Significance Low concentrations of the micronutrient iron in seawater are known to limit primary production and nitrogen fixation in large regions of the global ocean. Thus, it is important to ...constrain the sources and sinks controlling the marine dissolved iron distribution and consequent micronutrient supply to surface plankton. Although the major dissolved iron sources have been historically thought to be atmospheric dust inputs and fluxes from the continental margin, we show here the first data to our knowledge demonstrating that dissolved iron from hydrothermal vents can be transported thousands of kilometers from the venting site, which to date has only been suggested and modeled. Thus, hydrothermal vents must be considered when determining the marine dissolved iron inventory, especially in the abyssal ocean.
Until recently, hydrothermal vents were not considered to be an important source to the marine dissolved Fe (dFe) inventory because hydrothermal Fe was believed to precipitate quantitatively near the vent site. Based on recent abyssal dFe enrichments near hydrothermal vents, however, the leaky vent hypothesis Toner BM, et al. (2012) Oceanography 25(1):209–212 argues that some hydrothermal Fe persists in the dissolved phase and contributes a significant flux of dFe to the global ocean. We show here the first, to our knowledge, dFe (<0.4 µm) measurements from the abyssal southeast and southwest Pacific Ocean, where dFe of 1.0–1.5 nmol/kg near 2,000 m depth (0.4–0.9 nmol/kg above typical deep-sea dFe concentrations) was determined to be hydrothermally derived based on its correlation with primordial ³He and dissolved Mn (dFe: ³He of 0.9–2.7 × 10 ⁶). Given the known sites of hydrothermal venting in these regions, this dFe must have been transported thousands of kilometers away from its vent site to reach our sampling stations. Additionally, changes in the size partitioning of the hydrothermal dFe between soluble (<0.02 µm) and colloidal (0.02–0.4 µm) phases with increasing distance from the vents indicate that dFe transformations continue to occur far from the vent source. This study confirms that although the southern East Pacific Rise only leaks 0.02–1% of total Fe vented into the abyssal Pacific, this dFe persists thousands of kilometers away from the vent source with sufficient magnitude that hydrothermal vents can have far-field effects on global dFe distributions and inventories (≥3% of global aerosol dFe input).
Changes in chromium (Cr) isotope ratios due to fractionation between trivalent Cr(III) and hexavalent Cr(VI) are being utilized by geologists to infer oxygen conditions in past environments. However, ...there is little information available on Cr in the modern ocean to ground-truth these inferences. Transformations between the two chromium species are important processes in oceanic Cr cycling. Here we present profiles of hexavalent and trivalent Cr concentrations and stable isotope ratios from the eastern tropical North Pacific (ETNP) oxygen-deficient zone (ODZ) which support theoretical and experimental studies that predict that lighter Cr is preferentially reduced in low-oxygen environments and that residual dissolved Cr becomes heavier due to removal of particle-reactive Cr(III) on sinking particles. The Cr(III) maximum dominantly occurs in the upper portion of the ODZ, implying that microbial activity (dependent on the sinking flux of organic matter) may be the dominant mechanism for this transformation, rather than a simple inorganic chemical conversion between the species depending on the redox potential.
Although the cycling of chromium (Cr) in the ocean may have direct implications on the Cr isotope system's application as a paleo-redox proxy, little seawater Cr isotope data has been published. This ...may be partly due to the analytical challenges associated with the preconcentration and the purification of the nano-molar trace metal Cr from the seawater matrix. Here, we present a reliable method for the determination of accurate and precise 53Cr/52Cr isotope ratios of total chromium (=Cr(III) + Cr(VI)) in seawater, which could be modified to be redox species-specific. Seawater acidification induces the slow conversion of Cr(VI) to Cr(III). Total Cr of the sample and an equilibrated 50Cr-54Cr double spike is preconcentrated as Cr(III) using Mg(OH)2 coprecipitation. Cr captured by the Mg(OH)2 pellet is oxidized to Cr(VI) using (NH4)2S2O8 under carefully controlled pH, temperature and time settings, in order to avoid H2O2 generation and to control sulfur speciation. Taking advantage of the differing charges of Cr(III) and Cr(VI), three AG 1-X8 columns separate Cr from the salt matrix and isobaric interferences V, Ti, and Fe (column 1), sulfur polyatomic interferences (column 2), and Fe traces (column 3). Analysis is conducted on a MC-ICP-MS IsoProbe featuring a hexapole collision cell in low resolution mode. When pure solutions of SRM 979 are processed via this low blank method (~0.017 nmol of Cr), δ53Cr w.r.t. SRM 979 = +0.02 ± 0.06‰ (±2 SD) is obtained (n = 10). Using this technique, the first full water column profile of Cr isotope ratios at a station in the Pacific Ocean (SAFe station, 30°N 140°W) was generated and demonstrates high oceanographic consistency.
This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.
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Anthropogenic Lead Emissions in the Ocean BOYLE, EDWARD A.; LEE, JONG-MI; ECHEGOYEN, YOLANDA ...
Oceanography (Washington, D.C.),
03/2014, Volume:
27, Issue:
1
Journal Article
Peer reviewed
Open access
We review the current distribution of lead and lead isotopes in the ocean with regard to the evolving pattern of human emissions during the past decades and centuries.
Methanogenic burst in the end-Permian carbon cycle Rothman, Daniel H.; Fournier, Gregory P.; French, Katherine L. ...
Proceedings of the National Academy of Sciences,
04/2014, Volume:
111, Issue:
15
Journal Article
Peer reviewed
Open access
The end-Permian extinction is associated with a mysterious disruption to Earth’s carbon cycle. Here we identify causal mechanisms via three observations. First, we show that geochemical signals ...indicate superexponential growth of the marine inorganic carbon reservoir, coincident with the extinction and consistent with the expansion of a new microbial metabolic pathway. Second, we show that the efficient acetoclastic pathway in Methanosarcina emerged at a time statistically indistinguishable from the extinction. Finally, we show that nickel concentrations in South China sediments increased sharply at the extinction, probably as a consequence of massive Siberian volcanism, enabling a methanogenic expansion by removal of nickel limitation. Collectively, these results are consistent with the instigation of Earth’s greatest mass extinction by a specific microbial innovation.
Reconstructing the history of metal deposition in Singapore lake sediments contributes to understanding the anthropogenic and natural metal deposition in the data-sparse Southeast Asia. To this end, ...we present a sedimentary record of Pb, Pb isotopes and eleven other metals (Ag, As, Ba, Cd, Co, Cr, Cu, Ni, Tl, U and Zn) from a well-dated sediment core collected near the depocenter of MacRitchie Reservoir in central Singapore. Before the 1900s, the sedimentary Pb concentration was less than 2 mg/kg for both soil and sediment, with a corresponding 206Pb/207Pb of ∼1.20. The Pb concentration increased to 55 mg/kg in the 1990s, and correspondingly the 206Pb/207Pb decreased to less than 1.14. The 206Pb/207Pb in the core top sediment is concordant with the 206Pb/207Pb signal of aerosols in Singapore and other Southeast Asian cities, suggesting that Pb in the reservoir sediment was mainly from atmospheric deposition. Using the Pb concentration in the topmost layer of sediment, the estimated atmospheric Pb flux in Singapore today is ∼1.6 × 10−2 g/m2 yr. The concentrations of eleven other metals preserved in the sediment were also determined. A principal component analysis showed that most of the metals exhibit an increasing trend towards 1990s with a local concentration peak in the mid-20th century.
•The temporal variations of 12 trace metals and Pb isotopes are reported from Singapore.•Pb isotope changed from natural to regional atmospheric values.•Most metals in the sediment have increased greatly during the 20th century.
The sedimentary record from Singapore show that most metals increased during 20th century with Pb isotopes implied atmospheric sources.
The size partitioning of dissolved iron (dFe, <0.4μm) into soluble (sFe, <0.02μm) and colloidal (0.02μm<cFe<0.4μm) phases was investigated at seven stations in the tropical North Atlantic Ocean, and ...the results are compared to the dFe size fractionation study of Bergquist et al. (2007) in the same region. Downwind of the North African dust plumes, cFe comprised 80±7% of the surface dFe pool at six stations, supporting the hypothesis that atmospherically-derived Fe is maintained in the colloidal size fraction. At the deep chlorophyll maximum, colloidal Fe had minimum concentrations or was completely absent, suggesting that cFe was either preferentially taken up by microbes and/or scavenged/aggregated at these depths. At remineralization depths, sFe was the dominant fraction both in the subtropical gyre-like stations (76% sFe; sFe=0.42±0.03nmol/kg) and in the oxygen minimum zone (56% sFe; sFe=0.65±0.03nmol/kg). Only at remineralization depths of stations with intermediate oxygen concentrations (100–110μmol/kg) did colloidal Fe dominate (contributing 58% of dFe) , indicating that cFe may be serving as a conduit of dFe loss during mixing of high-Fe OMZ and low-Fe gyre waters. North Atlantic Deep Water (NADW) had a typical sFe concentration of 0.34±0.05nmol/kg. In the deepest samples composed of a NADW/Antarctic Bottom Water mixture where the bottom water may have attained a ∼0.1nmol/kg hydrothermal Fe input during transit past the Mid-Atlantic Ridge, sFe did not increase coincidentally with dFe, indicating that any potential hydrothermal Fe contribution was colloidal. In general, the results of this study counter the previous hypothesis of Bergquist et al. (2007) that the colloidal Fe fraction predominately controls dFe variability, instead suggesting that both soluble and colloidal Fe are variable, and both contribute to the observed dFe variability throughout the North Atlantic. The nearly 50–50% dFe partitioning into soluble and colloidal phases below the DCM suggest one of two partitioning mechanisms persists: (1) soluble and colloidal Fe exchange rates reach a “steady state,” over which regional, uniquely-partitioned Fe sources can be overlain, or (2) the partitioning of Fe-binding ligands between the two size fractions is variable in the open ocean and directly controls dFe partitioning.
Twenty years ago, measurements on ice cores showed that the concentration of carbon dioxide in the atmosphere was lower during ice ages than it is today. As yet, there is no broadly accepted ...explanation for this difference. Current investigations focus on the ocean's 'biological pump', the sequestration of carbon in the ocean interior by the rain of organic carbon out of the surface ocean, and its effect on the burial of calcium carbonate in marine sediments. Some researchers surmise that the whole-ocean reservoir of algal nutrients was larger during glacial times, strengthening the biological pump at low latitudes, where these nutrients are currently limiting. Others propose that the biological pump was more efficient during glacial times because of more complete utilization of nutrients at high latitudes, where much of the nutrient supply currently goes unused. We present a version of the latter hypothesis that focuses on the open ocean surrounding Antarctica, involving both the biology and physics of that region.
Material fluxes at the land-ocean interface impact seawater composition and global cycling of elements. However, most attention has been focused on the fluvial dissolved fluxes. For elements like ...lead (Pb), whose fluvial particulate flux into the ocean is two orders of magnitude higher than the dissolved counterpart, the role of particulates in elemental cycling is potentially important but currently less appreciated. Using both chemical analyses on samples collected from around equatorial Southeast Asia and model simulations, we show that particulate-dissolved exchange is an important mechanism controlling the concentration and isotopic composition of dissolved Pb in the ocean. Our model indicates that Pb contributed from particulate-dissolved exchange at ocean boundaries is larger than, or at least comparable to, other major Pb sources to the seawater before the Anthropocene, when the anthropogenic Pb was absent. Our work highlights the importance of boundary exchange in understanding marine element cycling and weathering-climate feedback.
Organic ligands dominate the speciation of iron in the ocean. Little is known, however, about the chemical composition and distribution of these compounds. Here, we describe a method to detect low ...concentrations of organic Fe ligands using reverse-phase high-performance liquid chromatography (HPLC) tandem multicollector inductively coupled plasma mass spectrometry. This technique can be used to screen seawater and marine cultures for target compounds that can be isolated and structurally characterized. Sensitive detection (<1 picomole Fe) is achieved using an iron-free HPLC system to reduce background Fe levels, minimizing 40Ar16O+ interferences on 56Fe with a hexapole collision cell, and introducing oxygen into the sample carrier gas to prevent the formation of reduced carbon deposits that decrease sensitivity. This method was tested with a chromatographic separation of five trace metal complexes that represent the polarity range likely found in seawater. Good separation was achieved with a 20 min water/methanol gradient, although sensitivity decreased by a factor of 2 at high organic solvent concentrations. Finally, Fe ligand complexes were detected from the organic extract of surface South Pacific seawater and from culture media of the siderophore producing cyanobacteria Synechococcus sp. PCC 7002.
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