THE MEDIATION OF SEAWATER AND ATMOSPHERE COMPOSITION BY BASALT-SEAWATER EXCHANGE REACTIONS THROUGHOUT THE PHANEROZOIC

Is long term global climate regulated primarily from within or without? Despite strong evidence of the fundamental importance of CO₂ in climate regulation (Crowley and Berner 2001), it has been recently argued that celestial forcing may be of equal or greater importance on geologic time scales (Shaviv and Veizer 2003). One means of evaluating these arguments is to reconcile atmospheric composition with proxies for the major element ratios of seawater (e.g., Lowenstein et al., 2001, 2003), through explicit incorporation of these data within models of long term geochemical cycling (Berner and Kothavala, 2001; Hansen and Wallmann, 2003; Berner, 2004). This work presents results from a geochemical cycling model for the Phanerozoic (MAGic). In its current state, this model incorporates rock weathering, basalt-seawater exchange reactions, and the formation and destruction of chemical sediments and organic matter. Hydrothermal reactions between seafloor and seawater involving calcium, magnesium, sodium, potassium, sulfate and carbon are the high temperature counterparts to low temperature redox, weathering, precipitation and diagenetic reactions. This model has basic scientific and philosophical roots in the earlier collaboration of Fred Mackenzie and Bob Garrels, and represents one of Fred's long-term goals.

MAGic's standard run is used as a point of departure from which to evaluate the sensitivity of the earth-ocean-atmosphere system to variations in seafloor spreading rate over the past 500 Ma. The goal is to understand how atmosphere and seawater composition are affected by variations in the fluxes of sedimentary carbonate to subduction and metamorphic regimes. The Mesozoic transition from a state in which biogenic carbonates occur solely as shelf and platform deposits to one in which carbonate sedimentation is partitioned between shelf and pelagic regimes, is also examined. The return fluxes of these components to the atmospheric and primary silicate reservoirs reflect not only the overall rates of basalt subduction and metamorphism, but also the distribution of authigenic minerals formed during basalt alteration, as well as the sedimentary burden itself. Model relationships suggest that the stoichiometry of basalt exchange fluxes (Mg/Ca and SO₄/Ca) may buffer atmospheric CO₂ and O₂ concentrations.