FURTHER EVIDENCE FOR A LONG-LIVED ORDOVICIAN ICEHOUSE CLIMATE SUGGESTS ALTERNATIVE KILL MECHANISMS FOR THE END-ORDOVICIAN MASS EXTINCTIONS

The emerging paradigm of a long-lived “Early Paleozoic Icehouse” challenges the notion of a dominantly greenhouse climate for much of the Ordovician. Extended cooling throughout most of the Ordovician can be demonstrated using data-numerical model comparisons. The data here consist of graptolite spatial distribution patterns, extracted from the CONOP stratigraphical database, that show a stepped retreat of these planktonic organisms from polar regions during the Ordovician, with key shifts in the mid-Floian and the Katian. We interpret this as the migration of an essentially warm-adapted fauna responding to a cooling climate. Our Earth system model has an innovative coupling method between ocean, atmosphere and land-ice and shows the emergence of the ice sheet happened in two pulses: as we lower pCO₂ in the model runs, the continental ice sheet suddenly expands to the mid-latitudes (≈45°S) and global temperature decreases by 3.9°C (from 23.9°C at 16 PAL to 20.0°C at 12 PAL). During a later second phase, sea surface temperatures dropped steeply (by c. 8°C in the tropics), the land-ice reached 30°S, and sea-ice extended to 45ºS. Comparing our model to new and published data, glacial onset may have occurred during the Mid Ordovician or even earlier. The second step in ice-sheet growth represents the Hirnantian glacial maximum. Prolonged cooling throughout the Ordovician, signaled both by the graptolite and model data, questions the notion of severe cooling as the key trigger for the Hirnantian mass extinction event. Although we here re-emphasize significant and sudden drops in global temperature during the Hirnantian, high-resolution analyses of the near field stratigraphic record have shown that these do not align with the main faunal turnovers. An alternative kill-mechanism for the mass extinction (supported by data presented in parallel), is likely related to changing oceanic redox conditions.