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摘要
Nitrogen isotopic compositions of upper Permian to lowermost Triassic rocks were analyzed at Chaotian in northern Sichuan, South China, in order to clarify changes in the oceanic nitrogen cycle around the Permian-Triassic boundary (P-TB) including the entire Changhsingian (Late Late Permian) prior to the extinction. The analyzed ca. 40 m thick interval across the P-TB at Chaotian consists of three stratigraphic units: the upper Wujiaping Formation, the Dalong Formation, and the lowermost Feixianguan Formation, in ascending order. The upper Wujiaping Formation, ca. 10 m thick, is mainly composed of dark gray limestone with diverse shallow-marine fossils such as calcareous algae and brachiopods, deposited on the shallow shelf. In contrast, the overlying Dalong Formation, ca. 25 m thick, is mainly composed of thinly bedded black mudstone and siliceous mudstone containing abundant radiolarians, deposited on the relatively deep slope/basin. Absence of bioturbation, substantially high total organic carbon contents (up to 15%), and abundant occurrence of pyrite framboids in the main part of the Dalong Formation indicate deposition under anoxic condition. The lowermost Feixianguan Formation, ca. 5 m thick, is composed of thinly bedded gray marl and micritic limestone with minor fossils such as ammonoids and conodonts, deposited on the relatively shallow slope. delta N-15(TN) values are in positive values around +1 to +2 parts per thousand in the upper Wujiaping Formation implying denitrification and/or anammox in the ocean. delta N-15(TN) values gradually decrease to -1 parts per thousand in the lower Dalong Formation and are consistently low (around 0 parts per thousand) in the middle Dalong to lowermost Feixianguan Formation. No clear delta N-15(TN) shift is recognized across the extinction horizon. The consistently low delta N-15(TN) values suggest the enhanced nitrogen fixation in the ocean during the Changhsingian at Chaotian. Composite profiles based on previous and the present studies demonstrate the substantial delta N-15 variation on a global scale in the late Permian to earliest Triassic; a systematic delta N-15 difference by low and high latitudes is particularly clarified. Although the enhanced nitrogen fixation throughout the Changhsingian at Chaotian was likely a regional event in northwestern South China, the composite delta N-15 profiles imply that the sea area in which fixed nitrogen is depleted has gradually developed worldwide in the Changhsingian, possibly acting as a prolonged stress to shallow-marine biota.
- 出版日期2014-10-15
- 单位中国地质科学院
