Early Cambrian seawater chemistry from fluid inclusions in halite from Siberian evaporites

Abstract

Thirty nine samples of Lower Cambrian marine sedimentary halite with zoned primary fluid inclusions were analyzed using the UMCA method introduced by Petrichenko (1973) [Petrichenko, O.I., 1973. Metody doslidzhennya u mineralakh galogennykh porid. Naukova Dumka, Kyiv, 91 pp.] in order to determine the contents of K +, Mg 2+, SO 4 2− and Ca 2+ ions and to interpret the composition of parent seawater. The analyzed samples were from the giant salt basin of eastern Siberia where five major phases of salt deposition can be distinguished, in the Late Vendian (Danilovo) and Early Cambrian (Usolye, Belsk, Angara, and Litvintsevo) basins. Our samples are all from Early Cambrian basins although the largest data set comes from the Angara Basin. The results indicate that Early Cambrian parent seawater was similar in all studied basins and was characterized by lower Mg 2+ and SO 4 2−, and higher Ca 2+ concentrations relative to modern seawater; the concentration of K + in Early Cambrian seawater was similar to that of modern seawater. Accordingly, the seawater in the entire Early Cambrian was Ca 2+-rich and SO 4 2−-poor. The change of composition of seawater–from SO 4 2−-rich, Ca 2+-poor in the latest Neoproterozoic to Ca 2+-rich, SO 4 2−-poor recorded throughout the Early Cambrian–probably occurred within a relatively short time span during the earliest Cambrian (Nemakit-Daldynian) time and caused a rise in Ca 2+ concentrations in the shelf seas leading to the onset of biocalcification and then the Cambrian explosion.