SCALLOP RECORDS OF MID MIOCENE AND EARLY PLIOCENE CLIMATE FROM THE MIDDLE ATLANTIC COASTAL PLAIN

Modern instrumental records show a trend of warm winters and an overall decrease in seasonality relative to preindustrial time. Is this pattern of modern global warming unique to today or have past intervals of global warming experienced similar patterns and trends? The Middle Atlantic Coastal Plain (MACP) provides an opportunity to develop new geochemical proxies to reconstruct climate change from two episodes of global warming: the Mid Miocene Climate Optimum (MMOC) and the early Pliocene warm interval (EPWI). We hypothesize that global warming shifts (1) the warm- and cold-temperate biogeographic boundary northward; and (2) the transition of summer versus winter growth cessation in marine bivalves northward. We have generated seasonal climate records of the MMCO and EPWI along a latitudinal (~27° to 37°N) and biogeographic province (tropical to cold-temperate) gradient within a tightly constrained temporal context. Absolute temperature was estimated from shell δ¹⁸O values using published temperature equations and assuming a δ¹⁸O_WATER value of -0.35‰ (VSMOW). Reconstructed seasonal temperatures of the MMCO ranged from 19.7 to 23.1 °C with average winter temperatures of 20.7 °C (n = 11, SD = 0.88) and average summer temperature of 22.4 °C (n = 11, SD = 0.86); whereas, seasonal temperature of the EPWI ranged from 3.1 to 18.7 °C with average winter temperatures of 9.8 °C (n = 9, SD = 4.49) and average summer temperatures of 15.7 °C (n = 11, SD = 1.93). Modern temperature data obtained from the National Oceanographic Data Center (NODC) ranged from average winter temperatures of 11.9 °C (n = 44, SD = 2.35) to average summer temperatures of 14.2 °C (n = 72, SD = 2.76) (NODC, 2006). Our data indicate that the MMCO shell records decreased seasonality and higher overall temperature relative to today, whereas the EPWI shell records seasonality and temperature similar to modern instrument records.