Stable isotopic evidence for the rise of oxygen and reorganization of the sulfur cycle from the ca. 2.4 Ga Duitschland Formation, South Africa.

摘要

Metasedimentary rocks from the Transvaal Supergroup, South Africa, were used in a stable isotopic study to investigate the evolution of the Earth's early atmosphere and early biogeochemical cycles. Sulfur isotopic evidence indicates that the transition from an anoxic to an oxic surface environment is recorded in the Duitschland Formation. The mass-independent fractionation signal (Δ33S and Δ36S) of Precambrian sulfide and sulfate samples diminishes from the Malmani Subgroup at the base of the study area to the top of the Duitschland Formation. Coincidental with the reduced mass-independent fractionation signal is a sizable jump in δ 34S and δ13C fractionation related to biological activity. These data suggest that the rise of atmospheric oxygen to levels above which mass-independent fractionation in sulfur species in not effectively recorded occurred at the time of the deposition of the upper Duitschland Formation and was associated with a fundamental reorganization of the carbon and sulfur cycles.