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FUTURE DIRECTIONS IN OCEAN SCIENCES 65 



The ocean is a chemical reactor, with inputs, internal reac- 

 tions, and outputs. Inputs are received from continents via rivers 

 and airborne transport. Other chemicals enter the ocean from 

 hydrothermal sources, primarily at mid-ocean ridges. As oceanic 

 crust is subducted beneath continents, elements are expelled from 

 both the crust and its overlying sediment layer. Finally, a minor 

 proportion of input to the ocean comes from cosmic sources. In- 

 puts are the least understood part of the reactor. 



Elements are redistributed in the ocean by circulation and 

 mixing, and are transformed through chemical reactions and bio- 

 logical activity. Chemicals finally exit the reactor through incor- 

 poration into seafloor sediments. The residence time of chemi- 

 cals in seawater and in the sediments depends on properties of the 

 chemicals, as well as chemical, physical, and biological condi- 

 tions. Chemical oceanographers seek to understand the present 

 reactor and then, from examination of changes in the outputs 

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

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

 tion, the limits of future oceanic changes under given climatic 

 and tectonic scenarios can be estimated. 



Future studies in chemical oceanography will be aided by new 

 instruments that are capable of analyzing a wide variety of ele- 

 ments and isotopes contained in sm.all samples. Greater knowl- 

 edge of processes controlling fluxes, redistribution, and removal, 

 and improvements in our ability to read the sedimentary record 

 are likely in the coming decade. 



Introduction 



Marine geochemistry integrates several oceanographic disci- 

 plines. The aims of marine geochemistry are (1) to understand 

 the inputs of elements from the continents, mantle, and cosmic 

 sources into the ocean over time; (2) to understand the process of 

 material removal from the ocean to the sediments and oceanic 

 crust; (3) to understand the process by which elements and their 

 isotopes are redistributed; (4) to determine the mechanisms of 

 chemical coupling between the ocean and the atmosphere, and 

 interpret the sedimentary record of past oceanic change; and (5) to 

 study marine organic compounds both in their relation to the 

 above factors and to the global carbon cycle, and as detectors of 

 oceanographic properties over time. 



Oceanic sedimxcnts record environmental events over the past 

 180 million years of Earth's history. Ocean water in which sedi- 



