Mineral associations and nutritional quality of organic matter in shelf and upper slope sediments off Cape Hatteras, USA: a case of unusually high loadings

Relationships among organic carbon (OC), enzymatically hydrolyzable amino acid (EHAA) concentrations and mineral surface area (SFA) were assessed for sediments from the shelf and slope region near Cape Hatteras, USA. Grain size, measured here as mineral-specific surface area, explained 55% of the variance in organic matter concentrations. Organic loadings, as ratios of organic carbon to surface area (OC:SFA), decrease with water column and core depth. OC:SFA ratios in this region are comparable to those found in areas with anoxic water columns, and are the highest reported for shelf-slope sediments underlying oxygenated water columns. With increasing water-column depth, organic matter becomes progressively incorporated into low-density (<2.4 g cm−3), organomineral aggregates, reflecting the increase in clay content in sediments with water-column depth. Organic coverage of mineral surfaces was determined by gas sorption methods; throughout the depth range minerals are essentially bare of organic coatings, in spite of high organic loadings. EHAA concentrations increase with water-column depth in a similar fashion as OC concentrations, and help to support intense heterotrophic communities at depth. Rapidly decreasing ratios of EHAA to total organic matter with water-column depth are consistent with previous inferences that largely refractory organic matter is exported from the shelf to the slope. Significant burial of EHAA downcore indicates protection of enzymatically hydrolyzable biopolymers with depth.