The great latest Ordovician extinction on the South China Plate: Chemostratigraphic studies of the Ordovician-Silurian boundary interval on the Yangtze platform

Abstract

Continuous samples from four Ordovician-Silurian boundary sections of graptolitic shales and mudstones in the Yangtze Basin (Yangtze Platform) on the South China Plate, spanning the latest Ordovician mass extinction of many unrelated groups of organisms, were chemically analyzed for the abundances of about 40 common and trace elements and iridium. It is observed that the abundances of Ir and other siderophile and chalcophile elements (e.g., Co, Cr, As, Mo, Sb and V) show elevated values in the extinction horizon at the base of the graptolite persculptus Zone, which corresponds to the stratigraphic level for the local systematic boundary in China between the “Ordovician” and “Silurian”. Iridium abundances show maxima at this extinction level in three of the four sections analyzed with varying strength, 230 ppT at Yichang, 92 ppT at Jinxian, and 88 ppT at Youyang (but 50 ppT at Xiushan). These geochemical patterns appear to be of global significance, since similar geochemical anomalies (including weak Ir anomaly) were also recorded at about the same stratigraphic level in many other sections previously studied in Quebec, Yukon, Arctic and Northwest Territories, Canada, and Scotland. Although impact by a comet with low platinum group element abundances could have been responsible for the geochemical anomalies, we find it is most plausible to attribute the geochemical signal to the effects of a lower sedimentation rate and reducing water conditions at this horizon, the beginning of a rapid transgression induced by rapid melting of the ice caps on Gondwana. Carbon isotope excursions in organic material at Yichang, together with other carbon isotope data from central Sweden, northwestern Canada, Latvia, Quebec and Wisconsin, suggest that, after a period of decreasing atmospheric and marine pCO 2 in the latest Ordovician (the Hirnantian), a sudden, large pCO 2 increase in the ocean and atmosphere may have occurred at the extinction horizon at the base of the persculptus Zone. A greenhouse effect might have existed for a short time. What triggered such a sudden pCO 2 increase is uncertain, but a rapid CO 2 degassing caused by volcanism, tectonics, and/or extraterrestrial impact could have been responsible.