78 



R. T. Prentki et al. 



Percent Organic Carbon 



The organic matter in the surface sediments of the intensively studied 

 ponds, measured as a percent of the ash-free dry weight (combusted at 525 

 to 550°C), was higher than that in other ponds in the vicinity (Figure 4-2). 

 It is likely that these high percentages, greater than 80% of the dry weight, 

 occur in ponds located near the center of the most recent former lake (see 

 discussion in Chapter 3). 



The changes in the bulk density of the surface sediments among ponds 

 were caused by changes in the organic content rather than by a change in 

 water content. Thus, the correlation between the percent organic matter 

 and the bulk density was —0.92 (n = 20). The percent organic matter (0 to 

 2 cm) equals (bulk density in mg dry wt cc ')( —0.01244) -I- 77.4. Organic 

 carbon was assumed to be 40% of the organic matter. Thus, a 1 -cm-thick 

 slice of the surface sediments in the intensively studied ponds contained 

 from300to566gCm"'. 



The percent organic carbon of sediments changes with depth but there 

 is an increase in the total amount of organic carbon (Table 4-2). This 

 apparent paradox is explained by the increase in bulk density with depth. 

 It is evident from these data that the top 4 cm are quite uniform with 

 respect to percent organic carbon; this is a result of the stirring activity of 



50 lOOm 



I I I I . I 



FIGURE 4-2. Percent of organic matter in the surface 

 sediments of Barrow ponds, 19 August 1973. 



