Biogeosciences
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2010
10.5194/bg-7-809-2010
http://www.biogeosciences.net/7/809/2010/
http://www.biogeosciences.net/7/809/2010/bg-7-809-2010.html
http://www.biogeosciences.net/7/809/2010/bg-7-809-2010.pdf
809
826
2010-03-02
The impact of Saharan dust on the particulate export in the water column of the North Western Mediterranean Sea
E. Ternon
C. Guieu
guieu@obs-vlfr.fr
M.-D. Loÿe-Pilot
N. Leblond
E. Bosc
B. Gasser
J.-C. Miquel
J. Martín
INSU-CNRS, UMR 7093, Laboratoire d'océanographie de Villefranche, 06230 Villefranche-sur-Mer, France
Université Pierre et Marie Curie-Paris 6, UMR 7093, Laboratoire d'océanographie de Villefranche, 06230 Villefranche-sur-Mer, France
CERES-ERTI, Ecole Normale Supérieure, Paris, France
IAEA Marine Environment Laboratories, Monaco
Simultaneous measurements of atmospheric deposition and of sinking particles
at 200 and 1000 m depth, were performed in the Ligurian Sea (North-Western
Mediterranean) between 2003 and 2007, along with phytoplanktonic activity derived from satellite images. Atmospheric
deposition of Saharan dust particles was very irregular and confirmed the
importance of sporadic high magnitude events over the annual average (11.4 g m<sup>−2</sup> yr<sup>−1</sup>
for the 4 years). The average marine total mass flux
was 31 g m<sup>−2</sup> yr<sup>−1</sup>, the larger fraction being the lithogenic one
(~37%). The marine total mass flux displayed a seasonal pattern
with a maximum in winter, occurring before the onset of the spring bloom.
The highest POC fluxes did not occur during the spring bloom nor could they
be directly related to any noticeable increase in the surface
phytoplanktonic biomass. Over the 4 years of the study, the strongest POC
fluxes were concomitant with large increases of the lithogenic marine flux,
which had originated from either recent Saharan fallout events (February
2004 and August 2005), from "old" Saharan dust "stored" in the upper water
column layer (March 2003 and February 2005), or alternatively from
lithogenic material originating from Ligurian riverine flooding (December
2003, Arno, Roya and Var rivers). Those associated export fluxes defined as
"lithogenic events", are believed to result from a combination of forcing
(winter mixing or Saharan events, in particular extreme ones), biological
(zooplankton) activity, and also organic-mineral aggregation inducing a
ballast effect. By fertilising the surface layer, mixed Saharan dust events
were shown to be able to induce "lithogenic events" during the
stratification period. These events would be more efficient in transferring
POC to the deeper layers than the spring bloom itself. The extreme Saharan
event of February 2004 exported ~45% of the total annual POC,
compared to an average of ~25% for the bloom period. This
emphasises the role played by these "lithogenic events", and in particular
those that are induced by the more extreme Saharan events, in the carbon
export efficiency in the North-western Mediterranean Sea.
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