Fluid inclusions from ten Cenozoic (Eocene-Miocene) marine halites are used to quantify the major-ion composition (Mg2+, Ca2+, K+, Na+, SO42−, and Cl−) of seawater over the past 36 My. Criteria used to determine a seawater origin of the halites include: (1) stratigraphic, sedimentologic, and paleontologic observations; (2) Br− in halite; (3) δ34S of sulfate minerals; (4) 87Sr/86Sr of carbonates and sulfates; and (5) fluid inclusion brine compositions and evaporation paths, which must overlap from geographically separated basins of the same age to confirm a “global” seawater chemical signal.
Changes in the major-ion chemistry of Cenozoic seawater record the end of a systematic, long term (>150 My) shift from the Ca2+-rich, Mg2+- and SO42−-poor seawater of the Mesozoic (“CaCl2 seas”) to the “MgSO4 seas” (with higher Mg2+ and SO42−>Ca2+) of the Cenozoic. The major ion composition of Cenozoic seawater is calculated for the Eocene-Oligocene (36-34 Ma), Serravallian-Tortonian (13.5-11.8 Ma) and the Messinian (6-5 Ma), assuming chlorinity (565 mmolal), salinity, and the K+ concentration (11 mmolal) are constant and the same as in modern seawater. Fluid inclusions from Cenozoic marine halites show that the concentrations of Mg2+and SO42− have increased in seawater over the past 36 My and the concentration of Ca2+ has decreased. Mg2+ concentrations increased from 36 mmolal in Eocene-Oligocene seawater (36-34 Ma) to 55 mmolal in modern seawater. The Mg2+/Ca2+ ratio of seawater has risen from ∼2.3 at the end of the Eocene, to 3.4 and 4.0, respectively, at 13.5 to 11.8 Ma and 6 to 5 Ma, and to 5 in modern seawater.
Eocene-Oligocene seawater (36-34 Ma) has estimated ranges of SO42− = 14–23 mmolal and Ca2+ = 11–20 mmolal. If the (Ca2+)(SO42−) product is assumed to be the same as in modern seawater (∼300 mmolal2), Eocene-Oligocene seawater had Ca2+ ∼16 mmolal and SO42− ∼19 mmolal. The same estimates of Ca2+ and SO42− for Serravallian-Tortonian seawater (13.5-11.8 Ma) are SO42− = 19–27 mmolal and Ca2+ = 8–16 mmolal and SO42− ∼24 mmolal and Ca2+ ∼ 13 mmolal if the (Ca2+)(SO42−) product is equal to that in modern seawater. Messinian seawater has an estimated range of SO42− ∼21–29 mmolal and Ca2+ ∼7–15 mmolal with SO42− ∼26 mmolal and Ca2+ ∼12 mmolal assuming the (Ca2+)(SO42−) product is equal to that in modern seawater. Regardless of the estimation procedure, SO42− shows progressively increasing concentrations from 36 Ma to the present values, which are the highest of the Cenozoic.