Cenomanian (Upper Cretaceous) carbon isotope stratigraphy of terrestrial organic matter for the Yezo Group, Hokkaido, Japan

Abstract Carbon isotope fluctuations of sedimentary organic matter along the two geological traverses in the Yezo Group, Hokkaido, northern Japan, elucidate a detailed chemostratigraphy for the Cenomanian Stage on the northwestern Pacific margin. Visual characterization of the kerogen from mudstone samples shows that the major constituents of sedimentary organic matter originated as terrestrial higher plants. The atomic hydrogen/carbon ratios of the kerogen suggest that the original δ¹³C values of terrestrial organic matter (TOM) have not been affected significantly by thermal diagenesis. The patterns in two δ¹³C_TOM curves are similar and independent of changes in lithology and total organic carbon contents, which suggests that TOM was mixed sufficiently before the deposition in the Yezo forearc basin for the δ¹³C composition having been homogenized. In addition, this implies that the Hokkaido δ¹³C_TOM profiles represent the averaged temporal δ¹³C variations of terrestrial higher‐plant vegetation in the hinterlands of northeast Asia during Cenomanian time. Three shorter‐term (ca. 0.1 my duration) positive‐and‐negative δ¹³C_TOM fluctuations of ∼1‰ are present in the Lower to Middle Cenomanian interval in the Yezo Group. On the basis of the age‐diagnostic taxa (ammonoids, inoceramids and planktic foraminifers), these discrete δ¹³C_TOM events are interpreted to be correlated with those in the δ¹³C curves of pelagic carbonates from European basins. The correlation of δ¹³C events between the European and Yezo Group sections suggests that the shorter‐term δ¹³C fluctuations in Cenomanian ocean‐atmosphere carbon reservoirs are useful for global chemostratigraphic correlation of marine strata. In particular, the correlation of δ¹³C fluctuations of the so‐called ‘Mid‐Cenomanian event’ (MCE) implies: (i) the δ¹³C variations of global carbon reservoir during the MCE are precisely recorded in the δ¹³C_TOM records; and (ii) the MCE δ¹³C_TOM event is an efficient chronostratigraphic index for the Lower/Middle Cenomanian boundary of the Mid‐Cretaceous sequences.