The composition of fluid inclusions in halite in four Tertiary and one Cretaceous marine evaporite shows that the Mg concentration of sea water increased from 32 mmol/kg sw to its present value of 53 mmol/kg sw during the past 60 m.y. They also show that mK⁺ was essentially constant at 10 mmol and that mMg⁺² + mCa⁺² ~ mSO₄^-2 + 35 mmol during this period. The values of mCa⁺² and mSO₄^-2 can be deconvolved if the product mCa⁺²*mSO₄^-2 is assumed to have been constant during the Tertiary. Although absolute constancy of this product is unlikely, the assumption leads to a mMg⁺²/mCa⁺² ratio in early Tertiary sea water that is consistent with the estimated value at the transition of aragonite to calcite seas and with an estimate of the mMg⁺²/mCa⁺² ratio in sea water based on the Mg content of the foraminifer O. umbonatus (Lear, Elderfield and Wilson, 2000).

Time (m.y.b.p.)	Concentration (mmol/kg sw)
	Mg+2	Ca+2	K+	SO4-2	(Cl- - Na+)
0	53	10	10	28	77
20	38	15	10	19	77
40	34	17	10	16	77
60	32	18	10	15	77