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 δ¹³C_org fluctuations are independent of lithological or total organic carbon variations. Thus, the Hokkaido δ¹³C_org profile is interpreted as representing the temporal δ¹³C changes in whole C3 plant vegetation in the provenance of East Asia during Albian time. The patterns and amplitudes in δ¹³C_wood values and their relationship with planktonic foraminiferal zones are conformable with coeval Tethyan δ¹³C records of pelagic carbonates. This observation reinforces the view that δ¹³C 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.