Microbial mats record magmatically driven sea level fall as a prelude to the end-Triassic mass extinction

Abrupt sea-level fall as a consequence of the Central Atlantic Magmatic Province (CAMP), is a characteristic of the end-Triassic mass extinction. In the SW UK and also, in most European basins, there was a short-lived replacement of the marine organisms with shallow and fresh to brackish ecosystems, comprising microbial mats shown by biomarkers and their 13C- values. The established carbon isotopic excursion, typically recognized as marking the extinction event, is not exclusively associated with light carbon input from the CAMP activity, but instead a result of the regional reduction in salinity and water depth. Thus the marine extinction is identified at a higher level deposited as sea-level rose and is accompanied with a major biocalcification crisis event. CAMP resulted in pulsed doublings-triplings in atmospheric pCO2 linked to the biocalcification crisis that led to almost total extinction of scleractinian corals, ammonoids and a massive loss in bivalve diversity and the extinction of conodonts. From biomarkers we show that the biocalcification event was related to persistent photic zone euxinia and anoxia in SW UK. The event is characterized by a Lilliput molluscan assemblage, followed by a deficiency in calcified fossils, and ending with the reappearance of aragonitic ammonites leading into the Jurassic. We also demonstrate that an older precursor carbon isotopic excursion is not of CAMP origin, but rather related to increased input of 12C-rich C3-plants. The marine extinction event at the onset of the biocalcification crisis is thus directly tied to extremely harsh changes in ocean chemistry and stratification attributed to CAMP CO2.