The end of the Paleoproterozoic carbon isotope excursion has been constrained between 2113±4 and 2062±2 Ma based on UPb zircon ages of volcanic units in Finland (Karhu, 1993). Three Paleoproterozoic successions: Nash Fork Fm., WY; Lower Albanel Fm., Quebec; and Upper Jatuli/Ludian groups. Fennoscandian Shield show a sharp decrease in carbon isotope values. Heavy carbonates of these units were deposited in shallow-marine environments in a warm arid climate as evidenced by stromatolites, pseudomorphs after sulfates and halite, tepee structures, dolomitic composition, and desiccation cracks. Overlying carbonates have ~0o/oo PDB and are associated with organic-rich shales with pyrite. Inorganic sea floor and cavity filling precipitates were found in the upper part of the Nash Fork Fm. along with decrease/disappearance of stromatolites. Similar decrease in stromatolites above organic-rich shales occurs in the Lower Albanel Fm. and on the Fennoscandian Shield (Melezhik et al., 1997). Based on similar d¹³C trend within these successions, and similar paleoenvironmental and lithological changes we suggest that the sharp decrease in d¹³C of carbonates associated with the appearance of organic-rich shales strongly depleted in ¹³C represents a global event.

We propose a following model to explain data. A low latitude position of continents led to a greenhouse event and a stratified anoxic ocean. Heavy carbonates were deposited in intracratonic basins and in the shallow zone of the ocean. Major paleogeographic reorganization occurred at ~2.1-2.0 Ga based on ages of collisions in the circum-South Atlantic region and ocean opening in several basins. New seaways induced circulation, overturned the ocean, and brought ¹²C-rich alkaline deep waters into rite shallow waters. Shales record deepening which was likely related to sea level rise. Oceanic overturn brought nutrients from the deeper waters and led to productivity bloom recorded by high organic content of shales.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah