Continental Margins — Review of Geochemical Settings

Abstract

The ocean margin, including the continental shelf, slope and rise, constitutes an essential boundary between the continents and the ocean basins and represents about 20% of the surface area of the marine system. It is characterized by an enhanced productivity and biological activity due to the input of nutrient from rivers and more importantly, from the transfer of nutrient-rich deep ocean waters. This transfer results from upwelling under favourable local wind conditions or from turbulent mixing at the shelf break due to wind-stress or internal waves mostly of tidal origin. Despite the great complexity of the ocean margin system, compilation of the literature provides coherent data sets indicating that the global primary production on the shelf may reach 6 – 7 GTC yr-1 and about 5 GTC yr-1 on the adjacent slope, as compared to 28 GTC yr-1 for the open ocean. Results on 15N incorporation experiments or nutrient budget calculations indicate that the f-ratio is most probably around 0.3 5 – 0.4. The export production estimates, mainly obtained from flux measurements in sediment traps deployed on the slope, are often lower than those for the new production, likely due to the rapid degradation of organic matter in the intermediate and deep water column and to the low trapping efficiency. Furthermore, organic matter may be exported from the shelf to the open ocean in surface waters or within the benthic boundary layer by resuspension and off-shore transport. These fluxes may not be recorded by the sediment traps. Exchange of CO2 with the atmosphere at the margins is complicated by the competition between high inputs of this-component from deep ocean water and river water, and its rapid removal by photosynthesis in a very productive area. In addition, a tentative mean global cycle for nitrogen on the shelf is presented and discussed. It confirms on a global scale the major role of deep open ocean water transfer versus continental input of this element. Finally the data collected in the literature demonstrate that the continental margins are privileged areas of production, deposition and burial of CaCO3, a component which has been often poorly considered in existing global carbon cycle assessments in the oceans.