Amino Acid Racemization in Ostracodes 

from Bear Lake Cores BL96-1 and BL96-2, 

Utah and Idaho 

By Darrell S. Kaufman 

Open-File Report 2005-1129 

U.S. Department of the Interior 

U.S. Geological Survey

 1 
U.S. Department of the Interior 

Gale A. Norton, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

U.S. Geological Survey, Reston, Virginia 2005 

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Suggested citation: 

Kaufman, D.S., 2005, Amino acid racemization in ostracodes from Bear Lake cores BL96-1 and BL96-2, Utah and Idaho: U.S. 

Geological Survey Open-File Report 2005-1129, 8p. 

Prepared by the U.S. Geological Survey in Denver, Colorado (http://climchange.cr.usgs.gov/) 

2 
Amino Acid Racemization in Ostracodes 

from Bear Lake Cores BL96-1 and BL96-2, 

Utah and Idaho 

By Darrell S. Kaufman 

Introduction 

The ostracode-rich sediment in Bear Lake cores provides an opportunity to evaluate the 

utility of amino acid racemization for geochronology in a stratigraphically and thermally 

controlled environment. Because dating lake cores can be difficult for sediments that lack 

macrofossils or are beyond the range of 14C dating, amino acid geochronology may serve as a 

useful chronostratigraphic tool in future studies ( McCoy, 1988). This new application of amino 

acid geochronology is now feasible because the technology needed for routine separation of the 

right-handed (D) and left-handed (L) forms in microgram quantities of shell has recently become 

available (Kaufman and Manley, 1998). 

Ostracodes are millimeter-size microcrustaceans that secrete a carbonate exoskeleton with 

a relatively high concentration of proteins. Their shells often are present in sediment cores, 

allowing amino acid geochronology to be integrated into the study of lake cores. Unlike 

materials collected from emerged deposits in terrestrial settings, the temperature history of 

ostracodes recovered in sediment cores taken from below the thermocline of stratified lakes can 

confidently be assumed to have remained close to 4° C, the temperature of maximum-density 

fresh water. Because the rate of transformation of amino acid enantiomers from their L- to Dconfiguration (racemization) is dependent upon temperature, a stable and predictable temperature 

history is essential to deriving ages. 

This study focuses on aspartic acid1

 (Asp) and glutamic acid (Glu) in the common 

ostracode genus Candona. These are among the most abundant amino acids in ostracode shell 

protein (Kaufman, 2000), which reduces sample-size requirements. They also span the range of 

amino acid racemization rates. Aspartic acid is among the most rapidly racemizing amino acids, 

which increases the temporal resolution of the technique (for example, Goodfriend, 1992), 

despite the relatively low ambient temperatures of bottom-water sites. Glutamic acid is one of 

the most slowly racemizing amino acids (Goodfriend and Meyer, 1991), which suggests that it is 

well suited to date old material from deep cores. Furthermore, Asp and Glu D/L ratios are the 

most consistently well-resolved chromatographically, with analytical precision of ~2% (Kaufman 

and Manley, 1998). Candona is the most common freshwater ostracode in Bear Lake and is 

present in many lakes. Because the rate of racemization is somewhat taxon dependent (although 

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the rates measured in several taxa are indistinguishable, Kaufman, 2000), using a cosmopolitan 

genus helps ensure the applicability of these results to other lakes. 

Methods 

Ostracodes were recovered from two ~5-m-long gravity cores from ~40 and ~50 m depth 

in Bear Lake, Utah and Idaho. The shells were prepared according to standard procedure 

(Kaufman, 2000), and the enantiomeric amino acid composition was analyzed for 343 

subsamples of ostracodes using reverse phase HPLC (Kaufman and Manley, 1998). From 3 to 

34 subsamples were analyzed from each of 26 levels in the cores, plus an additional 24 

subsamples of modern Candona shells, collected live. About one-third of the subsamples 

comprised ~10–40 individual shells and weighed 0.1–0.2 mg. Following the analysis of these 

samples, new refinements to the analytical technique (Kaufman, 2000) enabled accurate results to 

be obtained from a single ostracode shell (~5 μg of shell per injection), with detection in the 

subpicomole range. The D/L ratios measured in the single shells are indistinguishable from those 

in multiple-shell preparations. In all, data from 45 subsamples (13 percent of data) were rejected 

because their amino acid composition showed evidence of contamination by modern amino acids 

(high concentration of serine relative to glutamic acid, see Kaufman, 2000). The data reported 

here (table 1) are the mean values of multiple subsamples from each level. 

Results 

The results show a monotonic increase in Asp and Glu D/L with increasing depth, with 

higher D/L ratios for Asp compared to Glu (fig. 1). The D/L ratios in the uppermost samples are 

Figure 1. Extent of racemization expressed as the ratio of right-handed (D) to left-handed (L) 

forms of (A) aspartic acid (Asp), and (B) glutamic acid (Glu) in Candona from Bear Lake cores 

BL96-1 and BL96-2. Error bars are ±1σ. Data are listed in Table. 

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Table 1. Amino acid results. Aspartic acid (Asp) and glutamic acid 

(Glu) D/L ratios measured in Candona from Bear Lake cores and in 

modern Candona collected in springs. 

Note: Nearly all samples were made up of mono-specific subsamples 

(either multiple or single shells). The most common species analyzed was 

C. spp. 1 shown in Bright and others (2005). In a few cases, shells of two 

other undocumented species of Candona were analyzed (C. sp. 2 and 3). 

On the basis of results available to date (Kaufman, 2000; and Kaufman, 

unpublished data), the D/L ratios measured in different species of Candona 

are indistinguishable. 

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higher than for modern shells, suggesting that sediment was lost from the core top. The higher 

ratios at the surface of Core BL96-2, and the more rapid increase in D/L with depth, are 

consistent with the 14C ages for these cores (Colman and others, 2005). The uncertainty in the 

D/L ratios, as determined by the intershell variability, increases with sample depth. The higher 

standard deviations for the older shells might reflect the natural variability associated with 

progressive diagenesis. Small initial differences in the diagenetic reaction network might result 

in increasingly larger differences in the extent of racemization with time. Alternatively, the 

extent of sediment mixing by bioturbation might have decreased through the Holocene. Because 

the sediment is massive throughout the core, we have no evidence to exclude reworking. 

Fig. 2. Monthly temperature of Bear Lake bottom water and air temperature at Lifton, Idaho. 

Lake temperature data from Wurtsbaugh and Luecke (1995, and written communication); air 

temperature data from Utah State University Climate Center. Note the break in the time series 

between 1988 and 1994. 

Discussion 

Once these data are combined with the 14C age model for these cores, they will be used to derive 

a calibrated age equation for racemization in Candona at the temperature of Bear Lake bottom 

water. Water temperature of Bear Lake was measured during 6 years, between 1983–88 and 

1994–1995 (Wurtsbaugh and Luecke, 1995, and Wurtsbaugh and Luecke, written 

communication). Abyssal water from 40 to 60 m (the lake bottom) during 5 of 6 years fluctuated 

in the narrow range of 3.0° to 5.5°C, with a mean annual temperature of 4.6 ± 0.7°C (fig. 2). 

This compares to air temperatures at the north end of the lake that typically experienced seasonal 

temperature fluctuations of 30°C (based on mean monthly records obtained from the Utah State 

University Climate Center) (fig. 2). Stormy conditions during spring turnover of 1985 caused 

relatively warm water (up to 8°C) to penetrate the hypolimnion and reside at the lake bottom 

until late fall. Because the amplitude of seasonal and interannual temperature fluctuations is 

typically <±1°C at the depth of the core sites, the difference between the arithmetic mean annual 

temperature and the effective diagenetic temperature is negligible (Miller and Brigham-Grette, 

1989). Furthermore, the size of temperature fluctuations sensed in the subbottom sediment 

generally is less than at the that of the overlying water because: (1) the sediment/water mixture 

that blankets some lake bottoms tends to isolate the lake bottom from temperature changes, 

6 
however minor, in the main body of the bottom water (for example, Pugh, 1977); and (2) the 

temperature wave is attenuated exponentially with depth below the lake bottom (for example, 

Wang and others, 1986). Upon deeper burial, to many tens of meters, thermal conditions become 

increasingly controlled by the geothermal flux (for example, Katz and others, 1979; Bada and 

Man, 1980). 

Conclusion 

The data presented in this study demonstrate the feasibility of using amino acid 

racemization in ostracodes for geochronology. The sample size required for an analysis is an 

order of magnitude less than for AMS 14C dating and presents a new opportunity to derive ages 

for deposits that are organic poor or suffer from large 14C reservoir effects. Because ostracodes 

are common in subaqueous depositional settings, the technique can be integrated into studies of 

lake sediment cores. These environments are ideally suited for amino acid geochronology 

because, unlike materials collected from emerged deposits, the temperature history of ostracodes 

recovered in sediment cores from deep lacustrine and marine settings can be estimated 

confidently within narrow limits. 

Footnote 

1

During laboratory hydrolysis under ~6 M HCl, any asparagine (Asn) in fossil proteins 

undergoes deamidation to Asp (Zhao and others, 1989) and glutamine (Gln) is transformed to 

Glu. The Asp and Glu results might, therefore, include some component of Asn and Gln, which 

was converted during sample preparation. 

Acknowledgements 

Jordon Bright and Rick Forester identified the ostracodes; Jordon Bright prepared the 

samples for amino acid analysis and assisted in the laboratory. Funding was provided through 

National Science Foundation grant EAR-9896251 and the Earth Surface Dynamics Program of 

the U.S. Geological Survey. 

Literature Cited 

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paleotemperature and heat flow determinations: Earth Science Reviews, v. 16, p. 21–55. 

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BL96-2 from Bear Lake Utah and Idaho, U.S. Geological Survey Open-File Report ####. 

Colman, S., Kaufman, D., Rosenbaum, J.G., and McGeehin, J.P., 2005, Radiocarbon dating of 

cores collected in Bear Lake, Utah and Idaho, Geological Survey Open-File Report ####. 

7 
Goodfriend, G.A., 1992, Rapid racemization of aspartic acid in mollusc shells and potential for 

dating over recent centuries: Nature, v. 357, p. 399–401. 

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Geochemistry: Oxford University Press, New York, p. 145–160. 

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