Can sclerosponge skeletons record ocean acidification?: Boron and carbon isotope ratios (δ¹¹B and δ¹³C) in Acanthochaetetes wellsi from Okinoerabu Island, southwestern Japan

Abstract

Boron stable isotope ratios in biogenic calcium carbonate minerals are known to reflect the decreasing of seawater pH, and thus they can be a useful tracer to track the trend of the ocean acidification. While validation of boron isotopes as a tracer for seawater pH has mainly focused on foraminiferal and coral CaCO₃, a few studies examined CaCO₃ skeletons produced by sclerosponges. In this study, we investigated stable boron and carbon isotope ratios in two sclerosponge specimens (Acanthochaetetes wellsi), collected from Okinoerabu Island, Japan. Carbon isotope ratios in both specimens showed a continuous decrease over the estimated growth periods, indicating that the Suess effect is recorded in sclerosponge skeletons. In contrast, boron isotope ratios in one specimen decreased over time, but not in the other. These findings suggest further analysis of additional specimens is necessary to determine whether boron isotope ratios in sclerosponge skeletons are reliable recorder of ocean acidification.