Abstract
Abstract. In this study we report particulate organic carbon (POC) export fluxes for different biogeochemical basins in the North Atlantic as part of the GEOTRACES GA01 expedition (GEOVIDE, May–June 2014). Surface POC export fluxes were deduced by combining export fluxes of total Thorium-234 (234Th) with the ratio of POC to 234Th of sinking particles at the depth of export. Particles were collected in two size classes (>53 and 1–53 µm) using in situ pumps and the large size fraction was considered representative of sinking material. Surface POC export fluxes revealed latitudinal variations between provinces, ranging from 1.4 mmol m−2 d−1 in the Irminger basin, where the bloom was close to its maximum, to 12 mmol m−2 d−1 near the Iberian Margin, where the bloom had already declined. In addition to the state of progress of the bloom, variations of the POC export fluxes were also related to the phytoplankton size and community structure. In line with previous studies, the presence of coccolithophorids and diatoms appeared to enhance the POC export flux, while the dominance of picophytoplankton cells, such as cyanobacteria, resulted in lower fluxes. The ratio of POC export to primary production (PP) strongly varied regionally and was generally low (≤14 %), except at two stations located near the Iberian Margin (35 %) and within the Labrador basin (38 %), which were characterized by unusual low in situ PP. We thus conclude that during the GEOVIDE cruise, the North Atlantic was not as efficient in exporting carbon from the surface, as reported earlier by others. Finally, we also estimated the POC export at 100 m below the surface export depth to investigate the POC transfer efficiencies. This parameter was also highly variable amongst regions, with the highest transfer efficiency at sites where coccolithophorids dominated.
Highlights
Through the sinking of particulate biogenic material, the biological carbon pump (BCP) plays a major role in the sequestration of carbon-rich particles in the ocean’s interior
We investigated particulate organic carbon (POC) export fluxes derived from the Thorium-234 (234Th) approach along a transect in the high-latitude North Atlantic, from the Iberian Margin to the subarctic Irminger and Labrador seas. 234Th, a highly particle reactive element with a short half-life (24.1 days), is widely used to explore particle export over short-term events such as phytoplankton blooms (Bhat et al, 1969; Buesseler et al, 1992; Coale and Bruland, 1985; Cochran and Masqué, 2003)
The GEOVIDE section sampled a diversity of dynamic regimes (Zunino et al, 2017), including continental margins affected by strong zonal surface currents (LC, West Greenland Current (WGC) and East Greenland Current (EGC); Mercier et al, 2015; Reverdin et al, 2003), local and seasonal upwelling, and the deep convection zone in the Labrador Sea
Summary
Through the sinking of particulate biogenic material, the biological carbon pump (BCP) plays a major role in the sequestration of carbon-rich particles in the ocean’s interior. N. Lemaitre et al.: High variability of particulate organic carbon export tions in the North Atlantic (Buesseler et al, 1992; Buesseler and Boyd, 2009; Ceballos-Romero et al, 2016; Herndl and Reinthaler, 2013; Lampitt et al, 2008; Moran et al, 2003; Mouw et al, 2016; Thomalla et al, 2008), directly questioning how carbon export efficiency varies at a transatlantic scale and its controlling factors. The studied area crossed five basins, differentiated by their distinct biogeochemical and hydrodynamic characteristics: the Iberian basin, the western European basin, the Icelandic basin, the Irminger basin and the Labrador basin (Fig. 1)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
