into the surrounding water column. These clouds rise above the vents until their density equals that of the surrounding seawater. On ridges with high rift walls (e.g., the Mid-Atlantic Ridge), the plumes often stay within the confines of the bounding rift walls. The precipitates and dissolved material they scavenge then accumulate on the walls and floor of the rift valley. On fast-spreading ridges with low walls (e.g., the East Pacific Rise), the plumes rise above the walls and are dispersed in the middepth circulation. Fine-grained vent-derived particles can be transported for thousands of kilometers, slowly settling out to form a compositionally distinctive shadow in the sediments. Unreactive species, such as helium, delineate plume flow.

If redistribution of elements occurred only by the physical dispersal of dissolved material and suspended particles near their sources, geochemical patterns would be related solely to input functions. However, the chemistry of the ocean reactor is determined primarily by organisms and chemical kinetics rather than by thermodynamics. Essential nutrient cycles are controlled largely by the metabolic processes of living organisms. For nonnutrient elements, scavenging by nonliving organic materials is much more important. Settling particles are reactive enough to adsorb and transport a large variety of elements to the bottom. Because bacteria continue to degrade this material, the concentrations of many elements increase with depth and along current flow lines.

Surface productivity shows strong regional variability. The vertical particle flux and the intensity of scavenging and release also vary by region. For geochemical purposes, satellite pictures of ocean productivity must be projected into the vertical dimension to appreciate the fact that strong lateral variations in reactivity occur (reactivity is the intensity of the chemical reactions that are driven by biological activity in the ocean water column). Vast areas, such as the subtropical gyres, are quite unreactive; these are surrounded by a coastal rim of high reactivity. Other zones of high reactivity correspond to areas of physical upwelling.

The combination of lateral and vertical transport and continuous reaction of particles suggests that the water column distribution of elements at a particular location may have little influence on local processes but, instead, reflects the integration of processes over various time scales and distances. The best example of this effect is the deep silica maximum found throughout the Indian and Pacific oceans. It results from strong upwelling and associated high productivity of siliceous plankton in relatively

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