Sources of organic matter to surficial sediments from the Scotian Shelf and Slope, Canada

Abstract

On the basis of chemical analyses of sediments sampled by grab and by box corer on the Scotian Shelf and Slope off Nova Scotia, Canada, an attempt is made to assign to source the organic matter (OM) found in surface and near-surface sediments. Concentrations of organic carbon and nitrogen were higher in Shelf basins and the Laurentian Channel than on the Shelf banks and Slope. Organic carbon/nitrogen (C/N) ratios within the basins and Channel imply OM is predominantly marine in origin. There is no progressive decrease in C/N across the shelf and down the slope that could be interpreted as due to declining terrestrial influence offshore, but a decrease with distance west of Cabot Strait implicates the Gulf of St Lawrence as a major source of terrigenous OM to the Shelf east of 62°W. The distribution of less-polar organic fractions ( n-alkanes, isoprenoids, fatty acids, sterols) in the Gulf indicates that there is OM of both marine and terrestrial origin in surface unconsolidated sediments with a dominance of the terrigenous component. This study of a well-characterized continental margin illustrates that existing estimates of the mass and composition of organic sedimentation world-wide are far from being robust. The distribution of OM in marine sediments is complex and it is premature to generalize about the proportion of terrigenous to marine OM preserved in oceanic sediments.