Selenium isotope ratios, redox changes and biological productivity across the end-Permian mass extinction

• Selenium isotopes show a small positive and a large negative excursion associated with the Permian–Triassic mass extinction.
• Data are consistent with local euxinia followed by a decline of macro-biological productivity under (sub-)oxic conditions.
• Selenium isotopes can be a useful geobiological proxy in deep time.

The causes of the greatest mass extinction in Earth's history, in the latest Permian, remain actively debated. Here we use Se isotopes and abundances in marine sediments from an outer-shelf environment to test one of the most common hypotheses for the collapse of the biosphere, i.e. widespread euxinia in the open ocean. Our data show a small positive excursion in δ82/78Se prior to the extinction, consistent with local euxinia. However, this is followed by a significant negative excursion with a minimum of −1.8‰ (relative to NIST SRM 3149), immediately preceding the principal extinction horizon. A net fractionation of this magnitude likely resulted from partial reduction of Se oxyanions dissolved in the water column. Due to their low abundance, Se oxyanions are rapidly scavenged in anoxic basins or regions of high biological productivity with little net isotopic fractionation. We therefore interpret the uniquely negative fractionations in this section as an indicator for relatively oxygenated conditions in this marine basin at the time when biological productivity declined. The offset between the peak excursion and the major extinction horizon possibly reflects a slow-down in ocean circulation leading to nutrient limitation, which may thus have prohibited a rapid recovery of the local biosphere in the early Triassic. Although we are unable to extrapolate to the global ocean due to the short residence time of Se in seawater, our data are consistent with the newly emerging view that euxinia developed along ocean margins and in oxygen minimum zones before the extinction, but was probably replaced by (sub-)oxic conditions during the ~ 1 kyr peak productivity decline and was thus not solely responsible for the extinction event.