The Causes and Consequences of Ordovician Cooling

Abstract

A long-term cooling trend through the Ordovician Period, from 487 to 443 Ma, is recorded by oxygen isotope data. Tropical ocean basins in the Early Ordovician were hot, which led to low oxygen concentrations in the surface ocean due to the temperature dependence of oxygen solubility. Elevated temperatures also increased metabolic demands such that hot shallow water environments had limited animal diversity as recorded by microbially dominated carbonates. As the oceans cooled through the Ordovician, animal biodiversity increased, leading to the Great Ordovician Biodiversification Event. The protracted nature of the cooling suggests that it was the product of progressive changes in tectonic boundary conditions. Low-latitude arc-continent collisions through this period may have increased global weatherability and decreased atmospheric CO2 levels. Additionally, decreasing continental arc magmatism could have lowered CO2 outgassing fluxes. The Ordovician long-term cooling trend culminated with the development of a large south polar ice sheet on Gondwana. The timescale of major ice growth and decay over the final 2 Myr of the Ordovician is consistent with Pleistocene-like glacial cycles driven by orbital forcing. The short duration of large-scale glaciation indicates a high sensitivity of ice volume to temperature with a strongly nonlinear response, providing a valuable analog for Neogene and future climate change. ▪ Oxygen isotope data record progressive and protracted cooling through the Ordovician leading up to the onset of Hirnantian glaciation. ▪ The gradual cooling trend is mirrored by an Ordovician radiation in biological diversity, consistent with temperature-dependent oxygen solubility and metabolism as a primary control. ▪ Long-term cooling occurred in concert with low-latitude arc-continent collisions and an increase in global weatherability. Although CO2 outgassing may have also decreased with an Ordovician decrease in continental arc length, in the modern, CO2 outgassing is variable along both continental and island arcs, leaving the relationship between continental arc length and climate uncertain. ▪ Evidence for significant ice growth is limited to less than 2 Myr of the Hirnantian Stage, suggesting a high sensitivity of ice growth to pCO2 and temperature. ▪ Independent estimates for ice volume, area, and sea level change during the Hirnantian glacial maximum are internally consistent and comparable to those of the Last Glacial Maximum.