Angela Milne

Developing standards for dissolved iron in seawater

In nearly a dozen open- ocean fertilization experiments conducted by more than 100 researchers from nearly 20 countries, adding iron at the sea surface has led to distinct increases in photosynthesis rates and biomass. These experiments confirmed the hypothesis proposed by the late John Martin [Martin, 1990] that dissolved iron concentration is a key variable that controls phytoplankton processes in ocean surface waters. However, the measurement of dissolved iron concentration in seawater remains a difficult task [Bruland and Rue, 2001] with significant interlaboratory differences apparent at times. The availability of a seawater reference solution with well- known dissolved iron (Fe) concentrations similar to open- ocean values, which could be used for the calibration of equipment or other tasks, would greatly alleviate these problems [National Research Council (NRC), 2002]. The Sampling and Analysis of Fe (SAFe) cruise was staged from Honolulu, Hawaii, to San Diego, Calif., between 15 October and 8 November 2004 to collect data and samples that were later used to provide this reference material. Here we provide a brief report on the cruise results, which have produced a tenfold improvement in the variability of iron measurements, and announce the availability of the SAFe dissolved Fe in seawater standards.

Determination of Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb in seawater using high resolution magnetic sector inductively coupled mass spectrometry (HR-ICP-MS)

A novel method, combining isotope dilution with standard additions, was developed for the analysis of eight elements (Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb) in seawater. The method requires just 12 mL of sample and employs an off-line pre-concentration step using the commercially available chelating resin Toyopearl AF-Chelate-650M prior to determination by high resolution inductively coupled plasma magnetic sector mass spectrometry (ICP-MS). Acidified samples were spiked with a multi-element standard of six isotopes ((57)Fe, (62)Ni, (65)Cu, (68)Zn, (111)Cd and (207)Pb) enriched over natural abundance. In addition, standard additions of a mixed Co and Mn standard were performed on sub-sets of the same sample. All samples were irradiated using a low power (119 mW cm(-2); 254 nm) UV system, to destroy organic ligands, before pre-concentration and extraction from the seawater matrix. Ammonium acetate was used to raise the pH of the 12 mL sub-samples (off-line) to pH 6.4+/-0.2 prior to loading onto the chelating resin. The extracted metals were eluted using 1.0 M Q-HNO(3) and determined using ICP-MS. The method was verified through the analysis of certified reference material (NASS-5) and the SAFe inter-comparison samples (S1 and D2), the results of which are in good agreement with the certified and reported consensus values. We also present vertical profiles of the eight metals taken from the Bermuda Atlantic Time Series (BATS) station collected during the GEOTRACES inter-comparison cruise in June 2008.

Biogeochemical cycling of dissolved zinc along the GEOTRACES South Atlantic transect GA10 at 40°S

The biogeochemical cycle of zinc (Zn) in the South Atlantic, at 40°S, was investigated as part of the UK GEOTRACES program. To date there is little understanding of the supply of Zn, an essential requirement for phytoplankton growth, to this highly productive region. Vertical Zn profiles displayed nutrient-like distributions with distinct gradients associated with the water masses present. Surface Zn concentrations are among the lowest reported for the worlds oceans ( 2 = 0.97, n = 460). Our results suggest that the use of a global Zn-Si relationship would lead to an underestimation of dissolved Zn in deeper waters of the South Atlantic. By utilizing Si* and a new tracer Zn* our data indicate that the preferential removal of Zn in the Southern Ocean prevented a direct return path for dissolved Zn to the surface waters of the South Atlantic at 40°S and potentially the thermocline waters of the South Atlantic subtropical gyre. The importance of Zn for phytoplankton growth was evaluated using the Zn-soluble reactive phosphorus (SRP) relationship. We hypothesize that the low Zn concentrations in the South Atlantic may select for phytoplankton cells with a lower Zn requirement. In addition, a much deeper kink at ~ 500m in the Zn:SRP ratio was observed compared to other oceanic regions. Key Points Dissolved zinc biogeochemical cycle investigated in the South Atlantic Ocean ©2014. American Geophysical Union. All Rights Reserved.

Investigation of iron(III) reduction and trace metal interferences in the determination of dissolved iron in seawater using flow injection with luminol chemiluminescence detection

A detailed investigation into the performance of two flow injection-chemiluminescence (FI-CL) manifolds (with and without a preconcentration column) for the determination of sub-nanomolar dissolved iron (Fe(II)+Fe(III)), following the reduction of Fe(III) by sulphite, in seawater is described. Kinetic experiments were conducted to examine the efficiency of reduction of inorganic Fe(III) with sulphite under different conditions and a rigorous study of the potential interference caused by other transition metals present in seawater was conducted. Using 100microM concentrations of sulphite a reduction time of 4h was sufficient to quantitatively reduce Fe(III) in seawater. Under optimal conditions, cobalt(II) and vanadium(IV)/(III) were the major positive interferences and strategies for their removal are reported. Specifically, cobalt(II) was masked by the addition of dimethylglyoxime to the luminol solution and vanadium(IV) was removed by passing the sample through an 8-hydroxyquinoline column in a low pH carrier stream. Manganese(II) also interfered by suppression of the CL response but this was not significant at typical open ocean concentrations.

Iron stable isotopes track pelagic iron cycling during a subtropical phytoplankton bloom

Significance The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for approximately 40% of the global ocean; however, our knowledge of how it is transferred between chemical states and pools is poorly constrained. Here we utilize the isotopic composition of dissolved and particulate iron to fingerprint its transformation in the surface ocean by abiotic and biotic processes. Photochemical and biological reduction and dissolution of particulate iron in the surface ocean appear to be key processes in regulating its supply and bioavailability to marine biota. Iron isotopes offer a new window into our understanding of the internal cycling of Fe, thereby allowing us to follow its biogeochemical transformations in the surface ocean. The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for ∼40% of the global ocean. The redox state, organic complexation, and phase (dissolved versus particulate) of iron are key determinants of iron bioavailability in the marine realm, although the mechanisms facilitating exchange between iron species (inorganic and organic) and phases are poorly constrained. Here we use the isotope fingerprint of dissolved and particulate iron to reveal distinct isotopic signatures for biological uptake of iron during a GEOTRACES process study focused on a temperate spring phytoplankton bloom in subtropical waters. At the onset of the bloom, dissolved iron within the mixed layer was isotopically light relative to particulate iron. The isotopically light dissolved iron pool likely results from the reduction of particulate iron via photochemical and (to a lesser extent) biologically mediated reduction processes. As the bloom develops, dissolved iron within the surface mixed layer becomes isotopically heavy, reflecting the dominance of biological processing of iron as it is removed from solution, while scavenging appears to play a minor role. As stable isotopes have shown for major elements like nitrogen, iron isotopes offer a new window into our understanding of the biogeochemical cycling of iron, thereby allowing us to disentangle a suite of concurrent biotic and abiotic transformations of this key biolimiting element.

Coastal ocean and shelf-sea biogeochemical cycling of trace elements and isotopes: lessons learned from GEOTRACES

Continental shelves and shelf seas play a central role in the global carbon cycle. However, their importance with respect to trace element and isotope (TEI) inputs to ocean basins is less well understood. Here, we present major findings on shelf TEI biogeochemistry from the GEOTRACES programme as well as a proof of concept for a new method to estimate shelf TEI fluxes. The case studies focus on advances in our understanding of TEI cycling in the Arctic, transformations within a major river estuary (Amazon), shelf sediment micronutrient fluxes and basin-scale estimates of submarine groundwater discharge. The proposed shelf flux tracer is 228-radium (T1/2 = 5.75 yr), which is continuously supplied to the shelf from coastal aquifers, sediment porewater exchange and rivers. Model-derived shelf 228Ra fluxes are combined with TEI/ 228Ra ratios to quantify ocean TEI fluxes from the western North Atlantic margin. The results from this new approach agree well with previous estimates for shelf Co, Fe, Mn and Zn inputs and exceed published estimates of atmospheric deposition by factors of approximately 3–23. Lastly, recommendations are made for additional GEOTRACES process studies and coastal margin-focused section cruises that will help refine the model and provide better insight on the mechanisms driving shelf-derived TEI fluxes to the ocean. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Uncertainty contributions to the measurement of dissolved Co, Fe, Pb and V in seawater using flow injection with solid phase preconcentration and detection by collision/reaction cell-quadrupole ICP-MS.

A flow injection manifold incorporating a solid phase chelating resin (Toyopearl AF-Chelate-650) is reported for the preconcentration of dissolved metals from seawater, with a focus on investigating the effect of the loading pH, wash solution composition and wash time. Cobalt, iron, lead and vanadium have been used as target analytes with contrasting oceanographic behaviour. Quadrupole ICP-MS has been used for detection to make the approach accessible to most laboratories and a collision/reaction cell has been incorporated to minimise polyatomic interferences. Results for the seawater CRM NASS-6 and two GEOTRACES reference materials were in good agreement with the certified/consensus values, demonstrating the suitability of the approach for the determination of trace metals in seawater. The experimental design used allowed a thorough investigation of the uncertainty contribution from each method parameter to the overall expanded uncertainty of the measurement. The results showed that the parameters making the largest contributions were the precision of the peak area measurement and the uncertainty associated with the slope of the calibration curve. Therefore, these are the critical parameters that should be targeted in order to reduce the overall measurement uncertainty. For iron, the wash blank also gave a measureable contribution.

Return of naturally sourced Pb to Atlantic surface waters

Anthropogenic emissions completely overwhelmed natural marine lead (Pb) sources during the past century, predominantly due to leaded petrol usage. Here, based on Pb isotope measurements, we reassess the importance of natural and anthropogenic Pb sources to the tropical North Atlantic following the nearly complete global cessation of leaded petrol use. Significant proportions of up to 30–50% of natural Pb, derived from mineral dust, are observed in Atlantic surface waters, reflecting the success of the global effort to reduce anthropogenic Pb emissions. The observation of mineral dust derived Pb in surface waters is governed by the elevated atmospheric mineral dust concentration of the North African dust plume and the dominance of dry deposition for the atmospheric aerosol flux to surface waters. Given these specific regional conditions, emissions from anthropogenic activities will remain the dominant global marine Pb source, even in the absence of leaded petrol combustion.

Particulate phases are key in controlling dissolved iron concentrations in the (sub)tropical North Atlantic

The supply and bioavailability of iron (Fe) controls primary productivity and N2 fixation in large parts of the global ocean. An important, yet poorly quantified, source to the ocean is particulate Fe (pFe). Here we present the first combined dataset of particulate, labile-particulate (L-pFe), and dissolved Fe (dFe) from the (sub)tropical North Atlantic. We show a strong relationship between L-pFe and dFe, indicating a dynamic equilibrium between these two phases whereby particles “buffer” dFe and maintain the elevated concentrations observed. Moreover, L-pFe can increase the overall “available” (L-pFe + dFe) Fe pool by up to 55%. The lateral shelf flux of this available Fe was similar in magnitude to observed soluble aerosol-Fe deposition, a comparison that has not been previously considered. These findings demonstrate that L-pFe is integral to Fe cycling and hence plays a role in regulating carbon cycling, warranting its inclusion in Fe budgets and biogeochemical models.

Seasonal iron depletion in temperate shelf seas

Our study followed the seasonal cycling of soluble (SFe), colloidal (CFe), dissolved (DFe), total dissolvable (TDFe), labile particulate (LPFe) and total particulate (TPFe) iron in the Celtic Sea (NE Atlantic Ocean). Preferential uptake of SFe occurred during the spring bloom, preceding the removal of CFe. Uptake and export of Fe during the spring bloom, coupled with a reduction in vertical exchange, led to Fe deplete surface waters (<0.2 nM DFe; 0.11 nM LPFe, 0.45 nM TDFe, 1.84 nM TPFe) during summer stratification. Below the seasonal thermocline, DFe concentrations increased from spring to autumn, mirroring NO3- and consistent with supply from remineralised sinking organic material, and cycled independently of particulate Fe over seasonal timescales. These results demonstrate that summer Fe availability is comparable to the seasonally Fe limited Ross Sea shelf, and therefore is likely low enough to affect phytoplankton growth and species composition.