Hydrothermal circulation flanks

Off-axis hydrothermal circulation is responsible for 70% of the convectively driven heat flow from hydrothermal systems. This circulation is thought to occur on the flanks of the mid-ocean ridges and rises at temperatures on the order of 20 to 54°C. The... 

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). 

Figure 13 Schematic representation of an MOR hydrothermal system and its effects on the overlying water column. Circulation of seawater occurs within the oceanic crust, and so far three types of fluids have been identified and are illustrated here high-temperature vent fluids that have likely reacted at >400 °C high-temperature fluids that have then mixed with seawater close to the seafloor fluids that have reacted at intermediate temperatures, perhaps 150 °C. When the fluids exit the seafloor, either as diffuse flow (where animal communities may live) or as black smokers, the water they emit rises and the hydrothermal plume then spreads out at its appropriate density level. Within the plume, sorption of aqueous oxyanions may occur onto the vent-derived particles (e.g., phosphate, vanadium, arsenic) making the plumes a sink for these elements biogeochemical transformations also occur. These particles eventually rain-out, forming metalliferous sediments on the seafloor. While hydrothermal circulation is known to occur far out onto the flanks of the ridges, little is known about the depth to which it extends or its overall chemical composition because few sites of active ridge-flank venting have yet been identified and sampled (Von Damm, unpublished).

Mottl M. J. and Wheat C. G. (1994) Hydrothermal circulation through mid-ocean ridge flanks fluxes of heat and magnesium. Geochim. Cosmochim. Acta 58, 2225-2237. 

Both examples of the hydrothermal influence on seawater chemistry indicate substantial chemical alterations on the ridge flanks away from the axis. The current challenge for improving the marine mass balance is to devise a way to determine fluxes in enough areas so that some generalizations can be made about the role of the off-axis hydrothermal circulation in the geochemical mass balance. 

Wheat, C.G. and Mottl, M.J. (1994) Hydrothermal circulation, Juan de Fuca Ridge eastern flank Factors controlling basement water composition. Journal of Geophysical Research, 99(B2), 3067-3080. 

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