Systematic changes in the chemistry of evaporated seawater contained in primary fluid inclusions in marine halites indicate that seawater chemistry has fluctuated during the Phanerozoic. The fluctuations are in phase with oscillations in seafloor spreading rates, volcanism, global sea level, and the primary mineralogies of marine limestones and evaporites. The data suggest that seawater had high Mg2+/Ca2+ ratios (>2.5) and relatively high Na+ concentrations during the Late Precambrian [544 to 543 million years ago (Ma)], Permian (258 to 251 Ma), and Tertiary through the present (40 to 0 Ma), when aragonite and MgSO4 salts were the dominant marine precipitates. Conversely, seawater had low Mg2+/Ca2+ ratios (<2.3) and relatively low Na+ concentrations during the Cambrian (540 to 520 Ma), Silurian (440 to 418 Ma), and Cretaceous (124 to 94 Ma), when calcite was the dominant nonskeletal carbonate and K-, Mg-, and Ca-bearing chloride salts, were the only potash evaporites.
Major ion compositions of primary fluid inclusions from terminal Proterozoic (ca. 544 Ma) and Early Cambrian (ca. 515 Ma) marine halites indicate that seawater Ca 2؉ concentrations increased approximately threefold during the Early Cambrian. The timing of this shift in seawater chemistry broadly coincides with the ''Cambrian explosion,'' a brief drop in marine 87 Sr/ 86 Sr values, and an increase in tectonic activity, suggesting a link between the advent of biocalcification, hydrothermal mid-ocean-ridge brine production, and the composition of seawater. The Early Cambrian surge in oceanic [Ca 2؉ ] was likely the first such increase following the rise of metazoans and may have spurred evolutionary changes in marine biota.Figure 1. Geological and biological changes in oceanic realm during terminal Proterozoic and Cambrian. Stages in Early Cambrian are Nemakit-Daldynian (N-D), Tommotian-Atdabanian (T-A), and Botomian-Toyonian (B-T); Middle (M) and Late (L) Cambrian are also shown. There was dramatic increase in diversity of animal classes and orders starting in Tommotian time (from Knoll and Carroll, 1999). Phylogeny and corresponding crown group first appearances (time line) show diversification of animal clades (from Knoll and Carroll, 1999). Crown groups are ''the last common ancestor of all living members of a clade plus all its descendants'' (from Knoll and Carroll, 1999, p. 2136). Decrease in marine 87 Sr/ 86 Sr ratios ca. 530 Ma during otherwise increasing trend in 87 Sr/ 86 Sr ratios likely indicates increase in mid-ocean-ridge spreading rates (Walter et al., 2000; Nicholas, 1996). Bars indicate possible ranges of oceanic [Ca 2؉ ] determined in this study; open circles represent average values (Table 1) used for computer simulations shown in Figure 2. Concentrations are millimolal (millimoles per 1 kg H 2 O).
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) ␦ 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 ϳ2.3 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 (ϳ300 mmolal 2 ), Eocene-Oligocene seawater had Ca 2؉ ϳ16 mmolal and SO 4 2؊ ϳ19 mmolal. The same estimates of Ca 2؉ and SO 4 2؊ for Serravallian-Tortonian seawater (13.5-11.8 Ma) are SO42؊ ؍ 19 -27 mmolal and Ca 2؉ ؍ 8 -16 mmolal and SO 4 2؊ ϳ24 mmolal and Ca 2؉ ϳ 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؊ ϳ21-29 mmolal and Ca 2؉ ϳ7-15 mmolal with SO 4 2؊ ϳ26 mmolal and Ca 2؉ ϳ12 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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