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

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.