苏北浅滩钙结核的特征及其环境指示意义

Characteristics of calcareous concretions from the Northern Jiangsu Shoal and their significance on palaeoenvironment

对采自苏北浅滩的钙结核样品进行了岩石学、矿物学和碳氧同位素的分析。样品富含石英和长石砂屑,碳酸钙胶结,其碳和氧同位素组成分别为-8.38‰^-8.19‰V-PDB及-5.23‰^-5.03‰V-PDB。根据样品的氧同位素组成,利用碳酸盐-水体系氧同位素方程,结合现今底层水温度,并考虑可能存在的温度变化,计算得到古沉淀流体的δ18 O水范围为-4.72‰^-4.52‰VSMOW,较正常海水偏负,认为钙结核的形成可能受到了淡水影响。根据结核中碎屑矿物的成分及其成熟度,判断其包含的碎屑矿物源自古黄河,认为这些结核样品可能形成于海陆交互环境,形成时间约为7~6.5ka BP左右,当时古海平面高度比现今低10m左右,古黄河河道可能位于苏北浅滩附近。

This paper presented comprehensive petrological, mineralogical and C-and O-isotope analysis on calcareous concretions from the Northern Jiangsu Shoal. These samples were abundant with fragments of quartz and feldspar, and cemented by calcite. The C- and O-isotope ratios of the carbonates were as follows. δ13 C is --8. 38‰-8. 19‰ V--PDB andδ18 O is-5.23‰--- 5.03‰V-PDB. Based on their O-isotope compositions and the possible formation temperatures,the oxygen isotopic fractionation between calcite and water was used to estimate the equi- librium 8180 values of the precipitating fluids. The calculated equilibrium δ18O values of the precipitating fluids ranged from -- 4. 72‰ to -- 4. 52‰ V-SMOW, which were lighter than present seawater. It is considered that freshwater involved in the formation of calcareous concretions, and they formed in terrestrial environmentAccording to the composition of detrital minerals contained in the concretions and also the Quartz/feldspar ratios （index of maturity） ,it is suggested that the concretions were originated from Ancient Yellow River. About 7--6.5 ka BP, the Ancient Yellow River flew by the Northern Jiangsu Shoal.