4:15 PM - 4:30 PM
[BCG06-22] Paleoenvironmental variations recorded by molecular fossils in sedimentary rocks deposited across the Cretaceous/Paleogene boundary in the Nemuro Group, Hokkaido, Japan
Keywords:K-Pg boundary, Nemuro Group, Hokkaido, biomarker, northwestern Pacific, Kawaruppu
The Cretaceous-Paleogene boundary (K-Pg boundary) is thought to be the mass extinction event occurred by asteroid impact and large igneous activity. In this event, the scale of environmental disturbance and the duration for environmental recovery might be variable by the distance from the impact site (the Chicxulub crater, Yucatan Peninsula) and individual biota and ecosystem in each region. Thus, the paleoenvironmental investigation during the K-Pg boundary in distal regions from the impact site is significant for understanding the behavior of ecosystems against environmental disturbance such as the asteroid impact. In the present study, we analyzed molecular fossils (biomarkers) in the sediments of the Nemuro Group in Hokkaido to reconstruct paleoenvironmental and paleoecological variations in the northwestern Pacific region across the K-Pg boundary.
The samples used were mudstones of the Kawaruppu Formation distributed in the Kawaruppu Rivers and its tributary in the Shiranuka Hills, Hokkaido, Japan. In the tributary of the Kawaruppu Rivers, the K-Pg boundary was suggested to be identified based on magnetostratigraphic and Os isotope stratigraphic investigation (Ota et al., 2024). Powdered samples were extracted by solvents and then separated into fractions by silica-gel column chromatography. The aliphatic hydrocarbon fraction was separated into straight-chain and branched/cyclic fractions by urea adduction. These fractions were analyzed by GC-MS.
In the section of the tributary of the Kawaruppu Rivers, pristane/phytane (Pr/Ph) ratios, which is redox proxy, was 2.1-3.7, showing that the depositional environments were oxic. The values of higher plant parameter (HPP’’; sum of retene, cadalene, and isocadalene), which is coniferous vegetation indicator, were nearly constant across the K-Pg boundary. On the other hand, ar-AGI, which is angiosperm/gymnosperm ratio, considerably decreased in the long-term variations. These results suggested that the coniferous vegetation was constantly distributed but the angiospermous vegetation declined in hinterland across the K-Pg boundary. The ratios of algal steroids to bacterial hopanoids showed that the relative abundances of marine algae remarkably decreased. On the other hand, dinoflagellate-derived triaromatic dinosteroids relatively increased but C25 higher branched isoprenoid (HBI) alkane, which is a biomarker of pelagic diatom species, clearly decreased below the K-Pg boundary. Thus, dinoflagellate flourished but pelagic diatom declined across the K-Pg boundary, although the decline of diatom production might not be related to the asteroid impact.
In the section of ‘main river’ of the Kawaruppu River, the Pr/Ph ratios and C27/C29 sterane ratios indicate that the depositional setting was coastal (neritic) under oxic condition. The triaromatic dinosteroids and cyanobacterial 2-methyl hopanes were nearly constant, but the C25 HBI alkanes considerably decreased. These results implied that dinoflagellate and cyanobacteria were hardly affected but the pelagic diatom production was damaged by the asteroid impact. The HPP’’ values tended to increase but the ar-AGI values decreased during the Paleocene. Thus, it was presumed that the angiosperm vegitation declined, and the coniferous vegetation was relatively early recovered during the early Paleocene.
The samples used were mudstones of the Kawaruppu Formation distributed in the Kawaruppu Rivers and its tributary in the Shiranuka Hills, Hokkaido, Japan. In the tributary of the Kawaruppu Rivers, the K-Pg boundary was suggested to be identified based on magnetostratigraphic and Os isotope stratigraphic investigation (Ota et al., 2024). Powdered samples were extracted by solvents and then separated into fractions by silica-gel column chromatography. The aliphatic hydrocarbon fraction was separated into straight-chain and branched/cyclic fractions by urea adduction. These fractions were analyzed by GC-MS.
In the section of the tributary of the Kawaruppu Rivers, pristane/phytane (Pr/Ph) ratios, which is redox proxy, was 2.1-3.7, showing that the depositional environments were oxic. The values of higher plant parameter (HPP’’; sum of retene, cadalene, and isocadalene), which is coniferous vegetation indicator, were nearly constant across the K-Pg boundary. On the other hand, ar-AGI, which is angiosperm/gymnosperm ratio, considerably decreased in the long-term variations. These results suggested that the coniferous vegetation was constantly distributed but the angiospermous vegetation declined in hinterland across the K-Pg boundary. The ratios of algal steroids to bacterial hopanoids showed that the relative abundances of marine algae remarkably decreased. On the other hand, dinoflagellate-derived triaromatic dinosteroids relatively increased but C25 higher branched isoprenoid (HBI) alkane, which is a biomarker of pelagic diatom species, clearly decreased below the K-Pg boundary. Thus, dinoflagellate flourished but pelagic diatom declined across the K-Pg boundary, although the decline of diatom production might not be related to the asteroid impact.
In the section of ‘main river’ of the Kawaruppu River, the Pr/Ph ratios and C27/C29 sterane ratios indicate that the depositional setting was coastal (neritic) under oxic condition. The triaromatic dinosteroids and cyanobacterial 2-methyl hopanes were nearly constant, but the C25 HBI alkanes considerably decreased. These results implied that dinoflagellate and cyanobacteria were hardly affected but the pelagic diatom production was damaged by the asteroid impact. The HPP’’ values tended to increase but the ar-AGI values decreased during the Paleocene. Thus, it was presumed that the angiosperm vegitation declined, and the coniferous vegetation was relatively early recovered during the early Paleocene.