RAMIFICATIONS OF SECULAR VARIATIONS IN SEAWATER CHEMISTRY

The discovery of two 100 to 200 m.y. oscillations in Phanerozoic seawater chemistry (Mg, Ca, Na, K, SO₄ and Cl) between MgSO₄-rich (MgSO₄-seas) and CaCl₂-rich compositions (CaCl₂-seas) has led to a series of related results, which include: (1) Identification and modeling of the mechanism which controls the major ion composition of seawater. The major-ion composition of seawater is controlled by variations in the flux of mid ocean ridge brine and riverine inputs to the ocean; the composition of seawater over the past 550 Ma has been accurately modeled using this mechanism. (2) Identification of shorter "second order" variations in seawater composition. Permian brine inclusions in marine halites from North America and Central/Eastern Europe show variations in major-ions over periods of millions to tens of millions of years, specifically decreases in the oceanic Ca concentrations led to Mg/Ca ratios rising from 3.2-3.3 in the Lower Permian to 3.7 in the Upper Permian. The terminal Proterozoic-Early Cambrian transition was the time of a switch from MgSO₄ to CaCl₂ seas, due to a spike in oceanic Ca, coincident with the advent of biocalcification. (3) Connecting basinal brine compositions and paleo-seawater compositions. CaCl₂ basinal brines may have formed from evaporated paleoseawaters when the world oceans were CaCl₂ seas. CaCl₂ seas coincided with elevated sea levels, conditions that favored accumulation of marine CaCl₂ brines in marginal and interior continental basins. Typical basinal brines in Silurian-Devonian formations of the Illinois basin show the same compositional trends as those of progressively evaporated CaCl₂-rich Silurian seawater, modified by brine-rock reactions during burial, including dolomitization, and the precipitation of dolomite and K-feldspar cements. (4) Estimating atmospheric pCO₂ concentrations. Oscillations in both seawater Ca and Mg concentrations and calcite and aragonite seas were used to estimate that the atmospheric pCO₂ levels over the past 60 Ma were likely lower than previous published estimates that assume a constant total dissolved inorganic carbon.