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.
Highlights
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]
Grasby et al.[21] demonstrated that background Hg levels appear constant over geologic time, showing that Hg spikes associated with LIPs are truly anomalous features of the rock record
We argue that the two Hg spikes we observe, that are temporally coincident with the two main O-S extinction pulses, represent enhanced Hg loading to marine environments due to greatly increased volcanic activity
Summary
Our data show a covariant relationship between Hg and organic matter (OM) throughout the Ordovician, with the exception of two significant Hg spikes near the O-S boundary. The overall positive shifts of Δ199Hg values probably suggest that the spikes of Hg are related to the Hg2+ absorbed from the atmosphere by volcanic plume particles, instead of direct atmospheric deposition from volcanoes themselves. This finding is consistent with the stable Hg values of Buchanan Lake at the latest Permian extinction (LPE), which reflected a dominant volcanic source for an Hg spike[24]. We argue that the two Hg spikes we observe, that are temporally coincident with the two main O-S extinction pulses, represent enhanced Hg loading to marine environments due to greatly increased volcanic activity This interpretation is supported by our Hg stable isotope data. Ernst et al.[44] suggested candidate LIP events do occur at this time, most notably the Suordakh intraplate event in eastern Siberia which is ca. 440 Ma51, the mafic events of Ongnyeobong Formation volcanics in South Korea[52] and flood basalts in Argentina, southern South America[53] that are of approximately the right age, but without sufficiently precise age dates to confirm a direct link
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call