Evidence for Paleoproterozoic cap carbonates in North America

The early Paleoproterozoic Snowy Pass Supergroup of the Medicine Bow Mountains and Sierra Madre, Wyoming, USA and Huronian Supergroup, Ontario, Canada were deposited along the present-day southern flank of the Wyoming and Superior cratons. Whereas three discrete levels of glacial diamictite are developed in both successions, carbonate strata are known only directly above the middle diamictite (Vagner and Bottle Creek formations in Medicine Bow Mountains and Sierra Madre, respectively, and Espanola Formation in southern Ontario) in these thick correlative siliciclastics-dominated strata. The carbonates from each succession record negative δ¹³C values (-4.0 to -0.8‰, V-PDB) and attenuated carbon isotopic difference between organic and inorganic phases. Oxygen in carbonates is strongly depleted in ¹⁸O suggesting exchange with hot fluids, which is consistent with pervasive recrystallization of carbonates and remobilization of elements. However, the stratigraphic coherence of carbon isotopic compositions and the general lack of correlation between δ¹³C and either δ¹⁸O values or trace element concentrations supports a primary origin for ¹³C-depleted carbonates, which are interpreted here to reflect anomalous oceanic compositions. The intimate association of thick carbonate units containing abundant carbonate debris flows with immediately underlying glacial strata indicates that chemical precipitation resulted from a rapid flux of carbonate alkalinity onto ocean margins during post-glacial transgression. Although these early Paleoproterozoic carbonates are similar to Neoproterozoic 'cap dolomites' in stratigraphic position and carbon isotopic compositions, the older post-glacial accumulations begin with limestone and lack many of the sedimentary structures typical of Neoproterozoic deposits. Furthermore, it is not understood why carbonates only occur above the middle of the three glacial horizons whereas these deposits are ubiquitous above Neoproterozoic diamictites. The differences might reflect lower overall carbonate saturation in early Paleoproterozoic oceans which contrasts sharply with Archean and later Paleoproterozoic intervals and higher siliciclastic inputs in rift environments, which shut down carbonate deposition. Geological and geochemical indicators suggest a stepwise increase in atmospheric oxygen across the Paleoproterozoic glacial epoch. The tempo and mode of atmospheric oxygen rise has significant consequences for the abundance of the important greenhouse gases CH₄ and CO₂ and hence for oceanic acidity. If we accept that atmospheric oxidation of methane to carbon dioxide resulted in each of the three discrete glaciations, it implies that atmospheric CH₄ remained high throughout the interval and that pulsed oxidation events, plausibly linked to higher primary productivity and lower hydrothermal activity, led to surface refrigeration. If correct, the unique presence of cap carbonate above the middle Paleoproterozoic diamictite may reflect an appropriate balance of CO₂ and CH₄ sufficient to provide enough alkalinity to seawater through silicate weathering, but not so high that carbonate preservation would be inhibited by enhanced acidity.