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[BCG05-P01] A unique carbon isotope signature across the Mississippian-Pennsylvanian boundary in mid-Panthalassan paleo-atoll carbonates: A critical turning point in the Late Paleozoic Ice Age
キーワード：石炭紀-前期ペルム紀生物多様性イベント、ミシシッピ紀-ペンシルバニア紀境界、古礁性炭酸塩、炭素同位体
The Paleozoic Era, i.e., the first 300 Myr of the Phanerozoic, was a critical period in Earth history, characterized by significant changes in the biosphere. These changes include large and sustained diversifications of marine organism, such as Cambrian explosion, Great Ordovician Biodiversification Event, and Carboniferous-earliest Permian Biodiversification Event (CPBE). Within these events, the CPBE comprises two distinct pulses; especially, the first one occurred across the Mississippian-Pennsylvanian Boundary (MPB). In addition to the first pulse of the CPBE, several unique geological phenomena were recognized around the MPB; i.e., Gondwana glaciation (Late Paleozoic Ice Age), atmospheric pCO2decrease, and increase of primary production. Therefore, the establishment of carbon isotope profile around the MPB is desirable for tracking and interpreting important disruptions within the combined ocean-atmosphere-biosphere Earth system. However, previous δ13Ccarb data was mainly reported from shallow-marine shelf sequences around Pangea, which inherently reflect reginal event and does not provide a reliable picture of global environmental changes.Understanding global carbon cycle in the mid-Panthalassa across the MPB, we for the first time determined the δ13Ccarbvalues from Mississippian-Pennsylvanian paleo-atoll carbonate preserved in the on-land exposed Permian accretionary complex in SW Japan.
Our newly obtained δ13Ccarb values are almost stable, with a range from 4.23‰ to 5.09‰, around the MPB. On the other hand, positive δ13Ccarb shifts (1~3‰) around the MPB were recognized in shallow-marine Tethyan regions; i.e., Naqing (South China), Arrow Canyon Range (United States), Ural Mountains (Russia), and Moscow Basin (Russia). These positive shifts were interpreted as the consequences of the increase of primary production and subsequently accelerated burial of organic matter, in response to enhanced continental weathering. It indicates that the enhancement of continental weathering, probably triggered by increased Hercynian orogenic uplift during the formation of the supercontinent Pangaea by collision between Gondwana and Euramerica, was limited solely to shallow-marine settings. Additionally, these combinations of enhanced silicate weathering and increased organic carbon burial resulted in lower atmospheric pCO2 levels, triggering climate cooling that culminated in the first pulse of the Late Paleozoic Ice Age. From the above, we conclude that the unique geological events, such as enhanced continental weathering, increase of primary production, and biodiversification, around the MPB were restricted around the continental margins of Tethys, rather than global signature.
Our newly obtained δ13Ccarb values are almost stable, with a range from 4.23‰ to 5.09‰, around the MPB. On the other hand, positive δ13Ccarb shifts (1~3‰) around the MPB were recognized in shallow-marine Tethyan regions; i.e., Naqing (South China), Arrow Canyon Range (United States), Ural Mountains (Russia), and Moscow Basin (Russia). These positive shifts were interpreted as the consequences of the increase of primary production and subsequently accelerated burial of organic matter, in response to enhanced continental weathering. It indicates that the enhancement of continental weathering, probably triggered by increased Hercynian orogenic uplift during the formation of the supercontinent Pangaea by collision between Gondwana and Euramerica, was limited solely to shallow-marine settings. Additionally, these combinations of enhanced silicate weathering and increased organic carbon burial resulted in lower atmospheric pCO2 levels, triggering climate cooling that culminated in the first pulse of the Late Paleozoic Ice Age. From the above, we conclude that the unique geological events, such as enhanced continental weathering, increase of primary production, and biodiversification, around the MPB were restricted around the continental margins of Tethys, rather than global signature.