ABSTRACT

The Late Ordovician (Hirnantian) glaciation was the only major glaciation during the generally ice-free early Paleozoic. It had a profound effect on the earth system and is closely associated with a major Phanerozoic mass extinction. This glaciation has attracted attention in recent years because geochemical models and proxy data indicate ∼14–16 times the present atmospheric level (PAL) of atmospheric CO2, apparently coincident with the glaciation. Previous climate modeling studies postulated that glaciation was possible at high CO2, given the paleogeographic configuration of the Late Ordovician. With the coast of Gondwana positioned close to the South Pole, the ocean's thermal inertia would have prevented summer temperatures from rising above freezing, thus allowing snow to survive and glacial inception. However, recent stratigraphic and stable isotope studies indicate that major glaciation lasted perhaps less than 1 million years, meaning that mechanisms operating on tectonic timescales are unlikely to be the primary cause of the glaciation. A change in oceanographic circulation which led to an increase in organic carbon burial has recently been proposed as the cause of a brief drawdown of atmospheric CO2, which resulted in the Late Ordovician glaciation.

We present new general circulation model results for both Late Ordovician and Early Silurian paleogeographies, using identical solar luminosities (a 4.5% reduction from modern) and atmospheric CO2 levels. These experiments allow us to address the relative importance of paleogeography and CO2 in forcing the development of a positive snow mass balance on Gondwana.