Importance of continental margins in the marine biogeochemical cycling of carbon and nitrogen

THE continental margins occupy less than 20% of the surface area of the world ocean, and it is widely assumed that they do not play a significant part in the oceanic biogeochemical cycles of carbon and nitrogen. Data from 32 sediment-trap moorings, 16 in the deep sea and 16 on the continental slope1, suggest that at an average depth of 2,650 m on the slope, the combined rain of surviving shelf and slope particles yields a mean carbon flux of 6.9 g C m−2 yr−1—about ten times that at the same average depth in the deep sea (0.8 g C m−2 yr−1). Because the area of the deep sea is about ten times greater than that of the continental slopes, using the sediment-trap data and assuming a carbon/nitrogen ratio of 5:1, the equivalent total particulate offshore nitrogen loss is 0.5 × 1014 g N yr−1 at 2,650 m. If these trap observations are generally representative of the oceans and continental margins, then the supply of dissolved nitrate to the overlying euphotic zones should also be similar. Here I provide an independent estimate of the annual supply of onwelling nitrate from the deep sea to the shelves and find that it may balance the offshore flux of carbon, suggesting that the continental margins and deep sea are equally important in the carbon and nitrogen biogeochemical cycles.