Isotopic evolution of the biogeochemical carbon cycle during the Proterozoic Eon

Time-related changes in the Proterozoic sedimentary isotopic record of organic carbon and carbonates are consistent with the following: (1) an increase in the oxidation state of the environment, (2) a transition towards microbial populations dominated by producers and utilizers of O-2 and SO42-, and (3) a relatively minor increase in organic delta(13)C during postdepositional thermal alteration (delta(13)C increased similar to 3 parts per thousand as H/C declined from 0.8 to 0.1). The progressive oxidation of the environment was episodic. For example, between 2.2 and 1.9 Ga, crustal and atmospheric reservoirs of O-2, Fe3+ and SO42- increased relatively rapidly, and this increase was too large to be attributed solely to the declining rate of oxidant consumption by volcanic and hydrothermal fluxes of reduced species. Additional fundamental changes seem to be required in the tectonic style of organic sedimentation and preservation. Episodic changes in both the biogeochemical C cycle and atmospheric O-2 levels during the Proterozoic Eon likely influenced the evolution of Eukarya. (C) 1997 Elsevier Science Ltd. All rights reserved.