Reconstructed seasonality during the Mid Piacenzian Warm Interval and early Pleistocene cooling as recorded by growth temperatures from Mercenaria shells

The Mid Piacenzian Warm Interval (MPWI) has been identified as an analogue for future global warming because it had warmer temperatures and higher atmospheric CO2 levels than today, while subsequent early Pleistocene cooling is more similar to modern climate. However, reconstructions of these intervals lack seasonal-scale temperature reconstructions. Seasonal sea surface temperature (SST) reconstructions from deep-time archives can potentially provide insight into the nature of the climate system on sub-annual scales during critical climate states. Here we compared seasonal variability in growth temperatures during the MPWI and subsequent early Pleistocene cooling to modern climate reconstructed from oxygen isotope ratios recorded by the bivalve Mercenaria. Modern shells were collected from the University of North Carolina's Marine Sanctuary. Fossil shells from the Mid-Atlantic Coastal Plain (North Carolina) were collected from the Duplin Formation (MPWI) and the Waccamaw Formation (early Pleistocene). Oxygen isotope ratios range from -1 to 2.4 parts per thousand during the MPWI, -1-2.4 parts per thousand during the early Pleistocene, and -2.2 to 2.3 parts per thousand in modern shells. Stable carbon isotope ratios ranged from -0.91 to 1.67 parts per thousand during the MPWI, 0.19-1.87 parts per thousand during the early Pleistocene, and -2.46 to 0.60 parts per thousand in modern shells. Average seasonal growth temperature variations for the MPWI (similar to 11 +/- 3 degrees C) are reduced compared to modern instrument records (similar to 16 +/- 3 degrees C), while the early Pleistocene (similar to 14 +/- 2 degrees C) was more similar to today. Modern Mercenaria record dark increment formation during the summer while fossil Mercenaria record dark increment formation during the winter. We hypothesize that this difference is timing of dark increment formation may be due in part to differences in water depth (similar to 1m for modern shells and similar to 20m for fossil shells) or a shift in the location of the boundary between biogeographic zones. A -1.6 parts per thousand shift in mean stable carbon isotope ratios between fossil (0.92 +/- 0.66 parts per thousand) and modern (-0.65 +/- 0.44 parts per thousand) shells has been observed and may be a result of the "Suess Effect" and/or influences of estuarine/freshwater relative to marine settings.