-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.