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Radioisotopic dating and the Cretaceous time scale
Brad S Singer
University of Wisconsin-Madison, United States of America
Radioisotopic dating using single crystal U-Pb zircon and 40Ar/39Ar sanidine methods since publication of Geologic Time Scale 2020 (GTS2020) continues to improve the Cretaceous time scale and more firmly establish the ages and uncertainties associated with several stage boundaries and key phenomena. Examples here aim to highlight some of these recent contributions. Bayesian age-depth modeling of 19 U-Pb dates of tuffs in the Yezo Group, Hokkaido, Japan where Leupoldina cabri fossils occur above nearly synchronous carbon and osmium isotope excursions indicate that OAE1a began at 119.55 +0.072/−0.079 Ma and had a duration of 1116 +87/−93 kyr. These findings imply that Ontong Java volcanism is a probable driver of OAE1a. Collective U-Pb and 40Ar/39Ar dating of tuffs in Quingshan Group, Jiaolai Basin, China constrain the onset of magnetic chron M0r, and thus the Barremian-Aptian boundary, to 120.287 ± 0.088 Ma (±2s full external uncertainty) which is >1 myr younger, and nearly an order of magnitude more precise than in GTS2020. U-Pb and 40Ar/39Ar dates from 11 tuffs in 5 sections of the Yezo Group, Hokkaido, Japan have yielded an age for the Albian-Cenomanian boundary of 100.74 ± 0.10 Ma. This age, which exceeds that in GTS2020 by 240 kyr, relies on correlating distinctive positive δ13C excursions of OAE1d from the GSSP near Mt. Risou, France to those identified within Yezo Group strata where a first appearance of Thalmanninella globotruncanoides has been documented. A slight osmium isotope excursion in these mudstones suggests a role for Kerguelan LIP volcanism in driving OAE1d.
