Speleothem Evidence from Oman for Continental Pluvial Events During Interglacial Periods

Speleothem evidence from Oman for continental pluvial events during interglacial periods

Stephen J. Burns Dominik Fleitmann Albert Matter Institute of Geology, University of Bern, CH-3012 Bern, Switzerland Ulrich Neff Augusto Mangini Heidelberg Academy of Sciences, D-69120 Heidelberg, Germany

ABSTRACT Growth periods and stable isotope analyses of speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods extending back over the past four of Earth's glacial-interglacial cycles. Rapid speleothem growth occurred during the early to middle Holocene (6±10.5 ka B.P.), 78±82 ka B.P., 120±135 ka B.P., 180±200 ka B.P., and 300±325 ka B.P. The speleothem calcite deposited during each of these episodes is highly depleted in 18O compared to modern speleothems. The ␦18O values for calcite deposited within pluvial periods generally fall in the range of ؊4½ to ؊8½ relative to the Vienna Peedee belemnite standard, whereas modern speleothems range from ؊1½ to ؊3½. The growth and isotopic records indicate that during peak interglacial periods, the limit of the monsoon rainfall was shifted far north of its present location and each pluvial period was coincident with an interglacial stage of the marine oxygen isotope record. The association of continental pluvial periods with peak interglacial conditions suggests that glacial boundary conditions, and not changes in solar radiation, are the primary control on continental wetness on glacial-interglacial time scales.

Keywords: speleothems, stable isotopes, Oman, monsoon, uranium-series method.

INTRODUCTION rine and continental records show that its in- such as the extent of glaciation on the Hima- How climate in Earth's tropical regions var- tensity has varied considerably in the recent layan plateau or sea-surface temperatures? ied over the course of Earth's glacial-intergla- past. For example, an early Holocene inten- cial cycles of the past several hundred thou- si®cation of the southwest summer monsoon, GEOLOGIC SETTING AND METHODS sand years remains largely unknown. Even followed by its dramatic reduction in the mid- Hoti Cave is a through cave in northern more acute is the lack of knowledge of con- dle to late Holocene is recorded by changes in Oman, in the foothills along the southern mar- tinental, as opposed to marine, climate varia- upwelling indicators in marine sediments gin of the Oman Mountains (Fig. 1). This 4.5- tion. The most commonly used archive of con- (Clemens et al., 1991; Sirocko et al., 1993; km-long cave is hosted by the Natih Forma- tinental climate change, lakes, are generally Overpeck et al., 1996), by variations in lake not long-lived enough to provide information levels across the Sahel region of Africa (Gasse on glacial-interglacial time scales, nor are and Street, 1978; Ritchie et al., 1985), in Ara- lacustrine sediments easily dated beyond the bia (McClure, 1976), and in India (Bryson and range of the radiocarbon method. Speleo- Swain, 1981), and by speleothem growth in thems, calcium carbonate deposits in caves, northern Oman (Burns et al., 1998). On longer offer the potential for considerably longer, time scales, knowledge of monsoon variation interpretable records of continental climate comes almost entirely from marine sediments variation (e.g., Dorale et al., 1992) includ- (Anderson and Prell, 1993; Rosteck et al., ing reconstruction of paleorainfall intensity 1997). Yet, these records primarily register (Bar-Matthews et al., 2000). Here, we at- changes of wind intensity and may or may not tempt to develop a record of continental climate re¯ect changes in available moisture on the variation for southern Arabia, a region affected continents. As a result, they leave unanswered by one of the most important components of some fundamental questions concerning cli- tropical climate, the Indian Ocean monsoon. mate variations in continental regions affected The record covers the past four glacial-inter- by the monsoon. In particular, does continen- glacial cycles, and is based on observed periods tal pluviality vary in concert with marine up- Figure 1. Map showing location of Hoti Cave of rapid growth and stable isotope data in spe- welling and/or wind strength, and are periods (star) and generalized modern, summer, surface wind pattern. Dashed line is approxi- leothems from Hoti Cave in Oman. of increased continental wetness in the region mate location of intertropical convergence The Indian Ocean monsoon is one of the primarily controlled by changes in solar in- zone, the present northern limit of monsoon major weather systems on Earth, and both ma- solation or by glacial boundary conditions rainfall.

᭧ 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; July 2001; v. 29; no. 7; p. 623±626; 3 ®gures; Data Repository item 2001069. 623 tion, a Cretaceous limestone. The present climate is arid to semiarid; average rainfall is ϳ200±250 mm/yr (Food and Agriculture Or- ganization, 1987). Groundwaters in the region are recharged primarily by cyclonic low-pres- sure systems moving from the Mediterranean Sea southeast across the Arabian Peninsula during the winter months (Weyhenmeyer et al., 2000). The northern limit of the summer monsoon, the intertropical convergence zone, is located generally south of the Arabian land- mass (Hastenrath, 1985). Oxygen isotopic values of rainfall in the vicinity of Hoti Cave range from ϳ0toϪ2½ (Macumber et al., 1997; Weyhenmeyer, 2000). We collected whole or sampled by drilling seven large fossil stalagmites and one large ¯owstone from the cave. These inactive stalagmites ranged from ϳ30 cm to 3 m in height; the ¯owstone is 2.7 m thick. In addition, we collected drip waters from several locations in the cave together with small active Figure 2. Composite record of speleothem growth periods and oxygen isotope ratios for stalagmites beneath the drips. Only one active Hoti Cave. Samples H2 and H3 are modern; all other ages are based on U/Th data (see footnote 1). Also shown is SPECMAP (Imbrie et al., 1990) marine oxygen isotope curve and stalagmite taller than 30 cm was found. More marine oxygen isotope stage numbers. typically, active stalagmites are ϳ10 cm in height. Temperature and humidity measurements were taken at numerous locations in the analyzed with an on-line, automated, carbon- dle Holocene; four different stalagmites yield- cave. ate preparation system linked to a VG Prism ed ages between 6.2 and 10.5 ka. Prior periods The ages of deposition of the samples were ratio mass spectrometer. Results are shown as of growth of large speleothems took place determined by measuring their U/Th ratios by the per mil difference between the sample and during the intervals 78±82 ka B.P., 117±130 thermal ionization mass spectrometry; we the Vienna Peedee belemnite (VPDB) stan- ka B.P., 180±210 ka B.P., and 300±325 ka B.P. made a total of 51 measurements for all sam- dard in delta notation. Reproducibility of stan- (Fig. 2). An important feature of one sample, 18 ples (Table 11). Analytical procedures for the dard materials is 0.08½. Drip water ␦ O stalagmite H-13, is that it records three sepa- separation and puri®cation of thorium and ura- values were measured using an on-line, au- rate growth periods. As shown on the age ver- nium were as described by Ivanovich and Har- tomated equilibration system. The values are sus depth plot in Figure 3, the stalagmite grew mon (1993). U/Th measurements were per- reported against the Vienna standard mean rapidly during three relatively short periods of formed on a multicollector mass spectrometer ocean water (VSMOW) standard, and have a about 10±30 k.y.; intervening long periods of (Finnigan MAT 262 RPQ) with a double ®l- reproducibility of 0.08½. nondeposition are marked by thin, reddish, ament technique. Uranium and thorium were clay mineral±rich layers. Growth phases oc- measured in semi-peak-jump mode and peak- RESULTS curred during times equivalent to MIS (marine jump mode, respectively. Calibration of Far- Active stalagmites in Hoti Cave are small [oxygen] isotope stages) 5e, 7a, and 9. Note aday cup to ICM ef®ciency was made adopt- in comparison to fossil stalagmites and have that although two samples from the last 18 ing the natural 238U/235U of 137.88. To ␦ O values that range from about Ϫ1½ to growth period in H-13 date beyond MIS 9, the 18 determine the uranium and thorium concentra- Ϫ3½ (Fig. 2). Measured ␦ O values of drip ages of these samples have large errors and tions, de®ned quantities of a 233U/236U double waters associated with growing stalagmites are bracketed above and below by better- spike and a 229Th spike were added. Th/U vary mainly between 0½ and Ϫ2½, very determined ages (Fig. 3). Because growth was 18 ages were corrected for detritus following Iva- similar to rainfall ␦ O values in the region of reactivated multiple times within a single sam- novich and Harmon (1993), assuming a 232Th/ the cave despite high evaporation rates in arid ple, the intervening hiatuses are taken to have 238U isotope ratio of 3.8. The reproducibility climates. For the measured cave temperatures been continually dry. Each of the growth pe- 18 of the isotope ratio of 234U/238U and the con- of 25±27 ЊC, the modern speleothem ␦ O val- riods of large stalagmites is coincident with centration of 232Th of standard materials is ues are, thus, very close to expected equilib- an interglacial stage of the marine isotope re- 0.3% and 0.8% (2 ␴), respectively. For details rium calcite values and well re¯ect modern cord; growth took place during times equiva- about measurements of standard material see rainfall in the area. Transects of stable isotopic lent to MIS 1, 5a, 5e, 7a, and 9 (Fig. 2). Dur- Frank et al. (2000). measurements along growth lines of older sta- ing each phase of rapid speleothem growth, 18 More than 500 oxygen isotope analyses lagmites show no evaporative effect, suggest- the ␦ O values of speleothem carbonate are were done on the speleothems. For each, ϳ5 ing that they too precipitated calcite close to much more negative than those measured in mg of powder was drilled from the sample and isotopic equilibrium. modern, active speleothems. For the early to With one exception, large stalagmites in middle Holocene and MIS 5a, speleothem car- Hoti Cave are no longer actively growing. bonate ␦18O values generally range from 1 GSA Data Repository item 2001069, Table 1, Age determinations on large stalagmites show Ϫ4½ to Ϫ6½. For the periods associated Details of U-Th chemistry, is available on request from Documents Secretary, GSA, P.O. Box 9140, that they grew during discrete periods. The with the interglacial periods of MIS 5e, 7a, 18 Boulder, CO 80301-9140, [email protected], most recent growth period of large stalagmites and 9, the ␦ O values from stalagmites are or at www.geosociety.org/pubs/ft2001.htm. in Hoti Cave occurred during the early to mid- even more negative, ranging mainly from

624 GEOLOGY, July 2001 over the past four of Earth's glacial cycles. For each of these cycles, the peak interglacial periods, equivalent in time to MIS 5a, 5e, 7, and 9, are marked by a continental pluvial phase. This association strongly suggests that periods of continental wetness are controlled by some aspect of glacial boundary conditions. Wheth- er the most important of these might be sea- surface temperatures in the Indian Ocean, snow cover on the Himalayan plateau, atmo- spheric CO2 concentrations, or even conditions far a®eld from the Indian Ocean cannot be interpreted from our data. Perhaps the primary source of information on changes in the Indian Ocean monsoon system is marine sediments in the Arabian Sea and Indian Ocean. For nearly all of these re- Figure 3. Ages, with associated error bars, vs. sample depth for stalagmite H-13. Stalagmite grew rapidly during each of three peak interglacial periods, equivalent cords, the proxies for variation in monsoon to marine (oxygen) isotope stages (MIS) 5e, 7a, and 9. No growth took place during winds are based on indicators of wind strength intervening times. or wind-driven upwellingÐfor example, lithogenic grain size, carbon mass-accumulation rates, or relative percentages of foraminifera Ϫ6½ to Ϫ8½. The most negative ␦18O val- rapid speleothem growth, marked by very or coccolithophores. In some cases, the up- ues, from Ϫ9½ to Ϫ12½ were measured in negative ␦18O values for speleothem calcite, welling is driven by the southwest summer samples from the ¯owstone deposited during are separated by hiatuses marking periods of monsoon; in others, upwelling is a response MIS 5e. little or no speleothem growth, with much less to the northeast, or winter, monsoon winds. negative calcite ␦18O values. The periods of Several important differences exist between DISCUSSION rapid growth are interpreted to mark climate these marine, upwelling-based records and The highly negative ␦18O values that char- periods much wetter than at present, periods those based on studies of continental pluvial- acterize carbonate deposited during the pluvial during which the intertropical convergence ity. First, on glacial-interglacial time scales, periods as compared to modern speleothems zone and limit of monsoon rainfall were shift- the majority of these records vary primarily cannot be due to higher temperatures in the ed far north of their present location and with a periodicity of ϳ20 k.y., close to the past. Even a 1½ decrease in carbonate ␦18O brought monsoon rainfall to the region of Hoti precessional cycle in Earth's orbit records would require a 4.5 ЊC temperature increase, Cave. The growth hiatuses occurred during (Anderson and Prell, 1993; Rosteck et al., and marine oxygen isotope records from the arid periods similar to the present climate, 1997; Clemens and Prell, 1990; Reichart et al., Arabian Sea (e.g., Murray and Prell, 1992) when the summer intertropical convergence demonstrate that temperatures during the early zone was generally south of the Arabian 1997). This is true regardless of whether the Holocene and past interglacial were within 1± Peninsula. upwelling region in question is summer or 2 ЊC of recent temperatures. Rather, the neg- For the most recent speleothem growth pe- winter monsoon driven, although these two ative isotopic values of speleothem carbonate riod, the early to middle Holocene, a pluvial types of upwelling do not vary in phase. Near- re¯ect isotopic depletion of rainfall and period is also recorded in continental paleo- ly all of these records have been interpreted groundwaters during these periods. If we as- climate records from across northern Africa, as indicating that the dominant control on sume that cave temperatures during the peri- the Arabian Peninsula, and into India. In par- monsoon intensity is solar insolation and that ods of speleothem growth were within 1±2 ЊC ticular, lacustrine sediments in these regions glacial boundary conditions play a much sub- of modern cave temperatures, then the stalag- show high lake-level stands during the early ordinate role (Anderson and Prell, 1993; Ros- mite calcite ␦18O values yield calculated to middle Holocene (Gasse and Street, 1978; teck et al., 1997; Clemens and Prell, 1990; 18 Reichart et al., 1997). ␦ OVSMOW values for rainfall of about Ϫ4½ Ritchie et al., 1985; Bryson and Swain, 1981). for the early to middle Holocene and MIS 5a, Pollen data from marine sediments in the In- Second, there are speci®c time periods dur- and Ϫ5½ for MIS 5e, 7a, and 9. The ¯ow- dian Ocean (Van Campo et al., 1982; Prell and ing which marine upwelling indicators suggest stone ␦18O values suggest that rainfall values Van Campo, 1986) and the continental margin intensi®ed monsoon winds, yet the continental averaged about Ϫ7½ (VSMOW), during its of equatorial west Africa (Dupont et al., 2000) records of climate indicate dryness. For ex- deposition. also show a pattern of continental wetness re- ample, during MIS 3, we have thus far found The most likely source for rainfall with markably similar to our speleothem record. no evidence of speleothem growth in Hoti such negative ␦18O values is the Indian Ocean Studies of pollen contained in cores from both Cave, and pollen studies indicate that arid summer monsoon. The strong convection as- areas show peaks in species indicative of trop- conditions dominated eastern Africa (Van sociated with monsoonal circulation results in ical humid conditions during the early Holo- Campo et al., 1982; Prell and Van Campo, rainfall highly depleted in 18O. For example, cene and in sediments deposited during MIS 1986; Dupont et al., 2000). In contrast, a va- the weighted mean ␦18O rainfall value for 5a and 5e (Van Campo et al., 1982; Prell and riety of marine indicators of the intensity of New Delhi, India, is Ϫ5.81½ (International Van Campo, 1986; Dupont et al., 2000). Dur- monsoon-wind±driven upwelling along the Atomic Energy Agency, 1992), re¯ecting the ing the period between the end of MIS 5a and Oman margin suggest intensi®ed monsoon dominance of the summer monsoon rainfall. the early Holocene, pollen indicative of more winds. In fact, the marine indicators of mon- The composite record of speleothem growth arid conditions dominated. Our speleothem soon wind intensity during MIS 3 are often periods from Hoti Cave shows that periods of data extend the record of continental climate much higher than those during the early Ho-

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626 GEOLOGY, July 2001