Atmospheric, climatic, and biological evolution at both ends of the Proterozoic Eon

Category Climate system
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Kaufman, Alan
Holding Date 09 September 2008

Both ends of the Proterozoic Eon (2500 to 542 Ma) are characterized by inter-related environmental, climatic, and biological events likely associated with fluctuations in atmospheric concentrations of oxygen, carbon dioxide, and methane. In each interval there is abundant evidence for widespread glaciation and the alternation of redox sensitive lithologies. Isotopic studies across the Proterozoic Eon reveal the effects of major orogenic and hydrothermal events on ocean chemistry, as well as significant reorganization of the carbon and sulfur cycles. Detailed stratigraphic studies indicate a close association of the biogeochemical and climatic events with the rise and fall of atmospheric oxygen.
In the Ediacaran Period, carbonates worldwide record a remarkable negative carbon isotope anomaly that accompanies a rise in oceanic sulfate abundance and fall of δ34S values in carbonate associated sulfate and sulfides. The 12C-rich alkalinity delivered to the seawater pool was likely associated with a dramatic rise in atmospheric O2, which resulted in 1) the oxidation of exposed continental shelf sediments rich in fossil organic matter and sulfides, and 2) the ventilation of the oceans. The carbon isotope anomaly is recorded in broadly equivalent successions that post-date known Marinoan glacial deposits and pre-date the Precambrian-Cambrian boundary in Oman, India, South China, Australia, Namibia, Mexico, and the western USA, supporting the view that the oxygenation event was global. Large metazoan fossils first appear directly above this anomaly, suggesting that a critical threshold with respect to atmospheric O2 had been crossed at this time.
In the early Proterozoic, the rise of atmospheric oxygen coincides with Earth’s earliest glacial record, and is linked to strong variations in carbon and sulfur isotope compositions of seawater proxies. The glacial interval is notable for the occurrence of widespread diamictite in at least three stratigraphic levels, but also for the general absence of carbonates and iron-formation. Studies in South Africa and North America highlight the glacial changes in isotopic measurements of both rare carbonates and organic matter. Notably, organic matter in a post-glacial succession in the Huronian Supergroup is significantly depleted in 13C, suggesting that that methane production and recycling by methanotrophs may have been an important process at the redox boundary within the ocean. Accepting that the oxidative drawdown of methane resulted in each of the three Paleoproterozoic ice ages requires that methane fluxes continued throughout and that oxygen rose prior to each glaciation. This hypothesis suggests that the glacial cycles were driven by changes in the CH4/CO2 of the atmosphere and oceans associated with progressive oxygenation of the surface environment. Furthermore, high pCO2 may have resulted in lower oceanic pH and the general absence of carbonate lithologies during the glacial interval.