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

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.