Conference Agenda

Past seawater osmium isotopic record (¹⁸⁷Os/¹⁸⁸Os) is a useful chemostratigraphic tool. The Os isotopic composition of seawater reflects the balance between radiogenic Os from continental sources and less radiogenic Os from the mantle and extraterrestrial materials. Because the residence time of Os in seawater (~10⁴ years) is longer than the typical ocean circulation timescale (~10³ years), its isotopic composition is relatively homogeneous throughout the ocean. Osmium isotope ratios in seawater have varied over geological time, and these signatures are preserved in sediments. Therefore, the past-seawater Os isotopic variations recorded in the sedimentary rocks are useful for the stratigraphic correlations.

Although Cenozoic seawater Os isotopic records have been compiled over the past 30 years, Cretaceous Os isotopic data have been limited to the major oceanic anoxic events (e.g., OAE1a and OAE2) and the Cretaceous–Paleogene boundary, which is insufficient to establish a complete Os isotope stratigraphy during the Cretaceous. However, in recent years, an increasing number of Os isotopic datasets have been assembled, enabling the development of reference curves for the entire Cretaceous. In this presentation, I integrate previously published Os isotopic data with newly obtained data to provide an updated Os isotopic stratigraphy for the Cretaceous. Furthermore, I explain key Os isotopic variations and their possible causes.

The Valanginian is characterized by a period during the Cretaceous when the earliest carbon cycle perturbation event (the Weissert Event) as well as the earliest on-land LIP emplacement took place. In recent years, the GSSPs for the Valanginian and Hauterivian stages have been ratified, and stratigraphic and palaeoenvironmental studies of the Valanginian are becoming more active. On the other hand, there is little paleoceanographic information on the Valanginian Panthalassa, the largest ocean at the time. In Northeast Japan, Berriasian-Hauterivian marine sequences, accumulated in the northwest of the Panthalassa on the northeastern margin of Eurasia, are exposed sporadically. These sequences have not been chronologically well constrained in detail due to a scarcity of age-diagnostic fossils. We have attempted detailed international stratigraphic correlation based on zircon U-Pb ages of tuff as well as carbon isotopic stratigraphy for the Berriasian-Hauterivian sequences (Karakuwa, Oshima and Somanakamura groups) exposed in Northeast Japan. As a result, the Berriasian/Valanginian boundary and the Weissert Event were successfully identified. In addition, an approximate stratigraphic level for the Valanginian/Hauterivian boundary was recognized, although it was not conclusive. On the basis of TOC and DOP analyses for the studied sequences, the Weissert Event was dominated by an oxic environment in NW Panthalassa, while transient dysoxia occurred sporadically.

The Upper Cretaceous Kuji Group (300-400 m thick) distributed along the Pacific coast of north Tohoku region, Northeast Japan are examined through sedimentary facies and sequence stratigraphic analyses, and U-Pb age dating of detrital zircon from intercalated tuff layers. Identified twenty-six sedimentary facies and seven sedimentary facies associations show that the group is composed of terrestrial to shallow-marine sandstone-dominated siliciclastic deposits. The Tamagawa (1), Kunitan (2), and Sawayama (3) formations in ascending order are represented by talus, flood plain, gravelly river, sandy meandering river, and inner bay-estuary (1), shoreface to inner shelf (2), and meandering river (3) depositional systems, respectively. Their stratigraphical and geographical distributions indicate five depositional sequences with ten forth-order sequences, reflecting five third-order relative sea-level changes with some local tectonic influence along the north-south trending western margin of the Yezo forearc basin. The U-Pb age data including those of a previous research range 92.6 to 79.1 Ma, showing the middle Turonian to middle Campanian age. Our results provide valuable information on paleo- and sedimentary environments for reconstructing Late Cretaceous biota including terrestrial and marine vertebrates such as dinosaur and shark, etc., benthic molluscs, and sporophyte, gymnosperm, and angiosperm fossils.

Understanding the climate variability and stability during past ‘hothouse’ periods is essential for predicting future climate under ongoing global warming. However, information on millennial-scale (and shorter) climate variability during such periods is extremely limited due to the lack of appropriate high-resolution, deep-time archives. We have been conducting research to elucidate decadal- to orbital-scale continental climate variability from a Lower Aptian lacustrine varve record located in southeastern Mongolia (Hasegawa et al., 2022). The material used in this study is the CSH02 research core, which we consider now to cover the OAE1a interval based on new analyses.

We performed ultra-high-resolution elemental composition analysis (6-yr resolution) using an XRF core scanner (Itrax), mineral composition analysis of 1282 samples, carbonate δ¹⁸O and δ¹³C analysis of 822 samples, and organic matter δ¹³C measurements combined with palynofacies analysis of ~200 samples. The results indicate a decrease in the K/Al ratio, an increase in kaolinite, a shift in carbonate δ¹⁸O and δ¹³C towards more negative values, and a gradual increase in pollen and plant fragments near the stratigraphic level presumed to mark the onset of OAE1a, suggesting increased humidity and enhanced chemical weathering.

Furthermore, the Ca/Ti ratio (evaporation/precipitation proxy) shows that, during a 20 kyr-lasting interval prior to the estimated onset of OAE1a, centennial-scale variations were predominant, whereas during the 20 kyr following the onset of OAE1a, pronounced millennial-scale (~1,000–2,000 years) variations are observed. Our findings suggest that the state dependence of the millennial-scale climate instability also existed at the transitional phase toward the “hothouse” climate state.