A comparison of spatially resolved and global mean reconstructions of continental denudation under ice-free and present conditions

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Copyright 1997 by the American Geophysical Union. We assess the impact of continental-scale processes on global denudation through the use of spatially resolved information for both an ice-free paleoclimate and the present climate. Runoff from general circulation modeling cases representing the early Eocene is superimposed upon an Eocene paleogeologic reconstruction, and this information is combined with chemical denudation rates for silica (dissolved moles Si) and bicarbonate (dissolved moles HCO3-). Global fluxes of silica and bicarbonate to the global ocean are then calculated. A parallel procedure is carried out with present-day distributions of lithology and model-derived runoff. This work demonstrates that fluxes produced by a simple model such as the one used here are significantly different when calculated with spatially uniform runoff values versus those calculated with 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 spatially 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, suggesting that our model captures the first-order relationship accurately; however, the bicarbonate value compares less well to observations. Comparison of Eocene and present-day flux results shows that present-day fluxes 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 definition, not contributing to chemical weathering in our model) in the present case. The increase in Si global denudation fluxes from the Eocene to the present are caused primarily by the large increase in exposed granitic, basaltic, and shale lithologies, and a decrease in exposed sandstone areas, between the Eocene and present.

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