RECONCILING LATE ORDOVICIAN (440-MA) GLACIATION WITH VERY HIGH (14X) CO2 LEVELS

Geochemical data and models suggest a positive correlation between car bon dioxide changes and climate during the last 540 m.y. The most dram atic exception to this correlation involves the Late Ordovician (440 M a) glaciation, which occurred at a time when CO2 levels may have been much greater than present (14-16X?). Since decreased solar luminosity at that time only partially offset increased radiative forcing from CO 2, some other factor needs to be considered to explain the glaciation. Prior work with energy balance models (EBMs) suggested that the uniqu e geographic configuration of Gondwanaland at that time may have resul ted in a small area of parameter space permitting permanent snow cover and higher CO2 levels. However, the crude snow and sea ice parameteri zations in the EBM left these conclusions open to further scrutiny. He rein we present results from four experiments with the GENESIS general circulation model with CO2 levels 14X greater than present, solar lum inosity reduced 4.5%, and an orbital configuration set for minimum sum mer insolation receipt. We examined the effects of different combinati ons of ocean heat transport and topography on high-latitude snow cover on Gondwanaland. For the no-elevation simulations we failed to simula te permanent summer snow covet. However, for the slightly elevated top ography cases (300-500 m), permanent summer snow cover occurs where ge ological data indicate the Ordovician ice sheet was present. These res ults support the hypothesis based on EBM studies. Further results indi cate that although average runoff per grid point increases substantial ly for the Ordovician runs, the decreased land area results in global runoff 10-30% less than present, with largest runoff reductions for fl at topography This response has implications for CO2-runoff/weathering parameterizations in geochemical models. Finally, simulated tropical sea surface temperatures (SSTs) are the same or only marginally warmer than present. This result is consistent with evidence from other warm time intervals indicating small changes in tropical SSTs during time of high CO2.