MODELING ORDOVICIAN ICE SHEET AND THE SEA-LEVEL FINGERPRINT OF ITS COLLAPSE: TOWARD A CONSISTENT PICTURE OF THE ORDOVICIAN GLACIATION (Invited Presentation)

The Ordovician glaciation (~445 Ma) represents the acme of one of the three major icehouse periods in Earth's Phanerozoic history. The ensuing deglaciation and associated transgression deeply affected depositional environments and critically impacted marine living communities, contributing to the Late Ordovician Mass Extinction. Nevertheless, the geometry of the Ordovician ice sheet remains crudely constrained and the major transgressive event provoked by the collapse of the ice sheet is usually considered to be uniform (i.e. eustatic), which may lead to erroneous interpretations of the geological record. Here, using an Earth system model with an innovative coupling method between ocean, atmosphere and land ice accounting for climate and ice-sheet feedback processes, we simulate the response of the Ordovician global climate to a drop in atmospheric CO₂ concentrations and the growth of the Ordovician ice sheet. We subsequently use the simulated ice sheet to force a gravitationally self-consistent model of sea-level change, allowing us to propose the first numerical simulation of late Hirnantian regional sea-level rise.

1. Simulating Ordovician climate and land ice

We show that the emergence of the ice sheet happens in two discrete phases. The comparison with abundant sedimentological, geochemical and micropaleontological data suggests that glacial onset may have occurred as early as the Mid Ordovician Darriwilian stage (~460 Ma), in agreement with recent studies reporting third-order glacio-eustatic cycles during the same period. The second step in ice-sheet growth, typified by a sudden drop in tropical sea-surface temperatures by ~8 °C and the further extension of a single, continental-scale ice sheet over Gondwana, marks the onset of the Hirnantian glacial maximum.

2. Simulating the sea-level fingerprint of Late Ordovician ice-sheet collapse

Ice-sheet collapse triggers major departures from eustasy. We compare our modeling results to key sedimentary sections. We show that previously enigmatic opposite sea-level trends (i.e., regressive vs. transgressive) documented in the geological record during the deglaciation on each side of Laurentia are predicted by the model. These sections may thus reflect more complex patterns of sea-level change than the eustatic approximation considered so far rather than erroneous correlations.