Seasonal Sea Surface Temperatures from Mercenaria spp. During the Plio-Pleistocene: Oxygen Isotope Versus Clumped Isotope Paleothermometers

The Mid-Piacenzian Warm Interval (MPWI) is marked by warmer temperatures and higher atmospheric CO2 levels than today, making it an analogue for late-21st-century-warming, whereas the early Pleistocene cooling is more like today. We compare seasonal growth temperatures derived from oxygen isotope ratios (δ18O) and clumped isotopes (∆47) in Mercenaria. Modern shells were previously collected from coastal NC. The fossil shells are from the Duplin (MPWI) and Waccamaw Formations (early Pleistocene), NC. Oxygen isotope ratios range from −2.2‰ to 2.3‰ (modern), −0.9‰ to 2.4‰ (MPWI), and −0.9‰ to 2.9‰ (early Pleistocene). The values of Δ47 range from 0.576‰ to 0.639‰ (modern), 0.566‰ to 0.621‰ (MPWI), and 0.581‰ to 0.615‰ (early Pleistocene). We show that Mercenaria do not require a species-specific ∆47 calibration. Modern and MPWI ∆47-derived summer/winter temperatures (SST∆47) and seasonal amplitudes are indistinguishable from δ18O-derived temperatures. The early Pleistocene summer SST∆47 is indistinguishable from δ18O-derived temperatures, but the winter SST∆47 is warmer by 5 °C and may reflect within-shell time averaging. The modern summer/winter SST∆47 are indistinguishable from the MPWI, but the MPWI has a lower seasonal amplitude by 5 °C. Compared to our calculated δ18Osw values, modeled values for the MPWI are within error but are much lower, and they are not within error for the early Pleistocene.