CRETACEOUS PALEOENVIRONMENTS OF CONTINENTAL FAR EAST ASIA: A PERSPECTIVE FROM CARBON ISOTOPE RATIO OF TERRESTRIAL ORGANIC MATTER

Terrestrial organic matter from marine Cenomanian-Maastrichtian fore-arc basin successions in Japan and Russian Far East provides valuable information about paleoenvironments of continental Far East Asia. Carbon isotope ratios of the organic matter (exclusively higher plant tissues) generally show a stratigraphic profile concordant with that of Tethyan carbonate through time reflecting carbon-isotopic composition of CO2 in ocean-atmospheric reservoir. However, these stratigraphic profiles from different carbon species show remarkable episodes of "decoupling" non-parallel fluctuations of carbon isotope values during two periods. Physiological and ecological responses of terrestrial plants to humidity are the most plausible factors that explain such episodes. Gradual onset of the first "decoupling" is observed in the middle Cenomanian successions. Above the maximum decoupling (~1.5 permil) below the Cenomanian/Turonian boundary, the isotope values of those two carbon species regain their parallel fluctuations in the lower Turonian. Temporary onset of humid climate and associated turnover of plant community over the provenance of the organic matter during the global climatic optimum best explains this "decoupling" episode. The second episode in the lower Campanian is represented by an instantaneous ~1.5 permil negative shift observed only on profiles of terrestrial organic carbon, not on carbonate profiles. A remarkable lithological change showing region-wide shoaling of the Asian fore-arc basin associated with the second episode suggests that regional tectonic modification caused switching the provenance of the organic matter from wider continental inland to narrower local coastal area. The second carbon isotopic episode is also best explained by a different isotopic fractionation during carbon fixation under discrete humidity conditions.