DID VOLCANISM TRIGGER THE LATE ORDOVICIAN MASS EXTINCTION? MERCURY DATA FROM SOUTH CHINA

The Late Ordovician mass extinction (LOME) was one of the five largest Phanerozoic marine diversity depletions. The extinction event occurred in two pulses, associated with the expansion and contraction of ice sheets on Gondwana during the Hirnantian Stage. It is widely recognized that environmental changes associated with peak glacial conditions drove the extinctions; these environmental changes include global cooling, drained epicontinental seaways, and perhaps marine redox fluctuations. However, the causes of the glacial expansion itself are not well understood. Here we provide geochemical evidence linking Hirnantian sea level fall to a major pulse of volcanic activity. We measured Hg abundance normalized to organic carbon content (Hg/TOC) in marine strata exposed at the Wangjiawan Riverside section in South China, a section with excellent published biostratigraphic control and C and S isotope chemostratigraphy. Hg/TOC ratios in latest Katian and early Hirnantian strata are ~24 ppb Hg/%TOC, whereas ratios in mid-Hirnantian syn-glacial strata climb to a maximum of ~200 ppb Hg/%TOC. This Hg/TOC excursion is broadly synchronous with positive excursions in δ¹³C_org and δ³⁴S_pyr. The onset of rising Hg/TOC precedes a shift from deep to shallow water sedimentation, and the subsequent fall in Hg/TOC levels precedes reflooding of the basin. We suggest the elevated Hg loading may be the product of a substantial and prolonged volcanic eruption. We propose that sulfur aerosols emitted during the eruption disturbed the planet’s radiative balance and allowed for the expansion of Gondwanan ice sheets and sea level fall; cessation of volcanism allowed radiative balance and ice volume to relax to a state similar to that prior to the excursion. While elevated Hg concentrations have been linked to eruption of large igneous provinces coincident with other Phanerozoic mass extinctions, the climate response during the LOME may have been unique owing to different geological boundary conditions and the rates and magnitudes of volcanic forcing. Our observations are consistent with a volcanic trigger for the LOME mediated through the combined effects of glacioeustasy, changes in ocean temperature, and possibly metal toxicity.