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 δ15NO3− due to isotopic fractionation during phytoplankton uptake. Moored sediment traps placed at four locations crossing the polar front between 57° and 66°S provided a well-resolved time-series for both sinking particle flux and δ15N for comparison to water column results. time increases in near-surface δ15NO3− at each site ranged from 1.2‰ to 2.8‰ and corresponded to reductions in surface [NO3−] of 16% to 37%. Both seasonal variations in δ15NO3− as well as summertime vertical variations in the upper 100 m were used to estimate the fractionation factor (ε). ε ranged between 6‰ and 8‰ assuming closed system dynamics and 7‰ to 10‰ assuming open system dynamics. The lower range more likely reflects actual values for ε since NO3− drawdown is closely associated with stratification of the upper water column and is similar to recent estimates for this region. g particle δ15N varied with particle flux and ranged from −1‰ to 5‰. Average δ15N weighted by N fluxes were skewed toward summertime values, between −0.1‰ and 1.7‰. To compare with theoretical expectations, differences between average trap δ15N and the δ15N of NO3− prior to depletion (Δδ15N) were calculated for each site. Observed Δδ15N 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 ε. 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 δ15N in the polar Southern Ocean as evidence for a large increase in relative nutrient utilization during the last glacial maximum.