What triggered the Late Ordovician mass extinction (LOME)? Perspectives from geobiology and biogeochemical modeling

Recent studies have described an oceanic anoxic event during the Hirnantian (HOAE) and linked this event to the Late Ordovician mass extinction (LOME). However, the extent and duration of the HOAE remain under debate, as do questions about how oceanic anoxia impacted marine ecosystems. For this study, we investigated two previously unstudied sections adjacent to the open ocean in South China for iron speciation data and sulfur isotope signatures of pyrite and carbonate associated sulfate. Combined with published results, these data provide a view of both local and global oceanic redox landscapes during this interval. Previous work has challenged the idea that increased oxygen solubility under colder temperatures was the critical control of glacial marine oxygenation. Here we describe oxygenated Hirnantian shallow-water environments impacted by the effects of glacio-eustatic variation and deep-water anoxia induced by an enhanced biological pump. Decreased isotopic fractionation
between seawater sulfate and sedimentary pyrite has the potential to reveal a period of low oceanic sulfate levels and helps to constrain the duration and style of oceanic redox evolution during the HOAE. Moreover, our modeling work suggests another possibility that the Hirnantian sulfur cycle shifted less than +3‰ within 1 Ma as a result of decreased silicate weathering, increased volcanism, and enhanced pyrite burial. Correlations with data for marine faunal diversity are somewhat inconsistent with previous assertions that oceanic anoxia triggered the LOME. Nonetheless, cool-water tolerant animals (especially within benthos) may have suffered from oxygen- deficient seawater and lethal metal levels within their limited ecospace much later in the middle of the Hirnantian. Recovery or proliferation of some portions of the marine ecosystem coincided with oscillating redox conditions at the end of glaciation, including the deep-water Anji sponge-dominated benthic faunas. Our findings
contribute new insights into the processes operating during the LOME and, more generally, the evolutionary interactions between OAEs and mass extinctions over Earth history.