Secular variation in the major-ion chemistry of seawater: Evidence from fluid inclusions in Cretaceous halites

Abstract

The major-ion (Mg 2+, Ca 2+, Na +, K +, SO 4 2 - , and Cl −) chemistry of Cretaceous seawater was determined from analyses of seawater-derived brines preserved as fluid inclusions in marine halites. Fluid inclusions in primary halite from three evaporite deposits were analyzed by the environmental scanning electron microscopy (ESEM) X-ray energy dispersive spectrometry (EDS) technique: the Early Cretaceous (Aptian, 121.0–112.2 Ma) of the Sergipe basin, Brazil and the Congo basin, Republic of the Congo, and the Early to Late Cretaceous (Albian to Cenomanian, 112.2–93.5 Ma) of the Khorat Plateau, Laos, and Thailand. The fluid inclusions in halite indicate that Cretaceous seawater was enriched several fold in Ca 2+, depleted in SO 4 2 - , Na +, and Mg 2+, and had lower Na +/Cl −, Mg 2+/Ca 2+, and Mg 2+/K + ratios compared to modern seawater. Elevated Ca 2+ concentrations, with Ca 2+ > SO 4 2 - at the point of gypsum saturation, allowed Cretaceous seawater to evolve into Mg 2+–Ca 2+–Na +–K +–Cl − brines lacking measurable SO 4 2 - .The major-ion composition of Cretaceous seawater was modeled from fluid inclusion chemistries for the Aptian and the Albian-Cenomanian. Aptian seawater was extreme in its Ca 2+ enrichment, more than three times higher than present day seawater, with a Mg 2+/Ca 2+ ratio of 1.1–1.3. Younger, Albian-Cenomanian seawater had lower Ca 2+ concentrations, and a higher Mg 2+/Ca 2+ ratio of 1.2–1.7. Cretaceous (Aptian) seawater has the lowest Mg 2+/Ca 2+ ratios so far documented in Phanerozoic seawater from fluid inclusions in halite, and within the range chemically favorable for precipitation of low-Mg calcite ooids and cements. Results from halite fluid inclusions, together with Mg 2+/Ca 2+ ratios measured from echinoderm and rudist calcite, all indicate that Early Cretaceous seawater (Hauterivian, Barremian, Aptian, and Albian) had lower Mg 2+/Ca 2+ ratios than Late Cretaceous seawater (Coniacian, Santonian, and Campanian). Low Aptian-Albian Mg 2+/Ca 2+ seawater ratios coincide with negative excursions of 87Sr/ 86Sr ratios and δ 34S SO 4 , and peak Cretaceous ocean crust production rates, all of which suggests a link between seawater chemistry and midocean ridge hydrothermal brine flux.