Lemaitre Nolwenn

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Lemaitre
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Nolwenn
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  • Article
    The export flux of particulate organic carbon derived from 210Po∕210Pb disequilibria along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise
    (Copernicus Publications, 2019-01-24) Tang, Yi ; Lemaitre, Nolwenn ; Castrillejo, Maxi ; Roca-Martí, Montserrat ; Masqué, Pere ; Stewart, Gillian
    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.
  • Article
    Ironing out Fe residence time in the dynamic upper ocean
    (American Geophysical Union, 2020-08-26) Black, Erin E. ; Kienast, Stephanie S. ; Lemaitre, Nolwenn ; Lam, Phoebe J. ; Anderson, Robert F. ; Planquette, Helene ; Planchon, Frederic ; Buesseler, Ken O.
    Although iron availability has been shown to limit ocean productivity and influence marine carbon cycling, the rates of processes driving iron's removal and retention in the upper ocean are poorly constrained. Using 234Th‐ and sediment‐trap data, most of which were collected through international GEOTRACES efforts, we perform an unprecedented observation‐based assessment of iron export from and residence time in the upper ocean. The majority of these new residence time estimates for total iron in the surface ocean (0–250 m) fall between 10 and 100 days. The upper ocean residence time of dissolved iron, on the other hand, varies and cycles on sub‐annual to annual timescales. Collectively, these residence times are shorter than previously thought, and the rates and timescales presented here will contribute to ongoing efforts to integrate iron into global biogeochemical models predicting climate and carbon dioxide sequestration in the ocean in the 21st century and beyond.