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.