ALBERT

Description: The Ordovician glaciation represents the acme of one of only three major icehouse periods in Earth's Phanerozoic history, and is notorious for setting the scene for one of the “big 5" mass extinction events. Nevertheless, the mechanisms that drove ice-sheet growth remain poorly understood, and the final extent of the ice sheet crudely constrained. 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 report simulations portraying for the first time the detailed evolution of the Ordovician ice sheet. We show that the emergence of the ice sheet happened in two discrete phases. In a counter-intuitive sequence of events, the continental ice sheet appeared suddenly in a warm climate. Only during the second act, and set against a background of decreasing atmospheric CO 2 , followed steeply dropping temperatures and extending sea-ice. The comparison with abundant sedimentological, geochemical and micropaleontological data suggests that glacial onset may have occurred as early as the Mid Ordovician Darriwilian, 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, marked the onset of the Hirnantian glacial maximum. By suggesting the presence of an ice sheet over Gondwana throughout most of the Mid and Late Ordovician, our models embrace the emerging paradigm of an “Early Paleozoic Ice Age”.