Chemical Weathering Yields from Basement and Orogenic Terrains in Hot and Cold Climates

Abstract

The ¹⁸O record in benthonic foraminifera over the last 70 million years can only be interpreted in terms of a more-or-less continual climatic deterioration over the period of record.^1–2 Abyssal water temperatures record winter conditions in the oceanic areas where climatic extremes generate the densest waters. The isotopic temperatures in the abyssal waters in the early Eocene, 50 million years ago, were about 15°C higher than at present. It is unlikely that a Mediterranean-type “warm salty bottom water” could have dominated the global circulation in the face of freezing temperatures as ice-brine formation should have prevailed as the major source of convection, then much as it does today. Hence the mean surface temperature must have been much higher than at present, and the latitudinal climatic gradient much lower; this is in accord with the fossil record. The only feasible way to achieve this climatic optimum appears to be through an enhanced greenhouse effect involving greatly elevated atmospheric pCO₂, despite the fact that general circulation model (GCM) runs following this approach have been less than successful in simulating the conditions inferred from the geologic observations.^3–6 The problem then is to explain the mechanisms responsible for large oscillations (order tenfold) in pCO₂ on geologic timescales.