Respiration and denitrification in permeable continental shelf deposits on the South Atlantic Bight: Rates of carbon and nitrogen cycling from sediment column experiments

Nitrogen (N) cycling and respiration rates were measured in sediment columns packed with southeastern United States continental shelf sands, with high permeability (4.66 x 10(-11)m(2)) and low organic carbon (0.05%) and nitrogen (0.008%). To simulate porewater advection, natural shelf seawater was pumped through columns of different lengths to achieve fluid residence times of approximately 3, 6, and 12 h. Experiments were conducted seasonally at in situ temperature. Fluid flow was uniform in nearly all columns, with minimal dead zones and channeling. Significant respiration (O-2 consumption and Sigma CO2 production) occurred in all columns, with highest respiration rates in summer. Most (78-100%) remineralized N was released as N-2 in the majority of cases, including columns with oxic porewater throughout, with only a small fraction released as NO3- from some oxic columns. A rate of 0.84-4.83 x 10(10) mol N yr(-1), equivalent to 1.06-6.09 x 10(-6) mmol N cm(-2) h(-1), was calculated for benthic N-2 production in the South Atlantic Bight, which can account for a large fraction of new N inputs to this shelf region. Metal and sulfate reduction occurred in long residence time columns with anoxic outflow in summer and fall, when respiration rates were highest. Because permeable sediments dominate continental shelves, N-2 production in high permeability coastal sediments may play an important role in the global N cycle. (C) 2007 Elsevier Ltd. All rights reserved.