THE IMPACT OF HUMANS ON CONTINENTAL EROSION AND SEDIMENTATION (Invited)

Rock uplift and erosional denudation of orogenic belts have long been the most impor- tant geologic processes that serve to shape continental surfaces, but the rate of geomor- phic change resulting from these natural phe- nomena has now been outstripped by human activities associated with agriculture, con- struction, and mining. Although humans are now the most important geomorphic agent on the planet's surface, natural and anthro- pogenic processes serve to modify quite dif- ferent parts of Earth's landscape. In order to better understand the impact of humans on continental erosion, we have examined both long-term and short-term data on rates of sediment transfer in response to glacio-fl u- vial and anthropogenic processes. Phanerozoic rates of subaerial denuda- tion inferred from preserved volumes of sedimentary rock require a mean conti- nental erosion rate on the order of 16 m per million years (m/m.y.), resulting in the accumulation of ~5 gigatons of sediment per year (Gt/yr). Erosion irregularly increased over the ~542 m.y. span of Phanerozoic time to a Pliocene value of 53 m/m.y. (16 Gt/yr). Current estimates of large river sediment loads are similar to this late Neogene value, and require net denudation of ice-free land surfaces at a rate of ~62 m/m.y. (~21 Gt/yr). Consideration of the variation in large river sediment loads and the geomorphology of respective river basin catchments suggests that natural erosion is primarily confi ned to drainage headwaters; ~83% of the global river sediment fl ux is derived from the high- est 10% of Earth's surface. Subaerial erosion as a result of human activity, primarily through agricultural practices, has resulted in a sharp increase in net rates of continental denudation; although less well constrained than estimates based on surviving rock volumes or current river loads, available data suggest that present farmland denudation is proceeding at a rate of ~600 m/ m.y. (~75 Gt/yr), and is largely confi ned to the lower elevations of Earth's land surface, primarily along passive continental margins; ~83% of cropland erosion occurs over the lower 65% of Earth's surface. The conspicuous disparity between natu- ral sediment fl uxes suggested by data on rock volumes and river loads (~21 Gt/yr) and anthropogenic fl uxes inferred from measured and modeled cropland soil losses (75 Gt/yr) is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately downslope from erod- ing croplands over the history of human agriculture. Accumulation of postsettlement alluvium on higher-order tributary channels and fl oodplains (mean rate ~12,600 m/m.y.) is the most important geomorphic process in terms of the erosion and deposition of sedi- ment that is currently shaping the landscape of Earth. It far exceeds even the impact of Pleistocene continental glaciers or the cur- rent impact of alpine erosion by glacial and/ or fl uvial processes. Conversely, available data suggest that since 1961, global cropland area has increased by ~11%, while the global population has approximately doubled. The net effect of both changes is that per capita cropland area has decreased by ~44% over this same time interval; ~1% per year. This is ~25 times the rate of soil area loss antici- pated from human denudation of cropland surfaces. In a context of per capita food pro- duction, soil loss through cropland erosion is largely insignifi cant when compared to the impact of population growth.