Ocean euxinia and climate change "double whammy" drove the Late Ordovician mass extinction

The Late Ordovician mass extinction (LOME, ca. 445 Ma) was the first of the "Big Five" Phanerozoic extinction events and comprised two extinction pulses. Proposed kill mechanisms include glacially induced global cooling and the expansion of water-column anoxia and/or euxinia (sulfidic conditions), but no general consensus has been reached with regard to the precise role of these mechanisms. A more definitive understanding is hampered by poorly constrained temporal links between the extinction pulses and climate change, and by uncertainty over the spatial distribution and intensity of euxinia. Here, we utilize Fe speciation and Mo concentrations, in addition to the chemical index of alteration weathering proxy, to reconstruct ocean redox conditions and climate change across a Late Ordovician to Early Silurian shelf-to-slope transect on the Yangtze Shelf Sea (a siliciclastic-dominated shelf basin in South China). These data show two cycles of expanded euxinia corresponding to the two pulses of the LOME, suggesting a strong causal relationship. Significantly, we show that intermittent or weak euxinia developed during the first extinction pulse, which likely accounts for the loss of benthic fauna and some planktonic organisms and nektonic groups. By contrast, the development of more intense euxinia throughout the water column during the second pulse likely drove survival fauna to extinction. Superimposed upon this, significant global cooling occurred across the first extinction phase, reflecting a secondary role in driving the extinction of certain low-latitude taxa.