56 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 27 



Calcium Carbonate 



Dried and weighed sediment samples were treated with sulphuric 

 acid, heated, and the evolved carbon dioxide was measured volumetrically. 

 From these volumes the percentages of calcium carbonate were computed 

 on the assumption that all of the carbonate was combined with calcium. 



The results (Fig. 20) exhibit a range from to 36 per cent calcium 

 carbonate. Nearly all values lower than 10 per cent are from canyons 

 along the mainland. Most values higher than 10 per cent are from the 

 offshore Santa Cruz, San Clemente, and Tanner canyons. As a secondary 

 trend, the higher percentages for nearshore canyons occur in the finer- 

 grained samples, and for the offshore canyons they are in the coarser- 

 grained samples. Calcium carbonate grains coarse enough to be identified 

 as to source organism consist dominantly of shell fragments in the coarse 

 sediments and of foraminiferal tests in the fine sediments. 



Organic Matter 



The content of organic matter in the sediment samples was measured 

 as nitrogen using micro-Kjeldahl equipment and as carbon using a Leco 

 (Laboratory Equipment Company) carbon analyzer. The latter device 

 measures the carbon dioxide evolved by fusing the sample at 1300° C in 

 an induction furnace. Kjeldahl nitrogen would serve as an excellent 

 measure of total organic matter except that nitrogen constitutes only 

 about 6 per cent of total organic matter and it is more subject to oxida- 

 tion than is carbon, as indicated by an increase of C/N ratio with depth 

 of sediment burial or lapsed time (Emery, 1960a). Carbon comprises 

 about 55 per cent of total organic matter but it is very difficult to 

 measure satisfactorily, owing to the difficulty of combusting some car- 

 bonaceous materials and to the variable ease by which carbon is released 

 from calcium carbonate. As a result, organic carbon in samples was 

 measured in two different ways: by combusting the residue left from 

 carbonate analysis (direct method), and by combusting a total sample 

 and subtracting carbonate carbon (difference method) . The direct method 

 may yield results that are too low owing to partial breakdown of or- 

 ganic matter by the acid treatment for carbonate, or too high because of 

 incomplete breakdown of carbonate carbon by the acid. The second 

 method can yield erratic results because of the need for two separate 

 subsamples. 



In general, the results by the two methods of carbon analysis agree 

 (Fig. 21), but there are some individual variations and the direct method 

 is considered the more reliable. A plot of direct organic carbon against 

 Kjeldahl nitrogen (Fig. 22) reveals good agreement for about 95 per 



