End-Triassic calcification crisis and blooms of organic-walled 'disaster species'

The Triassic–Jurassic (T–J) mass-extinction event is marked by isotope anomalies in organic (δ13Corg) and carbonate carbon (δ13Ccarb) reservoirs. These have been attributed to a (rapid) 4-fold rise in pCO2 as a result of massive flood basalt volcanism and/or methane hydrate dissociation. Here we examine the response of marine photosynthetic phytoplankton to the proposed perturbation in the carbon cycle. Our high-resolution micropalaeontological analysis of T–J boundary beds at St Audrie's Bay in Somerset, UK, provides evidence for a bio-calcification crisis that is characterized by (1) extinction and malformation in calcareous nannoplankton and (2) contemporaneous blooms of organic-walled, green algal 'disaster' species which comprise in one case > 70% of the total palynomorph fraction. Blooms of prasinophytes and acritarchs occur at the onset and in association with a prominent negative shift in δ13Corg values close to the first appearance of the Early Jurassic ammonite Psiloceras planorbis. Across the same interval we obtained palaeotemperature and palaeosalinity estimates from oyster low-Mg calcite based on Mg/Ca, Sr/Ca and δ18O records. The results of our palynological and geochemical analyses strongly suggest that shallow marine basins in NW Europe during this period became salinity stratified, inducing anoxic conditions. The T–J boundary event shows similarities with the Permian–Triassic (P–T) mass-extinction event, which was also marked by extensive flood basalt volcanism, negative excursions in carbon isotope records, a bio-calcification crisis, the development of shallow-marine anoxia and mass abundances of acritarchs in the Early Triassic. This leads us to suggest that the proliferation of green algal phytoplankton may be symptomatic of elevated carbon dioxide levels in the atmosphere and oceans during mass-extinction events