Hydrothermal fluids plumes

Hydrothermal venting injects fluids into seawater as buoyant, jetlike pliunes. These turbulent flows mix rapidly with seawater becoming diluted by factors of lO" to 10. This mixing eventually makes the plumes neutrally buoyant, after which they are transported laterally through the ocean basins as part of the intermediate and deepwater currents. Hydrothermal plumes have the potential to greatly affect seawater chemistry. From global estimates of hydrothermal fluid emissions and dilution ratios, a volume of seawater equivalent to the entire ocean can be entrained in the hydrothermal plumes every few thousand years. 

The rest of the chapter is organized as follows. In Section 6.07.2 we discuss the chemical composition of hydrothermal fluids, why they are important, what factors control their compositions, and how these compositions vary, both in space, from one location to another, and in time. Next (Section 6.07.3) we identify that the fluxes established thus far represent gross fluxes into and out of the ocean crust associated with high-temperature venting. We then examine the other source and sink terms associated with hydrothermal circulation, including alteration of the oceanic crust, formation of hydrothermal mineral deposits, interactions/uptake within hydrothermal plumes and settling into deep-sea sediments. Each of these fates for hydrothermal material is then considered in more detail. Section 6.07.4 provides a detailed discussion of near-vent deposits, including the formation of polymetallic sulfides and... 

Hydrothermal plumes form wherever buoyant hydrothermal fluids enter the ocean. They represent an important dispersal mechanism for the thermal and chemical fluxes delivered to the oceans while the processes active within these plumes serve to modify the gross fluxes from venting, significantly. Plumes are of further interest to geochemists because they can be exploited in the detection and location of new hydrothermal fields and for the calculation of total integrated fluxes from any particular vent field. To biologists, hydrothermal plumes represent an... 

Mitra A., Elderfield H., and Greaves M. J. (1994) Rare earth elements in submarine hydrothermal fluids and plumes form the Mid-Atlantic Ridge. Mar. Chem. 46, 217-235. 

In comparison, Fe oxidation and deposition appear to be much less common in plumes. About half the Fe in the hydrothermal fluids combines with H2S and is rapidly transformed into Fe sulphides within a few seconds of release (e.g. Rudnicki Elderfield, 1993 James etal., 1995), and much of the Fe2+ that escapes sulphide precipitation is rapidly and spontaneously oxidized in well-oxygenated seawater making it difficult to evaluate the bacterial contribution to the redox transformations of hydrothermal Fe in plumes (Lilley etal., 1995 Winn etal., 1995). Nevertheless, high Fe/Mn particles and Fe-encrusted capsule forms have been observed in plumes at Axial Volcano (JDFR) the physicochemical characteristics of the capsules may be responsible for the passive or surface-enhanced deposition of iron (Cowen etal., 1999). Thiosulphate, the primary product of sulphide autooxidation, may also serve as a useful energy source, but this is yet to be documented in hydrothermal plumes (Winn etal., 1995 Cowen German, 2002). 

Lonsdale P, Becker K (1985) Hydrothermal plumes, hot springs, and conductive heat flow in the Southern Trough of Guaymas Basin. Earth Planet Sci Lett 73 211-225 Massoth GJ, Butterfield DA, Lupton JE, McDuff RE, Lilley MD, Jonasson IR (1989) Submarine venting of phase-separated hydrothermal fluids at Axial Volcano, Juan de Fuca Ridge. Nature 340 702-705 McKenzie DP, Davies D, Molnar P (1970) Plate tectonics of the Red Sea and east Africa. Nature 226 243-248... 

Klinkhammer, G., German, C. R., Elderfield, H., Greaves, M.J., and Mitra, A. (1994). Rare earth elements in hydrothermal fluids and plume particulates by inductively coupled plasma mass spectrometry. Mar. Chem. 45(3), 179-186. 

It is worth elucidating mineral particle behavior in hydrothermal plumes in order to consider the formation mechanism of chimney and massive ores on the seafloor. Using the grain size data on sulfides and sulfates, the density of the fluids and of the minerals, the relationship between vertical settling rate and grain size of sulfides and sulfates can be derived based on the following Stokes equation ... 

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).

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