CONTENT OF SILICA AND ORGANIC CARBON IN SEA-FLOOR SEDIMENTS 



123 J 20'W 



123"00' 



122'40' 



123'20'W 



123-00' 



122 40 



38 OO'N 



37 : '40' 



37 20' 



~~6& Contours of 

 silica content 

 (weight percent) 



Sample site 



^Contours of 

 organic-carbon 

 content (weight 

 percent) 



Sea-floor sediments on the Continental Shelf in the Gulf of the Farallones have high contents of silica 

 (Si0 2 ), especially adjacent to the Farallon Islands, where granitic sand has been shed from the islands. 

 The silica content of sediments is lower on the Continental Slope, because the clay minerals making 

 up much of the sea-bottom mud there are lower in silica. The higher silica content in sediments on the 

 slope at the south end of the map probably reflects sand moving down Pioneer Canyon. 



The content of organic carbon (carbon from the remains of organisms) in sea-floor sediments from the 

 Gulf of the Farallones is relatively high for an area of ocean adjacent to a continent, reflecting the high 

 biologic productivity supported by the nutrient-rich seasonal upwelling of the California Current system. 

 The finer-grained sediments on the Continental Slope in the gulf have higher contents of organic carbon 

 than the mostly sandy sediments on the Continental Shelf. 



Chemical Composition of Surface Sediments on the Sea Floor 27 



