IMPROVING THE AGE CONSTRAINTS ON CRETACEOUS CARBON ISOTOPE EXCURSIONS: A COMBINED U-PB DATING APPROACH USING PALEOSOL CRYPTOTEPHRA AND PEDOGENIC CARBONATES

The timing of Cretaceous carbon isotope excursions (CIEs) that provide crucial information on paleoclimate continues to be rather poorly constrained (e.g. Erba et al. 2015, GSA SP 511). We propose that a combined U-Pb dating approach using zircons from paleosol cryptotephra and pedogenic carbonates produces significant improvements. Mature paleosols have the ability to capture and preserve volcanogenic zircon and constitute cryptotephra if there is active volcanism upwind. They yield more deposition-age appropriate zircons than fluvial sandstones, which are dominated by older inherited grains (Smith et al. 2017 JSR 87, Joeckel et al. 2019, Geol. Soc. London SP 498). Zircon can only be as old or older than sediment deposition, while pedogenic carbonates within the paleosols can only be younger than deposition and the different datasets thus bracket the age of deposition.

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.