Particulate barium tracing of significant mesopelagic carbon remineralisation in the North Atlantic Nolwenn Lemaitre, Hélène Planquette, Frédéric Planchon, Géraldine Sarthou, Stéphanie Jacquet, Maribel García-Ibáñez, Arthur Gourain, Marie Cheize, Laurence Monin, Luc André, et al. To cite this version: Nolwenn Lemaitre, Hélène Planquette, Frédéric Planchon, Géraldine Sarthou, Stéphanie Jacquet, et al.. Particulate barium tracing of significant mesopelagic carbon remineralisation in the North Atlantic. Biogeosciences, European Geosciences Union, 2018, 15 (8), pp.2289-2307. 10.5194/bg-15- 2289-2018. hal-02024247 HAL Id: hal-02024247 https://hal-amu.archives-ouvertes.fr/hal-02024247 Submitted on 19 Feb 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution| 4.0 International License Biogeosciences, 15, 2289–2307, 2018 https://doi.org/10.5194/bg-15-2289-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. 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 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 Correspondence: Nolwenn Lemaitre ([email protected]) Received: 20 September 2017 – Discussion started: 4 October 2017 Revised: 13 February 2018 – Accepted: 16 March 2018 – Published: 19 April 2018 Abstract. The remineralisation of sinking particles by period and phytoplankton appear to be dominated by small prokaryotic heterotrophic activity is important for controlling and calcifying species, such as coccolithophorids. The Baxs oceanic carbon sequestration. Here, we report mesopelagic content, related to oxygen consumption, was converted into a particulate organic carbon (POC) remineralisation fluxes in remineralisation flux using an updated relationship, proposed the North Atlantic along the GEOTRACES-GA01 section for the first time in the North Atlantic. The estimated fluxes (GEOVIDE cruise; May–June 2014) using the particulate were of the same order of magnitude as other fluxes obtained biogenic barium (excess barium; Baxs/ proxy. Important using independent methods (moored sediment traps, incuba- mesopelagic (100–1000 m) Baxs differences were observed tions) in the North Atlantic. Interestingly, in the subpolar along the transect depending on the intensity of past blooms, and subtropical provinces, mesopelagic POC remineralisa- the phytoplankton community structure, and the physical tion fluxes (up to 13 and 4.6 mmol C m−2 d−1, respectively) forcing, including downwelling. The subpolar province was were equalling and occasionally even exceeding upper-ocean 234 characterized by the highest mesopelagic Baxs content (up to POC export fluxes, deduced using the Th method. These 727 pmol L−1/, which was attributed to an intense bloom av- results highlight the important impact of the mesopelagic eraging 6 mg chl a m−3 between January and June 2014 and remineralisation on the biological carbon pump of the studied by an intense 1500 m deep convection in the central Labrador area with a near-zero, deep (> 1000 m) carbon sequestration Sea during the winter preceding the sampling. This down- efficiency in spring 2014. welling could have promoted a deepening of the prokaryotic heterotrophic activity, increasing the Baxs content. In com- parison, the temperate province, characterized by the lowest −1 Baxs content (391 pmol L /, was sampled during the bloom Published by Copernicus Publications on behalf of the European Geosciences Union. 2290 N. Lemaitre et al.: Particulate barium tracing significant mesopelagic carbon remineralisation 1 Introduction intense (Bishop, 1988; Collier and Edmond, 1984; Dehairs et al., 1980; Ganeshram et al., 2003; Gonzalez-Munoz et al., The ocean represents the largest active CO2 sink (Sabine et 2003). Bacterial activity will result in the disruption of these al., 2004) partly materialised by the oceanic biological car- aggregates, thereby releasing the barite crystals in the ambi- bon pump (BCP), which controls the export of carbon and ent water. As a result, the concentration of Baxs relates with nutrients to the deep ocean through the production of bio- oxygen consumption rate (Dehairs et al., 1997; Shopova et genic sinking particles (Boyd and Trull, 2007; Sigman and al., 1995) and can be converted into a remineralisation rate Boyle, 2000; Volk and Hoffert, 1985). The North Atlantic of POC in the mesopelagic layer (Dehairs et al., 1997). Baxs sustains one of the most productive spring phytoplankton has been successfully used as a proxy of POC remineralisa- blooms of the world ocean (Esaias et al., 1986; Henson et al., tion flux in the Southern Ocean (Cardinal et al., 2005; Jacquet 2009; Longhurst, 2010; Pommier et al., 2009). The high pri- et al., 2008a, b, 2011a, 2015; Planchon et al., 2013) and Pa- mary productivity in combination with the water mass forma- cific Ocean (Dehairs et al., 2008). tion there as part of the thermohaline circulation (Seager et We examined mesopelagic POC remineralisation along al., 2002) results in a particularly efficient BCP in the North the GEOTRACES-GA01 section during the GEOVIDE Atlantic (Buesseler et al., 1992; Buesseler and Boyd, 2009; cruise (15 May–30 June 2014; R/V Pourquoi Pas?) by as- Herndl and Reinthaler, 2013; Honjo and Manganini, 1993; sessing particulate biogenic barium (excess barium; Baxs/ Le Moigne et al., 2013b), estimated to contribute up to 18 % contents. This study is the first one to report the use of of the global oceanic BCP (Sanders et al., 2014). However, the Baxs proxy in the North Atlantic. Regional variations the magnitude of the carbon transfer to the deep ocean de- in the Baxs distributions along the crossed biogeochemical pends on many factors including the efficiency of bacterial provinces are discussed regarding the stage and intensity of remineralisation within the mesopelagic layer (100–1000 m the bloom, the phytoplankton community structure, and the depth layer). In this layer, most of the particulate organic car- physical forcing. We reassessed the algorithm between Baxs bon (POC) exported from the upper mixed layer is respired content and oxygen consumption developed for the South- or released to the dissolved phase as dissolved organic car- ern Ocean, adapting it for the North Atlantic. We compared bon (DOC; Buesseler et al., 2007; Buesseler and Boyd, 2009; the remineralisation fluxes resulting from this new North Burd et al., 2016; Herndl and Reinthaler, 2013; Lampitt and Atlantic-specific algorithm with those obtained using other Antia, 1997; Martin et al., 1987). Mesopelagic remineralisa- methods in the same area. This comparison, in combination tion has often been reported to balance or even exceed the with surface primary production (PP) and POC export esti- carbon supply from the surface (i.e. POC and DOC; Aris- mates (Lemaitre et al., 2018; this issue), allowed us to eval- tegui et al., 2009; Baltar et al., 2009; Burd et al., 2010; uate the fate of POC to the deep ocean and to constrain the Collins et al., 2015; Fernández-castro et al., 2016; Giering BCP in the North Atlantic. et al., 2014; Reinthaler et al., 2006), highlighting the impact of mesopelagic processes on bathypelagic carbon seques- tration. Unfortunately, studies focusing on the mesopelagic 2 Methods layer are scarce, and the remineralisation process in this part of the water column remains poorly constrained. A variety 2.1 Study area of methods have been used to assess deep remineralisation. The attenuation of the particulate organic matter concentra- The GEOVIDE section (15 May–30 June 2014; R/V tion with depth can be deduced from POC fluxes recorded by Pourquoi pas?) crossed different biogeochemical provinces bottom-tethered or free-floating neutrally buoyant sediment in the North Atlantic including the North Atlantic subtrop- traps (e.g. Buesseler et al., 2007; Honjo