CONSTRAINING THE CAUSE OF THE END-GUADALUPIAN EXTINCTION WITH COUPLED RECORDS OF CARBON AND CALCIUM ISOTOPES

A negative δ¹³C excursion in carbonate sediments from Guadalupian (Middle Permian) and Lopingian (Late Permian) stratigraphic sections has been interpreted to result from a large carbon cycle disturbance during end-Guadalupian extinction event (ca. 260 Ma). However, the carbon isotope data alone are insufficient to uniquely determine the type and magnitude of perturbations to the global carbon cycle. The carbon and calcium cycles are coupled via CaCO₃ burial, so changes in calcium isotopes can be used to constrain the cause of a carbon isotope excursion. In this study, we present coupled carbon and calcium isotope records from three Guadalupian-Lopingian (G/L) sections in China and Turkey. Isotope records among our studied sections are inconsistent in both their δ¹³C and δ^44/40Ca records. Similar inconsistencies in δ¹³C among sections occur across previously published datasets. Sections with large (>3‰) changes in δ¹³C either show evidence for diagenetic alteration or do not show δ¹³C and δ^44/40Ca changes consistent with severe volcanic degassing from Emeishan or methane clathrate destabilization. We conclude that the large isotopic changes are more likely the result of local burial conditions or diagenetic effects, rather than a large carbon cycle disturbance. Perturbations to the global carbon and calcium cycles appear to have been much smaller across the G/L transition than across the subsequent Permian-Triassic boundary. This finding is consistent with recent paleobiological data showing that the end-Guadalupian extinction was much less severe than previously believed, and was indistinguishable in magnitude from background intervals. However, selective extinction of marine animals with passive respiratory physiology indicates that the G/L extinction cannot simply be due to background extinction or sampling failure, and that it was triggered by some environmental event. Therefore, any environmental event must have been small enough to not generate large changes in δ¹³C and δ^44/40Ca. A small acidification or warming event is therefore plausible, but difficult to confirm. Loss of marine habitat by way of sea-level change also remains a possible extinction trigger.