摘要
对宜昌地区寒武系和下奥陶统的碳酸盐碳、氧同位素进行测定 ,所得δ18O大于 - 10‰且与δ13C不相关 ,表明研究区碳酸盐样品基本上保存了碳、氧同位素的初始比值。δ18O在白云岩样品中呈现正漂移而在非白云岩样品中呈现负漂移 ,认为δ18O反映海水古盐度变化 ,即δ18O随着古盐度升高而增大 ;δ13C在缺氧沉积物中呈现负漂移 ,但负漂移的δ13C却不限于缺氧沉积物 ,推测δ13C可能反映古海洋的初级生产力 ,即δ13C随着初级生产力的提高而降低。研究还指出 ,由于Z值与δ13C密切正相关 (相关系数为 0 99) ,综合运用δ18O和δ13C的经典盐度公式不适用于研究区古盐度环境的重建。
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.[KH2D][WT5HZ]
出处
《高校地质学报》
CAS
CSCD
2000年第1期106-115,共10页
Geological Journal of China Universities
基金
国家自然科学基金!资助项目 (496 73182 )
关键词
氧同位素
寒武系
下奥陶统
湖北
碳同位素
carbon and oxygen isotopes
Cambrian and Lower Ordovician
Yichang area
