CHAP. 33] THE PRESERVED RECORD: PALEONTOLOGY OF PELAGIC SEDIMENTS 867 



chemically. In most sequences of sedimentary rocks of shallow-water origin, 

 planktonic microfossils are far outweighed in mass by the skeletons of benthic 

 organisms. These latter, owing principally to their usually larger size, have been 

 the subject of many investigations and form the basis for most interpretations 

 of paleoecology and age of sedimentary sequences. Finally, sediments deposited 

 in shallow water frequently have their character influenced profoundly by water 

 movements (shallow currents, wave action, etc.) which frequently change 

 intensity and direction; these movements rework, sort, and abrade the associated 

 fossils. 



In sediments of the deep-sea basins, on the other hand, skeletal remains of 

 benthic organisms practically never constitute a high proportion of the con- 

 tained fossils. The fossil assemblage here is composed principally of the remains 

 of a few groups of micro-planktonic organisms. The httle direct evidence at 

 present available indicates that in many areas the type of sediment accumula- 

 ting and the contained fossil assemblage have remained constant in most of their 

 characters over millions of years : and if a change of sediment type and fossil 

 content does occur, it is most frequently occasioned by a change in the calcium 

 carbonate compensation depth and concomitant increase or decrease of the 

 calcareous components as a whole. Another principal cause of changes in the 

 sediment at any locality is migration of the boundaries of the main surface 

 water masses of the open oceans: such changes apparently occur much less 

 frequently than their analogues in shallow and nearshore waters. The areal 

 extents of most of the uniform pelagic sedimentary units being deposited at the 

 present day are measurable in thousands of square miles, and there are reasons 

 to believe that this has been the case for as long as the ocean basins have 

 existed. All of the main groups of planktonic organisms contributing skeletal 

 remains to deep-sea sediments are ubiquitous, only their relative proportions 

 differing from one locality to another. Their frequency in sediments is controlled 

 largely by the extent of their dissolution during descent to the sea floor and 

 during their burial to a depth of a few centimeters below the sediment surface : 

 dilution by varying rates of supply of inorganic sedimentary components plays 

 a relatively minor role. Once deposited, deep-sea sediments and their associated 

 fossils are not so liable to disturbance by strong current action as are shaUow- 

 water deposits, though even at great dej)ths there are jjhysical agencies capable 

 of re-distributing sediments, as indicated in the section on "Reworking", 

 page 881. 



3. Objectives of Deep-Sea Paleontology 



Following the early observation that patterns of distribution of present-day 

 organisms are strongly dependent upon certain measurable physical and 

 chemical parameters, it was inevitable that natural historians should attempt 

 to deduce past environmental conditions through investigation of fossil 

 assemblages. A knowledge of past oceanic conditions is fundamental to many 

 aspects of oceanography, and, therefore, one of the most important objectives 

 29— s. m 



