CARBON EXPORT FROM CONTINENTAL SHELVES, DENITRIFICATION AND ATMOSPHERIC CARBON-DIOXIDE

To investigate the long-term role of continental shelves, a global carbon box model which included continental shelf waters and sediments, ocean surface waters, the deep-sea and the atmosphere was constructed. With nitrogen limiting oceanic primary production, the model included balanced nitrogen inputs (from the continents and atmosphere) and losses (primarily via denitrification). Carbon export to the deep-sea (without deposition or burial in sediments) affected the average conditions found in the shelves but did not influence atmospheric carbon dioxide content. Similarly, redistribution of nitrogen inputs to the surface oceans had no major effect on atmospheric pCO(2). Tn contrast, atmospheric pCO(2) changes were caused by redistribution of denitrification rates between the continental shelf sediments and the deep-sea. When denitrification in continental shelf sediments increased from 10% of the total oceanic rate to 95%, shelf denitrification removed more nitrogen from the surface ocean, supporting less of a flux of sinking particulate carbon (SPC) into the deep-sea. This increased atmospheric pCO(2) by II ppm. When overall rates of nitrogen cycling were decreased by half to today's level, the atmospheric pCO(2) content was increased by an additional 5 ppm. These effects may have influenced the ice-ages. As the glaciers expanded, shelf denitrification was lessened by the reduction in continental shelf area. Moving the site of denitrification from the shelves to the deep-sea would have increased both oceanic new production and the SPC flux into the deep-sea, thereby lowering atmospheric pCO(2) levels during the initial periods of glaciation. Increased new production may have enhanced water column denitrification which in turn lowered the oceanic inorganic nitrogen content and restricted oceanic productivity. With less oceanic nitrogen, water column denitrification would have decreased resulting in a more equal proportioning of the total removal rate between the deep-sea and the continental shelves. Also, the total cycling rate of nitrogen would have lessened. In addition, inundation of the continental shelves during glacial retreat would have increased shelf denitrification. From the model, these trends would have released CO2 from the ocean, accentuating global warming and hastening the return to the interglacial climate.