Deep-sea stylasterid <i>δ</i>¹⁸O and <i>δ</i>¹³C maps inform sampling scheme for paleotemperature reconstructions

Abstract

Deep-sea corals have the potential to provide high-resolution paleotemperature records to evaluate oceanographic changes in settings that are vulnerable to current and future ocean warming. The isotopic records preserved in coral skeletal carbonate, however, are limited by their large offsets from isotopic equilibrium with seawater. These “vital effects” are the result of biological influences (kinetic and metabolic) on the calcification of coral skeletons and are well known to drive oxygen and carbon stable isotope ratios (δ18O and δ13C, respectively) away from isotopic equilibrium with environmental variables. In this study, two calcitic stylasterid corals (Errina fissurata) are sampled via cross sections through their primary growth axes to create skeletal δ18O and δ13C maps. The maps reveal a consistent trend of increasing isotopic values toward the innermost portion of the cross sections, with minimal spatial change in carbonate mineralogy, the average center values being ∼1 ‰ and ∼3 ‰ closer to seawater δ18O and δ13C equilibrium values, respectively. We investigate possible mechanisms for these isotopic trends, including potential growth patterns that would drive spatial isotopic trends. Our results highlight the diversity of the stylasterid coral family, and because of our unique sampling strategy, we can prescribe that E. fissurata corals with minimal mineralogical variability be sampled from the center portions of their stems to achieve accurate paleotemperature reconstructions.