ISOTOPIC FRACTIONATION OF OXYGEN AND NITROGEN IN COASTAL MARINE-SEDIMENTS

The bulk and isotopic exchanges of O-2, N-2, NO3- and NH4+ between sed iments and the overlying water (benthic flux) were measured at three s hallow locations in Puget Sound using a benthic tripod. Oxygen consump tion by the sediments averaged 4.4 mmol m(-2) d(-1), while N-2 gas flu xes averaged 1.1 mmol m(-2) d(-1). The nitrogen gas flux out of the se diments was always greater than the corresponding NO(3)(-)flux into th e sediments, indicating a large nitrification contribution to the N, f lux. Isotopic changes in overlying water delta(18)O of O-2 were small, with an average apparent sediment respiration fractionation-factor, e psilon, of 3. This fractionation-factor is much smaller than that meas ured in either the open ocean or in the laboratory. The low degree of fractionation is hypothesized to occur because of the interaction of d iffusive layers around reactive microsites within these sediments. The re was no apparent isotopic NO3- (epsilon = 0), while the ammonium tha t diffused out of the sediments averaged 4.5% heavier than both source organic matter within the sediments and overlying water NO;. The larg e discrepancy in isotopic composition between the source organic mater ial and NH4- can be attributed to significant isotopic fractionation o f ammonium during nitrification in the oxic zone. Finally, the delta(1 5)N of N-2 did not change significantly over the course of the incubat ions, in agreement with the nitrogen isotopic budget for these sedimen ts. (C) 1997 Elsevier Science Ltd.