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70 OCEANOGRAPHY IN THE NEXT DECADE 



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 accu- 

 mulate 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 thou- 

 sands 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 deter- 

 mined 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 bac- 

 teria 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 dimen- 

 sion to appreciate the fact that strong lateral variations in reactiv- 

 ity 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 continu- 

 ous reaction of particles suggests that the water column distribu- 

 tion of elements at a particular location may have little influence 

 on local processes but, instead, reflects the integration of pro- 

 cesses 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 



