The fate of carbon in continental shelf sediments of eastern Canada: a case study

Abstract

This paper discusses the effects of organic carbon oxidation on the dissolution and precipitation of calcium carbonate (aragonite and calcite) in fine-grained continental shelf and slope sediments, using data obtained during the Canadian Joint Global Ocean Flux Study program in the Gulf of St. Lawrence and on the Scotia shelf. The oxygen-penetration depth in these sediments is on the order of 10-15 mm, indicating that organic carbon is mineralized aerobically within this interval. Below this depth, organic matter degradation proceeds mostly through anoxic mineralization processes. The organic carbon content of these sediments decreases smoothly with depth. At all sites, the bottom water is supersaturated with respect to calcite. However, in most cores, immediately below the sediment-water interface, the acidity produced by the aerobic degradation of organic matter is sufficient to overcome the supersaturation of the overlying waters and induce CaCO3 dissolution. This is most strongly reflected by an increase in the porewater calcium concentration near the sediment-water interface. Deeper in the cores, the saturation state of the porewaters increases at depth as a result of alkalinity generation by sulfate reduction and CaCO3 of precipitates. Unlike organic carbon, the inorganic carbon content of the sediments therefore varies little or even increases with depth because what is lost through dissolution near the sediment-water interface is replaced at depth by precipitation. The carbon precipitated as CaCO3 in the sulfate-reduction zone originates in part from the organic carbon, resulting in the preservation of a fraction of the original organic carbon as inorganic carbon. The precipitation of CaCO3 in the anoxic zone creates an additional source of CO2 to the porewaters. The geochemical significance of this source is discussed.