Abstract
Abstract. Continental shelves play a key role in the cycling of nitrogen and carbon. Here the physical transport and biogeochemical transformation processes affecting the fluxes into and out of continental shelf systems are reviewed, and their role in the global cycling of both elements is discussed. Uncertainties in the magnitude of organic and inorganic matter exchange between shelves and the open ocean is a major source of uncertainty in observation-based estimates of nitrogen and carbon fluxes. The shelf-open ocean exchange is hard to quantify based on observations alone, but can be inferred from biogeochemical models. Model-based nitrogen and carbon budgets are presented for the Northwestern North Atlantic continental shelf. Results indicate that shelves are an important sink for fixed nitrogen and a source of alkalinity, but are not much more efficient in exporting organic carbon to the deep ocean than the adjacent open ocean for the shelf region considered.
Highlights
Continental shelves, conventionally defined as ocean areas shallower than 200 m water depth, are the shallow, gently sloping extensions of the continental plates and include marginal seas like the North Sea and the East China Sea
The average horizontal divergence of organic carbon (OC) is 3.61, 9.21 and 4.73×1010 mol C yr−1 for the Mmixiidngdline thAe tBlaaynotifc Bight (MAB), the region combining Georges Bank and the Gulf of Maine, and the Scotian Shelf, respectively. These results indicate that the relative contribution of the biologically driven Continental Shelf Pump (CSP) is smaller on the Scotian Shelf than in the stratified MAB, and that tidal mixing on Georges Bank significantly increases the strength of the biological CSP, enhancing uptake of atmospheric CO2 locally
Two significant sources of uncertainty in the model simulations described above are (1) the choice of initial and boundary conditions for dissolved inorganic carbon (DIC) and alkalinity, which are based on climatological data mostly from the open ocean, and Observation-based budgets and biogeochemical model simulations suggest that continental shelves act as a barrier for nitrogen inputs to the ocean from land
Summary
Continental shelves, conventionally defined as ocean areas shallower than 200 m water depth, are the shallow, gently sloping extensions of the continental plates and include marginal seas like the North Sea and the East China Sea. Continental shelves serve as important link between land and the ocean interior and between the atmosphere and deep ocean (Liu et al 2000), and play a key role in the global cycling of biologically essential elements such as nitrogen and carbon. Human-induced pressures include coastal eutrophication and development of hypoxia and anoxia, overfishing and acidification The objective of this manuscript is to review the role of continental shelves in global nitrogen and carbon cycling by first summarizing insights based on direct observations The annual afimguprleitude in sea surface temperature is largest in coastal regions of the MAB and on the Scotian Shelf; tidal mixing in the Bay of Fundy (northern Gulf of Maine) and on Georges Bank lead to comparatively smaller seasonal temperature differences there
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