Clumped isotope signatures in modern brachiopod carbonate

Abstract

We present a temperature to Δ47 relationship for naturally occurring modern brachiopods, which was established using a suite of thirteen brachiopod specimens from twelve sampling localities that span a mean annual seawater temperature range of −0.8 to 29.0°C. Sample locations and depths were selected to maximize the range of temperatures covered by the calibration while minimizing the seasonal temperature variability impacting the samples. Linear least-squares regression analysis yields the following relationship expressed using the Ghosh et al. (2006) reference frame:Δ47=0.0478±0.0099⋅106/T2+0.1164±0.1203n=13;R2=0.91;95%CI.This equation lies within the 95% confidence interval of the original relationship determined using synthetic calcite (Ghosh et al., 2006), but is in better agreement with the newly revised calibration of Zaarur et al. (2013). However, despite this broad agreement, our equation has a shallower slope than the equations for synthetic calcites, and yields temperatures at the warm end of the calibration temperature range (Δ47=0.640‰) that are as much as 3.1°C offset from those predicted using the equation of Zaarur et al. (2013). On the other hand, our new equation has a steeper slope than the Henkes et al. (2013) equation for modern brachiopods and mollusks. This finding is consistent with the hypothesis that differences in the protocols employed by laboratories that use different temperatures of phosphoric acid digestion cause the observed discrepancies between the Δ47 values reported by various laboratories.