SEDIMENTARY AND ISOTOPIC CHARACTERIZATION OF THE PALEOPROTEROZOIC GLACIAL INTERVAL

The Paleoproterozoic glacial interval (ca. 2.45 - 2.22 Ga) in North America is notable for the occurrence of widespread diamictite at several stratigraphic levels, and also for the general absence of carbonates and iron-formation. This observation contrast markedly with lithologic associations from Neoproterozoic glacial successions. For example, in the Paleoproterozoic Huronian Supergroup of southern Ontario and equivalents in Wyoming there are three discrete horizons of glacial diamictite, but only one known cap carbonate. To chart temporal changes in carbon cycling across the oldest glacial deposit, which lacks carbonate, core and outcrop samples of laminated shale and siltstone from the McKim and Pecors formations (separated by the Ramsay Lake diamictite) were collected; organic isolates were combusted and resulting CO₂ was quantified, purified, and measured for ¹³C abundances. Organic matter comprises 0.18 to 0.46 mg C/g sample in the underlying McKim Formation and has δ¹³C values between -22.1 and -27.6‰ in drill core. In the Pecors Formation, organic matter concentrations range between 0.1 and 1.2 mg C/g sample and have a much wider range of δ¹³C variability. Samples from drill core within 6 meters of the diamictite are enriched in ¹³C with values up to -14.8‰, while those from the rest of the unit are markedly ¹³C-depleted with the lowest δ¹³C_org values around -40.5‰. Based on the core measurements and an assumption of the magnitude of carbon isotope fractionation, the pre-Ramsay Lake oceanic δ¹³C composition may have been moderately positive. Furthermore, ¹³C-enrichment in organic matter immediately above the diamictite may be related to carbon limitation during high nutrient driven organic productivity. On the other hand, ¹³C-depletion in overlying drill core and outcrop lithologies suggest that methane production and recycling by methanotrophs was an important process. These findings are consistent with the view 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, extremely high pCO₂ required to overcome lower luminosity at that time may have resulted in lower oceanic pH and the general absence of carbonate lithologies during the glacial interval.