CALCIUM ISOTOPE EVIDENCE OF AN EPISODE OF OCEAN ACIDIFICATION ACROSS THE SMITHIAN-SPATHIAN BOUNDARY

The Smithian-Spathian boundary (SSB) transition was an interval characterized by a major global carbon cycle perturbation, climatic cooling from a late Smithian hyperthermal condition, and a major setback in the recovery of marine biotas from the end-Permian mass extinction. Previous studies have provided evidence that these events were interconnected. The marine Ca cycle is closely related to marine alkalinity and atmospheric CO₂, playing an important role in the Earth’s climate system. The Ca isotopic composition (δ^44/40Ca) of seawater provides a proxy for changes in the global Ca cycle. Here, to explore perturbations to the global marine Ca cycle across the SSB, we generated a high-resolution Ca isotope profile from the upper Smithian to the lower Spathian of Jiarong section (South China). Our δ^44/40Ca profile shows a negative shift from ~+1.0 ‰ in the early late Smithian to ~+0.5 ‰ at the SSB, followed by a positive shift to ~+1.1 ‰ in the early Spathian. Traditional carbonate diagenetic indicators suggest that our Ca isotope record was not systematically altered by post-depositional diagenesis. We thus interpret these δ^44/40Ca trends as secular variations in the Ca isotopic composition of Early Triassic global seawater. Using a simple Ca isotope mass balance model, we infer a rapid increase in seawater Ca²⁺ concentrations during the late Smithian, coincident with the Smithian Thermal Maximum, and a subsequent decrease in Ca²⁺ concentrations across the SSB that was coincident with climatic cooling. This pattern may reflect an episode of ocean acidification that contributed to end-Smithian marine biodiversity losses. Our study highlights the potential influence of paleo-ocean acidification events on marine organisms and communities.