Fluxes and distribution of dissolved iron in the eastern (sub-) tropical North Atlantic Ocean

Aeolian dust transport from the Saharan/Sahel desert regions is considered the dominant external input of iron (Fe) to the surface waters of the eastern (sub-) tropical North Atlantic Ocean. To test this hypothesis, we investigated the sources of dissolved Fe (DFe) and quantified DFe fluxes to the surface ocean in this region. In winter 2008, surface water DFe concentrations varied between <0.1 nM and 0.37 nM, with an average of 0.13 ± 0.07 nM DFe (n = 194). A strong correlation between mixed layer averaged concentrations of dissolved aluminum (DAl), a proxy for dust input, and DFe indicated dust as a source of DFe to the surface ocean. The importance of Aeolian nutrient input was further confirmed by an increase of 0.1 nM DFe and 0.05 ?M phosphate during a repeat transect before and after a dust event. An exponential decrease of DFe with increasing distance from the African continent, suggested that continental shelf waters were a source of DFe to the northern part of our study area. Relatively high Fe:C ratios of up to 3 × 10?5 (C derived from apparent oxygen utilization (AOU)) indicated an external source of Fe to these African continental shelf waters. Below the wind mixed layer along 12°N, enhanced DFe concentrations (>1.5 nM) correlated positively with apparent oxygen utilization (AOU) and showed the importance of organic matter remineralization as an DFe source. As a consequence, vertical diffusive mixing formed an important Fe flux to the surface ocean in this region, even surpassing that of a major dust event

Interferences in the analysis of nanomolar concentrations of nitrate and phosphate in oceanic waters

This paper reports on investigations into interferences with the measurements of nanomolar nitrate + nitrite and soluble reactive phosphate (SRP) in oceanic surface seawater using a segmented continuous flow autoanalyser (SCFA) interfaced with a liquid-waveguide capillary flow-cell (LWCC). The interferences of silicate and arsenate with the analysis of SRP, the effect of sample filtration on the measurement of nanomolar nitrate + nitrite and SRP concentrations, and the stability of samples during storage are described. The investigation into the effect of arsenate (concentrations up to 100 nM) on phosphate analysis (concentrations up to 50 nM) indicated that the arsenate interference scaled linearly with phosphate concentrations, resulting in an overestimation of SRP concentrations of 4.6 ± 1.4% for an assumed arsenate concentration of 20 nM. The effect of added Si(OH)4 was to increase SRP signals by up to 36 ± 19 nM (at 100 [mu]M Si(OH)4). However, at silicate concentrations below 1.5 [mu]M, which are typically observed in oligotrophic surface ocean waters, the effect of silicate on the phosphate analysis was much smaller (&lt;=0.78 ± 0.15 nM change in SRP). Since arsenate and silicate interferences vary between analytical approaches used for nanomolar SRP analysis, it is important that the interferences are systematically assessed in any newly developed analytical system. Filtration of surface seawater samples resulted in a decrease in concentration of 1.7-2.7 nM (±0.5 nM) SRP, and a small decrease in nitrate concentrations which was within the precision of the method (±0.6 nM). A stability study indicated that storage of very low concentration nutrient samples in the dark at 4 °C for less than 24 h resulted in no statistically significant changes in nutrient concentrations. Freezing unfiltered surface seawater samples from an oligotrophic ocean region resulted in a small but significant increase in the SRP concentration from 12.0 ± 1.3 nM (n = 3) to 14.7 ± 0.6 nM (n = 3) (Student's t-test; p = 0.021). The corresponding change in nitrate concentration was not significant (Student's t-test; p &gt; 0.05).<br/

Aerosol time-series measurements over the tropical Northeast Atlantic Ocean: Dust sources, elemental composition and mineralogy

The North Atlantic receives the largest dust loading of any of the world's oceans due to its proximity to North African deserts and prevailing wind patterns. The supply of biologically important trace elements and nutrients via aerosols has an important influence on biogeochemical processes and ecosystems in this ocean region. In this study we continuously sampled aerosols between July 2007 and July 2008 at the Cape Verde Atmospheric Observatory (CVAO), which is situated on an island group close to the North African continent and under the Saharan/Sahelian dust outflow path. The aim of our work was to investigate temporal variations in aerosol concentration, composition and sources in the Cape Verde region over a complete seasonal cycle, and for this purpose we undertook mineralogical and chemical (42 elements) analyses of the aerosol samples and air mass back-trajectory calculations. Aerosol samples were also collected during a research cruise in the (sub-) tropical Northeast Atlantic Ocean in January 2008. The concentration of atmospheric Al, a proxy for mineral aerosol concentration, at CVAO was in the range of 0.01–66.9 μg m− 3 (maximum on 28–30 January 2008) with a geometric mean of 0.76 μg m− 3. It showed distinct seasonal variations, with enhanced Al concentrations in winter (geometric mean 1.3 μg m− 3), and lower concentrations in summer (geometric mean 0.48 μg m− 3). These observations have been attributed to dust transport occurring in higher altitude air layers and mainly north of the Cape Verde during summer, while in winter the dust transport shifts south and occurs in the lower altitude trade winds with consequent greater influence on the Cape Verde region. The elemental composition of the aerosols closely agreed with mean upper crustal abundances, with the exception of elements with pronounced anthropogenic sources (e.g. Zn and Pb) and major constituents of sea water (Na and Mg). Mineral analysis showed that clays were the most abundant mineral fraction throughout the whole sampling period, with an increase in quartz and clays during strong dust events and an associated decrease in calcite. This could have important implications for the estimation of release of for example Fe from mineral dust with clays having a higher Fe solubility than quartz. The elemental composition and mineralogy of aerosol samples collected during the cruise were indistinguishable from those collected at the CVAO during the same period, although mean atmospheric Al was 65% higher at the CVAO than those measured on the ship due to the irregular and uneven nature of dust transport. Air mass back-trajectories showed an important role for southern source regions of the North African deserts during summer, with 92.5% of the samples indicating a contribution from the Sahel. Significantly elevated ratios of V, Ni, Cu, Zn, Cd and Pb with Al were present in samples originating from the Sahel compared with samples with a more northerly origin. This was likely due to enhanced anthropogenic emissions related to the greater population densities in the Sahel compared with the less developed Saharan regions further north. Ratios of other elements and trends in rare earth elements could however not be used to distinguish differences in source regions. Similar source material compositions, the mixing of dust from different regions during transport, and the pooling of samples over a 1–3 day collection period appear to have diluted specific signals from source regions

Determination of nitrate and phosphate in seawater at nanomolar concentrations, TracTrend Anal

Over much of the worldÕs surface oceans, nitrate and phosphate concentrations are below the limit of detection (LOD) of conventional techniques of analysis. However, these nutrients play a controlling role in primary productivity and carbon sequestration in these waters. In recent years, techniques have been developed to address this challenge, and methods are now available for the shipboard analysis of nanomolar (nM) nitrate and phosphate concentrations with a high sample throughput. This article provides an overview of the methods for nM nitrate and phosphate analysis in seawater. We outline in detail a system comprising liquid waveguide capillary cells connected to a conventional segmented-flow autoanalyser and using miniaturised spectrophotometers. This approach is suitable for routine field measurements of nitrate and phosphate and achieves LODs of 0.8 nM phosphate and 1.5 nM nitrate.

Fluxes of particles and soluble elements in dry and wet deposition samples collected between September 2012 and April 2016 at Gran Canaria, Canary Islands.

Steps to reproduce - Sampling sites and aerosol collection Tafira (TF) station (28º 06' N, 15º 24' W; 269 m a.s.l) is an urban background site subject to anthropogenic influence and situated within the marine boundary layer (<1800 m a.s.l). A Total Suspended particles (TSP) concentration Time-series has been recorded since 1 December 2003(see Gelado-Caballero et al., 2012; López-García et al., 2013). High volume (60 m3 h-1) aerosol collectors (MCV, model CAV-A/M) were used to collect aerosol samples for TSP in air on glass fibre filters (Whatman GF/A). TSP concentrations were measured following the procedure described in Gelado-Caballero et al. (2012). For deposition measurements, an automatic wet and dry sampler (ARS 1000, MTX Italy) with cubic containers having a surface area of 490 and 660 cm2, respectively was used. The instrument is equipped with a rain sensor. The aerosols collected in the deposition samples were quantified by gravimetry after filtering. Nucleopore filters were dried under a Class100 horizontal laminar flow bench and weighed with an accuracy of ±0.01 mg (Sartorius CP225D), before washing with 0.1 M hydrochloric acid (Suprapure, Merck) prior to use. Dry deposition (DD) samples were collected throughout the period from September 2012 to April 2016 at Tafira, in 92 samples representing average time periods ranging of 11 days but only since February 2013 soluble elements were analysed. Wet deposition (WD) samples were collected after each rainfall over the same period (n=125) representing average sampling periods of 2 days. - Chemical Analysis Soluble major ions, soluble Fe and pH were determined in the deposition samples. For DD samples, 150 mL was added to the bucket trying to wash all the particles deposited on the walls. The bucket was placed in an orbital shaker for 15 minutes to extract all the soluble elements, after that, the pH was measured and the sample was filtered using an acid clean Nucleopore filter (47 mm diameter and 0.2 µm pore size). WD samples were analysed just after the rain event and the same type of filter was used. pH in wet and dry deposition unfiltered samples were determined using a combined electrode (Aquatrode Plus, Metrohm). Soluble major ions were determined using two ion chromatographers (883 Basic IC Plus, Metrohm), a Metrosep ASupp 4 column for anion separation (fluoride, formate, chloride, bromide, nitrite, nitrate, phosphate, sulphate, and oxalate) and a Metrosep C4 column for cation separation (sodium, ammonium, potassium, calcium and magnesium) with a detection limit of 1 µg L-1. Because most of the phosphate concentrations in wet and dry deposition samples were below the detection limits of the IC, a nanomolar nutrient system was used (Patey et al., 2008). Soluble Fe was measured using a FiALab-3500 flow-injection analysis (FIA) system adapted to measure Fe using luminol chemiluminescence

Dissolved silver in European estuarine and coastal waters

Silver is one of the most toxic elements for the marine microbial and invertebrate community. However, little is known about the distribution and behaviour of dissolved silver in marine systems. This paper reports data on dissolved and sediment-associated silver in European estuaries and coastal waters which have been impacted to different extents by past and present anthropogenic inputs. This is the first extended dataset for dissolved silver in European marine waters. Lowest dissolved silver concentrations were observed in the Gullmar Fjord, Sweden (8.9 ± 2.9 pM; x ± 1?), the Tamar Estuary, UK (9.7 ± 6.2 pM), the Fal Estuary, UK (20.6 ± 8.3 pM), and the Adriatic Sea (21.2 ± 6.8 pM). Enhanced silver concentrations were observed in Atlantic coastal waters receiving untreated sewage effluent from the city of A Cor?na, Spain (243 ± 195 pM), and in the mine-impacted Restronguet Creek, UK (91 ± 71 pM). Anthropogenic wastewater inputs were a source of dissolved silver in the regions studied, with the exception of the Gullmar Fjord. Remobilisation of dissolved silver from historically contaminated sediments, resulting from acid mine drainage or sewage inputs, provided an additional source of dissolved silver to the estuaries. The ranges in the log particle-water partition coefficient (Kd) values of 5–6 were similar for the Tamar and Mero estuaries and agreed with reported values for other estuaries. These high Kd values indicate the particle reactive nature of silver with oxic sediments. In contrast, low Kd values (1.4–2.7) were observed in the Fal system, which may have been due to enhanced benthic inputs of dissolved silver coupled to limited scavenging of silver on to sediments rich in Fe oxide

Changes in iron speciation following a Saharan dust event in the tropical North Atlantic Ocean

Concentrations of dissolved iron (DFe) and Fe-binding ligands were determined in the tropical Northeast Atlantic Ocean (12–30°N, 21–29°W) as part of the UK-SOLAS (Surface Ocean Lower Atmosphere Study) cruise Poseidon 332 (P332) in January–February 2006. The surface water DFe concentrations varied between 0.1 and 0.4 nM with an average of 0.22 ± 0.05 nM (n = 159). The surface water concentrations of total Fe-binding ligands varied between 0.82 and 1.46 nM with an average of 1.11 ± 0.14 nM (n = 33). The concentration of uncomplexed Fe-binding ligands varied between 0.64 and 1.35 nM with an average of 0.90 ± 0.14 nM (n = 33). Thus, on average 81% of the total Fe-binding ligand concentration was uncomplexed. The average logarithmic conditional stability constant of the pool of Fe-binding ligands was 22.85 ± 0.38 with respect to Fe3+ (n = 33). A transect (12°N, 26°W to 16°N, 25.3°W) was sailed during a small Saharan dust event and repeated a week later. Following the dust event, the concentration of DFe increased from 0.20 ± 0.026 nM (n = 125) to 0.25 ± 0.028 (n = 17) and the concentration of free Fe-binding ligands decreased from 1.15 ± 0.15 (n = 4) to 0.89 ± 0.10 (n = 4) nM. Furthermore, the logarithmic stability constants of the Fe-binding ligands south of the Cape Verde islands were distinctively lower than north of the islands. The absence of a change in the logarithmic stability constant after the dust event south of the Cape Verde islands suggests that there was no significant atmospheric input of new Fe-binding ligands during this dust event