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FUTURE DIRECTIONS IN OCEAN SCIENCES 71 



small areas at these oceans' northern boundaries, and subsequent 

 vertical transport and dissolution of their siliceous tests at depth. 

 The flux of particles to the seafloor is apparently the most 

 important mechanism by which elements are delivered to the sedi- 

 mentary reservoir. However, many complex processes, primarily 

 biological, can occur after initial deposition. Particles falling from 

 above are the only source of nutrition for many bottom-dwelling 

 organisms. Thus this material is ingested and excreted repeatedly 

 by a variety of species and is also subject to continuous bacterial 

 degradation, reducing its carbon content and destroying the func- 

 tional groups responsible for scavenging reactive elements from 

 the water column. 



Sinks : 



The two most important properties of the ocean system that 

 control the uptake of chemical species by the sediments and oce- 

 anic crust are its oxidation state and its temperature. 



Chemical reactions in sediments involving oxidation and re- 

 duction depend on the amount of oxygen present, which in turn 

 depends on metabolic activity and diffusion from the overlying 

 water column through the pore fluids. Where metabolic processes 

 exceed the oxygen supply through diffusion, the sediments be- 

 come anaerobic. This condition creates diffusion gradients in the 

 oxic-anoxic transition zone from above and below, affecting a range 

 of chemical reactions. The transition zone rises through the sedi- 

 ment column as sediment accumulation progresses. Understand- 

 ing and quantifying these processes are crucial for studies of glo- 

 bal change because of their implications for interpreting the 

 sedimentary record. 



The character of the crustal sink for oceanic dissolved mate- 

 rial changes with the temperature of the water-rock interactions. 

 At high temperatures, the reaction environment is anoxic. Sul- 

 fide-forming elements are precipitated; elements that form insoluble 

 oxides in the reduced state, such as uranium and chromium, are 

 also precipitated. Soluble materials, such as boron and alkali 

 compounds, are completely removed from the rocks. Magnesium 

 is removed but calcium is released. At the very high tempera- 

 tures (>400°C) associated with recent eruptions, phase separation 

 can occur, producing a dilute aqueous phase and a residual brine. 

 Subsequent mixing of these components appears to be responsible 

 for the large variations in salinity observed in hydrothermal flu- 



