EARLY PALEOPROTEROZOIC EVENTS AND PROCESSES: AN OVERVIEW

The composition of the atmosphere and the oceans changed considerably during the Paleoproterozoic. The disappearance of mass independent fractionation of the sulfur isotopes in excess of ca. 0.2% between 2.45 and 2.32 Ga is currently the most compelling evidence for the onset of the Great Oxidation Event (G.O.E.). The loss of easily oxidized minerals during weathering began ca. 2.3-2.4 Ga. The oxidation state of iron in paleosols increased dramatically between 2.45 and 2.20 Ga. Large excess quantities of the redox sensitive elements are absent in pre-2.3 Ga but are present in ca. 2.15 Ga and younger carbonaceous shales.

The average Fe₂O₃/FeO ratio in shales increased considerably between 2.3 and 2.1 Ga, as did the oxidation state of shallow water BIFs. The abundance of red beds and of evaporative sediments containing gypsum and/or anhydrite increased markedly during this time interval.

These changes are almost certainly related to the burial of excess organic carbon during the large positive d¹³C excursion in marine limestones between ca. 2.2 and 2.0 Ga, and to the resultant rise in the partial pressure of atmospheric oxygen.

The rates and the causes of the environmental changes during the Paleoproterozoic are still poorly defined. Biologic evolution as well as inorganic and biogeochemical processes have been proposed as causes for the initiation and for the termination of the G.O.E. CH₄ has been assigned a major role as a greenhouse gas, and variations in its partial pressure have been invoked as a cause of the three major Paleoproterozoic glaciations. Yet we are still short of a completely satisfactory explanation of the large, well documented environmental changes of the Paleoproterozoic era.