NUMERICAL MODEL FOR THE TEMPORAL EVOLUTION OF THE GEOHYDROLOGIC CYCLE AND ITS IMPACT ON OCEAN CHEMISTRY OVER THE PAST 100 MILLION YEARS

Analyses of seawater-bearing evaporite fluid inclusions suggest that the Mg/Ca ratio of global oceans has increased from 1.5 to 5.2 over the past 100 Ma. Ocean chemistry, including the Mg/Ca ratio, is controlled by a variety of processes, including surface runoff from the continents into the oceans, mid-ocean ridge (MOR) hydrothermal processes and biological activity. The amount of water transported into and/or out of the oceans by each of these processes has changed over geologic time, suggesting that the relative contribution of each of these processes to ocean chemistry has also changed with time. Previous workers have estimated variations in the amount of water involved in surface runoff, MOR hydrothermal processes, and marine biogenic activity over the past 100 Ma. Here, we describe a quantitative geochemical box model using these previously published estimates to assess the potential influence of these processes on the observed changes in ocean Mg/Ca ratios over the past 100 Ma. Input data for the model includes the amount of water in the ocean, oceanic crust, and surface water reservoirs, together with the fluxes of H₂O, Ca, and Mg between these reservoirs. Sensitivity analyses suggest that a 1.1% decrease in surface water runoff or a 2.3% decrease in MOR fluid flux over 100 Ma would increase ocean Mg/Ca ratios from 1.5 to 5.2. Some previous studies suggest that the average annual rate of seafloor spreading during the past 100 Ma has not been constant but, rather, has fluctuated between 45 to 75 mm/year. Assuming that hydrothermal fluid flux through MOR systems scales with spreading rate, the observed variation in ocean Mg/Ca ratios during the past 100 Ma is consistent with a minor net decline in spreading rates over that same time interval. To our knowledge, variations in surface water runoff to the oceans in the geologic past have not been reported, so it is not possible to compare our required variations in surface water runoff with prior estimates.