EVIDENCE FROM BIVALVE SCLEROCHRONOLOGY FOR A COOL LATE PLIOCENE CLIMATE IN THE WESTERN NORTH ATLANTIC

Pliocene temperature data from the US Atlantic coastal plain is currently insufficient for a detailed understanding of how global climatic shifts during the epoch affected the region. Previous studies based on planktonic assemblages, bryozoan zooid size analysis, and oxygen isotope proxy data from marine shells have provided some constraints on possible annual-scale sea surface temperatures (SST) for the region, but results are inconsistent. More data are required to fully understand the forcing mechanisms affecting regional Pliocene climate and more accurately evaluate modeled temperature projections.

Bivalve sclerochronology (growth increment analysis) is an alternative proxy for SST that can provide annually resolved multi-year time series. In this study we applied this technique to Pliocene Mercenaria shells and compared estimated growth rates with data from modern Mercenaria populations whose growth rates were controlled by known environmental parameters. We tested the hypotheses that: (1) shells from the Mid Pliocene Warm Period (3.5 - 3.3 Ma, upper Yorktown Fm., VA and NC) recorded higher SST relative to today; and (2) shells from the subsequent cooling interval (2.4 - 1.8 Ma., Chowan River Fm., NC) recorded lower SST than present. Preliminary results show regional average annual SST experienced by Chowan River shells were ~5 degrees cooler than today, as predicted. Unexpectedly, the growth rates of populations analyzed from multiple Yorktown Fm. localities also suggest the animals were living in cooler than modern conditions, with SST ~4 to 6 degrees cooler than today. Future work will validate our growth increment findings using stable isotope analysis, which will also provide estimates of regional seasonality.