400 



66 OCEANOGRAPHY IN THE NEXT DECADE 



ment particles are formed or through which they pass is both an 

 agent of transport and the site where information about environ- 

 mental conditions is imprinted onto components that eventually 

 become part of marine sediments. Geochemical signatures in the 

 sediments complement the sedimentological and paleontologic record 

 in special ways. Oxygen and carbon isotopes in marine calcare- 

 ous shells or tests and organic remains provide insight into ocean 

 surface temperatures, ocean circulation, and the extent of ice storage 

 in past glaciations. Trace elements such as cadmium and barium 

 add to our understanding of deep-ocean circulation and upwelling. 

 Isotopes produced by cosmic rays, those originating from uranium 

 decay, and those produced or enhanced by human activities, pro- 

 vide information on chronology, sediment properties, oceanic up- 

 welling, deep-water formation, and transport of elements. The 

 unique properties of the isotope ratios ^^Sr/*^Sr, ^"*5pb/^°''Pb, ''*^Nd/ 

 ^'^'^Nd, and ^^^Os/^^^Os permit insights into the geologic questions 

 of plate tectonics because the isotopic composition of seawater 

 reflects the isotopic composition and relative importance of each 

 source, for example, material derived from the mantle and conti- 

 nental rock weathering. 



Although present in greatly diluted concentrations, marine or- 

 ganic substances play a key role in the global carbon cycle as 

 modulators and tracers of oceanic processes. For example, or- 

 ganic remains account for approximately 20 percent of all carbon 

 buried in marine sediments and thus are an important sink for 

 atmospheric carbon dioxide. The burial of organic matter and its 

 subsequent oxidation essentially control atmospheric oxygen lev- 

 els over geologic time. Dissolved organic matter in seawater may 

 contain a mass of carbon comparable to that in terrestrial biomass 

 and could potentially affect atmospheric carbon dioxide concen- 

 trations on a time scale of a thousand years — the ocean's turnover 

 time. In addition, organic molecules in particulate and dissolved 

 forms are important vehicles for the transport of reducing power, 

 nutrients, and trace elements throughout the ocean and across the 

 air-sea and sea-sediment interfaces. Organic molecules in the 

 marine environment are couriers of unique information about the 

 sources, pathways, and histories of the associated particles and 

 water. 



The Ocean Reactor 



The ocean receives dissolved and particulate material from a 

 variety of sources and pathways. Traditionally, river inputs were 



