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Records of Carbon and Oxygen Isotopes from[KH5D]the Cambrian and Lower Ordovician Carbonates in Yichang Area, Hubei Province

WT5HZ]FENG Hong zhen, LIU Jia run, SHI Gui jun[KH3D][WT5BX](Department of Earth Sciences, Nanjing University, Nanjing, 210093,China)[KH2D][WT5HZ]  
The carbonate samples studied in this paper were collected from the classic sections of Cambrian and Lower Ordovician in the Yichang area, Hubei province. The results of C and O isotope analysis demonstrate that δ 18 O is smaller than or close to -10‰ and does not correlate with δ 13 C, suggesting a basically complete preservation of primary ratios of C and O isotopes in these carbonate samples. The δ 18 O profile from the Cambrian to the Lower Ordovician in the Yichang area drifts positively in dolomite horizons and negatively in non dolomite horizons. Such a fluctuation in O isotopic compositions probably reflects paleoceanic variations in salinity, i.e. large δ 18 O indicates increase in paleosalinity. The Z value is calculated in this study to trace quantitatively the changes of paleosalinity. However, the correlation coefficient is only 0.43 between the Z value and δ 18 O but up to 0.99 between the Z value and δ 13 C.This means that the Z value mainly reflects changes in C isotopic compositions and is not suitable for the reconstruction of paleosalinity in the investigated area. The δ 13 C profile from the Cambrian to the Lower Ordovician in the Yichang area drifts negatively in anoxic sediments (e.g. in black shales of the lower Shujingtuo Formation), but negative fluctuations are not limited to anoxic sediments. They also occur in oxic sediments (e. g. in the Shipai, Xiping and Nanjinguan Formation). This suggests that δ 13 C, whether in oxic or anoxic sediments, is possibly related to the primary productivity of the ancient marine environments. When the black shales of the Shuijingtuo Formation were deposited, abundant planktonic organisms were accumulated to the seafloor. The fresh organic materials from these planktons were then consumed by sulfate reduction bacterial, leading to the release of CO 2 with lower δ 13 C and the formation of carbonates by the combination of such CO 2 with some metal ions. We believe that the intensity of sulfate reduction largely depended on the supply of organic materials, i.e. the more sufficient the organic materials, the more intensive the sulfate reduction, and therefore the lower the value of δ 13 C in carbonates. When the oxic sediments were deposited on the sea floor, the remains of planktonic organisms and benthic algae were oxidized causing the release of light C isotope from these organic materials into the bottom seawater. Presumbly, the values of δ 13 C was very low in the carbonate system of the bottom seawater as long as the supply of dissolved oxygen and organic materials were sufficient.[KH2D][WT5HZ]
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