Glaciation, chemical weathering and the carbon cycle

Causes of glaciation through Earth history are usually assigned to changes in paleogeography or atmospheric pCO{dollar}sb2{dollar}. This study focuses on the interaction between these factors. Two glacial intervals are considered: the Late Quaternary, for which a wealth of data exists; and the Late Ordovician ({dollar}sim{dollar}440 Ma), during which a short, but intensive, glaciation occurred at time when the general climatic state was much warmer than today.; Geographically based calculations were made to examine the potential effects of changes in lithology and hydrology on chemical weathering on glacial-interglacial time-scales. General circulation model (GCM) predictions of runoff, global lithologic maps, and empirical relationships for runoff versus bicarbonate flux were used to calculate global chemical weathering rates.; Chemical weathering in ice-free areas at the last glacial maximum (LGM) was slightly greater than today. Although total ice-free land area was reduced, a relatively high proportion of carbonates (which weather faster than average) were exposed on the continental shelves. Incorporation of meltwater at ice margins doubles the global chemical weathering rate at the LGM. Such an increase would lead to an excessive deepening of the CCD; this scenario seems unrealistic.; During deglaciation faster weathering of fresh glacial deposits and a more vigorous hydrological cycle contributed to a riverine alkalinity flux one third greater than today at 11 ka. This increased flux might explain the observed coincidence between deglaciations and enhanced deep-sea carbonate preservation during the Quaternary.; Results are presented from a suite of GCM experiments that investigate the sensitivity of the Late Ordovician climate to changes in atmospheric pCO{dollar}sb2{dollar}. All experiments used a 4.5% lower solar luminosity and pCO{dollar}sb2{dollar} was set between 18X and 8X PAL. In the 8X experiment snow rapidly advances towards the tropics. In the 10X experiment substantial snow cover survives through summer (in areas consistent with the distribution of glacial deposits). No snow survives in the 18X experiment. These results suggest a strong sensitivity to relatively modest changes in pCO{dollar}sb2{dollar}, under the condition of reduced solar luminosity. They are also consistent with the Late Ordovician being caused by a drawdown of atmospheric CO{dollar}sb2{dollar}, due to a short-lived increase in organic-carbon burial.