Alternate Fate of Planktonlc Detritus: Organic Deposition and the Geological Record
Abstract:Extensive deposits of amorphous and fine grained organic matter called black shale are found in the geological record. These deposits were formed in open water marine and freshwater systems as evidenced by the presence of planktonic microfossils including zooplankton fecal pellets. Fecal pellets are easily identifiable particles found among sedimenting planktonic material. An examination of morphometric and ecological conditions favoring pellet deposition today allows us to reconstruct the paleoecological environments at the time black shale formed.
Pellet production is greatest where food is most abundant: during algal blooms and in areas of upwelling and outwelling. Because of their size, shape, and organic coat, pellets sink much more rapidly than their contents would if dispersed. Loss of organic matter from the pellets through microbial degradation and coprophagy is reduced by cool water temperatures and brief residence time in the water column. Pellet preservation in sediments is enhanced in areas with minimal physical disturbance, bioturbation, and microbial activity. These conditions are generally well met in anoxic, silled marine basins, trenches, fjords, and permanently anoxic lakes, especially if production, sedimentation, and burial occur at high rates. These are the likely paleoenvironments that produced black shales. An empirical model relating organic sedimentation to surface primary productivity and sedimentation depth is developed using sediment trap data from all planktonic environments.
Document Type: Research Article
Publication date: November 1, 1984
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