606 3CAPLAN AND RITTENBERG [CHAP. 23 



anaerobic zones only. This is probably the first in situ estimate that has been 

 obtained. Much more work is required to establish a reliable figure that can be 

 used on a world-wide basis in order to balance the nitrogen cycle. This tech- 

 nique should ultimately prove important in determining the amount of mole- 

 cular nitrogen being fixed, both in the euphotic zone as well as at the nnul- 

 water interfoce. 



Fractionation of jiydrogcni isotopes by bacteria has been shown in the labo- 

 ratory. Using facultative organisms isolated from sediments of the Bahama 

 Bank, Cloud et al. (1958) showed that there was an enrichment of the lighter 

 isotope by a factor of 20 over its occurrence in sea-water in the metabolic 

 hydrogen produced. Such drastic fractionations could have some influence on 

 the H/D ratio of semi-isolated bodies of water, such as lagoons. 



Studies on the isotopes of carbon are of particular interest in tracing break- 

 down and accumulation processes of organic matter. It has been confirmed by 

 a number of workers that growing organisms are able to fractionate the carbon 

 atoms by concentrating the lighter isotope in the protoplasm and the heavier 

 one in the carbonate skeleton. Craig (1953) has shown that marine plants and 

 animals have a definite range for the i^c/isQ ratio of their total organic con- 

 stituents. Certain extracts of phytoplankton, however, notably hydrocarbons 

 and lipids, were found by Silverman and Epstein (1958) to be considerably en- 

 riched (15 to 20%o) in the lighter isotope with respect to the total protoplasm. 

 The organic matter in basin sediments is also known to be enriched in the lighter 

 isotope with respect to marine organisms (Emery, 1960; Silverman and Epstein, 

 1958). Chloroform-methanol extracts of the organic matter gave slightly higher 

 12C concentrations than the average total material and closely coincided with 

 the measin-ements made for petroleum. This can be used as an argument for the 

 origin of oil directly from the hydrocarbon and lipid fractions of the organic 

 matter. Carbon isotope work should be able to determine the importance of 

 bacterial processes in the deposition of carbonate, since metabolic CO 2 is 

 probably enriched in ^-C. This process has been proposed for the formation of 

 the limestone associated with the salt-dome deposits of Texas and Louisiana 

 (Feely and Kulp, 1957). Laboratory experiments have not yet substantiated 

 these predictions. 



There is a considerable literatin-e on the isotopes of sulfur, ^\•hich has most 

 recently been reviewed by Ault and Kulp (1959). The investigations have been 

 largely restricted to sulfur compounds in ancient sediments, ores and volcanic 

 exhalations. Comparatively few results exist for recent marine sedimentary 

 deposits. The investigations carried out on some sediments at Milford Sound by 

 Kaplan, Rafter and Hulston (19(50) can be used as an indication of the im- 

 portance of such studies. The analyses on four samples are given in Table I. 

 These represent surface sediments from stations A and G and the water over- 

 lying them. The results show that the isotope ratios for the sea-water sulfate 

 fall very close to the average of 21.76 given by Ault and Kulp (1959). The 

 slightly higher ratio for sample R240/1 may be due to the influence of inter- 

 mixing with fresh water, since the area i)i Avhich this restricted basin lies has an 



