Humans as the third evolutionary stage of biosphere engineering of rivers
journal contributionposted on 13.04.2015, 09:20 by Mark Williams, Jan Zalasiewicz, N. Davies, I. Mazzini, J-P. Goiran, S. Kane
We examine three fundamental changes in river systems induced by innovations of the biosphere, these being: 1) the evolution of oxygenic photosynthesis; 2) the development of vascular plants with root systems; and 3) the evolution of humans. The first two innovations provide context for the degree of human-induced river change. Early river systems of the Precambrian Archean Eon developed in an atmosphere with no free oxygen, and fluvial sediments accumulated ‘reduced detrital’ minerals such as uraninite, siderite, gersdorffite and pyrite. By 2.4 Ga the evolution of oxygenic photosynthesis produced an oxygenated atmosphere and ‘reduced detrital’ minerals mostly disappeared from rivers, affording a distinct mineralogical difference from subsequent fluvial deposits. Rivers of the Precambrian and early Phanerozoic were dominantly braided, but from 0.416 Ga, the evolution of vascular plants with roots bound floodplain sediments and fostered fine-grained meandering rivers. Early meandering river deposits show extensive animal activity including fish and arthropod tracks and burrows. Homo sapiens, appearing about 150 Ka BP, has, in recent millennia, profoundly modified river systems, altering their mineralogical, morphological and sedimentary state. Changes in sediment fluxes caused by human ‘reverse engineering’ of the terrestrial biosphere include deforestation, irrigation and agriculture. Sediment retention has been encouraged by the construction of dams. Modern river systems are associated with extensive human trace fossils that show a developing complexity from ancient civilizations, through to the gigantic metro systems beneath rivers in modern megacities. Changes induced by humans rank in scale with those caused by earlier biosphere innovations at 2.4 and 0.416 Ga, but would geologically soon revert to a “pre-human” state were humans to become extinct.