Global perturbation of the marine calcium cycle during the Permian-Triassic transition

A negative shift in the calcium isotopiccomposition of marine carbonate rocksspanning the end-Permian extinction horizonin South China has been used to arguefor an ocean acidification event coincidentwith mass extinction. This interpretationhas proven controversial, both because theexcursion has not been demonstrated acrossmultiple, widely separated localities, and becausemodeling results of coupled carbon andcalcium isotope records illustrate that calciumcycle imbalances alone cannot accountfor the full magnitude of the isotope excursion.Here, we further test potential controlson the Permian-Triassic calcium isotoperecord by measuring calcium isotope ratiosfrom shallow-marine carbonate successionsspanning the Permian-Triassic boundary inTurkey, Italy, and Oman. All measured sectionsdisplay negative shifts in δ44/40Ca of upto 0.6‰. Consistency in the direction, magnitude,and timing of the calcium isotope excursionacross these widely separated localitiesimplies a primary and global δ44/40Ca signature.Based on the results of a coupled boxmodel of the geological carbon and calciumcycles, we interpret the excursion to reflect aseries of consequences arising from volcanicCO2 release, including a temporary decreasein seawater δ44/40Ca due to short-lived oceanacidification and a more protracted increasein calcium isotope fractionation associatedwith a shift toward more primary aragonitein the sediment and, potentially, subsequentlyelevated carbonate saturation statescaused by the persistence of elevated CO2delivery from volcanism. Locally, changingbalances between aragonite and calcite productionare sufficient to account for the calciumisotope excursions, but this effect alonedoes not explain the globally observed negativeexcursion in the δ13C values of carbonatesediments and organic matter as well. Onlya carbon release event and related geochemicalconsequences are consistent both withcalcium and carbon isotope data. The carbonrelease scenario can also account for oxygenisotope evidence for dramatic and protractedglobal warming as well as paleontologicalevidencefor the preferential extinction ofmarine animals most susceptible to acidification,warming, and anoxia