EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Nitrate isotopic constraints on routes of nutrient supply to global ocean pycnocline

Francois Fripiat1,2, Aflredo Martinez-Garcia2, Dario Marconi3, Sarah E. Fawcett4, Daniel M. Sigman3, and Gerald H. Haug2
Francois Fripiat et al.
  • 1Université Libre de Bruxelles, Department of Geosciences, Environment and Society, Belgium (
  • 2Max Planck Institute for Chemistry, Germany
  • 3Princeton University, Department of Geosciences, USA
  • 4University of Cape Town, Department of Oceanography, South Africa

The circulation of the ocean plays a fundamental role in restoring the surface nutrients necessary to maintain global ocean biological production. However, our quantitative understanding of the physical mechanisms that return deep-ocean water and nutrients to the upper ocean is currently limited. The nitrate isotopes are investigated here as a new data constraint on the percentage of gross water transport into the global pycnocline that derives from the Southern Ocean as opposed to the deep ocean (which we term the “pycnocline recipe”). Based on a comparison between large-scale observations of nitrate isotopes and the output of a box model, we estimate that the pycnocline recipe is 75 ± 10%; this result implies that ~ 64% of the nutrients supplied to the low latitude pycnocline pass through the Southern Ocean. Our simulations also highlight the shortcomings of a purely advective view of the ocean’s transport of water and nutrients, confirming that mixing with both the deep ocean and the Southern Ocean ventilating area are key to the exchange of water and nutrients between the pycnocline and higher-density deep and polar surface waters. Our calculations support a pure advective-diffusive balance in the deep ocean. In contrast, in the Southern Ocean, our findings provide independent evidence for the importance of air-sea fluxes of momentum and buoyancy in driving the circulation.

How to cite: Fripiat, F., Martinez-Garcia, A., Marconi, D., Fawcett, S. E., Sigman, D. M., and Haug, G. H.: Nitrate isotopic constraints on routes of nutrient supply to global ocean pycnocline, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3957,, 2020

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Presentation version 1 – uploaded on 19 Apr 2020
  • CC1: Comment on EGU2020-3957, Ivy Frenger, 14 May 2020

    Hi Francois,

    Thank you for the presentation, it is very interesting to me (given that I am looking at processes affecting nitrate in the Southern Ocean). Can you say sth about lateral transport of nitrate from the SO towards the lower latitudes near the surface rather than via the pycnocline (as highlighted by Letscher et al, 2016,

    I am looking forward to the publication.

    Best, Ivy

    • AC1: Reply to CC1, Francois Fripiat, 15 May 2020

      Dear Ivy,

      Thank you for your comment, your presentation was really interesting too.

      We didn’t specifically look at this lateral exchange term. A colleague is using the same rationale for the Subantarctic Zone and this exchange term is one of our targets.

      For the pycnocline, the big advantage of nitrate isotopes is that low-latitude export production and remineralization in the pycnocline have the same isotopic composition than that of the nitrate supply (due to the complete consumption in the low-latitude area). It will not change that much (for pycnocline d15N) if nitrate is either supplied into the pycnocline by the subduction of intermediate water or indirectly by lateral nitrate transport which is quantitatively exported and remineralized in the pycnocline.

      Thanks, and all the best,