Late Ordovician mass extinction caused by volcanism, warming, and anoxia, not cooling and glaciation

Abstract

The Ordovician saw major diversification in marine life abruptly terminated by the Late Ordovician mass extinction (LOME). Around 85% of species were eliminated in two pulses 1 m.y. apart. The first pulse, in the basal Hirnantian, has been linked to cooling and Gondwanan glaciation. The second pulse, later in the Hirnantian, is attributed to warming and anoxia. Previously reported mercury (Hg) spikes in Nevada (USA), South China, and Poland implicate an unknown large igneous province (LIP) in the crisis, but the timing of Hg loading has led to different interpretations of the LIP-extinction scenario in which volcanism causes cooling, warming, or both. We report close correspondence between Hg, Mo, and U anomalies, declines in enrichment factors of productivity proxies, and the two LOME pulses at the Ordovician-Silurian boundary stratotype (Dob’s Linn, Scotland). These support an extinction scenario in which volcanogenic greenhouse gases caused warming around the Katian-Hirnantian boundary that led to expansion of a preexisting deepwater oxygen minimum zone, productivity collapse, and the first LOME pulse. Renewed volcanism in the Hirnantian stimulated further warming and anoxia and the second LOME pulse. Rather than being the odd-one-out of the “Big Five” extinctions with origins in cooling, the LOME is similar to the others in being caused by volcanism, warming, and anoxia.

A high-resolution record of early Paleozoic climate

“…4D); this lag may repre-sent the long marine residence time of Sr (11). The brief warm interval that punctuates the Late Ordovician epoch is associated with repeated regional volcanic events, evidenced by the widespread deposition of well-dated K-bentonite ash layers (54)(55)63) and mercury chemostratigraphy (64)(65)(66). These volcanic eruptions may have led to enhanced climatic volatility, as perhaps evidenced by the regional "Boda event," which has been linked to warming (67).…”

The latest Ordovician Hirnantian brachiopod faunas: New global insights

“…The complexity is manifested by J o u r n a l P r e -p r o o f both external and biological factors. The main abiotic factors included substantial climate change (e.g., Finnegan et al, 2011;Algeo et al, 2016), sea-level fluctuations (e.g., Brenchley et al, 2006), changes in ocean chemistry and circulation (e.g., Berry and Wilde, 1978;Sheehan, 2001), atmospheric carbon and oxygen perturbations (e.g., Brenchley et al, 1994;Kaljo et al, 2008;Bergström et al, 2014 and therein), intense orogenic movements (Rasmussen and Harper, 2011a) and volcanic activity (McKenzie et al, 2016;Jones et al, 2017;Gong et al, 2017;Bond and Grasby, 2020), within a very short duration (e.g., Brenchley et al, 1994;Ling et al, 2019). The biological responses to these factors are largely characterized by taxonomic extinctions, phyletic origination, faunal turnovers, and changes in community structure.…”

Late Ordovician mass extinction caused by volcanism, warming, and anoxia, not cooling and glaciation: REPLY

“…are right to highlight that problems remain in global correlations of the Late Ordovician mass extinction (LOME)-and of course, the chronology at Dob's Linn requires improved biostratigraphy. However, their assertion that we (Bond and Grasby, 2020) placed the base of the Hirnantian Stage too high at Dob's Linn has little bearing on the extinction timing and our observed geochemical changes, and therefore on our conclusion that the two pulses of the were driven by volcanogenic warming, anoxia and productivity collapse. Here we respond to each of the four main criticisms:…”