VARIATIONS IN MAJOR-ION CHEMISTRY OF CRETACEOUS SEAWATER FROM FLUID INCLUSIONS IN HALITE MATCH OCEAN CRUST PRODUCTION RATES

The major-ion chemistry of Cretaceous seawater was determined from analyses of brine inclusions in crystals of Cretaceous marine halite. Three halites studied were: the Aptian (112.2 to 124 Ma) Loeme Formation, Congo basin and Muribeca Formation, Sergipe basin, Brazil, and the Albian to Cenomanian (93.5 to 112.2 Ma) Maha Sarakham Formation, Khorat Plateau, Thailand and Laos. Fluid inclusions were analyzed using the ESEM X-ray EDS technique, which measures absolute ionic concentrations of major cations and anions in frozen inclusions (Na, Ca, Mg, K, S in sulfate, and Cl). The parent seawaters of the Cretaceous halites were enriched in Ca and depleted in Na, Mg, and SO4 compared to modern seawater. Aptian seawater was extreme in its Ca-enrichment and SO4-depletion, with an estimated Mg/Ca ratio of 1.1 to 1.3. Later Albian-Cenomanian seawater had lower Ca, and a higher Mg/Ca ratio of 1.3 to 1.9. These results are close to those reported from echinoderm calcite (Dickson 2002) and rudist calcite (Steuber and Rauch 2003) all of which suggests that the low Mg/Ca in Cretaceous seawater was responsible for calcite being the important nonskeletal marine carbonate of that time. The maximum Mg/Ca ratio of Cretaceous (Aptian-Cenomanian) seawater was less than 2, primarily because of elevated Ca. Apparently, primary calcite formed in the warm Cretaceous oceans because the Mg/Ca ratio of seawater was low enough to override the influence of temperature favoring the precipitation of aragonite. Trends in Cretaceous seawater chemistry compare closely with records of marine S isotopes (Paytan et al. 2004), Sr isotopes (Jones and Jenkyns 2001), as well as estimates of ocean crust production (Larson 1991). Aptian seawater, with low Mg/Ca ratios, coincides with the Aptian-Albian negative excursion of strontium and sulfur isotopes, and peak Cretaceous ocean crust production. These connections suggest secular variations in the major ion chemistry of seawater are related to variations in the flux of midocean ridge hydrothermal brine driven by changes in the rate of ocean crust production.