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Ordovician seawater composition : evidence from fluid inclusions in halite

Meng F., Zhang Y., Galamay A. R., Bukowski K., Ni P., Xing E., Ji L.

Geological Quarterly

2018

Vol. 62, No. 2

344--352

Fluid inclusions in halite can directly record the major composition of evaporated seawater; however, Ordovician halite is very rare. The Ordovician is a key time during the evolution history because profound changes occurred in the planet’s ecosystems. Marine life was characterized by a major diversification, the Great Ordovician Biodiversification Event and the Late Ordovician Mass Extinction, the first of the “big five” mass extinctions. However, so far there is no data on the Ordovician seawater. Data from the Ordovician-Silurian boundary were available only. In this study, we report the major compositions from Middle Ordovician halite in China to give the exact composition of Ordovician seawater. The basic ion composition (K+, Mg2+, Ca2+, and SO42-) of inclusion brines was established with the use of ultramicrochemical analysis. The data on the chemical composition of the brines in the primary inclusions indicated that the brines were of Na-K-Mg-Ca-Cl (Ca-rich) type, and cover a huge gap in the evolution of seawater chemistry. The chemical composition of the primary inclusion brine in halite confirmed the earlier results for the Cambrian and Silurian halite originating from other salt basins and the previous speculation of “calcite sea” during the Ordovician, indicating a higher potassium content in the Lower Paleozoic seawater than in the seawater of other periods of the Phanerozoic.

The sulphur and oxygen isotopic composition of anhydrite from the Upper Pechora Basin (Russia) : new data in the context of the evolution of the sulphur isotopic record of Permian evaporites

Galamay A. R., Meng F., Bukowski K., Ni P., Shanina S. N., Ignatovich O. O.

Geological Quarterly

2016

Vol. 60, No. 4

990--999

This study describes a new determination of the S and O isotope composition of Lower Permian (Kungurian) anhydrites from the Upper Pechora Basin, Cis-Ural region, Russia. δ34S values in sulphate facies vary from +13.7 to +15.1‰; and δ18O values range from +9.3 to +10.4‰. The values of d34S and d18O of anhydrite from halite facies varies from +12.6 to +15.0‰ and +7.5 to +10.9‰ respectively. The quantitative ratio of pyrite content from the water-insoluble residue (silty-sand fraction) is characterized by extremely low (<<1%) to high (4–5%) steep gradation values. The increased presence of pyrite indicates the influence of bacterial sulphate reduction. The sulphate reduction process was more intense, especially when evaporites were formed in mud. The narrow fluctuation range of sulphur and oxygen isotopes values of the measured anhydrite indicates low levels of fractionation. It was established that during the Permian, evolutionary changes in the content of sulphate ions in sea water correlate with the sulphur isotopic composition of marine evaporites