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

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ID: 293551
2013
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Abstract
Fluid inclusions from ten Cenozoic (Eocene-Miocene) marine halites are used to quantify the major-ion composition (Mg^2+^, Ca^2+^, K^+^, Na^+^, SO~4~^2−^, 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) δ^34^S of sulfate minerals; (4) ^87^Sr/^86^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^2+^-rich, Mg^2+^- and SO~4~^2−^-poor seawater of the Mesozoic ("CaCl~2~ seas") to the "MgSO~4~ seas" (with higher Mg^2+^ and SO~4~^2−^\>Ca^2+^) 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 Mg^2+^and SO~4~^2−^ have increased in seawater over the past 36 My and the concentration of Ca^2+^ has decreased. Mg^2+^ concentrations increased from 36 mmolal in Eocene-Oligocene seawater (36-34 Ma) to 55 mmolal in modern seawater. The Mg^2+^/Ca^2+^ 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~4~^2−^ = 14--23 mmolal and Ca^2+^ = 11--20 mmolal. If the (Ca^2+^)(SO~4~^2−^) product is assumed to be the same as in modern seawater (∼300 mmolal^2^), Eocene-Oligocene seawater had Ca^2+^ ∼16 mmolal and SO~4~^2−^ ∼19 mmolal. The same estimates of Ca^2+^ and SO~4~^2−^ for Serravallian-Tortonian seawater (13.5-11.8 Ma) are SO~4~^2−^ = 19--27 mmolal and Ca^2+^ = 8--16 mmolal and SO~4~^2−^ ∼24 mmolal and Ca^2+^ ∼ 13 mmolal if the (Ca^2+^)(SO~4~^2−^) product is equal to that in modern seawater. Messinian seawater has an estimated range of SO~4~^2−^ ∼21--29 mmolal and Ca^2+^ ∼7--15 mmolal with SO~4~^2−^ ∼26 mmolal and Ca^2+^ ∼12 mmolal assuming the (Ca^2+^)(SO~4~^2−^) product is equal to that in modern seawater. Regardless of the estimation procedure, SO~4~^2−^ shows progressively increasing concentrations from 36 Ma to the present values, which are the highest of the Cenozoic.
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openalex_W1966573531 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors Sean T. Brennan, Tim K. Lowenstein, Diοni I. Cendón
Journal american journal of science
Year 2013
DOI
10.2475/08.2013.01
URL
https://doi.org/10.2475/08.2013.01
Keywords Keywords not found

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