AN ATMOSPHERIC PCO(2) THRESHOLD FOR GLACIATION IN THE LATE ORDOVICIAN

The Late Ordovician glaciation differs from other major Phanerozoic gl aciations because of its short duration (similar to 1 m.y.). A satisfa ctory explanation for this glaciation must be able to account for its sudden initiation, short duration, and rapid termination; mechanisms o perating on tectonic time scales appear to be precluded. Given recent studies of a major perturbation in the carbon cycle during the glaciat ion, a climate model investigation of the sensitivity of the Late Ordo vician climate to changes in atmospheric pCO(2) 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 dramat ically different between five experiments. The range of 18X present at mospheric level CO2 (ice free) to 8X (''runaway'' icehouse) lies withi n the uncertainty of previous geochemical estimates of Late Ordovician atmospheric pCO(2). The strong sensitivity to the modest direct forci ng from pCO(2) changes is due to the ice-albedo feedback. A plausible increase in organic carbon burial could have drawn down enough atmosph eric CO2 to have briefly lowered the climate system below a critical g lacial inception threshold at the end of the Ordovician. Conversely, a high pCO(2) is required for the rest of the early Paleozoic, which wa s essentially ice free.