Carbonate concretions of variable sizes occur in the upper member of the Senonian (Upper Cretaceous) Mishash formation in Israel. Eight concretions and their surrounding country rocks were examined in the field, in thin sections and by X-rays. The isotopic composition of carbon and oxygen in the carbonates, the amount of acid insoluble residue, as well as the concentrations of P2O5, Ca, Mg, Sr and Fe were determined. Other concretions and country rocks were analyzed for oxygen and carbon isotopes only.
The concretions are composed of almost pure, microsparitic calcite, whereas the country rocks are porcellanitic-phosphoritic chalks. Compared with the surrounding rocks, the concretions are strongly enriched in 12C and are depeleted in insoluble residue, P2O5, SiO2, Fe, Sr and Mg.
It is postulated that the concretions were formed by addition of CaCO3 to sites of anaerobic decomposition of organic matter, while CaCO3 was mobilized in the surrounding sediments, in which aerobic decomposition of organic matter prevailed.
Several consequences of this model are considered, concerning the quantitative volume changes, as well as the distribution of SiO2 and Mg between concretions and country rocks. 相似文献
Various investigators reported a decrease in pH as seawater is concentrated. A similar phenomenon was reported for Dead Sea waters which are about ten times more saline than seawater. The reasons for the low pH values of Dead Sea waters (pH 5.9–6.5), which precipitate CaCO3, were investigated by determining the apparent dissociation constants of carbonic acid in these brines. A new method, based on alkalinity titration and least-squares fitting, was used to estimate the proton activity coefficient (γH+) and the first and second dissociation constants of carbonic acid (K1′, K2′) in natural and artificial Dead Sea waters. It was found that as the salt content increases, pK′1 and pK′2 values progressively decrease whereas γH+ sharply increase. At the highest salinity investigated (TDS = 330 gl?1) γH+ pK′1 and pK′2 values are 24.5, 5.09 and 6.23, respectively, as compared to about 0.8, 5.9, 9.1 respectively for normal seawater (19‰ chlorinity) at the same temperature (30°C).The implication of the results of this study regarding solubility of CaCO3 and the general behavior of the carbonate system in hypersaline solutions is discussed. 相似文献