The export flux of particulate organic carbon derived from Po-210/Pb-210 disequilibria along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise

Yi Tang (1) , Nolwenn Lemaitre , Maxi Castrillejo (2) , Montserrat Roca-Marti (2) , Pere Masque (2, 3) , Gillian Stewart (1)

1 CUNY - City University of New York [New York]
2 UAB - Universitat Autònoma de Barcelona = Autonomous University of Barcelona = Universidad Autónoma de Barcelona
3 ECU - Edith Cowan University

The disequilibrium between Po-210 activity and Pb-210 activity in seawater samples was determined along the GEOTRACES GA01 transect in the North Atlantic during the GEOVIDE cruise (May-June 2014). A steady-state model was used to quantify vertical export of particulate Po-210. Vertical advection was incorporated into one version of the model using time-averaged vertical velocity, which had substantial variance. This resulted in large uncertainties for the Po-210 export flux in this model, suggesting that those calculations of Po-210 export fluxes should be used with great care. Despite the large uncertainties, there is no question that the deficits of Po-210 in the Iberian Basin and at the Greenland Shelf have been strongly affected by vertical advection. Using the export flux of Po-210 and the particulate organic carbon (POC) to Po-210 ratio of total (> 1 mu m) particles, we determined the POC export fluxes along the transect. Both the magnitude and efficiency of the estimated POC export flux from the surface ocean varied spatially within our study region. Export fluxes of POC ranged from negligible to 10 mmol Cm-2 d(-1), with enhanced POC export in the Labrador Sea. The cruise track was characterized by overall low POC export relative to net primary production (export efficiency < 1 %-15 %), but relatively high export efficiencies were seen in the basins where diatoms dominated the phytoplankton community. The particularly low export efficiencies in the Iberian Basin, on the other hand, were explained by the dominance of smaller phytoplankton, such as cyanobacteria or coccolithophores. POC fluxes estimated from the Po-210/Pb-210 and Th-234/U-238 disequilibria agreed within a factor of 3 along the transect, with higher POC estimates generally derived from Th-234. The differences were attributed to integration timescales and the history of bloom events.

Domaines: Microbiologie et Parasitologie

Yi Tang, Nolwenn Lemaitre, Maxi Castrillejo, Montserrat Roca-Marti, Pere Masque, et al.. The export flux of particulate organic carbon derived from Po-210/Pb-210 disequilibria along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise. Biogeosciences, 2019, 16 (2), pp.309-327. ⟨10.5194/bg-16-309-2019⟩. ⟨hal-02938598⟩