Isotopic fractionation of oxygen and nitrogen in coastal marine sediments

The bulk and isotopic exchanges of O-2, N-2, NO3- and NH4+ between sediments and the overlying water (benthic flux) were measured at three shallow locations in Puget Sound using a benthic tripod. Oxygen consumption by the sediments averaged 4.4 mmol m(-2) d(-1), while N-2 gas fluxes averaged 1.1 mmol m(-2) d(-1). The nitrogen gas flux out of the sediments was always greater than the corresponding NO(3)(-)flux into the sediments, indicating a large nitrification contribution to the N, flux. Isotopic changes in overlying water delta(18)O of O-2 were small, with an average apparent sediment respiration fractionation-factor, epsilon, of 3. This fractionation-factor is much smaller than that measured in either the open ocean or in the laboratory. The low degree of fractionation is hypothesized to occur because of the interaction of diffusive layers around reactive microsites within these sediments. There was no apparent isotopic NO3- (epsilon = 0), while the ammonium that diffused out of the sediments averaged 4.5% heavier than both source organic matter within the sediments and overlying water NO;. The large discrepancy in isotopic composition between the source organic material and NH4- can be attributed to significant isotopic fractionation of ammonium during nitrification in the oxic zone. Finally, the delta(15)N of N-2 did not change significantly over the course of the incubations, in agreement with the nitrogen isotopic budget for these sediments. (C) 1997 Elsevier Science Ltd.