186-6 Detecting Deep Time Ocean Acidification Events with High-Precision Calcium Isotope Measurements

Abstract:

Earth Scientists have long hypothesized that CO₂ outgassing during large igneous province eruptions can acidify seawater, perturb carbonate mineral saturation states, and suppress biocalcification. To test this hypothesis, our group has worked over the past decade to generate and interpret high-precision TIMS δ^44/40Ca records spanning multiple candidate ocean acidification (OA) events, including Ocean Anoxic Event 1a (OAE1a), the Aptian-Albian Boundary Interval (AABI), Ocean Anoxic Event 2 (OAE2), the Cretaceous-Paleogene boundary (K-Pg), and the Paleocene-Eocene Thermal Maximum (PETM). When aligned relative to accepted event boundaries/onsets, most records resolve similar fine-scale signals that oscillate around different average δ^44/40Ca values. The isotopic pattern comprises positive-to-negative shifts before the boundaries, negative-to-positive shifts that begin before and peak at the boundaries, negative excursions after the boundaries, and positive recoveries. Two K-Pg records yield the only exception, where peak δ^44/40Ca values occur immediately prior to the Chicxulub impact. The consistently repeated pattern is striking given that the records represent different depositional settings, CaCO₃ archives, and mineralogies (calcite and aragonite). Our dataset includes supporting measurements (e.g., δ^88/86Sr, Os_i, and Sr/Ca), paleontological observations, and model calculations, which allow us to constrain the pattern’s origin and meaning. The high-fidelity preservation of primary kinetic isotope effects (KIEs) offers the best explanation. Higher δ^44/40Ca values imply slower biocalcification rates, whereas lower δ^44/40Ca values imply faster biocalcification rates. Alternative explanations, such as diagenetic overprinting and rapid changes in the Ca isotope composition of seawater due to enhanced chemical weathering and/or reduced carbonate burial, appear less likely. The AABI dataset systematically eliminates these alternative explanations. Among multiple insights, we contend that discrepant views of the δ^44/40Ca tracer reflect artifacts of variable data quality and model assumptions that conflict with empirical evidence. Our findings illuminate the candidate OA events as volcanically triggered biocalcification crises. Implications exist for several first-order topics, including, but not limited to, linkages between solid and surface Earth processes, climate change, mass extinctions, OA dampening mechanisms, biomineralization, and even development of novel CO₂ removal strategies, such as artificial ocean alkalinization. 

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