The major-ion composition of silurian seawater

Abstract

One-hundred fluid inclusions in Silurian marine halite were analyzed in order to determine the major-ion composition of Silurian seawater. The samples analyzed were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and one formation in the Early Silurian Canning Basin (Australia), the Mallowa Salt of the Carribuddy Group. The results indicate that the major-ion composition of Silurian seawater was not the same as present-day seawater. The Silurian ocean had lower concentrations of Mg 2+, Na +, and SO 4 2−, and much higher concentrations of Ca 2+ relative to the ocean’s present-day composition. Furthermore, Silurian seawater had Ca 2+ in excess of SO 4 2−. Evaporation of Silurian seawater of the composition determined in this study produces KCl-type potash minerals that lack the MgSO 4-type late stage salts formed during the evaporation of present-day seawater. The relatively low Na + concentrations in Silurian seawater support the hypothesis that oscillations in the major-ion composition of the oceans are primarily controlled by changes in the flux of mid-ocean ridge brine and riverine inputs and not global or basin-scale, seawater-driven dolomitization. The Mg 2+/Ca 2+ ratio of Silurian seawater was ∼1.4, and the K +/Ca 2+ ratio was ∼0.3, both of which differ from the present-day counterparts of 5 and 1, respectively. Seawaters with Mg 2+/Ca 2+ <2 facilitate the precipitation of low-magnesian calcite (mol % Mg < 4) marine ooids and submarine carbonate cements whereas seawaters with Mg 2+/Ca 2+ >2 (e.g., modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg 2+/Ca 2+ ratio of seawater, not the pCO 2 of the Silurian atmosphere.