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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+, Mg2+, SO4 2− and Ca2+ 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 Mg2+ and SO4 2−, and higher Ca2+ 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 Ca2+-rich and SO4 2−-poor. The change of composition of seawater–from SO4 2−-rich, Ca2+-poor in the latest Neoproterozoic to Ca2+-rich, SO4 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 Ca2+ concentrations in the shelf seas leading to the onset of biocalcification and then the Cambrian explosion. [Copyright &y& Elsevier]

Abstract: Data on chemical composition of brines in primary inclusions of marine halites and on mineralogy of marine evaporites and carbonates lead to the conclusion that during the Phanerozoic two long-term cycles of chemical composition of seawater existed. During each of those cycles, seawater dominantly a Na-K-Mg-Ca-Cl (Ca-rich) type changed to a Na-K-Mg-Cl-SO4 (SO4-rich) type. Recrystallised halite from the uppermost Neoproterozoic Salt Range Formation (ca. 545Ma) in Pakistan, contains solitary inclusions indicating SO4-rich brines. This supports the concept derived from the study on primary fluid inclusions from the Neoproterozoic Ara Formation of Oman; SO4-rich seawater existed during latest Neoproterozoic time (ca. 545Ma). In contrast, samples of recrystallised halite from the Bitter Springs Formation (840–830Ma) in Australia contain inclusion brines that are entirely Ca-rich, indicating that basin brines and seawater were Ca-rich during deposition of central Australian evaporites. These combined data supported by the timing of aragonite and calcite seas suggest that during the Proterozoic, significant oscillations of the chemical composition of marine brines, and seawater, occurred, which are similar to those known to exist during the Phanerozoic. It is suggested that Ca-rich seawater dominated for a substantial period of time (more than 200Ma), at 650Ma, this was replaced by SO4-rich seawater, finally returning to Ca-rich seawater at 530Ma. [Copyright &y& Elsevier]