GLOBAL CHEMICAL EROSION DURING THE LAST GLACIAL MAXIMUM AND THE PRESENT - SENSITIVITY TO CHANGES IN LITHOLOGY AND HYDROLOGY

Geographically based calculations for 18,000 years ago (18 ka) and tod ay were made to examine the potential effects on terrestrial chemical erosion of changes in lithology and hydrology on glacial-interglacial timescales. Runoff fields were derived from general circulation model predictions of precipitation minus evaporation. Then global lithologic maps were prepared, so that empirical relationships for runoff versus bicarbonate flux for different rock types could be used to calculate global chemical erosion rates. Assuming that significant chemical eros ion does not occur beneath ice sheets, we find that chemical erosion i n ice-free areas at 18 ka was only slightly greater than today. This r esult arises because the amount of land covered by ice sheets is rough ly compensated for by exposed shelf areas and because there is little difference in global runoff. The small (approximately 20%) increase in the global chemical erosion rate during glacial conditions is due to exposure on the shelves of a relatively high proportion of carbonates (which weather faster than average). Data on glacial/interglacial move ments of the calcite compensation depth (CCD) are inconclusive but see m to indicate little change, whereas even a 20% increase in the riveri ne bicarbonate flux should cause an observable deepening. If the CCD d id not indeed change, then our predicted increase in the bicarbonate f lux from land during glaciations would have to be accommodated by othe r means, such as increased carbonate productivity. About half of the o bserved decrease in atmospheric pCO2 at 18 ka could be explained if in creased silicate chemical erosion accompanied increased total chemical erosion in ice-free areas. Consideration of the maximum possible effe ct of meltwater at ice margins leads to an 80% increase in the global chemical erosion rate at 18 ka. Such an increase would lead to a large r and faster drop in atmospheric PCO2 but also to an excessive deepeni ng of the global CCD; this scenario seems unrealistic. Our results are contrary to interpretations of a much higher silicate chemical erosio n rate during glaciations based on recently published records of Quate rnary marine Sr isotopic ratios. Therefore, if these records are indee d globally representative and entirely due to increased silicate chemi cal erosion (as opposed to changes in source areas or hydrothermal inp uts), then other factors, such as a shift in the global weathering reg ime which we do not explicitly consider here, may be involved. Sensiti vity tests show that considering spatial heterogeneities in lithology and runoff leads to lower predictions of global chemical erosion rates than what one would obtain by considering only global averages in a n on-spatially-resolved calculation.