An atmospheric pCO2 threshold for glaciation in the Late Ordovician

The Late Ordovician glaciation differs from other major Phanerozoic glaciations because of its short duration ([approximately]1 m.y.). A satisfactory explanation for this glaciation must be able to account for its sudden initiation, short duration, and rapid termination; mechanisms operating on tectonic time scales appear to be precluded. Given recent studies of a major perturbation in the carbon cycle during the glaciation, a climate model investigation of the sensitivity of the Late Ordovician climate to changes in atmospheric pCO2 was undertaken. Under the condition of a 4.5 % reduction in solar luminosity, permanent snow cover (taken as a key indicator of potential for glaciation) is dramatically different between five experiments. The range of 18X present atmospheric level C[O.sub.2] (ice free) to 8X ('runaway' icehouse) lies within the uncertainty of previous geochemical estimates of Late Ordovician atmospheric pC[O.sub.2]. The strong sensitivity to the modest direct forcing from pC[O.sub.2] changes is due to the ice-albedo feedback. A plausible increase in organic carbon burial could have drawn down enough atmospheric C[O.sub.2] to have briefly lowered the climate system below a critical glacial inception threshold at the end of the Ordovician. Conversely, a high pC[O.sub.2] is required for the rest of the early Paleozoic, which was essentially ice free.