Osmium is one of the rarer elements in seawater, with typical concentration of ≈10 x 10⁻¹⁵ g g⁻¹ (5.3 x 10⁻¹⁴ mol kg⁻¹). The osmium isotope composition (¹⁸⁷Os/¹⁸⁸Os ratio) of deep oceans is 1.05, ...reflecting a balance between inputs from continental crust (≈1.3) and mantle/cosmic dust (≈0.13). Here, we show that the ¹⁸⁷Os/¹⁸⁸Os ratio ratios measured in rain and snow collected around the world range from 0.16 to 0.48, much lower than expected (> 1), but similar to the isotope composition of ores (≈0.2) that are processed to extract platinum and other metals to be used primarily in automobile catalytic converters. Present-day surface seawater has a lower ¹⁸⁷Os/¹⁸⁸Os ratio (≈0.95) than deep waters, suggesting that human activities have altered the isotope composition of the world's oceans and impacted the global geochemical cycle of osmium. The contamination of the surface ocean is particularly remarkable given that osmium has few industrial uses. The pollution may increase with growing demand for platinum-based catalysts.
Dissolved iron and manganese and total dissolvable iron were measured in water column samples collected from the polynya region of the southern Ross Sea during cruises in November–December 1994 ...(spring 1994) and December 1995 to January 1996 (summer 1995). Iron and manganese addition bottle incubation experiments were also performed during these cruises in order to assess the nutritional sufficiency of ambient iron and manganese concentrations for growth of the phytoplankton community. Generally high dissolved iron concentrations (>0.5 nM) and relatively complex iron and manganese vertical profiles were obtained during the spring 1994 cruise, compared with the summer 1995 data. Dissolved iron concentrations in the upper water column averaged 1.0 nM during spring 1994 and 0.23 nM in summer 1995, excluding two stations where concentrations exceeding 1 nM are attributed to inputs from melting sea ice. The observed differences in the distribution of iron and manganese between spring 1994 and summer 1995 are attributed to seasonal decreases in the up welling of bottom waters and melting of sea ice, which supply these metals into the upper water column, combined with the cumulative removal of iron and manganese from the water column throughout the spring and summer, due to biological uptake, vertical export and scavenging by suspended and sinking particles. Results of the metal addition bottle incubation experiments indicate that ambient dissolved iron concentrations are adequate for phytoplankton growth requirements during the spring and early summer, when algal production is highest and Phaeocystis antarctica dominates the algal community, whereas low dissolved iron concentrations limit algal community growth later in the summer, except in the stratified, iron‐enriched waters near melting sea ice, where diatoms are able to bloom. Our observations and the inferred seasonal distributions of P. antarctica and diatoms in these waters suggest that iron availability and vertical mixing (i.e., irradiance) exert the primary controls on phytoplankton growth and community structure in the southern Ross Sea during the spring and summer.
Phytoplankton production in the Ross Sea is regulated by the availability of dissolved iron (dFe), a limiting micro-nutrient, whose sources include Circumpolar Deep Water, sea ice melt, glacial melt, ...and benthic sources (sediment efflux and remineralization). We employ a passive tracer dye to model the benthic dFe sources and track pathways from deep areas of the continental shelf to the surface mixed layer in simulations with and without tidal forcing, and at 5 and 1.5km horizontal resolution. This, combined with dyes for each of the other dFe sources, provides an estimate of total dFe supply to surface waters. We find that tidal forcing increases the amount of benthic dye that covers the banks on the continental shelf. Calculations of mixed layer depth to define the surface ocean give similar average values over the shelf, but spatial patterns differ between simulations, particularly along the ice shelf front. Benthic dFe supply in simulations shows an increase with tidal forcing and a decrease with higher resolution. The changes in benthic dFe supply control the difference in total supply between simulations. Overall, the total dFe supply from simulations varies from 5.60 to 7.95μmolm−2year−1, with benthic supply comprising 32–50%, comparing well with recent data and model synthesis. We suggest that including tides and using high horizontal resolution is important, especially when considering spatial variability of iron supply on the Ross Sea shelf.
•Dissolved iron (dFe) sources are tracked using a regional ocean model.•dFe supply increases with tides and decreases with horizontal resolution.•Tides and resolution affect dFe supply primarily through the benthic source.•Ross Sea dFe supply has high spatial variability.
High-resolution dissolved Fe (dFe) and dissolved Mn (dMn) distributions were obtained using a trace metal clean rosette during the GEOTRACES GA03 zonal transect cruises (USGT10 and USGT11) across the ...North Atlantic Ocean. This manuscript provides a general overview of the dFe, as well as dMn and dissolved Al (dAl) distributions that reveal several Fe inputs at varying depths across the study region. Elevated dFe concentrations correlate with elevated dAl concentrations in the surface waters of the subtropical gyre, indicating a significant atmospheric source of Fe, in contrast there is no apparent significant dust source for Mn. In the subsurface waters, dFe maxima are a result of the remineralization process, as revealed by their correspondence with dissolved oxygen minima. Within the oxygen minimum, the ratio of dFe to apparent oxygen utilization (AOU) is lower than would be expected from the measured Fe content of surface water phytoplankton, suggesting that a significant amount of dFe that is remineralized at depth (~63–90%) is subsequently scavenged from the water column. The rate of remineralization, which is based on the slope of dFe:AOU plot, is similar across a wide area of the North Atlantic. In addition to the remineralization process, sedimentary inputs are apparent from elevated dMn signals in the eastern basin, particularly near the African coast. In the western basin, sedimentary input is also occurring along the advective flow path of the Upper Labrador Sea Water (ULSW), as ULSW transits along the North American continental shelf region. The largest dFe anomaly (~68nM), which also corresponds to a dMn anomaly (up to ~33nM) is seen in the neutrally buoyant hydrothermal plume sampled over the Mid-Atlantic Ridge, and that signal is visible for ~500km to the west of the ridge.
Paytan et al. (2009) argue that the atmospheric deposition of aerosols lead to copper concentrations that are potentially toxic to marine phytoplankton in a large area of tropical and subtropical ...North Atlantic Ocean. A key assumption in their model is that all marine aerosols (mineral dust and anthropogenic particles) have a high (40%) fractional solubility of copper. Our data show that the fractional solubility of copper for Saharan dust over the Sargasso Sea and Bermuda is significantly lower (1–7%). In contrast, anthropogenic aerosols with non‐Saharan sources have significantly higher values (10–100%). Hence, the potential Cu toxicity in the tropical and subtropical North Atlantic should be re‐estimated, given the low fractional solubility of Cu in the Saharan dust that dominates aerosol deposition to this region.
During summer 1995–96, we measured iron in the water column and conducted iron‐enrichment bottle‐incubation experiments at a station in the central Ross Sea (76°30′S, 170°40′W), first, in the ...presence of melting sea ice, and 17 days later, in ice‐free conditions. We observed a striking temporal change in mixed‐layer dissolved iron concentrations at this station, from 0.72–2.3 nM with sea ice present, to 0.16–0.17 nM in ice‐free conditions. These changes were accompanied by a significant drawdown in macronutrients and an approximate doubling of algal (diatom) biomass. Our incubation experiments suggest that conditions were iron‐replete in the presence of sea ice, and iron‐deficient in the absence of sea ice. We surmise that bioavailable iron was released into seawater from the melting sea ice, stimulating phytoplankton production and the biological removal of dissolved iron from the mixed layer, until iron‐limited conditions developed. These observations suggest that the episodic release of bioavailable iron from melting sea ice is an important factor regulating phytoplankton production, particularly ice‐edge blooms, in seasonally ice‐covered Antarctic waters.
The bioavailability of iron influences the distribution, biomass and productivity of phytoplankton in the Ross Sea, one of the most productive regions in the Southern Ocean. We mapped the spatial and ...temporal extent and severity of iron-limitation of the native phytoplankton assemblage using long- (>24h) and short-term (24h) iron-addition experiments along with physiological and molecular characterisations during a cruise to the Ross Sea in December–February 2012. Phytoplankton increased their photosynthetic efficiency in response to iron addition, suggesting proximal iron limitation throughout most of the Ross Sea during summer. Molecular and physiological data further indicate that as nitrate is removed from the surface ocean the phytoplankton community transitions to one displaying an iron-efficient photosynthetic strategy characterised by an increase in the size of photosystem II (PSII) photochemical cross section (σPSII) and a decrease in the chlorophyll-normalised PSII abundance. These results suggest that phytoplankton with the ability to reduce their photosynthetic iron requirements are selected as the growing season progresses, which may drive the well-documented progression from Phaeocystis antarctica- assemblages to diatom-dominated phytoplankton. Such a shift in the assemblage-level photosynthetic strategy potentially mediates further drawdown of nitrate following the development of iron deficient conditions in the Ross Sea.
•Phytoplankton alter their photosynthetic physiology over the growing season to a strategy requiring less iron.•This results in fluorescence yields per chlorophyll and PSII both increasing as the growing season develops.•This observation may explain the well characterised progression from Phaeocystis spp. to diatoms over the growing season.•It will also have implications for the assessment of primary production from estimates of chlorophyll in this region.
Atmospheric deposition is an important pathway by which bioactive trace metals are delivered to the surface ocean. The proportions of total aerosol trace metals that dissolve in seawater, and thus ...become available to biota, are not well constrained and are therefore a key uncertainty when estimating atmospheric fluxes of these elements to surface waters. The aim of this study was to elucidate the main physico-chemical controls on the dissolution of the bioactive trace metals manganese (Mn), cobalt (Co), nickel (Ni) and lead (Pb). To this end, aerosol and surface seawater samples were collected in the Sargasso Sea and subsequently used in sequential seawater leach dissolution experiments to assess the role of aerosol source, seawater temperature, pH, and concentrations of dissolved oxygen and organic ligands, on aerosol trace metal dissolution.
Results reveal that changes in key physico-chemical parameters in seawater leaches had little effect on the proportions of Mn, Co, Ni and Pb released from aerosols, although organic ligand amendments impacted the size distribution of aerosol-derived Mn in solution. Conversely, aerosol source and composition had the most significant effect on the dissolution of aerosol Co and Pb, with the most ‘anthropogenic’ aerosol samples displaying the highest fractional solubilities in seawater (up to 58% for Co and 112% for Pb).
Fractional solubilities over the range of samples and conditions tested were in the range of 50–104% for Mn, 29–58% for Co, 40–85% for Ni and 67–112% for Pb. A large proportion (36–100%, median 89%) of the total dMn, dCo, dNi and dPb was mobilised rapidly during the first leaching step (5min), with less dTM being released in leaches 2 through 4. Furthermore, investigation of the size distribution of the aerosol-derived trace metals in seawater showed that dissolved Pb was mostly colloidal (0.02–0.4μm), dissolved Mn and Co were mostly soluble (<0.02μm), and dissolved Ni displayed a mixed size distribution. Good empirical relationships were observed between enrichment factors for aerosol antimony (Sb) and the fractional solubilities of aerosol Fe, Co and Pb, suggesting total aerosol Sb can be useful in estimating and modelling the fractional solubility of these metals using total aerosol trace metal concentrations from historical data.
•Simulated ocean warming, acidification and anoxia had little impact on aerosol trace metal dissolution in surface seawater.•Aerosol provenance and chemical speciation exerted the greatest control on Co and Pb dissolution.•Linear relationships between enrichment factors and solubility were found (useful in estimating Fe, Co and Pb solubility).•Size fractionation of seawater leaches showed aerosol dPb was mostly colloidal and aerosol dMn and dCo were mostly soluble.
The quotas of P, S, Mn, Fe, Ni, and Zn in individual Synechococcus cells collected from the surface and deep chlorophyll maximum (DCM) layer of three mesoscale eddies in the Sargasso Sea were ...measured using synchrotron X-ray fluorescence microscopy. Cells in a mode-water eddy had significantly higher P (57 ± 10 amol) and Mn (28 ± 7 zmol) cell quotas than cells collected from a cyclone (22 ± 2 amol and 10 ± 1 zmol, respectively) or anticyclone (25 ± 3 amol and 18 ± 3 zmol, respectively). Conversely, Ni and Zn quotas were significantly higher in the cells from the anticyclone (92 ± 19 and 561 ± 150 zmol, respectively) than in cells from the cyclonic (25 ± 4 and 35 ± 7 zmol, respectively) or mode-water (30 ± 9 and 21 ± 8 zmol, respectively) eddies. These changes may reflect biochemical responses (e. g., production of urease and alkaline phosphatase) to gradients in inorganic N and P supplies. Cellular quotas of Fe (111 ± 17 zmol in the cyclone) and S (52 ± 6 amol in the cyclone) did not vary significantly among eddies despite two-to threefold higher dissolved and particulate Fe concentrations in the anticylone. Cells collected from 10-m depth contained approximately 80% more Ni and S than cells collected from the DCM, potentially reflecting cell responses to heightened oxidative stress. Depthrelated trends varied by eddy for the other elements. Cellular P and Zn varied significantly during repeated samplings of the cyclone, with quotas of both elements dropping as bulk chlorophyll biomass in the DCM increased. These data demonstrate the dynamic responses of phytoplankton elemental composition to physical and chemical environmental gradients.
We report measurements of dissolved iron (dFe, <0.4
μm) in seawater collected from the upper 300
m of the water column along the CLIVAR SR3 section south of Tasmania in March 1998 (between 42°S and ...54°S) and November–December 2001 (between 47°S and 66°S). Results from both cruises indicate a general north-to-south decrease in mixed-layer dFe concentrations, from values as high as 0.76
nM in the Subtropical Front to uniformly low concentrations (<0.1
nM) between the Polar Front and the Antarctic continental shelf. Samples collected from the seasonal sea-ice zone in November–December 2001 provide no evidence of significant dFe inputs from the melting pack ice, which may explain the absence of pronounced ice-edge algal blooms in this sector of the Southern Ocean, as implied by satellite ocean-color images. Our data also allow us to infer changes in the dFe concentration of surface waters during the growing season. South of the Polar Front, a comparison of near-surface with subsurface (150
m depth) dFe concentrations in November–December 2001 suggests a net seasonal biological uptake of at least ∼0.14–0.18
nM dFe, of which ∼0.05–0.12
nM is depleted early in the growing season (before mid December). A comparison of our spring 2001 and fall 1998 data indicates a barely discernible seasonal depletion of dFe (∼0.03
nM) within the Polar Frontal Zone. Further north, most of our iron profiles do not exhibit near-surface depletions, and mixed-layer dFe concentrations are sometimes higher in samples from fall 1998 compared to spring 2001; here, the near-surface dFe distributions appear to be dominated by time-varying inputs of aerosol iron or advection of iron-rich subtropical waters from the north.
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