Abstract
Abstract. The disequilibrium between 210Po activity and 210Pb 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 210Po. 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 210Po export flux in this model, suggesting that those calculations of 210Po export fluxes should be used with great care. Despite the large uncertainties, there is no question that the deficits of 210Po in the Iberian Basin and at the Greenland Shelf have been strongly affected by vertical advection. Using the export flux of 210Po and the particulate organic carbon (POC) to 210Po ratio of total (> 1 µm) 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 C m−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 210Po∕210Pb and 234Th∕238U disequilibria agreed within a factor of 3 along the transect, with higher POC estimates generally derived from 234Th. The differences were attributed to integration timescales and the history of bloom events.
Highlights
The oceans play an essential role in the regulation of atmospheric CO2 and the buffering of the global climate system (e.g., Sabine, 2004) by removing carbon from the atmosphere via dissolution and photosynthesis in the surface ocean, and storing it in the dissolved or particulate forms
particulate organic carbon (POC) fluxes estimated from the 210Po/210Pb and 234Th/238U disequilibria agreed within a factor of 3 along the transect, with higher POC estimates generally derived from 234Th
Radionuclide data were produced by two collaborating laboratories to ensure higher counting statistics for 210Po activity in the samples: the Laboratori de Radioactivitat Ambiental at Universitat Autònoma de Barcelona (UAB) and the Stewart Laboratory at Queens College (QC)
Summary
The oceans play an essential role in the regulation of atmospheric CO2 and the buffering of the global climate system (e.g., Sabine, 2004) by removing carbon from the atmosphere via dissolution and photosynthesis in the surface ocean, and storing it in the dissolved or particulate forms. Lead-210 and 234Th are only adsorbed to particle surfaces, whereas 210Po is both adsorbed to surfaces and biologically reactive so it can be assimilated by organisms and even bioaccumulated (Fisher et al, 1983; Cherrier et al, 1995; Stewart and Fisher, 2003a, b). This behavior leads to a higher partitioning coefficient (relative association between the isotope and the particulate vs the dissolved phase) of 210Po compared to that of 210Pb (e.g., Masqué et al, 2002; Wei et al, 2014; Tang et al, 2017)
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