Subsurface iron accumulation and rapid aluminium removal in the Mediterranean following African dust deposition

Abstract. Mineral dust deposition is an important supply mechanism for trace elements in the low-latitude ocean. Our understanding of the controls of such inputs has been mostly built onto laboratory and surface ocean studies. The lack of direct observations and the tendency to focus on near surface waters prevent a comprehensive evaluation of the role of dust in oceanic biogeochemical cycles. In the frame of the PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea), the responses of the aluminium (Al) and iron (Fe) cycles to two dust wet deposition events over the central and western Mediterranean Sea were investigated at a timescale of hours to days using a comprehensive dataset gathering dissolved and suspended particulate concentrations, along with sinking fluxes. Dissolved Al (dAl) removal was dominant over dAl released from dust. Fe / Al ratio of suspended and sinking particles revealed that biogenic particles, and in particular diatoms, were key in accumulating and exporting Al relative to Fe. By combining these observations with published Al / Si ratios of diatoms, we show that adsorption onto biogenic particles, rather than active uptake, represents the main sink for dAl in Mediterranean waters. In contrast, systematic dissolved Fe (dFe) accumulation occurred in subsurface waters (~100–1000 m), while dFe input from dust was only transient in the surface mixed-layer. The rapid transfer of dust to depth (up to ~180 m d−1), the Fe-binding ligand pool in excess to dFe in subsurface (while nearly-saturated in surface), and low scavenging rates in this particle-poor depth horizon are all important drivers of this subsurface dFe enrichment. At the annual scale, this previously overlooked mechanism may represent an additional pathway of dFe supply for the surface ocean through diapycnal diffusion and vertical mixing. However, low subsurface dFe concentrations observed at the basin scale (< 0.5 nmol kg−1) questions the residence time for this dust-derived subsurface reservoir, and hence its role as a supply mechanism for the surface ocean, stressing the need for further studies. Finally, these contrasting responses indicate that dAl is a poor tracer of dFe input in the Mediterranean Sea.


atmospheric dFe input flux to the global ocean (~1-30 Gmol yr -1 ; Tagliabue et al., 2016), and hinder accurate predictions of the impact of dust on ocean productivity.
African dust deposition events have long been known to impact trace element concentrations and fluxes in the upper water column of the Mediterranean (e.g., Buat-Ménard et al., 1988;Davies and Buat-Ménard, 1990;Quétel et al., 1993;Guerzoni et al., 1999;Heimbürger et al., 2011). Our understanding of the role of dust in marine biogeochemical cycles remains 50 limited, however, partly resulting from the difficulty in quantifying atmospheric dust fluxes to the surface ocean at short timescales. In the absence of direct assessments of atmospheric inputs, marine concentrations of tracers such as aluminium (Al) have been widely used to constrain these fluxes (e.g., Measures and Brown, 1996;Han et al., 2008;Anderson et al., 2016;Menzel Barraqueta et al., 2019). Al is predominantly of crustal origin and is characterized by a similar fractional solubility than Fe with a longer residence time in seawater. Al could thus be used to constrain the integrated input of dust Fe 55 over seasonal timescales (Dammshäuser et al., 2011). However, the fact that the distribution of Al can itself be controlled by the biological activity (e.g., Mackenzie et al., 1978;Middag et al., 2015;Rolison et al., 2015) questions its quality as a tracer.
In addition, dust deposition being highly episodic in time and spatially patchy (Donaghay et al., 1991;Guieu et al., 2014a;Vincent et al., 2016), direct observations at sea are extremely challenging, and hence sparse (e.g., Croot et al., 2004;Rijkenberg et al., 2008). To overcome this limitation, a variety of small-volume enclosed systems have been used to quantify 60 Fe solubility from dust. Although yielding important insights into atmospheric trace element solubilities (Baker and Croot, 2010 and references therein), these systems do not fully simulate in situ conditions (de Leeuw et al., 2014), motivating the development of larger volume experiments (>100 L) where dust are free to sink and interact with dissolved and particulate organic matter while sinking Guieu et al., 2014b;Herut et al., 2016;Gazeau et al., in revision).
Two key findings emerged from these large volume experiments. First, they allowed demonstrating the pivotal role played 3 2013; Wuttig et al., 2013). The most striking and unexpected consequence is that upon deposition, dust can act as a net sink of dFe through scavenging (Wagener et al., 2010;Ye et al., 2011). Second, the large range in Fe solubility observed depending on the season, reveals that oceanic rather than atmospheric conditions, determine in fine the flux of 'truly' 70 bioavailable Fe to the surface ocean . However, these findings are valid in the first meters of the water column and direct observations of the whole water column are needed if we are to fully understand the role of dust in the oceanic iron cycle.
For this purpose, the Mediterranean Sea is a particularly relevant region. This semi-enclosed basin, characterized by a westto-east gradient in oligotrophy, receives some of the largest dust inputs of the ocean (Guerzoni et al., 1999), mostly under the 75 form of wet deposition in the central and western part of the Basin, and a few intense events may account for the bulk of the annual deposition (Loÿe-Pilot and Martin, 1996;Vincent et al., 2016). The PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea) and oceanographic campaign on board the R/V Pourquoi Pas? provided a unique opportunity to directly observe the biogeochemical effects of two mineral dust wet deposition events of contrasted intensity that occurred during late spring 2017 in the central and western open Mediterranean Sea (Guieu et al., 80 2020). The presence of the R/V before, during, and/or few days after deposition allowed investigating (1) the parameters and processes shaping the contrasting distributions of dAl and dFe, (2) the importance of the timescale considered when assessing the flux of bioavailable Fe to the surface ocean, and (3) the relevance of using dAl to constrain dFe input from dust.

Oceanographic cruise
The PEACETIME cruise (doi.org/10.17600/15000900) was conducted during late spring conditions in May and June 2017 aboard the R/V Pourquoi Pas? in the central and western Mediterranean Sea. In total, 10 short stations (~8 hours) and 3 long stations located in the Tyrrhenian Sea (TYR; occupation = 4 days), the Ionian Sea (ION; 4 days), and in the western Algerian basin (FAST; 5 days) were occupied (Fig. 1). FAST was an opportunistic station dedicated to investigate the biogeochemical 90 effects of a dust deposition event by combining atmospheric and oceanographic in situ measurements before, during and after deposition . At all stations, a 'classical' and a trace metal-clean (TMC) titanium rosette were deployed to sample the water column for biological and chemical parameters. Samples for aluminium and iron analyses were collected using the TMC titanium rosette mounted with GO-FLO bottles deployed on a Kevlar cable, while samples for particulate Al (pAl) determination were also collected at all the stations from the classical rosette (see section 2.3).  2. Note that the same color code is used in figures 2, 5 and 6.

Dissolved Al and Fe concentrations
Immediately after recovery, the GO-FLO bottles were transferred inside a class-100 clean laboratory container. Seawater samples were directly filtered from the GO-FLO bottles through acid-cleaned 0.2 µm capsule filters (Sartorius Sartobran-Pcapsule 0.45/0.2 µm). Dissolved Fe and Al samples were stored in acid-washed low-density polyethylene bottles and 105 immediately acidified to pH 1.8 (quartz-distilled HCl) under a laminar flow hood.
Dissolved Al analyses were conducted on board using the fluorometric method described by Hydes and Liss (1976). Briefly, the samples were buffered to pH 5 with ammonium-acetate and the reagent lumogallion was added. The samples were then heated to 80°C for 1.5 h to accelerate the complex formation. The fluorescence of the sample was measured with a Jasco FP-2020 Plus spectrofluorometer (excitation wavelength 495 nm, emission wavelength 565 nm). Calibration was realized with 110 additions of Al standard solution in seawater. The detection limit (DL; 3 times the standard deviation (SD) of the concentrations measured from the dAl-poor seawater used for calibration) varied between 0.2 and 0.5 nmol kg −1 . The reagent blank determined by measuring acidified ultrapure water varied between 0.9 and 1.7 nmol kg −1 .
Dissolved Fe concentrations were measured (mostly on board in the class-100 clean laboratory) using an automated Flow Injection Analysis (FIA) with online preconcentration and chemiluminescence detection (Bonnet and Guieu, 2006). The 115 stability of the analysis was assessed by analyzing daily an internal acidified seawater standard. On average, the DL was 15 pmol kg -1 (3 times the SD of the concentration measured 5 times from the same dFe-poor seawater) and the accuracy of the method was controlled by analysing on a regular basis the GEOTRACES seawater standards SAFe D1 (0.64 ±0.13 nmol kg -1 5 (n = 19), consensus value 0.67 ±0.04 nmol kg -1 ), GD (1.04 ±0.10 nmol kg -1 (n = 10), consensus value 1.00 ±0.10 nmol kg -1 ) and GSC (1.37 ±0.16 nmol kg -1 (n = 4), consensus value not available). 120

Suspended particulate trace elements
Just prior to sampling for particulate trace elements (pTM), GO-FLO bottles were gently mixed and pTM were sampled directly on-line from the pressurized (0.2 µm filtered N2) GO-FLO bottles onto acid-cleaned 25 mm diameter Supor 0.45 µm polyethersulfone filters mounted on Swinnex polypropylene filter holders (Millipore), following GEOTRACES recommendations. Filtration was stopped when the filter clogged or the bottle was empty. On average, each particulate 125 concentration was obtained by filtering 4.8 L (range 1.1-10.2 L). When the filtration was complete, filter holders were transferred under a laminar flow hood and residual seawater was removed using a polypropylene syringe. Filters were stored in acid-cleaned petri-slides, left open under the laminar flow hood for ~24 h to allow the filters to dry. Particulate samples were digested (10% HF / 50% HNO3 (v/v)) following the protocol described in the 'GEOTRACES Cookbook' and Planquette and Sherrell (2012). Procedural blanks consisted of unused acid-cleaned filters. Analyses were performed on a 130 HR-ICP-MS (High Resolution Inductively Coupled Plasma Mass Spectrometry; Element XR, Thermo-Fisher Scientific).
The accuracy of the measurements was established using the certified reference materials (CRM) MESS-4 and PACS-3 (marine sediments, National Research Council Canada) (Supp. Table 1).
In addition, pAl concentrations were also obtained at all the stations from the classical rosette. This additional pAl dataset already published by Jacquet et al. (in revision) was obtained according to the sampling, processing and analysis methods 135 described in Jacquet al. (2015). Briefly, 4 to 6 L of seawater collected with the Niskin bottles were filtered onto acid-cleaned 47 mm polycarbonate filters (0.4 µm porosity). Filters were rinsed with Milli-Q grade water and dried at 50°C. A total digestion of the membranes was performed using a tri-acid mixture (0.5 mL HF / 1.5 mL HNO3 / 1 mL HCl), and analyses were performed on the same HR-ICP-MS. A good agreement was obtained when comparing pAl concentrations obtained with the TMC and classical rosettes at ION and FAST (difference in sampling time at TYR prevents quantitative 140 comparison; see Sect. 4.1) (Supp. Fig. 2), demonstrating the absence of contamination for pAl when using the classical rosette.

Export fluxes and composition
Sinking particles were collected at ~200, 500, and 1000 m depth using PPS5 sediment traps (Technicap, France; 1 m 2 collection area) deployed on a free-drifting mooring for 4 (TYR and ION) and 5 days (FAST). Cups were filled with filtered 145 seawater and buffered formaldehyde (2% final concentration) as a biocide. Once recovered, each cup representing 24 hours of collection was stored in the dark at 4°C until processed. Samples were treated following the standard protocol followed at the national service "Cellule Piège" of the French INSU-CNRS (Guieu et al., 2005) following the JGOFS' protocol. After removing the swimmers, the remaining sample was rinsed three times with ultrapure water in order to remove salt, and then 6 freeze-dried. The total amount of material collected was weighted to quantify the total exported flux. Several aliquots were 150 then used to measure the following components: total and organic carbon, particulate Al and Fe, lithogenic and biogenic silica (LSi and BSi, respectively). Total carbon, particulate organic carbon (POC) (after removing inorganic carbon by acidification with HCl 2N), and particulate organic nitrogen (PON) were measured on an elemental analyzer CHN (2400 Series II CHNS/O Elemental Analyzer Perkin Elmer). For one sample (TYR 1000 m), 5 aliquots were analysed, yielding a coefficient of variation (CV) of 6%. Particulate inorganic carbon (PIC) was quantified by subtracting POC from total 155 particulate carbon. Particulate Fe and Al concentrations were determined by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry, Spectro ARCOS Ametek) after acid-digestion following the protocol described in Ternon et al. (2010). Blanks were negligible (<0.8% of the lowest Al and Fe concentrations of the digested aliquots) and the efficiency of the acid digestion was established using the CRM GBW-07313 (marine sediment, National Research Center for CRMs, China) (Supp. Table 1). Samples for BSi and LSi (2 or 3 aliquots) were digested (NaOH at 95°C and HF at ambient 160 temperature, respectively) and concentrations analysed by colorimetry (Analytikjena Specor 250 plus spectrophotometer) following the protocol described in Leblanc (2002). Mean export fluxes and composition of exported material are presented in Table 1.

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Discrete measurements at different depths were used to calculate the water column integrated Al and Fe inventories (µmol m -2 ) by trapezoidal integration. The concentration measured nearest to the surface was assumed to be constant up to 0 m. At FAST, six replicate measurements of dAl and dFe were performed at 5 and 400 m depth from 2 sets of 6 x GO-FLO bottles.
The CV obtained at 5 and 400 m depths were used to determine the uncertainties in the 0-20 m and 0-200 m inventories, respectively. Variability among replicates was higher for dFe (CV = 11.3 and 6.9% at 5 and 400 m depth, respectively) than 175 for dAl (CV = 5.3 and 1.1% at 5 and 400 m depth, respectively), potentially reflecting a small scale variability in the dFe distribution.
At the FAST station, the partitioning coefficient between the particulate and dissolved phases ( was used to investigate exchanges between dissolved and particulate pools of Al and Fe. Following the relative change over time of this metric allowed excluding potential artefacts related to change in water masses driven by lateral advection (Guieu 180 et al., 2020).

Biogeochemical conditions
The PEACETIME cruise took place in late spring when the stratification of the upper water column was well established with the mixed-layer depth ranging between ~10 and 20 m along the cruise track. Chlorophyll a concentrations were typical 185 of oligotrophic conditions . A diatom-dominated deep chlorophyll maximum (DCM) that coincided with a maximum in biomass and primary production was well developed and observed all along the cruise track (Marañón et al., 2021). POC downward fluxes measured at 200 m depth were similar at the 3 long stations, while downward fluxes of Al and LSi, two proxies for dust, were maximum at TYR (Table 1). At the surface, dAl distribution was characterized by a marked west-to-east increasing gradient (Supp. Fig. 1b) driven by advective mixing between (dAl poor) Atlantic and Mediterranean 190 waters and by the accumulation of dust, and reflected by a strong relationship between surface dAl concentrations and salinity (Guerzoni et al., 1999;Rolison et al., 2015). All along the transect, dFe concentrations were high in the upper 100 m (up to 2.7 nmol kg -1 ), and decreasing to levels <0.5 nmol kg -1 below the euphotic layer (Supp. Fig. 1c). Subsurface patches of high dFe concentrations previously observed in the eastern Mediterranean Basin, and attributed to hydrothermal activity and mud-volcanoes (Gerringa et al., 2017), were not observed along our cruise track.

Dust deposition over the central and western Mediterranean Sea
The impact in the water column of two dust deposition events of contrasting magnitudes could be studied during the cruise.
They occurred in the area of the TYR and FAST stations ( Fig. 1), on the 11-12 May and 3-5 June, respectively. The first 8 deposition event in the southern Tyrrhenian Sea was not directly observed but hypothesized based on satellite observations of intense dust plume transport and water-column Al inventory presented in the following. The combined analysis of timeseries of quick-looks of operational aerosol products from MSG/SEVIRI and from meteorological and dust transport models available during the campaign  allowed us to suspect that a red rain event likely occurred over the southern Tyrrhenian Sea on the 11 th of May and possibly on the early 12 th , as illustrated in Supp. Fig. 3. The daytime daily mean aerosol optical depth (AOD) product over oceanic areas (Thieuleux et al., 2005) shows that a large dust plume was The dust plume extension in the cloudy area on the 11 th is illustrated by Supp. Fig. 3b and c. Most meteorological models predicted significant precipitation over the Tyrrhenian Sea on the 11 th (Supp. Fig. 3d), and until the morning of the 12 th for some of them (not shown). Dust transport models producing dust deposition fluxes generally forecasted dust wet deposition 220 on that day between Tunisia and Italy, but with significant variability on the location, extent and schedule. The NMMB/BSC and SKIRON models predicted a significant wet deposition flux of dust, with up to 1.5 g m -2 over 6 h, or more in the area of our stations 5, TYR, and 6 in the afternoon of the 11 th of May (Supp. Fig. 3e and f). The DREAM model versions operated by the BSC and TAU, however, forecasted much lower values or even no dust wet deposition in the Tyrrhenian stations area (Supp. Fig. 3g). For simplicity, the 11 th of May 2017, 18:00 UTC will be considered as the time of deposition, that is 225 approximately 3 to 10 days before our sampling of the area.
During the early June deposition event in the western Algerian basin, precipitation was directly observed in the area of the R/V and even sampled onboard (Desboeufs et al., in prep.), associated with a dust transport event of moderate extent and intensity over the southwestern Mediterranean basin. The AOD550 peaked at about 0.40 in the area of the FAST station (Desboeufs et al., in prep.), corresponding to a maximum columnar dust load <0.4 g m -2 , assuming a non-dust background 230 AOD550 in the boundary layer of 0.10-0.15 as observed north of the plume or the day before the plume arrived. This dust plume encountered a massive rain front covering ~80,000 km 2 and moving eastward from Spain and North Africa regions 9 (Desboeufs et al.., in prep.). Direct atmospheric and oceanographic observations of this event were possible thanks to a dedicated 'fast action' strategy (see Guieu et al. (2020) for details). Two rain periods concomitant with the dust plume transported in altitude (1 to 4 km) allowed below-cloud deposition of dust in the FAST station area, as confirmed by on-235 board Lidar records (Desboeufs et al., in prep.). The first rain period occurred the 3 rd of June in the neighbouring area of the R/V, and the second one occurred from the 4 th (22:00 UTC) to the 5 th of June (9:00 UTC), and was sampled on board the R/V the 5 th of June from 00:36 to 01:04 UTC (Desboeufs et al., in prep.). This second rain event was characterized by a clear dust signature revealed by its chemical composition, representing a dust flux of about 40 mg m -2 (Desboeufs et al., in prep.).
This sampled flux, considered as relatively modest compared to the multi-year record in this area (Vincent et al., 2016), was 240 likely in the lower range of the total dust deposition flux that affected the whole area between the 3 rd and 5 th of June.

Reconstruction of the dust deposition fluxes
The absence of direct measurement of the dust deposition flux over the Tyrrhenian Sea and the limited spatial coverage of collection of atmospheric dust and rain at the FAST station call for an alternative approach to estimate dust deposition fluxes. For this purpose, we used the water-column Al inventory. We acknowledge that this approach involves uncertainties, 245 as all the observational approaches employed so far to quantify deposition (Anderson et al., 2016). Caveats include (1) Table   2). This spatial extent is in good agreement with the maps of precipitation and dust wet deposition provided for the 11 th of May by the ARPEGE, SKIRON, and NMMB/BSC models (Supp. Fig. 3).  Assuming that Al represents 7.1% of the dust in mass (Guieu et al., 2002), a dust deposition flux ranging between 1.7 (ST06) and 8.9 g m -2 (ST04) was derived from the Alexcess inventory (Table 2) Furthermore, this comparison with annual fluxes confirms that the annual deposition of African dust in the Mediterranean region is generally driven by only a few intense events (Loÿe-Pilot and Martin, 1996;Guerzoni et al., 1999;Kubilay et al., 2000;Desboeufs et al., 2018). The strong spatial variability of these dust flux estimates, with a marked west-to-east gradient, might result from the varying time lag between deposition and sampling of the water column at these different stations (Table 2), but also from the patchiness of the rainfalls associated with the rain front (Supp. Fig. 3). Indeed, Vincent et al.

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(2016) showed that high deposition events in the western Mediterranean are often limited spatially although the associated dust plumes may affect a large part of the basin. By assuming that the deposition was spatially homogeneous over the southern Tyrrhenian, an Al export flux of more than 4000 µmol m -2 d -1 is needed to explain the difference in the Alexcess inventory observed between ST04, ST05 and TYR (i.e., ~3.6 to 8.4 days after deposition). This order-of-magnitude difference with the Al export flux measured at TYR ~5 to 8 days after deposition (136 ±40 µmol m -2 d -1 ; Table 1) indicates 280 that the observed spatial variability was primarily driven by the precipitation patchiness rather than related to a sampling bias.

Western Mediterranean Sea
At the FAST station, dissolved and particulate Al and Fe concentrations were measured at high temporal and vertical resolutions before, during, and after the wet deposition of dust (Supp. Fig. 4). About 6 h after deposition, the total (dissolved  Fig. 3a and c).

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This increase in the 0-20 m inventories was consistent but higher than the atmospheric Al and Fe fluxes collected on the R/V (~98 and 25 µmol m -2 , respectively; Desboeufs et al., in prep.). Based on the increase in the 0-20 m total Al inventory and assuming 7.1% Al in the dust (Guieu et al., 2002), a total dust input of 55 mg m -2 was derived. Although direct collection of atmospheric dust aerosols represents the most straightforward approach for quantifying the dust flux, it has only a limited spatial coverage. At the opposite, the upper water-column inventory integrated most of the patchy rainfalls associated with 295 this large rain front. This difference in time and space integrations is best illustrated by the ~70% increase in the 0-20 m pAl and pFe inventories observed the 4 th of June (Fig. 3c), i.e., several hours before the rainfall collected onboard the R/V and probably associated with surrounding precipitations. It must be noted that the water-column approach is also subject to uncertainties and we cannot exclude an under-estimation of the deposition flux due to the rapid sinking of the largest dust particles (e.g., Bressac et al., 2012). However, no evidence of these fast-sinking particles was found deeper in the water 300 column (Fig. 3d), nor within the sediment traps (not shown).

Dust dynamic in the water column
In the Tyrrhenian Sea, deposition of dust was evidenced by the >3 times higher Al and LSi downward fluxes measured at 200 and 1000 m depth ~5 to 9 days after deposition relative to those measured at ION and FAST at the same depths (Table   1). At TYR, Al and LSi fluxes increased both by 35% between 200 and 1000 m depth, suggesting that a significant fraction 315 of the dust particles was rapidly transferred to depth. This trend is consistent with the pAl vertical profiles at the 4 stations likely impacted by this event, as a subsurface maximum was depicted between ~200 and 500 m depth ( Fig. 2a-d). In addition, three pAl vertical profiles performed at TYR over ~72 h showed a continuous decrease in surface pAl concentration of 20 µmol m -2 d -1 that was accompanied by subsequent increases within the ~150-500 m depth layer (Fig. 4).
It is worth noting that ~5.6 days after the event, remarkably high pAl concentration observed at 1000 m depth (~260 nmol 320 kg -1 (TYR_1); not shown) could indicate that dust particles were sinking at a rate of ~180 m d -1 . This finding confirms that dust particles can be rapidly transferred to depth either alone (Bressac et al., 2012) or incorporated into biogenic aggregates (e.g., Hamm, 2002;Bressac et al., 2014;Laurenceau-Cornec et al., 2019;van der Jagt et al., 2018). Together, these observations demonstrate the atmospheric origin of pAl observed in the southern Tyrrhenian (rather than sediment resuspension or advective inputs), and confirm that a significant fraction of the dust particles (coarse fraction) can rapidly 325 leave the surface mixed-layer when the stratification is strong (Croot et al., 2004;Ternon et al., 2010;Nowald et al., 2015), while the remaining fraction (small-sized particles) likely accumulates along the thermocline until the disruption of the stratification (Migon et al., 2002).
At the FAST station, a two-orders of magnitude lower dust deposition flux (~55 mg m -2 ) led to an increase by 78% of the 0-20 m pAl inventory (Fig. 3c). About 24 h after deposition, only ~40% of this signal was still present in the mixed-layer. This 330 is consistent with a short residence time in surface water for a significant fraction of the dust, although we cannot exclude the effect of lateral advection . Deeper in the water column, the trend is more complicated to interpret with a 40% decrease (~2000 µmol m -2 ) of the 0-200 m pAl inventory that occurred before/during deposition (Fig. 3d). This unexpected decrease cannot be explained by the vertical transfer of pAl, as only ~130 µmol m -2 of pAl were exported out of the upper 200 m over 5 days (data not shown). On the other hand, a southwestward flow disrupted the water column in the 335 ~25-100 m depth range from the 3 rd of June bringing water masses of distinct properties . Therefore, it is likely that the water mass sampled before deposition (2 nd of June) was different from the one sampled during the rest of the time-series. For this reason, inventories obtained the 4 th of June (instead of the 2 nd ) were used as background level to investigate the temporal evolution of Kd(Al) and Kd(Fe) in the 0-200 m depth range (Fig. 3f).

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A relatively large range in Al fractional solubility (1-15%; defined as the fraction of dust-derived Al that dissolves in rainwater or seawater) has been reported (e.g., Orians and Bruland, 1986;Baker et al., 2006;Measures et al., 2010;Han et al., 2012). Assuming a conservative Al fractional solubility of 1.5% in seawater (Wuttig et al., 2013), dust deposition over the Tyrrhenian Sea led to a dAl input ranging between 68 and 363 µmol m -2 (Table 2) (Fig. 5). Several mechanisms can be invoked here to explain the absence of dAl signal in the upper water column following the deposition. First, high surface dAl concentrations (>20 nmol kg -1 ) might mask any additional input. At ST06, the putative dAl input of 3.3 nmol kg -1 (Table 2) is within the range of variability of both published and observed 355 surface concentrations (30-43 nmol kg -1 ; Fig. 2h). At the other end (ST04), the situation is different with a dAl input of 17.7 nmol kg -1 that would represent more than 55% of pre-depositional surface dAl concentrations (31-32 nmol kg -1 ; Rolison et al., 2015). Considering the short time lag between deposition and observations at ST04 (~3.6 days), and the very close to 1D dynamic condition in the TYR station area (A. Doglioli, pers. comm., 2020), it is unlikely that advective mixing diluted any 15 elevated dAl signal from this event. Deeper in the water column, no clear trend was obtained with subsurface dAl 360 concentrations lower (ST04; Fig. 2e) or slightly higher than background levels (TYR; Fig. 2g). Similarly, no noticeable increase in dAl could be observed at the FAST station in the mixed-layer ( Fig. 3a and 5). In contrast to Kd(Fe), Kd(Al) was still higher than pre-depositional value 4 days after deposition (Fig. 3e), potentially reflecting a lower fractional solubility for dust-derived Al relative to Fe, and/or higher removal rate for dAl. Below the mixed-layer, Kd(Al) remained relatively constant and similar to initial value (Fig. 3f). Together, these observations indicate that wet deposition of dust over the FAST 365 station area had a limited impact on the dAl inventory.

Drivers of the rapid removal of dAl
An Al fractional solubility of 5% was measured in rainwater for dust aerosols collected at the FAST station (Desboeufs et al., in prep.), i.e., well above the conservative value of 1.5% used to estimate dAl inputs over the Tyrrhenian Sea. This further supports the need of rapid dAl removal via adsorption and/or biological uptake to explain the absence of dAl 375 anomaly following the dust events. In the Mediterranean Sea, a biological control on dAl distribution has been proposed to explain the strong coupling between dAl and orthosilicic acid (Si(OH)4) in subsurface waters (Chou and Wollast, 1997;Rolison et al., 2015). In addition, several laboratory and field studies have demonstrated that marine phytoplanktons, in particular diatoms (mainly incorporated into the frustules; Gehlen et al., 2002), can uptake and/or scavenge dAl (Mackenzie et al., 1978;Orians and Bruland, 1986;Moran and Moore, 1988;Loucaides et al., 2010;Twining et al., 2015b;Wuttig et al., 380 2013;Liu et al., 2019). To investigate the respective role of particle adsorption and biological uptake in removing dAl, Al was compared to Fe -a particle-reactive and bioactive element (Tagliabue et al., 2017) predominantly of crustal origin in the Mediterranean Sea -through the Fe/Al content of suspended and sinking particles collected at different depth horizons (Fig.   6).
For suspended particles, the median Fe/Al ratio was maximum within the surface mixed-layer, and minimum at the DCM 385 (60-100 m; Fig. 6a), highlighting a strong contrast in Fe/Al between the diatom-dominated particle assemblage at the DCM (Marañón et al., 2021) and detrital/lithogenic particles in the rest of the water column. This contrast supports the important role played by phytoplanktons, and in particular diatoms, in accumulating Al via active uptake (Gehlen et al., 2002;Liu et al., 2019) and/or adsorption onto cell membranes (Dammshäuser and Croot, 2012;Twining et al., 2015b). Regarding sinking particles collected at ION and FAST, Fe/Al was strongly correlated with the relative proportion of LSi and BSi (R 2 = 0.78, p 390 <0.001; Fig. 6b). Interestingly, this linear model predicts a Fe/Al ratio for BSi of ~0.22 mol mol -1 (y-intercept), similar to the value observed in the diatom-dominated DCM (Fig. 6a). At TYR, the large dust input likely masked the signature of diatoms, as the median Fe/Al ratio in sinking particles (0.25 mol mol -1 ) was similar to the Fe/Al ratio obtained for (1) suspended particles in the dust-impacted mixed-layer (Fig. 6a), and (2) particulate phase of the dusty rainwater sampled at FAST (0.26 mol mol -1 ; Desboeufs et al., in prep.). Sparse Al/Si ratios available for natural diatom communities range 395 between ~1 and 10 µmol mol -1 (van Bennekom et al., 1989;Gehlen et al., 2002;Koning et al., 2007).  Table 2) indicates that adsorption onto biogenic particles (including BSi), rather than active uptake by diatoms, was likely the main sink for dAl in that region.

Transient dFe increase in the surface mixed-layer
The absence of pre-depositional observations in the Tyrrhenian Sea is more problematic for Fe compared to Al, as no clear longitudinal trend has been reported in the Mediterranean Sea for that element. Dissolved Fe vertical profiles were thus compared to previously published data that were obtained at similar locations ( Fig. 1), and at the same period of the year for ST04 (mid-April) but about 2 months later at TYR and ST06 (early August) (Gerringa et al., 2017). Consequently, this 415 approach ignores interannual and seasonal variabilities in dFe, and cannot be used to strictly quantify dFe input but remains valuable to investigate its magnitude and vertical distribution.
Assuming a Fe content of 4.45% in dust (Guieu et al., 2002), this dust event over the Tyrrhenian Sea represented a Fe input of ~1300-7000 µmol m -2 (with a short retention time within the sea surface microlayer (Tovar-Sánchez et al., 2020)). Yet, dFe concentrations within the surface mixed-layer were at background levels (ST04 and ST06) or slightly below (TYR) (Fig.   420 2i-l). These observations made ~3 to 10 days after deposition indicate that this event had no impact on dFe in the surface mixed-layer at a timescale of days. At a shorter timescale, sampling performed at a high temporal resolution at the FAST station revealed two distinct increases of the 0-20 m dFe inventory that occurred during (+13 µmol m -2 ) and about 6 h after deposition (+15 µmol m -2 ; Fig. 3a). These ~50% increases were only transient and the pre-depositional level was rapidly recovered. Considering that Fe cycling in this LNLC system is dominated by physico-chemical rather than biological 425 processes, our findings are consistent with a rapid scavenging of dFe in surface Mediterranean waters following dust deposition, as already reported in some mesocosm and minicosm dust addition experiments (Wagener et al., 2010;Wuttig et al., 2013;Bressac and Guieu, 2013). Overall, the Fe-binding ligand pool is nearly saturated in surface Mediterranean waters (Gerringa et al., 2017). As a consequence, any new input of dFe will tend to precipitate, pointing to the importance of the initial dFe and Fe-binding ligand concentrations in setting the net effect of dust input on dFe in the surface mixed-layer (Ye 430 et al., 2011;Wagener et al., 2010;Wuttig et al., 2013).

Enrichment in dFe below the surface mixed-layer
A key feature in the southern Tyrrhenian was the systematic subsurface excess in dFe observed from ~40 m (ST04) and 200 m depth (TYR and ST06) (Fig. 2i-l), and mirroring the vertical distribution of Alexcess ( Fig. 2a-d). Similarly, wet dust deposition over the FAST station area led to a net input of dFe mainly below the mixed-layer, as revealed by the opposite 435 trends in Kd(Fe) observed in the 0-20 m and 0-200 m depth ranges (Fig. 3e-f). This increase in dFe relative to pFe was persistent on a timescale of days (Fig. 3f), and was primarily driven by dust dissolution (Fig. 3b) rather than ballasting of pre-existing pFe (Fig. 3d), as evidenced by the low export Fe flux collected at 200 m depth (1.7-12.3 µmol m -2 d -1 ). This systematic excess in dFe observed below the mixed-layer and extending to 1000 m suggests that the mechanisms involved are independent of the dust flux -that differed by two-orders of magnitude -and timescale considered (hours to week). Such 440 dust-related subsurface enrichment in dFe (without enhanced surface dFe concentrations) has already been observed in the subarctic Pacific and tropical North Atlantic. This feature was attributed either to low oxygen levels allowing Fe(II) to stay in solution (Schallenberg et al., 2017), or to remineralization of organic matter formed in the dust-laden surface ocean Fitzsimmons et al., 2013); two mechanisms that cannot be invoked here considering the oxygen levels in subsurface (170-200 µM), the short timescale considered, and the low mesopelagic Fe regeneration efficiency 445 (Bressac et al., 2019).
To account for this dFe excess below the surface mixed-layer, dust-bearing Fe must continue to dissolve as dust particles settle through the mixed-layer and reach the mesopelagic. The short residence time for dust in surface (Sect. 4.1), and the presence of a 'refractory' Fe pool within dust particles that dissolves over several days (Wagener et al., 2008) confirm that dust dissolution can occur in subsurface. It is also likely that low particle concentration encountered at these depths relative 450 to the particle-rich surface waters at the time of deposition prevented rapid removal of dFe (e.g., Spokes and Jickells, 1996;Bonnet and Guieu, 2004). Furthermore, the Fe-binding ligand pool is pivotal in setting the Fe fractional solubility (Rijkenberg et al., 2008;Wagener et al., 2008Wagener et al., , 2010Ye et al., 2011;Fishwick et al., 2014), and its magnitude, composition, and distribution likely shape patterns of dFe supply. While nearly saturated in surface, the Fe-binding ligand pool is in relatively large excess to dFe in subsurface Mediterranean waters (Gerringa et al., 2017), and hence available to stabilize 455 new dFe. Importantly, this subsurface pool is constantly replenished by bacterial degradation of sinking biogenic particles (Boyd et al., 2010;Velasquez et al., 2016;Bressac et al., 2019;Whitby et al., 2020). Thus, there is a permanent resetting of the ligand pool while dust particles settle (Bressac et al., 2019), and conceptually, we can imagine that the binding equilibrium between available ligands and Fe is rarely reached at these depths and timescale. This fundamental difference with the surface waters (and batch experiments) could explain the high Fe fractional solubility of 4.6-13.5% derived in the 460 southern Tyrrhenian from the increase in the 0-1000 m dFe inventories (relative to published data; Fig. 2i-l), and assuming 4.45% Fe in the dust (Guieu et al., 2002).
By feeding the subsurface dFe reservoir, dust deposition could represent an indirect supply route for the surface ocean through vertical mixing and diapycnal diffusion (e.g., Tagliabue et al., 2014). However, the residence time of this dustderived reservoir remains an open question. Relatively low subsurface dFe concentrations observed at the basin-scale (<0.5 465 nmol kg -1 ; Supp. Fig. 1), compared to Atlantic waters for instance (Gerringa et al., 2017), argue in favour of a short residence time. Scavenging by sinking (dust) particles (e.g., Wagener et al., 2010;Bressac et al., 2019), and bacterial removal of humic-like ligands (Dulaquais et al., 2018;Whitby et al., 2020) represent two potential sinks for this subsurface dFe reservoir that need to be explored.

470
During the PEACETIME cruise performed in May-June 2017 in the western and central Mediterranean, the determination of the Al and Fe water-column distributions allowed us the observation at sea of two atmospheric wet deposition events, providing important insights into the timescale and pattern of dAl and dFe inputs from African dust in the remote Mediterranean Sea. The use of water-column Al inventory was needed -and successful -to assess dust deposition fluxes in complement to atmospheric measurements and the 'fast-action' strategy used during the campaign to directly sample dusty 475 rain events. Our observations show that dAl removal through adsorption onto biogenic particles was dominant over dAl released from dust at a timescale of hours to days. While surface dAl concentrations reflect seasonal changes and large scale patterns in dust deposition, this finding indicates that this tracer may not be appropriate to trace the imprint of a single dust deposition event in highly dust-impacted areas. Furthermore, dust deposition represented a significant input of dFe in the surface mixed-layer only on a timescale of hours. On a longer timescale (days/weeks), dFe inputs occurred primarily below 480 the surface mixed-layer and extended until 1000 m depth where the Fe-binding ligand pool likely in excess to dFe allows stabilizing any additional input of dFe. This mechanism may represent an additional pathway of dFe resupply for the surface ocean (through vertical mixing and diapycnal diffusion), although the residence time of this dust-derived dFe reservoir still needs to be investigated.

485
Underlying research data are being used by researcher participants of the PEACETIME campaign to prepare other papers, and therefore data are not publicly accessible at the time of publication. Data will be accessible once the special issue is completed (June 2021) (http://www.obs-vlfr.fr/proof/php/PEACETIME/peacetime.php; last access 02/04/2021