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68 OCEANOGRAPHY IN THE NEXT DECADE 



able from developed regions of the world, they do not necessarily 

 represent areas less impacted by human activities. 



Many of these same uncertainties apply to airborne inputs. 

 Transport is strongly seasonal and diffuse, and thus is difficult to 

 measure. Wind erosion rates are sensitive to the nature of the 

 land cover and therefore to changes in land use. Windborne par- 

 ticles efficiently scavenge volatile chemicals released to the at- 

 mosphere by volcanism, biomass burning, and industrial activi- 

 ties. Thus the chemical composition of windborne dust is sensitive 

 to pollution. In addition, photochemical reactions occur in the 

 atmosphere, changing the chemistry of the atmosphere and par- 

 ticle-bound chemicals. 



Hydrothermal activity in the deep sea is driven by volca- 

 nic processes in the oceanic crust, predominantly at spreading 

 centers. Seawater seeps into crustal rocks, entering convection 

 cells in the rocks, and is heated to 300 to 400°C. In the course of 

 this circulation, elements in the seawater react with the hot ba- 

 saltic rocks; some chemicals are removed (e.g., magnesium, sul- 

 fate, and uranium) and others are added (e.g., mobile elements and 

 gases). The fluxes of elements at any one site are difficult to 

 quantify, and it is not feasible to measure the thousands of sites 

 that differ in rock temperature and composition. To account for 

 observed isotope concentrations in seawater (e.g., strontium), a 

 volume of water equal to the world's ocean must circulate through 

 the hydrothermal system at temperatures above 325''C every 10 

 million years. However, estimates of hydrothermal circulation 

 based on heat lost in the formation of the oceanic crust are about 

 five times lower. If the higher estimate for hydrothermal circula- 

 tion is correct, this process is as important as rivers for the input 

 of many elements to the ocean. Hydrothermal processes would 

 stabilize seawater composition and thus act as a geochemical fly- 

 wheel, potentially damping large-scale changes induced by long- 

 term climatic and tectonic changes. Yet, if the lower estimate of 

 hydrothermal circulation is correct, hydrothermal activity is a 

 minor factor in the cycling of ocean elements and is important for 

 only a few. The major inconsistency between the fluxes based on 

 isotopic and thermal constraints, apparent since the first hot springs 

 were found in the deep sea 15 years ago, remains to be resolved. 



There is evidence that hydrothermal circulation at relatively 

 low temperatures (a few tens of degrees) away from spreading 

 centers may also be important for fluxes of elements. However, it 

 is not yet possible to calculate even the vaguest estimate of the 

 chemical fluxes involved. Recent seafioor exploration and ocean 



