16:45 〜 17:00
[BCG09-18] Global perturbations of carbon cycle in the mid-Panthalassa during the Triassic-Jurassic transition
キーワード：大量絶滅、中央大西洋火成岩岩石区、深海酸素イベント、有機炭素同位体比、深海チャート、日本
The biodiversity crisis across the Triassic-Jurassic boundary (T-JB; ca. 201 Ma) has been regarded as the one of the biggest mass extinctions in the Phanerozoic life history. Extinction-related environmental changes across the T-JB occurred concurrently with significant perturbations in carbon cycle, as mirrored by a complex pattern of the positive and negative carbon isotope excursions in the fossiliferous shallow-marine strata deposited along continental margins. These investigations of the carbon-cycle reconstruction across the T-JB emphasized the causal relationships between the Central Atlantic Magmatic Provinces (CAMP) volcanism and T-JB event. However, the deep mid-Panthalassa, which occupied major portion of the global ocean at the Triassic-Jurassic transition, has not been much focused in reconstructing carbon cycle. Understanding global carbon cycle associated with the CAMP volcanism in the mid-Panthalassa across the T-JB, we determined the δ13Corg values from Rhaetian (Late Triassic) to Hettangian (Early Jurassic) shales interbedded within deep-sea cherts at Katsuyama section in Inuyama area, Mino-Tanba belt, SW Japan.
A high-resolution Rhaetian to Hettangian δ13Corg values in the mid-Panthalassa contain three distinct negative carbon isotopic excursions (NCIEs) before and across the T-JB; the Rhaetian NCIE1 and NCIE2 occurred for 5‰ from ca. -24.0‰ to -29.0‰, whereas the NCIE3 across the T-JB occurred for 3.5‰ from ca. -23.5‰ to -27.0‰. The newly observed NCIEs in the deep mid-Panthalassa can be correlated with the δ13Corg records in the shallow-marine Tethyan regions; i.e., St Audrie’s (England), Tiefengraben (Austria) and Čanj (Montenegro). This suggests that three NCIEs in the mid-Panthalassa reflect the global perturbations rather than local phenomena. Especially, the NCIE2 and NCIE3 occurred under high atmospheric CO2 levels, whereas atmospheric CO2 concentrations during the NCIE1 was around background values. This pCO2 level during the NCIE1 might be attributed to consumption of atmospheric CO2 by intensified weathering of the CAMP basaltic rocks. We thus consider that the NCIEs in the mid-Panthalassa before and across the T-JB were attributed to the volcanically CO2 outgassing from multiple emplacements of the CAMP volcanism. In addition, the high atmospheric CO2 level associated with the CAMP volcanism across the T-JB could promote the continental weathering in the shallow-marine regions, which might have accelerated burial of organic matter. This process likely resulted in deep-sea oxygenation in the mid-Panthalassa across the T-JB. From the above, the multiple emplacements of the CAMP volcanism during the Triassic-Jurassic transition had played a significant role for perturbations in carbon cycle in addition to the redox condition in the mid-Panthalassa.
A high-resolution Rhaetian to Hettangian δ13Corg values in the mid-Panthalassa contain three distinct negative carbon isotopic excursions (NCIEs) before and across the T-JB; the Rhaetian NCIE1 and NCIE2 occurred for 5‰ from ca. -24.0‰ to -29.0‰, whereas the NCIE3 across the T-JB occurred for 3.5‰ from ca. -23.5‰ to -27.0‰. The newly observed NCIEs in the deep mid-Panthalassa can be correlated with the δ13Corg records in the shallow-marine Tethyan regions; i.e., St Audrie’s (England), Tiefengraben (Austria) and Čanj (Montenegro). This suggests that three NCIEs in the mid-Panthalassa reflect the global perturbations rather than local phenomena. Especially, the NCIE2 and NCIE3 occurred under high atmospheric CO2 levels, whereas atmospheric CO2 concentrations during the NCIE1 was around background values. This pCO2 level during the NCIE1 might be attributed to consumption of atmospheric CO2 by intensified weathering of the CAMP basaltic rocks. We thus consider that the NCIEs in the mid-Panthalassa before and across the T-JB were attributed to the volcanically CO2 outgassing from multiple emplacements of the CAMP volcanism. In addition, the high atmospheric CO2 level associated with the CAMP volcanism across the T-JB could promote the continental weathering in the shallow-marine regions, which might have accelerated burial of organic matter. This process likely resulted in deep-sea oxygenation in the mid-Panthalassa across the T-JB. From the above, the multiple emplacements of the CAMP volcanism during the Triassic-Jurassic transition had played a significant role for perturbations in carbon cycle in addition to the redox condition in the mid-Panthalassa.