Hydrothermal circulation water fluxes

A solution, still controversial, has been recently proposed. This is the loss of sulfate from seawater during hydrothermal circulation through mid-ocean ridges (Edmond et al., 1979). The flow of water through these systems is estimated to be about 1.4 x 10 L/yr, about 0.4% of the flow of rivers. However, sulfate is quantitatively removed, yielding a flux of 125 Tg S/yr, capable of balancing the river flux. The controversy is whether the chemistry involved in removing sulfate is the formation of... 

More detailed examination and sampling allows association of hydrothermal circulation with specific vent fields. In such waters samples in the Galapagos Rift by the Alvin deep submersible, Jenkins, Edmond, and Corliss (1978) report juvenile He enrichments which dwarf the normal saturation concentrations by factors up to 11 for 4He and 60 for 3He (Figure 4.6). A particularly significant feature of this report is that added He occurs roughly in proportion to added heat AT up to 12°C in sampled water), corresponding to about 7.6 x 10 xcal/atom of 3He (Figure 4.7). Jenkins et al. note that if this value is representative, hydrothermal circulation may indeed account for the depression of conductive heat flow relative to models for total heat flux. As... 

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). 

To extrapolate the sea-floor weathering flux Thydro into the past requires a model of the water-rock reaction. There are two important possibilities, depending on whether the reactable cations are more or less abundant than CO2. In equation (11) we implicitly assume that CO2 is quantitatively removed from the sea water. In effect we presume fast reactions with superabundant cations. Walker (1985) and Fran9ois Walker (1992) assumed that CO2 is removed in linear proportion to its concentration. This differs from our picture only in the value of -4hydro in either model it is the delivery of CO2-rich waters to the oceanic crust by hydrothermal circulation that limits uptake, so that CO2 is quantitatively depleted in the circulating sea water until available CaO, MgO and FeO in the rock are exhausted. 

Determining the influence of hydrothermal circulation on the mass balance of chemical constituents in seawater boils down to knowing how much water flows through the hydrothermal areas and how important the reactions at high temperature are to the total hydrothermal flux. There is presently a lot of dehate about these issues. The rest of this chapter is devoted to an explanation of how one goes about interpreting hydrothermal fluxes from observed changes in hydrothermal chemistry. 

---