-Iournal of Coastal Research Charlottesville

Amino Acid Dating of Quaternary Marine Terraces, Bahia Asuncion, Baja California Sur, Mexico

Everly M. Keenan", Luc Ort.lieb" and John F. Wehmiller"

"Shell Development Co, -o.R. S. T. O. M. Instit ut Francais "Department of Geology P,O. Box 4R1 de Recherche Scientifique pour University of Delaware Houston, Texas 77001, USA Ie Developpment en Cooperation Newark, Delaware 19716, USA 24, rue Bayard 7 flOOR Paris, France ABSTRACT - _

KEENAN, E.M., ORTLIEB, L. and WEHMILLER, ,J.F., 19H7. Amino acid dating of Quaternary marine t('IT,H'eS, Hahia Asuncion, Raja California SUI'. Mexico. Journal ofCoasuil Research. :q;-l). 297­ :lO;l. Charlot I csville. ISSN 0749-020H. ~ • 8. .• In the area of Bahia Asuncion, on the Pacific coast of Baja California peninsula, amino acid •••4""""" racemization dating has been used to estimate ages of mollusks from Quaternary marine terr-aces. Eighteen molluscan samples (of the J{enera Tiuela; Saxidomus; and Chione) from ten localities have been analyzed. The high mean annual temperature for the region(greater than 20 C) has resulted in extensivc racemization ofsamples from what are considered to be late Middle and Late Pleistocene terrace localities. Racemization ofmost amino acids is effectively complete by about :-lOO,OOO years. However, two amino acids, leucine and valine. demonstrate enough resolvingpower to be used to delineate different age groups among the terrace sites. Where these apparent groups arc testable with stratigraphic or geomorphic evidence, they are generally consistent with the available geologic control. The ages estimated for the thr-ee aminostratigraphic groups recognized in this study are approx­ imat.ely 120,000, 200,000 and aoo,ooo years.

ADDITIONAl.. INDEX WORDS: (-.)uatl'rnnry marine terraces; H(Ua ('nlifornia Sur. Mexico, amino acid datiru: amimistratigrnphv. I/iz('aino n'flinswn

INrrRODUCTION " aminostratigraphic units" or "aminozones" within" aminozones" within a geographic region Because mollusks are the most common mac­ (MILLER and HARE, 1980). WEHMILLER (1982) roinvertebrates found in Quaternary deposits, they has recently reviewed many of the applications of have been used by numerous workers to apply and amino acid racemization dating to Quaternary evaluate the amino acid racemization (AAR) dating coastal chronologie problems. method. This technique relies upon the conversion Amino acid racemization dating has been em­ (racemization) of L-amino acids (" left- handed") ployed extensively in studies of Quaternary present in living mollusk shells to an equilibrium mix­ deposits of the Pacific coast of the United States ture of 50~' L-forms and 50/{ D-forms ("right­ (WEHMILLER et al., 1977; LA,JOIE et aL, 1979; handed"). The amino acids represent the residuum KARROW and BADA, 1980; KENNEDY et al., 1982; ofthe original calcification protein, and the racemiza­ and MUHS, 198:3). Results from VALENTINE (1980), tion of the amino acids is one of several diagenetic WOODS (1980), WEHMILLER and EMERSON (1980), reactions that occur. D/L values increase from 0.0 to and EMERSON et aL (1981), permit the extension of 1.0 with increasing age of the sample. and individual the United States aminostratigraphic sequences to clusters of D/L values are often recognized as lower latitudes (WEHMILLER, 1982: Figure 6). These studies have demonstrated that D/L ratios I The amino acid dating of mollusks in this study u'as funded by not only increase in samples of increasing age, but scuera! grants [rom the United States Gco!offica! Survey R40:29 received 11 September, 1YR·I: accepted in reoisicm 27 November also in samples of equal age at lower latitudes 19136. (higher temperatures). In addition, D/L analysis of 298 Keenan et al. different amino acids and different genera have tocene temperature change recorded in this region. permitted relative intrageneric and intergeneric Solitary corals have been dated by Uranium­ racemization rates to be established. This allows series methods at the following low terrace lo­ for the conversion of the results for one genus into calities along the Pacific coast of North America: equivalent results for another genus thus enabling Cayucos, Calif. (VEEH and VALENTINE, 1967); Nes­ direct aminostratigraphic comparison (LA,101E et tor Terrace, Calif. (KLJ and KERN, 1974); Eel Point al, 1980; WEHMILLER, 1982). Terrace, Calif. (MUHS and SZABO, 1982); and Calibration of the racemization kinetics to the Magdalena Bay, Baja California Sur, Mexico local effective temperature can be made by using (OMURA et al., 1979). The dates on these samples amino acid analyses of samples from localities that appear to represent the 120,000 to 130,000 year have been independently dated. Because several of high sea-stand that is recorded by the marine these localities now existfor the mid-latitude Pacific oxygen-isotope record (SHACKLETON and OPDYKE, coast of North America, aminostratigraphic age es­ 1973) and numerous tropical island coral terrace timates can be made by simple interpolation be­ records (e.g., BLOOM et al., 1974). Also, on the tween the D/L data points for the 120,000 year Sonoran coast ofthe Gulf of California, several last dated sites (WEHMILLER, 1982). Kinetic models of interglacial radiometric dates have been obtained racemization are needed to extrapolate to age es­ (BERNATet al., 1980). Absolute dating of mollusks timates for otherwise un- dated localities, and can be and coral fragments by Uranium-series has pro­ tested with data from calibration localities and in­ vided several anomalous results from the Vizcaino formation on the climatic history of the region. The Peninsula (ORTLIEH et al., 1984). mid-latitude Pacific coast of North America has Along the Vizcaino Peninsula, Baja California been an excellent region in which to study the com­ Sur, and, in particular, in the region of Bahia Asun­ bined effects of age and temperature on racemiza­ cion (Figure 1), a series of Quaternary marine tion. This is primarily due to the relative abundance terraces and superposed deposits exists that range of calibration sites and the moderate Late Pleis- in elevation from +2 to more than +100 meters

BAHIA SAN CRISTOBAL

PUNTA SAN PABLO PUNTA SAN ROQUE BAHIA SAN

PACIFIC OCEAN o 27 OO'N PUNTA o 10 20 30 SAN HIPOLITO KM

Figure 1. Sample locations, Bahia Asuncion, southern Vizcaino Peninsula (Baja California Sur, Mexico) .

•Journal of Coastal Research, Vol. ;~, No. :i, 1BR7 Amino Acid Dating 299

(TROUGHTON, 1974; ORTLIEB, 1978, 1979a, derivative procedures used (Table 1), although a 1979b, 1980; MALPICA, 1980; and MALPICA et al., slight conversion has been made (see WEHMILLER 1981). The lowest terraces have been tentatively and EMERSON, 1980) for the comparison of leucine correlated on the basis of elevation and morphology D/L values obtained by the different techniques. and are thought to representthe lastMiddle to Late The results reported in Table 1 represent the mean Quaternary high sea-stands corresponding to the values of peak height ratio determinations from at oxygen- isotope curve of SHACKLETON and OPDYKE least two chromatograms of each sample deriva­ (1973)(ORTLIEB, 1978, 1979b, and 1979c). Since tive. Precision of multiple sample analyses of well­ uplift rates have been relatively low in this region preserved shells is usually between 5ex) and 10exJ, (less than 100mm/10:~ years) there is some over­ depending on the amino acid lapping of marine units deposited during several interglacial transgressions; generally these marine DISCUSSION beds are separated by continental deposits. Where Pleistocene marine units occur in such strati­ The mollusks examined in this study generally graphic sequences, the oldest deposit is the lowest had relative intrageneric racemization rates similar lying one and elevation does not have the same to those observed by LA,JOIE et al: (1980): phenyl­ meaning as in a flight of staircase terraces. No local alanine> alanine> proline> leucine> glutamic faulting has been mapped within the study area. acid> valine. As the D/L values increased in the The present investigation was undertaken to samples studied, these intrageneric trends became evaluate the utility of the amino acid racemization less pronounced and sometimes were inverted in method in resolving discrete time intervals of the most extensively racemized samples. Valine, marine deposition in a high temperature environ­ leucine, and glutamic acid were the only amino ment such as found along the Vizcaino Peninsula (current mean annual temperature is about 21 0 C). Since samples from terraces of different elevations 1.0 are available, the consistency of the amino­ stratigraphy using various geologic criteria can be LR461 • LR527 evaluated, and the effective age limits at these high LR527~ 0.9 LR453 00 @LR458 temperatures can be determined. Three apparent • LR470 W LR530 aminostratigraphic units have been found for the Z Bahia Asuncion region and they generally conform o 0.8 • LR458 to local stratigraphic control. Based upon kinetic ::> LR46TLR464 modeling, it appears that the amino acid racemiza­ W LR468. tion dating method reaches its limit of usefulness in -I 0.7 fR525 this area by approximately 300,000 years. ..J ...... LR522 0 0.6 METHODS AND RESULTS ~ GROUP I AVERAGE

EEl GROUP II AVERAGE Ten localities (Figures 1 and 2) were sampled by 0.5 EE GROUP III AVERAGE one of us (L. 0.) in 1977. Eighteen mollusk samples (of the genera Tivela, Saxidomus, and Chione) were 0.5 0.6 0.7 0.90.8 1.0 analyzed from these sites. Ideally, more samples D/L VALINE should be studied to improve the statistics of each Figure 2. OIL leucine andvaline ratiosforallmollusks examined aminostratigraphic group. Amino acid enantiomeric inthisstudy. Average values arereportedformultiple samples of (D/L) ratio analyses were made by gas chromato­ the samespecies from the same locality. OIL leucine ratios for graphic methods described by KVENVOLDEN et al Saxidomus (a fast racemizing genus) have heen converted to equivalent values for slow racernizing genera by the regression (1972), FRANK et al. (1977L WEHMILLER et aL equations of LA,JOIE et al. (1980), to enable the comparison with (1977), and WEHMILLER and EMERSON (1980). ratios of Tioela and Chione (slow racemizing genera). Average Using different derivatization procedures, it has values andstandarddeviation aboutthe mean foreachgroup are been possible to determine the D/L values for as alsorepresentedonthis diagram. Symbols are as follows: Tioelo; many as seven amino acids in most of the samples single sample-solid circle; Saxidomus, single sample-solid square, multiple samples-square uithin square; and Chione, multiple studied here. D/ L data are reported in Table 1. samples-triangle within triangle. Generally, there is good agreement between the two

.Iournal of Coastal Research, Vol. 3, No. :~, 1987 300 Keenan ct al. acids that were not racemic in the oldest mollusks. be directly comparable to Tivela or Chione results. Valine and leucine form three apparent clusters of Valine differentiates the three populations more D/L data (Figure 2). Glutamic acid data are gen­ clearly than leucine because of its slower rate of erally consistent with these clusters but are more racemization. variable because of the systematic differences in The age represented by the lowest leucine-valine the results from the two analytical methods used group in Figure 2 (Group D is derived from the (WEHMILLER, 1984). Alanine, proline, phenyl­ isochron shown in Figure 3 (modified from alanine, and aspartic acid are all so extensively WEHMILLER, 1982). This isochron is drawn for one racemized (or analytically variable, as in the case of amino acid, leucine, and one genus, Protothaca, aspartic acid) that they are of little value in amino­ which is kinetically equivalent to Chione and Tivela, stratigraphic applications at these temperatures. and it represents the D/L data for the"early Stage The three well-resolved groups of leucine and 5," 120,000 year calibration localities between cen­ valine D/L values are plotted in Figure 2. The mean tral California and Magdalena Bay. The curve of the values and standard deviations for these three calibrated isochron demonstrates that D/L values groups are also shown in Figure 2. These mean D/L in age-equivalent samples increase with decreasing values, and the corresponding grou p designations, latitude and increasing temperature. The late are as follows: Group I, leucine, 0.660; valine, Stage 5 theoretical isochron also shown in Figure 3 0.508; Group II, leucine, 0.746; valine, 0.671; is derived from the leucine kinetic model of Group III, leucine, 0.878; valine, 0.860. Saxidomus WEHMILLER and BELKNAP (1978), using the 120,­ D/L values have been converted (by regression 000 year isochron to establish the kinetics (or effec­ equations in LA,JOIE et al., 1980) to ratios thatwould tive temperature, as discussed by WEHMILLER,

Washington are on California Baja California o 9 .m o 8 .n MB o 7 I BT a 6 PSR 7 0

l.J.J 0.5 Z t 0 -r-: VIZCAINO => 0.4 w PENINSULA ....J

....J 03 ...... 0 o 2

0.1

0 PRESENT MEAN ANNUAL TEMPERATURE, °c II 13.3 16 21.5

8 10 12 14 16 18 20 22 I I I I I

EFFECTIVE QUATERNARY TEMP. , °C Figure 3. Aminostratigraphiy and kinetic model interpretation of Pacific coast enantiometric data for Stage .S.All data are depicted as Protothaca D/L leucine values. The heavy line represents calibrated 120,000 year D/L data and the band laheled "Stage 5" shows the range of OIL values expected for terrace deposits formed during Stage 5. The lower isochron of the Stage [) hand depicts D/L leucine L'S. effective temperature (inversely proportional to latitude) as determined for D/L values with the kinetic model of WEHl\IILLER and HELK:-JAP (1978). Vizcaino Peninsula (Bahia Asuncion) sites are labeled with a solid circle. U-series solitary coral calibration localities are indicated by a solid sq uare and include Cayucos (C), San Nicholas Island (SND, Nestor Terrace (N) and Magdalena Bay (MB). Syrn bois for non-calibrated localities from Baja (open circle) are as follows: Camalu (em), Punta Santa Rosalillita (PSR), and Bahia Tortugas (HT). See the text for a more detailed reference to these localities. Modified from WEHMILLEH (19H2: Figure 4).

.Iournal of Coastal Research, Vol. ;), No. ;), 19H7 Amino Acid Dating :101

1982 and elsewhere) for a given latitude and tem­ are those at Camalu (VALENTINE, 1980), Punta perature range. Figure 3 can be used to determine if Santa Rosalillita (WOODS, 1980), Bahia Tortugas a D/L ratio group could be equivalent to 95,000 or (EMERSON et aL, 1980), and Magdalena Bay 120,000 years by making the assumptions: (1) (WEHMILLER and EMERSON, 1980). latitudinal gradients of effective temperature can The Group II leucine data plot above all the Baja be smoothly interpolated between calibration sites; results thought to represent any portion of Stage 5. and (2) effective temperatures for a local area can Because of the lack of calibration sites older than be considered equal (or nearly so) for samples of 120,000 years and the difficulty of extrapolating different ages but with the same present tempera­ the kinetic model to D/L leucine values as high as ture. Other pairs of isochrons can be drawn for the those in either Group II or Group III, we can only probable range of D/L data that could be observed tentatively propose an age of approximately 200,­ for samples deposited during Stages :3, 7, 9, etc. of 000 years (Stage 7) for the samples representing the marine isotope record by using the ages pro­ Group II. Elevation and morphologic characteris­ posed by SHACKLETON and OPDYKE (197:3) for tics of the Group II terrace suggest that it is older these stages. However, the use of the theoretical than the Group I terrace by a whole glacial/in­ isochrons for age assignments greaterthan 120,000 terglacial cycle (ORTLIEB, 1978, 19793, 1979b). years in Baja California, Mexico is currently limited Because the valine and glutamic acid D/L values for because the leucine kinetic model has not yet been the Group II samples are significantly greater than tested sufficiently in this region of relatively high those observed in nearby Stage 5 samples mean annual temperatures. (WEHMILLER and EMERSON, 1980; EMERSONet al., The Group I leucine data fall in the early Stage 5 1981), the Group II terrace must represent a pre­ portion of the isochron band shown in Figure 3. 120,000 event rather than Groups I and II rep­ Therefore, those data are interpreted to represent a resenting a late and early Stage 5 terrace time of deposition approximately 120,000 years complex. before present. Other Baja California sites that fall Group III plots above the Group II average and within the same region of the 120,000 year isochron can probably be assigned to either Stage 9 or Stage

Table 1. Amino acid cn antiomrtric (f)/IJ ratiox· [rom Tivela, Saxidomus, and Chione [rom Bahia Asuncion; Baj« Co lifurnia Sur. M exico.

Locality' Elevation Genus, sp. Sample Method* LEU (;LlJ PRO VAL ALA PHE ASP Group (m) no.

--- LRS:.!2 ;l Tiivla s. 7H-67 H (l.(,:W 0.620 0.740 0.510 0.910 0.810 I LR:)2[) 12 HO-2 H O.G89 O.G2G O.76!) O.S06 0.H;)5 0.818 0.679 I LR468 fi 80-2!) P O.7G8 0.7:)1 nd** 0.672 1.0:~O 0.689 0.660 II LR468 6 Sax. n. 80-1 H O.7!)7 0.782 0.909 0.678 0.822 0.882 0.688 II LR464 11 Tiveto s. 78-:)2 H n.7{i!) 0.70n 0.84:~ O.7:W 0.980 0.940 II LR4;")H 10 78-(-)0 H O.H80 0.870 0.900 1.070 0.960 III LR4fJ8 10 79-20 H O.R~J:~ 0.991 O.9f):~ 0.857 0.856 0.992 0.807 III LR4;")H 10 ~~'(1X n. 79-21 H O.tQH O.97:~ 0.900 0.862 O.8:H 0.97:~ 0.798 III LR461 12 Chione sp. HO-47 P O.H9:~ O.RS7 nd 0.891 0.898 0.748 III LR461 12 HO-48 P 0.894 O.82S nd O.H76 0.980 0.716 III LR461 12 HO-!)O H 0.894 0.88:) O.9Hj 0.791 0.916 0.960 0.79;) III LR527 12-1!) 80-41 P O.~n4 0.8S0 nd 0.907 1.020 0.784 0.748 III LRC)27 12-1f) 80-;)!) p O.8S:) 0.77:l nd 0.801 0.987 0.96~) 0.721 III LR!)27 12-1;) Sax. n. 80-40 P O.9!)8 0.798 nd 0.879 1.014 0.8;)] 0.705 III LR5:)() If) Tiivta s. 80-!)9 H O.8!)8 O.H2R 0.940 0.862 0.99S 0.989 0.861 III LR470 :Hl 7H-S() H 0.870 0.870 o.sro 0.895 0.960 1.000 III LR4S;l :t2 Sax. n. 80-8 H 0.R7;) 0.79;) 0.911 0.72:1 0.954 0.899 0.760 TIl LR4fl:~ :l2 RO-9 P O.9();~ 0.875 nd 0.882 I.nOO 0.852 0.805 III

*Methods: H---- (+)-2-butv I derivat ive P-isoprop~:l derivative; D/L leucine values listed have beenconverted to equivalent butanol leucine values using the equation of WEHMILLEJ{ and EM":I{SO' (1980). **nd-not determined LEU-leucine; GL lJ-glutamic acid; PRO-proline; VAL-valine; ALA-alanine; PHE-phenylalanine;ASP-aspartic acid Tiuela s. - Tirela stultorum

Sax n.-sSaxidomus nuttalli

.Iournal of Coastal Research, VoL :~, No. ;~, 1987 302 Keenan et al:

11 of the marine isotope record (Figure 3). There­ from the higher, 200,000-year deposit or both fore, we propose that the age of the samples in this LR464 and LR468 are penecontemporaneous an d group is at least 300,000 years. In most cases the the aminostratigraphic data cannot resolve small aminostratigraphic groups recognized here are con­ age differences between what appear (from D/ L sistent with the relative ages that could be inferred data alone) to be the Group 11,200,000 year, terrae: from local elevation differences between terrace deposits. sites. However, it is clear that elevation differences Localities LR458 and LR461 (Figure 1 and Plat, alone do not correspond to the aminostratigraphic 1) both fall into aminostratigraphic Group III groups when comparisons are made in regions of (Figure 2 and Table 1), yet they are stratigraph­ faulting or over distances of a few kilometers. ically separated by alluvial sediments that coul d The most rigorous test of the relationship be­ represent either one full glacial stage or substage tween aminostratigraphic data and the morpho­ (Plate 1). It appears that the leucine and valin e stratigraphy of the terrace sites is found at sites enantiomeric ratios simply cannot resolve the age LR464, LR470, and LR468 (Figure 1 and Plate 1). A difference (which could be as much as 100,000 sample from the 11 meter terrace (LR464) falls into years) between these samples. There is a sugges­ Group II(Figure2; Table 1) while a sample from the tion that the glutamic acid data indicate some age 30 meter terrace (LR470) (Figure 2; Table 1) falls difference. Clearly, the high effective temperature" into Group Ill, thus demonstrating consistency be­ at these latitudes limit the utility of amino­ tween relative aminostratigraphic and geomorphic stratigraphic methods in resolving age differences ages. Two samples from LR4 68, a depositional sur­ to about the last 250,000 years. face at6 meters on the flank of the 11 meter terrace, In this area of the Pacific coast of Baja California, also fall into aminostratigraphic Group II (Figure 2; amino acid racemization dating gives better geo­ Table 1), even though geomorphic relationships chronological results than U- series methods imply a younger age (Stage 5) for the LR468 site (ORTLIEB, 1982). Mollusks are the most common (ORTLIEB, 1978, 1979a). Either the ageofLR468 is fossil material in marine deposits of this area and . 120,000 years and the shells have been reworked although suitable for amino acid analysis, they can

r I MARINE ! DEPOSITS 30 CONTINENTAL 30 DEPOSITS PRE­ PLEISTOCENE

I I 20 20 I I f I 10 l'a I I ------~ O M I

Plate l. Cross sections at localities with Quaternary ma rine deposits showing stratigraphic relationships. differences in elevation abo", present mean sealevel, and amino acid assignments (in parenthesis). Modified from OHTLIEIl (1978): Figure7 (LR464 and 468) and F'igur. 10 (LR458-461 and LR530-527); and from ORTLlF.1l (l979a): Figure 4 (LR453, LR470-LR464-LR468 and LR458-46J) . Amino Acid Dating be controversial when used for U-series dating. Institute de Geologia, Universidad Nacional Amino acid racemization dating provides internally Autonoma de Mexico. Part of the field studies has consistent results which, in most cases, are in agree­ been completed in collaboration with V. Malpica ment with morphostratigraphic interpretations. (Inst, Geol., U.N.A.M.). This paper was presented Also, amino acid racemization dating can dis­ at the symposium "Neotectonis and Sea Level tinguish samples of the last two interglacials (Stages Variations in the Gulf of California Area" that was 5 and 7) from older samples (Stage 9 and older). held in Hermosillo, Mexico (April, 1984) and a pre­ Four of the samples examined in this study have liminary version appears in a symposium volume of been submitted for Uranium series dating. Two of the same title. these (LR459=458 and LR5:30) had an excess of Ionium while the othertwo yielded apparent ages of LITERATURE CITED 2:35,000 +40,000/-:30,000 years (LR522) and greater than or equal to 400,000 years (LR470) BERNArr, M.; GAVEN, C. and ORTLEIB, L., 1980. (ORTLIF.B et al., 1984). Radiometric ages cannot be Datation de depots littoraux du dernier Interglaciaire (Sangamon) sur la cote orientale du Golfe de California, calculated for the first two localities because of the Mexique. Bulletin de la Societe Geologique de Prance excess of Ionium due to contamination. These two 7(XXII): 219-224. localities (LR458 and LR5:l0) fall within amino­ BLOOM, A. L.; BROECKER, W. S.; CHAPPELL, /1. M. stratigraphic Group III (Figure 2; Table 1). T'he lJ/ A.; MA'l'THEWS, R. K. and MESOLELLA, K. J., Th apparent age of the third locality, LR522, sug­ 1974. Quaternary sea level fluctuations on a tectonic coast: 2:)(J T'h!234 LJ dates from the Huon Peninsula, gests that it corresponds to the Stage 7 high sea New Guinea. Quaternary Research 4: 18fl-'20fl. stand but both the morphostrat.igraphic interpreta­ EMERSON, W. K.; KENNEDY, G. L.; WJ1~HMILLER; tion and the aminostratigraphy of LRI>22favor a Stage ,1. F. and KEENAN, E., 1981. Age relationships and 5 age (120,000 years). The last locality, LR470, may zoogeographic implications of late Pleistocene inverte­ brate faunules, Turtle Ray, Baja California Sur, Mexico. be considered to be a minimum of400,000 years old Nautilus 95: 105-116. by Uranium-series dating which agrees with the FRANK, H. G.; NICHOLSON, ~J. and HAYER, E., 1977. minimum amino acid age of :~OO,OOO years. Rapid gas chromatographic separation of amino acid cnantiomers with a novel stationary phase. Journal of CONCLUSIONS Chromatographic Science 15: 174-176. KARROW, P. F. and BADA, ~J. L., 1980. Amino acid ra­ cemization dating of Quaternary raised marine terraces in Amino acid racemization dating methods have San Diego County, California Geology, 8: 200-204. been applied to Quaternary marine terrace local­ KAUFMAN, A.; BROECKER, W. S.; KLJ, T. L. and ities (with elevations between :3 and :-{o meters) at THURBER, D. L., 1971. The status of U-series methods of mollusk dating. Geochimica et C08­ Bahia Asuncion, Baja California Sur. Three amino­ mochimica Acta :35: 1155-118:3. stratigraphic age groups have been recognized, the KENNEDY, G. L.; LA~JOIE, K. R. and WEHMILLER, ,1. youngest of which appears correlative with Ll-series F., 1982. Aminostratigraphy and faunal correlations of dated 120,OOO-year terrace localities at higher and late Quaternary marine terraces, Pacific coast, U. S. A. lower latitudes. The older aminostratigraphic Nature 299: 54fl-547. KU, T. I.... and KERN, ,1. P., 1974. Uranium-series age of groups probably represent ages of about 200,000 the upper Pleistocene Nestor Terrace, San Diego, years and 300,000 years (or older). These apparent California. Geological Society of America Hulletin 85: ages are generally consistent with geomorphic 17 1 ;~ -17 16. criteria for relative terrace ages and with local KVENVOLDEN, K. A.; PETERSON, E., and POL­ LOCK, G. E., 1972. Geochemistry of amino acid enan­ stratigraphy. The high effective temperature for tiomers: gas chromatography of their diastereomeric the central and southern Baja California region derivatives. In: von Gaertner, H. and Werner, H. (eds.), limits the utility of amino acid racemization dating Advances in Organic Geochemistry, 1971. :387-401. but it can be used to define ages representing either Oxford: Pergamon Press. ~J. Stage 5 (early and late) or pre- Stage 5 depositional l..AJOIE, K. R.; KERN, J. P.; WEHMILLER, F.; KEN­ NEDY, G. L.; MATHIESON, S. A.; SARNA­ events without much difficulty. WO~JCICKI,A. ~1. ; YERKES, R. F. and MCCRORY, P. A., 1979. Quaternary shorelines and crustal deforma­ ACKNOWLEDGEMENTS tion, San Diego to Santa Barbara, California. In: Abbott, P. L. (ed.), Geological Excursions in the Southern Califor­ nia Area, 3-15. San Diego: San Diego State Univ. The field work and collection of samples have LAJOIE, K. R., WEHMILLER, ~J. F. and KENNEDY, G. been completed through a cooperative program L., 1980. Inter- and intrageneric trends in apparent (GEOCORTEZ) ofO.R.S.T.O.M. (France) and the racemization kinetics of amino acids in Quaternary

.Iournal of Coastal Research, Vol. :~, No. :~, 1987 Keenan dol.

mollusks. In: Hare, P. E., Hoering, T. C. and King, K. /Jr. vol.tz): 229. (eds.), Biogeochemistry of Amino Acids. ;~05-:340. New ORTLIEB, L.; CARRO, O. and CAUSSE, C., 1984. Don­ York: John Wiley. nees radiochronologiques U/Th de t errasses marines de MALPICA-CRUZ, V., 1980. Etude de depots littoraux la cote occidentale de Basse Californie Mexique. In: pleistocenes du Sud-ouest de la Basse Californie, Mexi­ Malpica-Cruz, V.; Celis-Gutierrez, S.; Guerrero-Garcia que. 3rd cycle thesis dissert., unpubl., 131 p. Bordeaux: S., and Ortlieb, L. (cds.), Neotectonics and Sea Leuel Univ. Bordeaux I. Variations in the Gulf of Califcmiia Area, a Symposium. MALPICA-CRUZ, V., CELIS, S. and ORTLIEB, L., 19H1. 22G-240. Mexico, D. F: Univ. Nal. Auton. Mexico, Caracteristicas diageneticas de depositos litorales pleis­ Inst. Geologia. tocenicos en la costa occidental de Baja California Sur, SHACKLETON, N. ~J. and OPDYKE, N. D., 197:L Mexico. Aetas Va Reunion Grupo espanol de trabajo del Oxygen isotope and paleomagnetic stratigraphy of Cuaternario, 151-161. Sevilla: Univ. Sevilla. equatorial Pacific core V2:~-2:~8: Oxygen isotope tem­ MILLER, G. H. and HARE, P. E., 1980. Amino acid peratures and ice volumes on a 10;) and 10 l; year scale. geochronology: Integrity of the carbonate matrix and (iualernary Research :~: ;~~)-ss. potential of molluscan fossils. In: Hare, P. E., Hoering, TROUG H'r'ON, G. H., 1974, Stratigraphy of the Vizcaino T. C., and King, K. /Jr. (eds.), Biogeochemistry ofAmino Peninsula, near Asuncion Bay, Territorio de Baja Acids, 415-444. New York: .Iohn Wiley. California, Mexico. M. S. thesis dissert., unpubl., p.8:-L MUHS, D. R., 19B:). Quaternary sea-level events on San Diego: San Diego State l Iniv. northern San Clemente Island, California. Quaternary VALENTINE, ~J. W., 1980. Camalu: A Pleistocene Research 20: 322<~41. terrace fauna from Haja California. Journal ofPaleontol­ MUHS, D. R. and Szabo, H. ~J., 1982. Uranium- series age ogv G4: 1;~10-1;~IH. of the Eel Point terrace, San Clemente Island, Califor­ VEEH, H. H. and VALENTINE, /J. W., 1967. Radiomet­ nia. Geology 10: 23-26. ric ages of Pleistocene fossils from Cayucos, California. OMURA, A., EMERSON, W. K. and KU, T. L., 1979. Geological Socie(v 0/ America nul/din 78: {)47 -5;")0. lJranium-series ages of echinoids and corals from the WEHMILLER, ~J. F" 19H2. A review of amino acid upper Pleistocene Magdalena Terrace, Baja California racemization studies in Quaternary mollusks: Strati­ Sur, Mexico. Nautilus 9:~: 184-189. graphie and chronologie applications in coastal and ORTLIEB, L. 197B. Reconocimiento de las terrazas interglacial sites, Pacific and Atlantic coasts, United marinas cuaternarias en la parte central de Baja Califor­ States, United Kingdom, Haffin Island, and Tropical nia. u-«. NaL Auton. Mexico, lnst. Geol., Rev. 2(2): 200­ Islands. Quaternary Science Heviell',...,· 1: H:~-1:20. 211. WEHMILLER, ,I. W., 1~)84. Interlaboratory comparison ORTLIEB, L., 1979a. T'errasses marines dans Ie nord­ of amino acid enantiomeric ratios in fossil Pleistocene ouest mexicain: Etude au long d'une transversale entre mollusks. (/ualernary Research 22: 109-120. la cote Pacifique et le Sonora en passant par la Peninsule WEHMILLER, ,1. F, and BELKNAP, D. F., 1978. Alter­ de Basse-California, In: Suguio, K. et al. (eds.), Pro­ native kinetic models for the interpretation of amino ceedings ofthe 1978 International Symposium on Coastal acid enantiomeric ratios in Pleistocene mollusks: exam­ Evolution in the Quaternary, 45;~-474. Sao Paulo, ples from California, Washington, and Florida. Quater­ Brazil. nary Research 9: :~:~O-:~48. ORTLIEB, L., 1979b. Quaternary marine terraces in the WEHMILLER, .J. F., and EMERSON, W. K., 1980. southwestern Vizcaino Peninsula, Baja California Sur, Calibration of amino acid racemization in late Pleis­ Mexico. In: Abbott, P. L. and Gastil, R. G. (eds.), Baja tocene mollusks: results from Magdalena Bay, Baja California Geology, Field Guides and Papers. 89-9:1. San California Sur, with dating applications and paleo­ Diego: San Diego State Univ. climatic implications. Nautilus 94: ;~ 1-:~6. ORTLIEB, L., 1979 c. Quaternary shorelines around Baja WEHNIILLER, ,I. F.; LA~JOIE, K. R.; KVENVOLDEN, California Peninsula Mexico: Neotectonic implications. K. A.; PETERSON, E.; BELKNAP, D. F.; KENNEDY, Geological Society of America Abstracts u.itli Programs G. L.; ADDICOTT, W.O.; VEDDER, .J. G. and 11:490. WRIGHT, R. W., 1977. Correlation and chronology of ORTLIEB, L., 1980. Neotectonics from marine terraces Pacific coast marine terraces of continental United along the Gulf of California. In: N.-A. Marner, (ed.) States by amino acid stereochemistry-technique Earth Rheology: Isostasy and Eustasy, Proceedings of evaluation, relative ages, kinetic model ages, and Earth Rheology and Late Cenozoic Isostatic Move­ geologic implications, 19Gp. U. S. Geol. Survey Open ments Symposium (Stockholm, 1977), 497-504. New File Report 77-680. York: Wiley-Interscience. WOODS, A. ~J., 1980. Geomorphology, deformation, and ORTLIEB, L., 1982. Geochronology of Pleistocene ter­ chronology of marine terraces along the Pacific coast of races in the GulfofCalifornia region, northwestern Mex­ central Baja California, Mexico. Quaterna1)' Research ico. XI Congress INQUA (Moscow, 1982), abstr, 1:-3: ;~ 46-:~64.

o RESUMEN 0 Se hautilizado una determinacion genealogica de aminoacidos (amino acid recernizat ion, AAP) para estimarlas epocas de los moluscos de las terrazas marinas Cuaternarias en el area de la Bahia Asuncion, en la costa Pacifica de la Baja Califor­ nia. Se ha analizado dieciocho muestras de moluscos (del genero Tioela: Saxulomus y Chione) de diez Jocalizaciones diferentes. La alta temperatura media anual de la region (mayor de 2(t C) ha dado lugar a una variada genealogia en las muestras consideradas como pertenecientes a terrazas del lejano Medio y allejano Pleistoceno. La variacion geneal6gica de la mayoria de los arninoacidos queda completada efectivamente alrededor de hace ;~OO.OOO anos. Sin embargo, dos

.Iournal of Coastal Research, Vol. :l. No.4, 19B7 Amino Acid Dating

aminoacidos, leunina y valina, muestran dernasiada variahilidad como para ser usados para dcterminar los diferentes grupos de edades entre las localidades de las t errazas, Cuando estos grupos son comparables con evidencias estratigraficas 0 geomorficas, son generalmente consistentes con el control geologico disponible. Las edades es­ timadas para los tres grupos aminoest ratigraficos reconocidos es este estudio son aproximadamente 120.000, 200.000 y ;~OO.OO{) aI1os.--Miguc! A" Losada. l lniucrsitlad til' Santander, Santander. Spain

D ZUSAMMENFASSUNC; D Aminsaure-Racernisierendatierung wird zur Schatzung des Alters von Mollusken benutzt; 18 Molluskespezies (von del' Genera Tiocla. Saxidomus und Chione) wird in Quartargelandestufen in del' Nahe von Bahia Asuncion (Pazifische Kuste von Californie) gefunden; die Proben entstehen von 10 Orte. Die hohe mittlere .Iahrestemperatur des Ge bietes (li bel' 20° C) hatte aussetgewohnliche Racemisierung del' Proben bewirkt; diese Proben sind bekannt, aus spateren Mitt.elholozan- und Spatholozanorte entstanden Zll haben. Obwohl meiste Aminsaure nach :100.000 .l ahre vollig racemisiert wird, dernonstrieren zwei von diesel' Saure-Leuzin und Walin- geniigende Auf­ losungskraft, den Entwurf verschiedenen Zeit gruppen unter den Gelandestufen zu crmoglichen. Insofern als diese scheinliche Gruppen pruf'har mit stratigraphischern oder geomorphologischem Zeugnis sind, ubereinstim men die Gruppe mit verfugbarer geologischcr Kontrolle. Die schatzt.e Alter del' drei erkennten aminstrati­ graphischen Gruppen diesel' Forschung sind ungefahr 120.000, 200.000 und :HlO.OOO -Iahre.>- ...Stephen A. Murdock. Cr-,'RF, Cha rlottcsuiItc, Virginia lISA

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