Nitrate isotope distributions on the US GEOTRACES North Atlantic cross-basin section: Signals of polar nitrate sources and low latitude nitrogen cycling

Dario Marconi, M. Alexandra Weigand, Patrick A. Rafter, Matthew R. McIlvin, Matthew Forbes, Karen L. Casciotti, Daniel Mikhail Sigman

Research output: Contribution to journalArticle

32 Scopus citations

Abstract

We report and interpret the nitrate δ15N and δ18O distributions in the cross-basin transect of the US GEOTRACES North Atlantic expedition. Lownitrate δ15N observed in the shallowthermocline of the centralwestern portion of the basin (reaching as lowas 2.5%vs. air; ~2.3%lower than deep Atlantic nitrate) reflects the remineralization of nitrogen with a low δ15N, presumably mostly from nitrogen fixation. A strong correlation between the δ15N and concentration of nitrate argues that the focusing of the low δ15N signal in the upper thermocline of the North Atlantic subtropical gyre is at least partly due to the low concentration of nitrate in those waters, such that the east-to-west δ15N decrease in shallow thermocline nitrate does not require an east-to-west increase in nitrogen fixation rate. In underlying intermediate-depthwater, which enters the section largely fromthe South, the nitrate δ15N is ~1%higher than in deep water while the δ18O is similar to that in deep water. The δ15N elevation derives from the partial assimilation of nitrate in Southern Ocean surface waters prior to Antarctic Intermediate and Subantarctic Mode Water formation. The δ18O elevation of preformed nitrate in southern-sourced intermediate waters has been diluted by the addition of nitrate fromregeneration/nitrification in the lowlatitudes, which does not similarly weaken the δ15N elevation. In deepwater, the nitrate δ15N in sampleswith a high relative contribution of North Atlantic DeepWater is almost indistinguishable from that of samples with a higher contribution of Antarctic BottomWater (4.8-4.9%vs. air). Thus, the high nitrate δ15N innorthward flowing intermediate waters is almost entirely erased by the input of low-δ15N newly fixed nitrogen to the Atlantic (including that from the Mediterranean) prior to the incorporation of those intermediate depth waters into southward flowing North Atlantic Deep Water. The data appear consistent with a nitrogen isotopic balance between northward and southward nitrate transports across the GEOTRACES section, suggesting that most Atlantic nitrogen fixation occurs south of the section.

Original languageEnglish (US)
Pages (from-to)143-156
Number of pages14
JournalMarine Chemistry
Volume177
DOIs
StatePublished - Oct 15 2014

All Science Journal Classification (ASJC) codes

  • Oceanography
  • Chemistry(all)
  • Environmental Chemistry
  • Water Science and Technology

Keywords

  • Intermediate water
  • Internal nitrogen cycle
  • Nitrate isotopes
  • Nitrogen fixation

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