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To assess the impacts of Amazon River discharge, Saharan dust deposition, N2‐fixation and mixed‐layer deepening on the biological carbon pump, sediment traps were moored from October 2012 to November 2013 at two sites in the western tropical North Atlantic (49°W,12°N/57°W,12°N). Particle exports interpreted along with satellite‐ and Argo‐float data show peak fluxes in biogenic silica (31 mg m−2d−1) and organic carbon (25 mg m−2d−1) during the fall of 2013 that were ten to five times higher than any time earlier during the year. These high export fluxes occurred in tandem with high surface chlorophyll a concentrations associated with the dispersal of the Amazon River plume, following retroflection into the North‐Atlantic‐Counter‐Current. High fucoxanthin fluxes (> 80 μg m−2d−1) and low δ15N‐values (−0.6‰) suggest a large contribution by marine diatom‐diazotrophic‐associations, possibly enhanced by wet Saharan dust deposition. During summer, the Amazon River plume resulted in high mass fluxes at 57°W that were enriched in biogenic silica but weakly influenced by diazotrophic‐associations compared to the fall event at 49°W. High carbonate‐carbon fluxes (17 mg m−2d−1) dominated a second single event at 49°W during spring that was likely triggered by mixed‐layer deepening. Rain‐ratios of BSi/Ccarb amounted to 1.7 when associated with high export fluxes linked to the Amazon River plume. Compared to an annual average of 0.3, this indicates a more efficient uptake of CO2 via the biological pump compared to when the plume was absent, hence supporting earlier observations that the Amazon River plume is important for ocean CO2 sequestration.

Massive amounts of Saharan dust are blown from the coast of northern Africa across the Atlantic Ocean towards the Americas each year. This dust has, depending on its chemistry, direct and indirect effects on global climate which include reflection and absorption of solar radiation as well as transport and deposition of nutrients and metals fertilizing both ocean and land. To determine the temporal and spatial variability of Saharan dust transport and deposition and their marine environmental effects across the equatorial North Atlantic Ocean, we have set up a monitoring experiment using deep-ocean sediment traps as well as land-based dust collectors. The sediment traps were deployed at five ocean sites along a transatlantic transect between north-west Africa and the Caribbean along 12° N, in a downwind extension of the land-based dust collectors placed at 19° N on the Mauritanian coast in Iouîk. In this paper, we lay out the setup of the monitoring experiment and present the particle fluxes from sediment trap sampling over 24 continuous and synchronized intervals from October 2012 through to November 2013. We establish the temporal distribution of the particle fluxes deposited in the Atlantic and compare chemical compositions with the land-based dust collectors propagating to the downwind sediment trap sites, and with satellite observations of Saharan dust outbreaks. First-year results show that the total mass fluxes in the ocean are highest at the sampling sites in the east and west, closest to the African continent and the Caribbean, respectively. Element ratios reveal that the lithogenic particles deposited nearest to Africa are most similar in composition to the Saharan dust collected in Iouîk. Downwind increasing Al, Fe and K contents suggest a downwind change in the mineralogical composition of Saharan dust and indicate an increasing contribution of clay minerals towards the west. In the westernmost Atlantic Ocean, admixture of re-suspended clay-sized sediments advected towards the deep sediment trap cannot be excluded. Seasonality is most prominent near both continents but generally weak, with mass fluxes dominated by calcium carbonate and clear seasonal maxima of biogenic silica towards the west. The monitoring experiment is now extended, with autonomous dust sampling buoys for better quantification of Saharan dust transport and deposition from source to sink and their impact on fertilization and carbon export to the deep ocean.

Incubation experiments comprising Saharan dust additions were conducted in the tropical North Atlantic Ocean along an east-west transect at 12° N to study the phytoplankton response to nutrient release in oligotrophic seawater conditions. Experiments were performed at three stations (M1, M3, M4), mimicking wet and dry deposition of low and high amounts of Saharan dust deposition from two different dust sources (paleo-lake and sand dune). Dust particle sizes were adjusted to resemble dust that is naturally deposited over the ocean at the experiment sites. For wet dust deposition, the dust was pre-leached in acidified ‘artificial rainwater’ (H2SO4) for 16 to 24 hours, mimicking acid cloud processing at different pH values. Experiments were run up to eight days. Daily nutrient measurements of phosphate (PO43−), silicate (SiO44−), nitrate (NO3−) and cell abundances were performed in addition to measurements of concentrations of total dissolved iron (DFe), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) at the start and at the end of the experiments. A significant initial increase and subsequent gradual decrease in PO43−, SiO44− and DFe concentrations were observed after wet dust deposition using high amounts of dust previously leached in low pH rain (H2SO4, pH = 2). Remarkably, the experiments showed no nutrient release (PO43−, SiO44− and DFe) from dry-dust addition and the NO3− concentrations remained unaffected in all (dry and wet) experiments. The prokaryotic cyanobacterium Synechococcus spp. was the most prominent picophytoplankton in all mixed layer experiments. After an initial increase in cell abundance, a subsequent decrease (at M1) or a slight increase (at M3) with similar temporal dynamics was observed for dry and wet dust deposition experiments. The POC concentrations increased in all experiments and showed similar high values after both dry and wet dust deposition treatments, even though wet dust deposition is considered to have a higher potential to introduce bioavailable nutrients (i.e. PO43−, SiO44− and DFe) into the otherwise nutrient-starved oligotrophic ocean. Our observations suggest that such nutrients may be more likely to favor the growth of the phytoplankton community when an additional N-source is also available. In addition to acting as a fertilizer, our results from both dry and wet dust deposition experiments suggest that Saharan dust particles might be incorporated into marine snow aggregates leading to similar high POC concentrations.

In September 2009, a major dust storm passed over the southeast coast of Australia and covered the large city of Sydney that is located on the eastern coast of Australia. It was referred to as the 'Red Dawn' event. Along its course across the state of New South Wales, the dust plume passed over Canberra, the capital of Australia located some 300 km from Sydney. In this study we identified the sources of the dust and tracked the progression of the dust storm using satellite imagery. We also examined the meteorological conditions that led to the formation of the dust plume. We also investigated the microbial and lipid composition, grain-size distribution, pollen content and geochemical composition of several samples of the dust that fell in Canberra (called 'Canberra dust'), with particular attention paid to the ratio of some rare earth elements as well as strontium and neodymium isotopes. This was done to identify a geochemical and palynological 'fingerprint' of this dust to determine the source of the material. Collectively, the meteorological and fingerprinting analyses identified the principal location of dust entrainment as between the large playa Lakes Gairdner and Torrens in arid South Australia, some 1600 km away from Canberra. We also determined through Sr isotope analyses and other elements measured in the dust that fell in Canberra and the township of Eden, located south of Sydney, that the dust changed composition slightly over a few hours, being the consequence of local dust entrainment along the path of the dust plume across South Australia and New South Wales. In addition, we present a scenario which explains dust transport away from the direction of the major dust plume. Those investigations carried out on the Canberra dust are compared with a previously documented event when dust was also sampled in Canberra in 2002. The origin and composition of the 2002 dust plume was clearly different from that which occurred in 2009, as demonstrated by the different chemical (organic and inorganic) composition, grain-size characteristics and palynological fingerprints, as well as microbial composition. © 2014 Elsevier B.V. All rights reserved.

Dust storms are a major source of aerosolized bacteria, especially in the drought conditions experienced in Australia in the decade to 2009. The major aims of this project were to identify the culturable bacteria in environmental samples and to genetically fingerprint all isolates using repetitive element PCR (Rep-PCR) to investigate the possibility of tracking isolates from their source into the atmosphere. Four field trips were conducted to a dry lake in western Victoria, Australia to sample aerosols and sediments. Aerosols were collected at heights up to 150 m using vacuum pumps with filters attached to a tethered helium balloon, while corresponding sediments were collected in sterile polypropylene tubes. Isolates were cultivated on Tryptic Soy Agar, R2 Agar and Marine Agar, and grown in dark conditions at ambient temperature. By sequencing the 16S rRNA gene of 270 isolates, fifteen different bacterial families were identified, with both the aerosols and sediments dominated by the Bacillaceae family. Four sets of Rep-PCR primers were tested, with the ERIC and (GTG)5 primers proving to be the most suitable for fingerprinting the cultured taxa. Rep-PCR revealed very high strain diversity in the samples collected, however some strains were still able to be tracked from sediments up to 150 m in height. This shows the potential of Rep-PCR, however very large reference databases would be required for the technique to be more useful.

Massive amounts of Saharan dust are blown from the African coast across the Atlantic Ocean towards the Americas each year. This dust has, depending on its chemistry, direct and indirect effects on global climate including reflection and absorption of solar radiation as well as transport and deposition of nutrients and metals fertilizing both ocean and land. To determine the temporal and spatial variability of Saharan dust transport and deposition and their marine environmental effects across the equatorial North Atlantic Ocean, we have set up a monitoring experiment using deep-ocean sediment traps as well as land-based dust collectors. The sediment traps were deployed at five ocean sites along a transatlantic transect between northwest Africa and the Caribbean along 12⁰ N, in a down-wind extension of the land-based dust collectors placed at 19⁰ N on the Mauritanian coast in Iwik. In this paper, we lay out the setup of the monitoring experiment and present the particle fluxes from sediment trap sampling over 24 continuous and synchronised intervals from October 2012 through to November 2013. We establish the temporal distribution of the particle fluxes deposited in the Atlantic and compare chemical compositions with the land-based dust collectors propagating to the down-wind sediment trap sites, and with satellite observations of Saharan dust outbreaks. First-year results show that the total mass fluxes in the ocean are highest at the sampling sites in the east and west, closest to the African continent and the Caribbean, respectively. Element ratios reveal that the lithogenic particles deposited nearest to Africa are most similar in composition to the Saharan dust collected in Iwik. Down-wind increasing Al, Fe and K contents suggest a downwind change in the mineralogical composition of Saharan dust and indicate an increasing contribution of clay minerals towards the west. In the westernmost Atlantic, admixture of re-suspended clay-sized sediments advected towards the deep sediment trap cannot be excluded. Seasonality is most prominent near both continents but generally weak, with mass fluxes dominated by calcium carbonate and clear seasonal maxima of biogenic silica towards the west. The monitoring experiment is now extended with autonomous dust sampling buoys for better quantification Saharan dust transport and deposition from source to sink and its impact on fertilization and carbon export to the deep ocean.