Multi-proxy approach (234Th, Baxs) of export and remineralization fluxes of carbon and biogenic elements associated with the oceanic biological pump

  • Nolwenn Christiane LEMAîTRE ((PhD) Student)
  • Frank Dehairs (Promotor)
  • Géraldine Sarthou (Co-promotor)
  • Helene Planquette (Co-promotor)
  • Frederic Planchon (Co-promotor)
  • Ian Salter (Jury)
  • Frédéric Lemoigne (Jury)
  • Stéphane Blain (Jury)
  • Lionel Guidi (Jury)
  • Filip Meysman (Jury)

Student thesis: Doctoral Thesis

Abstract

The aim of this thesis is to improve our understanding of the different controls that affect the biological pump in the North Atlantic and around the Kerguelen Island in the Southern Ocean. The GEOTRACES GA01 (GEOVIDE, May-June 2014 from Portugal to Newfoundland) cruise provided a good framework to investigate export fluxes of POC (particulate organic carbon), PN (particulate nitrogen), BSi (biogenic silica) and particulate trace elements (pTEs), as well as remineralization fluxes using two proxies: Thorium-234 (234 Th) and excess barium (Ba xs ). The North Atlantic Ocean, which is an important environment influencing the thermohaline oceanic circulation and thus climate on Earth (Seager et al., 2002), is characterized by a strong spring bloom generating a pulse of sinking particles and thus impacting the carbon export from surface to the deep ocean (Henson et al., 2009; Pommier et al., 2009; Sanders et al., 2014). The observed POC export fluxes were highest in the western European basin and the Labrador Sea but they were similar or lower than values reported by others in previous North Atlantic studies, probably due to a high temporal variability of production and export in the region. We observed the higher export fluxes to be associated with the presence of biogenic (biogenic silica or calcium carbonate) and lithogenic particles which have been shown to have a ballasting effect on particle settling. As the ballasting effect is different for silica and calcium carbonate (it is smaller for the former), the composition of the phytoplankton and the sinking particles will impact the export efficiency of photosynthetically produced organic carbon to the deep (>1000m) ocean. Export efficiency was generally low (< 10%), and appeared inversely related to primary production, highlighting that a phase lag exists between production and export, but also probably that the North Atlantic biopump is not as efficient as previously thought. The highest transfer efficiencies, i.e. the fraction of C that reached the 400 m depth horizon, appeared to be driven by sinking particles ballasted by calcite or lithogenic minerals. We also observed an important regional variation of the mesopelagic remineralization across the whole North Atlantic, which could be attributed to the intensity of the bloom, the phytoplankton cell size and community structure and the physical forcing, including the vertical advection (downwelling). Surprisingly, carbon remineralization fluxes balanced or even exceeded upper ocean POC export fluxes. Excess remineralization, relative to export can be explained by the fact that primary production, export and remineralization measurements integrate different timescales. Overall our results highlight the important impact of mesopelagic remineralization on the biological carbon pump with a near-zero, deep (> 1000 m) carbon sequestration efficiency for spring 2014. We then investigated the export of trace elements such as P, Fe, Zn, Mn, Cu, Co, Ni or Cd which are essential nutrients for metalloprotein synthesis (e.g., Morel, 2003; Sunda, 1989; Twining and Baines, 2013). As observed for POC, biogenic trace element exports are strongly influenced by lithogenic particles advected from the ocean margins, yet to a different extent. Trace element fluxes appeared to depend considerably on phytoplankton activity and biomass, as we observed similarities between intracellular TE/C quotas and the TE/C composition of exported material at open ocean stations, not influenced by lithogenic matter. The stage of the bloom and the phytoplankton community composition could also explain the variability of the export fluxes between trace elements. Finally, a last study focused on particulate nitrogen, biogenic silica and iron export fluxes in the Southern Ocean Kerguelen Island area. This area is characterized by a naturally Fe-fertilization of a recurrent large phytoplankton bloom extending up to 1000 km downstream of the Island (KEOPS2 cruise, austral spring 2011). Our findings indicate that Fe-fertilization increase N, biogenic Si, Fe and POC export fluxes and also that flux variability inside the fertilized area are related to phytoplankton community composition. Interestingly, the highest pFe export flux was estimated at the early bloom, low-productivity station over the Plateau, largely sustained by Fe-rich lithogenic particles. This thesis therefore provides one of the first attempts to characterize export and remineralization fluxes in the North Atlantic and a good reference point for subsequent studies of the biological carbon pump that will be conducted in this area. Moreover, similarities and contrasts in the biological pump functioning between the Sub-Arctic and the Antarctic are discussed.
Date of Award20 Jan 2017
Original languageEnglish
Awarding Institution
  • Université de Bretagne Occidentale
  • Vrije Universiteit Brussel

Keywords

  • Oceanic Biopump, Carbon export, trace elements

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