Spatial variation of carbon isotopic compositions of carbonate and organic matter from the Late Ordovician sedimentary succession in the Yangtze Platform, South China: Implications for sea - Level eustacy and shoaling of marine chemocline

摘要

Spatial correlation of isotopic chemostratigraphy across ancient epeiric basins is of particular interest in the field of geology. This study examines the carbon isotopic compositions of carbonate and organic matter from the Late Ordovician sedimentary succession in the Yangtze Platform, including the Linxiang (LX) Formation deposited in the bottom and the Tiezufeike (TZFK)/Daduhe (DDH)/Wufeng (WF) formations in the top. Paired delta C-13(carb) and delta C-13(org) analyses of the Upper Ordovician strata on the Yangtze Platform were recorded, and the results show that the rocks in the LX Formation have high delta C-13(carb) and delta C-13(org) values relative to those in the DDF/TZFK/WF formations. Additionally, a variation of similar to 29 parts per thousand was found between shallow- and deep- water settings during the Late Ordovician: heavier delta C-13(carb) and delta C-13(org), values are observed at the Wukemuchang (WKMC) and Wanhe (WH) sections, representing the shallow regions of the Yangtze Platform, relative to the Tianjiawan (TJW) and Tianba (TB) sections representing the deeper margin of the Yangtze Platform. We suggest that the variations of delta C-13 in carbonate and organic matter are related to the sea - level eustacy and shoaling of marine chemocline: (1) sea - level rise driven decrease in local scale carbonate weathering; (2) decreased decomposition of organic matter under anoxic conditions; and (3) incorporation of non - photosynthetic chemoautorophic bacteria living in deeper waters. The Delta C-13 (Delta C-1(3) = delta C-13(carb) - delta C-13(org)) values in both the WH and WKMC sections display an increasing trend upward from the LX Formation to the overlying strata, varying from 27.8 parts per thousand to 31.2 parts per thousand and from 28.1 parts per thousand to 31.1 parts per thousand, respectively. These increased Delta C-13 values are related to the enhanced nutrients and correspondingly high primary productivity, and thus that, the anoxic water conditions and high primary productivity may be the significant factors in the deposition of Late Ordovician black shales.