Calcium isotope evidence of increased carbonate saturation state during the Frasnian–Famennian boundary event

The Frasnian–Famennian (F–F; Late Devonian) mass extinction (ca. ∼375 Ma) was one of the largest biocrises of the Phanerozoic, yet the mechanism that drove environmental change during this event remains in debate. Here, we report new geochemical data from two Upper Devonian marine carbonate successions (Baisha and Yangdi) that span the Upper Kellwasser Event (UKE) in Guangxi Province, South China to investigate the trigger mechanism of the F-F mass extinction. The measurements include carbonate carbon and oxygen isotopes (δ13Ccarb and δ18Ocarb) and calcium isotopes (δ44/40Cacarb), the major- and trace-element geochemistry of both bulk rocks and their carbonate fractions, and the strontium isotopes of conodonts (87Sr/86Srconodont). The δ13Ccarb and δ44/40Cacarb profiles of the study sections were not strongly affected by diagenetic alteration and are readily correlated with marine isotope records globally. The temporal resolution of our new δ44/40Cacarb record is ∼3 to 5 × higher than those of prior studies, thereby providing more nuanced insight into carbonate saturation state changes during the F‒F mass extinction event. We observe a ∼3 ‰ positive δ13Ccarb excursion and near-constant 87Sr/86Srconodont in both study sections, coinciding with δ44/40Cacarb excursions of ∼‒0.2 ‰ at Baisha and ∼‒0.4 ‰ at Yangdi. Results of a Ca-cycle box model that accounts for kinetic Ca isotope fractionation suggest that a δ44/40Cacarb excursion of ∼‒0.2 ‰ during the UKE can be explained by a 0.25 ‰ increase in the magnitude of Ca isotope fractionation between carbonate and seawater (i.e., Δ44/40Cacarb-sw changes from ∼‒0.98 to ∼‒1.23 ‰). We infer that changes in Ca isotope fractionation were due to increased carbonate precipitation rates resulting from decreased atmospheric pCO2 and higher oceanic carbonate saturation state. The proximate driver of environmental changes during the F‒F transition was likely transient climatic cooling, with the ultimate driver being enhanced organic carbon burial linked to the rapid spread of arborescent land plants.