Seawater circulation through oceanic crust

Using 2.5 X lO g/m.y. as oceanic production rate and 5-20 as seawater/rock ratio and assuming that 30% of oceanic crust interacts with circulating seawater, and the crustal production rate is (0.8-1.1) x 10 kg/m.y., then the rate of seawater cycling through back-arc basin is estimated to be (4-22) x lO kg/m.y. Using this value and the CO2 concentration of hydrothermal solution ((0.05-0.3) mol/kg -H20) (Table 3.2), hydrothermal CO2 flux into the ocean is estimated to be (0.2-6) x lO kg/m.y. 

While most studies of seafloor hydrothermal systems have focused on the currently active plate boundary ( 0-1 Ma crust), pooled heat-flow data from throughout the world s ocean basins (Figure 1) indicate that convective heat loss from the oceanic lithosphere actually continues in crust from 0-65 Ma in age (Stein et al, 1995). Indeed, most recent estimates would indicate that hydrothermal circulation through this older (1-65 Ma) section, termed flank fluxes, may be responsible for some 70% or more of the total hydrothermal heat loss associated with spreading-plate boundaries—either in the form of warm (20-65 °C) altered seawater, or as cooler water, which is only much more subtly chemically altered (Mottl, 2003). 

However, most of the sedimentary mass is of detrital origin. Nutrient elements (Si, P, V, N, and trace metals) are removed in the ocean as biological debris to sediments. The main sink of substances is by hydrothermal reactions (volcanic activity) at locations of seafloor spreading and circulation through the ocean crust. Volcanic activities in the oceans are extensive, and produce submarine lava flows which are unstable in seawater. The high temperature (200-400 °C) is not only important for basalt-seawater reactions but also triggers circulation. Subduction (see Chapters 2.2.1.1 and 2.6.4.3) transfers seawater and its constituents back to the magma. 

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