The biogeochemical carbon cycle is one of the most studied elemental cycles due to the intimate links between geology, biology, and climate. During the Late Ordovician (444-458 Ma) large perturbations in carbonate carbon isotopes (d13Ccarb), a proxy for the carbon cycle, are concurrent with major climatic changes and a mass extinction. The largest of these environmental shifts is the terminal Ordovician Hirnantian Glaciation, an event that caused the disappearance of ~85% of invertebrate species from the rock record and ended a ~140 My period of greenhouse conditions. Recent work suggests that the shift to icehouse conditions may have occurred millions of years prior to the major pulse of the Hirnantian glaciation, but a detailed climatic reconstruction is hindered by limited data sets and ambiguous stratigraphic correlations.
Here, I discuss geochemical and petrographic methods for establishing a high-resolution d13Ccarb record used for stratigraphic correlation. This provides a temporal context within which we interpret the environmental changes recorded in the lithology. We suggest that change in eustatic sea level (driven by glacial formation) may have begun ~10 Myr prior to the major pulse of glaciation in the Hirnanation. We use numerical box models to simulate biogeochemical responses of the carbon cycle to changes in sea level and temperature. Model results are consistent with an increase in organic carbon burial, perhaps related to a temperature-dependent decrease in microbial degradation of organic matter in the ocean. An increase in the weathering of carbonate platforms exposed during sea level lowstand may have been an alterative mechanism or one that acted simultaneously with changes in organic carbon burial.