Stable Ca and Sr isotope responses to ocean acidification during Oceanic Anoxic Event 2

Le résumé fourni par la source

Volcanism-driven ocean acidification has repeatedly disrupted Earth's carbon cycle and ecosystems. Calcium isotopes (δ 44/40 Ca) provide a potential means of tracking these perturbations, since both the weathering and burial of CaCO 3 and the isotopic compositions associated with these fluxes are sensitive to changes in seawater pH. However, the use of δ 44/40 Ca to reconstruct acidification and infer biocalcification stress is often complicated from overprinting by diagenetic processes. In this paper, we investigate stable Ca (δ 44/40 Ca) and Sr (δ 88/86 Sr) isotope responses to ocean acidification during Oceanic Anoxic Event 2 (OAE 2) in a pelagic marine carbonate succession from the Western Interior Seaway. The records are reconstructed using the Iona-1 core in Texas, which fully captures Os isotope evidence for the onset of LIP volcanism that is missing from a previously published Ca isotope record in the Western Interior Seaway in Colorado. Here we show that a negative Ca isotope excursion occupies the missing part of the previously published Western Interior Seaway record, and that it the begins with the onset of large igneous province volcanism as represented by the abrupt shift to lower initial 187 Os/ 188 Os isotopic ratios in the core. Box-model simulations show that the negative Ca isotope excursion could be produced through reduced Ca isotope fractionation affecting pelagic carbonate production under conditions of transient acidification stress, followed by rapid recovery through carbonate alkalinity feedback. δ 88/86 Sr values broadly covary with δ 44/40 Ca values but deviate from the expected kinetic slope, instead defining a mixing array consistent with seawater-buffered recrystallization. Together, the coupled stable Ca-Sr isotope systematics distinguishes primary acidification-driven signals from diagenetic modification and demonstrate the utility of dual-isotope approaches for tracing volcanism-induced perturbations to the marine carbon cycle.

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