Enhanced Age Resolution of the Marine Quaternary Record in the Arctic Using Aspartic Acid Racemization Dating of Bivalve Shells

QUATERNARY RESEARCH ARTICLE NO.

45, 176–187 (1996)

0018

Enhanced Age Resolution of the Marine Quaternary Record in the Arctic Using Aspartic Acid Racemization Dating of Bivalve Shells GLENN A. GOODFRIEND Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, DC 20015-1305

JULIE BRIGHAM-GRETTE Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003-0026 AND

GIFFORD H. MILLER Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0450 Received July 13, 1995

Aspartic acid (Asp) racemization occurs at a significantly higher rate than isoleucine epimerization and consequently provides better temporal resolution of Arctic marine deposits (from Alaska, Spitsbergen, and Baffin Island). Heating experiments (at 1007C) on the bivalves Mya and Hiatella show the Asp racemization rate decreases with increasing D/L values, as is typical for biogenic carbonates. Based on these experimental racemization rates and rates determined from racemization of samples radiocarbon dated to ca. 10,000–12,000 yr B.P., activation energies for Mya and Hiatella are estimated to be 30.6 and 30.0 kcal/mol, respectively, for Asp racemization, and 29.0 and 29.5 for isoleucine epimerization. Analysis of a time series of Plio–Pleistocene Hiatella from the north coast of Alaska shows that last-interglacial mollusks can be readily distinguished from modern samples by Asp but not by isoleucine. D/L Asp values indicate a younger age for the Fishcreekian transgression than does isoleucine epimerization. For Spitsbergen, D/L Asp shows a slight age difference (ca. 12,000 yr) between two units of the ‘‘episode B’’ interstadial and suggests that the age of these units may be closer to 65,000 than to 80,000 yr B.P., two possible ages suggested by other evidence. The age of the Loks Land Interstadial on Baffin Island is likely to be greater than that indicated by radiocarbon ages. Within deposits from each region, D/L Asp values are less variable among individual shells than isoleucine epimerization values. This may indicate better reliability of Asp for geochronology. q 1996 University of Washington.

INTRODUCTION

Amino acid racemization dating has been used extensively for analysis of the chronology of Quaternary marine deposits in the Arctic. These studies have generally measured the epimerization of isoleucine (conversion of L-isoleucine to Dalloisoleucine) in mollusk shells (e.g., Miller, 1982, 1985;

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Miller et al., 1977, 1992; Miller and Hare, 1980; Boulton et al., 1982; Kaufman, 1992; Kaufman and Brigham-Grette, 1993; Brigham-Grette and Hopkins, 1995) or in Foraminifera (Sejrup and Haugen, 1992). However, a significant problem with this method is that, because of low ambient temperatures in the Arctic, isoleucine epimerizes quite slowly and consequently does not provide good temporal resolution for younger samples. For example, on the very cold north coast of Alaska, modern and last-interglacial samples are only marginally distinguishable on the basis of isoleucine epimerization (Brigham and Miller, 1983). One commonly employed solution to this problem is to measure the A/I (D-alloisoleucine/L-isoleucine) values of the free amino acid fraction (Miller and Hare, 1980; Miller, 1982, 1985; Miller et al., 1977; Boulton et al., 1982). This fraction has higher A/I values than the total amino acids obtained from hydrolysis of samples in the laboratory and thus improves temporal resolution. However, this approach fails where increased resolution is most needed—in samples with very low A/I values (net epimerization of less than ca. 0.02 in the total amino acids). In such samples, natural hydrolysis has not proceeded far enough to produce amounts of free amino acids sufficient for analysis. The free amino acid fraction is also likely to be more susceptible to leaching. An alternative approach to this problem is to analyze samples for other, faster racemizing amino acids, such as aspartic acid (Asp). Amino acid racemization analyses of late Holocene samples from warm regions have shown that, at low D/L values, Asp racemizes much faster than isoleucine (Goodfriend, 1992; Goodfriend et al., 1992, 1995; Goodfriend and Stanley, 1996). The same can be expected for very cold areas, where even relatively old shells show little isoleucine epimerization. However, until now, little informa-

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tion on Asp racemization in mollusks from cold regions has been available. Rutter et al. (1980) measured D/L Asp values (but not isoleucine epimerization) in a series of freshwater mollusk samples from the Old Crow Basin in the northern Yukon Territory of Canada. Shells of various bivalve and gastropod species and mixed-species samples from below the Old Crow tephra (dated at ca. 140,000 yr B.P. by fission track; Westgate et al., 1990) showed D/L Asp values in the range of ca. 0.20–0.40. Similar values were also found in overlying late Pleistocene sediments having radiocarbon ages ranging from ca. 30,000 yr B.P. to beyond the limit of radiocarbon dating (ú Ç50,000 yr B.P.). These D/L Asp values were reportedly higher than those measured for other amino acids. The present study investigates the use of Asp racemization for improving upon the resolution of the chronology of Arctic marine deposits provided by isoleucine epimerization. Analyses were carried out on time series of samples (the bivalves Hiatella arctica and Mya truncata) from three areas: northern Alaska, Baffin Island, and Spitsbergen (Fig. 1). In addition, the latitudinal gradient in D/L Asp values for last-interglacial samples from northern to southwestern Alaska is documented. These taxa are widespread in Arctic marine deposits and are the taxa most commonly used in aminostratigraphic studies in the region. Most of these sample series had previously been analyzed for isoleucine epimerization.

One of the major questions concerning the chronology of the Spitsbergen sequence is the age of episode B (summarized in Miller et al., 1989). Radiocarbon ages range from finite ages in the range 30,000–45,000 yr B.P. for shell carbonate to ú61,500 yr B.P. for whalebone collagen. U– Th ages of bone appear to be reliable and place the age at ca. 60,000–65,000 yr B.P. TL ages place the deposits at ca. 60,000–80,000 yr B.P. The low d18O value of forams points to a very warm period. But according to the SPECMAP chronology (Imbrie et al., 1984), 65,000 yr B.P. represents a full glacial phase and thus a low sea stand; a relatively warm phase (with low d18O values) occurs only at 80,000 yr B.P. (substage 5a). Furthermore, it is expected that mainly high sea stands, associated with warm phases, will be represented in onshore sections. Miller et al. (1989) concluded that the age of this phase is most likely in the range of 70,000 { 10,000 yr B.P., which does not resolve the question of whether a glacial or interstadial phase is represented. A similar problem exists in the Baffin Island sequence: the age of the Loks Land Interstadial (Miller, 1985). This unit is represented in Frobisher Bay (SE Baffin Island) by nonglacial marine sediments containing an in situ mollusk fauna. Radiocarbon analyses of bivalve shells gave finite 14C ages of ca. 32,000 to 41,000 yr B.P. Isoleucine epimerization analysis of Mya shells gave values slightly higher than (but not statistically separable from) those contained in overlying till radiocarbon dated to ca. 10,000 yr B.P. (Miller, 1985; Miller and Kaufman, 1990) and slightly lower (õ0.01) than shells in the last-interglacial Kogalu Formation at the Clyde Foreland to the north. It was hoped that the use of Asp racemization might better resolve the age of the Loks Land Interstadial in relation to these other units and allow evaluation of the reliability of the radiocarbon ages. Heating experiments were carried out to establish the form of racemization kinetics. The rate of aspartic acid racemization typically decreases with time as D/L values increase, so that plots show a convex upward trend, with the degree of convexity of the trend varying among different mollusk taxa (Goodfriend, 1992; Goodfriend et al., 1995) as well as among different types of samples, such as corals (Goodfriend et al., 1992) and ostrich eggshells (Goodfriend and Hare, 1995). The value of heating experiments is that they provide constant temperature conditions, so that the intrinsic form of racemization kinetics can be determined. In natural series, the form of the kinetics is complicated by changes in temperature and differences in the temperature histories of samples making up the series. By placing samples under relatively high temperatures in the laboratory (in this case, 1007C), the racemization rate is greatly increased, so that the racemization reaction can be followed up to the relatively high D/L values found in natural samples in the age range of 104 –106 yr. Establishment of the form of racemization kinetics allows transformation of D/L values to values that are linearly related

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FIG. 1. Map showing the locations of samples analyzed in this study. A Å Nome, AK; B Å Cape Espenburg, AK; C Å Kuk Inlet, AK; D Å Skull Cliff, AK; E Å Barrow, AK; F Å Clyde Forelands, Baffin I.; G Å Loks Land, Baffin I.; H Å s. Maine; I Å Brøggerhalvøya, Spitzbergen; J Å Tromsø, Norway.

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Samples from the Arctic Coastal Plain (north coast) of Alaska were collected from the Gubik Formation. This formation consists largely of beach and shelf marine deposits that record, in superposition, six high sea stands of Pliocene through late Pleistocene age (Brigham, 1985; Carter et al., 1986; Brigham-Grette and Carter, 1992). Samples were analyzed from five of these transgressive units: from oldest to youngest, the Colvillian, Bigbendian, Fishcreekian, Wainwrightian, and Pelukian. Most of the samples were collected in 1980 and 1981 by J.B.G. from nearly continuous seacliff exposures (Skull Cliff) stretching 80 km SW of Barrow, Alaska (Brigham, 1985). The Wainwrightian sample was collected from the type section in Kuk Inlet, near Wainwright. The Bigbendian sample was collected by L. David Carter (USGS, Anchorage) near Ocean Point on the Colville River, ca. 150 km SE of Barrow (Brigham-Grette and Carter, 1992). The relatively continuous last-interglacial (isotope substage 5e) Pelukian shoreline at ca. 10 m a.s.l. is generally recognized as the first major terrace or beach landward of the Holocene or modern beach that yields mollusks beyond the range of radiocarbon dating. Samples of the Pelukian from Nome (87-23A), Cape Espenberg (92-BELA-13C), and Barrow (81Akb258) were analyzed (see Brigham-Grette and Hopkins, 1995, for sample information), as well as a sample from Skull Cliff (see above). From western Spitsbergen, bivalves from the three most recent sedimentation cycles (episodes A, B, and C) from Brøggerhalvøya were analyzed (Miller, 1982; Miller et al., 1989). Shells representing episode A came from a sublittoral deposit containing mostly paired valves (M83-208; 14C age of 10,850 { 90 yr B.P.) and from glacio-marine sediments (SSh88; Miller, 1982). Shells representing episodes B and C were collected from sections where deposits from episodes A, B, C, and D are superposed. For both episodes, shells from ice-proximal stony glacio-marine silt (SSh70, SSh69), as well as from overlying ice-distal sublittoral sand (SSh71, SSh74), were analyzed (Miller, 1982). A mean annual air temperature (MAT) of 067C (Miller, 1982) was used for radiocarbon-dated episode A samples in calculating Arrhenius parameters. Three collections were analyzed from Baffin Island, Arctic Canada. From the east coast of Loks Land in the southeast (Fig. 1), samples from the Loks Land Interstadial (BS24)

and the overlying Gold Cove drift (BS28; 14C age, 10,550 { 75 yr B.P.; Kaufman et al., 1993) were analyzed. The third collection (BSh44) consists of specimens from the next older marine unit on Baffin, representing the Kogalu amino zone, from the Clyde Foreland 600 km to the north. A value of 067C for MAT (Miller, 1985) is used for the Arrhenius calculations. Estimation of Arrhenius parameters (describing the temperature dependence of racemization rates) was based on heating experiments at 1007C (see below) and the radiocarbon-dated samples from Spitsbergen and Baffin, described above. In addition, clam samples from Tromsø, Norway (AAL-3924; 14C age, 11,990 { 130 yr B.P.; MAT, 37C) and Maine (AAL-7791; 14C age, 12,610 { 100 yr B.P.; MAT, 77C; Stuiver and Borns, 1975) were also analyzed to widen the range of temperatures used in estimation of the temperature dependence of racemization rates (Table 1). Modern specimens of Hiatella arctica and Mya arenaria collected from Skull Cliff on the north coast of Alaska were used for heating experiments and for measurement of D/L and A/I values in modern shells (for modern Mya [n Å 3], mean values were 0.056 for D/L Asp and 0.010 for A/I; for modern Hiatella [n Å 7], the values were 0.057 and 0.011, respectively). These values represent racemization induced by the preparation procedures (mainly the hydrolysis step). These results were also taken as the modern values for these taxa at other locations. For the heating experiments, shells were cleaned (see below) and broken into small pieces (2–3 mm), ca. 8–12 of which were sealed (under N2) into each of a series of glass tubes partially filled with ca. 25 mm of sand and 120 ml distilled water. The tubes were heated in an aluminum block in the oven of a gas chromatograph at 1007C and a single tube was withdrawn at intervals. Because of the slower racemization rate in Hiatella, experiments for this species were carried out for a longer period of time (20 days) than for Mya (12 days). Temperature was monitored to 0.17C by a thermocouple thermometer with the probe inserted into the block to the same depth as the samples. Details of the heating experiment procedures are given in Goodfriend and Meyer (1991). Fossil shells were cleaned of secondary carbonates and chalky areas under a stereo microscope using a hand-held Dremel motorized tool fitted with fine dental bits. After a brief dip in dilute HCl, the shell pieces were washed with distilled water, dried under vacuum, and ground to a coarse powder using a small agate mortar and pestle. Aliquots of ca. 20–40 mg were weighed out for amino acid racemization analysis. Samples were hydrolyzed in 6N HCl in N2-filled screw-top tubes at 1007C for 20 hr, desalted with HF, derivatized to N-trifluoracetyl isopropyl ester amino acid derivatives, and analyzed on an HP 5790 or 5890 series II gas chromatograph with a Chirasil-val column (Goodfriend, 1991). Peak area ratios for D/L Asp and A/I were determined

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to ages (at a given temperature) and thus removes the effect of rate changes with increasing D/L values that are intrinsic to the racemization reaction. The basic assumption in using this approach is that the form of the kinetics is independent of temperature, a hypothesis supported by previous studies on racemization kinetics in fossil and modern land snails (Goodfriend and Meyer, 1991). MATERIALS AND METHODS

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TABLE 1 Measured Radiocarbon Ages, Calibrated Radiocarbon Ages, Current Mean Annual Air Temperatures, and D/L Aspartic Acid and D-alloisoleucine/L-isoleucine (A/I) Values for Samples of the Bivalves Mya and Hiatella Used for Estimation of Activation Energies of Racemization and Epimerization Site

Genus

Spitsbergen

Mya Hiatella Mya Mya Hiatella

Baffin Norway Maine

C lab no.

Age (14C yr B.P.)

Agea (cal yr B.P.)

Temperature (7C)

DIC-3122

10,850 { 90

12,270 { 150c

06

AA-5835 T-2378 Y-2205

10,550 { 75 11,990 { 130 12,610 { 100 f

11,530 { 180d 13,450 { 160e 14,370 { 150g

06 3 7

14

D/L

Aspb

0.153 0.146 0.193 0.319 0.322

A/Ib 0.0118 0.0123 0.0142 0.0313 0.0488

a Radiocarbon ages were calibrated using the CALIB 3.0 program (Stuiver and Reimer, 1993) with the marine sample data set of Bard et al. (1993). DR values (representing the local deviation of the marine reservoir age from the worldwide average) from Stuiver et al. (1986) were used in the calibration procedure. b Amino acid racemization values are means of single measurements of one to two preparations of each of three individual shells from each site. c DR Å 70 14C yr. d DR Å 140 14C yr (data for Iceland used). e DR Å 60 14C yr. f Corrected for isotopic fractionation assuming d13C Å 0‰. g DR Å 85 14C yr.

from a PC-based integration program. A/I values were calibrated by multiplying the peak area ratio by 0.87, a calibration based on analysis of A/I standards prepared from weighed amounts of D-alloisoleucine and L-isoleucine. A similar calibration procedure for D/L Asp showed that peak area ratios are unbiased, so no calibration is required. Replicate preparations and analysis of standard shell powders give overall analytical errors that average 3% for D/L Asp and 4% for A/I. For each sample, one to five (usually three) individual shells were analyzed. Racemization data for these individual shells, based on a single analyses of one to two preparations of each shell, are plotted. It should be noted that these procedures differ from those used previously by Miller and Brigham-Grette. They carried out hydrolyses at 1107C for 22 hr and used peak height ratios (without calibration) to determine A/I values. The three interlaboratory comparison shell powder samples of Wehmiller (1984) were analyzed both at the Carnegie Institution of Washington (CIW) (n Å 2) as well as previously at the Weizmann Institute of Science (WIS) (n Å 3) using the same procedures with an HP 5890 gas chromatograph. Results for aspartic acid are as follows [mean ({standard error)]: ILC-A, 0.401 ({0.013) (CIW), 0.398 ({0.002) (WIS); ILC-B, 0.721 ({0.005) (CIW), 0.736 ({0.007) (WIS); ILC-C, 0.878 ({0.001) (CIW), 0.901 ({0.016) (WIS). Results for A/I (calibrated, as indicated above) are ILC-A, 0.157 ({0.002) (CIW), 0.157 ({0.003) (WIS); ILCB, 0.504 ({0.012) (CIW), 0.533 ({0.007) (WIS); ILC-C, 1.059 ({0.015) (CIW), 1.134 ({0.007) (WIS). KINETICS OF RACEMIZATION IN ARCTIC CLAMS

isoleucine epimerization (Fig. 2). Isoleucine epimerization shows a linear trend with time (with R2 values of 0.998 and 0.994 for Mya and Hiatella, respectively). Mya shows a higher rate of isoleucine epimerization (0.00383/day) than

Heating experiments on modern Mya and Hiatella at 1007C show that Asp racemization occurs much faster than

FIG. 2. Heating experiments (at 1007C) showing aspartic acid racemization and isoleucine epimerization in clams: (A) Mya, and (B) Hiatella. Note the difference in time scales.

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Hiatella (0.00261/day). The trend in D/L Asp values is convex upward (i.e., the rate of racemization decreases with increasing D/L), with a stronger bend for Mya than for Hiatella. Power-function transformations of D/L Asp values generally linearize these trends well (Goodfriend et al., 1995; Goodfriend and Stanley, 1996), with the best fits being found for (D/L)3.6 for Mya and (D/L)2.8 for Hiatella (Fig. 3). After transformation, the trend for Hiatella fits well throughout the time range and has a high R2 value (0.992). For Mya, the R2 value is even higher (0.998), but the trend deviates from the model at the lowest D/L values—a concave upward trend can be seen in the data for samples heated for less than 1 day. This poor fit for the samples with low D/L values is also indicated by the estimated intercept for the regression line (00.0030), which is below the value measured for unheated samples ([D/L]3.6 Å 0.00003, for D/L Å 0.057). The higher exponent for Mya reflects the stronger bending of the trend of D/L Asp with time. A much improved fit for Mya for D/L Asp values less than 0.25 is obtained by a lower

order transformation. For these data, a best fit was found for (D/L)1.7 (Fig. 3A, inset). This gives a clearly linear trend and an R2 value of 0.999. The activation energy, Ea , for Asp racemization (which gives the temperature dependence of the racemization rate) for Mya and Hiatella was calculated from the 1007C heating experiments and several fossil samples with radiocarbon ages between ca. 10,000 and 12,000 yr B.P. (Table 1). The average temperature history of these latter samples should approximate present mean annual air temperature (Miller, 1985). For Hiatella, D/L Asp values were transformed to (D/L)2.8 to linearize them with respect to time. Rates were calculated as net (D/L)2.8 per unit time for fossil samples, where the net D/L is the D/L value measured in the sample minus the D/L value measured in modern samples. For the heating experiment samples, a simple linear regression of (D/L)2.8 vs time was used to estimate the rate. The Arrhenius plot (Fig. 4A) shows the natural log of the rate in relation to the inverse of the temperature in Kelvin. The slope (15,075) was estimated from a simple linear regression and multiplied by the gas constant R (0.001987 kcal/degrmol) to obtain an activation energy (Ea) estimate of 30.0 kcal/mol. For Mya, the procedure is not so straightforward, as no transformation of D/L values satisfactorily linearizes them over the full range of interest. Consequently, an alternative approach was used to determine the temperature dependence of Asp racemization. The rate at 1007C, as determined from the heating experiments, was arbitrarily assigned a value of 1. The rates of natural samples were calculated relative to the heating experiment samples: the time (in years) at 1007C required to reach the D/L value measured in the fossil sample was divided by the age of the fossil sample. So, for example, if a 10,000-yr-old fossil had a D/L value the same as that reached at 1007C in 3.65 day (0.01 yr), then the relative rate of racemization in the fossil sample would be 1 1 1006; i.e., the fossil sample would have racemized one million times slower than a sample at 1007C. An analogous method was used by Wehmiller and Belknap (1978) for dealing with nonlinear kinetics. An Arrhenius plot was constructed from these relative rates and their associated temperatures (Fig. 4B). The slope (15,397) gives an estimated Ea of 30.6 kcal/ mol, which is close to that for Hiatella. Activation energies were also estimated for isoleucine epimerization for the two bivalve taxa. Rates of epimerization were estimated as net A/I per unit time. No first-order reversible kinetic transformation was used to account for the slowing of the rate with increasing A/I, since at the low A/I values involved, the reverse reaction of D-alloisoleucine to L-isoleucine is inconsequential. For Mya (Fig. 4D), an Ea value of 29.0 kcal/mol was estimated, and for Hiatella (Fig. 4C), a value of 29.5 kcal/mol was obtained. The value for Mya is higher than that determined in an earlier study (28.1)

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FIG. 3. Heating experiments (at 1007C) showing D/L aspartic acid values transformed to linearize them with respect to time: (A) Mya, plotted as (D/L)3.6 (inset, plotted as (D/L)1.7 for D/L õ 0.25), and (B) Hiatella, plotted as (D/L)2.8.

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FIG. 4. Arrhenius plots for aspartic acid racemization and isoleucine epimerization in the bivalves Hiatella and Mya: (A) Asp racemization in Hiatella, (B) asp racemization in Mya, (C) isoleucine epimerization in Hiatella, and (D) isoleucine epimerization in Mya. Centigrade temperatures corresponding to the Kelvin temperatures are also shown.

by Miller (1985). This difference is largely because, in the present study, the radiocarbon ages were calibrated to convert them to a sidereal time scale concordant with the time scale of the heating experiments. When the earlier studies were done, a calibration for marine samples in the range 10,000–12,000 yr B.P. (now based on high-precision U– Th dating; Bard et al., 1993) was not available. The same calculation done with uncalibrated radiocarbon ages gives a value of 27.8, which is close to the estimate of Miller (1985). Because of the difference in the activation energy for isoleucine epimerization between Mya and Hiatella, the relative rates of epimerization of these two taxa change according to temperature: at 1007C, Mya epimerizes faster than Hiatella, whereas at ambient temperatures in the Arctic, Hiatella epimerizes faster (Miller, 1982, and data presented below for Spitsbergen). From these results it can be seen that in both taxa studied, Asp racemization has a higher activation energy than isoleucine epimerization. This difference is greater in Mya (30.6 vs 29.0) than in Hiatella (30.0 vs 29.5).

A Plio–Pleistocene time series of Hiatella samples from the north coast of Alaska shows a pattern of D/L Asp values

increasing with higher A/I values (Fig. 5A; Table 2). In general, the Asp racemization rate considerably exceeds that of isoleucine epimerization (Fig. 6). The rate difference is particularly dramatic in the youngest samples. The last-interglacial (Pelukian) samples have A/I values that are analytically only marginally distinguishable from modern shells, as was shown by Brigham-Grette and Hopkins (1995). However, the D/L Asp values differ by 0.08, showing that these ages can be readily distinguished by Asp racemization. The other samples follow a general monotonic trend of increasing D/L Asp with increasing A/I, except for the Bigbendian samples, which have relatively high D/L Asp values in relation to their A/I values. Thus, whereas the A/I data suggest that the Bigbendian is rather closer in age to the Fishcreekian than to the Colvillian, the Asp data suggest the converse. Between the Bigbendian and Colvillian samples, the Asp racemization rate is similar to the isoleucine epimerization rate (Fig. 6). In the Colvillian samples, a rather wide spread of A/I and D/L Asp values is observed. Variation between these two amino acids is concordant, i.e., individuals with high D/L Asp values also have high A/I values. Within each of the deposits, the Asp data show less variability among specimens (as measured by the coefficient of variation [C.V.]; Table 2) than the isoleucine data. For the

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FIG. 5. D/L aspartic acid values in relation to D-alloisoleucine/L-isoleucine (A/I) values for fossil and modern Hiatella samples from the north coast of Alaska: (A) D/L Asp vs A/I, and (B) (D/L Asp)2.8 vs A/I. mo Å modern; Pe Å Pelukian; Wa Å Wainwrightian; Fi Å Fishcreekian; Bi Å Bigbendian; Co Å Colvillian.

five fossil sample sets, the C.V. for A/I values averages 11.9%, whereas for D/L Asp, the average is only 4.3%. Leaving out the Colvillian sample set, which may contain a range of ages, the C.V. for the Asp values is only 3.7%, which is very close to the analytical error (3%). The age of the Bigbendian is fairly well established at 2.6 myr, based on the apparent occurrence of the Gauss– Matuyama polarity boundary within this unit (BrighamGrette and Carter, 1992). The next older unit, the Colvillian, is considered to be less than 3.1 million yr old (Fyles et al., 1991), whereas the age of the next younger unit, the Fishcreekian, has been the subject of some controversy. Vertebrate remains and previous analyses of isoleucine epimerization suggest an age of ca. 2.1–2.6 myr (Brigham-Grette and Carter, 1992), whereas analysis of benthic forams suggests a younger age of ca. 1.2–1.7 myr (McDougall, 1995). The A/I and D/L Asp data presented in the present study do not give concordant results for the ages of these units. In Fig. 5B, D/L Asp values have been transformed to (D/L Asp)2.8 and plotted against A/I values, so that relative distances

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between the racemization or epimerization values of samples would be proportional to age differences, if temperature conditions for the various intervals were the same. The A/I data suggest either that the Colvillian is substantially older than the Bigbendian or that the intervening period was very much warmer than that subsequent to the Bigbendian (the interpretation favored by Brigham-Grette and Carter, 1992). The A/I data also suggest that the Fishcreekian is rather closer in age to the Bigbendian than to the subsequent Wainwrightian (dated at ca. 300,000–500,000 yr BP; Kaufman and Brigham-Grette, 1993), as was shown previously by Brigham– Grette and Carter (1992). However, the scatter in the A/I values of the Fishcreekian samples is quite large (C.V. Å 22.2%; Table 2) and calls into question the reliability of this age estimate. If we interpret ages based on the D/L Asp data, the Colvillian does not appear to be as dramatically older (or the interval dramatically warmer) than the Bigbendian as is indicated by the A/I values. However, a very different picture for the age of the Fishcreekian is indicated by the D/L Asp data: the age of this unit appears to be very much closer to that of the Wainwrightian than to the Bigbendian. The D/L Asp values for the Fishcreekian also show a much narrower range than the A/I values, although the scatter still exceeds analytical error (C.V. Å 6.6%). The Asp results (but not the A/I results) are thus consistent with McDougall’s (1995) interpretation of a younger age for this unit. The geographic gradient in Asp racemization rates along the temperature gradient from the north slope of Alaska (Skull Cliff, at 717N) south to Nome (647N) (see Fig. 1 in Brigham-Grette and Hopkins, 1995) was documented in bivalves (mostly Mya) from four last-interglacial sites. A regular increase of D/L Asp values southward, with increasing temperature, is seen (Table 3). Transformation of the net D/L values to the 1.7 power gives values that are proportional to racemization rates. The samples from Cape Espenberg, presently some 77C warmer than Skull Cliff or Barrow on the north coast, racemize about twice as fast. At Nome, presently some 107C warmer than the north coast, the rate is 2.6 times faster. These rates, while showing a general increase with temperature, are less than the expected rate differences (4.7 and 8.9 times the rate at Barrow, respectively) based on the activation energy and the differences between current mean annual temperatures. This may reflect greater similarity of temperatures among sites over the past 125,000 years compared to the present differences or else a greater similarity of effective diagenetic temperatures (see Wehmiller, 1982, for a discussion of effective diagenetic temperatures). A similar gradient in isoleucine epimerization rates in various bivalve taxa was shown recently for this region by Brigham-Grette and Hopkins (1995). SPITSBERGEN

Asp racemization and isoleucine epimerization results for Spitsbergen Hiatella and Mya are plotted in Fig. 7A. In-

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TABLE 2 Mean D/L Aspartic Acid and D-alloisoleucine/L-isoleucine (A/I) Values, Number of Specimens Analyzed, and Coefficients of Variation (C.V.) (Å[S.D./Mean] 1 100%) for Arctic Bivalve Samples Hiatella D/L

Asp

Mya A/I

Sample no.

Unit

Mean

C.V.

Mean

81Akb258 81AKB547 81Akb410 USGS-8359 80Akb152

Pelukian Wainwrightian Fishcreekian Bigbendian Colvillian

0.126 0.206 0.281 0.439 0.496

3.7 2.9 6.6 1.5 6.9

0.0078 0.0293 0.0680 0.0913 0.206

M79 M83 M79 M79 M79 M79

Episode Episode Episode Episode Episode Episode

0.153 0.146 0.182 0.204 0.273 0.281

— 2.9 — 3.1 2.3 3.0

0.0111 0.0110 0.0216 0.0223 0.0370 0.0384

D/L

C.V.

N

6.2 12.8 22.2 5.4 13.1

3 3 5 3 4

Asp

A/I

Mean

C.V.

Mean

C.V.

N

0.168 0.153 0.210 0.227

4.8 14.2 3.3 2.2

0.0102 0.0118 0.0177 0.0192

9.3 14.9 0.4 22.8

3 3 4 2

0.193 0.230 0.273

1.5 5.4 0.5

0.0142 0.0239 0.0374

9.0 47.8 2.8

2 3 2

Alaska

Spitsbergen SSh88 208 SSh71 SSh70 SSh69 SSh74

A A B B C C

— 7.3 — 20.4 8.1 5.0

1 2 1 3 3 3

Baffin Island M89 BS38 M89 BS24 M74 BSh44

Eglinton Loks Land Kogalu

0.222

6.4

0.0260

37.3

2

cluded are data for modern shells and fossil shells from episode A (radiocarbon dated to 12,270 cal yr B.P.), two units of episode B (both plotted at alternative ages of ca. 65,000 and 80,000 yr B.P.), and two units of episode C (considered to represent the last-interglacial isotope stage 5e at ca. 125,000 yr B.P.). Whereas isoleucine in the episode A samples shows less than half a percent (0.005) more epimerization than modern samples, Asp shows a difference of some 10%. Episodes B and C are also rather better separated by Asp than by isoleucine. The two units of episode B consist of glacio-marine sediments and overlying sublittoral sediments (Miller et al., 1989; their Fig. 11). Previous work using isoleucine failed to find a significant difference in the A/I values of either the total fraction or the free fraction between these two units (Miller et al., 1989). However, the Asp data presented here point to a measurable age difference. Using the average D/L value in Mya for the two units (net [D/L]1.7) and assuming an age of 70,000 yr for the sublittoral unit, we estimate that the underlying glacio-marine unit would be some 12,000 yr older. A large age difference is also suggested by the Hiatella data. However, since data on only one specimen were available for the sublittoral unit, no age calculations were performed for this taxon. In contrast, no difference in D/L Asp values is seen for the glacio-marine and sublittoral units for episode C. In most cases, the D/L Asp

values show considerably less variability among individuals than the A/I values and, except for M83 208 Mya, are within the range of analytical error (Table 2). The Asp racemization data shed some light on the age of episode B but do not lead to a resolution of the problem, due to uncertainties in the temperature histories of the samples. Using the Asp data for Hiatella, we examine the implications for paleotemperatures on Spitsbergen during the lastglacial cycle under alternative scenarios of 65,000- and 80,000-yr ages for episode B. Plotting the Hiatella Asp data as (D/L)2.8, the slopes of the lines connecting the average D/L value for each episode represent the average rate of racemization during the intervening period (Fig. 7B). Under the scenario of a 65,000-yr age, the period from 12,000 to 65,000 yr B.P. would have to be on average ca. 4.47C cooler than the Holocene, represented by the racemization rate of 12,000-yr-old samples. The period from 65,000 to 125,000 yr B.P. (isotope stage 5) would be ca. 1.37C cooler on average than the Holocene. If an 80,000-yr age is assumed for episode B, then the period from 12,000 to 80,000 would need to be some 5.27C colder on average than the Holocene, whereas the period from 80,000 to 125,000 yr B.P. would average about the same temperature as the Holocene. While reliable paleoclimatic indicators are not available for this region, the latter scenario (80,000 yr age) seems less reason-

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age stage 5 temperature similar to the Holocene. However, a further complication arises in considering the amount of time the samples have spent submerged by seawater. Since sea temperatures are generally no lower than ca. 07C, the initial temperature (and that during later submergence, should this occur) under marine water will be warm relative to conditions after emergence to the cooler terrestrial realm (present MAT: 067C). Thus, although the Asp data tend to favor the younger age for episode B indicated by the U–Th dates, the results are not inconsistent with the older age, assuming very cold conditions during isotope stages 4 to 2 and the influence of warm marine waters and glacial ice cover early in the history of the samples. Mya show slightly higher rates of racemization than Hiatella but otherwise show a similar pattern. However, because no Mya samples from episode C were available, this species is not used for age and paleotemperature calculations. BAFFIN ISLAND

FIG. 6. D/L aspartic acid values in relation to A/I values for fossil samples (from Alaska, Spitsbergen, and Baffin Island) and for heating experiment samples, plotted so the axes are equally scaled: (A) Hiatella, and (B) Mya. The diagonal dashed line shows what the trend would be if Asp racemization and isoleucine epimerization rates were the same.

able than the former. For temperature to average at least 57C cooler during the 12,000- to 80,000-yr period than during the Holocene, an extremely cold last-glacial maximum is implied. On the other hand, the average temperature throughout stage 5 would have to average about the same as the Holocene. Although the last interglacial (substage 5e) may have been slightly warmer than the Holocene, the stadial substages 5b and 5d could not have been much cooler than average Holocene temperatures and still maintained an aver-

D/L

Asp racemization and isoleucine epimerization results for Baffin Island Hiatella and Mya are plotted in Fig. 8A. As in other deposits, Asp shows significantly better resolution between the different ages than does isoleucine and also shows less variability among individuals (Table 2). Except for the Loks Land Interstadial samples, plotted at alternative presumed ages of 40,000 and 80,000 yr B.P., the D/L Asp values are rather uniform among individual shells within each sample. In Fig. 8B, the D/L Asp values for Mya are transformed to the 1.7 power so that slopes between groups of points are proportional to racemization rates. However, the last-interglacial samples have values slightly above the portion of the trend (in the heating experiment series) that is linearized by this transformation (see Fig. 3A), so the slope between this and the Loks Land sample cannot be taken as precisely proportional to the racemization rate. Additional problems in interpretation preclude any definite con-

TABLE 3 Aspartic Acid Values of Bivalves from Last Interglacial Sites in Alaska

Site

MATa (7C)

Mean ({S.E.) of D/L Asp

Netb (D/L Asp)1.7

Nc

Species

Skull Cliff Barrow Cape Espenburg Nome

013 013 06 03

0.138 ({0.003)d 0.136 ({0.007) 0.188 ({0.006) 0.219

0.0258 0.0267 0.0514 0.0686

3 4 3 1

Hiatella arctica Mya sp. Mya arenaria Mya sp.

a

Current mean annual temperature. Asp values were linearized with respect to time by transforming them to the 1.7 power (see text) and the racemization measured in modern Mya ([D/L]1.7 Å 0.0075, for D/L Å 0.056) was subtracted to obtain the net racemization. c Number of individual shells analyzed; one to two preparations per shell were analyzed. d The D/L Asp value of Hiatella is presented here after conversion to the equivalent value for Mya (11.09), based on difference between the two taxa in Spitsbergen samples. b

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significantly younger than the Kogalu and definitely older than the earliest Holocene samples, as indicated by their radiocarbon ages. Whatever the assumed age of the Loks Land Interstadial is, rate differences indicate that isotope stages 2 and 3 were very much colder (8 to 127C) than the Holocene (Fig. 8B). Consideration of implied rate differences and associated paleotemperature differences for the alternative age scenarios for the Loks Land Interstadial tends to favor an older age. If the 40,000 yr B.P. radiocarbon age were correct, then a very slow racemization rate and correspondingly very low average temperatures (as much as 127C less than the Holocene) would be implied for the period 125,000–40,000 yr B.P. Still rather low but more reasonable

FIG. 7. Aspartic acid racemization and isoleucine epimerization in Spitsbergen Hiatella and Mya (for episodes A, B [2 units], and C [2 units], and modern specimens) plotted in relation to measured or presumed ages. Episode B samples are plotted under scenarios of both 65,000- and 80,000year ages. (A) D/L Asp and A/I values of Mya and Hiatella. (B) D/L Asp in Hiatella samples, plotted as (D/L)2.8, and showing estimated racemization rates (in 1007 [D/L Asp]2.8 yr01) and relative temperatures (in 7C) for alternative scenarios of 65,000- and 80,000-year ages for episode B. Temperature estimates are given relative to the temperature for the period 0–12,300 cal yr B.P., as represented by the racemization rate in episode A shell samples.

clusions about the actual age of the Loks Land Interstadial based on the Asp data. First, the scatter of D/L Asp values for the Loks Land samples is significantly larger (C.V. Å 5.4%; Table 2) than the analytical error, so interpretation depends on whether the specimen with the higher value represents an older, redeposited shell, or whether the spread of values is due to variation in diagenetic alteration of the shells. Second, the last-interglacial sample is from 600 km north of the other samples, where the mean annual temperature is 47C cooler. If adjusted for this temperature difference, the D/L values of last-interglacial samples would be significantly higher. However, it is not clear whether the summer temperatures, which largely determine the racemization rate, are significantly different between the sites. In any case, the Asp data do indicate that the Loks Land Interstadial is

FIG. 8. Racemization in Baffin Island Hiatella and Mya plotted in relation to measured or presumed ages. Loks Land Interstadial (LLI) samples are plotted under alternative scenarios of 40,000- and 80,000-year ages. D/L Asp and A/I values for modern specimens of the two taxa are also shown. (A) D/L Asp and D-alloisoleucine/L-isoleucine (A/I) values. (B) 1.7 D/L Asp in Mya samples, plotted as (D/L) , and showing estimated racemization rates (in 1007 [D/L Asp]1.7 yr01) and relative temperatures (in 7C) for scenarios of 40,000- and 80,000-year ages for the Loks Land Interstadial. Temperature estimates are given relative to the temperature for the period 0–11,500 cal yr B.P., as represented by the racemization rate in Eglinton shell samples. For the period between the LLI and the last interglacial, rates are minimum estimates and temperature depressions are maximum estimates, due to the fact that the last-interglacial samples are from a more northerly (therefore presumably cooler) locality.

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Brigham, J. K. (1985). ‘‘Marine Stratigraphy and Amino Acid Geochronol-

ogy of the Gubik Formation, Western Arctic Coastal Plain, Alaska.’’ U.S. Geological Survey Open-file Report 85–381. Brigham, J. K., and Miller, G. H. (1983). Paleotemperature estimates of the Alaskan Arctic Coastal Plain during the last 125,000 years. In ‘‘Proceedings of the IV International Conference on Permafrost,’’ National Academy of Sciences, Fairbanks, AK, Vol. 1, pp. 80–85. Brigham-Grette, J., and Carter, L. D. (1992). Pliocene marine transgressions of northern Alaska: Circumarctic correlations and paleoclimatic interpretations. Arctic 45, 74–89. Brigham-Grette, J., and Hopkins, D. M. (1995). Emergent marine record and paleoclimate of the last interglaciation along the northwest Alaskan coast. Quaternary Research 43, 159–173. Carter, L. D., Brigham-Grette, J., and Hopkins, D. M. (1986). Late Cenozoic marine transgressions of the Alaskan Arctic Coastal Plain. In ‘‘Correlation of Quaternary Deposits and Events around the Beaufort Sea’’ (J. A. Heginbottom and J. S. Vincent, Eds.), pp. 21–26. Geological Survey of Canada Open-file Report 1237. Fyles, J. G., Marincovich, L., Jr., Matthews, J. V., Jr., and Barendregt, R. (1991). Unique mollusc find in the Beaufort Formation (Pliocene) on Meighen Island, Arctic Canada. Current Research, Part B, Geological Survey of Canada No. 91-1B, 105–112. Goodfriend, G. A. (1991). Patterns of racemization and epimerization of amino acids in land snail shells over the course of the Holocene. Geochimica et Cosmochimica Acta 55, 293–302. Goodfriend, G. A. (1992). Rapid racemization of aspartic acid in mollusk shells and potential for dating over recent centuries. Nature 357, 399– 401. Goodfriend, G. A., and Hare, P. E. (1995). Reply to the comment by K. L. F. Brinton and J. L. Bada on ‘‘Aspartic acid racemization and protein diagenesis in corals over the last 350 years.’’ Geochimica et Cosmochimica Acta 59, 417–418. Goodfriend, G. A., and Meyer, V. R. (1991). A comparative study of amino acid racemization/epimerization kinetics in fossil and modern mollusk shells. Geochimica et Cosmochimica Acta 55, 3355–3367. Goodfriend, G. A., and Stanley, D. J. (1996). Reworking and discontinuities in Holocene sedimentation in the Nile Delta: Documentation from amino acid racemization and stable isotopes in mollusk shells. Marine Geology 129, 271–283. Goodfriend, G. A., Hare, P. E., and Druffel, E. R. M. (1992). Aspartic acid racemization and protein diagenesis in corals over the last 350 years. Geochimica et Cosmochimica Acta 56, 3847–3850. Goodfriend, G. A., Kashgarian, M., and Harasewych, M. G. (1995). Aspartic acid racemization and the life history of deep-water slit shells. Geochimica et Cosmochimica Acta 59, 1125–1129. Hare, P. E., and Mitterer, R. M. (1969). Laboratory simulation of aminoacid diagenesis in fossils. Carnegie Institution of Washington Yearbook 67, 205–208. Imbrie, J., Hays, J. D., Martinson, D. G., McIntyre, A., Mix, A. C., Morley, J. J., Pisias, N. G., Prell, W. L., and Shackleton, N. J. (1984). The orbital theory of Pleistocene climate: Support from a revised chronology of the marine d18O record. In ‘‘Milankovitch and Climate, Part I’’ (A. Berger, J. Imbrie, J. Hays, G. Kukla, and B. Saltzman, Eds.), pp. 269–305. Reidel, Dordrecht. Kaufman, D. S. (1992). Aminostratigraphy of Pliocene–Pleistocene highsea-level deposits, Nome coastal plain and adjacent nearshore area, Alaska. Geological Society of America Bulletin 104, 40–52. Kaufman, D. S., and Brigham-Grette, J. (1993). Aminostratigraphic correlations and paleotemperature implications, Pliocene–Pleistocene high-sealevel deposits, northwestern Alaska. Quaternary Science Reviews 12, 21– 33. Kaufman, D. S., Miller, G. H., Stravers, J. A., and Andrews, J. T. (1993).

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paleotemperatures for isotope stage 5 would be indicated if the Loks Land Interstadial occurred at the end of stage 5, ca. 80,000 yr B.P. Thus the Asp data suggest that the finite radiocarbon ages of shells from this unit should be regarded only as minimum ages. CONCLUSIONS

Results presented here indicate that for most applications to low-temperature situations, aspartic acid racemization has distinct advantages over isoleucine epimerization for establishing chronologies. Because of its much faster rate, especially at low D/L values, Asp provides significantly better temporal resolution than isoleucine. Because of differences in activation energies between Asp and isoleucine racemization, these rate differences are more pronounced at higher temperatures (Fig. 6). But even at the lowest temperatures considered in this study (the Alaskan samples), Asp shows a higher racemization rate through most of the time range studied. D/L Asp values also show significantly lower variability among individuals than A/I values in all the areas studied. This suggests that they may be more reliable age indicators. This result is unexpected, since Asp is a less stable amino acid than isoleucine (Hare and Mitterer, 1969) and thus is expected to be more subject to variations in diagenetic rates or pathways. A disadvantage of Asp racemization is that it sometimes shows a complicated kinetic pattern and, in any case, the form of the kinetics needs to be established experimentally for each taxon studied. Both methods are limited by uncertainties in the temperature histories of samples, which are further complicated in Arctic coastal settings due to large temperature differences between submergent and emergent conditions. ACKNOWLEDGMENTS Analytical facilities were kindly provided by P. E. Hare. F. Keimig and W. F. Manley assisted with figure preparation and sample procurement, respectively. Many helpful comments on the manuscript were provided by J. F. Wehmiller and D. S. Kaufmann. Collection of Alaskan samples was supported by the Alaska Branch of the U.S. Geological Survey and by grants from the National Science Foundation (DPP87-14671 and 90-15234) and U.S. National Park Service.

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Glacial history and marine environmental change during the last interglacial–glacial cycle, western Spitsbergen, Svalbard. Boreas 18, 273–296. Rutter, N. W., Crawford, R. J., and Hamilton, R. (1980). Correlation and relative age dating of Quaternary strata in the continuous permafrost zone of northern Yukon with D/L ratios of aspartic acid of wood, freshwater molluscs, and bone. In ‘‘Biogeochemistry of Amino Acids’’ (P. E. Hare, T. C. Hoering, and K. King, Jr., Eds.), pp. 463–475. Wiley, New York. Sejrup, H. P., and Haugen, J.-E. (1992). Foraminiferal amino acid stratigraphy of the Nordic Seas: geological data and pyrolysis experiments. DeepSea Research 39, Suppl. 2, S603–S623. Stuiver, M., and Borns, H. W., Jr. (1975). Late Quaternary marine invasion in Maine: Its chronology and associated crustal movement. Geological Society of America Bulletin 86, 99–103. Stuiver, M., and Reimer, P. J. (1993). Extended 14C data base and revised CALIB 3.0 14C age calibration program. Radiocarbon 35, 215–230. Stuiver, M., Pearson, G. W., and Braziunas, T. (1986). Radiocarbon age calibration of marine samples back to 9000 CAL YR BP. Radiocarbon 28, 980–1021. Wehmiller, J. F. (1982). A review of amino acid racemization studies in Quaternary mollusks: Stratigraphic and chronologic applications in coastal and interglacial sites, Pacific and Atlantic coasts, United States, United Kingdom, Baffin Island, and tropical islands. Quaternary Science Reviews 1, 83–120. Wehmiller, J. F. (1984). Interlaboratory comparison of amino acid enantiomeric ratios in fossil Pleistocene mollusks. Quaternary Research 22, 109–120. Wehmiller, J. F., and Belknap, D. F. (1978). Alternative kinetic models for the interpretation of amino acid enantiomeric ratios in Pleistocene mollusks: Examples from California, Washington, and Florida. Quaternary Research 9, 330–348. Westgate, J. A., Stemper, B. A., and Pe´we´, T. L. (1990). A 3 m.y. record of Pliocene–Pleistocene loess in interior Alaska. Geology 18, 858–861.

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McDougall, K. (1995). Age of the Fishcreekian transgression. Palaios 10, 199–220. Miller, G. H. (1982). Quaternary depositional episodes, western Spitsbergen, Norway: Aminostratigraphy and glacial history. Arctic and Alpine Research 14, 321–340. Miller, G. H. (1985). Aminostratigraphy of Baffin Island shell-bearing deposits. In ‘‘Quaternary Environments, Eastern Canadian Arctic, Baffin Bay and Western Greenland’’ (J. T. Andrews, Ed.), pp. 394–427B. Allen and Unwin, Boston. Miller, G. H., and Hare, P. E. (1980). Amino acid geochronology: Integrity of the carbonate matrix and potential of molluscan fossils. In ‘‘Biogeochemistry of Amino Acids’’ (P. E. Hare, T. C. Hoering, and K. King, Jr., Eds.), pp. 415–443. Wiley, New York. Miller, G. H., and Kaufman, D. S. (1990). Rapid Fluctuations of the Laurentide Ice Sheet at the mouth of Hudson Strait: New evidence for ocean/ ice sheet interactions as a control on the Younger Dryas. Paleoceanography 5, 907–919. Miller, G. H., Andrews, J. T., and Short, S. K. (1977). The last interglacial– glacial cycle, Clyde foreland, Baffin Island, N. W. T.: Stratigraphy, biostratigraphy, and chronology. Canadian Journal of Earth Sciences 14, 2824–2857. Miller, G. H., Funder, S., de Vernal, A., and Andrews, J. T. (1992). Timing and character of the last interglacial-glacial transition in the eastern Canadian Arctic and northwest Greenland. In ‘‘The Last Interglacial–Glacial Transition in North America’’ (P. U. Clark and P. D. Lea, Eds.), pp. 223–231. Geological Society of America Special Paper 270, Boulder, CO.

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