) BC 72(-87)

9 Temple Scroll 43(43) BC 0 9(-9)

10 Genesis Apocryphon 24(24) BC 0 22(-22)

11 Thanksgiving Scroll AD 16(-6) 10 68(-58)

12 Wadi Seyal AD 79(51) 130(S) 121(9)

13 Murabba'at AD 99(35) 134(5) 135(-1)

14 Kh. Mird AD 685(59) 744(5) 735(9)t * Values in brackets are differences with respect to the paleographic or specified ages **P/S age = paleographic or specified age tValue based on higher conventional 14C age than determined in adjustment procedure (see text) ponding cal ages (AD 79 and AD 99). By contrast, my adjustment of 42 yr brings the new values of 1875 and 1850 BP into a steeper area of the curve. The effect is more apparent with the higher- precision curves and data of Stuiver and Becker (1986) than for the values given in Table 1 (derived from Stuiver and Pearson (1986)). The values obtained are AD 126 and 131, which closely 14C correspond to the specified values of AD 130-31 and AD 134. The conventional ages must be associated with a steep part of the calibration curve for such different values to give similar calibrated ages.

CONCLUSION

I based this analysis on specific age values and have not taken error margins into account. However, these results indicate the 14C dating of the Dead Sea Scroll documents may be more informative than initially indicated (Bonani et al. 1991). I also suggest that a general accuracy of about ± 25 yr has been achieved, making the method especially useful for documents whose ages are otherwise in doubt.

ACKNOWLEDGMENT

I wish to thank Dr. Igor Gonda for helpful comments. 338 G. A. Rodley

REFERENCES

Bonani, G., Broshi, M., Carmi, I., Ivy, S., Strugnell, J. Stuiver, M. and Pearson, G. W. 1986 High-precision and Woelfli, W. 1991 Radiocarbon dating of the calibration of the radiocarbon time scale, AD 1950 - Dead Sea Scrolls. 'tigot 20: 27-32. 500 BC. In Stuiver, M. and Kra, R. S., eds., Proceed- Bonani, G., Ivy, S., Wolfli, W., Broshi, M., Carmi, I. ings of the 12th International 14C Conference. Radio- and Strugnell, J. 1992 Radiocarbon dating of fourteen carbon 28(2B): 805-838. Dead Sea Scrolls In Long, A. and Kra, R. S., eds., Stuiver, M. and Polach, H. A. 1977 Discussion: Report- 14C Proceedings of the 14th International Conference. ing of 14C data. Radiocarbon 19(3): 355-363. Radiocarbon 34(3): 843-849. Wolfli, W. 1987 Advances in accelerator mass spec- Stuiver, M. and Becker, B. 1986 High-precision decadal trometry. In Gove, H. E., Litherland, A. E. and calibration of the radiocarbon time scale, AD 1950 - Elmore, D., eds., Proceedings of the 4th International 2500 BC. In Stuiver, M. and Kra, R. S., eds., Pro- Symposium on Accelerator Mass Spectrometry, ceedings 14C of the 12th International Conference. Nuclear Instruments and Methods in Physics Re- Radiocarbon 28(2B): 863-910. search B29: 1-13. [RADIOCARBON, VOL. 35, No. 2, 1993, P. 339-342]

BOOK REVIEWS The Quaternary of China. Edited by Zhang Zonghu, Shao Shixiong, Tong Ghobang and Cao Jiadong. Institute of Hydrogeology and Engineering Geology, Zhengding, China. Beijing 1991 China Ocean Press, 575 pages. Explanatory Notes of the Quaternary Geologic Map of the People's Republic of China and Adjacent Sea Area. Edited by Zhang Zonghu, Shao Shixiong, Zhou Mulin and Fan Yi. Beijing 1990 China Cartographic Publishing House (9 maps and 78-page manual).

These book(s) and maps were produced in association with the 1991 INQUA XIII Congress in China. The primary work, The Quaternary of China, includes 16 chapters by various authors. The companion work, Explanatory Notes of the Quaternary Geologic Map of the People's Republic of China and Adjacent Sea Area, consists of nine 1:2,500,000 high-quality color maps (104 x 76 cm) of the Quaternary Geology of China, along with an explanatory 78-page volume. The Quaternary of China bears many similarities to Late Quaternary Environments of the Soviet Union, edited by A. A. Velichko (1984). Zhang et al.'s work is more comprehensive and larger, and has the wonderful maps, but the format is the same. Following a general introduction by the Chief Editor (Zhang), there are chapters covering a wide range of topics, from tectonism to vertebrate paleontology. The design of the figures and correlation charts is even the same. Like the Soviet volume, The Quaternary of China provides a glimpse of prolific research in a region of great interest for Quaternary studies. Velichko's volume has photographs, Zhang et al.'s does not, and neither has an index.

As with Velichko's volume, The Quaternary of China provides a valuable counterpoint for Western Quaternary studies. Many conclusions seem familiar, but others are novel or exotic. The Quaternary time scale presented is entirely familiar. Major subdivisions are based on magnetostratigraphy and the marine oxygen isotope stages, and the Holocene subdivisions clearly are descended from the European Blytt-Syrnander sequence. Also familiar are the environmental reconstructions for the Last Glacial Maximum. In Tibet, lake basins dried ca. 18.9 ka BP, and trees were replaced by cold- and drought-resistant herbs. The periglacial limit was 800 m lower, and the snow line descended 350-1100 m, with many regional variations. In eastern coastal areas, sea level was 130-150 m lower.

Climatic events during the deglaciation are unclear. Whereas there are indications of climatic fluctuations between 18 and 10 ka BP, and some evidence for the Younger Dryas event (p. 232), I found no mention of whether lake levels were higher during deglaciation (like the northern Great Basin), or in the early Holocene (like southern Sahara). In fact, I found little information on the history of the Asian monsoon, beyond a general statement regarding its Neogene intensification due to uplift of the Tibetan plateau.

In contrast to recent claims of intensified monsoonal precipitation during the early Holocene (An et al. 1991), the climatic chronology presented in this book indicates greater aridity in the early Holocene (pp. 150-154). The middle Holocene, 5.5-2.5 ka BP, was warm and wet in most regions, and the late Holocene cold and dry. The mountainous regions of western China contain extensive deposits left by Holocene glacial advances dated 5500, 3900, 2800, 1700, 1100 and 400-70 yr BP. Many records of (relative) Holocene sea level from eastern China indicate water depths ca. 4 m greater than today during the middle Holocene (8-4 ka BP). Several aspects of the book and maps warrant special mention. Chapter 6, titled "Quaternary Geology in Offshore Areas of China", is the most detailed and informative of the book. The errors

339 340 Book Reviews are minimal, and there are more figures, radiometric dates, and detailed diagrams of ostracods and pollen than in any other chapter. I also appreciated Chapter 15 by Zang Zonghu, Zhang Zhiyi and Wang Yunsheng on "Loess in China". It includes a detailed history of loess research, regional descriptions of loess stratigraphy, summaries of fossils preserved in loess, discussions of soil- forming processes and implications of loess stratigraphy for the environmental chronology.

I recommend highly the chapters on fossil hominids in China, "The Other Cradle of Humanity". Chapter 6 in Explanatory Notes ... lists the Ziaochangliang Culture, dated 2.5 Ma by paleomagnetism, as the oldest evidence (tools only) for humans in China. Chapter 10 in The Quaternary of China mentions, more conservatively, the earliest skeletal remains of Homo erectus yuanmouensis, dated 1.7 Ma by paleomagnetism. These are followed by many discussions of other human fossils throughout the Quaternary.

Chapter 13, by Han Tonglin, includes a discussion of a unique aspect of Chinese Quaternary studies: the early Pleistocene "Great Ice Sheet". This extensive ice cap formed before the uplift of the Tibetan Plateau had blocked monsoon moisture from the Indian Ocean. Its deposits include several continental-scale glacial features, such as bedrock drumlins and till-covered plains. The ice is estimated to have been 1000-2000 m thick, covering an area of 2-3 million km2. Other topics include neotectonics, volcanism, stratigraphy, paleogeography, palynology and laterites. There are two regional syntheses: one for the Qinghai-Tibet Plateau, another for the Eastern China Plain. I was surprised by the absence of some topics, such as Quaternary faunal extinctions, and the minimal coverage given to the history of monsoon climate and pluvial lakes, but overall, the coverage is thorough.

Although The Quaternary of China compares favorably with Velichko's Soviet volume, it would have benefitted from English-language editing. Wright and Barnosky (Velichko 1984) provided a conceptual interface for Western readers and revised the English. The errors in The Quaternary of China range from distracting to obscuring. Some sections must be read very carefully, and figure captions are particularly error-prone. For example, the axes of Figure 3.12 are labeled "Age (Ma BP)", but the units are actually 10,000 yr. Most errors appear to result from the typesetters' unfamiliarity with the English alphabet, but these mistakes should have been caught in proof. Explanatory Notes ... (Fan Yi, English Editor) is comparatively error-free.

The volumes are valuable sources of information on the Pleistocene of China, but they fall short as a resource for further study. The "big picture" is there, but without the specifics. Most references cited in the text are not included in the "Main References" at the end of the book. Even the radiocarbon dates are given without laboratory numbers. Despite these shortcomings, I strongly recommend these books and maps to any Quaternary scientist interested in Asia, in particular, or global change, in general. They provide a broad introduction over a wide array of topics for this fascinating region, and they have heightened my interest in more detailed studies.

REFERENCES

An, Z. Kukla, G. J., Porter, S. C. and Xiao, J. 1991 Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of central China during the last 130,000 years. Quaternary Research 36: 29-38. Velichko, A. A. 1984 Late Quaternary Environments of the Soviet Union. Wright, H. E., Jr, and Barnosky, C. W., (eds., English edition). Minneapolis, University of Minnesota Press: 327 p. Owen K. Davis Department of Geosciences The University of Arizona Tucson, Arizona 85721 USA Book Reviews 341

The Last Deglaciation: Absolute and Radiocarbon Chronologies. Edited by Edouard Bard and Wallace S. Broecker. Nato ASI Series I: Global Environmental Change, Vol. 2. Proceedings of the NATO Advanced Research Workshop, Erice, Sicily (Italy), December 9-13,1990. New York 1992 Springer-Verlag, 344 pages, $159.00.

As with many topics in the Quaternary sciences, chronology limits our ability to understand the complicated relations among events that occurred during the termination of the most recent ice age. Although abrupt changes in oceans, atmosphere, biosphere and cryosphere are inextricably linked in ways that now appear to have global consequences, we are not yet able to resolve critical questions about the precise order in which the changes occurred. This useful book brings together a diverse but focused collection of papers in three sections dealing with (I) radiocarbon and absolute chronologies, (II) past changes in cosmonuclide production, and (III) climate changes during the last deglaciation. 14C Section I contains papers by Kromer and Becker (tree-ring calibration); Johnsen and Dansgaard (flow-model dating of ice cores); Bjorck et al. (Swedish varve chronology); Lotter et al. (annually laminated sediments from Switzerland); Rozanski et al. (annually laminated sediments from 14C Poland); Zolitschka et al. (varve-dated records from Germany); and Bard et al. (230Th/234U and dating of corals from Barbados, Galapagos and French Polynesia). Section II has papers by Lal (variations in global production rate of 14C); Raisbeck et al. (10Be variations in 50,000 years of the Vostok core); Beer et al. (10Be peaks in polar ice cores); Salis and Bonhommet (geomagnetic field intensity from 8-60 ka); and Mazaud et al. (geomagnetic calibration of the 14C time scale). Section III includes Broecker (strength of the Nordic heat pump); Sarnthein et al. (180 meltwater anomalies in the North Atlantic; Duplessy et al. (a new method to reconstruct sea-surface salinity); Southon et al. (past ocean-atmosphere 14C differences); Jouzel et al. (evidence of a "Younger Dryas" event in Antarctica); Fisher (ice-core evidence for an early Holocene freshwater cap in the Atlantic); Gasse and Fontes (climate changes in northwest Africa during the last deglaciation); and Peteet (palynological evidence for the Younger Dryas in Europe and North America). The chapters are unusually concise and to-the-point, with key conclusions supported well by data and figures. Because the papers were typeset camera-ready by authors, they vary in font and format; nevertheless, printing is of high quality and most chapters are easy to read. The first two sections include discussions of such topics as 1) the plateaus in 14C ages that occur at 10,000, 9600 and 8800 BP, according to dendrochronologic data, and at 12,700, 10,000 and 9500 BP, according to data from annually laminated Swiss lake sediments; 2) the duration of the Younger Dryas, which is measured as 260-400 yr (Swedish varves), 450 yr (Greenland ice-core chronology), 680 yr (annually laminated lake sediments from Switzerland), and at least 1200 yr (annually laminated lake sediments from Poland); 3) estimates for the absolute age of the Younger Dryas/Holocene transition, which vary from 10,630 cal BP to 211,090 cal BP; 4) support for the accuracy and precision of 230Th/234U age determinations (dating of corals); and 5) stratigraphic markers that may be reliably used to date ice cores.

Chapters in the third section are interesting and provocative. All in all, this book provides first- hand insights that are both fascinating and useful - to this reader, at least. Despite its $159.00 cost, many Quaternary scientists will be pleased to have this book readily available.

George L. Jacobson, Jr. Institute for Quaternary Studies University of Maine Orono, Maine 04469 USA 342 Book Reviews

Isotopic Techniques in Water Resources Development 1991. Edited by G. V. Ramesh. Proceedings of a Symposium, Vienna, 11-15 March 1991. Vienna, Austria, International Atomic Energy. Agency, 1992, 789 pages. (757 text pages) Paperbound, $240.

This volume consists of 84 papers presented (37 oral, 47 poster) at the symposium of the same name. The topics for the oral presentation are: Interface Processes between the Atmosphere and the Hydrosphere (2 papers); Surface Water and Sediments (6 papers); Groundwater Dating: Problems and New Approaches (9 papers); Groundwater Dating: Problems and New Approaches - Methodological Aspects and Models (3 papers); Groundwater (6 papers); Environmental Problems and Water Pollution (5 papers); and Paleohydrology and Paleoclimatology (6 papers). The poster presentations are listed in their separate categories, and are more concise than the papers from the oral presentations. Seven papers are in French, 2 in Spanish and 2 in Russian. Only the three non-English papers from the oral presentations have English-language abstracts. This volume will be valuable reading for those who utilize environmental isotopes, and need to learn about the latest isotopic tools and new applications of the traditionally applied isotopes. 14C, 160/160, 36C1, Topics range from applications of the well-studied 3H, 2H/1H, and to U-series isotopes and noble gases. The papers contain a few examples of the use of artificial tracers in water-tracer experiments; one illustrates the propitious use of Chernobyl-generated isotopes as large-scale "artificial" tracers. At least three papers deal with in-situ production of environmental isotopes. Recent years have seen the evolution of the hydrological applications of environmental isotopes from strict isotopic considerations to more integrated studies that include not only all available geochemical parameters, but also the geological and hydrological context. Environmental isotopes have assumed their proper role in testing hypotheses based on all available data. This volume continues this trend.

A particularly vexing aspect of groundwater isotope data is the interpretation of the results in terms of real uncertainties. Several previous groundwater investigations have pointed out possible sources of errors, but until recently, few efforts have dealt with uncertainties quantitatively. The article by Brian Payne shows how statistical methods can be applied to stable oxygen and hydrogen isotope data. The techniques are transferrable to other isotopes. This chapter should be required reading for users of environmental isotopes. The section on environmental problems includes studies of saline mine discharge, artificial radionuclide migration, and public water supply degradation. The Paleoclimatology section has particular relevance to global change research, as papers in this section illustrate that aquifers and groundwater-deposited travertine and permafrost can be archives of paleoclimatic information.

This volume will be a valuable addition to libraries of universities and research institutions in two senses of the word. First, it illustrates the current state of acceptance of the use of environmental isotopes in practical situations involving questions of water quality and quantity. Second, the price all but precludes this volume's occupancy on personal library shelves. At $240 (US), even university libraries, many of which are currently trimming budgets, will be circumspect about its purchase. Current and potential users of environmental isotopes in water resources, including hydrology students and environmental consultants, should take note of the variety of ground and surface water studies illustrated in this volume.

Austin Long [RADIOCARBON, VOL. 35, No. 2, 1993, P. 343-344]

RADIOCARBON UPDATES Relocation Robert M. Kahn has moved from the Environmental Radioisotope Laboratory at The University of Arizona to the Center for Applied Isotope Studies (CAIS), The University of Georgia, where he joins John E. Noakes in the direction of the Radiocarbon Laboratory (UGa). His E-mail address is: [email protected]. We wish Bob great success in his new position!

Conference Announcements The 6th International Conference on Accelerator Mass Spectrometry will be held in Canberra and Sydney, Australia, 27 September to 1 October 1993. For more information, contact:

L. K. Fifield, S. H. Sie or C. Tuniz AMS 6 ACTS GPO Box 2200 Canberra, ACT 2601 61 6 6 249

The International Conference on Tree Rings, Environment and Humanity: Relationships and Processes will be held at the Hotel Park Tucson in Tucson, Arizona, USA on 17-21 May 1994, and will be hosted by the Laboratory of Tree-Ring Research at The University of Arizona. The conference offers an opportunity for individuals interested in tree-ring research to meet and discuss current progress and future directions of dendrochronology. The meeting will address aspects of the past and future Earth, including its physical, biological and social systems. The five-day program will be organized into paper and poster sessions on Tuesday, Wednesday, Friday and Saturday. Thursday will be devoted to field trips, workshops and other optional activities. Longer field trips will be offered before and after the meeting. Participation by students and young scientists is especially encouraged. For further information, please contact: International Tree Ring Conference Laboratory of Tree-Ring Research Building 58 The University of Arizona Tel: 602-621-2191 Tucson, Arizona 85721 USA Fax: 602-621-8229

LSC 94 - Advances in Liquid Scintillation Spectrometry will be held in Glasgow, Scotland, 8-12 August 1994, the week preceding the 15th International Radiocarbon Conference. Please see our ad for more details, or contact: Gordon Cook Scottish Universities Research & Reactor Centre East Kilbride 44 3551-23332 Glasgow G75 OQU, Scotland Fax: 44 3552 29898

343 344 14C Updates

Directly following the Liquid Scintillation Conference will be the 15th International Radiocarbon Conference in Glasgow, Scotland, 15-19 August 1994. Please see our ad for more details, or contact:

Marian Scott or Mrs. M. Smith Tel: 44 41 339 Ext. 5024 Department of Statistics Fax: 44 41 330 University of Glasgow E-mail: [email protected], or Glasgow, G12 80W, Scotland [email protected] Forthcoming Publications

1) Liquid Scintillation Spectrometry 1992, edited by J. E. Noakes, H. A. Polach and F. Schonhofer, will be published in 1993. This volume contains the Proceedings of the Conference, LSC 92, held in Vienna, Austria in September 1992. The volume will be hardcover and is outside the regular journal series; 2) Late Quaternary Chronology and Paleoclimates of the Eastern Mediterranean, edited by Ofer Bar-Yosef and R. S. Kra, will be published early in 1994. The volume is based on our workshop at the Tucson 14C Conference. It is also outside the journal series; 3) Applications of Radiocarbon Dating in the Former Soviet Union and Eastern Europe, Special Editor, J.-M. Punning, will appear as Volume 35, No. 3, 1993; 4) Volume 36, No. 1, 1994 will contain Radiocarbon Dynamics in Soils, with Special Editors Peter Becker-Heidmann, D. D. Harkness, Eldor Paul and Susan Trumbore. This issue results from a NASA-sponsored workshop held in Tucson in June 1992.

14C Database Service

We have had a communication from Francesco Paolo Bonadonna, Dipartimento di Scienze della Terra, University degli Studi di Pisa, which announces the ASTRA 14C Database Service. Apparently, this is a detailed list of 14C dates of cultural and climatic events from European countries over the period, 25,000-2000 BP, compiled mostly from RADIOCARBON up to 1991. Dates are stored in CRON and references are stored in META. These can be accessed through user interface or E-mail. For more information, contact:

Guiseppe A. Romano Tel: 39 50 593248 CNUCE - CNR :: GARR - NIS Fax: 39 50 904052 Via Santa Maria 36 Telex: 500371- CNUCE 56126 Pisa, Italy E-mail: [email protected], or [email protected]. GARR.IT Price Change

What seems to be too good to be true really is not. The price of CALIBRATION 1993 will increase from $40.00 to $50.00 on June 1. The response is so positive, we may have to go to a second printing! [RADIOCARBON, VOL. 35, No. 2, 1993, P. 345]

LETTER TO THE EDITOR

12 April 1993 Dear Editor,

I recently 14C came across an abstract of H. E. Gove's article on the dating of the Shroud of Turin in Art and Archaeology Technical Abstracts. Luckily, I retrieved a copy of the issue of RADIOCARBON in which it appeared (Gove 1990) just before it was off to the bindery. I enjoyed his review of the details of the whole process, especially after reading his earlier outline (Gove 1987). I noted that he had some questions about the art historical nature of painting and the possible image formation methods of the Shroud. An article that I and T. B. Kahle published in 1989 covers many of the questions he raised. I also would like to share some additional information that I have not yet published and which was not in our article.

This new information relates to the effect of deterioration products of various organic substances, including paint media and wood resins, which can produce similar images, such as the shroud. The interaction of peroxides and hydroperoxides on cellulosic materials can produce the type of discoloration seen on shroud fibers (Daniels 1988). The Conservation Analytical Laboratory (CAL) of the Smithsonian Institution has investigated the complexity of the transfer of images onto adjacent materials (Padfield, Erhardt & Hopwood, ms.). In images formed in frames by transfer from pictures of various kinds, one often notes on the glazing that a substance has formed on the surface of the glazing. This substance can be rubbed off with a finger. Padfield and others at the CAL analyzed one example of such an image transfer and found the substance to be mainly sodium chloride and an organic material with surfactant properties. The salt was present from the picture's salted silk. A liquid, mobile phase of deliquescence was apparently the main agent of transfer, aided by the surfactant. Relative humidity and even weave structure were transfer factors. As we noted in our study (Caldararo & Kahle 1989), textile fibers and finished textiles are often treated prior to manufacture to prepare the fibers for weaving or use. Chemical interactions of agents used in such treatment can be active in image transfer and image formation.

The CAL study involves only one type of image transfer in picture frames. Several very different types exist, for example, image transfer to back mats in matted works of art and photographic latent image transfer of un-neutralized reagents. I hope this information is of interest to your readers. Sincerely, Niccolo Caldararo Adjunct Professor of Anthropology San Francisco State University San Francisco, California 94132 USA

REFERENCES

Caldararo, N. and Kahle, T. B. 1989 An analysis of the Methods in Physics Research B29: 193-195. present status of research into the authenticity of the 1990 Dating the Turin shroud - An assessment. Shroud of Turin. Restauro 95(4): 297-305. Radiocarbon 32(1): 87-92. Daniels, V. 1988 The discoloration of paper on ageing, Padfield, T., Erhardt, D. and Hopwood, W. (ms.) Joan The Paper Conservator 12: 93-100. of Arc and the bicycle race. Conservation Analytical Gove, H. E. 1987 Turin workshop on radiocarbon Laboratory Reports, Smithsonian Institution, unpub- dating the Turin shroud. Nuclear Instruments and lished.

345

15th International Radiocarbon Conference Glasgow, Scotland 15-19 August 1994 The 15th International Radiocarbon Conference will be held in Glasgow, Scotland in the Royal Scottish Academy of Music and Drama (RSAMD) from 15-19 August 1994, the week immediately following the International Conference on Advances in Liquid Scintillation Spectrometry.

The broad themes of the conference are ...... 14C in Archaeology Advances ::in Beta Counting Accelerator Techniques Calibration of the 4C Time Scale ...... As a ...Tracer of the Dynamic Carbon Cycle in Current Environments 14C In the Reconstruction of Past Environmentsx

There will be both poster and oral sessions, with no parallel sessions. There will also be opportunities for exhibiting equipment, computer software and books. To complement the main conference program, a number of workshops will be held on the weekend of 13 and 14 August, on topics including 14C in soils, archaeology, and the 14C intercomparison (TIRI). Other workshop topics are currently under discussion. Accommodation, all within walking distance of the RSAMD, ranges from University Halls of Residence to five star hotels. A full social program is also planned, with a welcome party, civic reception, outing, and conference dinner. Submission of Papers We welcome contributed papers within the broad themes outlined above. Authors will be required to submit a 500-word abstract for refereeing by the Scientific Committee. Within the time constraints posed by our aim of having no parallel sessions for all presentations, the committee will determine the appropriate session and manner of presentation for the accepted papers.

For more information please contact Marian Scott or the Conference Secretariat, Mrs. M. Smith, Department of Statistics University of Glasgow, Glasgow G12 8QW, Scotland Telephone: (44)41339 8855 Ext. 5024; Fax: (44)41 330 4814 E-mail: [email protected] or [email protected] RADIOCARBON Announces the Publication of the Following: CALIBRATION 1993 Volume 35, No. 1, 1993 @ $40.00 ($50.00 as of June 1, 1993)

Calibration 1993 (Minze Stuiver, editor) is a hardcover update of the 1986 Calibration Issue (Volume 28, No. 2B), one of our most frequently ordered issues. It contains new papers with 14C data sets that extend and refine the most widely used source of radiocarbon age calibrations. A diskette of CALIB 3.0.3, the IBM PC-based calibration program (Stuiver and Reimer) is provided with the issue. The program allows for calibrations from "conventional radiocarbon years" to calendar dates for the past 18,36014C years.

Radiocarbon After Four Decades: An Interdisciplinary Perspective Editors: R. E. Taylor, Austin Long and R. S. Kra Special Hardcover Edition Published by Radiocarbon and Springer-Verlag, New York $89.00 List Price; $66.25 for Subscribers to RADIOCARBON Radiocarbon After Four Decades: An Interdisciplinary Perspective commemorates the 40th anniversary of radiocarbon dating and documents the major contributions of 14C dating to archaeology, biomedical research, earth sciences, environmental studies, hydrology, the natural carbon cycle, oceanography and palynology. The volume serves as a synthesis of past, present and future research in the field. It has been co-published with Springer-Verlag, and is offered outside of our regular issues; RADIOCARBON subscribers receive a 25% discount off the $89.00 list price and pay only $66.25.596 pages.

Proceedings of the 14th International Radiocarbon Conference Tucson, Arizona, 20-24 May 1991 Volume 34, No. 3, 1992 @ $65.00 This Conference Proceedings documents state-of-the-art research in 14C dating and cosmogenic isotopes. It contains papers on recent developments in Sample Preparation and Measurement Techniques, Applied Isotope Geochemistry, Global 14C Production and Variation, Paleoclimatology and Archaeological Applications; Workshop Reports are included. 665 pages. The Proceedings issue is part of the 1992 subscription.

FORTHCOMING...

LSC 92: Proceedings of the International Conference on Advances in Liquid Scintillation Spectrometry, 14-18 September 1992, Vienna, Austria. Volume editors: John E. Noakes, Henry A. Polach and Franz Schonhofer. (Outside journal series) Applications of Radiocarbon Dating in the Former Soviet Union and Eastern Europe. Special editor: Jaan-Mati Punning, Tallinn, Estonia. RADIOCARBON, Vol. 35, No. 3, 1993. Late Quaternary Chronology and Paleoclimates of the Eastern Mediterranean. Volume editors: Ofer Bar-Yosef and Renee Kra, 1994 (Outside journal series) 14C Dynamics in Soils. Special editors: Peter Becker-Heidmann, D. D. Harkness, Eldor Paul and S. E. Trumbore. RADIOCARBON, Vol. 36, No. 1, 1994. RADIOCARBON 1993 PRICE LIST

NEW CALIBRATION 1993 (Vol. 35, No. 1, 1993) - - before 1 June 1993 - $40.00; after 1 June 1993 $ 50.00

Proceedings of the 14th International Radiocarbon Conference (Vol. 34, No. 3, 1992) $ 65.00 14C Proceedings of the International Workshop on Intercomparison of Laboratories $ 40.00 (Vol. 32, No. 3, 1990)

Proceedings of the 13th International Radiocarbon Conference (Vol. 31, No. 3, 1989) $ 60.00

Proceedings of the 12th International Radiocarbon Conference $ 60.00 (Vol. 28, Nos. 2A & 2B, 1986 - available separately at $30.00 each)

Proceedings of the 11th International Radiocarbon Conference (Vol. 25, No. 2, 1983) $ 50.00

Proceedings of the 10th International Radiocarbon Conference $ 60.00 (Vol. 22, Nos. 2 & 3, 1980)

Vol. 35, Nos. 1-3,1993 (includes NEW CALIBRATION 1993) $105.00/vol. Inst.* $ 73.50/vol. Ind. $ 36.75/voL Stud** Vol. 34, Nos. 1-3,1992 (includes Proceedings) $105.00/vol. Inst. $ 73.50/vol. Ind.

Vol. 33, Nos. 1-3,1991 $ 94.50/vol. Inst. $ 63.00/vol. Ind.

Vol. 32, Nos. 1-3,1990 (includes Glasgow Proceedings) $ 94.50/vol. Inst. $ 63.00/vol. Ind.

Vol. 31, Nos. 1-3,1989 (includes Proceedings) $ 90.00/vol. Inst. $ 60.00/vol. Ind.

Vol. 30, Nos. 1-3,1988 $ 85.00/vol. Inst. $ 55.00/vol. Ind. Vols. 15-29,1973-1987 $ 75.00/vol. Inst.t $ 50.00/vol. Ind.

Vols. 10-14,1968-1987 $ 50.00/vol.t

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Single Issue $ 25.00 Single copy of out-of-print Issue $ 35.00

SPECIAL PACKAGE OFFER (now includes 9 out-of-print issues): Full set, Vols. 1-34 $650.00 (1959-1992) + free subscription for 1993! Postage & handling for Package Offer: USA $ 25.00 Foreign $ 50.00 *Foreign postage on current volumes $ 10.00 **New Student Rate - with student identifica- tplus additional charge for copy of out-of-print issue(s) tion and letter from subscribing sponsor. Back Postage & handling will be added to back orders issues available at 1/2 the individual rate.

NOTICE TO READERS AND CONTRIBUTORS

14C The purpose of RADIOCARBON is to publish technical and interpretative articles on all aspects of and other cosmogenic isotopes. In addition, we present regional compilations of published and unpublished dates along with interpretative text. Besides the triennial Proceedings of Radiocarbon Conferences, we publish Proceedings of conferences in related fields. Organizers interested in such arrangements should contact the Managing Editor for information.

Our regular issues include NOTES AND COMMENTS, LETTERS TO THE EDITOR, RADIOCARBON UPDATES and ANNOUNCEMENTS. Authors are invited to extend discussions or raise pertinent questions regarding the results of investigations that have appeared on our pages. These sections also include short technical notes to disseminate information concerning innovative sample preparation procedures. Laboratories may also seek assistance in technical aspects of radiocarbon dating. Book reviews are encouraged.

Manuscripts. Papers may be submitted on floppy diskettes and as printed copy. When submitting a manuscript, include three printed copies, double-spaced. When the final copy is prepared after review, please provide a floppy diskette along with one printed copy. We will accept, in order of preference, WordPerfect 5.1 or 5.0, Microsoft Word, Wordstar or any IBM word-processing software program. ASCII files, MS DOS and CPM-formatted diskettes are also acceptable. The diskettes should be either 3%" (720 k or 1.44 mb) or 51/4" (360 k or 1.2 mb). Papers should follow the recommendations in INSTRUCTIONS TO AUTHORS (RADIOCARBON, 1992, vol. 34, no. 1, p. 177-185). Offprints are available upon request. Our deadlines for submitting manuscripts are:

For Date Vol. 36, No. 1, 1994 September 1, 1993 Vol. 36, No. 2, 1994 January 1, 1994 Vol. 36. No. 3, 1994 May 1, 1994

Half-life of 14C. In accordance with the decision of the Fifth Radiocarbon Dating Conference, Cambridge, England, 1962, all dates published in this volume (as in previous volumes) are based on the Libby value, 5568 yr, for the half-life. This decision was reaffirmed at the 11th International Radiocarbon Conference in Seattle, Washington, 1982. Because of various uncertainties, when 14C measurements are expressed as dates in years BP, the accuracy of the dates is limited, and refinements that take some but not all uncertainties into account may be misleading. The mean of three recent determinations of the half-life, 5730 ± 40 yr, (Nature, 1962, vol. 195, no. 4845, p. 984), is regarded as the best value presently available. Published dates in years BP can be converted to this basis by multiplying them by 1.03.

AD/BC Dates. In accordance with the decision of the Ninth International Radiocarbon Conference, Los Angeles and San Diego, California, 1976, the designation of AD/BC, obtained by subtracting AD 1950 from conventional BP determinations is discontinued in RADIOCARBON. Authors or submitters may include calendar estimates as a comment, and report these estimates as cal AD/BC, citing the specific calibration curve used to obtain the estimate. Calibrated dates should be reported as "cal BP" or "cal AD/BC" according to the consensus of the Twelfth International Radiocarbon Conference, Trondheim, Norway, 1985.

Measuring PC. In Volume 3, 1961, we endorsed the notation, A (Lamont VIII, 1961), for geochemical measurements of 6140 14C activity, corrected for isotopic fractionation in samples and in the NBS oxalic-acid standard. The value of that entered the calculation of A was defined by reference to Lamont VI, 1959, and was corrected for age. This fact has been 614C lost sight of, by editors as well as by authors, and recent papers have used as the observed deviation from the standard. 814C At the New Zealand Radiocarbon Dating Conference it was recommended to use only for age-corrected samples. Without an age correction, the value should then be reported as percent of modern relative to 0.95 NBS oxalic acid (Proceedings of the 8th Conference on Radiocarbon Dating, Wellington, New Zealand, 1972). The Ninth International Radiocarbon Conference, Los Angeles and San Diego, California, 1976, recommended that the reference standard, 0.95 NBS 813C oxalic acid activity, be normalized to = -19%0.

614C In several fields, however, age corrections are not possible. and A, uncorrected for age, have been used extensively in oceanography, and are an integral part of models and theories. Thus, for the present, we continue the editorial policy of using A notations for samples not corrected for age. VOL. 35, No. 2 RADIOCARBON 1993

CONTENTS

FROM THE EDITOR - Journal Cost Crisis Austin Long ...... iii

OBITUARY - Elizabeth K. Ralph Henry N. Michael ...... v

ARTICLES Intra-Annual Variability of the Radiocarbon Content of Corals from the Galapagos Islands T. A. Brown, G. W. Farwell, P. M. Grootes, F. H. Schmidt and M. Stuiver ...... 245 Radiocarbon Age of Lacustrine Deposits in Volcanic Sequences of the Lomas Coloradas Area, Socorro Island, Mexico J. D. Farmer, M. C. Farmer and R. Berger ...... 253 S13C Late Pleistocene-Recent Atmospheric Record in C4 Grasses L. J. Toolin and C. J. Eastoe ...... 263 Carbon Isotopic Composition of Deep Carbon Gases in an Ombragenous Peatland, Northwestern Ontario, Canada R. Aravena, B. G. Warner, D. J. Charman, L. R. Belyea, S. P. Mathur and H.Dinel ...... 271 Isotopic Analysis of Groundwater and Carbonate System in the Surdulica Geothermal Aquifer M. Hadzifehovk, N. Miljevid, V. Sipka, D. Golobocanin and R. Popovi5 ...... 277 Radiocarbon Dating of Paleoseismocity Along an Earthquake in Southern Italy G. Calderoni and V. Petrone ...... 287 A Batch Preparation Method of Graphite Targets With Low Background for AMS 14C Measurements H. Kitagawa, T. Masazawa, T. Nakamura and E. Matsumoto ...... 295 AMS-Graphite Target Production Methods at the Woods Hole Oceanographic Institution During 1986-1991 A. R. Gagnon and G. A. Jones ...... 301 Radiocarbon to Calendar Date Conversion: Calendrical Bandwidths as a Function of Radiocarbon Precision F. G. McCormac and M. G. L. Baillie ...... 311 A Simplified Approach to Calibrating 14C Dates A. S. Talma and J. C. Vogel ...... 317 Radiocarbon Dates from American Samoa J. T. Clark ...... 323

NOTES AND COMMENTS 14C Dating of Laser-Oxidized Organics A. L. Watchman, R. A. Lessard, A. J. T. Jull, L. J. Toolin and W. Blake, Jr...... 331 An Assessment of the Radiocarbon Dating of the Dead Sea Scrolls G. A. Rodley ...... 335

BOOK REVIEWS ...... 339

RADIOCARBON UPDATES ...... 343

LETTER TO THE EDITOR ...... 345