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CARBON ISOTOPE RECORDS OF TERRESTRIAL ORGANIC MATTER AND OCCURRENCE OF PLANKTONIC FORAMINIFERA FROM THE ALBIAN STAGE OF HOKKAIDO, JAPAN: OCEAN-ATMOSPHERE ¹³C TRENDS AND CHRONOSTRATIGRAPHIC IMPLICATIONS

¹ Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
² Department of Earth Sciences, Faculty of Science, Chiba University, Chiba 263-8522, Japan
³ Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan ando{at}chikyu.ac.jp

Carbon isotope compositions of sedimentary organic matter (average –24.1) from an Albian marine siliciclastic succession in Hokkaido, Japan, exhibit a distinct anomaly by +1.2 with a trifurcate shape across the Albian–Cenomanian boundary and two relatively small shifts (<+1) in the middle and upper Albian, respectively. The organic matter consist predominantly of woody materials with an insignificant degree of thermal alteration, judged from the visual and elemental characteristics of kerogen; the stratigraphic ¹³C_org fluctuations are independent of lithological or total organic carbon variations. Thus, the Hokkaido ¹³C_org profile is interpreted as representing the temporal ¹³C changes in whole C3 plant vegetation in the provenance of East Asia during Albian time. The patterns and amplitudes in ¹³C_wood values and their relationship with planktonic foraminiferal zones are conformable with coeval Tethyan ¹³C records of pelagic carbonates. This observation reinforces the view that ¹³C compositions of marine and terrestrial carbon reservoirs fluctuated simultaneously by the same amplitude within the ocean-atmosphere-biosphere system regardless of changes in such paleoenvironmental parameters as pCO₂. From a chemostratigraphic viewpoint, time-equivalent levels of Oceanic Anoxic Events and stage boundaries are constrained for the Hokkaido sections, allowing for the proposal of a detailed chronostratigraphic framework for future advanced paleoceanographic research in the mid-Cretaceous northwestern Pacific region.