Aptian–albian Integrated Stratigraphy of the Forearc Basin Succession in Hokkaido, Northern Japan

The mid-Cretaceous Aptian–Albian interval was characterized by increased igneous activity and the frequent occurrence of pronounced environmental and carbon-cycle perturbations, known as Oceanic Anoxic Events (OAEs). Despite their global significance, high-resolution Aptian–Albian sedimentary records from the Pacific Ocean are exceptionally scarce due to the subduction of most of the seafloor, leaving a critical gap in our understanding of how these events manifested in the largest ocean basin of the time. Since the Pacific occupied a vast proportion of the Cretaceous global ocean, the burial of organic carbon and the development of anoxic water masses are expected to have had significant impacts on the carbon cycle and biological evolution. In fact, the paleo-Pacific Ocean is suggested to have represented a “hidden budget” in estimates of organic carbon burial during OAE 2. Additionally, rapid warming during OAEs is thought to have intensified continental weathering, which in turn enhanced the nutrient supply to the oceans, promoting marine productivity and organic carbon burial. Recent studies have emphasized that the weathering of volcanic arcs along active continental margins may have played a particularly important role in regulating global carbon cycling and nutrient fluxes. To address this, we conducted an integrated stratigraphic analysis of the marine sedimentary sequence of the Yezo Group, a forearc basin succession deposited along the active continental margin of the northwestern Pacific in Hokkaido, Japan. Since these strata accumulated adjacent to an active volcanic arc, they offer an excellent opportunity to evaluate the role of arc weathering during OAEs. The Yezo Group, which reaches a thickness of approximately 10,000 m, is widely distributed along the axial zone of Hokkaido Island. It consists mainly of terrigenous mudstone and sandstone accumulated along the eastern Eurasian continental margin from the Cretaceous to the Paleocene. In this study, we investigated planktic foraminifera as well as carbon- and osmium-isotope stratigraphy in the Aptian–Albian interval of the Yezo Group in the Oyubari area. By combining previously studied macrofossil stratigraphy with our high-resolution microfossil and isotope stratigraphies and the radio-isotopic ages of silicic tuffs, we established a robust stratigraphic correlation between the Yezo Group and the well-documented Tethyan standard framework. Our multi-proxy approach enabled the precise identification of stratigraphic horizons in the Yezo Group equivalent to the OAE 1a, Fallot, OAE 1b, OAE 1c, and OAE 1d levels. Since abundant silicic tuffs occur within these event horizons in the studied successions, their radio-isotopic ages will greatly improve the numerical age model for the Early Cretaceous in the future. In addition, the study of environmental proxies for these event horizons is expected to provide substantial insights into how OAEs impacted the Pacific Ocean.