Nitrogen isotope chemostratigraphy across the Permian-Triassic boundary at Chaotian, Sichuan, South China
Friday, 19 December 2014
Nitrogen isotopic compositions of upper Permian to lowermost Triassic rocks were analyzed at Chaotian, Sichuan, China, to clarify changes in the oceanic N 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 is composed of dark gray limestone with diverse shallow-marine fossils, such as calcareous algae, 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 TOC contents, and abundant occurrence of pyrite framboids in the Dalong Formation indicate deposition under anoxic conditions. The lowermost Feixianguan Formation is composed of thinly bedded gray marl and micritic limestone with minor fossils, deposited on the relatively shallow slope. δ15N values are in positive values around +1 to +2‰ in the upper Wujiaping Formation implying denitrification and/or anammox in the ocean. δ15N values gradually decrease to -1‰ in the lower Dalong Formation and are consistently low (around 0‰) in the middle Dalong to lowermost Feixianguan Formation. No clear δ15N shift is recognized across the extinction horizon. The consistently low δ15N values suggest the enhanced N fixation in the ocean during the Changhsingian at Chaotian. Composite profiles based on previous and the present studies demonstrate the substantial δ15N variation on a global scale in the late Permian to earliest Triassic; a systematic δ15N difference by low and high latitudes is particularly clarified. Although the enhanced N fixation throughout the Changhsingian at Chaotian was likely a regional event in northwestern South China, the composite δ15N profiles imply that the sea area in which fixed N is depleted has gradually developed worldwide in the Changhsingian, possibly acting as a prolonged stress to the shallow-marine biota.