288 



Sediments 



from sewage and other media (Young, Pratt, 

 and Biale, 1951; Davis and Squires, 1954). 



Carbon Atoms 



A comparison of the nature of carbon 

 atoms in organic matter and in extracts of 

 it provides information about diagenetic 

 changes not obtainable only a few years ago. 

 Data on three aspects are available: number 

 of carbon atoms per molecule; radiocarbon 

 isotope, C^^; and stable carbon isotope, C'^ 



When samples of hydrocarbons are exam- 

 ined with a mass spectrometer, the number 

 of carbon atoms per molecule can be meas- 

 ured. Using this method, Stevens, Bray, and 

 Evans (1956) discovered that the normal 

 paraffins (w-heptane fraction of chromatog- 

 raphy) from soils and marine muds of the 

 Gulf of Mexico have a decided predomi- 

 nance of molecules containing an odd num- 

 ber of carbon atoms. Frequency peaks are 

 highest for 29 and 3 1 carbon atoms but are 

 present for 27, 25, and 23 also. The odd- 

 even ratio over this range for the sample of 

 marine mud that was figured is 3.4. In con- 

 trast, no such preference is exhibited by 

 petroleums that were studied. The paraffin 

 fraction from a plankton sample collected off" 

 the east coast of Florida exhibited only a 

 slight predominance of odd numbers of car- 

 bon atoms, the odd-even ratio being 1.1. 

 Thus, the odd carbon atoms are accentuated 

 in marine muds and soils over those in 

 plankton, and no preference seems to be 

 present in petroleum. A similar variation of 

 odd-even preference in sediments from the 

 southern California basins has been found 

 by Mr. Ellis Bray of Magnolia Petroleum 

 Company, but results have not yet been 

 published. 



The radiocarbon isotope, C^\ has been 

 measured in a few hydrocarbon extracts 

 from organic matter in sediments. First was 

 the discovery by Smith (1954) that hydrocar- 

 bons present in a 33-meter core of Recent 

 sediments at Grande Isle, Louisiana, had 

 radiocarbon ages of 1 1,800 to 14,600 ± 1400 

 years. A composite sample of nonextracta- 

 ble organic matter had an average age of 

 9200 ± 1000 years, and of carbonate 12,300 



± 1200 years. Similarly recent ages of 3100 

 to 9300 years were obtained by Stevens, 

 Bray, and Evans (1956) for hydrocarbons 

 extracted from muds flooring the continental 

 shelf of the Gulf of Mexico. Age determi- 

 nations of hydrocarbons from sediments of 

 the California basins are not available, but 

 Magnolia Petroleum Company (N. P. Ste- 

 vens, personal communication) made meas- 

 urements on 15 organic extracts with result- 

 ing ages ranging between 4310 and 13,880 

 years and averaging 8550 years. 



The stable isotope C^^ is far more abun- 

 dant than the radioisotope C", but it is only 

 about 1 per cent as abundant as C^-. The 

 diff'erence in atomic weight permits measure- 

 ments of the relative abundance of C'^ and 

 C^- by means of a mass spectrometer. 

 Studies of the C^VC^- ratio by Craig (1953) 

 and others have shown a range of values for 

 different materials. Because of the aid that 

 C^VC^- ratios might provide in understand- 

 ing the diagenesis of organic matter, part of 

 a grant from the National Science Founda- 

 tion was used to defray the costs of their 

 measurement along with the radiocarbon 

 ages of basin sediments by T A. Rafter of 

 New Zealand Department of Science and 

 Industrial Research. His analyses (Table 

 28) are accurate within about \%c. Special 

 analyses of extracts and hydrocarbon frac- 

 tions were kindly made by S. Silverman of 

 California Research Corporation with an 

 especially sensitive mass spectrometer con- 

 sidered to be accurate to 0. \%o. The results, 

 given in Table 28, are expressed as 



8 in per mil (%o) = 



C^VO- sample - C/O' standard 



C^VC>- standard 



- X 1000 



The standard to which all values have been 

 adjusted is the one in use at the University 

 of Chicago, a Cretaceous belemnite. Values 

 of 8 in nature range mostly between -|- 5 and 

 — 50, with the more negative ones indicating 

 lower contents of C" with respect to C^- (or 

 fighter carbon). 



Craig (1953) showed that 8 in marine 

 plants (measured mostly for sessile algae) 

 ranges between —7.6 and — 16.5%o, about 

 the same as in marine invertebrates but far 



