Source Of DFe Research Articles

Mechanistic Constraints on the Drivers of Southern Ocean Meridional Iron Distributions Between Tasmania and Antarctica

AbstractWhile modeling efforts have furthered our understanding of marine iron biogeochemistry and its influence on carbon sequestration, observations of dissolved iron (dFe) and its relationship to physical, chemical and biological processes in the ocean are needed to both validate and inform model parameterization. Where iron comes from, how it is transported and recycled, and where iron removal takes place are critical mechanisms that need to be understood to assess the relationship between iron availability and primary production. To this end, hydrographic and trace metal observations across the GO‐SHIP section SR3, south of Tasmania, Australia, have been analyzed in tandem with the novel application of an optimum multiparameter analysis. From the trace‐metal distribution south of Australia, key differences in the drivers of dFe between oceanographic zones of the Southern Ocean were identified. In the subtropical zone, sources of dFe were attributed to waters advected off the continental shelf, and to recirculated modified mode and intermediate water‐masses of the Tasman Outflow. In the subantarctic zone, the seasonal replenishment of dFe in Antarctic surface and mode waters appears to be sustained by iron recycling in the underlying mode and intermediate waters. In the southern zone, the dFe distribution is likely driven by dissolution and scavenging by high concentrations of particles along the Antarctic continental shelf and slope entrained in high salinity shelf water. This approach to trace metal analysis may prove useful in future transects for identifying key mechanisms driving marine dissolved trace metal distributions.

Investigation of hydrothermal activity in the South West Indian ridge region using Ra isotopes and 227Ac as tracers

Hydrothermal vents have been shown to be important vectors for various chemical elements into the ocean. However, both the intensity of the chemical fluxes associated with these systems and the fate of the chemical elements along the plume are still largely overlooked. At two stations located above the South West Indian Ridge (SWIR), we investigate the distributions of the Ra quartet (223Ra, 224Ra, 226Ra and 228Ra) and 227Ac that have been used as tracers of hydrothermal activity. While the vertical distributions of 226Ra, 228Ra and 227Ac do not show a clear enrichment at depth, unusual signatures of excess 223Ra and 224Ra near the seafloor are attributed to the presence of a hydrothermal activity. The discrepancy observed between the different isotopes is attributed to different chemical reactivity when seawater circulates within the crust and/or to different regeneration rates within the fluid. A 1D diffusion model applied to the vertical profiles of short-lived Ra isotopes provided an estimation of the vertical eddy diffusivity coefficients (KZ) between 38 cm2 s−1 and 149 cm2 s−1. These high values suggest strong mixing likely favored by the complex bathymetry in the region. By combining these KZ with the vertical gradient of dissolved Fe (dFe), we estimate a vertical flux of dFe that ranges from 139 to 1173 nmol m−2 d-1. These results confirm that low-expansion-rate ridges could be significant sources of dFe to the deep ocean.

Permafrost Wetlands Are Sources of Dissolved Iron and Dissolved Organic Carbon to the Amur‐Mid Rivers in Summer

AbstractDissolved iron (dFe) transported by the Amur River greatly contributes to phytoplankton growth in the Sea of Okhotsk. Nevertheless, there has been little research on the source of dFe to rivers, especially in the Amur‐Mid Basin, which is situated in a sporadic permafrost area. In the Amur‐Mid Basin, permafrost generally exists in wetlands in flat valleys, and these permafrost wetlands could be a source of dFe to rivers. To assess the importance of permafrost wetlands for dFe export, we conducted a local survey on land and soil characteristics of wetlands, and moreover analyzed the chemical composition (dFe, dissolved organic carbon [DOC], pH, and electrical conductivity [EC]) of 24 rivers with different watershed sizes in summer. As a result of local survey, the thickly accumulated peat soils in the permafrost wetland were almost saturated and rich in organic matter from the surface to a greater depth near the permafrost table. In addition, the coverage of such permafrost wetlands in watersheds showed significant positive correlations with dFe (r2 = 0.67, p = 1.7e−6) and DOC (r2 = 0.48, p = 1.8e−4) and a negative correlation with EC (r2 = 0.52, p = 7.7e−5). The dFe concentration was also correlated well with DOC concentration (r2 = 0.68, p = 7.3e−7) but not correlated with pH and watershed area. These findings are the first to indicate that permafrost wetlands in the Amur‐Mid Basin considerably contribute to dFe and DOC export to rivers, and their coverage primarily determines riverine dFe and DOC concentrations in summer.

A proposed seasonal cycle of dissolved iron-binding ligands in Antarctic sea ice

Iron (Fe) is an essential micronutrient to oceanic microalgae, and its dissolved fraction (DFe) is retained in surface waters by Fe-binding ligands. Previous work has suggested that ligands may also bind Fe within sea ice, although supporting data are limited. This study investigates distribution, concentration, and potential drivers of Fe-binding ligands in Antarctic sea ice, considering the ice type, location and season. Results suggest that the concentration of ligands (CL) varies throughout the year, both spatially and seasonally. The lowest CL (3.3–8.0 nM) and DFe concentrations (0.7–3.5 nM) were recorded in newly formed winter sea ice in the Weddell Sea, likely due to the early stage of sea-ice growth and low biological activity. The highest CL (1.7–74.6 nM), which follows the distribution of DFe (1.0–75.5 nM), was observed during springtime, in the Eastern Antarctic Sector. There, consistently higher values for CL in bottom ice depths were likely associated with enhanced algal biomass, while aeolian deposition may have acted as an additional source of DFe and ligands near Davis station. In summer, the senescence of ice algae and advanced sea-ice melting led to intermediate CL (1.0–21.9 nM) and DFe concentrations (0.6–13.3 nM) both on and off the East Antarctic coast. Regardless of time and location, >99% of DFe was complexed, suggesting that CL controls the distribution of DFe in sea ice. This study represents a first attempt at a year-round investigation of CL in sea ice, providing results that support the premise that sea ice acts as a potential biogeochemical bridge between autumn and spring phytoplankton blooms.

Release of tephra-hosted iron during early diagenesis fingerprinted by iron isotopes

The micronutrient iron (Fe) plays a fundamental role controlling primary productivity in the upper ocean, with volcanic eruptions and deposition of airborne volcanic material (termed tephra) a potential source of Fe. Here, we investigate the geochemical and Fe isotopic (δ56Fe) composition of tephra layers, sediments, and mixed tephra-sediment samples from the Integrated Ocean Drilling Program (IODP) Hole 1396C, located offshore the volcanically active island of Montserrat in the Lesser Antilles, Caribbean Sea. We find that buried tephras, which have experienced diagenesis, exhibit lighter δ56Fe (relative to standard IRMM-524a) compositions (down to −0.26 ± 0.04‰, 2SD) than fresh tephra deposited in Montserrat (δ56Fe = 0.02 ± 0.02‰, 2SD). Such negative values suggest that isotopically heavier Fe has been lost from the originally deposited material. Using multivariate statistical modelling and mass balance constraints, we identify the outward Fe flux (with calculated δ56Fe of 0.21 ± 0.31‰, 2SD, n = 12) during non-reductive dissolution of tephra as the likely cause of the retention of these light δ56Fe compositions. Due to the widespread nature of tephra deposition, tephra diagenesis may provide an important source of isotopically heavy dissolved Fe (dFe) to the oceans. This process contrasts with more commonly considered reductive dissolution processes, which provide a source of dFe enriched in light isotopes to the oceans.

Dissolved, Labile, and Total Particulate Trace Metal Dynamics on the Northeast Greenland Shelf

AbstractWe present high‐resolution profiles of dissolved, labile, and total particulate trace metals (TMs) on the Northeast Greenland shelf from GEOTRACES cruise GN05 in August 2016. Combined with radium isotopes, stable oxygen isotopes, and noble gas measurements, elemental distributions suggest that TM dynamics were mainly regulated by the mixing between North Atlantic‐derived Intermediate Water, enriched in labile particulate TMs (LpTMs), and Arctic surface waters, enriched in Siberian shelf‐derived dissolved TMs (dTMs; Co, Cu, Fe, Mn, and Ni) carried by the Transpolar Drift. These two distinct sources were delineated by salinity‐dependent variations of dTM and LpTM concentrations and the proportion of dTMs relative to the total dissolved and labile particulate ratios. Locally produced meltwater from the Nioghalvfjerdsbræ (79NG) glacier cavity, distinguished from other freshwater sources using helium excess, contributed a large pool of dTMs to the shelf inventory. Localized peaks in labile and total particulate Cd, Co, Fe, Mn, Ni, Cu, Al, V, and Ti in the cavity outflow, however, were not directly contributed by submarine melting. Instead, these particulate TMs were mainly supplied by the re‐suspension of cavity sediment particles. Currently, Arctic Ocean outflows are the most important source of dFe, dCu, and dNi on the shelf, while LpTMs and up to 60% of dMn and dCo are mainly supplied by subglacial discharge from the 79NG cavity. Therefore, changes in the cavity‐overturning dynamics of 79NG induced by glacial retreat, and alterations in the transport of Siberian shelf‐derived materials with the Transport Drift may shift the shelf dTM‐LpTM stoichiometry in the future.

Seasonal Dynamics of Dissolved Iron on the Antarctic Continental Shelf: Late-Fall Observations From the Terra Nova Bay and Ross Ice Shelf Polynyas.

Over the Ross Sea shelf, annual primary production is limited by dissolved iron (DFe) supply. Here, a major source of DFe to surface waters is thought to be vertical resupply from the benthos, which is assumed most prevalent during winter months when katabatic winds drive sea ice formation and convective overturn in coastal polynyas, although the impact of these processes on water-column DFe distributions has not been previously documented. We collected hydrographic data and water-column samples for trace metals analysis in the Terra Nova Bay and Ross Ice Shelf polynyas during April-May 2017 (late austral fall). In the Terra Nova Bay polynya, we observed intense katabatic wind events, and surface mixed layer depths varied from ∼250 to ∼600m over lateral distances <10km; there vertical mixing was just starting to excavate the dense, iron-rich Shelf Waters, and there was also evidence of DFe inputs at shallower depths in the water column. In the Ross Ice Shelf polynya, wind speeds were lower, mixed layers were <300m deep, and DFe distributions were similar to previous, late-summer observations, with concentrations elevated near the seafloor. Corresponding measurements of dissolved manganese and zinc, and particulate iron, manganese, and aluminum, suggest that deep DFe maxima and some mid-depth DFe maxima primarily reflect sedimentary inputs, rather than remineralization. Our data and model simulations imply that vertical resupply of DFe in the Ross Sea occurs mainly during mid-late winter, and may be particularly sensitive to changes in the timing and extent of sea ice production.

Isotopic fingerprinting of dissolved iron sources in the deep western Pacific since the late Miocene

Iron (Fe) is a productivity-limiting nutrient in the ocean. However, the sources of dissolved Fe (dFe) in the deep ocean and how they respond to tectonic and climate changes are still poorly understood. In the northern hemisphere, dust flux to the low-latitude western Pacific has increased dramatically since the late Miocene associated with intense aridification of the Asian inland. Meanwhile, the terrigenous material supply to the open ocean might have also changed as a result of the reorganization of the Pacific circulation due to the gradual closure of seaways in the low latitudes. Therefore, the western Pacific is a characteristic region for understanding the sources of dFe in the deep ocean and their responses to long term climate changes. Here, we present data on isotopic evolution of dFe and dissolved Pb since ∼8 Ma based on ferromanganese crust METG-03 (16.0°N, 152.0°E, 3850 m water depth) in the western Pacific deep water. Our results show that δ56Fe of the crust remains fairly stable since the late Miocene, i.e., about −0.32±0.08‰ (2SD). We infer that δ56Fe of dFe in the deep western Pacific is relatively invariant at ∼0.45 ±0.1‰ based on the Fe isotopic fractionation between hydrogenetic crust and the seawater dissolved component. The reconstructed isotope signature is similar to the measured δ56Fe value (0.37±0.15‰) of the intermediate to deep waters in the modern low-latitude western Pacific region close to the island arcs, but is significantly higher than that of the eastern Pacific deep waters near South America which is controlled by the reductive dissolution of continental shelf sediments and the hydrothermal inputs (δ56Fe<−0.1‰). The deep-water 206Pb/204Pb ratio recorded by METG-03 displays systematic increase at about 8–4 Ma, reflecting increased input from sediment dissolution of low-latitude island arcs associated with reorganization of the western Pacific deep circulation. Notably, Fe isotopes of terrigenous materials from different sources are similar, while their dissolved Fe isotopic signatures released to the ocean are mainly controlled by the mechanism of particle dissolution. The stability of δ56Fe and systematic changes in Pb isotopes over the last ∼8 Ma thus suggest that Asian dust dissolution and hydrothermal inputs are likely only minor sources of dFe in the low-latitude deep western Pacific, while the acquisition and transport of dFe from shelf sediments by organic ligand binding in the oxic environment is the major dFe source which keeps stable on tectonic time scales since the late Miocene.

Meteoric water promotes phytoplankton carbon fixation and iron uptake off the eastern tip of the Antarctic Peninsula (eAP)

Both bioavailable iron (Fe) and water stability have been proposed to regulate primary production in large parts of the Southern Ocean. Freshwater is not only an important factor regulating water stability, but is also an important source of dissolved iron (DFe) for phytoplankton growth. In this study, stable oxygen isotopic composition (δ18O) and salinity were used to distinguish meteoric water (glacial discharge and precipitation) from sea ice meltwater in the northernmost part of the eastern Antarctic Peninsula (eAP). The carbon fixation rate (CFR) and Fe uptake rate (FeUR) were measured via 14C and 55Fe tracer assay, respectively. We observed positive responses of FeURs to the fraction of meteoric water and sea ice meltwater, and a stronger correlation between FeUR and meteoric water, indicating that freshwater input could relieve the Fe limitation in this area, and meteoric water was likely to be the main source of DFe. In addition, we also observed that both meteoric water and sea ice meltwater increased CFR, but CFR only showed slightly positive response to sea ice meltwater. This suggested that the primary effect of increased sea ice meltwater was to enhance the stability of water column, providing a favorable condition for phytoplankton growth. Since the potential meteoric water inventory is much greater than sea ice meltwater and meteoric water input is expected to rise further as climate warming continues as mentioned in Meredith et al’s studies (2008, 2010), the meteoric water is likely increasingly important in Antarctic coastal zones. Smaller phytoplankton responded more obviously to the freshwater input, and we speculate that this would influence on the carbon export in the ambient and nearby oceanic areas. The low Fe demand (Fe:C ratio = 8.04 μmol mol−1) suggested that phytoplankton in the eAP were adapted to a low Fe environment. In addition, primary production estimated from high Fe:C ratios would underestimate the real situation in the southern ocean. These findings help to understand how meteoric water and sea ice meltwater could affect the Antarctic ecosystem in response to climate change.

Dissolved iron and manganese in the Canadian Arctic Ocean: On the biogeochemical processes controlling their distributions

Dissolved iron (DFe) and manganese (DMn) are essential micronutrients involved in vital phytoplankton physiological pathways, and their deficit can limit primary production in otherwise nutrient-replete surface ocean waters. In this work we present the spatial distributions and biogeochemical cycling of these metals across the Canadian GEOTRACES transect in the Canadian Arctic Ocean during the summer and autumn of 2015. Surface concentrations are dominated by freshwater inputs showing a strong negative correlation with salinity, especially for DMn which behaves more conservatively than DFe. The highest surface concentrations were measured in the Canadian Arctic Archipelago (Fe: 0.401–1.91 and Mn: 4.33–9.54 nmol kg−1) and the Canada Basin (Fe: 0.225–0.479 and Mn: 3.93–7.02 nmol kg−1), regions highly influenced by riverine inputs, whereas the lowest values were found in the Labrador Sea (Fe: 0.106–0.362 and Mn: 0.450–1.09 nmol kg−1) where freshwater inputs diminished and phytoplankton uptake increased. Subsurface and deep water distributions for both metals are largely controlled by a complex balance between sources (advective inputs and organic matter remineralization) and removal processes. The subsurface peaks (∼100–300 m) observed in the Canada Basin (Fe: 0.541 ± 0.060 and Mn: 1.38 ± 0.42 nmol kg−1) and Baffin Bay (Fe: 0.753–1.03 nmol kg−1) were advected from the Chukchi Sea and the Canadian Arctic Archipelago respectively, where DFe and DMn are released from the benthic boundary layer in these shelf-dominated environments. Advective sources associated with the Arctic Circumpolar Boundary Current, rather than vertical fluxes of DFe and DMn in sinking particles, dominate metal distributions in the deep Canada Basin waters (>300 m). In the highly productive Baffin Bay and the Labrador Sea, organic matter remineralization is a notable source of DFe and DMn to deep waters. In the deepest waters (>1000 m), scavenging of DFe and DMn govern their vertical distributions; a pseudo-first order scavenging model explained the continuous removal of DMn in the Canada Basin, where the concentrations reach uniformly low concentrations (0.150 ± 0.004 nmol kg−1) after ∼400 years. Applying this DMn scavenging model we were able to estimate the age (120–190 years) of deep Baffin Bay waters, a topic of discussion for many years.

Evidences of strong sources of DFe and DMn in Ryder Bay, Western Antarctic Peninsula.

The spatial distribution, biogeochemical cycling and external sources of dissolved iron and dissolved manganese (DFe and DMn) were investigated in Ryder Bay, a small coastal embayment of the West Antarctic Peninsula, during Austral summer (2013 and 2014). Dissolved concentrations were measured throughout the water column at 11 stations within Ryder Bay. The concentration ranges of DFe and DMn were large, between 0.58 and 32.7 nM, and between 0.18 and 26.2 nM, respectively, exhibiting strong gradients from the surface to the bottom. Surface concentrations of DFe and DMn were higher than concentrations reported for the Southern Ocean and coastal Antarctic waters, and extremely high concentrations were detected in deep water. Glacial meltwater and shallow sediments are likely to be the main sources of DFe and DMn in the euphotic zone, while lateral advection associated with local sediment resuspension and vertical mixing are significant sources for intermediate and deep waters. During summer, vertical mixing of intermediate and deep waters and sediment resuspension occurring from Marguerite Trough to Ryder Bay are thought to be amplified by a series of overflows at the sills, enhancing the input of Fe and Mn from bottom sediment and increasing their concentrations up to the euphotic layer.This article is part of the theme issue 'The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change'.

Dissolved Fe in the Deep and Upper Arctic Ocean With a Focus on Fe Limitation in the Nansen Basin

Global warming resulting from the release of anthropogenic carbon dioxide is rapidly changing the Arctic Ocean. Over the last decade sea ice declined in extent and thickness. As a result, improved light availability has increased Arctic net primary production, including in under-ice phytoplankton blooms. During the GEOTRACES cruise PS94 in the summer of 2015 we measured dissolved iron (DFe), nitrate and phosphate throughout the central part of the Eurasian Arctic. In the deeper waters concentrations of DFe were higher, which we relate to resuspension on the continental slope in the Nansen Basin and hydrothermal activity at the Gakkel Ridge. The main source of DFe in the surface was the Trans Polar Drift (TPD), resulting in concentrations up to 4.42 nM. Nevertheless, using nutrient ratios we show that a large under-ice bloom in the Nansen basin was limited by Fe. Fe limitation potentially prevented up to 54% of the available nitrate and nitrite from being used for primary production. In the Barents Sea, Fe is expected to be the first nutrient to be depleted as well. Changes in the Arctic biogeochemical cycle of Fe due to retreating ice may therefore have large consequences for primary production, the Arctic ecosystem and the subsequent drawdown of carbon dioxide.

Pathways and supply of dissolved iron in the Amundsen Sea (Antarctica)

AbstractNumerous coastal polynyas fringe the Antarctic continent and strongly influence the productivity of Antarctic shelf systems. Of the 46 Antarctic coastal polynyas documented in a recent study, the Amundsen Sea Polynya (ASP) stands out as having the highest net primary production per unit area. Incubation experiments suggest that this productivity is partly controlled by the availability of dissolved iron (dFe). As a first step toward understanding the iron supply of the ASP, we introduce four plausible sources of dFe and simulate their steady spatial distribution using conservative numerical tracers. The modeled distributions replicate important features from observations including dFe maxima at the bottom of deep troughs and enhanced concentrations near the ice shelf fronts. A perturbation experiment with an idealized drawdown mimicking summertime biological uptake and subsequent resupply suggests that glacial meltwater and sediment‐derived dFe are the main contributors to the prebloom dFe inventory in the top 100 m of the ASP. The sediment‐derived dFe depends strongly on the buoyancy‐driven overturning circulation associated with the melting ice shelves (the “meltwater pump”) to add dFe to the upper 300 m of the water column. The results support the view that ice shelf melting plays an important direct and indirect role in the dFe supply and delivery to polynyas such as the ASP.

Isotopic signature of dissolved iron delivered to the Southern Ocean from hydrothermal vents in the East Scotia Sea

Abstract It has recently been demonstrated that hydrothermal vents are an important source of dissolved Fe (dFe) to the Southern Ocean. The isotopic composition (δ56Fe) of dFe in vent fluids appears to be distinct from other sources of dFe to the deep ocean, but the evolution of δ56Fe during mixing between vent fluids and seawater is poorly constrained. Here we present the evolution of δ56Fe for dFe in hydrothermal fluids and dispersing plumes from two sites in the East Scotia Sea. We show that δ56Fe values in the buoyant plume are distinctly lower (as low as −1.19‰) than the hydrothermal fluids (−0.29‰), attributed to (1) precipitation of Fe sulfides in the early stages of mixing, and (2) partial oxidation of Fe(II) to Fe(III), &amp;gt;55% of which subsequently precipitates as Fe oxyhydroxides. By contrast, the δ56Fe signature of stabilized dFe in the neutrally buoyant plume is −0.3‰ to −0.5‰. This cannot be explained by continued dilution of the buoyant plume with background seawater; rather, we suggest that isotope fractionation of dFe occurs during plume dilution due to Fe ligand complexation and exchange with labile particulate Fe. The δ56Fe signature of stabilized hydrothermal dFe in the East Scotia Sea is distinct from background seawater and may be used to quantify the hydrothermal dFe input to the ocean interior.

Seasonal distribution of dissolved iron in the surface water of Sanggou Bay, a typical aquaculture area in China

Sanggou Bay (SGB) is one of the largest aquaculture centers in China. Although iron is an essential biological element, little is known about its distribution in this bay. In this study, seasonal distributions of dissolved iron (dFe) in the surface water of SGB and its surrounding rivers and groundwater were investigated during the four seasons from April 2013 to January 2014. Our results indicate that SGB is a source of dFe on the bay scale. The average dFe concentrations in the surface water of SGB were 12.0±6.29nM in spring, 5.00±2.92nM in summer, 1.83±0.42nM in autumn, and 3.36±2.06nM in winter. External sources of dFe in SGB were ranked in the following order: atmospheric deposition>sediment diffusion>groundwater input>riverine input. The biological iron of the cultivated species (kelp, scallops, and oysters) was the main sink for dFe in SGB. The calculation of Fe⁎P (a tracer defined to quantify the degree of iron limitation in water relative to phosphorus) suggested that dFe was sufficient to support the biological utilization of dissolved inorganic phosphorus in spring and summer but was a potentially limiting factor for primary production in autumn.

Ferrioxamine Siderophores Detected amongst Iron Binding Ligands Produced during the Remineralization of Marine Particles

The microbial degradation of marine particles is an important process in the remineralization of nutrients including iron. As part of the GEOTRACES process study (FeCycle II), we conducted incubation experiments with marine particles obtained from 30 and 100 m depth at two stations during austral spring in the subtropical waters east of the North Island of New Zealand. The particles were collected using in-situ pumps, and comprised mainly of suspended and slow sinking populations along with associated attached heterotrophic bacteria. In treatments with live bacteria, increasing concentrations of Fe binding ligands were observed with an average stability constant of logKFeL,Fe3+ = 21.11±0.37 for station 1 and 20.89±0.25 for station 2. The ligand release rates varied between 2.54 and 11.8 pmol L-1 d-1 (calculated for ambient seawater particle concentration) and were similar to those found in two Southern Ocean subsurface studies from ~110 m depths in subpolar and polar waters. Dissolved iron (DFe) was released at a rate between 0.33 and 2.09 pmol Fe L-1 d-1 with a column integrated (30 -100 m) flux of 107 and 58 nmol Fe m-2 day-1 at station 1 and 2, respectively. Given a mixed layer DFe inventory of ~48 µmol m-2 and ~4 µmol m-2 at the time of sampling for station 1 and 2, this will therefore result in a DFe residence time of 1.2 and 0.18 years, assuming particle remineralization was the only source of iron in the mixed layer. The DFe release rates calculated were comparable to those found in the previously mentioned study of Southern Ocean water masses. Fe-binding ligand producing bacteria (CAS positive) abundance was found to increase throughout the duration of the experiment of 7 to 8 days. For the first time ferrioxamine type siderophores, including the well-known ferrioxamine B and G, have been quantified using chemical assays and LC-ESI-MS. Our subtropical study corroborates prior reports from the Southern Ocean of particle remineralization being an important source of DFe and ligands, and adds unprecedented detail by revealing that siderophores are probably an important component of the ligands released into subsurface waters during particle remineralisation.

Iron supply and demand in an Antarctic shelf ecosystem

AbstractThe Ross Sea sustains a rich ecosystem and is the most productive sector of the Southern Ocean. Most of this production occurs within a polynya during the November–February period, when the availability of dissolved iron (dFe) is thought to exert the major control on phytoplankton growth. Here we combine new data on the distribution of dFe, high‐resolution model simulations of ice melt and regional circulation, and satellite‐based estimates of primary production to quantify iron supply and demand over the Ross Sea continental shelf. Our analysis suggests that the largest sources of dFe to the euphotic zone are wintertime mixing and melting sea ice, with a lesser input from intrusions of Circumpolar Deep Water and a small amount from melting glacial ice. Together these sources are in approximate balance with the annual biological dFe demand inferred from satellite‐based productivity algorithms, although both the supply and demand estimates have large uncertainties.

A ventilation‐based framework to explain the regeneration‐scavenging balance of iron in the ocean

AbstractOur understanding of the processes driving the patterns of dissolved iron (DFe) in the ocean interior, either in observations or models, is complicated by the combined influences of subduction from the surface mixed layer, notable subsurface sources, regeneration, and scavenging loss. We describe a ventilation‐based framework to quantify these processes in a global ocean biogeochemical model including diagnostics along potential density surfaces. There is a prevailing control of subsurface DFe by the subduction of surface DFe as preformed DFe augmented by benthic sources of DFe from hydrothermal activity and sediments. Unlike phosphate, there is often a first‐order balance with a near cancelation between regeneration and scavenging with the remaining “net regeneration” controlled by the ventilation of surface excesses in Fe‐binding ligands. This DFe framework provides a more stringent test of how the total DFe distribution is mechanistically controlled within a model and may be subsequently used to interpret observed DFe distributions.

The contribution of aeolian sand and dust to iron fertilization of phytoplankton blooms in southwestern Ross Sea, Antarctica

AbstractIron is a limiting micronutrient for primary production in the Ross Sea, Antarctica. Recent observations reveal low dissolved Fe (dFe) concentrations in the Ross Sea polynya following high initial rates of primary production in summer, after the dFe winter reserve has been consumed. Significant new sources of dFe are therefore required to further sustain phytoplankton blooms. Iron from aeolian sand and dust (ASD) released from melting sea ice is one potential source. To constrain aeolian Fe inputs, we determined ASD mass accumulation rates and the total and soluble Fe content of ASD on sea ice in McMurdo Sound, southwestern (SW) Ross Sea. The mean mass accumulation rate was ~1.5 g m−2 yr−1, total Fe content of this ASD was 4 ± 1 wt %, and the percentage of soluble Fe was 11 ± 1%. Our mean estimate of the bulk aeolian dFe flux of 122.1 µmol m−2 yr−1 for the McMurdo Sound region suggests that aeolian Fe can support between 9.0 × 109 and 4.1 × 1011 mol C yr−1 (0.1–4.9 Tg C yr−1) of new primary production. This equates to only ~15% of new primary production in the SW Ross Sea, suggesting that aeolian dFe is a minor component of seasonal Fe supply. The very high ASD accumulation on sea ice in McMurdo Sound compared to other regions of Antarctica suggests that our results represent the upper limit of dFe supply to the ocean from this source in the Ross Sea.

Controls on dissolved cobalt in surface waters of the Sargasso Sea: Comparisons with iron and aluminum

Dissolved cobalt (dCo), iron (dFe) and aluminum (dAl) were determined in water column samples along a meridional transect (∼31°N to 24°N) south of Bermuda in June 2008. A general north‐to‐south increase in surface concentrations of dFe (0.3–1.6 nM) and dAl (14–42 nM) was observed, suggesting that aerosol deposition is a significant source of dFe and dAl, whereas no clear trend was observed for near‐surface dCo concentrations. Shipboard aerosol samples indicate fractional solubility values of 8–100% for aerosol Co, which are significantly higher than corresponding estimates of the solubility of aerosol Fe (0.44–45%). Hydrographic observations and analysis of time series rain samples from Bermuda indicate that wet deposition accounts for most (&gt;80%) of the total aeolian flux of Co, and hence a significant proportion of the atmospheric input of dCo to our study region. Our aerosol data imply that the atmospheric input of dCo to the Sargasso Sea is modest, although this flux may be more significant in late summer. The water column dCo profiles reveal a vertical distribution that predominantly reflects ‘nutrient‐type’ behavior, versus scavenged‐type behavior for dAl, and a hybrid of nutrient‐ and scavenged‐type behavior for dFe. Mesoscale eddies also appear to impact on the vertical distribution of dCo. The effects of biological removal of dCo from the upper water column were apparent as pronounced sub‐surface minima (21 ± 4 pM dCo), coincident with maxima in Prochlorococcus abundance. These observations imply that Prochlorococcus plays a major role in removing dCo from the euphotic zone, and that the availability of dCo may regulate Prochlorococcus growth in the Sargasso Sea.