PAIRED RACEMIZATION AND RADIOCARBON ANALYSIS OF PLEISTOCENE AND HOLOCENE SHELLS, US ATLANTIC COASTAL PLAIN: IMPLICATIONS FOR SHELL GEOCHRONOLOGY, PLEISTOCENE SEA LEVEL HISTORY, AND PALEOCLIMATE
For Holocene shells, there is good agreement between the extent of racemization and radiocarbon age, with the exception of a few beach shells that appear to have been exposed to elevated temperature for a substantial portion of their history. For Pleistocene shells, there is much more ambiguity. Thirty-three shells with AAR results indicating either “”Pleistocene” or “last interglacial” based on independent U-series calibration yield 14C ages between ~25 kyr to >52 kyr. Fifteen of these returned ages greater than laboratory detection limits (~35 kyr to ~52 kyr).
The interpretation of finite 14C shell ages for Pleistocene ACP samples is controversial, particularly for emergent samples with apparent marine isotope stage 3 (~45-50 kyr) ages. Nevertheless, some shells from offshore units and from beach sites have AAR values that could equate with an MIS 3 age, consistent with their 14C age. In many cases, however, the geochemical integrity of the shell can be questioned because of the inconsistency of the 14C and AAR results, with small (<1%) modern inorganic carbon contamination being the explanation. Among the several taxa used for these paired analyses, Astarte, Mercenaria, and Spisula usually return results near or beyond 14C lab detection limits, implying that these are geochemically robust shell types.
Plots of D/L values vs. 14C age show predicted trends for Holocene samples. Early Holocene samples have D/L values only slightly below (10-15% at most) those for shells of comparable taxa with Pleistocene 14C ages. This small difference can be explained by the colder temperatures that the Pleistocene shells have experienced for a majority of their history. Although model-dependent, full-glacial temperatures of ~10o +/- 3o are consistent with these observations.