The export flux of particulate organic carbon derived from <sup>210</sup>Po∕<sup>210</sup>Pb disequilibria along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise
<p>The disequilibrium between <span class="inline-formula"><sup>210</sup></span>Po activity and <span class="inline-formula"><sup>210</sup></span>Pb activity in seawater samples was determined along the GEOTRACES GA01 transect...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-01-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/16/309/2019/bg-16-309-2019.pdf |
| Summary: | <p>The disequilibrium between <span class="inline-formula"><sup>210</sup></span>Po activity and <span class="inline-formula"><sup>210</sup></span>Pb 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 <span class="inline-formula"><sup>210</sup></span>Po.
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 <span class="inline-formula"><sup>210</sup></span>Po export flux in this model,
suggesting that those calculations of <span class="inline-formula"><sup>210</sup></span>Po export fluxes should be used
with great care. Despite the large uncertainties, there is no question that
the deficits of <span class="inline-formula"><sup>210</sup></span>Po in the Iberian Basin and at the Greenland
Shelf have been strongly affected by vertical advection. Using the export
flux of <span class="inline-formula"><sup>210</sup></span>Po and the particulate organic carbon (POC) to <span class="inline-formula"><sup>210</sup></span>Po
ratio of total (> 1 <span class="inline-formula">µ</span>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 C m<span class="inline-formula"><sup>−2</sup></span> d<span class="inline-formula"><sup>−1</sup></span>, 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 <span class="inline-formula"><sup>210</sup>Po∕<sup>210</sup>Pb</span> and <span class="inline-formula"><sup>234</sup>Th∕<sup>238</sup>U</span>
disequilibria agreed within a factor of 3 along the transect, with higher POC
estimates generally derived from <span class="inline-formula"><sup>234</sup></span>Th. The differences were attributed
to integration timescales and the history of bloom events.</p> |
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| ISSN: | 1726-4170 1726-4189 |