Stratigraphic carbon isotope fluctuations of detrital woody materials during the Aptian Stage in Hokkaido, Japan: Comprehensive δ ¹³C data from four sections of the Ashibetsu area

Geological and organic geochemical studies were performed on an Aptian (mid-Cretaceous) sedimentary succession of the Sorachi and Yezo groups in the Ashibetsu area, central Hokkaido, northern Japan. Microscopic observations of kerogen samples indicate that the constituents of sedimentary organic matter (SOM) are predominantly detrital woody materials (opaque and translucent phytoclasts). Hydrogen/carbon atomic ratios of the kerogen samples indicate that the SOM have not been affected by thermal alteration significantly and thus preserved the original δ¹³C compositions. It is found that δ¹³C_wood profiles of four sections from the Ashibetsu area fluctuate systematically with stratigraphy. The δ¹³C_wood profiles show a remarkable negative shift followed by a positive excursion of ∼3.6‰ (from -25.4 to -21.8‰) exhibiting a bifurcate shape in the early Aptian. δ¹³C _wood values become nearly constant between -23.0 and -23.5‰ and then approximate to ∼ -24.0‰ in the late Aptian. A relatively small positive anomaly is also identified in the latest Aptian. The similarity in δ¹³C_wood profiles among different sections suggests that SOM with homogenized δ¹³C values of C3 plants were deposited in sediments. Transportation for a long distance from the sufficiently large provenance is postulated to mix C3 plant materials, resulting in the homogenization of δ¹³C_wood values. The δ¹³C _wood profile is essentially conformable with that of Pacific marine carbonates, with their isotopic difference almost constant at all times. Such conformable marine and terrestrial δ¹³C variations support the previous hypothesis that the temporal δ ¹³C change of the Aptian ocean-atmosphere-biosphere system is precisely traced by bulk isotopic analyses of detrital woody materials, and the supplemental paleoenvironmental effects onto δ¹³ C_wood composition, such as pCO₂ changes, are probably negligible.