TIMING OF OCEAN ACIDIFICATION AT THE LATEST PERMIAN MASS EXTINCTION: EVIDENCE FROM FOSSILS

The most catastrophic mass extinction in Earth’s history, the latest Permian mass extinction (252 million years ago) resulted in the loss of approximately 81% of all species on Earth. Currently, the explanation favoured by most researchers is that the cause of this extinction event in the oceans was a combination of ocean anoxia, high sea surface temperatures, and ocean acidification. Nevertheless, despite several provocative studies, there is no robust evidence as to whether ocean acidification was a contributing factor. To test for evidence of acidification at the latest Permian mass extinction, this research seeks to identify dissolution and repair marks on post-extinction fossils, which would indicate shell growth in acidic conditions. An exceptionally well-preserved fossil assemblage collected from the Permian/Triassic boundary in Svalbard (Arctic Norway) provides the opportunity to assess both shell dissolution and repair, and thus investigate whether ocean acidification was present at the extinction event. Because ocean acidification events are rapid (typically less than 20 kyr), the samples collected from horizons closest to the latest Permian extinction are expected to show dissolution scars and repairs, caused by an acidified water column, while those after the hypothesized acidification interval, i.e. at the Permian/Triassic boundary, would not. Samples from the hypothesised ocean acidification pulse are dominated by lingulids, but very little else. Lingulids would not have been affected by the undersaturation of calcium carbonate because of their organophosphatic shell and use of hemerythrin. Samples collected from the strata deposited immediately after the expected ocean acidification pulse contain gastropods, bivalves, lingulids, and sponge spicules. This composition change, therefore, may suggest the presence of acidification during the extinction as recovery occurs rapidly following the theoretical return to supersaturation. The lack of dissolution and repair marks in the well-preserved mollusks at the Permian/Triassic boundary (11.9 and 12.6 m and roughly 60 kyrs above the extinction horizon) clearly demonstrates that neither the planktic nor benthic habitats were affected by the ocean acidification at this time, contrary to some geochemical evidence from other sections.