A COMPARISON OF SPATIALLY-RESOLVED AND GLOBAL MEAN RECONSTRUCTIONS OFCONTINENTAL DENUDATION UNDER ICE-FREE AND PRESENT CONDITIONS

We assess the impact of continental-scale processes on global denudati on through the use of spatially resolved information for both an ice-f ree paleoclimate and the present climate. Runoff from general circulat ion modeling cases representing the early Eocene is superimposed upon an Eocene paleogeologic reconstruction, and this information is combin ed with chemical denudation rates for silica (dissolved moles Si) and bicarbonate (dissolved moles HCO3-). Global fluxes of silica and bicar bonate to the global ocean are then calculated. A parallel procedure i s carried out with present-day distributions of lithology and model-de rived runoff. This work demonstrates that fluxes produced by a simple model such as the one used here are significantly different when calcu lated with spatially uniform runoff values versus those calculated wit h a spatially varying runoff distribution having the same global mean value. Use of a uniform runoff distribution produces denudation rates that are significantly higher than the global results derived from a s patially varying runoff distribution. We show that present-day fluxes of silica calculated by our model containing spatially varying runoff and lithology are similar to observations of current fluxes, suggestin g that our model captures the first-order relationship accurately; how ever, the bicarbonate value compares less well to observations. Compar ison of Eocene and present-day flux results shows that present-day flu xes of Si are greater than Eocene values, while calculated present-day HCO3- fluxes are greater than or equal to Eocene values. This result occurs despite the existence of greater global mean annual runoff for the Eocene cases and despite the existence of ice-covered areas (by de finition, not contributing to chemical weathering in our model) in the present case. The increase in Si global denudation fluxes from the Eo cene to the present are caused primarily by the large increase in expo sed granitic, basaltic, and shale lithologies, and a decrease in expos ed sandstone areas, between the Eocene and present.