Nitrogen isotope biogeochemistry of the antarctic polar frontal zone at 170°W

During the US JGOFS AESOPS project, an intensive study was conducted of the nitrogen isotope biogeochemistry of the polar frontal region south of New Zealand and north of the Ross Sea. Repeated cruises and crossings of the study area permitted observation of the seasonal drawdown of NO3- and corresponding increases in delta (NO3-)-N-15 due to isotopic fractionation during phytoplankton uptake. Moored sediment traps placed at four locations crossing the polar front between 57 degrees and 66 degreesS provided a well-resolved time-series for both sinking particle flux and delta N-15 for comparison to water column results. Summertime increases in near-surface delta (NO3-)-N-15 at each site ranged from 1.2 parts per thousand to 2.8 parts per thousand and corresponded to reductions in surface [NO3-] of 16% to 37%. Both seasonal variations in delta (NO3)-N-15 as well as summertime vertical variations in the upper 100 m were used to estimate the fractionation factor (epsilon). epsilon ranged between 6 parts per thousand and 8 parts per thousand assuming closed system dynamics and 7 parts per thousand to 10 parts per thousand assuming open system dynamics. The lower range more likely reflects actual values for epsilon since NO3- drawdown is closely associated with stratification of the upper water column and is similar to recent estimates for this region. Sinking particle delta N-15 varied with particle flux and ranged from -1 parts per thousand to 5 parts per thousand. Average delta N-15 weighted by N fluxes were skewed toward summertime values, between -0.1 parts per thousand and 1.7 parts per thousand. To compare with theoretical expectations, differences between average trap delta N-15 and the delta N-15 of NO3- prior to depletion (Delta delta N-15) were calculated for each site. Observed Delta delta N-15 vs. relative surface NO3- utilization (u) for the four sites studied plotted at the upper envelope of the theoretical relationships for both closed and open system equations using observed ranges in epsilon. The nitrogen isotopic signal transmitted to the sediments is quantitatively consistent with the degree of NO3- depletion in Southern Ocean waters. This comparison was also relatively insensitive to choice of open- or closed-system models due to the relative low values of u (0.2-0.4). These results validate previous interpretations of downcore delta N-15 in the polar Southern Ocean as evidence for a large increase in relative nutrient utilization during the last glacial maximum. (C) 2001 Elsevier Science Ltd. All rights reserved.