HUMANS AS GEOLOGIC AGENTS: A DEEP TIME PERSPECTIVE

Among major geologic processes, humans most profoundly augment the sedimentary arc of the geological cycle through both construction and agricultural activities, and it has been suggested that humans have increasingly become the premier geomorphic agents at the Earth's surface. In this regard, Hooke (2000) estimates that human activity has resulted in the transport of some 16 thousand cubic kilometers of soil and rock, with current rates on the order of about 140 cubic kilometers per year, an amount equivalent to the net movement of some 725 meters of exposed continental surface per million years. As geologic context for these numbers, Ronov (1985) tabulates data on total amounts of sediment presently comprising continental and oceanic reservoirs, material that was ultimately derived from subaerial erosion. Knowledge of ages and amounts of surviving sediment allow for estimation of Phanerozoic rates of continental denudation. These indicate that over the past half billion years, subaerial erosion has resulted in the net transport of about 3.6 billion cubic kilometers of soil and rock material. However, because the geologic cycle is largely cannibalistic, only about 18% of this material remains as Phanerozoic sedimentary successions. Plots of surviving sediment amount versus age define power law decreases in surviving amounts of sandstone, shale, and limestone in continental and oceanic reservoirs, and these relations indicate that Phanerozoic rates of denudation have been on the order of about 6.5 million cubic kilometers per million years, over much of the past 550 million years. This is an amount equivalent to the net movement of some 34 meters of exposed continental surface per million years, an amount about 5% of that effected by human activity. On the basis of these considerations, and given past patterns of global population growth, it seems apparent that humans are undoubtedly the most important agents of material transport on the modern Earth's surface, that the magnitude of these actions surpassed the net importance of all natural mechanisms of sediment movement sometime in the in the mid 1500s, and that human activities will increasingly dominate over all natural processes in the decades to come.