IMPROVING THE AGE CONSTRAINTS ON CRETACEOUS CARBON ISOTOPE EXCURSIONS: A COMBINED U-PB DATING APPROACH USING PALEOSOL CRYPTOTEPHRA AND PEDOGENIC CARBONATES
Zircons from 13 paleosols (total n>4000) from two well-characterized sections of the Cretaceous Ruby Ranch Member (Ruby Ranch Road: RRR, Dinosaur National Monument: DNM) in Utah were dated by LA-ICP-MS. These sections have been shown to contain several prominent CIEs (Ludvigson et al., 2010 JSR 80; Ludvigson et al., 2015 Cret.Res. 56; Suarez et al. 2020, Geol. Soc. London SP 507) and interpreted to encompass the Aptian-Albian boundary. The U-Pb data was interpreted using the Bayesian approach of Keller et al. 2018 (Geochem. Persp. L. 8) to identify the most likely age of volcanic eruption (=deposition) from large, dispersed zircon datasets. Late Aptian zircon ages (ca. 118 Ma) for the interpreted C10 CIE at RRR are consistent with the U-Pb age of a silicified carbonate horizon underlying the C10 horizon. These results are consistent with the latest interpretations from the marine record. They are also consistent with our U-Pb zircon results from DNM, while more work is needed to refine the age of the Ap7 and other CIEs.
The study shows the potential for significant improvement of the timing of paleoclimate events from continental strata with this approach, complementing the marine record. A more detailed and complete chronostratigraphic and paleoclimate record of this time interval can be obtained from deeper western parts of the basin and will be subject of future studies.