401 



FUTURE DIRECTIONS IN OCEAN SCIENCES 67 



regarded as the major contributor of dissolved material. Wind 

 transport of dust was recognized as adding to the particle flux 

 borne by streams, and the input of altered volcanic material to 

 deep-sea sediments was clearly identified by the 1960s. Within 

 the past 15 years, deep-sea hydrothermal activity, predominantly 

 at spreading centers, was found to be important as both a source 

 and a sink of elements. In addition, a significant flux of dissolved 

 material may be expelled from sedimentary wedges and underly- 

 ing oceanic crust as they descend into the mantle through sub- 

 duction zones. Cosmic dust and cosmic ray-produced nuclides 

 4 are not a major input to the ocean in terms of volume, but are 

 important as tracers and for understanding episodic influxes of 

 significance to the history of our planet. 



Elements are redistributed within the ocean in dissolved form 

 by horizontal and vertical advection, diffusive mixing, and incor- 

 poration into particles. Chemical species are removed from the 

 ocean as particles settle to the seafloor,- they also react directly 

 with the seafloor as the result of diffusion and circulation through 

 sediments and the oceanic crust. Seafloor sediments and altered 

 rocks undergo secondary reactions over time, and their composi- 

 tion may change substantially. From a chemical point of view 

 then, the oceans are a large chemical reactor with multiple feeds 

 and outputs. Chemical oceanographers want to understand the 

 present reactor and then, from examination of changes in the out- 

 puts over time, determine variations in the reactor's behavior and 

 the compositions and fluxes of inputs in the past. With this 

 information, the limits •^f future oceanic changes under given cli- 

 matic and tectonic scenarios can be estimated. 



Geochemical study of the oceanic water column and the out- 

 put of elements to the sediments is now relatively mature. The 

 descriptive phase is largely complete, and studies of mechanisms 

 are growing more numerous. For inputs of elements to the ocean, 

 however, quantitative research is difficult and, in the case of in- 

 puts from the continents, may not correctly reflect disturbances 

 in terrestrial inputs caused by humankind. These disturbances 

 have occurred on time scales of a few years to several centuries, 

 and the ocean has not yet equilibrated to the altered inputs. 



Fluxes 



Quantitative measurement of river inputs is difficult because 

 measurements of fluid discharge from rivers are uneven in qual- 

 ity, frequency, and distribution. Because the best data are avail- 



