John F. Wehmiller

Racemization of Amino Acids in Marine Sediments

Isoleucine, one of several amino acids isolated from a suite of welldated deep-sea cores, shows a progressive increase in the degree of racemization with the age of the sediment. Amino acids in sediments show an initial rate of racemization almost an order of magnitude faster than the rate observed for free amino acids at a comparable pH and temperature. The observed kinetics depend on a variety of diagenetic processes, but it appears that the ratio of alloisoleucine to isoleucine is a reliable indicator of age for samples less than 400,000 years old; for older samples the results are more ambiguous. Isoleucine is racemic in samples older than about 15 x 106 years.

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

Abstract Amino acid racemization (AAR) studies of Quaternary mollusks (mostly marine mollusks from coastal deposits) are reviewed. Data obtained in regional studies from the U.S. Atlantic and Pacific coasts, Baffin Island, the United Kingdom, and tropical islands are discussed as they relate to method evaluation, kinetic model development, and aminostratigraphic applications. U.S. Pacific coast studies have been employed for estimating tectonic deformation rates, study of molluscan zonation patterns in the late Pleistocene, and in kinetic model evaluation. U.S. Atlantic coast studies have identified multiple aminostratigraphic zones in areas with complex morphostratigraphic relationships, and have also revealed some important conflicts between aminostratigraphic temperature concepts and radiometric data. Studies in the United Kingdom, involving coastal sites and non-marine interglacial sites, identify some stratigraphic and kinetic conflicts but provide a preliminary chronostratigraphic model for the classical interglacial terminology of the U.K. middle to late Pleistocene. Baffin Island studies, involving AAR data and Uue5f8Th dating of shells, have resulted in some significant revisions of previous chronologic models. The Arctic environment has also stimulated some novel approaches in the collection and interpretation of AAR results. Tropical island studies, though few in number, aid in calibration of observed racemization and also aid in correlation of coastal deposits with late Pleistocene isotopic ice-volume records.

Interlaboratory comparison of amino acid enantiomeric ratios in fossil Pleistocene mollusks

Abstract Enantiomeric ( d/l ) ratios of eight amino acids in three homogeneous powdered fossil Pleistocene mollusk samples have been compared. Eleven laboratories have contributed results to this effort. Three gas chromatographic methods and one ion-exchange chromatographic method were used. In general, the coefficients of variation (c.v.) are best for alanine, glutamic acid, and aspartic acid (3–8%). For leucine and phenylalanine, the c.v.s are between 5 and 10%. For isoleucine, proline, and valine the c.v.s range from 10 to 18%. Individual laboratories usually report analytical precision of between 2 and 5%, but it is clear that significant differences between laboratories results are often encountered, and that these analytical differences could, in some cases, lead to very significant differences (25% or more) in age estimates based upon enantiomeric ratios. Multiple analyses of desalted hydrolyzates of these powder samples suggest that interlaboratory differences are often caused by instrumental and/or derivatization procedures rather than the wet-chemical sample preparative steps. It is proposed that the powder samples described here be used in all future publications of fossil amino acid enantiomeric ratio data.

Comparative taphonomy of bivalves and foraminifera from Holocene tidal flat sediments, Bahia la Choya, Sonora, Mexico (northern Gulf of California): Taphonomic grades and temporal resolution

We compare the preservation (taphonomic grade) and age of Chione (bivalve) and fo- raminifera from modern siliciclastic tidal flat sediments of Bahia la Choya, Sonora, Mexico (north- ern Gulf of California). Disarticulated shells of Chione collected from the sediment-water interface of Choya Bay exhibit a substantial range in taphonomic grade and age, several hundred years to -80-125 ka based on Accelerator Mass Spectrometer 4C dates and D-Alloisoleucine/L-Isoleucine values. There is not, however, a one-to-one correspondence between age and taphonomic alteration of Chione: old (or young) valves may be highly altered or they may be relatively pristine. In contrast to Chione, most foraminiferal tests at Choya Bay are quite pristine, which suggests a quite young age, but tests are surprisingly old (up to -2,000 calendar years based on Accelerator Mass Spec- trometer 4C dates). We suggest that following seasonal pulses in reproduction, some foraminiferal tests are rapidly incorporated into a subsurface shell layer by Conveyor Belt deposit feeders and preserved there, while the rest of the reproductive pulse rapidly dissolves. Ultimately, some of these buried tests, along with Chione, are transported back to the surface by biological activity and storms. The much greater range of taphonomic grades and ages among Chione shells suggests that they, unlike fora- minifera, are sufficiently large and preservable (low surface/volume ratio and chemical reactivity) to undergo many cycles of degradation, burial, and exhumation before complete destruction. The age of foraminiferal tests indicates that time-averaging of microfossil assemblages at Choya Bay is much more insidious than would be expected considering the relatively pristine state of the tests alone. Based on our studies, the lower limit of temporal resolution of shallow shelf microfossil assem- blages appears to be -1000 years. We caution, however, that each depositional setting (taphofacies) should be evaluated on a case-by-case basis before gross generalizations are made. Indeed, the dis- crepancy between age and taphonomic grade of fossil assemblages at Choya Bay suggests that nei- ther hardpart size or taphonomic grade are infallible indicators of test preservability or likely tem- poral resolution of the host assemblage, and that the dynamics of hardpart input and loss must also be evaluated.

Comparison of amino acid racemization geochronometry with lithostratigraphy, biostratigraphy, uranium-series coral dating, and magnetostratigraphy in the Atlantic Coastal Plain of the southeastern United States

The results of an integrated study comprising litho- and biostratigraphic investigations, uranium-series coral dating, amino acid racemization in molluscs, and paleomagnetic measurements are compared to ascertain relative and absolute ages of Pleistocene deposits of the Atlantic Coastal Plain in North and South Carolina. Four depositional events are inferred for South Carolina and two for North Carolina by all methods. The data suggest that there are four Pleistocene units containing corals that have been dated at about 100,000 yr, 200,000 yr, 450,000 yr, and over 1,000,000 yr. Some conflicts exist between the different methods regarding the correlation of the younger of these depositional events between Charleston and Myrtle Beach. Lack of good uranium-series dates for the younger material at Myrtle Beach makes the correlation with the deposits at Charleston more difficult.

Amino acid racemization geochronology of reworked Quaternary mollusks on U.S. Atlantic coast beaches: implications for chronostratigraphy, taphonomy, and coastal sediment transport

Approximately 200 shells (primarily Mercenaria) from 21 beach sites between New Jersey and Florida have been analyzed for the extent of racemization (epimerization) of their fossilized amino acids. The greatest concentration of sites is in North Carolina, in regions least affected by human modification of natural beach sediment-transport processes. These results can be used to estimate the frequency of age mixing of Pleistocene shell material in Holocene depositional environments. Selected shells have also been dated by 14C (conventional and/or AMS) to provide direct calibration of the amino acid epimerization ratios. Shell taphonomic characteristics (particularly color) can be qualitatively related to apparent ages. Data for shell fragmentation, combined with amino acid age estimates, provide insights into probable transport distances of reworked shells. Shells with apparent Pleistocene ages have epimerization values equivalent to those seen in onshore exposures of Pleistocene coastal units. Amino acid epimerization measurements are a comparatively rapid and inexpensive chronologic tool for use in taphonomic and sediment(shell-) transport studies. Paired amino acid-14C analyses on single shells provide insights into geochemical alteration of shells and permit modelling of the residence time of shells on beaches. Greater numbers of analyses at each site are needed before statistically valid estimates of age mixing can be obtained, but the apparent distribution of Pleistocene shells on Atlantic coast beaches appears to be related to the distribution of Pleistocene units in the shoreface and inner shelf, and the thickness of Holocene sedimentary cover in specific areas. These observations provide useful criteria for the evaluation of possible age mixing of shells collected at outcrops of Pleistocene units in the Coastal Plain.

Daily, seasonal, and annual relationships between air and subsurface temperatures

[1] Inversions of borehole temperature profiles that reconstruct past ground surface temperature (GST) changes have been used to estimate historical changes in surface air temperature (SAT). Paleoclimatic interpretations of GST reconstructions are based on the assumption that GST and SAT changes are closely coupled over decades, centuries, and longer. This assumption has been the subject of some debate because of known differences between GST and SAT at timescales of hours, days, seasons, and years. We investigate GST and SAT relationships on daily, seasonal, and annual timescales to identify and characterize the principal meteorological changes that lead to short-term differences between GST and SAT and consider the effects of those differences on coupling between the two temperatures over much longer time periods. We use observational SAT and subsurface data from Fargo, North Dakota; Prague, Czech Republic; Cape Henlopen State Park, Delaware; and Cape Hatteras National Seashore, North Carolina. These records comprise intradaily observations that span parts of one or two decades. We compare subsurface temperature observations to calculations from a conductive subsurface model driven with daily SAT as the surface boundary condition and show that daily differences exist between observed and modeled subsurface temperatures. We also analyze year-to-year spectral decompositions of daily SAT and subsurface temperature time series and show that dissimilarities between mean annual GST and SAT are attributable to differences in annual amplitudes of the two temperature signals. The seasonal partitioning of these amplitude differences varies from year to year and from site to site, responding to variable evapotranspiration and cryogenic effects. Variable year-to-year differences between mean annual GST and SAT are closely estimated using results from a multivariate regression model that associates the partial influences of seasonal meteorological conditions with the attenuation of annual GST amplitudes.

Amino Acid Age Estimates, Quaternary Atlantic Coastal Plain: Comparison with U-Series Dates, Biostratigraphy, and Paleomagnetic Control

Amino acid enantiomeric (DL) ratios in the mollusk Mercenaria are compared with recently published biostratigraphic and/or U-series solitary coral data from 22 Quaternary localities on the central and southern Atlantic Coastal Plain. In all cases, local relative aminostratigraphic sequences are consistent with relative ages inferred from U-series or biostratigraphic data, although occasionally more depositional events are recognized by aminostratigraphic than biostratigraphic methods. However, if the U-series data are used as age calibrations for the DL values, latitudinal trends of “isochronous” DL values are highly variable and conflict with trends expected from the present temperature gradient, which is smooth and nearly linear between 45° and 25° N. Age estimation can be performed independently of the U-series data using a kinetic model that relies on the assumption that Pleistocene temperature gradients have also been smooth functions of latitude, although significantly steeper than the present temperature gradient. Within the uncertainties of this assumption, kinetic model age estimates for localities in the coastal plain fall into the following groups: 70,000–130,000 yr, 200–250,000 yr, 300,000–400,000 yr, 500,000–600,000 yr, 700,000–800,000 yr, and > 1,000,000 yr. Major conflicts between these model age estimates are observed for localities near Charleston, South Carolina and in central Virginia. These conflicts could indicate that the basic temperature assumptions of aminostratigraphy are incorrect, and that apparent local aminostratigraphic sequences (clusters of different DL values) could be due to factors other than age difference. Alternatively, some of the U-series dates may be only minimum ages for these localities.

Alternative kinetic models for the interpretation of amino acid enantiomeric ratios in Pleistocene mollusks: Examples from California, Washington, and Florida

Alternative kinetic models for amino acid racemization in Pleistocene molluscan samples are compared by examination of results for samples from marine Pleistocene deposits in California, Washington, and Florida. Linear kinetic models previously have been applied to the Florida samples [Mitterer, R.M. (1975). Earth and Planetary Science Letters 28, 275–282.] because these kinetics were observed in laboratory pyrolysis experiments with the particular genus involved (Mercenaria). Nonlinear kinetic models, extrapolated from deep-sea foraminifera racemization kinetics, are applied to samples of Protothaca and Saxidomus from California and Washington and seem more consistent with their local chronologic and stratigraphic control. Average or effective diagenetic temperatures can be estimated by each of these models if reliably dated samples are available. Linear models applied to such samples from California and Florida suggest average diagenetic temperatures that are cooler (by as much as 10°C) than would be inferred from available paleoclimatic records. Nonlinear kinetic models yield estimates of average diagenetic temperatures that are more consistent with these records: full-glacial (i.e., approximately 18,000 yr BP) temperature reductions of between 2 and 6°C are inferred for coastal California and southern Florida. The nonlinear kinetic model is used to expand (by a factor of 2.5 to 3.0) the time scale proposed by Mitterer (1975) for five marine Pleistocene units of Florida.

Amino acid studies of the Del Mar, California, midden site: Apparent rate constants, ground temperature models, and chronological implications

Amino acid enantiomeric ratios in shell samples from the Del Mar, California, midden are interpreted in terms of apparent rate constants (derived from observed radiocarbon ages), exponentially decreasing ground temperatures (to values equal to those of mean annual air temperature at depths of approximately 1 m), and sample burial histories involving exposure to high ground temperatures during periods of shallow burial. These models, while approximate, are important in establishing the utility of shell material in amino acid racemization studies. The radiocarbon chronology (∼4000 to ∼9000 years) of the Del Mar shell samples (shell carbonate analysis) is stratigraphically consistent and quite reasonable within the uncertainties of these interpretive models. One level (the lowest) in the Del Mar site has yielded samples whose enantiomeric ratios indicate great antiquity. It is possible that these samples have a geological rather than archaeological origin.