Evidence exits from several parts of Gondwana for at least one major, continental ice sheet during the Late Ordovician and Early Silurian. Yet various sources of geological evidence suggest high atmospheric pCO2 at this time, and Berner's (1990,1991) Geochemical cycling model indicates that the amount may have been as high or higher than 13 times the present atmospheric level. To resolve this apparent paradox (high pCO2 would be expected to imply an ice- free Earth) the following approach will be followed: 1) Two periods will be studied: the Middle Ordovician, which lacks evidence for polar ice, and the Ashgill (Late Ordovician) which has evidence for the largest ice extend. 2) An atmospheric general circulation model GCM will be applied to the study of Ordovician climates, and the results of this will then be used to drive a spatially resolved geochemical cycling model (which considers geographical variations in climate and paleolithology). We will first model the climates of both periods with a CO2 level of 13xPAL. If differences in geography cannot account for the climate difference then we will progressively lover the Late Ordovician CO2 levels until we find that snow remains throughout the year where the geological evidence indicates an ice cap. In this way we can determine the range of pCO2 values which is climatically compatible with the glaciological evidence . We will then run a global geochemical cycling model, in part to provide an additional means of estimating/confirming pCO2 by seeing which climate, i.e., weathering regime, generates a CO2 consumption rate that is consistent with the inferred rate of production via volcanism and metamorphism. In particular we will examine the hypothesis that increased (terrestrial) volcanism in the Middle Ordovician led to increased (terrestrial) silicate weathering in the Late Ordovician. Increased weathering would mean that the carbon cycle was in balance at a lower pCO2 level than current model indicate, and thus allow glaciation. This study of the Middle and Late Ordovician will allow us to look at the evolution to a glaciated state, and will be a test of the importance of atmospheric CO2 as a prime governor of climate on Earth.
