THE COMPOSITION OF TERTIARY SEA WATER

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