REFINING A ∆47 PALEOTEMPERATURE CALIBRATION USING MODERN AND FOSSIL MERCENARIA SPP. (BIVALVIA) TO RECONSTRUCT DEEP-TIME SEASONALITY

Traditional oxygen isotope (δ¹⁸O) measurements in biogenic carbonates have been the dominant seawater (sw) temperature proxy in marine archives since the 1950s and have been used to reconstruct Cenozoic climate. However, temperature reconstructions can be complicated by assumptions made to constrain δ¹⁸O_sw values. Clumped isotope geochemistry (∆₄₇) shows promise as a novel paleothermometer, as it does not rely on constraining the δ¹⁸O_sw value. This study tests the viability of ∆₄₇ as a paleothermometer in the marine/estuarine bivalve Mercenaria mercenaria from coastal NC, USA. We analyzed δ¹⁸O_shell values of 4 live-collected shells from UNC-Wilmington’s marine sanctuary providing sub-monthly snapshots spanning the past 15 years. Values ranged from +2.3‰ to −2.2‰ (VPDB). We calculated predicted δ¹⁸O_shell values using in situ water data (temperature, salinity, and published salinity-δ¹⁸O_sw relationship) measured from a nearby location and an equilibrium fractionation equation for aragonite. Predicted values and the commonly used “wiggle matching” approach allowed us to assign dates to δ¹⁸O_shell values and estimate measured water temperature for the ∆₄₇ calibration. We found shells track maximum daily temperatures and do not record winter temperature below ~10ºC. Our ∆₄₇ sampling strategy targets peaks and valleys along the δ¹⁸O_shell time-series to capture average maximum summer (30.8±0.7°C) and average minimum winter (7.2±2.2°C) temperatures recorded in the shells. A total of 27 ∆₄₇ samples will be used to construct the calibration. Once constructed, our calibration will be compared to several published ∆₄₇ constructions using an ANOVA statistical test. We will compare ∆₄₇-derived paleotemperatures to published δ¹⁸O-derived paleotemperatures recorded in fossil Mercenaria spp. shells from the Mid Pliocene Warming Interval (Duplin Formation) and subsequent cooling in the early Pleistocene (Waccamaw Formation). Temperatures derived from our ∆₄₇ analysis of fossil shells, along with measured δ¹⁸O_shell values, will allow us to validate previously published δ¹⁸O_sw values based on global ice volume estimates.