CAPTURING CHANGES IN THE MESOPROTEROZOIC EARTH SYSTEM: δ13CCARB AND 87SR/86SR STRATIGRAPHY OF THE ~1254 MA BASS FORMATION, GRAND CANYON SUPERGROUP, AZ., USA

Evidence of the oldest life in the Grand Canyon is recorded in the Bass Formation of the Grand Canyon Supergroup. The Bass Formation is a mixed siliciclastic-carbonate unit (~100 m thick) that provides information on the Earth system in which these microbes thrived during the ‘boring billion’. Application of carbon, oxygen, and strontium isotope geochemistry will help to determine local vs. global connectivity with the ocean, as well as carbon burial and weathering rates at this time. A previously reported ID-TIMS U-Pb zircon age of 1254 ± 2 Ma from a tephra bed at the base of the Bass Formation provides a rare age-calibration, lacking in many Mesoproterozoic successions. Carbon and strontium isotope ratios from the Bass Formation can contribute age-calibrated data to the limited middle-late Mesoproterozoic global reference curves.

Preliminary isotopic data are consistent with other middle-late Mesoproterozoic successions around the globe. 87Sr/86Sr values range from 0.70659 to 0.70806, with the lowest value coming from the Hance Rapids locality, consistent with reported marine 87Sr/86Sr values from late Mesoproterozoic records. δ13C values from the Hance Rapids locality range from -0.47 to 1.52 ‰ and δ13C values from the Bass Canyon locality range from 0.10 to 4.14 ‰. δ18O values range from -0.60 to -9.8 ‰ and suggest that diagenesis did not affect δ13C values. The δ13C variability of 4 ‰ indicates that the carbon cycle had become perturbed by 1254 Ma. Analysis of other localities is currently in progress and results will be presented.