Cécile Guieu

Revisiting atmospheric dust export to the Southern Hemisphere ocean: Biogeochemical implications

Concentrations of dust were 6.1 ± 2.4 ng m 3 for SEPS and 13.0 ± 6.3 ng m 3 for SOKS. Dust fluxes, derived from those concentrations, were 9.9 ± 3.7 m gm 2 d 1 for SEPS and 38 ± 14 m gm 2 d 1 for SOKS and are shown to be representative of actual fluxes in those areas. Dust and iron deposition are up to 2 orders of magnitude lower than former predictions. A map of dust deposition on the Southern Hemisphere is proposed by incorporating those in situ measurements into a dust model. This study confirms that dust deposition is not the dominant source of iron to the large high-nutrient low-chlorophyll Southern Ocean.

Bacterial response to dust pulses in the western Mediterranean: Implications for carbon cycling in the oligotrophic ocean

event (2.6 g m � 2 ) induced a 1.5-fold increase in bacterial abundance (BA) and a 2-fold increase in bacterial respiration (BR). Experimental dust additions (equivalent to fluxes of 5 and 20 g m � 2 ) to bacteria natural assemblages also stimulated BA (between 2- and 4-fold increases) and BR (between 1.5- and 3-fold increases). Pooling the in situ and experimental data, linear relationships were obtained between dust concentration and BA (r 2 = 0.86; p < 0.01) and BR (r 2 = 0.89; p < 0.001). The dust-induced bacterial bloom resulted in a C mineralization of 0.5 g m � 2 , which may represent up to 70% of bioavailable DOC annually exported to the depth in the Mediterranean. These results demonstrate that heterotrophic bacteria may play a much larger role in the connections between dust and the ocean carbon cycle than previously recognized and highlight the need for a more accurate understanding of how dust pulses may affect C export in the oligotrophic ocean.

Impact of high Saharan dust inputs on dissolved iron concentrations in the Mediterranean Sea

During the PROSOPE cruise (Sept. 1999) in the Mediterranean Sea, dissolved iron concentrations in seawater and iron and aluminium concentrations in aerosols collected on board were investigated. Concentrations in aerosols were about two times higher in the Tyrrhenian Sea than in the west (Alboran Sea). This was in good agreement with the observed increase in dissolved iron concentrations in the surface waters from West to East. Depth profiles were characterised by a maximum in the surface mixed layer. Using an in vitro experiment, iron released from Saharan dust during the season characterized by a stratified water column and a low primary productivity was estimated: it resulted in an accumulation of 0.5-0.8 nM dissolved iron, in good agreement with the observed iron enrichment in the surface water (0.8 nM). This study confirms the significance of atmospheric input of Saharan origin on the iron cycle in the Mediterranean Sea.

Trend in total atmospheric deposition fluxes of aluminium, iron, and trace metals in the northwestern Mediterranean over the past decade (1985–1997)

The total atmospheric deposition of aluminium (Al), cadmium (Cd), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn) on the northwestern coast of Corsica (Pirio) was sampled for two years, and their respective concentrations were measured during that period. The sampling station was chosen for its isolation from any local and regional sources of contamination. The year-to-year variability of the total atmospheric deposition was found to be high (up to a factor of 2). Using At as the crustal reference indicates that Pb, Cd, and Zn are mainly associated with man-made aerosols (> 85%), and Fe is mainly associated with crustal aerosols (> 70 %). However, our results indicate that Saharan dust is a potential source of natural lead, especially in the case of a major input of dust to the northwestern Mediterranean. To determine the spatiotemporal variability of the trace metals over the northwestern Mediterranean Sea, the observed fluxes were compared to those found in the past decade using the same methodology. The comparison indicated a relative homogeneity of Cu and Ni fluxes over the northwestern Mediterranean and over the past decade. The decrease found in At and Fe since 1985 (by a factor of 4 to 6) can be related to the decrease in Saharan dust fallout. The limitation in the use of lead additives in gasoline may have resulted in a decrease in the European atmospheric lead emissions by a factor of about 6 since 1985 and in a maximum decrease in the total atmospheric flux by a factor of 12 at most if the natural interannual variability of atmospheric deposition in this region is taken into account. So the recorded decrease in the atmospheric lead flux since 1985 (by a factor of ∼30) reveals a slight local contamination at the earlier Corsican station. In the case of Cd and Zn, there was a decrease by a factor of 30 and 23, respectively, between the data obtained at La Tour du Valat (July 1988 to June 1989) and those at Pirio (March 1996 to March 1997); such a decrease cannot be the consequence of either a reduction in emissions in Europe as a whole (the factor being at most 4), or the distance from the emission sources. Hence, we can confirm that Zn and Cd fluxes at La Tour du Valat and Zn fluxes at Cap Ferrat were not representative of a long-range transport to the Mediterranean but the result of a local/regional contamination. The large decrease observed for metals (Cd, Zn, and Pb) mainly mobilized by human activities, results from a combination of the actual diminution of the concentrations due to a reduction in the emissions and the occurrence of local/regional contamination for some elements at some sampling sites.

The significance of the episodic nature of atmospheric deposition to Low Nutrient Low Chlorophyll regions

In the vast Low Nutrient Low-Chlorophyll (LNLC) Ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be one order of magnitude greater over short timescales. The short turnover time of atmospheric Fe and N supply (<1 month for nitrate) further supports deposition being an important source of nutrients in LNLC regions. Yet, the extent to which atmospheric inputs are impacting biological activity and modifying the carbon balance in oligotrophic environments has not been constrained. Here, we quantify and compare the biogeochemical impacts of atmospheric deposition in LNLC regions using both a compilation of experimental data and model outputs. A metadata-analysis of recently conducted field and laboratory bioassay experiments reveals complex responses, and the overall impact is not a simple “fertilization effect of increasing phytoplankton biomass” as observed in HNLC regions. Although phytoplankton growth may be enhanced, increases in bacterial activity and respiration result in weakening of biological carbon sequestration. The application of models using climatological or time-averaged non-synoptic deposition rates produced responses that were generally much lower than observed in the bioassay experiments. We demonstrate that experimental data and model outputs show better agreement on short timescale (days to weeks) when strong synoptic pulse of aerosols deposition, similar in magnitude to those observed in the field and introduced in bioassay experiments, is superimposed over the mean atmospheric deposition fields. These results suggest that atmospheric impacts in LNLC regions have been underestimated by models, at least at daily to weekly timescales, as they typically overlook large synoptic variations in atmospheric deposition and associated nutrient and particle inputs. Inclusion of the large synoptic variability of atmospheric input, and improved representation and parameterization of key processes that respond to atmospheric deposition, is required to better constrain impacts in ocean biogeochemical models. This is critical for understanding and prediction of current and future functioning of LNLC regions and their contribution to the global carbon cycle.

A Reassessment of Trace Metal Budgets in the Western Mediterranean Sea

This paper presents inputs and output fluxes of dissolved metals (As, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn) into and out the Western Mediterranean. These flux estimates are based on the most recently published concentrations and fluxes for the atmosphere, the rivers and the straits. Comparison of the different sources shows the predominance of the inputs through the straits over other sources. The river input is smaller than the atmospheric input except for As. For all elements except Fe, output flux and input flux are balanced; iron budget indicates transfer from the dissolved to the particulate phase.

Dissolved input of manganese to the ocean : aerosol source

Aerosol particle samples representative of polluted air, dust-laden air, and clean marine air were collected in marine regions and used in a series of dissolution studies. These samples were exposed to seawater for varying lengths of time and to deionized (Milli-Q®) water at various values of pH. The percentage of aerosol Mn dissolved in Milli-Q® water increased from 55 to 80% between pH 8 and pH 2 for pollution aerosols. Less dissolution occurred with the mineral aerosol particles, for which the dissolved Mn increased from 25 to 50% between pH 8 and pH 2. As similar behavior is found for particles collected in clean marine air, we conclude that the dissolution process for aerosol particles from a remote marine area, where crustal Mn dominates pollution Mn, is controlled by the background mineral content. The kinetics of Mn dissolution in seawater are rapid for all the samples: a concentration plateau is reached after 10 min or less of exposure. Release of dissolved Mn from polluted aerosols in seawater was approximately twice the value obtained with mineral particles (55 and 30%, respectively). Apparently, no additional Mn dissolution beyond that which has clearly taken place in rain occurs when rainwater enters the ocean. A positive relationship evidently exists between the total Mn concentration of the aerosol and the dissolved concentration after exposure in seawater. Extrapolating the relationship from the available data suggests that the dissolved saturation value is approximately 60 nmol L−1. Considering the different behavior found for the different types of particles, at least two cases must be considered when assessing mass balances or calculating atmospheric fluxes of Mn to the ocean. Based on the results of our dissolution studies, the resulting dissolved fluxes of Mn of atmospheric origin during a pulse of Saharan dust range from 0.16 to 0.33 μmol m−2d−1 over the duration of the pulse. The total dissolved Mn flux from anthropogenic sources in western North Europe is estimated to be 0.3 μmol m−2d−1. Fluxes of dissolved Mn of mineral and anthropogenic origin resulting from this calculation in the two examples selected are of the same order of magnitude. The spatial and temporal patterns of these different sources and the dynamics of the upper water column have to be taken into account to estimate quantitatively the impact of atmospheric input on Mn concentrations in the water column.

Transient fertilizing effect of dust in P-deficient LNLC surface ocean

In much of the worlds low-nutrient low-chlorophyll (LNLC) oceans, including the Mediterranean Sea, surface dissolved inorganic phosphorus (DIP) is below the detection limit of conventional techniques. Although dust deposition has been generally recognized as a major source of P to the Mediterranean Sea, the lack of DIP data at nanomolar levels has so far precluded a quantification of this effect. This work reports the first one-year time series of surface nanomolar DIP in the Mediterranean Sea. Moreover, by combining nanomolar DIP data from two field studies (the above cited time-series and an experimental addition of Saharan dust to large mesocosms) and one in vitro dust dissolution experiment, we show that dust pulses may indeed provoke transient increases in DIP concentration (up to 80 nM) in P-starved surface waters of this LNLC region. Citation: Pulido-Villena, E., V. Rerolle, and C. Guieu (2010), Transient fertilizing effect of dust in P-deficient LNLC surface ocean, Geophys. Res. Lett., 37, L01603, doi: 10.1029/2009GL041415.

Strong stimulation of N 2 fixation in oligotrophic Mediterranean Sea: results from dust addition in large in situ mesocosms

The response of N 2 (dinitrogen) fixation to contrasted (wet and dry) Saharan dust deposition was studied in the framework of the DUNE project (a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem) during which realistic simulations of dust deposition (10 g m -2 ) into large mesocosms (52 m 3 ) were performed. Three distinct experimental dust additions were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition, DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R: simulation of 2 successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. Here we show that wet and dry dust deposition induced a rapid (24 h or 48 h after dust additions), strong (from 2- to 5.3-fold) and long (at least 4-6 days duration) increase in N 2 fixation, indicating that both wet and dry Saharan dust deposition was able to relieve efficiently the nutrient limitation(s) of N 2 fixation. This means in particular that N 2 fixation activity was not inhibited by the significant input of nitrate associated with the simulated wet deposition (~ 9 mmol NO 3 - m -2 ). The input of new nitrogen associated with N 2 fixation was negligible relative to the atmospheric NO 3 - input associated with the dust. The contribution of N 2 fixation to primary production was negligible (≤ 1%) before and after dust addition in all experiments, indicating that N 2 fixation was a poor contributor to the nitrogen demand for primary production. Despite the stimulation of N 2 fixation by dust addition, the rates remained low, and did not significantly change the contribution of N 2 fixation to new production since only a maximum contribution of 10% was observed. The response of N 2 fixation by diazotrophs and CO 2 fixation by the whole phytoplankton community suggests that these metabolic processes were limited or co-limited by different nutrients. With this novel approach, which allows us to study processes as a function of time while atmospheric particles are sinking, we show that new atmospheric nutrients associated with Saharan dust pulses do significantly stimulate N 2 fixation in the Mediterranean Sea and that N 2 fixation is not a key process in the carbon cycle in such oligotrophic environments.

Does phosphate adsorption onto Saharan dust explain the unusual N/P ratio in the Mediterranean Sea?

A Saharan soil, considered as a proxy for Saharan aerosols, was used to perform radio-labelled phosphate adsorption experiments using 33 PO4 3– : leached particles were exposed to poisoned western Mediterranean seawater for varying lengths of time. The measured adsorption capacity of Saharan dust for phosphate was 0.13 µmol g –1 . Considering this value and an annual Saharan dust deposition of 12.5 t km –2 year –1 , we show that Saharan particles do not represent a significant sink for seawater phosphate in the western Mediterranean Sea. This result is in agreement with that determined from a similar approach conducted in the eastern basin. As a consequence, the unusual N/P ratio measured in the whole Mediterranean Sea (up to 29) cannot be explained by the adsorption process of seawater phosphate onto Saharan dust.