Tim K. Lowenstein
Dioni I. Cendon
Sean T. Brennan
2013
Fluid inclusions from ten Cenozoic (Eocene-Miocene) marine halites are used to quantify the major-ion composition (Mg<sup>2+</sup>, Ca<sup>2+</sup>, K<sup>+</sup>, Na<sup>+</sup>, SO<sub>4</sub><sup>2−</sup>, and Cl<sup>−</sup>) 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<sup>−</sup> in halite; (3) δ<sup>34</sup>S of sulfate minerals; (4) <sup>87</sup>Sr/<sup>86</sup>Sr 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 Ca<sup>2+</sup>-rich, Mg<sup>2+</sup>- and SO<sub>4</sub><sup>2−</sup>-poor seawater of the Mesozoic (“CaCl<sub>2</sub> seas”) to the “MgSO<sub>4</sub> seas” (with higher Mg<sup>2+</sup> and SO<sub>4</sub><sup>2−</sup>>Ca<sup>2+</sup>) 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<sup>+</sup> concentration (11 mmolal) are constant and the same as in modern seawater. Fluid inclusions from Cenozoic marine halites show that the concentrations of Mg<sup>2+</sup>and SO<sub>4</sub><sup>2−</sup> have increased in seawater over the past 36 My and the concentration of Ca<sup>2+</sup> has decreased. Mg<sup>2+</sup> concentrations increased from 36 mmolal in Eocene-Oligocene seawater (36-34 Ma) to 55 mmolal in modern seawater. The Mg<sup>2+</sub>/Ca<sup>2+</sup> 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 SO<sub>4</sub><sup>2−</sup> = 14–23 mmolal and Ca<sup>2+</sup> = 11–20 mmolal. If the (Ca<sup>2+</sup>)(SO<sub>4</sub><sup>2−</sup>) product is assumed to be the same as in modern seawater (∼300 mmolal<sup>2</sup>), Eocene-Oligocene seawater had Ca<sup>2+</sup> ∼16 mmolal and SO<sub>4</sub><sup>2−</sup> ∼19 mmolal. The same estimates of Ca<sup>2+</sup> and SO<sub>4</sub><sup>2−</sup> for Serravallian-Tortonian seawater (13.5-11.8 Ma) are SO<sub>4</sub><sup>2−</sup> = 19–27 mmolal and Ca<sup>2+</sup> = 8–16 mmolal and SO<sub>4</sub><sup>2−</sup> ∼24 mmolal and Ca<sup>2+</sup> ∼ 13 mmolal if the (Ca<sup>2+</sup>)(SO<sub>4</sub><sup>2−</sup>) product is equal to that in modern seawater. Messinian seawater has an estimated range of SO<sub>4</sub><sup>2−</sup> ∼21–29 mmolal and Ca<sup>2+</sup> ∼7–15 mmolal with SO<sub>4</sub><sup>2−</sup> ∼26 mmolal and Ca<sup>2+<sup> ∼12 mmolal assuming the (Ca<sup>2+</sup>)(SO<sub>4</sub><sup>2−</sup>) product is equal to that in modern seawater. Regardless of the estimation procedure, SO<sub>4</sub><sup>2−</sup> shows progressively increasing concentrations from 36 Ma to the present values, which are the highest of the Cenozoic.
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10.2475/08.2013.01
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The major-ion composition of Cenozoic seawater: the past 36 million years from fluid inclusions in marine halite
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