INITIATION OF PALEOPROTEROZOIC GLACIAL CYCLES: INSIGHTS FROM OSMIUM ISOTOPES

The Huronian Supergroup of Ontario, Canada is one of the best-preserved sedimentary successions that span the Paleoproterozoic glaciations and the Great Oxidation Event (GOE) and was deposited between 2.45 and 2.32 Ga. This broad window of time encompasses at least three global glaciations and associated interglacial periods. Deducing the cause(s) and timing of these major changes in Earth’s atmosphere-ocean system, as documented in the Huronian Supergroup, is crucial for understanding the GOE and associated glaciations, as well as for global correlation of Paleoproterozoic sedimentary successions.

The oldest Huronian glaciation is recorded by the Ramsey Lake Formation. We studied Os isotope systematics to constrain the link between this glacial event and changes in the redox state of the atmosphere-ocean system at this time. We found an increase from unradiogenic ¹⁸⁷Os/¹⁸⁸Os values (~0.1) in the underlying McKim Formation to more radiogenic values (~0.7) in the post-glacial Pecors Formation. In addition we find a two-fold enrichment in both Re and Os abundances in the Pecors Formation. The pre-glacial unradiogenic, mantle-like ¹⁸⁷Os/¹⁸⁸Os values require either overwhelmingly large juvenile hydrothermal sources of Os or levels of atmospheric oxygen that are too low to oxidize crustal sulfides and induce sufficient riverine flux of radiogenic Os from continental weathering. In contrast, during the post-glacial period, oxidative dissolution of glacial flour generated during the oldest Huronian glaciation could have caused a major influx of radiogenic Os from the continents. Our results suggest oxidizing surface conditions in the immediate aftermath of the oldest Huronian glaciation and provide additional support for the link between changes in the redox state of the atmosphere-ocean system and climate at the beginning of the GOE.