Cretaceous terrestrial paleoenvironments of northeastern Asia suggested from carbon isotope stratigraphy: Increased atmospheric pCO 2-induced climate

Abstract

Terrestrial organic carbon from Cenomanian–Turonian sequences in Hokkaido Island, Japan has retained an isotopic record of the response of terrestrial higher plants to the paleoenvironment, superimposed on a carbon isotope ratio fluctuation of atmospheric CO 2. The stratigraphic record of organic carbon isotopic compositions from the Hokkaido sections has been correlated to the carbonate carbon isotope record from Europe and North America; however, considerable discrepancy in the long-term, time-stratigraphic pattern of these isotope ratio curves exists through the Middle–Upper Cenomanian and Lower Turonian. Contrary to the marine carbonate profiles from Europe and North America which show consistent, positive migration through the Upper Cenomanian, terrestrial organic carbon isotope values display a negative shift through the Upper Cenomanian followed by a positive recovery in the Lower Turonian. Environmental pressures that only affected carbon isotope values of terrestrial organic carbon but not those of carbonate carbon are causal factors for this discrepancy. Variations in atmospheric temperature and humidity, and related conditions of carbon cycling in forests, may have acted as factors to shift isotope values of terrestrial organic carbon negatively. This causal relationship between climate and carbon isotope ratio fractionation is best explained when an increasing atmospheric pCO 2, as suggested from a decoupling between isotope ratio curves of marine organic carbon and carbonate, is understood as a driving force for a climatic optimum during the Middle Cenomanian through Early Turonian.