Oxygen isotopic evidence for greater seasonality in Holocene shells of Donax variabilis from Florida

Donax variabilis, the variable coquina clam, has been a common inhabitant of exposed sandy beach intertidal and shallow subtidal zones in the southeastern United States throughout the Pleistocene and Holocene. It is ideally suited for paleotemperature studies because it is restricted to environments of well-mixed, normal-marine seawater with a fairly uniform isotopic composition. As a result, oxygen isotopic variability in D. variabilis shells is largely explained by temperature variation. Although D. variabilis is small and short-lived, its shell represents an important paleoclimate archive because of its unique habitat preference. High-resolution sampling of individual D. variabilis shells and comparison of oxygen isotopic temperature profiles with historical seawater temperatures from the northeastern Florida coast indicate rapid shell growth over a brief life span of three to five or six months. Analysis of two modern shells reveals a close correspondence between isotopically determined water temperatures and historical water temperatures during the spring-summer growing season. Paleotemperature profiles from four archaeological shells, however, suggest a longer growth interval spanning summer-autumn. Two Preceramic Archaic Period shells (ca. 4240 and 5570 14C yr BP) and two Orange Period Archaic shells (ca. 3600 and 3760 14C yr BP), from four different archaeological sites, yield paleotemperatures that average 3.5 °C higher than present summer-autumn water temperatures. These warm paleotemperatures highlight seasonality differences associated with the mid-Holocene Hypsithermal climatic interval in this region.