Barium in deep-sea bamboo corals: Phase associations, barium stable isotopes, & prospects for paleoceanography

Abstract

Reconstruction of past seawater δ138/134BaNIST (barium-isotopic compositions) can augment existing proxies of water mass provenance and deep-ocean circulation. Deep-sea bamboo corals are uniquely poised to record Ba-isotopic variations, given their widespread oceanographic distribution and incorporation of ambient Ba in approximate proportion to that in surrounding seawater. However, the utility of such records requires knowing: the phases hosting Ba in deep-sea coral skeletons, that specimens faithfully capture modern Ba-isotopic chemistry, and that internal skeletal variability relates principally to historical variations in the composition of ambient seawater. We investigated each of these requirements using a stepped cleaning experiment, a ‘core-top’ comparison of eight live-collected specimens from the California margin (870–2,055 m) against ambient seawater, and through examination of historical variability in skeletal Ba chemistry, respectively. First, we report that non-carbonate phases minimally contribute to bamboo coral Ba/Ca, obviating the need for chemical cleaning of live-collected specimens. Second, using newly-obtained profiles of northeast Pacific Ba-isotopic chemistry, we observe that bamboo corals faithfully reflect ambient seawater with a taxonomically- and environmentally-invariant Ba-isotopic offset, Δ138/134Bacoral–SW, of −0.37±0.03‰ (±2 SD, n=8). The partition coefficient for Ba, KDBa, is similarly insensitive to taxonomy, but linearly decreases with depth. The driving mechanism is unresolved. Third, we find minimal Ba/Ca and Ba-isotopic variability in historical growth representing the past century. We interpret this invariance as evidencing the overall fidelity of deep-sea bamboo corals for ambient Ba chemistry over their long lifespans. The insensitivity of Δ138/134Bacoral–SW to environmental gradients indicates that the Ba-isotopic composition of bamboo corals can be solely interpreted in terms of seawater composition, which should find myriad applications to the study of past ocean circulation over a range of timescales.