NITRATE \delta15N AND \delta18O

PI:

  • Daniel Sigman (Princeton University)

Samplers:

  • Francesca Alatorre

  • Cassondra DeFoor

  • Paige Hoel

  • Elana Perez

Nitrate (NO3-) is the primary form of fixed nitrogen (N) in the sea and an essential macronutrient, the supply of which can limit primary production and carbon export from the surface ocean. The dual isotopes of NO3- (\delta15N and \delta18O) record biogeochemical and physical processes on different time scales. In general, nitrate consuming processes tend to raise the \delta15N and \delta18O of nitrate equally while nitrate producing processes tend to decouple the dual isotopes. Since different processes leave different imprints on the isotopic composition of nitrate, the dual isotopes can be used to separate and quantify the impact of multiple N fluxes acting on the nitrate pool.

Seawater samples for nitrate isotope analyses were collected from all depths at about every two degrees of latitude. Two 30mL samples were collected from each niskin bottle fired at depths shallower than 300 m. One 30mL sample was taken from all other depths. All bottles were rinsed once with half their full volume before being filled with seawater. The samples were stored onboard at -20°C in order to preserve them for land based analysis.

Analysis

The denitrifier method [Casciotti2002] [Sigman2001] will be used to analyze NO3-, \delta15N, and \delta18O. Briefly, this method converts all NO3 to nitrous oxide (N2O) via denitrifying bacteria before the sample is analyzed by an IRMS. Samples were collected at stations 9, 16, 19, 22, 24, 28, 33, 38, 44, 46,49, 51, 54, 56, 59,62, 64, 67, 69, 77, 88. At station 9 a problem with the CTD cable prevented bottles from tripping in the upper 100 m, therefore samples for the upper 100 m were collected on station 10.