Oceanic water column, reacting

In water and sediments, the time to chemical steady-states is controlled by the magnitude of transport mechanisms (diffusion, advection), transport distances, and reaction rates of chemical species. When advection (water flow, rate of sedimentation) is weak, diffusion controls the solute dispersal and, hence, the time to steady-state. Models of transient and stationary states include transport of conservative chemical species in two- and three-layer lakes, transport of salt between brine layers in the Dead Sea, oxygen and radium-226 in the oceanic water column, and reacting and conservative species in sediment. 

Ocean. Despite the fact that no changes in the chemical composition of the ocean water have been established in recent time, it is instructive to consider the transient behavior of reacting chemical species in the oceanic water column. The case of the nuclear fallout products transported through the surface and thermocline layer of the ocean is the best known, although not yet completely understood, case of a transient chemical event on a world-wide scale (34, 35,36,37). 

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chemical composition because few sites of active ridge-flank venting have yet been identified and sampled (Von Damm, unpublished).

While in the ocean transient states of chemical species are primarily characteristic of the surface waters where seasonal variations in biological productivity greatly affect the concentrations of dissolved nutrients (33), in lakes subject to seasonal turnovers transient concentrations are probably a rule. The effects exerted by the physical characteristics of the environment on the chances of a reacting chemical species to attain a steady-state concentration profile will be discussed for a lake, ocean, and sediment column. 

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