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

The major-ion (Mg2+, Ca2+, Na+, K+, SO42-, and Cl-) chemistry of Cretaceous seawater was determined from analyses of seawaterderived 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 Ca2+, depleted in SO42-, Na+, and Mg2+, and had lower Na+/Cl-, Mg2+/Ca2+, and Mg2+/K+ ratios compared to modern seawater. Elevated Ca2+ concentrations, with Ca2+ > SO42- at (t)he point of gypsum saturation, allowed Cretaceous seawater to evolve into Mg2+-Ca2+-Na+-K+-Cl- brines lacking measurable SO42-.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 Ca2+ enrichment, more than three times higher than present day seawater, with a Mg2+/Ca2+ ratio of 1.1-1.3. Younger, Albian-Cenomanian seawater had lower Ca2+ concentrations, and a higher Mg2+/Ca2+ ratio of 1.2-1.7. Cretaceous (Aptian) seawater has the lowest Mg2+/Ca2+ 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 Mg2+/Ca2+ ratios measured from echinoderm and rudist calcite, all indicate that Early Cretaceous seawater (Hauterivian, Barremian, Aptian, and Albian) had lower Mg2+/Ca2+ ratios than Late Cretaceous seawater (Coniacian, Santonian, and Campanian). Low Aptian-Albian Mg2+/Ca2+ seawater ratios coincide with negative excursions of Sr-87/Sr-86 ratios and delta S-34(SO4), and peak Cretaceous ocean crust production rates, all of which suggests a link between seawater chemistry and midocean ridge hydrothermal brine flux. (c) 2006 Elsevier Inc. All rights reserved.