Articles | Volume 15, issue 8
Biogeosciences, 15, 2289–2307, 2018

Special issue: GEOVIDE, an international GEOTRACES study along the OVIDE...

Biogeosciences, 15, 2289–2307, 2018

Research article 19 Apr 2018

Research article | 19 Apr 2018

Particulate barium tracing of significant mesopelagic carbon remineralisation in the North Atlantic

Nolwenn Lemaitre1,2,a, Hélène Planquette1, Frédéric Planchon1, Géraldine Sarthou1, Stéphanie Jacquet3, Maribel I. García-Ibáñez4,5, Arthur Gourain1,6, Marie Cheize1, Laurence Monin7, Luc André7, Priya Laha8, Herman Terryn8, and Frank Dehairs2 Nolwenn Lemaitre et al.
  • 1Laboratoire des Sciences de l'Environnement Marin (LEMAR), UMR 6539, IUEM, Technopôle Brest Iroise, 29280 Plouzané, France
  • 2Vrije Universiteit Brussel, Analytical, Environmental and Geo-Chemistry, Earth System Sciences research group, Brussels, Belgium
  • 3Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO), UM110, 13288 Marseille, France
  • 4Uni Research Climate, Bjerknes Centre for Climate Research, Bergen 5008, Norway
  • 5Instituto de Investigaciones Marinas, IIM-CSIC, Eduardo Cabello 6, 36208 Vigo, Spain
  • 6Ocean Sciences Department, School of Environmental Sciences, University of Liverpool, Liverpool L69 3GP, UK
  • 7Earth Sciences Department, Royal Museum for Central Africa, Leuvensesteenweg 13, Tervuren, 3080, Belgium
  • 8Vrije Universiteit Brussel, SURF research group, department of Materials and Chemistry, Brussels, Belgium
  • anow at: Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH-Zürich, Zürich, Switzerland

Abstract. The remineralisation of sinking particles by prokaryotic heterotrophic activity is important for controlling oceanic carbon sequestration. Here, we report mesopelagic particulate organic carbon (POC) remineralisation fluxes in the North Atlantic along the GEOTRACES-GA01 section (GEOVIDE cruise; May–June 2014) using the particulate biogenic barium (excess barium; Baxs) proxy. Important mesopelagic (100–1000 m) Baxs differences were observed along the transect depending on the intensity of past blooms, the phytoplankton community structure, and the physical forcing, including downwelling. The subpolar province was characterized by the highest mesopelagic Baxs content (up to 727 pmol L−1), which was attributed to an intense bloom averaging 6 mg chl a m−3 between January and June 2014 and by an intense 1500 m deep convection in the central Labrador Sea during the winter preceding the sampling. This downwelling could have promoted a deepening of the prokaryotic heterotrophic activity, increasing the Baxs content. In comparison, the temperate province, characterized by the lowest Baxs content (391 pmol L−1), was sampled during the bloom period and phytoplankton appear to be dominated by small and calcifying species, such as coccolithophorids. The Baxs content, related to oxygen consumption, was converted into a remineralisation flux using an updated relationship, proposed for the first time in the North Atlantic. The estimated fluxes were of the same order of magnitude as other fluxes obtained using independent methods (moored sediment traps, incubations) in the North Atlantic. Interestingly, in the subpolar and subtropical provinces, mesopelagic POC remineralisation fluxes (up to 13 and 4.6 mmol C m−2 d−1, respectively) were equalling and occasionally even exceeding upper-ocean POC export fluxes, deduced using the 234Th method. These results highlight the important impact of the mesopelagic remineralisation on the biological carbon pump of the studied area with a near-zero, deep (> 1000 m) carbon sequestration efficiency in spring 2014.

Short summary
We present the particulate biogenic barium distributions in the North Atlantic for the first time with the objective of estimating mesopelagic carbon remineralisation fluxes. The remineralisation fluxes balanced or slightly exceeded the upper-ocean carbon export fluxes. This is a key result as the North Atlantic is generally assumed to be efficient in transferring carbon to the deep ocean, but during our study, the North Atlantic was characterized by a near-zero carbon sequestration efficiency.
Final-revised paper