SECT. 3] BASIN SEDIMENTATION AND DIAGENESIS 597 



However, some interesting results were recently presented by Hodgson (1959) 

 who was able to carry out the conversion in the laboratory by varying the 

 temperature, pH and trace -metal content, using a lake gyttja. 



In the southern California basins, the porphyrins and hydrocarbons have a 

 similar distribution (Emery, 1960); their content is highest in the basins closest 

 to shore, they are preserved best in a reducing environment and they diminish 

 with depth most rapidly in the first few centimeters. 



B. Bacteriological Processes 



a. Aerobic and anaerobic metabolism 



Bacteria are without doubt the most efficient scavengers on the ocean floor. 

 In aerobic environments they compete for organic matter with fungi and 

 protozoa, as well as metazoa, but under stagnant conditions they carry out 

 their processes without challenge. It is of little value to enumerate aU the known 

 organisms, at the best such an effort would be totally inadequate. Reviews on 

 the subject have been written by Waksman (1934), ZoBell (1946), Brisou (1955) 

 and Wood (1958). As with many other aspects of oceanography, the studies 

 have so far been largely empirical, and the field is open for the perusal of 

 fundamental problems. 



It is generally accepted that the phytoplankton and the zooj)lankton con- 

 stitute the major source of organic matter to oceanic sediments. Close to shore, 

 land plants and sessile algae may also be washed into the sediments. This 

 matter represents organic molecules ranging from very simple structures to 

 those which are stable and highly complex. 



Lithified sediments, like shales, generally do not contain unaltered organic 

 matter. The changes that have occurred can in part be due to the lithification 

 process and in part to diagenetic changes which took place prior to lithification. 

 Since the Organic matter even in very recent unconsolidated sediments has 

 largely lost its identity, it can be supposed that the diagenetic processes, having 

 bacteria among their main agents, are the most significant. 



Two groups of bacterial processes are involved in degradation, the aerobic 

 in the presence of molecular oxygen and the anaerobic in its absence. During 

 aerobic metabolism reduced substances of diverse structure are converted by 

 stepwise processes to a few common intermediates which enter into terminal 

 oxidation pathways, the tricarboxylic acid cycle being the most important, 

 with their ultimate conversion to carbon dioxide. Since the energy release is 

 greatest during the aerobic processes, the destruction of organic matter under 

 these conditions is more complete and generally more rapid. Anaerobic 

 metabolism must result in the formation of end products more reduced as well 

 as more oxidized than the starting substrates, and the typical fermentation 

 process causes the accumulation of simple organic molecules such as fatty 

 acids. Thus the breakdown of organic matter is not usually as complete 

 anaerobically, although in the special situations of sulfate reduction and de- 

 nitrification, where the reduced product is inorganic, this need not be so. 



