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FUTURE DIRECTIONS IN OCEAN SCIENCES 79 



areas such as sequence stratigraphy and fault dynamics, provide 

 an opportunity for the development of fundamental new insight 

 into margin structure and evolution. What is needed is a coordi- 

 nated, interdisciplinary research effort involving both land-based 

 and sea-based research programs over the next decade. 



Marine sediments provide an important record of geological 

 processes including past global climates. For example, analysis of 

 marine sediment cores has provided critical information on the 

 importance of Earth's orbit in short-term climate change. Marine 

 sediments also provide a record of global sea-level changes, sea 

 surface and bottom water temperature variations, changes in ocean 

 current patterns, the volume of water locked in polar ice caps, 

 and the effects of a changing physical and chemical environment 

 on the evolution of marine life. Through drilling and coring, 

 especially in high-latitude regions, paleoceanographers are poised 

 to make major advances in our understanding of the natural vari- 

 ability in global climate systems in the coming decade. 



Introduction 



The plate tectonic paradigm forms an integrated and linked 

 physical and chemical framework for the flow of energy and mass 

 through Earth (Figure 3-1). Radioactive decay of material within 

 Earth's interior produces heat and creates a convective system 

 that transports heat and material from deep within Earth to shal- 

 low levels. Upper mantle rocks partially melt, producing basaltic 

 magma. Much of this melt is preferentially focused along the 

 world-encircling mid-oceanic ridge, where oceanic crust is cre- 

 ated. In time, the oceanic lithosphere (the oceanic crust and up- 

 per mantle) will be recycled into the mantle at convergent mar- 

 gins. The oceanic crust and some sediment are carried back into 

 the mantle, and the crustal components dehydrate, pumping wa- 

 ter and gases into the overlying mantle, causing partial melting 

 and fractionation, and creating silica-rich rocks, ore bodies, and 

 explosive volcanism along the overlying volcanic arc. 



As oceanic crust ages and moves away from the ridge axis, it 

 modifies Earth's environment. The chemistry of seawater is al- 

 tered as the oceanic crust cools and exchanges elements with the 

 seawater that circulates through it (see "Directions for Marine 

 Geochemistry"). At convergent margins, some sediment is scraped 

 off the subducting crustal plates, injecting fluids rich in dissolved 

 constituents into the overlying ocean waters. Moreover, the ag- 

 ing oceanic lithosphere serves as a repository for sediments that 



