T159 Volcanism, Impacts, Mass Extinctions, and Global Environmental Change III

Tuesday, 20 October 2009
8:00 AM - 12:00 PM
Portland Ballroom 253

THE LATE ORDOVICIAN GLACIATION AND MASS EXTINCTION: RELATION TO BASALTIC WEATHERING AND VOLCANIC DEGASSING?

Paper number: T159-5
Program: Technical Sessions
Time: 9:00 AM – 9:15 AM

Authors

Abstract

The Late Ordovician mass extinction occurred during an enigmatic, short-lived (~ 106-107 yrs) glacial episode in a greenhouse world. We propose a link between global cooling during the Late Ordovician and relative rates of volcanic degassing and weathering. Evidence for volcanic (basaltic) weathering is based on the evidence for a large Ordovician drop in seawater 87Sr/86Sr (from ~ 0.709 to 0.708) that is among the most rapid in the entire Phanerozoic. 87Sr/86Sr measurements from eastern and western Laurentia indicate the rapid drop began in the P. serra conodont zone of the upper Darriwilian Stage (Middle Ordovician). The late Darriwilian is also a time of substantial volcanism as seen in the abundant deposition of altered volcanic ash deposits (K-bentonites).

We use a numerical model to explore the hypothesis that CO2 consumption during Middle to Late Ordovician volcanic weathering (which provided the flux of non-radiogenic Sr to the oceans) was closely balanced by volcanic outgassing. This produces steady pCO2 levels and climate through the middle Katian Stage of the Late Ordovician, consistent with recent Ordovician paleotemperature estimates based on conodont δ18O. In the late Katian, outgassing is reduced, consistent with the lack of K-bentonites in sedimentary successions, while volcanic weathering continues to draw down pCO2 and results in a cooling episode leading into the well known end-Ordovician (Hirnantian) glaciation.