A Clumped Isotope and SIMS Perspective on Bamboo Coral “Lines” Paleothermometry

Abstract:

The pattern and causes of intermediate and deep ocean temperature variability are poorly understood, in part because of a paucity of reliable paleothermometers. Long-lived, cosmopolitan deep-sea bamboo corals (Keratoisis sp.) may be valuable paleotemperature archives, but realizing their potential has been complicated by evidence for vital and kinetic effects. Among these is a strong linear correlation in δ¹⁸O and δ¹³C along radial growth transects. The “lines” method assumes a common mechanism causes proportional δ¹⁸O and δ¹³C deviations from equilibrium, and empirically accounts for them to estimate the mean growth temperature across a coral’s lifespan. Recent work suggests similar δ¹⁸O - δ¹³C trends occur in contemporaneous skeleton, raising the possibility that timeseries of “lines” paleotemperatures may be possible. Here, we explore the mechanisms and paleoclimatic utility of the “lines” method using 1) traditional IRMS-based analyses of micromilled samples 2) SIMS-based analyses of δ¹⁸O and δ¹³C and 3) carbonate clumped isotopes. Consistent with previous studies, micromilled data show that clear linear δ¹⁸O - δ¹³C correlations in skeleton deposited during approximately the same multidecadal interval yield accurate, albeit relatively imprecise, “lines” paleotemperatures. However, SIMS data show no clear correlation suggesting that other processes affect δ¹⁸O - δ¹³C variability at spatial scales of tens of microns. Clumped isotope data will help evaluate if these patterns are consistent with variations in equilibrium dissolved inorganic carbon (DIC) speciation associated with pH, a non-equilibrium DIC pool and/or kinetic effects at the solution-skeleton interface.