Codeposition of organic carbon and arsenic in Bengal Delta aquifers

We present data showing that arsenic (As) was codeposited with organic carbon (OC) in Bengal Delta sediments as As and OC concentrations are highly (p <0.001) positively correlated in core profiles collected from widely dispersed geographical sites with different sedimentary depositional histories. Analysis of modern day depositional environments revealed that the As-OC correlations observed in cores are due to As retention and high OC inputs in vegetated zones of the deltaic environment. We hypothesize that elevated concentrations of As occur in vegetated wetland sediments due to concentration and retention of arsenate in aerated root zones and animal burrows where copious iron(III) oxides are deposited. On burial of the sediment, degradation of organic carbon from plant and animal biomass detritus provides the reducing conditions to dissolve iron(III) oxides and release arsenite into the porewater. As tubewell abstracted aquifer water is an invaluable resource on which much of Southeast Asia is now dependent, this increased understanding of the processes responsible for As buildup and release will identify, through knowledge of the palaeosedimentary environment, which sediments are at most risk of having high arsenic concentrations in porewater. Our data allow the development of a new unifying hypothesis of how As is mobilized into groundwaters in river flood plains and deltas of Southeast Asia, namely that in these highly biologically productive environments, As and OC are codeposited, and the codeposited OC drives As release from the sediments.

Co-deposition of organic carbon and arsenic in Bengal Delta aquifers.

We present data showing that arsenic (As) was codeposited with organic carbon (OC) in Bengal Delta sediments as As and OC concentrations are highly (p < 0.001) positively correlated in core profiles collected from widely dispersed geographical sites with different sedimentary depositional histories. Analysis of modern day depositional environments revealed that the As/OC correlations observed in cores are due to As retention and high OC inputs in vegetated zones of the deltaic environment. We hypothesize that elevated concentrations of As occur in vegetated wetland sediments due to concentration and retention of arsenate in aerated root zones and animal burrows where copious iron(III) oxides are deposited. On burial of the sediment, degradation of organic carbon from plant and animal biomass detritus provides the reducing conditions to dissolve iron(III) oxides and release arsenite into the porewater. As tubewell abstracted aquifer water is an invaluable resource on which much of Southeast Asia is now dependent, this increased understanding of the processes responsible for As buildup and release will identify, through knowledge of the palaeosedimentary environment, which sediments are at most risk of having high arsenic concentrations in porewater. Our data allow the development of a new unifying hypothesis of how As is mobilized into groundwaters in river flood plains and deltas of Southeast Asia, namely that in these highly biologically productive environments, As and OC are codeposited, and the codeposited OC drives As release from the sediments. We present data showing that arsenic (As) was codeposited with organic carbon (OC) in Bengal Delta sediments as As and OC concentrations are highly (p < 0.001) positively correlated in core profiles collected from widely dispersed geographical sites with different sedimentary depositional histories. Analysis of modern day depositional environments revealed that the As?OC correlations observed in cores are due to As retention and high OC inputs in vegetated zones of the deltaic environment. We hypothesize that elevated concentrations of As occur in vegetated wetland sediments due to concentration and retention of arsenate in aerated root zones and animal burrows where copious iron(III) oxides are deposited. On burial of the sediment, degradation of organic carbon from plant and animal biomass detritus provides the reducing conditions to dissolve iron(III) oxides and release arsenite into the porewater. As tubewell abstracted aquifer water is an invaluable resource on which much of Southeast Asia is now dependent, this increased understanding of the processes responsible for As buildup and release will identify, through knowledge of the palaeosedimentary environment, which sediments are at most risk of having high arsenic concentrations in porewater. Our data allow the development of a new unifying hypothesis of how As is mobilized into groundwaters in river flood plains and deltas of Southeast Asia, namely that in these highly biologically productive environments, As and OC are codeposited, and the codeposited OC drives As release from the sediments.

Reduction of arsenic uptake by lettuce with ferrous sulfate applied to contaminated soil

Soil contamination by arsenic (As) presents a hazard in many countries and there is a need for techniques to minimize As uptake by plants. A proposed in situ remediation method was tested by growing lettuce (Lactuca sativa L. cv. Kermit) in a greenhouse pot experiment on soil that contained 577 mg As kg(-1), taken from a former As smelter site. All combinations of iron (Fe) oxides, at concentrations of 0.00, 0.22, 0.54, and 1.09% (w/w), and lime, at concentrations of 0.00, 0.27, 0.68, and 1.36% (w/w), were tested in a factorial design. To create the treatments, field-moist soil, commercial-grade FeSO4, and ground agricultural lime were mixed and stored for one week, allowing Fe oxides to precipitate. Iron oxides gave highly significant (P < 0.001) reductions in lettuce As concentrations, down to 11% of the lettuce As concentration for untreated soil. For the Fe oxides and lime treatment combinations where soil pH was maintained nearly constant, the lettuce As concentration declined in an exponential relationship with increasing FeSO4 application rate and lettuce yield was almost unchanged. Iron oxides applied at a concentration of 1.09% did not give significantly lower lettuce As concentrations than the 0.54% treatment. Simultaneous addition of lime with FeSO4 was essential. Ferrous sulfate with insufficient lime lowered soil pH and caused mobilization of Al, Ba, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sr, and Zn. At the highest Fe oxide to lime ratios, Mn toxicity caused severe yield loss.

Reduction of arsenic uptake by lettuce with ferrous sulfate applied to contaminated soil

Soil contamination by arsenic (As) presents a hazard in many countries and there is a need for techniques to minimize As uptake by plants. A proposed in situ remediation method was tested by growing lettuce (Lactuca sativa L. cv. Kermit) in a greenhouse pot experiment on soil that contained 577 mg As kg(-1), taken from a former As smelter site. All combinations of iron (Fe) oxides, at concentrations of 0.00, 0.22, 0.54, and 1.09% (w/w), and lime, at concentrations of 0.00, 0.27, 0.68, and 1.36% (w/w), were tested in a factorial design. To create the treatments, field-moist soil, commercial-grade FeSO4, and ground agricultural lime were mixed and stored for one week, allowing Fe oxides to precipitate. Iron oxides gave highly significant (P < 0.001) reductions in lettuce As concentrations, down to 11% of the lettuce As concentration for untreated soil. For the Fe oxides and lime treatment combinations where soil pH was maintained nearly constant, the lettuce As concentration declined in an exponential relationship with increasing FeSO4 application rate and lettuce yield was almost unchanged. Iron oxides applied at a concentration of 1.09% did not give significantly lower lettuce As concentrations than the 0.54% treatment. Simultaneous addition of lime with FeSO4 was essential. Ferrous sulfate with insufficient lime lowered soil pH and caused mobilization of Al, Ba, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sr, and Zn. At the highest Fe oxide to lime ratios, Mn toxicity caused severe yield loss.

Deciphering the weathering processes using environmental mineralogy and geochemistry: Towards an integrated model of laterite and podzol genesis in the Upper Amazon Basin

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Stable isotope fractionation of selenium by biomethylation in soil

Selen ist in geringen Mengen ein essentielles Nährelement, das aber in höheren Gehalten toxisch wird. Der Se-Kreislauf in der Umwelt ist eng mit Redoxreaktionen wie der Reduktion von Se-Oxyanionen zu Methylselenid verknüpft. Flüchtige Methylselenide sind weit verbreitet und stellen einen wichtigen Se-Fluss in der Umwelt dar. Das übergeordnete Ziel meiner Dissertation war, die Stabilisotopenfraktionierung von Se durch Biomethylierung verschiedener oxidierter Se-Spezies (Se[IV] und Se[VI]) im Boden aufzuklären. Zunächst wurde eine Methode entwickelt, die es erlaubte flüchte Methylselenide quantitativ zurückzuhalten. Es zeigte sich, dass alkalische Peroxid-Lösung dafür geeignet war. Mit alkalischer Peroxid-Lösung wurde eine Wiederfindung von 95,6 ± Standardabweichung 5,4% in Verflüchtigungsexperimenten mit Methylselenid-Standards erreicht. Bei Einsatz von alkalischer Peroxid-Lösung in geschlossenen Mikrokosmos-Experimenten kam es zu keinen Se-Verlusten und ausgeglichenen Se-Isotopenbilanzen. Die massengewichteten δ82/76Se-Werte lagen für Se(IV) und Se(VI) am Ende der Mikrokosmos-Inkubationen bei -0,31 ± 0,05‰ (n = 3) und -0,76 ± 0,07‰ (n = 3) verglichen mit -0,20 ± 0,05‰ und -0,69 ± 0,07‰ im jeweils zugegebenen Se. Im zweiten Teil meiner Dissertation wurde die Pilzart Alternaria alternata mit Se(VI) und Se(IV) in geschlossenen Mikrokosmen für 11-15 und Se(IV) zusätzlich für 3-5 Tage bei 30°C inkubiert. In 11-15 Tagen wurden 2,9-11% des Se(VI) und 21-29% des Se(IV) und in 3-5 Tagen, 3-5% des Se(IV) methyliert. Die anfänglichen δ82/76Se-Werte von Se(VI) und Se(IV) lagen bei -0,69 ± 0,07‰, und -0,20 ± 0,05‰. Die δ82/76Se-Werte der Methylselenide unterschieden sich nach 11-15 Tagen Inkubation signifikant zwischen Se(VI) (-3,97 bis -3,25 ‰) und Se(IV) (-1,44 bis -0,16‰) als Quellen. Die δ82/76Se-Werte der Methylselenide zeigen also die Quellen der Biomethylierung von Se an. Die kürzere Inkubation von Se(IV) für 3-5 Tage führte zu einer ausgeprägten Se-Isotopenfraktonierung von mindestens -6‰, bevor ein Fließgleichgewicht erreicht wurde. Im dritten Teil bestimmte ich die Bindungsformen von Se mit drei operativ definierten sequentiellen Extraktionen und die δ82/76S-Werte des gesamten Selens in zehn urbanen Oberböden mit 0,09-0,52 mg/kg Se, die fünf verschiedene Landnutzungstypen repräsentierten (Überschwemmungsgrünland, Garten, Park, Straßenrand und Wald). Nur ein kleiner Teil des Seleniums lag in austauschbarer und damit direkt bioverfügbarer und in residualer, wenig reaktiver Form vor. Das meiste Se war an die organische Substanz und Fe-(Hydr-)Oxide gebunden (42-77% des gesamten Selens). Der mittlere δ82/76Se-Wert des gesamten Selens in den Oberböden lag mit -0,03 ± 0,38‰ nahe beim Mittelwert der gesamten Erde. Geringfügig niedrigere Se-Isotopensignale von -0,59 bis -0,35‰ v.a. in Waldböden und geringfügig höhere von 0,26 to 0,45‰ in Überschwemmungsgrünland wurden vermutlich durch Boden-Pflanze-Recycling und Se-Kontaminationen durch das Flusswasser verursacht. Der vierte Teil umfasste ein “Natural Attenuation”-Experiment und Mikrokosmos-Inkubationen von Bodenproben mit A. alternata. Die Equilibrierung von zum Boden gegebenem Se(IV) und Se(VI) für drei Tage führte zu abnehmenden wasserlöslichen Se-Gehalten um 32-44% bzw. 8-14, die mit kleinen Isotopenfraktionierung (ε = -0,045 bis -0,12 ‰ and -0,05 to -0,07‰ verbunden waren. In zwei der inkubierten Böden mit mäßig sauren pH-Werten wurden zwischen 9,1 und 30% des zugefügten Se(IV) und 1,7% des zugefügten Se(VI) methyliert während in einem stark sauren Boden keine Methylierung auftrat. Das aus Se(IV) entstandene Methylselenid war deutlich gegenüber dem zugegebenen Se-Standard (0,20‰) an 82Se verarmt (δ82/76Se = -3,3 bis -4,5‰). Meine Ergebnisse zeigen, dass die stabilen Isotopenverhältnisse von Se neue Einblicke in Se-Transformationsprozesse erlauben.rn

High exchangeable calcium concentrations in soils on Barro Colorado Island, Panama

The soils on four lithologies (basaltic conglomerates, Bohio; Andesite; volcanoclastic sediments with basaltic agglomerates, Caimito volcanic; foraminiferal limestone, Caimito marine) on Barro Colorado Island (BCI) have high exchangeable Ca concentrations and cation-exchange capacities (CEC) compared to other tropical soils on similar parent material. In the 0–10 cm layer of 24 mineral soils, pH values ranged from 5.7 (Caimito volcanic and Andesite) to 6.5 (Caimito marine), concentrations of exchangeable Ca from 134 mmolc kg− 1 (Caimito volcanic) to 585 mmolc kg− 1 (Caimito marine), and cation exchange capacities from 317 mmolc kg− 1 (Caimito volcanic) to 933 mmolc kg− 1 (Caimito marine). X-ray diffractometry of the fraction < 2 μm revealed that smectites dominated the clay mineral assemblage in soil except on Caimito volcanic, where kaolinite was the dominant clay mineral. Exchangeable Ca concentrations decreased with increasing soil depth except on Caimito marine. The weathering indices Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Weathering Index of Parker (WIP) determined for five soils on all geological formations, suggested that in contrast to expectation the topsoil (0–10 cm) appeared to be the least and the subsoil (50–70 cm) and saprolite (isomorphically weathered rock in the soil matrix) the most weathered. Additionally, the weathering indices indicated depletion of base cations and enrichment of Al-(hydr)oxides throughout the soil profile. Tree species did not have an effect on soil properties. Impeded leaching and the related occurrence of overland flow seem to be important in determining clay mineralogy. Our results suggest that (i) edaphic conditions favor the formation of smectites on most lithologies resulting in high CEC and thus high retention capacity for Ca and (ii) that there is an external source such as dust or sea spray deposition supplying Ca to the soils.

Comparisons of z values between fine-grained titanomagnetites within interstitial glass and large-grained titanomaghemtes in older ocean-floor basalts

Transmission electron microscopy observations and rock magnetic measurements reveal that alteration of fine- and large-grained iron-titanium oxides can occur at different rates. Fine-grained titanomagnetite occurs as a crystallization product within interstitial glass that originated as an immiscible liquid within a fully differentiated melt; in several samples with ages to 32 Ma it displays very little or no oxidation (z = ca. 0). In contrast, samples with ages of 10 Ma or older are observed to also contain highly oxidized (z >/= 0.66) large-grained titanomaghemite. These large grains, having originated by direct crystallization from melt, are associated with pore space. Such pore space can serve as a conduit for fluids that promote alteration, whereas fine grains may have been "armored" against alteration by the glass matrix in which they are embedded. Apparently, alteration of oceanic crust is a heterogeneous process on a microscopic scale. The existence of pristine, fine-grained titanomagnetite in the interstitial glass of older ocean-floor basalts that have undergone significant alteration implies that such glassy material is capable of carrying original thermal remanent magnetization and may be suitable for paleointensity determinations.

Solid phase phosphorus geochemistry in surface sediments of sediment cores GeoB3718-4, GeoB3702-2 and GeoB3707-4

In this study we investigate benthic phosphorus cycling in recent continental margin sediments at three sites off the Namibian coastal upwelling area. Examination of the sediments reveals that organic and biogenic phosphorus are the major P-containing phases preserved. High Corg/Porg ratios just at the sediment surface suggest that the preferential regeneration of phosphorus relative to that of organic carbon has either already occurred on the suspension load or that the organic matter deposited at these sites is already rather refractory. Release of phosphate in the course of benthic microbial organic matter degradation cannot be identified as the dominating process within the observed internal benthic phosphorus cycle. Dissolved phosphate and iron in the pore water are closely coupled, showing high concentrations below the oxygenated surface layer of the sediments and low concentrations at the sediment-water interface. The abundant presence of Fe(III)-bound phosphorus in the sediments document the co-precipitation of both constituents as P-containing iron (oxyhydr)oxides. However, highly dissolved phosphate concentrations in pore waters cannot be explained, neither by simple mass balance calculations nor by the application of an established computer model. Under the assumption of steady state conditions, phosphate release rates are too high as to be balanced with a solid phase reservoir. This discrepancy points to an apparent lack of solid phase phosphorus at sediment depth were suboxic conditions prevail. We assume that the known, active, fast and episodic particle mixing by burrowing macrobenthic organisms could repeatedly provide the microbially catalyzed processes of iron reduction with authigenic iron (oxyhydro)oxides from the oxic surface sediments. Accordingly, a multiple internal cycling of phosphate and iron would result before both elements are buried below the iron reduction zone.

Geochemistry and minerals at DSDP Hole 76-534A

Leg 76 sampled 31.5 m of basaltic basement at Deep Sea Drilling Project Hole 534A in the Blake-Bahama Basin. The basalts represent a short section of mineralogically uniform, sparsely plagioclase-phyric pillow flows, composed mainly of plagioclase, augitic clinopyroxene, iron-titanium oxides with variable amounts of alteration products (smectite ± carbonate ± quartz). Their major element chemistry is typical of mid-ocean ridge tholeiites and has normative compositions of olivine tholeiites. Mg/(Mg + Fe**2+) ratios range from 0.58 to 0.60, which suggests that these basalts are evolved compared to primitive mantle melts.

Geochemical and rock magnetic investigation of sediment cores of the western South Atlantic

Geochemical and rock magnetic investigations of sediments from three sites on the continental margin off Argentina and Uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane (AOM). The western Argentine Basin represents a suitable sedimentary environment to study nonsteady-state processes because it is characterized by highly dynamic depositional conditions. Mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals. As a typical feature of these deposits, distinct minima in magnetic susceptibility (k) are observed. Pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate/methane transition (SMT) where HS- is generated by the process of AOM. The released HS- reacts with the abundant iron (oxyhydr)oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility. Modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate (SR) which occurred during the Pleistocene/Holocene transition. We assume that the strong decrease in mean SR encountered during this glacial/interglacial transition induced a fixation of the SMT at a specific depth. The stagnation has obviously enhanced diagenetic dissolution of iron (oxyhydr)oxides within a distinct sediment interval. This assumption was further substantiated by numerical modeling in which the mean SR was decreased from 100 cm/kyr during glacial times to 5 cm/kyr in the Holocene and the methane flux from below was fixed to a constant value. To obtain the observed geochemical and magnetic patterns, the SMT must remain at a fixed position for ~9000 yrs. This calculated value closely correlates to the timing of the Pleistocene/Holocene transition. The results of the model show additionally that a constant high mean SR would cause a concave-up profile of pore water sulfate under steady state conditions.

Porewater and solid-phase phosphorus geochemistry in surface sediments of sediment core GeoB9510-3, GeoB9518-4 and GeoB9519-6

Despite intensive research on the different domains of the marine phosphorus (P) cycle during the last decades, frequently discussed open questions still exist especially on controlling factors for the benthic behaviour of P and its general distribution in sediment-pore water systems. Steady state or the internal balance of all relevant physical and (bio)geochemical processes are amongst the key issues. In this study we present and discuss an extended data set from surface sediments recovered from three locations on the NW African continental slope. Pore water data and results from sequential sediment extractions give clear evidence to the well-known close relationship between the benthic cycles of P and iron. Accordingly, most of the dissolved phosphate must have been released by microbially catalyzed reductive dissolution of iron (oxhydr)oxides. However, rates of release and association of P and iron, respectively, are not directly represented in profiles of element specific sediment compositions. Results from steady-state based transport-reaction modelling suggest that particle mixing due to active bioturbation, or rather a physical net downward transport of P associated to iron (oxyhydr)oxides, is an essential process for the balance of the inspected benthic cycles. This study emphasizes the importance of balancing analytical data for a comprehensive understanding of all processes involved in biogeochemical cycles.