Mid-latitude seasonality during the Middle Pliocene Warm Interval recorded in the extinct hard clam, Mercenaria corrugata, from the US Middle Atlantic Coastal Plain

The Middle Pliocene Warm Interval (3.2-2.8 Ma) has been identified by the Intergovernmental Panel on Climate Change in its 2007 report as an analog for Earth's climate in the late 21st century. Sclerochronology (analysis of growth patterns) of fossil molluscs that deposit seasonal shell growth increments in isotopic equilibrium with ambient water can serve as archives of past climate conditions. A specimen of the extinct hard clam, Mercenaria corrugata, was collected from the Moore House Member of the Yorktown Formation in Virginia along the US Middle Atlantic Coastal Plain. Oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C) recorded in the M. corrugata shell reflect seasonal changes in the ambient temperature of its habitat over approximately four years of growth. Estimated water temperature is close to the modern day average. The translucent increment of shell growth, reflecting an interval of slowed growth, is formed in early spring to early summer, and rapid growth occurs during the optimal temperatures during summer and early fall. The δ¹³C record also varies seasonally and corresponds with the seasonal variation of the δ¹⁸O time series; however, we were unable to determine the processes that control the variation in δ¹³C values due to confounding environmental and metabolic processes (i.e., vital effects). Further study of additional shells from this region will potentially provide insights into atmospheric and oceanographic circulation (e.g., the position of the Arctic Front and Gulf Stream) during the Middle Pliocene Warm Interval and its effect on biogeography, seasonal temperature variation, and climate.