11:00 〜 13:00
[BCG05-P13] 大分県東部秩父帯の層状チャートを用いた 前期～中期三畳紀の海洋酸化還元状態の復元
★招待講演
キーワード：秩父帯、三畳紀、海洋無酸素事変、放散虫
The globally recognized ocean anoxic event (OAE) at the Permian/Triassic boundary (PTB) resulted in the extinctions of 90% of marine invertebrate fossil species. Previous studies of Upper Permian to Lower Triassic pelagic sedimentary rocks (i.e., bedded chert) showed high concentrations of redox-sensitive trace-elements (e.g., vanadium, uranium, and molybdenum), which indicate anoxic to euxinic condition in the deep-water environments. Although a long-lasted OAE may have delayed the recovery of marine ecosystems in the Triassic, the total duration of this oxygen-depleted condition remains unclear. To document the recovery from PTB anoxia, here we show the results of radiolarian biostratigraphy and chemostratigraphy of redox-sensitive trace elements from the upper Olenekian (Lower Triassic) to lower Ladinian (Middle Triassic) bedded chert succession from the AJ and AJR sections in Ajiro Island in the Chichibu Belt, eastern Kyushu, Japan. Previous paleomagnetic and biostratigraphic studies of the bedded cherts suggest that these deposits accumulated in a low- to middle-latitudinal zone of the pelagic Panthalassa Ocean. AJ section is approximately 20 m thick and mainly consists of alternation beds of white shale and black chert. Previous study has recognized four conodont biozones in this section: the upper Olenekian Triassospathodus brevissimus Zone and Triassospathodus homeri Zones, the lower Anisian (Aegean) Chiosella timorensis Zone, and the middle Anisian Paragondolella bulgarica Zone. AJR section is approximately 40 m thick and consists of greenish gray, red, and purple bedded chert with thin intercalations of siliceous shales. Previous biostratigraphic analyses recognized three conodont biozones in this section: the middle Anisian (Bithynian to Pelsonian) Paragondolella bulgarica Zone and the upper Anisian (Illyrian) Paragondolella excelsa Zone and the uppermost Anisian to lowermost Ladinian Paragondolella trammeri Zone. Our new biostratigraphic data collected from above the previously sampled horizons indicate that uppermost AJR sections are correlated to the Ladinian TR 3B to TR 4A radiolarian biozones, and this result is consistent with previous conodont biostratigraphic work.
We investigated the sedimentary redox changes through the study sections using redox sensitive elements such as manganese (Mn) and iron (Fe). These elements form insoluble hydroxides or oxides under oxygenated conditions, but under suboxic to anoxic conditions, they are reduced and forms soluble cations. Our data show low enrichment factors (EF: normalized by the composition of upper continental crust) of Fe and Mn in the upper Olenekian to middle Anisian AJ section. Compared with the general trend of FeEF in the AJ section, AJR section (middle Anisian-lower Ladinian) show higher FeEF (FeEF>1) thorough the section. MnEF values are generally less than 1 in the lower part of the AJR section. However, MnEF rapidly increase across the P. bulgarica and P. excelsa zones, suggesting the deep-water environments changed from suboxic to oxic conditions in the middle (Pelsonian) to late (Illyrian) Anisian. PTB anoxia recorded in the Upper Permian bedded chert successions in Japan is thought to have started at ca. 252 Ma. Since the deep-ocean environments changed from suboxic to oxic conditions near the middle/late Anisian boundary (ca. 243 Ma) in the study sections, the total duration of this episode of oxygen depletion was likely up to 9 million years until its recovery in the Anisian.
We investigated the sedimentary redox changes through the study sections using redox sensitive elements such as manganese (Mn) and iron (Fe). These elements form insoluble hydroxides or oxides under oxygenated conditions, but under suboxic to anoxic conditions, they are reduced and forms soluble cations. Our data show low enrichment factors (EF: normalized by the composition of upper continental crust) of Fe and Mn in the upper Olenekian to middle Anisian AJ section. Compared with the general trend of FeEF in the AJ section, AJR section (middle Anisian-lower Ladinian) show higher FeEF (FeEF>1) thorough the section. MnEF values are generally less than 1 in the lower part of the AJR section. However, MnEF rapidly increase across the P. bulgarica and P. excelsa zones, suggesting the deep-water environments changed from suboxic to oxic conditions in the middle (Pelsonian) to late (Illyrian) Anisian. PTB anoxia recorded in the Upper Permian bedded chert successions in Japan is thought to have started at ca. 252 Ma. Since the deep-ocean environments changed from suboxic to oxic conditions near the middle/late Anisian boundary (ca. 243 Ma) in the study sections, the total duration of this episode of oxygen depletion was likely up to 9 million years until its recovery in the Anisian.