CALCIUM ISOTOPE VARIABILITY ACROSS ANCIENT CANDIDATE OCEAN ACIDIFICATION EVENTS

CO₂ emissions during large igneous province (LIP) eruptions are expected to affect biocalcification by acidifying seawater, decreasing [CO₃^2-], and lowering carbonate saturation states. Marine Ca isotope (δ^44/40Ca) records may provide insight into biocalcification crises because rate-dependent effects govern Ca isotope fractionation during carbonate mineral formation. However, input/output flux imbalances and early diagenesis can produce similar signals. To resolve various controls, we compared five high-precision TIMS δ^44/40Ca records spanning candidate ocean acidification events: the end-Permian mass extinction (EPME, ~252 Ma), the Cretaceous Oceanic Anoxic Events (OAE1a and OAE2, ~120 and ~94 Ma), the end-Cretaceous mass extinction (ECME, ~66 Ma), and the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma). The EPME exhibits a large (~0.6‰) and rapid negative δ^44/40Ca excursion. The other four records show similar patterns, with positive excursions (~0.10 – 0.15‰) that begin before major boundaries indicated by the onset of δ¹³C excursions, negative shifts after the boundaries, and positive recoveries. The apparent similarities are striking given that the four records represent different archives (bulk sediments and fossils), mineralogies (calcite and aragonite), and event durations (~0.2 to 1 Myr). These data are consistent with preservation of primary kinetic isotope effects and provide less support for a diagenetic origin. In particular, diagenesis of molluscan aragonite drives Sr/Ca and Ca isotope ratios in the opposite direction compared to bulk sediments and calcite microfossils. The ECME δ^44/40Ca record correlates with sedimentological indicators of saturation state. PETM foraminiferal δ^44/40Ca show no correspondence with sedimentological indicators of diagenesis. In the case of OAE1a, bulk carbonate δ^44/40Ca and [Sr], as well as δ^44/40Ca and δ^88/86Sr, strongly correlate and yield slopes expected for variable mass-dependent fractionation. Noting that atmospheric CO₂ provides a first-order control on the carbonate geochemistry of seawater, we hypothesize that the similar δ^44/40Ca patterns reflect a causal connection between LIP eruptions and biocalcification feedbacks. The EPME record appears anomalous and likely reflects additional effects.