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 δ13Cwood profiles of four sections from the Ashibetsu area fluctuate systematically with stratigraphy. The δ13Cwood 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 δ13Cwood 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 δ13Cwood 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 Cwood composition, such as pCO2 changes, are probably negligible.
|Number of pages||13|
|Journal||Journal of Asian Earth Sciences|
|Publication status||Published - 2003 Jun|
- Organic carbon
- Stable isotopes
ASJC Scopus subject areas
- Earth-Surface Processes