Hydrothermal venting along mid-ocean ridges exerts an important control on the chemical composition of sea water by serving as a major source or sink for a number of trace elements in the ocean. Of ...these, iron has received considerable attention because of its role as an essential and often limiting nutrient for primary production in regions of the ocean that are of critical importance for the global carbon cycle. It has been thought that most of the dissolved iron discharged by hydrothermal vents is lost from solution close to ridge-axis sources and is thus of limited importance for ocean biogeochemistry. This long-standing view is challenged by recent studies which suggest that stabilization of hydrothermal dissolved iron may facilitate its long-range oceanic transport. Such transport has been subsequently inferred from spatially limited oceanographic observations. Here we report data from the US GEOTRACES Eastern Pacific Zonal Transect (EPZT) that demonstrate lateral transport of hydrothermal dissolved iron, manganese, and aluminium from the southern East Pacific Rise (SEPR) several thousand kilometres westward across the South Pacific Ocean. Dissolved iron exhibits nearly conservative (that is, no loss from solution during transport and mixing) behaviour in this hydrothermal plume, implying a greater longevity in the deep ocean than previously assumed. Based on our observations, we estimate a global hydrothermal dissolved iron input of three to four gigamoles per year to the ocean interior, which is more than fourfold higher than previous estimates. Complementary simulations with a global-scale ocean biogeochemical model suggest that the observed transport of hydrothermal dissolved iron requires some means of physicochemical stabilization and indicate that hydrothermally derived iron sustains a large fraction of Southern Ocean export production.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
The organic complexation of dissolved iron was determined from full water column depth profile samples collected on the U.S. GEOTRACES North Atlantic Section cruises in 2010 and 2011 (GEOTRACES ...GA03). The concentrations of iron-binding ligands and their conditional stability constants were determined using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) with salicylaldoxime as the added competitive ligand. Across the basin, iron-binding ligands were found in excess of dissolved iron concentrations in all samples except those with the highest dissolved iron in the Trans-Atlantic Geotraverse (TAG) hydrothermal vent plume, where dissolved iron concentrations exceeded ligand concentrations. Ligand results were categorized based on conditional stability constants into three ligand classes (L1: logKFeLi,Fe′cond>12; L2: logKFeL2,Fe′cond=11–12; L3: logKFeL3,Fe′cond=10–11). The stronger L1-type ligand class tracked closely with dissolved iron, with the strongest ligands (i.e., highest logKFeL1,Fe′cond) found in the vicinity of the Trans-Atlantic Geotraverse (TAG) hydrothermal vent plume. All three ligand classes, including the stronger L1-type ligands, were observed through the water column. These measurements indicate that iron-binding ligands are indeed a ubiquitous feature of iron speciation in the North Atlantic.
Aerosol deposition provides a major input of the essential micronutrient iron to the open ocean. A critical parameter with respect to biological availability is the proportion of aerosol iron that ...enters the oceanic dissolved iron pool – the so-called fractional solubility of aerosol iron (%FeS). Here we present a global-scale compilation of total aerosol iron loading (FeT) and estimated %FeS values for ∼1100 samples collected over the open ocean, the coastal ocean, and some continental sites, including a new data set from the Atlantic Ocean. Despite the wide variety of methods that have been used to define ‘soluble’ aerosol iron, our global-scale compilation reveals a remarkably consistent trend in the fractional solubility of aerosol iron as a function of total aerosol iron loading, with the great bulk of the data defining an hyperbolic trend. The hyperbolic trends that we observe for both global- and regional-scale data are adequately described by a simple two-component mixing model, whereby the fractional solubility of iron in the bulk aerosol reflects the conservative mixing of ‘lithogenic’ mineral dust (high FeT and low %FeS) and non-lithogenic ‘combustion’ aerosols (low FeT and high %FeS). An increasing body of empirical and model-based evidence points to anthropogenic fuel combustion as the major source of these non-lithogenic ‘combustion’ aerosols, implying that human emissions are a major determinant of the fractional solubility of iron in marine aerosols. The robust global-scale relationship between %FeS and FeT provides a simple heuristic method for estimating aerosol iron solubility at the regional to global scale.
The results of several recent studies challenge the reigning paradigm that continental soil dust provides the only significant atmospheric source of dissolved iron to the surface ocean. This evidence ...includes correlations between the operational solubility of aerosol iron and atmospheric loadings of black carbon and aluminum-normalized vanadium and nickel, each of which are associated with emissions from the combustion of fossil fuel oil. These observations suggest that the relative solubility of aerosol iron, hence the eolian flux of soluble iron to the surface ocean, may be significantly impacted by anthropogenic oil combustion products. Using recent field data from the Bermuda region, we have developed an empirical method to estimate the solubility of aerosol iron using bulk aerosol concentrations of Fe, V and Al. We apply this method to a large body of published data from the
AEROCE program for North Atlantic island sites on Tenerife, Barbados, Bermuda and Ireland, where the relative proportions of anthropogenic aerosols range from minor to major, respectively. Our aerosol iron solubility estimates suggest that anthropogenic emissions contribute approximately 70% and 85% of the annual dry deposition of soluble iron to the surface ocean near Bermuda and Ireland, respectively, implying that human activities have profoundly affected the iron budget of the North Atlantic region. The annual mean dry deposition of soluble iron at Barbados and Izana is dominated by soil dust. The anthropogenic contribution at these two sites ranges from 12% to 30% and is highly dependent on the soil dust solubility of Fe employed in the model. The low end (∼12%) estimate appears to be more representative of these high-dust sites.
We report empirical estimates of the fractional solubility of aerosol iron over the Sargasso Sea during periods characterized by high concentrations of Saharan dust (summer 2003) and by low ...concentrations of aerosols in North American/maritime North Atlantic air masses (spring 2004 and early summer 2004). We observed a strong inverse relationship between the operational solubility of aerosol iron (defined using a flow‐through deionized‐water leaching protocol) and the total concentration of aerosol iron, whereby the operational solubility of aerosol iron was elevated when total aerosol iron loadings were low. This relationship is consistent with source‐dependent differences in the solubility characteristics of our aerosol samples and can be described by a simple mixing model, wherein bulk aerosols represent a conservative mixture of two air mass end‐members that carry different aerosol types: “Saharan air,” which contains a relatively high loading of aerosol iron (27.8 nmol Fe m−3) that has a low fractional solubility (0.44%), and “North American air,” which contains a relatively low concentration of aerosol iron (0.5 nmol Fe m−3) that has a high fractional solubility (19%). Historical data for aerosols collected on Bermuda indicate that the low iron loadings associated with North American air masses are typically accompanied by elevated V/Al, Fe/Al, and V/Mn mass ratios in the bulk aerosol, relative to Saharan dust, which are indicative of anthropogenic fuel‐combustion products. The identification of similar compositional trends in our Sargasso Sea aerosol samples leads us to suggest that the elevated solubility of iron in the aerosols associated with North American air masses reflects the presence of anthropogenic combustion products, which contain iron that is readily soluble relative to iron in Saharan soil dust. We thus propose that the source‐dependent composition of aerosol particles (specifically, the relative proportion of anthropogenic combustion products) is a primary determinant for the fractional solubility of aerosol iron over the Sargasso Sea. This hypothesis implies that anthropogenic combustion emissions could play a significant role in determining the atmospheric input of soluble iron to the surface ocean.
Dissolved iron, organic iron-binding ligands, and organic carbon were determined in full water column depth profiles across the US GEOTRACES Eastern Pacific Zonal Transect (GEOTRACES cruise GP16) in ...late 2013. Dissolved iron concentrations exhibited subsurface maxima associated with the remineralization of organic matter at the Peru shelf and with hydrothermal inputs from the East Pacific Rise. Iron-binding organic ligands are described as ligand classes based on defined ranges in conditional stability constants. The stronger L1-type ligands were measured in large excesses in surface and intermediate waters, and these excesses were negatively correlated with Si*, a biogeochemical proxy for iron limited diatom growth. These data suggest sources of strong iron-binding ligands from iron limitation of diatom communities, both locally and in waters originating from the Southern Ocean. Benthic sources of strong ligands were associated with new iron inputs from hydrothermal activity at the East Pacific Rise and from bottom sediments. In contrast to most studies in the Atlantic basin but consistent with previous datasets from the Pacific, stronger L1 ligands in this dataset were generally restricted to the upper water column and did not show large excesses through the water column. At depth, iron-binding ligands on GP16 were instead best described as L2 and L3 ligands. Concomitant decreases in excess L1, excess total ligands and dissolved organic carbon suggest similar degradation pathways of these pools below the surface.
Marine dinitrogen (N2)-fixing cyanobacteria have large impacts on global biogeochemistry as they fix carbon dioxide (CO2) and fertilize oligotrophic ocean waters with new nitrogen. Iron (Fe) and ...phosphorus (P) are the two most important limiting nutrients for marine biological N2 fixation, and their availabilities vary between major ocean basins and regions. A long-standing question concerns the ability of two globally dominant N2-fixing cyanobacteria, unicellular Crocosphaera and filamentous Trichodesmium, to maintain relatively high N2-fixation rates in these regimes where both Fe and P are typically scarce. We show that under P-deficient conditions, cultures of these two cyanobacteria are able to grow and fix N2 faster when Fe deficient than when Fe replete. In addition, growth affinities relative to P increase while minimum concentrations of P that support growth decrease at low Fe concentrations. In Crocosphaera, this effect is accompanied by a reduction in cell sizes and elemental quotas. Relatively high growth rates of these two biogeochemically critical cyanobacteria in low-P, low-Fe environments such as those that characterize much of the oligotrophic ocean challenge the common assumption that low Fe levels can have only negative effects on marine primary producers. The closely interdependent influence of Fe and P on N2-fixing cyanobacteria suggests that even subtle shifts in their supply ratio in the past, present and future oceans could have large consequences for global carbon and nitrogen cycles.
The proportion of aerosol iron (Fe) that dissolves in seawater varies greatly and is dependent on aerosol composition and the physicochemical conditions of seawater, which may change depending on ...location or be altered by global environmental change. Aerosol and surface seawater samples were collected in the Sargasso Sea and used to investigate the impact of these changing conditions on aerosol Fe dissolution in seawater. Our data show that seawater temperature, pH, and oxygen concentration, within the range of current and projected future values, had no significant effect on the dissolution of aerosol Fe. However, the source and composition of aerosols had the most significant effect on the aerosol Fe solubility, with the most anthropogenically influenced samples having the highest fractional solubility (up to 3.2%). The impact of ocean warming and acidification on aerosol Fe dissolution is therefore unlikely to be as important as changes in land usage and fossil fuel combustion. Our experimental results also reveal important changes in the size distribution of soluble aerosol Fe in solution, depending on the chemical conditions of seawater. Under typical conditions, the majority (77–100%) of Fe released from aerosols into ambient seawater existed in the colloidal (0.02–0.4 µm) size fraction. However, in the presence of a sufficient concentration of strong Fe‐binding organic ligands (10 nM) most of the aerosol‐derived colloidal Fe was converted to soluble Fe (<0.02 µm). This finding highlights the potential importance of organic ligands in retaining aerosol Fe in a biologically available form in the surface ocean.
Key Points
Aerosol source and composition have the greatest effect on iron dissolutionOcean warming and acidification may not impact aerosol iron dissolutionStrong ligands draw most of the labile aerosol iron into the soluble fraction
THE ROSS SEA IN A SEA OF CHANGE SMITH, WALKER O.; SEDWICK, PETER N.; ARRIGO, KEVIN R. ...
Oceanography (Washington, D.C.),
09/2012, Letnik:
25, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The Ross Sea, the most productive region in the Antarctic, reaches farther south than any body of water in the world. While its food web is relatively intact, its oceanography, biogeochemistry, and ...sea ice coverage have been changing dramatically, and likely will continue to do so in the future. Sea ice cover and persistence have been increasing, in contrast to the Amundsen-Bellingshausen sector, which has resulted in reduced open water duration for its biota. Models predict that as the ozone hole recovers, ice cover will begin to diminish. Currents on the continental shelf will likely change in the coming century, with a projected intensification of flow leading to altered deep ocean ventilation. Such changes in ice and circulation will lead to altered plankton distributions and composition, but it is difficult at present to predict the nature of these changes. Iron and irradiance play central roles in regulating phytoplankton production in the Ross Sea, but the impacts of oceanographic changes on the biogeochemistry of iron are unclear. Unlike other Southern Ocean regions, where continental shelves are very narrow and Antarctic krill dominates the herbivorous fauna, the broad shelf of the Ross Sea is dominated by crystal krill and silverfish, which are the major prey items for higher trophic levels. At present, the Ross Sea is considered to be one of the most species-rich areas of the Southern Ocean and a biodiversity "hotspot" due to its heterogeneous habitats. Despite being among the best-studied regions in the entire Southern Ocean, accurate predictions of the impacts of climate change on the oceanography and ecology of the Ross Sea remain fraught with uncertainty.
Particulate phases transport trace metals (TM) and thereby exert a major control on TM distribution in the ocean. Particulate TMs can be classified by their origin as lithogenic (crustal material), ...biogenic (cellular), or authigenic (formed in situ), but distinguishing these fractions analytically in field samples is a challenge often addressed using operational definitions and assumptions. These different phases require accurate characterization because they have distinct roles in the biogeochemical iron cycle. Particles collected from the upper 2,000 m of the northwest subtropical Atlantic Ocean over four seasonal cruises throughout 2019 were digested with a chemical leach to operationally distinguish labile particulate material from refractory lithogenics. Direct measurements of cellular iron (Fe) were used to calculate the biogenic contribution to the labile Fe fraction, and any remaining labile material was defined as authigenic. Total particulate Fe (PFe) inventories varied <15% between seasons despite strong seasonality in dust inputs. Across seasons, the total PFe inventory (±1SD) was composed of 73 ± 13% lithogenic, 18 ± 7% authigenic, and 10 ± 8% biogenic Fe above the deep chlorophyll maximum (DCM), and 69 ± 8% lithogenic, 30 ± 8% authigenic, and 1.1 ± 0.5% biogenic Fe below the DCM. Data from three other ocean regions further reveal the importance of the authigenic fraction across broad productivity and Fe gradients, comprising ca. 20%–27% of total PFe.
Plain Language Summary
The availability of dissolved iron controls phytoplankton growth in much of the ocean. Most oceanic iron is contained in particles that are a mixture of live and dead cells (biogenics), crustal material (lithogenics), and minerals that have precipitated in seawater (authigenics). Each of these materials has different chemical properties that determine how iron cycles through the ocean and regulate how much iron is available for uptake by phytoplankton. Specifically, authigenic and biogenic forms are more accessible to phytoplankton but are often measured as one fraction despite formation by different processes. We measured different forms of iron in particles across seasons and environmental gradients and found that 20%–30% of iron is authigenic.
Key Points
Iron in phytoplankton accounts for <80% of chemically labile Fe and does not exceed 0.1 nM
Biogenic Fe is a small and relatively invariant component of particulate Fe (PFe) pools, even across seasonal and regional gradients
Authigenic Fe is a significant fraction of particulate Fe across seasons and productivity gradients
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