Dissolved Fe Concentrations Research Articles

Incubation Experiments Characterize Turbid Glacier Plumes as a Major Source of Mn and Co, and a Minor Source of Fe and Si, to Seawater

AbstractGlaciers are a source of fine‐ground rock flour to proglacial and coastal marine environments. In these environments, suspended rock flour may affect light and (micro)nutrient availability to primary producers. Due to high loads of glacier rock flour, the particulate metal load of glacier runoff typically exceeds the dissolved metal load. As glacier rock flour is deposited in downstream environments, short‐term exchange between particulate and dissolved metal phases may have a moderating influence on dissolved metal concentrations. Here we compare the behavior of iron (Fe), manganese (Mn), cobalt (Co) and silica (Si) following the addition of different glacier‐derived sediments into seawater under conditions of varying sediment load (20–500 mg L−1), time (0.5 hr–21 days), temperature (4–11°C) and light exposure (dark/2,500 Lux). Despite a moderately high labile Fe content across all particle types (0.28–3.50 mg Fe g−1 of dry sediment), only 0.27–7.13 μg Fe g−1 was released into seawater, with less efficient release as sediment load increased. Conversely, Si, Mn, and Co exhibited a more constant rate of release, which was less sensitive to sediment load. Dissolved Si release was equivalent to 17% ± 22% of particulate amorphous Si after 1–2 weeks. Dissolved Mn concentrations in most incubations exceeded dissolved Fe concentrations within 1 hr despite labile Mn content being 12‐fold lower than labile Fe content. Our results show the potential for glacier‐derived particles to be a large source of Mn and Co to marine waters and add to the growing evidence that Mn may be the bio‐essential metal most affected by glacier‐associated sources.

Overlooked effects of chlorides and bicarbonates on the intensity of peroxydisulfate activation in Fe(II)/citric acid-S2O82− process

The influence of chlorides (Cl−) and bicarbonates (HCO3−) on the Fe(II)/citric acid peroxydisulfate based (Fe(II)/CA-S2O82−) process has been commonly attributed to the formation of secondary reactive species, such as Cl• and CO3•, and their different reactivity with the target pollutant compared to primary reactive species (e.g., SO4•−). However, this conclusion has been entirely based only on analyzing target pollutants removal without the context of S2O82− consumption and dissolved Fe concentration throughout the process. To address this knowledge gap, we conducted a series of batch experiments to investigate the influence of Cl− (2 mmol∙L-1) and HCO3− (1 mmol∙L-1) on the intensity of S2O82− activation within the Fe(II)/CA-S2O82− process with tetrachloroethene (PCE) as the target pollutant. Throughout the experiments, PCE removal efficiency, S2O82− consumption, pH, redox potential, and dissolved Fe were analyzed. Equilibrium hydrochemical modelling (Visual MINTEQ 4) and high-resolution transmission electron microscopy (TEM) were employed to analyze and interpret the data obtained. We found that both Cl− and HCO3− significantly affected the intensity of S2O82− activation. While Cl− influenced the steady-state dissolved Fe(II) concentration, HCO3− primarily increased solution pH, which led to the formation of CA-stabilized ferrihydrite nanoparticles. This resulted in the inaccessibility of dissolved Fe(III) to the Fe(III)/Fe(II) regeneration cycle, consequently suppressing S2O82− activation intensity and PCE removal. Overall, the findings in this study have deepened the fundamental knowledge of how Cl− and HCO3− influence the Fe(II)/CA-S2O82− process, which may help improve the design and operation of S2O82−-based advanced oxidation processes using the Fe(II)/CA activation method.

Role of carbon treating in piperazine oxidation

Bench-scale experiments were performed to test the effects of carbon treating (8 × 30 mesh, lignite-based granular activated carbon from Nowata Filtration) on the oxidation of aqueous piperazine (PZ) at conditions of CO2 capture. The combined mitigation with carbon treating and N2 sparging was also tested. When carbon treating is applied, the UV–Vis absorbance at 320 nm, the NH3 production rate, and the accumulation rate of dissolved Fe concentration and total formate concentration decrease. The effects of carbon treating last even when the carbon is bypassed, which is possibly due to the removal of catalytic degradation products. Therefore, it is better to apply carbon treating in the early stages of oxidation. Combining carbon treating and nitrogen sparging is more effective in oxidation mitigation than applying the methods separately. The amount of Fe removed by the carbon bed is greater than the result calculated from the solvent inventory and concentration difference of Fe in solvent, indicating that Fe colloids or other “soluble” Fe will dissolve into the solvent and replace the dissolved Fe that is adsorbed and removed. The carbon removes the degradation products that can complex with Fe3+, even if they are not complexed. Since the source of Fe includes fly ash and corrosion, removing the complexing agents instead of all the available Fe is a more feasible target.

Biogeochemistry of iron in coastal Antarctica: isotopic insights for external sources and biological uptake in the Amundsen Sea polynyas

Seasonal phytoplankton blooms in the Antarctic Amundsen Sea Polynyas are thought to be supported by an external supply of iron (Fe) from circumpolar deep waters, benthic sediments, and/or ice shelf meltwaters. However, largely due to the limited amount of Fe data reported for the Amundsen Sea Polynyas, understanding of the sources and processes that affect the biogeochemistry of Fe in this region (notably within the ice shelf system) remains limited. Here, we present the first investigation of dissolved Fe isotope distributions (δ56Fe) along the conveyer belt of waters into and through the Amundsen Sea, via the Dotson Ice Shelf, from samples collected during austral summer (2017–2018). Our dataset allows us to characterize and compare the dissolved δ56Fe signatures of incoming modified Circumpolar Deep Water (mCDW) and of sedimentary sources on the continental shelf. The range in dissolved δ56Fe (–1 to +0.1 ‰) observed in the Amundsen Sea close to the seafloor, coupled with elevated dissolved Fe concentrations (up to 1.6 nmol/L), suggests that Fe is released from shelf sediments via a combination of reductive and non-reductive processes, with non-reductive dissolution input being relatively more important (20–56 %) than reductive dissolution (4–12 %). Near the Dotson Ice Shelf, the δ56Fe in the mCDW inflow (–0.70 ‰) was lower than the mCDW outflow (–0.23 ‰), whereas any change in dissolved Fe concentrations was negligible. We speculate that this shift in dissolved δ56Fe underneath the ice shelf is driven by a combination of enhanced preservation (and addition) of lithogenic colloidal Fe(III) and/or complexation with Fe-binding ligands, together with a differential loss of Fe2+. We also found distinct δ56Fe signatures in surface waters of the polynya, with apparent preferential uptake of isotopically light Fe in a bloom dominated by diatoms leading to a relatively heavy remnant dissolved δ56Fe signature of +1.06 ‰, compared to a bloom dominated by haptophytes where more modest and variable isotope fractionation was observed. The different isotopic composition between the two regions could be related to the dominance of different species, but this remains speculative. Despite prominent biological uptake, we suggest that other factors such as rapid recycling (e.g., adsorption and regeneration), bacterial regeneration, and complexation with organic ligands, together with the supply of lithogenic particles also play important roles in setting surface dissolved δ56Fe in the Amundsen Sea Polynyas. Overall, this study provides a further understanding of the external Fe sources and the biogeochemical processes in the Amundsen Sea and thus a baseline on how changing conditions in Antarctica can affect Fe cycling in the Southern Ocean and beyond.

Strong geochemical anomalies following active submarine eruption offshore Mayotte

Submarine volcanic activity releases large amounts of gases and metals in the water column, affecting biogeochemical cycles and ecosystems at a regional and local scale. In 2018, Fani Maoré submarine volcano erupted 50 km offshore Mayotte Island (Comoros Archipelago, Indian Ocean). Active eruptive plumes were observed in May 2019 at and around the summit with acoustic plumes rising 2 km into the water column coupled to strong geochemical anomalies. Between May 2019 and October 2020, three research cruises monitored the eruptive activity. Here, we report spatial and temporal variability of water column chemistry above the volcano, focusing on dissolved gases, trace metal concentrations, and physico-chemical parameters. In May 2019, concentrations above 800 nM in CH4 and H2 were measured throughout the water column, with Total Dissolvable Mn and Total Dissolvable Fe concentrations above 500 nM, and CO2 values of 265 μM. Strong water column acidification was measured (0.6 pH unit) compared to the regional background. From May 2019 to October 2020, we observed a general decrease in gas concentrations, and an evolution of the TDMn/TDFe ratios similar to previously reported values in other submarine volcanic contexts, and consistent with a decrease of the eruptive activity at the volcano. In October 2020, a rebound of high H2 concentrations resulted from new lava flows, which were identified by seafloor observation using deep-towed camera, 5 km further than the volcano summit. During 2 years timespan of our observations (2019–2020), He, CO2 and CH4 concentrations correlate highlighting a magmatic origin of dissolved gases. δ13C-CH4 values of −34‰ vs. vPDB might suggest magma/sediments interaction during the magma ascent, and potential thermal cracking of organic matter, although abiotic methane generation cannot be ruled out given the volcanic context. Weak correlations between H2 and excess of 3He suggest complex processes of H2 from magmatic degassing, lava/seawater interaction, and oxidation processes in the water column. Strong and correlated Fe, Mn and Si water column anomalies are also consistent with fluid-rock reactions induced by acidic fluids rich in magmatic volatiles. Water column acidification appears to be associated with the release of CO2-rich fluids. A year after the main eruptive event, the system seems to be back to steady-state highlighting the buffer capacity and resilience of the seawater column environment.

A novel recycling route for aluminum alloys: Synthesis of Fe/Al bimetallic materials and magnetic separation

Aluminum (Al) is one of the most important non-ferrous metals used in modern society, but its production from primary ores (e.g., bauxite) requires high energy input, emits large CO2, and generates huge amounts of solid wastes. In contrast, the production of Al from recycled scrap—also known as secondary Al production—only requires 5–8% of the energy for producing Al from bauxite and generates less amount of solid waste than that of the primary Al production, thus having both economic and environmental benefits. In general, scrap Al alloys are treated by physical-based separation techniques to remove impurities, and then mixed aluminum alloys are charged into the furnace for further refining. The ideal way of recycling Al scrap is to process each Al alloy by remelting/refining processes. To this end, this study investigated the surface modification of aluminum alloys by iron (Fe) deposition, followed by their separation by magnetic separation.When Al powder was treated with an acidified ferrous chloride solution (FeCl2-HCl-NaCl solution), dissolved Fe concentration decreased with time while releasing Al3+ into the solution, indicating that Fe cementation on Al occurred. X-ray photoelectron spectroscopy (XPS) analysis of treated Al powder showed that its surface was covered with zero-valent iron (ZVI) as well as its oxidation products (e.g., Fe(II)/Fe(III)-oxyhydroxide). The synthesis of Fe/Al bimetallic materials from Al alloys—1050, 2024, 3003, 5083, 6061, and 7075—was strongly dependent on HCl concentration as well as reaction temperature. Moreover, the kinetics of Fe cementation on Al alloys and the amounts of Fe cemented were determined not only by the rest potentials of the Al alloys but also their corrosion resistance. After treating Al alloys with FeCl2-HCl-NaCl solution, each Al alloy could be sequentially recovered by magnetic separation.

The Impact of Sea Ice Cover on Microbial Communities in Antarctic Shelf Sediments.

The area around the Antarctic Peninsula (AP) is facing rapid climatic and environmental changes, with so far unknown impacts on the benthic microbial communities of the continental shelves. In this study, we investigated the impact of contrasting sea ice cover on microbial community compositions in surface sediments from five stations along the eastern shelf of the AP using 16S ribosomal RNA (rRNA) gene sequencing. Redox conditions in sediments with long ice-free periods are characterized by a prevailing ferruginous zone, whereas a comparatively broad upper oxic zone is present at the heavily ice-covered station. Low ice cover stations were highly dominated by microbial communities of Desulfobacterota (mostly Sva1033, Desulfobacteria, and Desulfobulbia), Myxococcota, and Sva0485, whereas Gammaproteobacteria, Alphaproteobacteria, Bacteroidota, and NB1-j prevail at the heavy ice cover station. In the ferruginous zone, Sva1033 was the dominant member of Desulfuromonadales for all stations and, along with eleven other taxa, showed significant positive correlations with dissolved Fe concentrations, suggesting a significant role in iron reduction or an ecological relationship with iron reducers. Our results indicate that sea ice cover and its effect on organic carbon fluxes are the major drivers for changes in benthic microbial communities, favoring potential iron reducers at stations with increased organic matter fluxes.

Statewide Assessment Reveals Spatiotemporal Variability of Iron in Iowa Lakes

AbstractThe micronutrient iron has been noted to play a crucial role in regulating phytoplankton growth; however, most studies have focused on large lakes with persistent phytoplankton blooms that are known to undergo iron limitation, such as Lake Erie. Iron abundance in boreal lakes is also known to correlate with dissolved organic carbon and increased iron concentrations causing “browning.” To assess the spatial distribution of dissolved Fe (DFe) in lakes throughout Iowa, a landscape once dominated by prairies, DFe was measured in surface waters of 124 lakes distributed across the state over the 2018 summer season. Thirty lakes were selected for 15 weeks of weekly DFe monitoring to assess temporal trends over the summer season. Dissolved Fe concentrations in surface waters ranged from 5 to 1000 μg L‐1. Iowan lakes exhibited temporal trends in DFe, with decreasing concentrations from May to mid‐July and an increase into August. Unsupervised learning method (k‐means) identified three main groups of lakes based on temporal DFe trends. In this study, surface water temperature was associated with DFe trends in some lakes. This study serves as a baseline for DFe in Iowa’s lakes and can provide insights into iron biogeochemical cycling and its role in phytoplankton blooms, which are important to ecosystem and public health.

Dissolved Iron Concentration and the Solubility Inferred by Humic‐Like Fluorescent Dissolved Organic Matter in the Intermediate Water in the North Pacific Including the Marginal Seas

AbstractThe marginal seas have been found to be important external sources of dissolved iron (Fe) in the North Pacific through circulation of intermediate water. Here, we show comprehensive spatial distributions of dissolved Fe concentrations and Fe(III) solubilities inferred from humic‐like fluorescent dissolved organic matter (FDOMH) over the North Pacific, including the marginal seas, the Sea of Okhotsk, and the Bering Sea. FDOMH was used as a proxy of chemical speciation of dissolved Fe in the intermediate and deep waters where liner relationships were previously observed between FDOMH and Fe(III) solubility. When dissolved Fe concentration exceeds Fe(III) solubility, Fe(III) solubility is assumed to be equivalent to concentration of FDOMH‐Fe complexes, and excess dissolved Fe concentration above the Fe(III) solubility is assumed to be colloidal Fe which is not complexed with FDOMH. In the intermediate water, the dissolved Fe concentration exceeded the Fe(III) solubility in the marginal seas, while excess Fe(III) solubility was evident downstream of the intermediate water circulation, suggesting that the major dissolved Fe chemical form derived from shelf and slope sediments in the marginal seas changed from colloidal Fe to FDOMH‐Fe complexes. With the previous findings, namely the dominance of labile particulate Fe in total Fe in the intermediate water of the Sea of Okhotsk and the Bering Sea, we hypothesized that the average size of sediment‐derived Fe decreases during transportation by intermediate water, most likely due to reversible scavenging with the highest removal rate for labile particulate Fe and the lowest removal rate for FDOMH‐Fe complexes.

Thermodynamics-CFD-DPM modeling of dross formation and diffusion in a Zn-0.3Al galvanizing bath

In order to further reduce the production cost and enhance product of steel strip marketing competitiveness, industrial enterprises will choose to increase production rate, leading to the more dross generation. This paper proposes a numerical simulation based on thermodynamics, computational fluid dynamics (CFD) and discrete particle method (DPM) to simulate the η-Fe2Al5 dross formation distribution, and to track the trajectories of different sized dross particles in a Zn-0.3Al galvanizing bath. An optimized modeling of interfacial reactions involving the instantaneous rate of iron dissolution and aluminum uptake is firstly set up and validated with the previous experimental results and theoretical calculation. On the basis of the analysis on interfacial reaction, a potential influence factor (strip velocity) is discussed. The results show that higher strip velocity will result in much more η-Fe2Al5 dross around the strip entry side and the average dross formation within the V-typed region increases with the increase of the strip velocity due to the higher dissolved Fe concentration in molten zinc. Additionally, the trajectories of dross particles are discussed by injecting particles in the specific generation area. It can be concluded that the η-Fe2Al5 is likely to adhere to the steel-strip and roll surface. Furthermore, the trajectories of the dross particles are affected significantly by the flow of zinc and the particle size, thus the trajectories of particles resemble the streamlines. The average residence time of the suspended dross particles in the outer area of steel strip exit is longest. It is illustrative that the dross build-up region is the front wall area of the zinc pot.

Iron cycling during the decline of a South Georgia diatom bloom

The Southern Ocean is the largest high nutrient low chlorophyll (HNLC) oceanic region, where iron limits phytoplankton growth and productivity and ultimately influences the Biological Carbon Pump (BCP). Natural exceptions to the HNLC regime occur where island wakes cause iron to be mixed into surface waters from sediments, enabling large, prolonged phytoplankton blooms and increased carbon drawdown. Interactions between iron and phytoplankton are reciprocal in blooms: with plankton regulating the (re)cycling of iron through cellular uptake and remineralisation. The depth of iron remineralisation then influences either re-supply to the surface mixed layer biota or sequestration into deeper waters. Water column trace metal observations and shipboard experiments, using bioassays and radioisotope (55Fe, 32Si, 14C) cycling, were undertaken to investigate surface mixed layer phytoplankton iron limitation, iron uptake, and mesopelagic iron remineralisation relative to carbon and silica within the November 2017 bloom downstream of South Georgia. Surface phytoplankton residing in the iron depleted mixed layer were iron limited throughout the four-week sampling period. Experiments designed to investigate particulate water column (re)cycling revealed limited iron remineralisation from freshly produced upper ocean particles. The main pathway of iron transfer from particulates into the dissolved phase was through rapid (<2 d) release of extra-cellular adsorbed iron, which, if occurring in situ, could contribute to observed higher sub-surface dissolved Fe concentrations. This was accompanied by a small loss of cellular carbon, likely through respiration of the fixed 14C, and limited dissolution of particulate 32Si to dissolved 32Si. Decoupling of the remineralisation length scales for Fe, C and Si, with Fe having the fastest turnover, is thus likely in the upper mesopelagic zone beneath the bloom.

The distribution of 227Ac along the GA01 section in the North Atlantic

Actinium-227 is a powerful tool to study vertical mixing in the ocean, and was more recently proposed as a tracer of hydrothermal plumes. However, because 227Ac activities are especially low in the ocean, relatively few studies have been conducted to date, and none has been done on the large scale, as is reported in the present study. Here, we report a section of dissolved 227Ac in the North Atlantic Ocean between Portugal-Greenland-Canada based on nine full-depth profiles obtained in the framework of the international GEOTRACES program (GA01 section - GEOVIDE cruise, May–July 2014). The simultaneous determination of 231Pa, the parent nuclide of 227Ac, along the section allowed us to report a section of excess 227Ac activities (227Acex).Actinium-227 activities were especially low along the section (<0.4 dpm m−3) compared to other ocean basins. In most cases, 227Ac activities reached secular equilibrium with 231Pa in the water column 500 m above the seafloor. Secular equilibrium is considered reached after ∼five half-lives of the daughter. Secular equilibrium therefore suggests no external input over the last ∼100 years. The highest 227Ac activities were often found close to the seafloor, indicating that 227Ac diffuses out of the sediments. However, two different patterns question the traditional one-dimensional vertical mixing model typically applied to 227Ac released by deep-sea sediments: first, significant 227Acex were found in the upper 500 m of the water column at several stations located near the Iberian margin. In the upper 500 m, the high 227Acex activities could indicate lateral advection of waters that interacted with the margins. Second, mid-water peaks of 227Acex were occasionally observed along the transect. At station 44 in the Irminger Basin, a peak of 227Acex activity at 2500 m that was co-located with high dissolved Fe concentrations could be interpreted as the signature of a hydrothermal plume. Near the seafloor, we often observed bottom nepheloid layers and we cannot exclude that these layers impact the 227Ac distributions.

The effects of long‐term drainage and short‐term restoration on dissolved organic C concentrations and optical properties in the Baijianghe Peatland

AbstractPeatlands are important reservoirs of terrestrial carbon (C), whose fate can have implications for the global C cycle and climate change. Many peatlands around the world have been drained for agriculture and forestry and restored later on hoping to reduce C emissions and improve water quality. However, the actual effects of these management practices remain unclear. To address this, we investigated the concentration and quality of dissolved organic C (DOC) and water chemistry (nutrient and Fe concentrations) in the surface water (creeks and ditches) of the natural, drained, and restored areas along an elevation gradient of the Baijianghe (BJH) peatland in northeast China from 2017 to 2020. We found significantly higher DOC concentrations in the natural than the drained area, with a heavier, more fulvic, and less aromatic DOC composition upslope, but the difference in DOC quality became smaller downslope. Restoration, on the other hand, did not bring DOC concentrations or quality back to the natural conditions within 2 years. These variations in DOC concentration and quality were closely associated with dissolved Fe concentration, highlighting the role of the Fe redox cycle, as Fe(II) can catalyse phenol oxidation activities and promote DOC production. Vegetation also affected DOC characteristics, as the drained and restored areas were dominated by shrubs with woodier tissues, which probably released less in amount but more recalcitrant DOC. Our study features rare side‐by‐side comparisons between pristine, drained, and restored areas of the same peatland, and sheds light on factors affecting peatland DOC under different management practices.

Examining ecological succession of diatoms in California Current System cyclonic mesoscale eddies

AbstractThe California Current System is a diatom‐dominated region characterized by seasonal coastal upwelling and additional elevated mesoscale activity. Cyclonic mesoscale eddies in the region trap productive coastal waters with their planktonic communities and transport them offshore with limited interaction with surrounding waters, effectively acting as natural mesocosms, where phytoplankton populations undergo ecological succession as eddies age. This study examines diatom community composition within two mesoscale cyclonic eddies that formed in the same region of the California Current System 2 months apart and in the California Current waters surrounding them. The diatom communities were analyzed in the context of shifting environmental gradients and through a lens of community succession to expand our understanding of biophysical interactions in California Current System cyclonic eddies. Diatom communities within each eddy were different from non‐eddy communities and varied in concert with salinity and dissolved iron (Fe) concentrations. The younger, nearshore eddy displayed higher macronutrient and dissolved Fe concentrations, had higher values for diatom Shannon diversity and evenness, and had nutrient ratios indicative of either eventual silicic acid (Si) or Fe limitation or possibly co‐limitation. The older, offshore eddy displayed low macronutrient and dissolved Fe concentrations, was likely nitrate‐limited, and had lower diatom Shannon diversity and evenness indices. Sequences from the genus Rhizosolenia, some of which form vertically migrating mats to bypass nitrate limitation, dominated in the older eddy. This is of potential significance as the prevalence of Rhizosolenia mats could impact estimates of carbon cycling and export in the wider California coastal area.

Speciation, leachability and bioaccessibility of tungsten in tungsten ore processing residue.

Tungsten ore processing residue (TOPR) poses a potential risk due to tungsten (W) leaching. However, the leachability of W in TOPR is not well understood. Herein, the mechanism of W leachability from TOPR was investigated using complementary characterization techniques and leaching experiments. Our X-ray absorption near edge structure (XANES) analysis resolved wolframite in TOPR with a distorted octahedral coordination. The sequential extraction procedure showed that 78% of mobile fraction W in TOPR were bound to Fe oxides, and consequently W leachability was positively correlated with dissolved Fe concentration as evidenced by the general acid neutralizing capacity (GANC) test. The GANC results showed that the W release was negatively correlated with Ca concentration due to CaWO4 precipitation. The in vitro gastrointestinal procedure (IVG) results indicated that organic acids, abundant in fruits and vegetables, significantly improved the bioaccessibility of W from 10% to 20% of total W in TOPR. As a consequence, accidental ingestion of TOPR with a chemical daily intake at 0.8mgkg-1 day-1 evidenced its emerging concern in the environment and human health.

The role of the Dotson Ice Shelf and Circumpolar Deep Water as driver and source of dissolved and particulate iron and manganese in the Amundsen Sea polynya, Southern Ocean

Coastal areas around Antarctica such as the Amundsen Sea are important sources of trace metals and biological hotspots, but are also experiencing the effects of climate change, including the rapid thinning of ice sheets. In the central Amundsen Sea Polynya (ASP), both bio-essential dissolved Fe (DFe) and dissolved Mn (DMn) were found to be depleted at the surface, indicating substantial biological uptake and/or precipitation. Close to the Dotson Ice Shelf (DIS) there were elevated surface concentrations of DMn (>3 nM) but surprisingly not for DFe (<0.3 nM). While Fe-binding ligand data suggests that ligands were abundant near the DIS, these were most likely not strong enough to outcompete scavenging and thus increase DFe substantially in the outflow. In contrast to the dissolved phase, particulate Fe (PFe) and Mn (PMn) concentrations (both labile and refractory fractions) were elevated over the entire water column close to the DIS and partly in the central ASP. We hypothesize that DFe was released from the DIS and immediately established an equilibrium with the labile particulate Fe (L-PFe)pool, via (reversible) scavenging, as indicated by a positive correlation between L-PFe and DFe in the outflow. This scavenging results in relatively low DFe concentrations, but the pool of labile PFe likely buffers the DFe pool when DFe is decreasing, e.g. due to uptake by phytoplankton. The DFe distribution also shows that inflowing modified circumpolar deep water (mCDW) and benthic sediments are clear and important sources for both DFe and DMn in the ASP. Refractory Fe and Mn likely have a lithogenic source, whereas the labile fractions are mostly biogenic in surface waters, and authigenic in deep waters (>100 m depth). We compared different uptake ratios, underlining that uptake ratio estimates do not necessarily capture natural variability and it is likely better to use a range of values. In the future, climate change may increase the heat flux of mCDW and thereby the melting of the DIS. This will most likely cause an increased input of Fe and Mn into the ASP, which may fuel increased levels of primary productivity in the ASP.

Time series of hydrothermal vent fluid chemistry at Main Endeavour Field, Juan de Fuca Ridge: Remote sampling using the NEPTUNE cabled observatory

Seafloor hydrothermal fluids play a critical role in regulating a wide range of geochemical and biological processes. Chemical and physical changes to venting fluids undoubtedly occur in response to geological events, at depth or near the seafloor, with corresponding effects on vent fluid chemistry, mineralization, and biological activity. However, the uncertain timing of such events makes it impractical to anticipate their occurrence. Thus, the temporal evolution of seafloor vent fluids and coexisting mineral deposits is most often inferred from observations made using conventional deep submergence assets that may be non-continuous for a specific vent field. The recent development of submarine volcanic observatories operated by Ocean Networks Canada (ONC) and the Ocean Observatory Initiative (OOI) in the U.S. that deliver power to instruments on the seafloor and communications via high-speed fiber optic cable permits real-time communication and monitoring of seafloor vents. Here, we describe the development and application of a remotely operated hydrothermal fluid sampler that enables repeat on-demand sampling of high-temperature vent fluids triggered through the internet. The sampler was deployed from September 2019 to June 2020 at the S&M vent area, in the southern portion of Main Endeavour Field (MEF), Juan de Fuca Ridge, on ONC's NEPTUNE ocean observatory. Nine vent fluid samples were acquired over a period of nine months. Analyses of these samples confirm the moderately high-temperature origin of the source fluid, as indicated, for example, by dissolved chloride depletion relative to seawater (vapor-rich) and moderately high silica concentrations. These and other dissolved species, such as methane, a known hallmark of MEF vent fluids, are in excellent agreement with the reported composition of S&M vent fluids over the past 15 years or more, suggesting overall chemical stability and rock-dominated alteration processes. On the other hand, short-term variability of dissolved Mg, sulfate and barium indicate entrainment of conductively heated and partially reacted seawater. Both before and after the incursion of the “secondary” seawater derived fluid, however, samples reveal the lowest Mg and sulfate concentrations yet reported for seafloor hydrothermal vent fluid samples. Accordingly, these data provide new insight on the solubility of these and other elements coexisting with minerals close to the Axial Magma Chamber (AMC), from which the primary source fluid is ultimately derived. A separate noteworthy event characterized by ∼20 °C decrease in temperature and associated 24–77% decrease in dissolved Fe concentration was observed toward the end of the deployment, with implications for hydrothermal transition metal fluxes and linked biogeochemical processes. On-demand remote acquisition of a continuous series of high-temperature vent fluid samples from a single vent permits the temporal evolution of heat and mass transfer processes to be studied with a heretofore unavailable perspective. Furthermore, deployment of this sampler within the NEPTUNE ocean observatory opens the door to future studies based on comparative analysis of contemporaneous datasets.

Advanced coagulation with liquid ferrate as SWRO desalination pretreatment during severe algal bloom. Process performance, environmental impact, and cost analysis

Harmful algal bloom (HABs) events represent a major operational challenge for seawater reverse osmosis (SWRO) desalination plants, leading to severe membrane fouling, increased chemical consumption and plant shutdowns. Current pretreatment practice applying conventional coagulation, flocculation, and sedimentation (CFS) with ferric chloride has shown limited capability in reducing seawater fouling potential during HABs events. The use of in-situ generated liquid ferrate (Fe(VI)) has recently been proposed for CFS pretreatment due to its capability of acting simultaneously as a coagulant, oxidant, and disinfectant. In this study, the use of ferric chloride (conventional coagulation) was compared to liquid ferrate (advanced coagulation) during HABs events, in terms of process performance, life-cycle assessment (LCA) and cost analysis. Results show that a 10 times lower concentration of Fe salt was required in advanced coagulation compared to conventional coagulation to achieve a similar performance in terms of turbidity and TOC removal. Using liquid ferrate also resulted in a higher total ATP removal (>99%) and lower dissolved Fe concentration. The advanced coagulation reduced dry sludge production by 88%, resulting in a significant decrease in environmental impacts and operational costs.

Assessing the contribution of diazotrophs to microbial Fe uptake using a group specific approach in the Western Tropical South Pacific Ocean

Diazotrophs are often limited by iron (Fe) availability in the oligotrophic ocean. The Western Tropical South Pacific (WTSP) ocean has been suggested as an intense N2 fixation area due to Fe fertilizations through shallow hydrothermal activity. Yet, the Fe demand of diazotrophs in their natural habitat, where they cohabit with other microbial organisms also requiring Fe, remains unknown. Here we develop and apply a method consisting of coupling 55Fe uptake experiments with cell-sorting by flow cytometry, and provide group-specific rates of in situ Fe uptake by the microbial community in the WTSP, in addition to bulk and size fractionation rates. We reveal that the diazotrophs Crocosphaera watsonii and Trichodesmium contribute substantially to the bulk in situ Fe uptake (~33% on average over the studied area), despite being numerically less abundant compared to the rest of the planktonic community. Trichodesmium had the highest cell-specific Fe uptake rates, followed by C. watsonii, picoeukaryotes, Prochlorococcus, Synechococcus and finally heterotrophic bacteria. Calculated Fe:C quotas were higher (by 2 to 52-fold) for both studied diazotrophs compared to those of the non-diazotrophic plankton, reflecting their high intrinsic Fe demand. This translates into a diazotroph biogeographical distribution that appears to be influenced by ambient dissolved Fe concentrations in the WTSP. Despite having low cell-specific uptake rates, Prochlorococcus and heterotrophic bacteria were largely the main contributors to the bulk Fe uptake (~23% and ~12%, respectively). Overall, this group-specific approach increases our ability to examine the ecophysiological role of functional groups, including those of less abundant and/or less active microbes.

Multiphase processes in the EC-Earth model and their relevance to the atmospheric oxalate, sulfate, and iron cycles

Abstract. Understanding how multiphase processes affect the iron-containing aerosol cycle is key to predicting ocean biogeochemistry changes and hence the feedback effects on climate. For this work, the EC-Earth Earth system model in its climate–chemistry configuration is used to simulate the global atmospheric oxalate (OXL), sulfate (SO42-), and iron (Fe) cycles after incorporating a comprehensive representation of the multiphase chemistry in cloud droplets and aerosol water. The model considers a detailed gas-phase chemistry scheme, all major aerosol components, and the partitioning of gases in aerosol and atmospheric water phases. The dissolution of Fe-containing aerosols accounts kinetically for the solution's acidity, oxalic acid, and irradiation. Aerosol acidity is explicitly calculated in the model, both for accumulation and coarse modes, accounting for thermodynamic processes involving inorganic and crustal species from sea salt and dust. Simulations for present-day conditions (2000–2014) have been carried out with both EC-Earth and the atmospheric composition component of the model in standalone mode driven by meteorological fields from ECMWF's ERA-Interim reanalysis. The calculated global budgets are presented and the links between the (1) aqueous-phase processes, (2) aerosol dissolution, and (3) atmospheric composition are demonstrated and quantified. The model results are supported by comparison to available observations. We obtain an average global OXL net chemical production of 12.615 ± 0.064 Tg yr−1 in EC-Earth, with glyoxal being by far the most important precursor of oxalic acid. In comparison to the ERA-Interim simulation, differences in atmospheric dynamics and the simulated weaker oxidizing capacity in EC-Earth overall result in a ∼ 30 % lower OXL source. On the other hand, the more explicit representation of the aqueous-phase chemistry in EC-Earth compared to the previous versions of the model leads to an overall ∼ 20 % higher sulfate production, but this is still well correlated with atmospheric observations. The total Fe dissolution rate in EC-Earth is calculated at 0.806 ± 0.014 Tg yr−1 and is added to the primary dissolved Fe (DFe) sources from dust and combustion aerosols in the model (0.072 ± 0.001 Tg yr−1). The simulated DFe concentrations show a satisfactory comparison with available observations, indicating an atmospheric burden of ∼0.007 Tg, resulting in an overall atmospheric deposition flux into the global ocean of 0.376 ± 0.005 Tg yr−1, which is well within the range reported in the literature. All in all, this work is a first step towards the development of EC-Earth into an Earth system model with fully interactive bioavailable atmospheric Fe inputs to the marine biogeochemistry component of the model.