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72 OCEANOGRAPHY IN THE NEXT DECADE 



ids. Details of this process are not understood. A major un- 

 known is the mechanism responsible for maintaining the salinity 

 and temperature of the vent fluids at a given site at stable levels 

 over periods of years. 



Low-temperature weathering of the oceanic crust appears to 

 constitute a major sink for alkali compounds and is accompanied 

 by extensive hydration of the rocks. When the crust is subducted, 

 this chemically bound water, along with the elements it can transport, 

 most likely is released and migrates. The water may accelerate 

 melting and participate in the eruption process in volcanic arcs. 



Marine Organic Substances 



The biochemicals that fuel marine organisms are photosyn- 

 thesized and then respired in the upper ocean on time scales of 

 hours to days. Only about 20 percent of the photosynthetic prod- 

 uct escapes from the sunlit surface ocean as sinking particles, and 

 less than 0.5 percent is ultimately preserved in marine sediments. 

 Living organisms comprise only about 1 percent of the organic 

 matter in the ocean. The remaining organic matter is primarily a 

 dilute solution (about 1 part per million) of "dissolved" macro- 

 molecules (i.e., material that passes through filters with a pore 

 size of 0.5 micrometer). The turnover rate of this dissolved pool 

 is now under discussion; the traditional view is that the pool 

 turns over at a rate of thousands of years. The alternative view is 

 that, because the pool of dissolved organic material contains ex- 

 cess carbon- 14 relative to what is expected in thousand-year-old 

 organic material, it must turn over more rapidly. Because of the 

 challenges of isolating or directly characterizing this extremely 

 dilute component of seawater, only about 20 percent of the or- 

 ganic molecules have been described. 



A little over a decade ago, a novel suite of organic lipids was 

 first reported in sediments from the Atlantic Ocean and the Black 

 Sea. The component molecules have a linear sequence of 37 to 39 

 carbon atoms containing one to four double bonds, with an oxy- 

 gen atom doubly bonded to the second or third carbon in the 

 chain. These long-chain alkenones were found to be produced by 

 the marine coccolithophorid algae Emiliania huxleyi and related 

 species that are widely distributed in tropical and subtropical oceans. 

 The same molecules were also discovered in sediments dating 

 back to the Miocene (about 20 million years ago). 



It was later demonstrated in the laboratory that the average 

 number of double bonds (extent of unsaturation) in these alkenones 



