Because humanity and its activities have transformed the vegetated face of our home planet, caused its climate to warm and pushed an increasing number of other species over the edge of extinction, some circles have coined the name “Anthropocene” for the last half of the Holocene Epoch. Human induced change almost certainly began as soon as settled agriculture arose to dominate most societies (see Did the earliest agriculture kick-start global warming?, in EPN of April 2005). In terms of atmospheric emissions and mobilizing metals we now push natural rates close: facts that emerge from annual reviews of mining and energy use. But are we truly significant geological agents as well as influences on the atmosphere and biosphere? Two articles in April 2005 suggest that we are.
Quarries, mines and other excavations are obvious signs of human erosive power, but our farming activities produce insidious results by inducing soil erosion. Although its effects are well known from such areas as the Ethiopian Highlands and the 1930’s “Dust Bowl” of the US mid-west, a global measure of the rates involved requires a careful compilation of quantitative data. Bruce Wilkinson of the University of Michigan has made the first attempt (Wilkinson, B.H. 2005. Humans as geological agents: A deep-time perspective. Geology, v. 33, p. 161-164). Throughout the Phanerozoic, the volume of sedimentary rocks suggests that enough erosion has taken place to have stripped a uniform blanket 3 km deep from the continental surface. That gives an average erosion rate for the last half-billion years of Earth history of the order of tens of metres per million years. Assembling information about current rates of human-induced stripping, roughly divided 30:70 between excavation and soil erosion, Wilkinson arrives at a staggering figure for anthropogenic denudation: hundreds of metres per million years. Our activities in the outer part of the rock cycle are an order of magnitude greater than purely natural rates of weathering, erosion and transportation. He suggests that humanity began to outpace sedimentology sometime around the time of the Norman Conquest.
This awesome picture might seem to indicate that rates of sediment deposition on continental margins are also tremendously elevated by our actions. That aspect has been studied by geoscientists from the US and Holland (Syvitski, J.P.M. 2005. Impact of humans on the flux of terrestrial sediment to the global coastal ocean. Science, v. 308, p. 376-380). The opposite is now happening. Syvitski et al.’s analysis of historical sediment loads in the catchments and lower reaches of the worlds major rivers shows that while overall sediment transport has increased by 2.3 billion t per year, since human effects became noticeable in the sedimentary record, the amount delivered to the sea has fallen. Some 1.4 billion t no longer add to marine sedimentation each year. Instead, that mass ends up behind dams of one kind or another. In the last 50 years, more than 100 billion t, containing 1 to 3 billion t of carbon is in silted up reservoirs, or redistributed to farmland by irrigation diversions. One of the outcomes is that natural coastal protection by spits and sand bars is growing less effective. Another is that less nutrients are getting to the near-shore marine biosphere, with possible effects on fish stocks, coral reefs and other habitats.
Caring among the Erects
Dmanisi in Georgia provided one great surprise in human evolution by yielding abundant remains of 1.7 Ma old Homo erectus where they might be least expected: north of the Caucasus mountains that would have formed a tremendous barrier to any migration from further south. The archaeological sites have provided another surprise in the form of a well-preserved skull of a completely toothless individual. It is clear from the regrowth of bone into the sockets that this “masticatorily impaired” individual survived for years after losing all their teeth (Lordkipanidze, D. et al. 2005. The earliest toothless hominin skull. Nature, v. 434, p. 717-718). It is impossible to believe that the individual could have survived on a tough meat and vegetable diet without special preparation of soft victuals. Although the person’s survival cannot prove that other Erects helped out, that is a distinct possibility. Losing teeth through dental disease or trauma would have been immensely painful and debilitating, yet the individual did survive. We have to move forward to around 40 thousand years ago for compelling evidence that Neanderthal society cared for disadvantaged people, when several near-complete skeletons show evidence of long-term, crippling damage.