403 



FUTURE DIRECTIONS IN OCEAN SCIENCES 69 



drilling show that the ocean crust and its overlying sediment ex- 

 pel water in the subduction process. Given the great composi- 

 tional and tectonic diversity of active subduction zones, it will be 

 impossible to estimate the geochemical significance of these pro- 

 cesses to the oceanic element cycles until much more exploratory 

 work has been done. 



Redistribution 



All the inputs will react with seawater. Reaction processes in 

 the zone of mixing between river water and the upper ocean are 

 quite well described qualitatively. These processes include de- 

 sorption of elements from suspended particles, coagulation and 

 precipitation of colloidal material, scavenging by organisms, and 

 vertical transport. Processes associated with the formation of 

 particle plumes above hydrothermal vents have been studied exten- 

 sively. These particles oxidize rapidly and appear to scavenge both 

 a large proportion of the hydrothermically transported trace metals 

 and a significant component of elements from ambient seawater. 



As mixing progresses, ocean circulation is increasingly domi- 

 nant in dispersing inputs until they cannot be traced directly back 

 to their source. Large rivers can be considered as point sources oi 

 material to the ocean superimposed on a diffuse background input 

 from smaller streams. Penetration of the river signal into the 

 deep sea follows complex pathways that are regionally diverse, 

 depending on the current regime and the configuration of the coastline 

 and the continental shelf. Because even unpolluted rivers usually 

 carry elevated nutrient loads relative to coastal seawater, their 

 discharge induces large phytoplankton blooms. The phytoplank- 

 ton settle toward the seafloor, carrying nutrients and scavenging 

 dissolved substances. In confined systems with high nutrient in- 

 puts, settling organic material can fuel bacterial activity and lead 

 to oxygen depletion of bottom waters. The complex coupling of 

 inorganic and biological processes, postdepositionai reactions in 

 the sediments, and strong seasonality of inputs produce a system 

 whose chemical transport is difficult to quantify. Estuarine pro- 

 cesses make it difficult to estimate river inputs of the more reac- 

 tive elements to the open ocean. Because of this complexity, 

 chemical oceanography in the coastal ocean has been relatively 

 neglected even though that is the site for some of the most in- 

 tense biogeochemical interactions in the entire ocean. 



High-temperature hydrothermal fluids create great buoyant clouds 

 of fine-grained sulfides and oxides upon their turbulent injection 



