Radioisotopic age, osmium isotopes, and global correlation of the Albian-Cenomanian boundary

Zircon U-Pb and sanidine ⁴⁰Ar/³⁹Ar dates of individual crystals from 11 tuffs in sections of the Yezo Group, Hokkaido, Japan, yield an age for the Albian-Cenomanian boundary of 100.74 ± 0.10 Ma. This age, which exceeds that in the current geologic time scale (GTS2020) by 240 k.y., relies on correlation of distinctive positive δ¹³C excursions known as the Albian-Cenomanian boundary event from the global stratotype section and point (GSSP) near Mont Risou, France, to those identified within Yezo Group strata. We find that the Albian-Cenomanian boundary in the Yezo Group is associated with a sharp negative δ¹³C excursion as well as the onset of a progressive transition toward less radiogenic Os isotope (¹⁸⁷Os/¹⁸⁸Os) ratios. The subdued expression of the Albian-Cenomanian boundary event δ¹³C isotope excursions, and that of oceanic anoxic event (OAE) 1d, relative to those of OAE1a and OAE2, coupled with a lack of prior evidence for an Os isotope shift, has led to the inference that monsoons, rather than submarine volcanism, propelled the lesser mid-Cretaceous OAEs. The high-resolution Os isotope profile of the Albian-Cenomanian boundary event from the Yezo Group provides potential evidence that submarine volcanism, likely associated with a high-flux phase of Kerguelen Plateau growth, played a role in perturbing the ocean carbon reservoir beginning during the final ~100,000 yr of the Albian stage. If the modest Os isotope shift reflects a volcanic source, its impact on seawater Os isotope composition was modulated by a drop in atmospheric and sea-surface temperatures, and a decrease in continental weathering at a global sequence boundary, features that distinguish the Albian-Cenomanian boundary event and OAE1d from the more prominent OAE1a and OAE2.