411 



FUTURE DIRECTIONS IN OCEAN SCIENCES 77 



tion processes and the mechanisms of sediment uptake must be 

 established for each tracer; this will be difficult experimental work. 

 Postdepositional redistribution processes must be characterized 

 for the range of sedimentary environments. The establishment of 

 reliable proxy records merits high priority because it is the most 

 direct constraint on the modeled mechanisms of previous global 

 changes in environmental processes. 



DIRECTIONS FOR MARINE GEOLOGY 

 AND GEOPHYSICS 



Summary 



The plate tectonic paradigm, first quantitatively described more 

 than 25 years ago, provides an integrated physical and chemical 

 framework for understanding the geological evolution of Earth. 

 Marine geologists and geophysicists played a critical role in the 

 development of this paradigm. By linking marine magnetic anomalies 

 to geomagnetic reversals of Earth's magnetic field, marine geo- 

 physicists were able to confirm seafloor spreading and provide 

 quantitative estimates of seafloor spreading rates. Through holes 

 drilled by the Deep Sea Drilling Project, marine geologists were 

 able to extend the geomagnetic reversal time scale back nearly 

 200 million years, providing a framework within which to recon- 

 struct the past positions of the continents and the opening and 

 closing of ocean basins. 



Throughout most of the 1970s, the major emphasis in marine 

 geology and geophysics was on large-scale kinematic descriptions 

 of relative plate motions and their consequences for the geologi- 

 cal evolution of ocean basins. However, by the 1980s, the focus 

 of the field had shifted toward more process-oriented studies cen- 

 tered around understanding how oceanic crust and lithosphere are 

 created, how these processes are related to the underlying mantle, 

 and the consequences of seafloor spreading on ancient ocean cir- 

 culation and climate. Four themes currently dominate research 

 in marine geology and geophysics: (1) the formation of oceanic 

 crust and lithosphere along mid-ocean ridges, and the associated 

 volcanic, hydrothermal, and biological processes; (2) off-ridge pro- 

 cesses and their relation to mantle convection; (3) the structure 

 and tectonics of active and passive continental margins; and (4) 

 the record of past climate change and ocean circulation preserved 

 in marine sediments. In addition to these four themes, there is 

 increasing interest in the study of coastal processes including sediment 



