The major-ion composition of Cenozoic seawater: the past 36 million years from fluid inclusions in marine halite

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 Mg^{2+/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 Ca^{2+ ∼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.}