New Re-Os constraint for the Bitter Springs anomaly in NW Canada: Exploring connections between continental weathering and the Tonian Os cycle

The Tonian Period (1000−717 Ma) was a crucial episode in the coevolution of Earth and life, and well-preserved sedimentary strata provide an opportunity to interrogate the paleoenvironmental conditions in the lead up to eukaryote proliferation. The Bitter Springs negative carbon isotope anomaly (BSA) represents the first documented prominent disturbance to the carbon cycle during the Tonian and is an important chemostratigraphic marker and tie point for studying ocean biogeochemistry and calibrating biostratigraphy during this period. Current radiometric ages for the BSA are from strata bracketing the excursion, which provide maximum and minimum constraints but do not permit a complete evaluation of the excursion’s timing and duration. We report a Re-Os age of 799.6 ± 3.5 Ma from within the BSA preserved in the Wynniatt Formation of the Shaler Supergroup, Northwest Territories, Canada. This age is the first precise radiometric age constraint from strata that host the BSA, thereby providing an important new tie point for improving mid-Tonian age models and stratigraphic correlation. In addition, we present an Os isotope (187Os/188Os) chemostratigraphic dataset suggesting that the source of Os fluctuated prior to the BSA in the Amundsen Basin, possibly linked to local changes in sea level. Our findings highlight that basin-scale sequence stratigraphy should be considered when interpreting radiogenic isotopic trends and their biogeochemical implications in deep-time sedimentary records.