Global chemical erosion over the last 250 my: Variations due to changes inpaleogeography, paleoclimate, and paleogeology

We utilize predictions of runoff from two series of GENESIS (version 1.02) climate model experiments to calculate chemical erosion rates for 12 time s lices that span the Mesozoic and Cenozoic. A set of "control" experiments w here geography is altered according to published paleogeographic reconstruc tions and atmospheric pCO(2) is held fixed at the present-day value was des igned to elucidate climate sensitivity to geography alone. A second series of experiments, where the (elevated) atmospheric CO2 level for each time sl ice was adapted from Berner (1991), was executed to determine the additiona l climate sensitivity to this parameter. By holding other climate forcing f actors (for example, vegetation) constant throughout the sequence of experi ments we evaluate the effects of systematic/coherent paleogeographic change s on runoff and temperature, and thus on global rates of chemical weatherin g. By using empirical relationships between runoff and bicarbonate fluxes for different rock types and maps of paleogeology we calculate global bicarbona te fluxes, taking into account spatial variations in lithology and hydrolog y. The climate model predicts that many regions experienced dramatic change s in runoff (for example, from wet to very arid) since the Early Triassic. In general, the supercontinent (Pangean) paleogeographic regime was driest, times of dispersed continents (Tethyan regime) wettest, and the modern geo graphy intermediate. Total bicarbonate fluxes, as well as those from silica te mineral weathering only, closely parallel these trends. We find that spatial variations in lithology account for little variation i n the total or silicate chemical erosion rates. In contrast, changes in hyd rology due to differences in paleogeography are significant and are the mai n clear trend in our results. Here we also add an Arrhenius-type temperatur e dependency that modifies the runoff-determined fluxes for each rock type. High activation energies for the weathering reactions substantially increa se absolute values, but we find that the ratio between the flux for a parti cular time slice and the Present Day remains very similar for different tem perature dependencies, following the paleogeographic-controlled trend in ru noff. Our calculations suggest a weaker-than-expected CO2-climate weathering feed back. The reasonable atmospheric pCO(2) variations specified for the climat e-model simulations do not lead to climatic effects that support large chan ges in the chemical erosion rate, compared to those generated by changing p aleogeography. In general, however, we find that silicate weathering rates are similar to outgassing rates of volcanic and metamorphic CO2. Times of s upercontinental stasis represent low outgassing but also high aridity due t o extreme continentality and thus low chemical erosion fluxes. In contrast, times of continental dispersion represent high outgassing as well as high runoff (and fluxes) due to increased proximity to moisture sources. Where a mismatch occurs, particularly in the case of the early to mid-Cretaceous, we infer higher CO2 levels than those used in our GCM simulations to balanc e the carbon cycle.