EARTH'S EARLIEST ICE AGES: NEW ZIRCON GEOCHRONOLOGY OF ARCHEAN GLACIAL DIAMICTITES FROM MONTANA

The timing, duration, and location of Earth's earliest glaciations are poorly understood. To date, the oldest well-established glacial deposits are found in the Pongola Supergroup of South Africa. These units are ~2.9 Ga. Here, we revisit purported glacial units in the Beartooth Mountains in Montana, USA. Initial zircon dating from the 1970s suggested an age of 3.0 Ga. Our work both supports a glacial origin, through identification of dropstones and till, and a new detrital zircon age of ~2.7 Ga. Our work has direct bearing on the Faint Young Sun Paradox, as it suggests at least periods of widespread glacial activity in the Neoarchean. The extent and duration of such glaciation remains for future work, but there are intriguing possible relationships to biogeochemistry and atmospheric composition. Should our 2.7 Ga date remain robust, it would then be coincident with both a mantle super-plume event and some of the earliest geochemical evidence for oxygenic photosynthesis. Perhaps the eruption of fresh material, and enhanced weathering, drew down CO2 to low enough levels to initiate glaciation. Perhaps the presence of O2 from photosynthetic organisms reacted with CH4 in the atmosphere, causing a crash in this greenhouse gas and initiating a glaciation.