MERCURY SPIKES SUGGEST VOLCANIC DRIVER OF THE ORDOVICIAN-SILURIAN MASS EXTINCTION

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.

In the laboratory, the TOC content was measured by a Elementar® vario Macro cube. Total mercury concentrations (THg) were measured by a LECO® AMA254 mercury analyzer. Hg isotopic ratios were determined by a Nu-Plasma multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS).

The analyses of Hg and TOC values from sections in the Yangtze Platform show consistent background values and Hg/TOC ratios through the Early and Middle Ordovician. However, distinct spikes in both Hg concentrations and Hg/TOC occur in the Late Katian and Late Hirnantian, coincident with the two main extinction pulses across the O-S boundary. This result was reproduced in two locations (Wangjiawan and Dingjiapo). 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. Therefore, we conclude that similar to other Phanerozoic mass extinctions, major volcanic eruptions may have triggered the O-S mass extinction events.