Denitrification in continental shelf sediments has major impact on the oceanic nitrogen budget

Abstract

Denitrification rates in sediments within the oxygen deficient waters off Mexico and from the Gulf of Maine were investigated on the basis of interstitial nutrient profiles. Nitrate fluxes into the sediments were calculated from gradients across the sediment‐water interface and vertical molecular diffusion coefficients and averaged 0.151 (Mexico) and 0.0920 (Gulf of Maine) pmol NO−3 cm−2 s−1. These are minimum values, since these gradients may have been underestimated. In the Gulf of Maine, bottom water irrigation by macrobenthos increases the nitrate supply well above this estimate. In addition, only 15‐22% of the expected ammonium is present in Gulf of Maine sediments perhaps because of removal by a rapid coupling of nitrification with denitrification. This large apparent loss of the regenerated ammonium appears to be ubiquitous in shelf sediments with oxygenated bottom water. The global denitrification rate in continental shelf sediments was reassessed to be >50 Tg N yr−1 (1 Tg = 1012 g), demonstrating that sediments are an important sink for oceanic nitrogen. Globally, current nitrogen losses from the oceans may exceed inputs by 60‐90 Tg N yr−1. Over the glacial‐interglacial cycle the global sedimentary denitrification rate probably varied commensurately with the changing continental shelf area. An oscillating oceanic nitrogen budget over these time scales could occur given the sequence of (1) scouring and dumping of terrestrial nitrogen into the oceans during glacial advance, (2) removal of oceanic combined nitrogen to the atmosphere by denitrification following glacial retreat, and (3) reincorporation of this N into terrestrial biomass during the interglacial period.