Mercury Spikes Suggest Volcanic Driver of the Ordovician-Silurian Mass Extinction

Abstract

The second largest Phanerozoic mass extinction occurred at the Ordovician-Silurian (O-S) boundary. However, unlike the other major mass extinction events, the driver for the O-S extinction remains uncertain. The abundance of mercury (Hg) and total organic carbon (TOC) of Ordovician and early Silurian marine sediments were analyzed from four sections (Huanghuachang, Chenjiahe, Wangjiawan and Dingjiapo) in the Yichang area, South China, as a test for evidence of massive volcanism associated with the O-S event. Our results indicate the Hg concentrations generally vary in parallel with TOC, and that the Hg/TOC ratios remain low and steady state through the Early and Middle Ordovician. However, Hg concentrations and the Hg/TOC ratio increased rapidly in the Late Katian, and have a second peak during the Late Hirnantian (Late Ordovician) that was temporally coincident with two main pulses of mass extinction. Hg isotope data display little to no variation associated with the Hg spikes during the extinction intervals, indicating that the observed Hg spikes are from a volcanic source. These results suggest intense volcanism occurred during the Late Ordovician, and as in other Phanerozoic extinctions, likely played an important role in the O-S event.Refinements in radio-isotopic dating techniques has demonstrated a strong temporal link between Large Igneous Province (LIP) volcanism and four out of the five largest mass extinctions in the Phanerozoic, leading to suggestions that LIP events have devastating effects on global ecosystems that in some ways mirrors concerns over modern anthropogenic impacts on the planet [1][2][3][4] . The one anomaly is the Ordovician-Silurian (O-S) mass extinction, the second largest of the "Big Five" extinctions. In terms of species loss the O-S represented about 86% of marine life 5,6 , including the elimination of graptolites 7-9 and brachiopods 10, 11 . The triggering mechanism of the O-S mass extinction has been extensively debated, and various hypotheses have been proposed, including gamma ray burst and bolide impact 12,13 ; however, the cause still remains highly controversial. Intense Late Ordovician volcanism was also suggested as a potential extinction mechanism 14,15 , although no clear geologic record of a Late Ordovician LIP event has been demonstrated. The age of the event makes it possible that any geologic record has been lost 15 . Given this, we tested for indirect evidence of volcanism at the O-S extinction boundary by examining the Hg record from classic sections in south China. Here we show that significant spikes in Hg concentrations occur at the O-S extinction, along with Hg stable isotope data that indicate a volcanic source for these spikes. We suggest that this reflects Hg loading by a LIP event. If correct, these results demonstrate that LIP events have indeed played a critical role in the evolution of life through time. Mercury as a Proxy for LIP eventsVolcanic eruptions are the main natural source of Hg to the environment 16 . Sanei et al. 17 demonstrated that th...

Large mass-independent sulphur isotope anomalies link stratospheric volcanism to the Late Ordovician mass extinction

“…The stratospheric eruptions may have much more severe and prolonged impacts on atmospheric and ocean chemistry, terrestrial environments, and climate change than tropospheric eruptions [52][53][54][55] . We note that, based on Hg abundances and Hg/TOC ratios, a LIP event has been hypothesised as a driver for the LOME 13,14 . However, Hg enrichments were interpreted to have been sulphide-hosted rather than of volcanic origin, which is against the causal linkage between the LOME and volcanism 15 .…”

“…Oceanic anoxia has been linked to the LOME [9][10][11][12] , however, temporal changes in global redox conditions that could synchronise with each pulse of the LOME remains elusive. The emplacement of a large igneous province (LIP) also has been hypothesised as a driver for the LOME based on mercury (Hg) enrichments 13,14 . However, the Hg enrichments in the sedimentary rocks of the Ordovician-Silurian boundary from South China have been argued to be sulphide-hosted rather than of volcanic origin, challenging the hypothesis of a volcanic driver for the LOME 15 .…”

Effect of Intense Weathering and Postdepositional Degradation of Organic Matter on Hg/TOC Proxy in Organic‐rich Sediments and its Implicationsfor Deep‐Time Investigations

“…During the last few years, mercury (Hg) contents have been increasingly used to investigate the complex relationship between environmental change and the intensity and temporal extension of large igneous province (LIP) activity (e.g., Charbonnier et al, ; Charbonnier et al, ; Charbonnier & Föllmi, ; Font et al, ; Gong et al, ; Grasby et al, ; Jones et al, ; Percival et al, ; Percival et al, ; Percival et al, ; Sabatino et al, ; Sanei et al, ; Scaife et al, ; Sial et al, ; Sial et al, ; Sial et al, ; Thibodeau et al, ). Hg emissions from volcanoes represent an important natural source of Hg to the atmosphere (Pirrone et al, ).…”

Sedimentary Mercury Enrichments as a Marker for Submarine Large Igneous Province Volcanism? Evidence From the Mid‐Cenomanian Event and Oceanic Anoxic Event 2 (Late Cretaceous)

“…Events studied include the end‐Permian, end‐Triassic, Early Toarcian, Valanginian, Early Aptian, and end‐Cretaceous global environmental perturbations, with Hg spikes linked to the Siberian Traps, the Central Atlantic Magmatic Province, the Karoo‐Ferrar igneous province, the Parana‐Etendeka province, the Ontong‐Java Plateau, and the Deccan Traps, respectively (e.g., Charbonnier et al, ; Charbonnier & Föllmi, ; Grasby et al, ; Percival et al, , , ; Sanei et al, ; Sial et al, ; Thibodeau et al, ). End‐Ordovician Hg enrichments have also been postulated to be evidence for volcanic activity of an as‐yet‐unidentified Palaeozoic LIP (Gong et al, ; Jones et al, ). Mercury is strongly enriched in volcanic emissions, which are one of the major natural sources of Hg to the troposphere and stratosphere (Pyle & Mather, ).…”