Did changes in atmospheric CO2 coincide with latest Ordovician glacial–interglacial cycles?

The Late Ordovician Hirnantian Stage (∼444 million years ago) was one of three time periods during the past half billion years in which large continental glaciers formed over Earth's polar regions. The effects of this glaciation were far-reaching and coincided with one of the largest marine mass extinction events in Earth history. The cause of this ice age is uncertain, and a paradoxical association with evidence for high atmospheric CO2 levels has been debated. Precise linkages between sea level, ice volume, and carbon isotope (δ13Ccarb and δ13Corg) proxy records of pCO2 have been poorly understood due in part to uncertainties in stratigraphic correlation and the interpretation of globally important sections. Although correlation difficulties remain, recent Hirnantian biostratigraphic studies now allow for improved correlations. Here we show that consistent trends in both δ13Ccarb and δ13Corg from two well-dated stratigraphic sequences in Estonia and Anticosti Island, Canada coincide with changes in Late Ordovician (Hirnantian) climate as inferred from sea level and the extent of ice sheets. The integrated datasets are consistent with increasing pCO2 levels in response to ice-sheet expansion that reduced silicate weathering. Ultimately, the time period of elevated pCO2 levels is followed by geologic evidence of deglaciation.