A complete Os excursion across a terrestrial Cretaceous–Paleogene boundary at the West Bijou Site, Colorado, with evidence for recent open system behavior

The few previously reported values of 187Os/188Os ratios from non-marine K–Pg boundary sections are distinctly higher than the range of 187Os/188Os ratios measured in chondrites and the range of ratios predicted by models of physical mixing between chondrites and upper crust. Here, Re–Os data from the West Bijou continental K–Pg boundary site, located within the Denver Basin, are used to better constrain the Os isotopic composition of fallout from the Chicxulub ejecta plume. For the first time, full vertical profiles of 187Os/188Os ratios and Re and Os concentrations across a continental K–Pg boundary section are reported. Within this section of lignite, the lowest measured 187Os/188Os ratio (0.182) coincides with the K–Pg boundary interval as previously determined by palynology and the distribution of shocked quartz in a nearby outcrop. However, sediments with elevated Os concentrations and measured 187Os/188Os below 0.23 extend over a 30 cm interval, from ~ 5 cm above the clay-rich boundary interval (~ 5 cm thick) to ≈ 25 cm below. Maximum Ir and Os concentrations occur 3 cm and 10 cm below the K–Pg boundary, respectively, demonstrating greater diagenetic mobility of Os relative to Ir. Low 187Os/188Os above the K–Pg boundary suggests that accumulation of impact derived Os at this site persisted after the impact event, perhaps due to redistribution of impact debris in the surrounding area. Importantly, calculated initial 187Os/188Os ratios throughout most of the section are impossibly low and require open system behavior, likely in the form of Re addition within the last 10 million years. Within the boundary interval, Re concentrations are among the lowest measured at Bowring Pit and calculated initial 187Os/188Os ratios are very similar to those previously reported from the western interior of the United States K–Pg boundary material (~ 0.14), suggesting super-chondritic 187Os/188Os is characteristic of the Chicxulub ejecta plume. Beyond implications for our understanding of Os as an impact tracer, this study emphasizes the dramatic effect diagenesis can have on the Re–Os system in organic-rich environments, especially through post-depositional addition of Re.