drilling show that the ocean crust and its overlying sediment expel water in the subduction process. Given the great compositional and tectonic diversity of active subduction zones, it will be impossible to estimate the geochemical significance of these processes to the oceanic element cycles until much more exploratory work has been done.
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 desorption 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 extensively. 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 dominant in dispersing inputs until they cannot be traced directly back to their source. Large rivers can be considered as point sources of 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 phytoplankton settle toward the seafloor, carrying nutrients and scavenging dissolved substances. In confined systems with high nutrient inputs, settling organic material can fuel bacterial activity and lead to oxygen depletion of bottom waters. The complex coupling of inorganic and biological processes, postdepositional reactions in the sediments, and strong seasonality of inputs produce a system whose chemical transport is difficult to quantify. Estuarine processes make it difficult to estimate river inputs of the more reactive 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 intense biogeochemical interactions in the entire ocean.
High-temperature hydrothermal fluids create great buoyant clouds of fine-grained sulfides and oxides upon their turbulent injection