Venting black smokers

Macdonald, K.C., Becker, K., Spiess, F.N. and Ballard, R.D. (1980) Hydrothermal heat flux of the black smoker vents on the East Pacific Rise. Earth Planet. Sci. Lett., 48, 1-7. 

Recent microanalysis by laser probe and ion microprobe documents one of the main problems. Sulfides forming at low to intermediate temperatures show extreme zonation of over 50%o in less than 100 pm in some diagenetic pyrites (McKibben and Riciputi 1998) and 4-5%o/cm in a hydrothermal black smoker vent (Shanks et al. 

The deposit is capped by a large black smoker complex with as many as 100 black smoker vents at a temperature of more than 360°C. 

At typical black smoker vents, a very large proportion (at least 90%) of the metals and sulfur carried in solution are lost to a hydrothermal plume in the overlying water column rather than deposited as chimneys. The metals are precipitated as sulfide particles in the plume above the black smokers and are rapidly oxidized and dispersed over distances of several kilometers from the vent (Feely et al. 1987, 1994a,b Mottl and McConachy 1990). Due to oxidation and dissolu-... 

Of the major solids formed from melts, many, but not all, at equilibrium, the overwhelming influence is of cooperative interaction between ionic units of similar shape and size as we see in crystals. Trace elements apart from forming isolated minerals are fractioned in bulk oxides, for example, in particular orders as the melt solidifies, and this reduces the relative availability of some elements such as Cr and Ni (see Williams, and Williams and Frausto da Silva (1999) in Further Reading). Again the interaction of selective molten minerals and water creates extremely reactive environments and such environments still exist, especially in the deep sea black smokers (hydrothermal vents), around which particular mixed minerals form, which could also have been involved in prebiotic chemistry and are still involved in the peculiarities of life in these smokers . In Figure 1.6 we summarise... 

Seawater is circulated below the sea bed where it is heated by volcanic activity before being re-injected into the sea at high pressure and temperature. The pressure prevents water from boiling until the temperature reaches 725 K. The superheated water dissolves minerals from around the vent that then precipitate as the water temperature cools. This gives the vents their black smoker appearance. Many small molecules such as H2, H2S and Mn2+ do not precipitate but remain in their reduced formed and are available as electron donors. 

Location Mud holes geothermal power plants deep terrestrial oil fields Submarine hot springs vents or black smokers deep submarine oil fields... 

In this chapter, we develop geochemical models of two hydrothermal processes the formation of fluorite veins in the Albigeois ore district and the origin of black smokers, a name given to hydrothermal vents found along the ocean floor at midocean ridges. 

Where fluids discharge from hot springs and mix with seawater, they cool quickly and precipitate clouds of fine-grained minerals. The clouds are commonly black with metal sulfides, giving rise to the term black smokers. Some vents give off clouds of white anhydrite these are known as white smokers. Structures composed of chemical precipitates tend to form at the vents, where the hot fluids discharge into the ocean. The structures can extend upward into the ocean for several meters or more, and are composed largely of anhydrite and, in some cases, sulfide minerals. 

A typical hydrothermal vent system. Included are basal mounds of mineral precipitates, pillow basalts, a black smoker, and various vent organisms. Source From Haymon, R. M. and K. C. Macdonald (1985). American Scientist, 73, 441 49. 

We also know that a considerable enrichment of prebiotic moieties may have come from submarine vents and other hydrothermal sources (see, for example, Miller and Bada, 1988 Holm and Andersson, 1998 Stetter, 1998). Let s start with the 1979 discovery of deep-sea vents with black smokers, which are associated with an extraordinary abundance of the most phylogenetically primitive organisms on Earth. This ecosystem is sulphur based, and is distinct from the more familiar, photosynthetically-based ecosystem that dominates Earth s surface. Corliss et al. (1981) were struck by the richness of the vent biota, based on chemosynthesis, and proposed that these were the origin of life. 

Directed evolution is an iterative process that mimics the natural evolution process in vitro, by generating a diverse library of enzymes and selecting those with the desired features. Natural evolution is very effective in the long term (bacteria adapt to every environment, living even in so-called black smokers, deep-ocean vents where temperatures can reach 350°C and the pressure is 200bar [93]). Unfortunately, it typically takes millions of years. Happily, directed evolution can be carried out within weeks or months and with an unlimited number of parents. Importantly, and unlike rational design, directed evolution is a stochastic method. It does not require any structural or mechanistic information on the enzyme of interest (although such information can help). 

Figure 10.1 Hydrothermal vents, so-called black smokers, carry high concentration of sulphur compounds with traces of methane and hydrogen together with suspended sulphide particles. (Picture by courtesy of Professor Verena Tunnidiffe, University of Victoria, Canada)...

Figure 8 End-member REE concentrations in vent fluids from four different black smokers at tbe 21° N vent site, EPR, normalized to chondrite (REE data from Klinkhammer et ah, 1994). NGS = National Geographic Smoker, HG = Hanging Gardens, OBS = Ocean Bottom Seismometer, SW = South West vent.

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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