Vineyard (10 to 15 ppm and 100 to 2,000 ppb), at canyon heads, and along the 

 slope and rise (10 to 30 ppm and 50 to 500 ppb). In general, the PAH 

 compounds are of a nonfossil source indicating combustion as a likely origin. 



SEASONAL TRENDS 



Seasonal trends are different for total and PAH hydrocarbons. Total 

 hydrocarbons and terrigenous plant materials do not vary seasonally and appear 

 to be in an overall steady state (Figure 17). Pristane (a marine biogenic 

 hydrocarbon) does vary seasonally from a spring high to a winter low (Figure 

 18). This seasonality is presumably determined by seasonal cycles of erosion 

 and deposition. Pristane is eventually transported to depositional areas to 

 the west and southwest of Georges Bank. Sporadic incidence of petroleum 

 residues (e.g., tar balls) are observed but are relatively short-lived. 

 Indirect evidence (hydrocarbon chromatograms in conjunction with sediment 

 textural data) suggests that anthropogenic hydrocarbons are associated with 

 the easily resuspended fine sediments while biogenic hydrocarbons associate 

 with the coarser fractions (Table 2). It has been estimated that 40 to 50 

 percent of the organic matter is resuspended and removed from the shelf to the 

 slope during fine-grained sediment transport. 



53 



