A CALCIUM ISOTOPE EXCURSION ASSOCIATED WITH THE END-TRIASSIC MASS EXTINCTION: IMPLICATIONS FOR OCEAN ACIDIFICATION AND CARBON CYCLE DYNAMICS

The end-Triassic mass extinction preferentially affected heavily calcified marine animals, suggesting ocean acidification was an important kill mechanism. Carbon isotope fluctuations across the Triassic-Jurassic boundary and into the Lower Jurassic are consistent with input of volcanic CO₂ from the Central Atlantic Magmatic Province (CAMP) as an underlying driver. However, changes in δ¹³C cannot be uniquely attributed to volcanic carbon release, and the ocean acidification scenario has yet to be tested using other geochemical proxies. Here we present a high-resolution calcium isotope record from 100 m of marine carbonate sediments spanning the Triassic-Jurassic boundary in two stratigraphic sections from the Lombardy Basin of the southern Alps. Just above the Triassic-Jurassic boundary and the 6‰ negative δ¹³C excursion, δ^44/40Ca decreases approximately 0.7‰ over less than 20 m, then recovers by 0.8‰ over the next 20 m. There are several potential controls on the δ^44/40Ca of limestone, including the δ^44/40Ca of seawater, carbonate mineralogy, precipitation rate, groundwater discharge, and early diagenesis. The most plausible local control is a fluctuation in the proportion of calcite and aragonite sediment being produced locally. Whereas this mechanism can account for variations in δ^44/40Ca, it does not explain the coupled variation in δ^44/40Ca and δ¹³C, the coeval mass extinction event, or the evidence for climate warming. Overall, the most parsimonious explanation for the coupled behavior of the carbon and calcium isotope data is that elevated CO₂ emissions from CAMP volcanism resulted in perturbations to the global carbon and calcium cycles via ocean acidification and enhanced continental weathering. Coupled numerical modeling of the carbon and calcium cycles demonstrates that the δ¹³C and δ^44/40Ca records can reflect the input of large volumes of CO₂ during emplacement of CAMP and gives us quantitative constraints on the volume and isotopic composition of emitted CO₂. This acidification scenario also explains the selective extinction of marine calcifiers, the temporary reduction in carbonate deposition, and the evidence for fluctuating pCO₂ during the end-Triassic crisis.