The impact of Saharan dust on the particulate export in the water column of the North Western Mediterranean Sea

Abstract

Abstract. 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 −2 yr −1 for the 4 years). The average marine total mass flux was 31 g m −2 yr −1 , 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

Quantification of the lithogenic carbon pump following a simulated dust-deposition event in large mesocosms

“…Although little evidence for a direct protective mechanism has emerged from in situ observations (Ingalls et al, 2006;Ploug et al, 2008;, laboratorycontrolled experiments have demonstrated the potential for minerals to inhibit the microbial degradation of OM (Arnarson and Keil, 2005;Engel et al, 2009a;Le Moigne et al, 2013). On the other hand, it has been demonstrated that minerals could increase the velocity at which aggregates sink (e.g., De La Rocha and Passow, 2007;De La Rocha et al, 2008;Thomalla et al, 2008;Engel et al, 2009b; and drive large POC flux events (e.g., Thunell et al, 2007;Lee et al, 2009;Sanders et al, 2010;Ternon et al, 2010). Such fast-sinking POC, negligible within the euphotic zone, is sufficient to explain deep-ocean POC fluxes (Honda and Watanabe, 2010;Riley et al, 2012); this emphasizes the importance of the mode by which carbon is transferred downward when considering carbon flux parameterization.…”

“…In light of this fact, an alternative explanation states that correlations between POC and mineral fluxes may result from OM, which acts as "glue" for aggregate minerals that would otherwise not sink Passow and De la Rocha, 2006). Without determining the catalyst for aggregation, Ternon et al (2010) concluded that the simultaneous presence of OM and lithogenic particles is required for the formation of large and fast-sinking particles in the western Mediterranean Sea. Considering the strong spatial and temporal variability of dust-deposition events (Jickells et al, 2005;Mahowald et al, 2009), the water column into which atmospheric particles are deposited is characterized by a wide-ranging OM composition and abundance.…”

“…Since only a few studies have reported the enhancement of export production following dust-deposition pulses in lownutrient, low-chlorophyll (LNLC) areas (e.g., Ternon et al, 2010), we sought to improve our understanding of this mechanism through this work. As a result of the episodic nature of dust deposition, artificial seeding in mesocosms constitutes a suitable approach for following, on an environmentally relevant timescale, the surface POC export associated with dust-deposition events.…”

Deep ocean mass fluxes in the coastal upwelling off Mauritania from 1988 to 2012: variability on seasonal to decadal timescales

“…Another explanation for the missing relationship could be that fine-grained dust accumulates in surface waters until the biological pump produces sufficient organic particles to allow the formation of larger particles which then settle into the deep ocean (Bory et al, 2002;Ternon et al, 2010;Nowald et al, 2015). Cape Blanc dust particles have predominant grain sizes between 10 and 20 µm (Ratmeyer et al, 1999a, b;Friese et al, 2016) and, thus, would sink too slowly to build a deep ocean flux signal.…”

Dust deposition: iron source or sink? A case study

“…They are also involved in particle aggregation and act as ballast for sinking organic material (e.g. Armstrong et al, 2002;Ternon et al, 2010), changing the settling velocity of iron adsorbed on particle surfaces. The net effect of dust deposition on DFe in surface waters is therefore influenced by various factors: while the input flux of iron is mainly determined by the Fe solubility and content in dust particles, the loss flux depends on the size and composition of sinking particles, rates of particle aggregation as well as of the Fe adsorption and desorption at particle surfaces.…”