Environmental Changes in Times of Biotic Crisis

Abstract

The fossil record indicates accelerated rates of extinction and of evolution at era transitions or times of biotic crisis. Geochemical anomalies suggest that the dissolved CO₂ content in ocean surface waters was then abnormally high, probably because the ocean was almost devoid of plankton. The oxygen-minimum zone in the oceans expanded, and the surface waters were unusually corrosive. Meanwhile the sea bottom underlying the expanded oxygen-minimum zone became locally anoxic. The environmental changes curtailed drastically the fertility of marine organisms and were probably the cause of mass extinctions. On land, pollen evidence indicates serious disruption of the terrestrial plant ecosystem at the end of Cretaceous; there was widespread destruction of the vegetation, although only a few plant species became extinct. We have no positive indications of comparable changes at other era boundaries, although a total elimination of the upiquitous algal community at the end of the Precambrian era has been speculated upon. Oxygen istotope data suggest that temperature may have dropped and/or risen abruptly at times of biotic crisis, but systematic trend has not yet been established. Of the numerous ideas advanced to explain the environmental catastrophes ending geological eras, I find the theory of large-body impact a most attractive working hypothesis.