High-resolution upper Maastrichtian carbon isotope stratigraphy of terrestrial organic matter from northern Japan

摘要

High-resolution stable carbon isotope stratigraphy of terrestrial organic matter was established for the upper Maastrichtian Senpohshi Formation of the Nemuro Group in eastern Hokkaido, northern Japan. The Senpohshi Formation, approximately 1,300 m thick, is dominated by hemipelagic mudstone deposited along an active margin in the North Pacific region. Microscopic observations of extracted kerogen samples from the formation revealed the presence of sedimentary organic matter (SOM), predominantly phytoclasts and a minor amount of non-fluorescent amorphous organic matter, indicating material of a terrestrial higher plant origin. The atomic hydrogen/carbon ratios of the kerogen samples indicated a coalification rank at the anthracite stage or below. Therefore, the stable carbon isotope values of the bulk SOM obtained for the Senpohshi Formation represent the unmodified, original values of terrestrial organic matter. The stable carbon isotope profile reconstructed for the formation provides the first high-resolution terrestrial record of the Mid-Maastrichtian Event (MME), which is comparable to high-resolution marine carbon isotope data from other sections. The carbon isotopic signatures defined by the marine records are recognized in the terrestrial data from the formation, especially in middle to upper part of the event. However, the terrestrial record showed a discrepancy from the marine data in the lower part of the MME, suggesting local variation of the hinterland environment in the North Pacific region. This study provides new insight into environmental changes during the late Maastrichtian by establishing a detailed carbon isotope record of terrestrial materials.