Tracing volcanic emissions from the Central Atlantic Magmatic Province in the sedimentary record

Abstract

The end-Triassic mass extinction (ETME) is thought to have been caused by voluminous, pulsed volcanic activity of the Central Atlantic Magmatic Province (CAMP). Over the last decades, various geochemical signals and proxy records, including δ13C, pCO2, iridium and other platinum-group elements, mercury, polycyclic aromatic hydrocarbons (PAH), charcoal and SO2, have been directly or indirectly attributed to CAMP magmatism. Here, we compile these various records in a stratigraphic framework to present a cohesive chain of events for the CAMP and the end-Triassic mass extinction. Mercury and iridium anomalies in sediments indicate that CAMP activity commenced prior to the onset of the marine extinctions (as marked by the last occurrence of the Triassic ammonoid Choristoceras marshi or closely related species), and a negative δ13C excursion in organic matter (the Marshi CIE). This CIE may be explained by input of light carbon to the atmosphere from CAMP lavas of the Tiourjdal and Prevalent groups. Pedogenic carbonate below and above the Prevalent group in North America indicates a more than twofold increase in atmospheric pCO2. Subsequent n-alkane C-isotopes, and stomatal pCO2 data seem to indicate a temporary cooling after the Marshi CIE, which is consistent with climate models incorporating volcanic emissions of both CO2 and SO2. Records of excess iridium and Hg/TOC indicate intensified magmatism during the extinction interval. Tectonic and perhaps epeirogenic (i.e. doming due to rise of magma) activity is suggested by the occurrence of multiple and widespread seismites in Europe. Atmospheric pCO2 proxies indicate global warming, which culminated contemporaneously with the Spelae CIE. Global warming is corroborated by increased wildfire activity testified by charcoal and pyrolytic PAH records. Increased isotopic ratios of Os and Sr from sections that record global ocean signatures suggest increased weathering of continental crust likely due to climatic changes. Just prior to the increase in pCO2 from stomatal proxy data, fossil plants exhibit SO2-induced damage indicating excess sulfur dioxide deposition priot to and across the Triassic–Jurassic boundary. At the same time, increased ratios of heavy molecular PAHs (coronene/benzo(a)pyrene) in sediments suggest metamorphism of organic sediments also occurred across the Triassic–Jurassic boundary. These proxies may suggest that thermogenic release of light carbon and sulfur from sill intrusions in the Trans-Amazonian basins, where both evaporate- and organic-rich sediments are known to have been intruded, may have played an important role during the course of the ETME. Geochemical traces of magmatism, i.e. Ir and Hg, appear to have gradually disappeared during the Hettangian, suggesting that later phases of CAMP were less voluminous. Stomatal proxy data from Greenland and n-alkane C-isotope data from the UK, together with oxygen isotope data from carbonate fossils in the UK, may indicate that the global warming at the Spelae CIE was succeeded by another short-term cooling event. A gradual decrease in δ13C culminated at the top-Tilmanni CIE, marking the beginning of a long-term steady state with more negative C-isotope values than prior to the ETME. At this time, terrestrial ecosystems appear to have stabilized globally and ammonoids had begun to rediversify.