[M-IS34] 古気候・古海洋変動

[MIS34-P01] 北西部パンサラッサ海におけるトアルシアン海洋無酸素事変時の古環境変動：豊浦地域の層序と地球化学分析

ポスター講演3分口頭発表枠

*泉 賢太郎¹、ケンプ デイビッド² (1.国立環境研究所　生物・生態系環境研究センター、2.アバディーン大学　地質学・石油地質学専攻)

The early Toarcian (Early Jurassic) oceanic anoxic event (T-OAE) was a significant palaeoenvironmental perturbation that led to marked changes in ocean chemistry and climate, and which also had a severe impact on marine ecosystems. In addition, this event is characterized by the widespread occurrence of a ~3?7‰ negative excursion in the carbon-isotope (δ¹³C) composition of marine organic and inorganic matter and terrestrial plant material. This feature of the event indicates a pronounced perturbation to the global carbon cycle. Despite such global impacts of the event, the precise palaeoenvironmental changes during the event from sections outside of the Boreal and Tethys realms are uncertain. Thus, to resolve this issue and further expand our understanding of the nature of the event, here we investigated the Nishinakayama Formation of the Toyora area, southwest Japan, which represents shallow-marine strata deposited at the northwestern margin of the Panthalassa Ocean. First, we established high-resolution carbon-isotope chemostratigraphy. A characteristic δ¹³C negative excursion was recognized around the middle part of the Nishinakayama Formation, allowing accurate international correlation. Then, we carried out geochemical analyses to reconstruct palaeoenvironmental conditions at the northwestern Panthalassic margin. Our results indicate that in the studied succession, organic-matter enrichment persisted through the early Toarcian, but elemental redox proxies and ichnofabrics do not support persistent bottom-water anoxia through the δ¹³C excursion. Analysis of terrigenously derived major and trace element abundances and palynology, coupled with sedimentological observations, revealed an increase in coarse-grained sediment, phytoclast size, and terrestrial organic-matter close to the onset of the δ¹³C negative excursion. These lines of evidence potentially suggest a marked strengthening of detrital sediment flux and hence hydrological cycling and continental weathering. This is consistent with previously published evidence from Boreal and Tethys realms.