Marine Boron Isotope Ratios of the Toarcian Oceanic Anoxic Event Captured in Different Archives - Biology Matters

Calcite of marine macrofossils is critical for the creation of geochemical proxy records used for Paleozoic and Mesozoic palaeoenvironmental reconstructions, because it is least likely to be compromised by diagenesis and contamination among carbonate phases in the sedimentary record. Additionally, techniques to trace effects of alteration on their geochemistry are well established, and fossil specimens are typically large, allowing for the creation of proxy records even for tracers that require relatively large sample sizes. Important obstacles for their use, however, arise from the comparatively high complexity of calcite secreting animals, which tend to induce deviations in isotope ratios from equilibrium with ambient water. While some of these vital effects can be controlled either by sampling of particular portions of fossil shell materials or by the application of corrections for isotope fractionation when using radiogenic isotope proxies, understanding of optimal sampling strategies for novel and metal stable isotope analysis is still limited. Here, we present a case study on the palaeo-pH proxy δ11B from bulk carbonate, brachiopod and oyster calcite across a likely phase of ocean acidification in the early Toarcian stage, the Toarcian Oceanic Anoxic Event. The chosen archives all record distinct fluctuations, but show offsets in absolute δ11B values, and only bulk carbonate data highlight a significant reduction of δ11B during the Toarcian Oceanic Anoxic Event, which can be interpreted as a decrease in ambient water pH. These results indicate that the ability of larger organisms such as bivalves and brachiopods to control the conditions in the liquid from which their shell calcite precipitates can blur or erase the isotopic signatures of changes in ocean chemistry and physical parameters across climate change events in their fossil remains. Chemostratigraphic records of δ11B from macrofossil calcite are therefore likely to be problematic for correlation and faithful reconstruction of ocean pH. While macrofossils will continue to be of great importance to palaeoclimate reconstruction, these results highlight that transferability of knowledge about geochemical proxies in calcite from single-celled organisms to the macrofossil world is limited. Instead, further detailed modern analogue work is needed to more comprehensively understand the expression, opportunities and limitations of novel (and traditional) geochemical proxies in the calcite of marine macrofauna.