Sensitivity of δ15N of nitrate, surface suspended and deep sinking particulate nitrogen to seasonal nitrate depletion in the Southern Ocean -: art. no. 1081

[1] We report measurements of the delta(15)N of nitrate, suspended particulate nitrogen (PN), and sinking PN from cruises and moored sediment traps in the Subantarctic Zone (SAZ) and Polar Frontal Zone (PFZ) south of Australia. As expected, surface water nitrate delta(15)N increased as nitrate was consumed during the spring/summer bloom. In contrast, the seasonal cycles of surface water suspended and sinking PN delta(15)N did not fit expectations from nitrate assimilation alone. Rather than increasing, the delta(15)N of surface suspended PN was relatively constant in the SAZ (at similar to1parts per thousand), and decreased during the summer in the PFZ (from similar to0 to similar to -4parts per thousand), most likely due to the production of low N-15 PN by summertime ammonium recycling. Deep sediment trap PN delta(15)N also displayed seasonal decreases (from similar to4 to similar to1parts per thousand in the SAZ, and from similar to3.5 to similar to0.5parts per thousand in the PFZ), which correlated with PON flux magnitude. During high-flux periods, exported PN delta(15)N values were close to expectations from nitrate-based export, but low-flux periods exhibited higher delta(15)N, consistent with either a reduction in the isotope effect of nitrate assimilation or more extensive isotopic alteration of the sinking material during low-flux periods. The mass balance between net nitrate supply and exported PN that links sinking flux delta(15)N to nitrate utilization requires only that the annually integrated ( rather than the seasonally varying) sinking flux of PN delta(15)N correlates with nitrate depletion. While a correlation between annually integrated sinking PN delta(15)N to nitrate depletion was observed in both the SAZ and PFZ, the sensitivity of sinking PN delta(15)N to nitrate depletion was lower than expected. Moreover, the seasonal observations raise the possibility that loss of the summertime high-flux period represents an alternative explanation to increased nitrate utilization for the high sedimentary PN delta(15)N observed during glacial periods.