A sea-level fingerprint of the Late Ordovician ice-sheet collapse
Résumé
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, con-tributing 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 gravita-tionally 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 rep-resentative 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.