Link to pdf: Pohl2018_Geology
The Hirnantian glacial acme (445–444 Ma) represents the glacial maximum of the long-lived Ordovician glaciation. The ensuing deglaciation and associated transgression deeply affected depositional environments and critically impacted marine living communities, contributing to the Late Ordovician Mass Extinction. In the absence of a better model, this transgressive event is usually considered to be a uniform (i.e., eustatic) rise in sea level, at least at low to intermediate paleolatitudes. This assumption may lead to erroneous interpretations of the geological record. Here we use a land-ice model and a gravitationally self-consistent treatment of sea-level change to propose the first numerical simulation of spatially varying late Hirnantian sea-level rise. We demonstrate significant departures from eustasy and compare our modeling results to key sedimentary sections. We show that previously enigmatic opposite sea-level trends (i.e., transgressive versus regressive) documented in the geological record are predicted by the model. Such sections may thus reflect patterns of sea-level change more complex than the eustatic approximation considered so far, rather than erroneous correlations. Our simulations also predict the locations where values of relative sea-level change are closest to the values predicted by a globally uniform rise and hence most representative of the volume of the ice sheet that collapsed. We identify these regions as preferential loci for future fieldwork investigating the ice volume during the Hirnantian glacial peak.