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

David P.G Bond,Stephen E Grasby

doi:10.1130/g47377.1

Abstract

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.

Highlights

Ordovician Earth experienced major diversification in the oceans (Sepkoski, 1981), abruptly terminated by the first of the “Big Five” extinctions—the Late Ordovician mass extinction (LOME)
The Jones et al (2017) record from Nevada and South China sees episodes of Hg enrichment in pre-extinction Katian strata and in the mid-Hirnantian, before the second extinction pulse (Fig. 1). They suggested that chemical weathering of a Katian large igneous province (LIP) drove Gondwanan cooling and extinction, while LIP activity in the Hirnantian resulted in a glacial maximum caused by the albedo effects of sulfate aerosols
The temporal scale of such climatic changes is at odds with the brevity of the two LOME pulses, and that the second pulse has been linked to warming is problematic for the Jones et al (2017) model

Summary

INTRODUCTION

Ordovician Earth experienced major diversification in the oceans (Sepkoski, 1981), abruptly terminated by the first of the “Big Five” extinctions—the Late Ordovician mass extinction (LOME). The Jones et al (2017) record from Nevada and South China sees episodes of Hg enrichment (diachronous between these regions) in pre-extinction Katian strata and in the mid-Hirnantian, before the second extinction pulse (Fig. 1) They suggested that chemical weathering of a Katian LIP drove Gondwanan cooling and extinction, while LIP activity in the Hirnantian resulted in a glacial maximum caused by the albedo effects of sulfate aerosols. The short atmospheric residence time of SO2 makes its cooling effects minor compared to CO2-driven warming, and modeling suggests that cooling resulting from LIP volcanism is an unlikely extinction driver (Schmidt et al, 2016) In their South China record, Gong et al (2017) showed an increase in Hg from background levels of ∼1 ppb to ∼100 ppb coincident with a Katian facies change from limestone to shale. Further Hg records are required in order to tease apart their spatial and temporal variability

MATERIAL AND METHODS

C C 13 carb

RESULTS

CONCLUSIONS