Superoxide reactions in seawater: evaluating the impact of superoxide on trace metal redox cycles and dust dissolution in the open ocean

The major external source of iron (Fe), which is an essential nutrient for all aquatic organisms, to the Ocean is through aeolian deposition of dust. The Superoxide radical (O2-) is a key species affecting metal redox speciation and has been shown to cycle Fe between the thermodynamically favoured, but poorly soluble, Fe(III) species and the more bioavailable reduced Fe(II) form. The primary hypothesis that this thesis examines is whether there is significant dissolution of recently deposited aeolian dust by O2- in the surface waters around Cape Verde in the Atlantic Ocean. A secondary goal was to fully identify and determine the significance of all reactions involving O2- in the open ocean. Due to problems with the methods in the existing published literature I was compelled to develop and evaluate new techniques and methods which could be successfully used for the field expeditions in this work.This includes a new theoretical basis for the interpretation of the data as in many cases the existing theory was too simplistic giving rise to serious artifacts. The first manuscript describes the optimized methodology I developed for the calibration and determination of O2- fluxes in seawater which avoids most of all the discrepancies found in previous works. This was achieved through the application of an O2- thermal source for use as calibration source for both concentration and flux measurements. O2- reactions with dissolved organic matter is the subject of my 2nd manuscript as little is known about these reactions which may be important for the cycling and oxidation of organic matter. This work found in waters close to Cape Verde a relative small but significant contribution from organic material to the O2- decay. The 3rd manuscript examines the question of which processes control dust dissolution in seawater. My results indicate that while ligand promoted and thermal dissolution appeared to be clearly important, the experiments in natural waters were highly variable and most of this variability was apparently related to dynamic factors attributed to changes in surface seawater chemistry occurring over both spatial and temporal scales. Key amongst these factors was most likely the abundance of colloidal metal species and ligands. The last manuscript describes work on superoxide decay kinetics in the Southern Ocean where measurements of O2- decay kinetics throughout the water column were performed. In this work Cu was found to be the major sink of O2- in the Southern Ocean despite being strongly organically complexed. This indicates that the Cu organic complexes react directly with O2-. Contrastingly the reaction with Fe was relatively slow throughout the water column suggesting that organic complexes of iron do not react significantly with O2-

Superoxide Decay Kinetics in the Southern Ocean

Measurements of superoxide (O(2)(-)) reaction kinetics were made during a transect with the research icebreaker Polarstern (ANT24-3) in the Antarctic through the Drake Passage in austral autumn 2008. Our sampling strategy was designed to investigate the sinks of superoxide in Polar waters; principally through reactions with dissolved organic matter (DOM) or metals (copper and iron). We modified an existing chemiluminescence flow injection system using methyl Cypridina luciferin analog (MCLA) for the detection of O(2)(-) and added O(2)(-) using KO(2) as the source. Our results indicate that O(2)(-) in ambient seawater had a half-life ranging from 9.3 to 194 s. DTPA additions to seawater, to remove the effects of reactions with metals, revealed O(2)(-) decay rates consistent with a second order reaction, indicating that the dismutation reaction dominated and that reactions with DOM were not significant. Titrations of seawater by the addition of nanomolar amounts of iron or copper revealed the importance of organic chelation of Fe and/or Cu in controlling the reactivity with O(2)(-). Throughout the water column reactions with Cu appeared to be the major sink for superoxide in the Southern Ocean. This new strategy suggests an alternative approach for speciation measurements of Fe and Cu in seawater

Reactivity of inorganic Mn and Mn Desferrioxamine B with O2, O2-and H2O2in seawater

Manganese (Mn) is a required element for oceanic phytoplankton as it plays a critical role in photosynthesis, through its unique redox chemistry, as the active site in photosystem II and in enzymes that act as defences against reactive oxygen species (ROS), most notably for protection against superoxide (O2-), through the action of superoxide dismutase (SOD), and against hydrogen peroxide (H2O2) via peroxidases and catalases. The distribution and redox speciation of Mn in the ocean is also apparently controlled by reactions with ROS. Here we examine the connections between ROS and dissolved Mn species in the upper ocean using field and laboratory experimental data. Our results suggest it is unlikely that significant concentrations of Mn(III) are produced in the euphotic zone, as in the absence of evidence for the existence of strong Mn(III) ligands, Mn(II) reacts with O2- to form the short lived transient manganous superoxide, MnO2+, which may react rapidly with other redox species in a similar manner to O2-. Experiments with the strong Mn(III) chelator, desferrioxamine B (DFB), in seawater indicated that the Mn(III) species is unlikely to form, as the pre-cursor Mn(II) complex under natural ambient conditions due to the high side reaction of DFB with Ca

Die Reaktionen von Superoxid in Seewasser : Abschätzung des Einflusses von Superoxid auf die Redoxkreisprozesse von Spurenmetallen und auf die Auflösung von Staub im offenen Ozean

The major external source of iron (Fe), which is an essential nutrient for all aquatic organisms, to the Ocean is through aeolian deposition of dust. The Superoxide radical (O2-) is a key species affecting metal redox speciation and has been shown to cycle Fe between the thermodynamically favoured, but poorly soluble, Fe(III) species and the more bioavailable reduced Fe(II) form. The primary hypothesis that this thesis examines is whether there is significant dissolution of recently deposited aeolian dust by O2- in the surface waters around Cape Verde in the Atlantic Ocean. A secondary goal was to fully identify and determine the significance of all reactions involving O2- in the open ocean. Due to problems with the methods in the existing published literature I was compelled to develop and evaluate new techniques and methods which could be successfully used for the field expeditions in this work.This includes a new theoretical basis for the interpretation of the data as in many cases the existing theory was too simplistic giving rise to serious artifacts. The first manuscript describes the optimized methodology I developed for the calibration and determination of O2- fluxes in seawater which avoids most of all the discrepancies found in previous works. This was achieved through the application of an O2- thermal source for use as calibration source for both concentration and flux measurements. O2- reactions with dissolved organic matter is the subject of my 2nd manuscript as little is known about these reactions which may be important for the cycling and oxidation of organic matter. This work found in waters close to Cape Verde a relative small but significant contribution from organic material to the O2- decay. The 3rd manuscript examines the question of which processes control dust dissolution in seawater. My results indicate that while ligand promoted and thermal dissolution appeared to be clearly important, the experiments in natural waters were highly variable and most of this variability was apparently related to dynamic factors attributed to changes in surface seawater chemistry occurring over both spatial and temporal scales. Key amongst these factors was most likely the abundance of colloidal metal species and ligands. The last manuscript describes work on superoxide decay kinetics in the Southern Ocean where measurements of O2- decay kinetics throughout the water column were performed. In this work Cu was found to be the major sink of O2- in the Southern Ocean despite being strongly organically complexed. This indicates that the Cu organic complexes react directly with O2-. Contrastingly the reaction with Fe was relatively slow throughout the water column suggesting that organic complexes of iron do not react significantly with O2-.Der Eintrag von Staub in das Oberflächenwasser der Ozeane ist als die wichtigste externe Quelle für z.B. Eisen, dass für alle im Wasser vorkommenden Organismen essentiell ist, bekannt. Gerade in Regionen mit hohem regelmäßigem Staubeintrag (z.B. subtropischer Atlantik) spielen daher die mit dem eingetragenen Material zusammenhängenden Stoffflüsse eine zentrale Rolle. Das Superoxid Radikal (O2-) gilt als Mediator zwischen verschiedenen Redox Stufen wichtiger Spurenmetalle (z.B. Fe, Cu). In dem Fall von Eisen vermittelt O2- zwischen dem thermodynamisch begünstigten, allerdings schlecht löslichen Fe(III) Spezies und der besser bioverfügbaren Form Fe(II).Grundlegend für die vorliegende Dissertation war daher die Frage, ob O2- signifikant die Auflösung von Saharastaub beeinflussen kann, welcher durch Wind in das Oberflächenwasser um die Kapverdischen Inseln eingetragen wird. Ein weiteres Ziel dieser Arbeit war die Identifikation sowie Quantifizierung aller Reaktionen in O2- im offenen Ozean involviert ist. Bisher publizierte Methoden zur Messung von O2- fanden in dieser Arbeit aufgrund von Inkonsistenzen der Detekionsmethoden und Auswertungen keine Anwendung. Nach eingehender praktischer sowie theoretischer Analyse der bestehenden Verfahren stellte sich heraus, dass es nötig war, eine neue Methode zur Bestimmung von O2- im Seewasser zu erstellen. Diese methode wurde zuerst in Laborversuchen etabliert und evaluiert und im weiteren Verlauf dieser Arbeit auch erfolgreich bei den Feldexpeditionen im offenen Ozean (Kapverdischen Inseln, Südpolarmeer) genutzt. Die Etablierung dieser Methode beinhaltet darüber hinaus eine neue theoretische Basis für die Interpretation der Daten, in welcher –im Gegensatz zu früher publizierten Datenanalysen- den verschiedenen komplexen Reaktionsmechanismen des Superoxides Rechnung getragen wird. Das erste Manuskript dieser Arbeit beschreibt die optimierte Methodik, welche für die Kalibrierung und Ermittlung von Superoxid-Stoffflüssen in Seewasser entwickelt wurde. Dieses wurde durch den Einsatz einer thermischen O2--Quelle erreicht, die einerseits als System-Kalibrierung und andererseits sowohl für die Bestimmung der Konzentration als auch für die Stoffluss-Messungen verwendet werden kann. Reaktionen von O2- mit gelösten organischen material im offenen Ozean sind das Thema des zweiten Manuskripts. Bisher ist wenig über diese Reaktionen bekannt, die möglicherweise eine zentrale Rolle im Kreislauf und bei der Oxidation von organischem Material spielen. In dieser Arbeit wurde im subtropischen Atlantik nahe der Kapverdischen Inseln ein kleiner aber signifikanter Beitrag von organischem Material für die Reaktionen von O2- gefunden. Das dritte Manuskript untersucht in einem experimentellen Ansatz welche Prozesse die Auflösung von Staub in Seewasser kontrollieren. Die Ergebnisse deuten darauf hin, dass eine durch Liganden und eine thermisch geförderte Auflösung wichtig zu sein scheint. Die Experimente in natürlichem Wasser zeigen allerdings variable Ergebnisse und der größte Teil dieser Variabilität ist offenbar mit dynamischen Faktoren verbunden die mit Änderungen der chemischen Zusammensetzung des Obeflächenwassers zusammenhängen und die sowohl auf einer räumlichen als auch zeitlichen Skala ablaufen. Der bestimmende Faktor ist hierbei höchst wahrscheinlich das Vorhandensein von colloidalen Spezies und Liganden. Das vierte Manuskript beschreibt die Arbeit über die Zerfallskinetik von O2- im Südpolarmeer wobei Messungen hierfür in der gesamten Wassersäule durchgeführt wurden. Hierbei wurde eine neue Versuchsstrategie angewandt um die Reaktionswege von O2- in polaren Gewässern zu klären. Diese Arbeit zeigt, dass Kupfer die Hauptsenke von O2- im Südpolarmeer darstellt obwohl es stark durch organische Liganden komplexiert ist. Die Ergebnisse deuten darauf hin, dass organische Cu-Komplexe direkt mit O2- reagieren. Die Reaktion mit Eisen hingegen war in allen untersuchten Tiefen relativ gering, was darauf hinweist, dass organische Fe-Komplexe nicht signifikant mit O2- reagieren

The Importance of Kinetics and Redox in the Biogeochemical Cycling of Iron in the Surface Ocean

It is now well established that Iron (Fe) is a limiting element in many regions of the open ocean. Our current understanding of the key processes which control iron distribution in the open ocean have been largely based on thermodynamic measurements performed under the assumption of equilibrium conditions. Using this equilibrium approach, researchers have been able to detect and quantify organic complexing ligands in seawater and examine their role in increasing the overall solubility of iron. Our current knowledge about iron bioavailability to phytoplankton and bacteria is also based heavily on carefully controlled laboratory studies where it is assumed the chemical species are in equilibrium in line with the free ion association model and/or its successor the biotic ligand model. Similarly most field work on iron biogeochemistry generally consists of a single profile which is in essence a “snap-shot” in time of the system under investigation. However it is well known that the surface ocean is an extremely dynamic environment and it is unlikely if thermodynamic equilibrium between all the iron species present is ever truly achieved. In sunlit waters this is mostly due to the daily passage of the sun across the sky leading to photoredox processes which alter Fe speciation by cycling between redox states and between inorganic and organic species. Episodic deposition events, dry and wet, are also important perturbations to iron cycling as they bring in new iron to the system and alter the equilibrium between iron species and phases. Here we utilize new field data collected in the open ocean on the complexation kinetics of iron in the surface ocean to identify the important role of weak iron binding ligands (i.e., those that cannot maintain iron in solution indefinitely at seawater pH: αFeL < αFe′) in allowing transient increases in iron solubility in response to iron deposition events. Experiments with the thermal O2- source SOTS-1 also indicate the short term impact of this species on iron solubility also with relevance to the euphotic zone. This data highlights the roles of kinetics, redox, and weaker iron binding ligands in the biogeochemical cycling of iron in the ocean

Near-field iron and carbon chemistry of non-buoyant hydrothermal plume particles, Southern East Pacific Rise 15°S

Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Marine Chemistry 201 (2018): 183-197, doi:10.1016/j.marchem.2018.01.011.Iron (Fe)-poor surface waters limit phytoplankton growth and their ability to remove carbon (C) from the atmosphere and surface ocean. Over the past few decades, research has focused on constraining the global Fe cycle and its impacts on the global C cycle. Hydrothermal vents have become a highly debated potential source of Fe to the surface ocean. Two main mechanisms for transport of Fe over long distances have been proposed: Fe-bearing nanoparticles and organic C complexation with Fe in the dissolved (dFe) and particulate (pFe) pools. However, the ubiquity and importance of these processes is unknown at present, and very few vents have been investigated for Fe-Corg interactions or the transport of such materials away from the vent. Here we describe the near-field contributions (first ~100 km from ridge) of pFe and Corg to the Southern East Pacific Rise (SEPR) plume, one of the largest known hydrothermal plume features in the global ocean. Plume particles (> 0.2 μm) were collected as part of the U.S. GEOTRACES Eastern Pacific Zonal Transect cruise (GP16) by in-situ filtration. Sediment cores were also collected to investigate the properties of settling particles. In this study, X-ray absorption near edge structure (XANES) spectroscopy was used in two complementary X-ray synchrotron approaches, scanning transmission X-ray microscopy (STXM) and X-ray microprobe, to investigate the Fe and C speciation of particles within the near-field non-buoyant SEPR plume. When used in concert, STXM and X-ray microprobe provide fine-scale and representative information on particle morphology, elemental co-location, and chemical speciation. Bulk chemistry depth profiles for particulate Corg (POC), particulate manganese (pMn), and pFe indicated that the source of these materials to the non-buoyant plume is hydrothermal in origin. The plume particles at stations within the first ~100 km down-stream of the ridge were composites of mineral (oxidized Fe) and biological materials (organic C, Corg). Iron chemistry in the plume and in the core-top suspended sediment fluff layer were both dominated by Fe(III) phases, such as Fe(III) oxyhydroxides and Fe(III) phyllosilicates. Particulate sulfur (pS) was a rare component of our plume and sediment samples. When pS was detected, it was in the form of an Fe sulfide mineral phase, composing ≤ 0.4% of the Fe on a per atom basis. The resuspended sediment fluff layer contained a mixture of inorganic (coccolith fragments) and Corg bearing (lipid-rich biofilm-like) materials. The particle morphology and co-location of C and Fe in the sediment was different from that in plume particles. This indicates that if the Fe-Corg composite particles settle rapidly to the sediments, then they experience strong alteration during settling and/or within the sediments. Overall, our observations indicate that the particles within the first ~ 100 km of the laterally advected plume are S-depleted, Fe(III)-Corg composites indicative of a chemically oxidizing plume with strong biological modification. These findings confirm that the Fe-Corg relationships observed for non-buoyant plume particles within ~ 100 m of the vent site are representative of particles within this region of the non-buoyant plume (~100 km). These findings also point to dynamic alteration of Fe-Corg bearing particles during transport and settling. The specific biogeochemical processes at play, and the implications for nutrient cycling in the ocean are currently unknown and represent an area of future investigation

Pathways of Superoxide (O2-) decay in the Eastern Tropical North Atlantic

Superoxide (O2-: IUPAC name dioxide (•1−)) is an important transient reactive oxygen species (ROS) in the ocean formed as an intermediate in the redox transformation of oxygen (O2) into hydrogen peroxide (H2O2) and vice versa. This highly reactive and very short-lived radical anion can be produced both via photochemical and biological processes in the ocean. In this paper we examine the decomposition rate of O2- throughout the water column, using new data collected in the Eastern Tropical North Atlantic (ETNA) Ocean. For this approach we applied a semi factorial experimental design, to identify and quantify the pathways of the major identified sinks in the ocean. In this work we occupied 6 stations, 2 on the West African continental shelf and 4 open ocean stations, including the CVOO time series site adjacent to Cape Verde. Our results indicate that in the surface ocean, impacted by Saharan aerosols and sediment resuspension, the main decay pathways for superoxide is via reactions with Mn(II) and organic matter

The GEOTRACES Intermediate Data Product 2014

The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-? data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes