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

Abstract

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.

Aluminium in the North Atlantic Ocean and the Labrador Sea (GEOTRACES GA01 section): roles of continental inputs and biogenic particle removal

“…In our study region, 5 diatoms dominate the phytoplankton communities at the early stage of the spring bloom (Brown et al, 2003), and are an important producer of biogenic silica (bSiO 2 ) (Nelson et al, 1995) which is the main carrier for scavenged Al (Moran and Moore, 1988b;Stoffyn, 1979). Elevated bSiO 2 concentrations and associated high export rates of bSiO 2 were measured in the ENAB (up to 1.19 µM BSi) and in the IrB and LB (up to 4.27 and 4.63 µM bSiO 2 respectively) (Lemaitre et al, 2017). .…”

Particulate Rare Earth Element behavior in the North Atlantic (GEOVIDE cruise)

“…The North Atlantic Drift region (NADR) is located between the NAST and the Reykjanes ridge, with higher nutrient concentrations than in the NAST (Longhurst, 1995). A strong bloom of coccolithophorids, with a maximum intensity in the Icelandic basin, occurred during the cruise, generating the highest primary production rate observed 85 on GEOVIDE (1740 molC.m -2 .d -1 , station #26, Fonseca-Batista et al, 2019) and high carbon export (up to 80 molC.m -2 .d -1 , station #32, Lemaitre et al, 2018). Four open ocean stations were sampled in this province: in the northern branch of the NAC (station #21), at the Subpolar front (station #26), in the northern branch of the NAC (station #32) and on the Reykjanes Ridge (station #38).…”

“…1. This figure also presents the main surface currents, as described in details (Zunino et al, 2017) and , together with the three main 70 biogeochemical provinces identified by Longhurst (1995) and described in details by Lemaitre et al (2018). The position of the stations where PREEs were sampled (Fig.…”

“…The North Atlantic Subtropical (NAST) province is characterized by warm and salty waters 80 Longhurst, 1995;Reygondeau et al, 2018;Zunino et al, 2017). This province is depleted in nutrients, yet under influence of margin inputs, displayed a declining bloom of cyanobacteria during the cruise (Lemaitre et al, 2018). Stations #1 and #13 were sampled in the NAST.…”

Dissolved Organic Nitrogen Production and Export by Meridional Overturning in the Eastern Subpolar North Atlantic

“…If this balance is positive for net organic matter production, as it is the case, the fate of these materials is either burial in the sediments or/and horizontal export to other ocean regions. Despite sinking particles representing the major contribution to the export of organic matter from the euphotic zone in the eSPNA (Giering et al, 2014;Martin et al, 2011), sinking particulate material suffers remineralization in the mesopelagic and bathypelagic waters before reaching the sediments (Alkire et al, 2012;Lemaitre et al, 2018). As a consequence, the burial of organic nitrogen in the sediments of the open ocean (0.20 kmol/s; Brunnegård et al, 2004) and shelves (1.2 kmol/s; Wollast, 1993) is one order of magnitude lower than the net nitrate uptake computed here, and the organic nitrogen budget is then dominated by lateral fluxes of DON.…”