ids. Details of this process are not understood. A major unknown 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 photosynthesized and then respired in the upper ocean on time scales of hours to days. Only about 20 percent of the photosynthetic product 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" macromolecules (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 excess 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 organic 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 oxygen 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

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