William M. Landing

The contrasting biogeochemistry of iron and manganese in the Pacific Ocean

Abstract Vertical and horizontal distributions of dissolved and suspended particulate Fe and Mn, and vertical fluxes of these metals (obtained with sediment traps) were determined throughout the Pacific Ocean. In general, dissolved Fe is low in surface and deep waters (0.1 to 0.7 nmol/kg), with maxima associated with the intermediate depth oxygen minimum zone (2.0 to 6.6 nmol/kg). Vertical distributions of dissolved Mn are similar to previous reports, exhibiting a surface maximum, a subsurface minimum, a Mn maximum layer coincident with the oxygen minimum zone, and lowest values in deep waters. Near-shore removal processes are more intense for dissolved Fe than for dissolved Mn. Dissolved Mn in the surface mixed layer remains elevated much farther offshore than dissolved Fe. Elevated near-surface dissolved Mn concentrations occur in the North Pacific Equatorial Current, suggesting transport from the eastern boundary. Near-surface mixed-layer dissolved Mn concentrations are higher in the North Pacific gyre reflecting enhanced northern hemisphere aeolian sources. Residence time estimates for the settling of refractory paniculate Fe and Mn from the upper water column are 62–220 days (Fe), and 105–235 days (Mn). In contrast, residence times for the scavenging of dissolved Fe and Mn are 2–13 years (Fe) and 3–74 years (Mn). Scavenging residence times for dissolved Mn based on horizontal mixing in the surface mixed layer of the northeast Pacific are 0.4 years (nearshore) to 19 years (1000 km offshore). There is no evidence for in situ Fe redox dissolution within sub-oxic waters in the eastern tropical North Pacific. Dissolved Fe appeared to be controlled by dissolution from sub-oxic sediments, with oxidative scavenging in the water column or upper sediment layers. However, in situ Mn dissolution within the oxygen minimum zone was evident.

The biogeochemistry of manganese and iron in the Black Sea

The solution speciation and solid-phase suspended particulate fractionation of Mn and Fe were investigated in the Black Sea in an effort to understand the biogeochemical cycling of Mn and Fe across redox boundaries and to study the scavenging/precipitation reactions affecting their distributions. The redox cycling of Mn (in a distinct “suboxic” zone from 15 to 50 m thick) and the redox cycling of Fe (coincident with total sulfide concentrations exceeding 0.4 μM) occur along isopycnal surfaces which deepen sharply towards the Turkish coast. Dissolved Mn behaves primarily as the free hydrated Mn2+ species and approaches saturation with respect to MnS2 (haurite) in the deep anoxic waters. In the oxic zone, colloidal and organically-complexed Fe species account for 10–30% of the total dissolved Fe, while colloidal Fe-sulfides account for 30–60% of the total in the mid-depth dissolved Fe(II) maximum. The deep waters are close to saturation with respect to FeS (mackinawite) or Fe3S4 (greigite). A weak-acid soluble Mn phase dominates in the broad particulate Mn maximum in the suboxic zone and appears to be associated with Mn-oxidizing bacteria. More resistant Mn and Fe phases, presumed to be sulfide precipitates, were found in the deep anoxic waters. Particulate Al showed a broad maximum below the sulfide interface, presumably due to offshore transport of resuspended sediment. A vertical mixing model for dissolved Mn in the central basin of the Black Sea yields removal rates consistent with measured bacterial Mn oxidation rates. The redox cycling of Fe occurs somewhat deeper in the water column. The vertical supply of oxygen cannot account for the Mn oxidation rate. However, horizontal advection and/or seasonal vertical mixing could provide enough oxidizing equivalents.

Manganese in the North Pacific

A quantitative and precise method for determination of dissolved Mn at the nanomole(nmol)/kg level in seawater has been developed and used to study the distribution of Mn in the northeast Pacific. Mn concentrations in the surface mixed layer decrease from 1.0 to 0.6 nmol/kg between the central gyre and the western boundary of the California Current, then increase to values from 2 to 6 nmol/kg near the coastal boundary (in contrast to the distribution of210Pb). Particulate Mn in the surface waters accounts for only about 1% of the total. Vertical distributions of Mn are characterized by surface maxima, minima near 300 m, maxima at mid-depth coinciding with the oxygen minimum and the labile nutrient maxima, and concentrations in Pacific bottom waters of approximately 0.2 nmol/kg. The oceanic distribution of Mn appears to be dominated by external inputs superimposed upon overall scavenging which can lead to Mn maxima in (1) the surface waters due to riverine and atmospheric sources; (2) the deep ocean as a result of hydrothermal injection and/or sediment resuspension; and (3) the oxygen minimum region resulting from in-situ breakdown of organic matter, in-situ MnO2 reduction, and/or advective-diffusive transport of dissolved Mn from anoxic slope sediments.

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.

International field intercomparison measurements of atmospheric mercury species at Mace Head, Ireland

Eleven laboratories from North America and Europe met at Mace Head, Ireland for the period 11–15 September 1995 for the first international field intercomparison of measurement techniques for atmospheric mercury species in ambient air and precipitation at a marine background location. Different manual methods for the sampling and analysis of total gaseous mercury (TGM) on gold and silver traps were compared with each other and with new automated analyzers. Additionally, particulate-phase mercury (Hgpart) in ambient air, total mercury, reactive mercury and methylmercury in precipitation were analyzed by some of the participating laboratories. Whereas measured concentrations of TGM and of total mercury in precipitation show good agreement between the participating laboratories, results for airborne particulate-phase mercury show much higher differences. Two laboratories measured inorganic oxidized gaseous mercury species (IOGM), and obtained levels in the low picogram m-3 range.

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.

Aerosol iron and aluminum solubility in the northwest Pacific Ocean: Results from the 2002 IOC cruise

Dust aerosol samples were collected across the western North Pacific Ocean during May–June 2002. Samples were analyzed for soluble aerosol Fe(II), Fe(II) + Fe(III), and Al as well as major cations and anions. The aerosol samples were leached using a 10 second exposure to either filtered surface seawater or ultrapure deionized water yielding a measure of the “instantaneous” soluble fraction. A variety of analytical methods were employed, including 57Fe isotope dilution high-resolution ICP-MS, energy dispersive X-ray fluorescence, graphite furnace AAS, ion chromatography, and the FeLume chemiluminescent technique. Fe was found to be more soluble in ultrapure deionized water leaches, especially during periods of higher dust concentrations. Fe solubility averaged 9 ± 8% in ultrapure water leaches and 6 ± 5% in seawater leaches. Significant correlations were found between both soluble aerosol FeT and soluble Fe(II) concentrations and aerosol acidity; however, the percentages of soluble aerosol FeT and Fe(II) did not correlate with aerosol acidity We also did not observe significant correlations between total and soluble aerosol Fe concentrations and the concentrations of either particulate Fe or dissolved Fe in surface waters.

Mercury associated with colloidal material in an estuarine and an open-ocean environment

We have used traditional filtration and tangential flow ultrafiltration to isolate mercury into 4 size fractions: particulate (> 0.4 μm), high molecular weight (>10 kD, 1kD = 1000 daltons), medium molecular weight (1–10 kD), and low molecular weight ( 1 kD) represented 35–87% of the total dissolved Hg within the estuary and 10–50% of the total dissolved Hg in the North Atlantic. Equilibrium Hg speciation modeling supports speculation that colloidal Hg is bound by thiol-type functional groups associated with the colloidal organic carbon.

Colloidal trace metals, organic carbon and nitrogen in a southeastern U.S. estuary

We have used cross-flow ultrafiltration on eleven 0.45-μm-filtered samples taken across the salinity gradient in the Ochlockonee Estuary. We sequentially ultrafiltered the samples using 10- and 1-kD filters (1 kD = 1000 daltons). The results indicate that total Fe and Mn behave nonconservatively in the estuary and that the removal is from the high-molecular-weight (HMW; > 10 kD) fraction although Mn is removed at lower salinity than Fe. For Ni, Cu and Cd, the HMW fraction is very important in the river but these elements are quickly converted from HMW to low-molecular-weight (LMW) species with increasing salinity. Carbon in the HMW fraction is strongly correlated with Fe but only weakly correlated with Fe in the smaller size fractions. The two important processes controlling the behavior of metals, carbon and nitrogen in the estuary are colloid aggregation and desorption or dissociation.

THE BEHAVIOR OF BARIUM IN ANOXIC MARINE WATERS

Abstract The present day distributions of Ba in the water columns at three anoxic marine sites, namely the Cariaco Trench, Framvaren Fjord, and Black Sea, are presented. Dissolved Ba levels generally increase with depth, ranging from 45–85, 64–280, and 180–460 nM in surface and bottom waters for the three basins, respectively. Small maxima are observed in the vicinity of the in redox interface in both the Framvaren Fjord and Black Sea. Comparison of the dissolved and particulate Ba, Fe, and Mn distributions show that the maxima do not result from adsorption onto freshly precipitated Fe and/or Mn oxyhydroxides. As for the open ocean, Ba cycling in all three basins is dominated by its uptake, primarily in the form of barite, into particulate matter associated with productivity in surface waters, followed by its regeneration at depth or in the sediments. Microbiological activity near the redox interface promotes the breakdown of settling particulate matter and the release of barite just above the O 2 H 2 S interface in the Black Sea, and most likely in the Framvaren Fjord, thus providing in part for the observed maxima. Dissolution of such barite in the marginal sediments of these basins probably also contributes to the maxima. Thermodynamic calculations show deep Black Sea Ba concentrations exceed saturation with respect to pure barite by at least a factor of 2. However, the uniformity of the deep water concentrations suggests thermodynamic control by some phase; it is likely that impurities, incorporated into barite during its rapid formation near the surface in microenvironments provided by decaying organisms, are responsible for the levels observed. Additional factors controlling the distributions of Ba in each basin are also discussed.