Sedimentary And Isotopic Characterization Of The Paleoproterozoic Glacial Interval
Kaufman, Alan J., Geology Department, University of Maryland, College Park, MD 20742
Bekker, Andrey, Geophysical Laboratories, Carnegie Institution of Washington, Washington, D.C. 20015
The Paleoproterozoic glacial interval (ca. 2.45 - 2.22 Ga) in North America and equivalents worldwide is notable not only for the occurrence of widespread diamictite at several stratigraphic levels, but also for the general absence of carbonates and iron-formation. These lithologies dominate the earlier sedimentary record and resume after the Paleoproterozoic ice ages. This observation contrast markedly with lithologic associations from Neoproterozoic glacial successions, and warrants further investigation of oceanic and atmospheric chemistry during Earth’s earliest glacial epoch. In the Huronian Supergroup of southern Ontario and equivalents in Wyoming there are three discrete horizons of glacial diamictite, but only one known cap carbonate (the Espanola Formation atop the middle diamictite). Chemostratigraphic correlation of these glacial deposits with examples from other cratons is notably hindered by the lack of carbonate or other seawater proxies. Thus, to chart temporal changes in carbon cycling across the oldest glacial deposit in the Huronian Supergroup, 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. The core (Kerr-McGee drillhole 156-1) was drilled into deep water lithofacies while stratigraphic outcrop samples were taken in shallower lithofacies near an apparent basement high. Organic matter comprises 0.18 to 0.46 mg C/g sample in the underlying McKim Formation and has 13C values between –22.1 and –27.6‰ in drill core and between –25.6 and –31.5‰ in outcrop. 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 13C variability. In particular, 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 13Corg values around –40.5‰. In contrast, organic matter isolated from outcrop samples of the Pecors Formation have 13Corg values ranging between –30.3 and –19.2‰. Based on the core measurements and an assumption of the magnitude of carbon isotope fractionation, the pre-Ramsay Lake ocean 13C composition may have been moderately positive, although this is difficult to constrain in the absence of well preserved carbonate. The 13C-enrichment in organic matter immediately above the diamictite may be related to carbon limitation during high organic productivity, perhaps stimulated by a high nutrient during oceanic upwelling or riverine inputs at the end of the glaciation, a trend also noticed in some Neoproterozoic and Paleoproterozoic post-glacial successions. The 13C-depletion in overlying drill core lithologies suggest that methane production and recycling by methanotrophs may have been an important process at the redox boundary in the deeper-water environment. 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, high pCO2 may have resulted in lower oceanic pH and the general absence of carbonate lithologies during the glacial interval.
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