SIMULTANEOUS NITRATE AND OXYGEN RESPIRATION IN COASTAL SEDIMENTS - EVIDENCE FOR DISCRETE DIAGENESIS

Oxygen and nitrate porewater profiles from sediments of Puget Sound, the Washington continental margin, and the Chukchi Sea were determined using a whole core squeezing apparatus. The two oxidants were observed to have equal sediment penetration depths and similar profile shapes in nearly all cores. Oxygen and nitrate, therefore, behaved in a similar manner in these shallow sediments: an observation that is not consistent with existing models of sediment diagenesis. A two-dimensional model was constructed in which nearly all oxidant (O-2 and NP3-) consumption took place in scattered, highly reactive discrete micro-sites. The model produced a sedimentary environment in which reactions at each micro-site were limited by oxidant concentrations with oxidant gradients extending well beyond the micro-sites into relatively nonreactive bulk sediments. Thus for a given depth surface within the sediment, oxygen concentrations were much lower at micro-sites than average concentrations on that surface. Furthermore, at most micro-sites oxygen concentrations were sufficiently low enough to permit simultaneous denitrification, which explained the apparent similarity between oxygen and nitrate concentration profiles within these sediments. The model suggests that a relatively few, short-lived reaction sites are responsible for most oxygen and nitrogen reduction within oxic sediments under shallow seas, and it is consistent with emerging concepts about the fate of organic carbon in coastal sediments.