Humans as geologic agents A deep time perspective Bruce H Wilkinson Department of Geological Sciences University of Michigan Ann Arbor Michigan 48109 USA ABSTRACT Humans move increasingly large amounts of rock and sediment during various construction activities and mean rates of cropland soil loss may exceed rates of formation by up to an order of magnitude but appreciating the actual importance of humans as agents of global erosion necessitates knowledge of prehistoric denudation rates imposed on land surfaces solely by natural processes Amounts of weathering debris that compose continental and oceanic sedimentary rocks provide one such source of information and indicate that mean denudation over the past half billion years of Earth history has lowered continental surfaces by a few tens of meters per million years In comparison construction and agricultural activities currently result in the transport of enough sediment and rock to lower all ice free continental surfaces by a few hundred meters per million years Humans are now an order of magnitude more important at moving sediment than the sum of all other natural processes operating on the surface of the planet Relationships between temporal trends in land use and global population indicate that humans became the prime agents of erosion sometime during the latter part of the first millennium A D Keywords denudation erosion humans deep time INTRODUCTION Rates of soil erosion associated with agricultural practices are generally thought to exceed soil loss tolerances over most of Earth s cropland regions Troeh et al 1999 However the magnitude and significance of this difference remain matters of debate Some studies e g Pimentel et al 1995 Pimentel and Skidmore 2004 U S Department of Agriculture USDA 1994 indicate that rates of soil loss from United States croplands exceed those of soil formation by over an order of magnitude implying that current agricultural practices are far removed from sustainable levels Others e g Brady and Weil 1999 conclude that rates of agricultural denudation and soil generation are sufficiently similar that U S soil erosion does not yet constitute a land use calamity The issue here is one of determining magnitudes of natural rates of rock and sediment erosion these are the significant processes of land surface denudation across most of Earth s surface One of the major difficulties in evaluating the degree to which human activities have augmented continental erosion is that in a given area the onset of such anthropogenic influences frequently predated historical records Assessing the magnitude of human induced erosion is of obvious pragmatic as well as intellectual importance However in order to appreciate changes induced by a growing world population it is also necessary as a basis for comparison to determine the importance of natural denudation processes An estimate of a deep time baseline rate of natural erosion such as that associated with glaciers and rivers has been attempted 1 through radionuclide dating of land surfaces and 2 by examination of data on river borne sediment fluxes Some ambiguity is inherent in either approach The determination of erosion rates using cosmogenic isotopes is still in its infancy available data are sparse and current rates are primarily from orogenic regions e g Kirchner et al 2001 Small et al 1997 Denudation rates calculated from river borne sediment loads e g Meybeck 1988 Harrison 2000 Summerfield and Hulton 1994 are also somewhat biased because current sediment loads reflect both natural and anthropogenic processes the latter of which have influenced drainage sediment yields for thousands of years Ruddiman 2003 and are also changing rapidly in response to societal evolution and population growth Much of the geologic cycle entails transfer of rock materials into the sedimentary reservoir through the weathering of preexisting rocks the movement of particulate and dissolved components resulting from weathering to sites of sediment accumulation and the deposition of this material as terrigenous clastic carbonate and evaporite units Moreover subsequent uplift and erosion result in a progressive decrease in epoch long interval rock volume with increasing age Data on surviving amounts of sedimentary rock Ronov 1983 therefore allow for estimation of epoch long rates of sediment accumulation which in turn relate to rates of physical and chemical denudation over Earth s subaerially exposed sur face for at least the past half billion years Sedimentary rock volumes are here employed in this way to derive deep time global denudation rates e g Harrison 1994 These values are then compared to approximations of near recent rates of denudation derived from major river borne particulate and dissolved loads as well as estimates of human induced erosion from construction and agricultural practices GLOBAL PHANEROZOIC SEDIMENT FLUXES Continental 85 of total and oceanic 15 sediments make up the current 630 3 106 km3 of terrigenous clastic 69 carbonate 28 and chert and evaporite 3 units that have accumulated on Phanerozoic oceanic basaltic and continental granitic crust Ronov 1983 an amount sufficient to blanket all continental surfaces to a depth of 3 km and all ocean basins to a depth of 300 m This amount of sedimentary material embodies the surviving volume of products from Earth surface erosion spanning the past 542 m y Moreover rock volumes of any given age in both oceanic and continental settings decrease with increasing age a relationship that reflects the progressive destruction of sedimentary and other rocks with the passage of geologic time Amounts of surviving oceanic and continental sediment decrease exponentially with increasing age relationships reflecting the fact that first order cycling rates are primarily dependent on the amounts of material that make up different parts of the q 2005 Geological Society of America For permission to copy contact Copyright Permissions GSA or editing geosociety org Geology March 2005 v 33 no 3 p 161 164 doi 10 1130 G21108 1 3 figures 161 Figure 1 Data from Ronov 1983 on volumes of sediment of given age deposited on oceanic shaded and continental unshaded crust Dashed line is best fit model of decrease in global volume of sedimentary rock under assumptions that mean oceanic and continental sediment fluxes are 1 1 and 2 3 3 106 km3 m y respectively y intercepts and sediment destruction by subduction and erosion of 2 28 m y and 0 06 m y