Sensitivity of δ~(15)N of nitrate, surface suspended and deep sinking particulate nitrogen to seasonal nitrate depletion in the Southern Ocean

摘要

We report measurements of the δ~(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 δ~(15)N increased as nitrate was consumed during the spring/summer bloom. In contrast, the seasonal cycles of surface water suspended and sinking PN δ~(15)N did not fit expectations from nitrate assimilation along. Rather than increasing, the δ~(15)N of surface suspended PN was relatively constant in the SAZ (at ～1‰), and decreased during the summer in the PFZ (from ～0 to ～-4‰), mist likely due to the production of low ~(15)N PN by summertime ammonium recycling. Deep sediment trap PN δ~(15)N also displayed seasonal decreases ( from ～4 to ～1‰ in the SAZ, and from ～3.5 to ～0.5‰ in the PFZ), which correlated with PON flux magnitude. During high-flux periods, exported PN δ~(15)N values were close to expectations from nitrate-based export, but low-flux periods exhibited higher δ~(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 δ~(15)N to nitrate utilization requires only that the annually integrated (rather than the seasonally varying) sinking flux of PN δ~(15)N correlates with nitrate depletion. While a correlation between annually integrated sinking PN δ~(15)N to nitrate depletion was observed in both the SAZ and PFZ, the sensitivity of sinking PN δ~(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 δ~(15)N observed during glacial periods.