The delta N-15 of nitrate in the Southern Ocean: Consumption of nitrate insurface waters

We report nitrogen isotope data for nitrate from transects of hydrocast and surface samples collected in the eastern Indian and Pacific sectors of the Southern Ocean, focusing here on the data from the upper water column to s tudy the effect of nitrate consumption by phytoplankton. The delta(15)N of nitrate increases by 1-2 parts per thousand from deep water into the Antarc tic summertime surface layer, due to kinetic isotopic fractionation during nitrate uptake. Estimation of the nitrate uptake isotope effect from Antarc tic depth profiles yields values in the range of 5-6 parts per thousand in east Indian sector and 4-5 parts per thousand in the east Pacific sector. S urface transect data from the Pacific sector also yield values of 4-5 parts per thousand. The major uncertainty in the profile-based estimation of the isotope effect involves the delta(15)N of nitrate from the temperature min imum layer below the summertime Antarctic surface layer, which deviates sig nificantly from the predictions of simple models of isotope fractionation. For the Subantarctic surface, it is possible to distinguish between nitrate supplied laterally from the surface Antarctic and nitrate supplied vertica lly from the Subantarctic thermocline because of the distinctive relationsh ips between the delta(15)N and concentration of nitrate in these two potent ial sources. Our Subantarctic samples, collected during the summer and fall , indicate that nitrate is supplied to the Subantarctic surface largely by northward transport of Antarctic surface water. Isotopic data from the Paci fic sector of the Subantarctic suggest an isotope effect of 4.5 parts per t housand, indistinguishable from the Antarctic estimates in this sector.