NEW CONSTRAINTS ON THE DURATION AND MAGNITUDE OF LATE ORDOVICIAN-EARLY SILURIAN GLACIATION FROM CARBONATE “CLUMPED” ISOTOPE PALEOTHERMOMETRY

One of the most severe mass extinctions in the fossil record occurred near the end of the Ordovician Period (~443.7 Mya). This episode has been linked to cooling and glaciation of the southern supercontinent of Gondwana, but the nature of this relationship is obscured by uncertainties about the timing of glaciation, the magnitude of the Gondwanan ice sheet(s), and the impact of global cooling on tropical ocean temperatures. Here we place new constraints on these variables by applying carbonate “clumped” isotope paleothermometry to well-preserved Late Ordovician and Early Silurian calcitic fossils from tropical paleolatitudes in Laurentia (U.S. midcontinent and Anticosti Island, Canada). We used microscopy (light, SEM) and trace element analysis to select the least-altered material from a large suite of fossils representing a range of lithofacies, taxa, and preservational states. We find evidence for tropical near-surface ocean temperatures (SST) in the 33-37° C range throughout this period except during the Hirnantian stage, when a sharp ~5° C drop occurred in conjunction with a major eustatic regression, a globally recognized positive carbon isotope excursion, and the major pulse of mass extinction. In contrast, inferred trends in the oxygen isotopic composition of seawater imply the presence of substantial continental ice for at least 6 million years, beginning before the latest Ordovician and continuing into Early Silurian time. These data also suggest that the drop in SST occurred during a pronounced glacial maximum; inferred ice volumes during this maximum exceed those of the last Pleistocene glacial maximum. The coexistence of substantial continental ice sheets with very warm tropical SSTs throughout much of Late Ordovician-Early Silurian time implies that the meridional temperature gradient during this interval may have been steeper than during other major Phanerozoic glaciations. We suggest that these observations may be linked to changes in the intensity of southern ocean meridional overturning circulation.