The Anthropocene: a comparison with the Ordovician–Silurian boundary

Abstract

Analysis and definition of the Anthropocene as a potential formal unit within the Geological Time Scale necessitates overt comparison of present day, geologically significant environmental changes with geologically earlier perturbations. Previous comparisons have focused on the glacial-interglacial changes of the Quaternary and the ‘hyperthermal’ events such as the Toarcian and Paleocene–Eocene thermal maximum, the Pliocene, and more recently the Precambrian–Cambrian boundary, though all these include significant differences from, as well as similarities with, the contemporary phenomenon. We here examine another geological boundary that in some respects bears a closer resemblance to the Anthropocene: the Ordovician–Silurian boundary. This major warming event starts from a global icehouse state, unlike the Meso-Cenozoic hyperthermals, and so includes a rapid, marked deglacial sea level rise associated with increased marine anoxia, as forecast for the near-future Earth. Both the deglaciation and prior glacial inception are linked with mass extinction events on a scale likely comparable with the ongoing event. In defining the Ordovician–Silurian boundary, it is notable that the boundary event selected is environmentally negligible by comparison with the major Earth system changes taking place around this level—the appearance and wide dispersal of a couple of distinctive graptolite species. Similar utilitarian rationale might be employed in seeking an Anthropocene boundary.