Microbes, mud and methane: cause and consequence of recurrent <scp>E</scp>arly <scp>J</scp>urassic anoxia following the end‐<scp>T</scp>riassic mass extinction

Abstract

Abstract The end‐ T riassic mass extinction ( c . 201.6 Ma) was one of the five largest mass‐extinction events in the history of animal life. It was also associated with a dramatic, long‐lasting change in sedimentation style along the margins of the T ethys O cean, from generally organic‐matter‐poor sediments during the T riassic to generally organic‐matter‐rich black shales during the J urassic. New core material from G ermany provides biomarker evidence of persistent photic‐zone euxinia during the H ettangian, the onset of which is associated with a series of both negative and positive carbon isotope excursions. Combined inorganic and organic geochemical and micropalaeontological analyses reveal strong similarities between the H ettangian and the better‐known T oarcian anoxic event. These events appear to be the most clearly expressed events within a series of anoxic episodes that also include poorly studied black shale intervals during the S inemurian and P liensbachian. Both the H ettangian and T oarcian events are marked by important changes in phytoplankton assemblages from chromophyte‐ to chlorophyte‐dominated assemblages within the E uropean E picontinental S eaway. Phytoplankton changes occurred in association with the establishment of photic‐zone euxinia, driven by a general increase in salinity stratification and warming of surface waters. For both events, the causes of large negative carbon isotope excursions remain incompletely understood; evidence exists for both variation in the δ 13 C of atmospheric CO 2 and variation in the sources of organic carbon. Regardless of the causes of δ 13 C variability, long‐term ocean anoxia during the E arly J urassic can be attributed to greenhouse warming and increased nutrient delivery to the oceans triggered by flood basalt volcanism.