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

American Journal of Science
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Abstract

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.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title The major-ion composition of Cenozoic seawater: the past 36 million years from fluid inclusions in marine halite
Series title American Journal of Science
DOI 10.2475/08.2013.01
Volume 313
Issue 8
Year Published 2013
Language English
Description 62 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title American Journal of Science
First page 713
Last page 775