B21B-0340
Iron Oxide Biominerals in Protein Nanocages, the Ferritins: Easing Into Life With Oxygen?
Organisms with ferritins could represent the progenitors of organisms that successfully made the transition to aerobic life. Ferritins are protein nanocages (8 or 12 nm diameter) that catalyze reactions between Fe(II) and O2 or H2O2 to synthesize ferrihydrite-like biominerals of Fe2O3(H2 O)n; phosphate is sometimes incorporated during mineralization. All groups of organisms, archea, bacteria, plants and animals have ferritins. Catalytic reactions between Fe and O occur in the protein cage with the products moving into the central protein cavity (5 or 8 nm diameter) where mineralization occurs; mineral sizes reach 4500 Fe with more than 7000 O atoms in the large cavities of maxi-ferritins and 500 Fe with more than 800 O atoms in the smaller, mini-ferritins, also called Dps proteins. H2O2 is preferentially used by mini-ferritins in archea and bacteria, contrasting with O2, preferentially used by maxi-ferritins in bacteria plants and animals, and some bacterial mini-ferritins that use either H2O2 or O2, to oxidize Fe(II) during biomineralization. The study of ferritins in contemporary organisms can illuminate mechanisms for oxygen and oxidant responses in changing environments now and in the past. Multiple genes encoding ferritins are often regulated by different environmental stimuli and in multi-cellular organisms, by tissue-specific, differentiation programs. The single celled E.coli has four ferritin genes, encoding three maxi-ferritins, one with a heme cofactor (bacterioferritin), and one mini-ferritin (Dps), expressed at different points in the culture cycle and/or in response to different stresses. Environmental iron, oxygen and peroxide all change the amounts of ferritin. When iron is plentiful, mineralized ferritin accumulates. Ferritin iron is recovered during periods of iron deficiency, apparently by selective unfolding of gated pores in ferritin protein nanocage that expose the mineral to reductants. Gene (DNA) transcription is the genetic target for iron or oxidant. Vertebrates use an additional genetic target, ferritin mRNA translation, to increase the range of sensitivity to iron and oxidant signals. The response of ancient organisms to increased peroxide or oxygen in the evolving terrestrial atmosphere may be recapitulated by the responses of contemporary, pathogenic microorganisms to human defense mechanisms. For example, human hosts sequester iron in ferritin to restrict pathogen growth, while releasing hydrogen peroxide to kill the invading cells. Successful pathogens resist the host by synthesizing iron chelators (siderophores) to compete effectively for iron and synthesizing ferritins that consume H2O2 with Fe(II) during biomineralization. New data on Fe (II) binding stoichiometry, iron biomineralization and recovering iron from ferritin minerals will be presented. The data will be related to environmental/cellular iron deficiency and iron repletion and to the evolution of ferritins that consume O2 during iron biomineralization. References: 1. Liu, X. and Theil, E.C. (2005). Acc. Chem. Res. 38:167-175. 2. Theil, E.C., Liu, X.S., Tosha, T. (2008) Inorg. Chim. Acta. 361: 868-874.
B21B-0341
Geochemical and Biochemical Co-Evolution of Uranium and Thorium Minerals
The origins and near-surface distributions of the ~220 known uranium and thorium minerals elucidate the
principles of mineral evolution. We divide this history into three phases, the first of which encompassed
Earth's earliest billion years and involved successive concentrations of uranium from its initial uniform trace
distribution into fluids from which the first U4+ minerals, uraninite (ideally UO2) and coffinite
(USiO4), precipitated in the crust. The second period, from ~3.5 to 2.2 Ga, saw abiotic alteration of
these two minerals, including auto-oxidation through the production of radiogenic Pb, as well as possible
near-surface oxidation and the formation of a very limited suite of uranyl oxide-hydroxides. The Earth's third
phase of uranium mineral evolution, during which most known U minerals first precipitated from reactions of
soluble uranyl (U6+O2)2+ ions, followed the Great Oxidation Event at ~2.2 Ga and thus was
biologically mediated. The multiple oxidation states of U (4+ vs. 6+) are a sensitive indicator of global redox
conditions. In contrast, the behavior of thorium, which has only a single oxidation state (4+) that has a very
low solubility, cannot reflect changing redox conditions, and there are no effective means of geochemical
concentration into major ore deposits. Uranium near-surface mineralogy provides a sensitive indicator of a
planet's geotectonic and geobiological history. In the absence of extensive aqueous fluid reworking of the
crust and upper mantle, uranium will not become sufficiently concentrated to form its own minerals or ore
deposits. Furthermore, in the absence of extensive surface oxidation, all but a handful of the known uranium
minerals are unlikely to form.
http://hazen.ciw.edu/research/mineral_evolution
B21B-0342
Element availability of bivalve with symbiotic zooxanthellae in coral sea area as studied by multielement profiling analysis
In coral sea, a characteristic ecosystem is formed by many kinds of marine animals and plants, although seawater is uneutrophic. This may be explained by the fact that various chemical species with bioessentiality are effectively taken and used by lower animals and plants in coral sea area. A symbiotic relationship often found among different animals and plants in this area is considered to be working as one of such processes. However, the specific bioavailability of the elements for the marine animals and plants in coral reef area has not been studied from the viewpoints of trace and ultratrace elements. It is found by the present authors that bivalve with symbiotic zooxanthellae (Tridacna crocea) living on coral reef had relatively higher bio- accumulation factors for many bio-essential elements than other kinds of bivalves, although they live in the uneutrophic sea area. The present authors focused on Tridacna crocea as one of the symbiotic animals. Thus, in the present study, at first, multielement determination of major-to-ultratrace elements (about 20 elements) in each organ of Tridacna crocea with symbiotic zooxanthellae, were carried out by ICP-AES, ICP- MS, and CHN coder. At Second, the specific bioavailability of trace and ultratrace elements in Tridacna crocea was discussed on the multielement data for seawater, seaweeds, and other bivalves in coral sea area.
B21B-0343
Molybdenum Storage in Cyanobacteria: "Mopping" Up Excess Mo
The heavy metal molybdenum (Mo) plays an essential role in the nitrogen (N) cycle. In order to acquire N from the environment, microorganisms utilize Mo-containing enzymes such as nitrogenase (for N2 fixation) and nitrate reductase (for NO3- assimilation). N2-fixing cyanobacteria likely played an important role in both marine and terrestrial N cycling throughout Earth history. Low Mo levels in Precambrian oceans may have necessitated evolution of cyanobacterial Mo-storage (“Mop”) proteins. If so, mop genes were likely lost in marine cyanobacteria after the rise of Mo (to approximately 100 nanomolar (nM) 600 Ma (Scott et al., 2008)). The distribution of mop genes in N2-fixing cyanobacteria supports this hypothesis; freshwater species contain this gene whereas marine species lack it. To assess the importance of Mo-storage in modern environments we are integrating laboratory and field investigations. In the lab, we grew N2-fixing freshwater cyanobacteria possessing the mop gene at high Mo levels (1500 nM) to assess whether excess Mo is accumulated Extremely high intracellular accumulation of Mo, much higher than could be accounted for by nitrogenase requirements alone, was observed. Investigation of mop gene expression and protein localization in Mo-replete and deplete conditions is underway. In the field, we are isolating indigenous cyanobacteria and cloning mop genes from Castle Lake, California. This field locale is of particular interest because previous studies show that periphyton consortia in this lake assimilate large quantities of N, even though Mo concentrations are <5 nM. This adaptation to low Mo may be aided by intracellular Mo storage.
B21B-0344
Iron, Sulfur, Arsenic and Water: Geochemical Implications of Facultative Anoxygenic Photosynthesis in Cyanobacteria and the Slow Rise of Oxygen
Over geologic time, the global rise in atmospheric oxygen (O2) is attributed to the evolution and wide
spread proliferation of oxygenic photosynthesis in cyanobacteria. However, cyanobacteria maintain a
metabolic flexibility that may not always result in O2 release. Specifically, cyanobacteria can use a
variety of alternative electron donors, rather than water, that are also readily oxidized. These may include
sulfur, iron, and arsenic. Cyanobacteria are thus not uniquely constrained towards O2 production.
Changes in the bioavailability of these key elements may have had dramatic consequences for and resulted
in the slow accumulation of O2 in the atmosphere. In particular, by using facultative anoxygenic
photosynthesis the cells maintain advantageous anaerobic conditions for N2-fixation. Although other
types of bacteria are capable of N2-fixation, cyanobacteria singularly possess the dynamic capability of
generating and surviving O2. These two processes "pull" the cells in opposite directions, metabolically
speaking, around an aerobic-anaerobic continuum. Such a strategy also confers a distinct competitive
advantage for cyanobacteria over photosynthetic eukaryotes, as they can endure widespread euxinia and
maintain their cellular N quota. In an anoxic and/or sulfidic ocean, cyanobacteria would be expected to
dominate over eukaryotic algae. Here we present Bayesian constructed phylogenetic distribution of specific
genes and the metabolic role of key enzymes that form the basis of this hypothesis. We further suggest that
the consequences of this proposed ecosystem structure altered the redox balance of the fluid Earth
(atmosphere and oceans) and can help explain the observed long-term geochemical stasis and slow rates of
eukaryotic diversification. We suggest that the underlying control for global oxygenation was a synergistic
interplay between the evolution and elastic physiology of cyanobacteria as they impacted the redox state of
early Earth.
http://www.geobiochemistry.org
B21B-0345
Thermophiles as Candidate Iron-Reducing Bacteria For the Putative Biogenetic Magnetite in Banded Iron Formations
The temperature of the Archaean-Palaeoproterozoic ocean was likely consistent with physiological requirements of thermophilic species being present. In this study, we compared the crystallochemistry and lattice constants of magnetite crystals produced by Thermoanaerobacter sp. TOR39, Geobacter and Shewanella and the slightly altered magnetite from BIF of Hamersley, Western Australia. The lattice constants of TOR39-magnetite and the BIF-magnetite were similar, being 8.3901 and 8.3869 Å respectively. The lattice constant of magnetite produced by Geobacter is more close to perfect stoichiometry (8.4038 Å), however, the magnetite produced by Shewanella experienced oxidization has a much smaller value (8.3522 Å). The stoichiometries of TOR39-magnetite was Fe3+[Fe3+1.1217Fe2+0.8175--0.0608]O4 and that of BIF-magnetite was quite similar being Fe3+[Fe3+0.9963Fe2+1.0056]O4. The stoichiometry, lattice constant and crystal size collectively indicated that TOR39-magnetite was similar to BIF-magnetite. The Mssbauer spectroscopy indicated the existence of a Fe(III)-salt, possibly Fe3+OH(CH3COO)2 in the magnetite lamina of BIF which was widely detected in the magnetite-assemblages of iron-reducing bacterial cultures that contained acetate. This is evidence that supports a potential role for thermophiles such as Thermoanaerobacter in the biogenesis of magnetite in BIF. The magnetite crystals produced by cultures of Shewanella, Geobacter, magnetotactic bacteria and those synthesized from green rust appeared less similar to BIF-magnetite by either their crystallochemistry or their optimized growth temperatures.
B21B-0346
Phosphorus Redox on the Early Earth: First Identification of Low-Oxidation State Phosphorus Compounds in Terrestrial Samples
Phosphorus is one of the key elements in biochemical systems, playing an important role in metabolism as ATP and other coenzymes, in replication as DNA and RNA, and in cellular structure as phospholipids. The geochemical cycling of phosphorus on the Earth is usually confined to the rock cycle- redox reactions of phosphorus are never considered. However, it has been proposed that redox reactions of phosphorus were important on the early Earth (Pasek, PNAS 2008). Indeed, such a suggestion is buttressed by the discovery of condensed phosphate formation linked to the oxidation of reduced P compounds. However, prior to the present work, there has been no report of these P compounds in geologic samples. Here we report the first occurrence of reduced P in samples of fulgurites, the glassy material resulting from the fusion of sand, soil, or rock during a lightning strike. On average, lightning strikes the Earth's surface at a rate of approximately 65 times per second (Krider et al., J. Geophys. Res.,1968) exposing target areas to extreme energy dissipation and temperatures. Through electron microprobe analyses and NMR we have identified naturally formed metal droplets containing Fe and P within several fulgurite samples and Ca-phosphite compounds. These droplets are highly reduced compared to the original material and are not naturally present in the target area, rather they were formed through the rapid, intense heating and quenching experienced during fulgurite formation. This process provides a natural means to create localized environments with greater than normal abundances of reduced Fe and P, less commonly found on Earth's surface than their oxidized counterparts. In particular, small areas that receive repeated lightning strikes due to topography or local weather patterns (e.g. hilltops) could potentially house unique microhabitats with reduced elements available for biological use.
B21B-0347
Do Secular Trends in the Nickel Content of Banded Iron Formation Record a Methanogen Famine?
As ancient chemical sediments whose composition was dictated by contemporaneous seawater, Banded Iron Formations (BIF) may prove to be one of the most useful indicators of changing oceanic trace element concentrations over geological timescales. We report here new trace element analyses of over 20 BIF spanning roughly 3 billion years of ocean history. Our data indicate a progressive decline in nickel abundance in BIF with age; we suggest that after the most intense period of mantle plume magmatism and continental crustal growth in Earth's history ca. 2.7 billion years ago, a cooler upper mantle led to decreased eruption of Ni-rich ultramafic rocks (i.e., komatiites), and consequently a reduced flux of dissolved Ni to the oceans. These results, combined with experimentally-determined Ni partition coefficients between simulated Precambrian seawater and diverse iron oxides, indicate that dissolved Ni concentrations may have been as high as 400 nM throughout much of the Archean, but dropped significantly to ~120 nM by 2.5 Ga, and then slowly approached modern day values (~9 nM) by ~500 Ma. The observed decline in the availability of Ni, a key metal cofactor in the enzymes of methanogens, would have progressively stifled methanogenic activity in the oceans and severely disrupted the supply of biogenic methane sometime between 2.7 and 2.5 Ga. Did a nickel famine at the end of the Archean cause catastrophic collapse of atmospheric methane and thereby facilitate the rise of atmospheric oxygen at 2.4 billion years ago, the so-called 'Great Oxidation Event' (GOE)?
B21B-0348 [WITHDRAWN]
Uptake and Requirements of Molybdenum and Vanadium in Nitrogen Fixing Bacteria: Implications for the Nitrogen Cycle Now and in the Past.
Three nitrogenases (Mo-, V- and Fe-Nase) have thus far been identified. The requirement and use efficiency of those metals are key parameters for the nitrogen cycle. Within present terrestrial ecosystems, the Mo- Nase is considered to be dominant and the so called alternative nitrogenases (V- and Fe-Nase) have heretofore been neglected, likely resulting in misconceptions about the soil nitrogen cycle. Here, I present an overview of recent findings on trace metals speciation in soils and requirements, homeostasis, and uptake of these metals by free-livng nitrogen fixing bacteria. Our data show that Mo in soils associates strongly with organic matter, contrary to the classic view of Mo being associated with iron oxides. We also find that free- living nitrogen fixers, such as Azotobacter vinelandii, acquire both Mo and V through highly regulated uptake systems using released ligands specifically targeting the required metals, similar to that of iron. Finally, our findings demonstrate that nitrogen fixers, e.g. A. vinelandii, use Mo and V to fix nitrogen with close efficiency. This, and recent work showing that Mo may be limiting N2 fixation in a variety of terrestrial systems suggest that the worldwide dominance of Monitrogenase may have been overestimated, and the role of the alternative nitrogenases in present environments deserves more attention. Interestingly, two decades after the identification of the alternative V and Fe nitrogenases, their evolution and exact role in the terrestrial nitrogen cycle over geologic time are still unclear. As crustal V abundance is about 100 times higher than Mo, nitrogen fixers might have benefited throughout geologic time from being able to utilize this additional metal source to sustain nitrogen fixation. A better understanding of the past and present nitrogen cycle is critical to anticipate the possible responses of terrestrial environments to global changes due to recent and future anthropic activities.
B21B-0349
MoS2 and ReS2 Stabilities as Indicators of the Oxidation State on the Early Earth
Anomalies in the geochemical abundances of Mo and Re in late Archean shales from Western Australia have been used to infer enhanced availability of these elements due to oxidation by O2 in the atmosphere during weathering of MoS2 and ReS2 [1]. More specifically, the oxidation of MoS2 to MoO42- was suggested as indicating a "whiff" of O2 in the environment more than 50 million years before the start of the Great Oxidation Event (GOE) [1]. This raises the question of the thermodynamic stabilities of MoS2 and ReS2 at near surface conditions during the late Archean. Under what conditions will MoS2 and ReS2 oxidize to aqueous molybdate and rhenate respectively? We calculated the equilibrium constants for the reactions MoS2 + 3H2O + 9/2O2 (g) = MoO42- + 2SO42- + 6H+ , logK = 236.4, and ReS2 + 5/2H2O + 19/4O2 (g) = ReO4- + 2SO42- + 5H+ , logK = 245.3, using thermodynamic data for the sulfide minerals from [2] and for the aqueous species from [3]. Assuming that the aqueous oxyanions have activities of 10-6 and that the pH = 7.0, results in calculated values of the logfO2 (g) equal to -66 and -63, respectively. Regardless of kinetic considerations, these results indicate that there is a thermodynamic driving force for the oxidation of MoS2 and ReS2 for logfO2 values greater than the above values. In other words, the oxidation of the metal sulfides to soluble oxyanions could take place in the absence of any significant quantity of O2. This implies that anomalies in the abundances of Mo and Re do not necessarily indicate any O2 in the Archean environment before the GOE. [1] Anbar A. et al. (2007) Science 317, 1903-1906. [2] Mills K. C. (1974) Thermodynamic data for inorganic sulfides, selenides and tellurides, Butterworths, 845 pp. [3] Shock et al. (1997) Geochim et Cosmochim. Acta 61, 907-950.
B21B-0350
The methane cycle in tropical Lake Matano: An analogue for Precambrian methane cycling?
In the persistently stratified and Fe-rich Lake Matano, Indonesia, authigenic organic matter generated by primary production is largely degraded through methanogenesis despite the high abundance of Fe (hydr)oxides in the lake's sediments. The methane produced accumulates to high concentrations in the lake's anoxic bottom waters and radiocarbon measurements demonstrate that this methane is modern. In lakes, methane produced under anoxia during seasonal stratification is typically aerobically oxidized during convective overturn. In Lake Matano, dissolved carbon concentration profiles and stable isotope patterns indicate that some of the methane is consumed by anaerobic oxidation (AOM). The absence of electron acceptors (nitrate and sulfate), known to oxidize methane anaerobically, and the abundant supply of Fe (and Mn) (hydr)oxides suggest that methane oxidation may be coupled to the reduction of Fe (and/or Mn) in Lake Matano. To our knowledge, this is the first documentation of this pathway in a natural environment. Our results demonstrate that methane can be an integral component of the C cycle, even in Fe-rich environments. By analogy to Lake Matano, methanogens and methanotrophs could have formed an important part of the ferruginous Archean ocean ecosystem in which they would have played an important role in regulating atmospheric chemistry and global climate.
B21B-0351
Spatial Patterns of Tungsten and Cobalt on Leaf Surfaces of Trees in Fallon, Nevada
Spatial patterns of airborne tungsten and cobalt are described from leaf surface chemistry of trees in Fallon, Nevada, where a cluster of childhood leukemia has been ongoing since 1997. In earlier research, airborne tungsten and cobalt have been shown to be elevated in total suspended particulates, surface dust, and lichens of Fallon. To update spatial patterns of airborne tungsten and cobalt in Fallon, leaves were collected in October 2007 from trees growing throughout Fallon. Collected leaves were measured for metals accumulated onto their surfaces. On Fallon leaf surfaces, tungsten and cobalt show maxima of 17 ppm and 6 ppm, respectively, near the center of town, north of Highway 50 and west of Highway 95. Background levels of tungsten and cobalt on leaves are ~0.50 ppm for both metals. These two peaks overlap spatially, and given the dense and widespread pattern of collection, the source area of these two airborne metals can be pinpointed to the vicinity of a hard-metal industry located north of Highway 50 and west of Highway 95. As an update of environmental research in Fallon, this leaf surface chemistry study adds to years of studies showing elevated airborne tungsten and cobalt in Fallon. Given the cluster of childhood leukemia in Fallon, it stands to reason that additional biomedical research is in order to test directly the leukogenicity of combined airborne tungsten and cobalt particulates.
B21B-0352
An initial examination of tungsten geochemistry along groundwater flow paths
Groundwater samples were collected along groundwater flow paths from the Upper Floridan (Florida), Carrizo Sand (Texas), and the Aquia (Maryland) aquifers and analyzed for tungsten (W) concentrations by high- resolution inductively couple plasma mass spectrometry. At each well head, groundwater samples were also analyzed for pH, specific conductance, temperature, alkalinity, dissolved oxygen (DO), oxidation-reduction potential (Eh), dissolved iron speciation, and dissolved sulfide [S(-II)] concentrations. Sediment samples from the Carrizo Sand and Aquia aquifers were also collected and subjected to sequential extractions to provide additional insights into the solid-phase speciation of W in these aquifers. Tungsten concentrations varied along the groundwater flow paths chiefly in response to changing pH, and to a lesser extent, variations in the redox conditions. For groundwater from the Carrizo Sand aquifer, W ranges between 3.64 and 1297 pmol/kg, exhibiting the lowest values proximal to the recharge zone. Tungsten concentrations progressively increase along the flow path, reaching 1297 pmol/kg in the sulfidic groundwaters located approximately 60 km downgradient from the recharge area. Tungsten is strongly correlated with S(-II) concentrations and pH in Carrizo groundwaters (r = 0.95 and 0.78, respectively). Within the Aquia aquifer, however, W generally occurs at lower concentrations than the Carrizo (14 to 184 pmol/kg; mean = 80 pmol/kg), and shows no systematic trends along the flow path (e.g., r = 0.08 and 0.4 for W vs. S(-II) and pH, respectively). Our data are consistent with the increase in W concentrations in Carrizo groundwaters reflecting, in part, pH-related desorption, which has been shown to be substantial for pH greater than 8. Moreover, because of the broad similarities in the chemistry of W and Mo, which forms thiomolybdates in sulfidic waters, we suggest that thiotungstate complexes may form in sulfidic groundwaters, thus partially explaining the elevated W in sulfidic waters of the Carrizo aquifer. We propose that the substantially lower W concentrations in Aquia groundwaters reflect the fact that these waters are suboxic and have not undergone sulfate reduction. Hence, the evolution of W concentrations in the Aquia aquifer is consistent with conservative behavior in these generally oxic to suboxic groundwaters. In summary, our data indicate that pH related adsorption/desorption reactions are the key factors controlling W concentrations in oxic and sub-oxic waters, whereas formation of thiotungstate complexes may be important in sulfidic/anoxic waters.
B21B-0353
Immobilisation of arsenic by iron(II)-oxidizing bacteria
Arsenic-contaminated groundwater is an environmental problem that affects about 1-2% of the world's population. As arsenic-contaminated water is also used for irrigating rice fields, the uptake of arsenic via rice is in some cases even higher than via drinking water. Arsenic is often of geogenic origin and in many cases bound to iron(III) minerals. Microbial iron(III) reduction leads to dissolution of Fe(III) minerals and thus the arsenic bound to these minerals is released to the environment. In turn, iron(II)-oxidizing bacteria have the potential to co-precipitate or sorb arsenic during iron(II) oxidation followed by iron(III) mineral formation. Here, we present work on arsenic co-precipitation and immobilization by anaerobic and aerobic iron(II)-oxidizing bacteria. Co-precipitation batch experiments with pure cultures of nitrate-dependent, phototrophic, and microaerophilic Fe(II)-oxidizing bacteria are used to quantify the amount of arsenic that can be immobilized during microbial iron mineral precipitation. Iron and arsenic speciation and redox state are determined by X- ray diffraction and synchrotron-based X-ray absorption methods (EXAFS, XANES). Microcosm experiments are set-up either with liquid media or with rice paddy soil amended with arsenic. Rice paddy soil from arsenic contaminated rice fields in China that include a natural population of Fe(II)-oxidizing microorganisms is used as inoculum. Dissolved and solid-phase arsenic and iron are quantified, Arsenic speciation is determined and the iron minerals are identified. Additionally, Arsenic uptake into the rice plant is quantified and a gene expression pattern in rice (Oryza sativa cv Gladia) is determined by microarrays as a response to the presence of Fe(II)-oxidizing bacteria.
B21B-0354
Dissolved and colloidal oxyanion trace elements in the Yukon River system
Dissolved and colloidal trace elements were determined in rivers and streams within the Yukon River Basin in Alaska and Canada. Among the oxyanions, we have determined As, Cr, Mo, Re, and V in the dissolved (< 0.02 μm) and colloidal (0.02 - 0.45 μm) phases. Among these elements, Re and Mo are found almost entirely in the dissolved phase. For the other three elements, while most of their < 0.45 μm concentration is in the dissolved phase, they nonetheless have significant colloidal suspended fractions. In general, these colloidal oxyanion concentrations follow colloidal Fe, with somewhat higher colloidal oxyanion/Fe ratios for glacial streams. This is likely a result of significant alumino-silicate colloidal Fe in the glacial streams versus dominant colloidal FeOOH in the non-glacial streams. In the dissolved phase, Mo and Re tend to be correlated as do V and As (and to a lesser extent Cr). V, As, and Cr all show positive correlations with DOC for high DOC streams. However, streams with low DOC show no such trend and frequently have higher V, As, and Cr than the high DOC streams. This suggests control by organic complexation (including retention in soils) for the high DOC systems, and by other processes (e.g., redox chemistry and mineral weathering) in low DOC systems.
B21B-0355
Arsenic mobility in soils contaminated with metallurgical wastes as a function of variable chemical conditions
Arsenic is a pervasive contaminant of natural aqueous systems, such as groundwater and soils, its sources being both natural and anthropogenic. The present investigation was performed on soils contaminated with residues from ore processing activities and revealed the presence of arsenate [As(V)] species with a very low mobility, through natural attenuation processes. The stability of this attenuation was investigated by varying two specific equilibrium chemical conditions: pH and presence of bicarbonate ions. One-unit changes in equilibrium pH generally caused small increases in As mobility, whereas the presence of bicarbonate ions considerably increased this mobility. The results were compared to thermodinamic simulations of equilibrium conditions using the total elemental composition of each individual soil, but excluding sorption reactions. Close matches between experimental data and simulations revealed the predominance of solubility-controlled As mobility via heavy-metal arsenate solid formation. Bicarbonate ions were found to be highly unsuitable for extraction of sorbed arsenate fractions due to indirect As release from solid arsenates, via heavy-metal carbonate precipitation processes.
B21B-0356
Geochemical Control on Cr(VI) Desorption From Sediments: a Macroscopic, Microscopic and Spectroscopic Investigation
Two sets of Cr contaminated sediments were used in this study: sediments that were or were not exposed to acidic waste fluids. Pre and post-treatment sediment samples were characterized with XRD, Focused Ion Beam SEM/EDS, TEM, XPS, X-ray microprobe, XANES and with wet chemical extraction techniques. In the sediments that were not exposed to acidic waste fluids: i.) Water extractions, alkaline extractions, and ultrafiltration indicated that Cr(VI) was dominantly present in the porewater; ii.) Majority of Cr(VI) mass was easily transported through sediments; iii.) At least two significant Cr(VI) pools with different leaching behavior (a fast and a slow releasing pools) were present in all tested sediments.; iv.) A two-site model described well the Cr(VI) desorption profiles of both aged and freshly contaminated sediment; v.) Calculated equilibrium and kinetic site Kd and rate constants were sediment dependent; vi.) Ba was not detected in the effluents, suggesting that moderately soluble BaCrO4 (hashemite) or other less soluble solid solutions of BaCrO4 BaSO4, which usually form under high Cr(VI) concentrations, were not controlling Cr(VI) solubility and mobility. vii.) Cr was concentrated around grain boundaries and in high concentration zones within the sediment matrix; viii.) Evidence of Cr associated with Fe oxides [most likely magnetite, which is a redox sensitive mineral with structural Fe(II)], and as insoluble BaCrO4 was also found in the contaminated sediments; ix.) Cr was also present in areas rich in alumino-silicates and/or Fe-rich alumino-silicates (most likely ferroan clinochlore or biotite); x.) XANES measurement confirmed that reduced Cr(III) via an abiotic pathway was present in small concentrated zones of mixed valence [Cr(III)/Cr(VI)] within the fine-grained coatings. The results from the experiments conducted with sediments that were exposed to acidic waste fluids demostrated that the sediments had appreciable amountss of acid extractable Cr and total Cr determined with microwave digestion. Water extractable Cr was significantly less, suggesting that it was relatively immobile. The results from a series of column experiments confirmed that Cr was mostly immobile in these sediments. The mechanistic aspects of such a behavior are yet to be determined.
B21B-0357
Abiotic Reduction of Selenite and Antimonate Under Controlled Oxygen Conditions
Laboratory and field studies have reported the oxidation of elemental Se to selenite or selenate or that of antimonite to antimonate but the reduction studies of the two elements, especially in absence of bacteria are more scarce. We have performed experiments on the abiotic reduction of Se(IV) and Sb(V) under controlled oxygen conditions in presence of naturally-encountered reducing agents such as Fe(II) and dissolved sulfide. In the case of selenite, the reduction by ferrous iron is barely detectable at very low concentrations of oxygen. However, at concentrations of 200 50 ppmv in the controlled atmosphere glove box, more iron oxide particles were formed at a higher initial Fe(II) concentration in the system and with time. In the pellets collected after filtration, a significant amount of Se(0) was found. Our field geochemical studies on Se also showed the same phenomenon, i.e. a higher level of Se(0) in lake sediments was accompanied by a higher presence of iron oxides. In the case of antimony, the reduction of Sb(V) by dissolved sulfide was extensive and far more rapid at more acidic pH values. Half lives for Sb(V) in the presence of excess dissolved sulfide at pH values of 5 to 7 were calculated and the reaction was found to be first order with respect to all three of [Sb(V)], [dissolved sulfide] and [H+]. Metastibnite precipitated after reduction of Sb(V) in working experimental samples at buffered pH of 5 and 6. The oxidation product of dissolved sulfide was identified as elemental sulfur. This study has demonstrated the ability of dissolved sulfide to reduce Sb(V) under a variety of environmentally relevant concentrations and conditions.
B21B-0358
Trace Metal Associations in an Anoxic Lake: the Relative Roles of Organic Carbon and Reduced Sulfur
We investigate the geochemistry of the trace elements Mo, U, and Re in sediments from a transect through the chemocline of Lake Tanganyika, East Africa. In addition to these relatively shallow cores (70 to 330m), we present data from a longer core representing ~30,000 years of lake history, which was taken within the sulfidic waters that lie well below the chemocline (~900m water depth). Our goal is to establish a framework for trace metal deposition within the context of organic carbon and sulfur burial - two important carrier phases for these metals. Sediment organic carbon contents are high, generally between 5 and 10 wt% at the shallow sites, and up to 16 wt% in the deep basin. Despite the very low sulfate (~35 μM) and sulfide (~30 μM) concentrations in the lake water, sediment reduced sulfur contents are up to 1.5 wt% in the shallow sites and as high as 5 wt% in the deepest sediments. Sediment C:S ratios for all study sites are consistent with these sediments generally being sulfur limited. Sediment C:S ratios decrease from ~22, which agree well with previously published freshwater values, to ~6 with increasing site depth. The lower C:S ratios are more comparable to the marine value (2.8), and suggest that a considerable amount of organic carbon must be decomposing via sulfate reduction. C:S ratios in the deepest site are highly variable, with some even lower than the marine threshold. In light of the sedimentary organic carbon and sulfur data, trace metal distributions imply that U deposition is closely associated with organic carbon deposition and is independent of sulfur cycling. In contrast, Mo behavior suggests both an association with organic carbon as well as sulfur, but is subject to poor preservation where the sediment C:S ratios are highest. Rhenium accumulation only appears significant at the deepest most sulfur-rich site, and there is a close correspondence between Mo and Re distributions. These latter observations suggest that sulfur burial is particularly important for the authigenic accumulation of Mo and Re in this lacustrine system, in contrast to marine anoxic sediments which show a much tighter coupling between Mo and organic carbon. Despite our expectation of sulfur limitation in this system, there is evidence of substantial reduced sulfur accumulation and corresponding metal enrichment within the lake sediments.
B21B-0359
Development of Arsenic and Iron Biogeochemical Gradients upon Anaerobiosis at Soil Aggregate Scale
In aerated soils, As release is limited due to the strong interaction between As(V) and soil minerals. However, under anaerobic conditions, As desorption is stimulated by As(V) reduction to As(III) and reductive dissolution/transformation of Fe (hydr)oxides, common hosts of As. The effect of As(V) and Fe(III) reduction on As release has been extensively studied in laboratory batch and column systems; correlation of apparent Fe and As reduction, with concomitant release to pore water, has also been noted under field conditions. What remains unresolved is the coupling of biogeochemical and physical processes that ultimately control As transport within structured media such as soils. Soils are heterogeneous porous media that are comprised of individual aggregates having pores that are dominated by diffusive (aggregate interiors) or advective (aggregate exteriors) transport. As a consequence of physical and chemical differences in the interior and the exterior of aggregates, As(III,V) and Fe(II,III) chemical gradients develop. Here, we examine As release from constructed aggregates exposed to fluctuating redox conditions. Artificial aggregates were made with As(V) adsorbed ferrihydrite-coated sand homogeneously inoculated with Shewanella sp. ANA-3 (model As(V) and Fe(III) reducer) and then fused using an agarose binder into spheres. Aggregates were placed in a flow reactor and saturated flow of aerobic or anaerobic artificial groundwater media was initiated. Redox fluctuated in select systems to examine changes in chemical gradient under changing aeration status. Our results show that within aerated solutions, oxidized aggregate exteriors provide a gprotective barrierh against As release despite anoxia within diffusively constrained aggregate interiors. During a transition to anaerobic conditions in advective zones, however, As is released and transport is promoted. Our study illustrates the microscale variation in biogeoechemical processes within soils and the importance of appreciating the spatial connection between reaction and transport fronts.
B21B-0360
Structural Identification of Thioarsenates and Their Differentiation From Thioarsenites by EXAFS
Thioarsenic complexes play an important role in regulating arsenic solubility, mobility, and toxicity in sulfidic systems. Despite their importance, there is little consensus on their thermodynamic properties and structural identification. A major focus of current research is the unambiguous identification of the members of the two homologue series of monomeric thioarsenic species that are conceptually postulated to exist under sulfidic conditions, (oxy)thioarsenites and (oxy)thioarsenates. Here we report the unambiguous identification of synthetic mono-, di, and tetrathioarsenate using a combination of X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). As-O and As-S coordination numbers confirmed the structure as the expected mono-, di- and tetrathioarsenate compounds. The As-O bond distances of 1.69 to 1.70 Å are comparable with those stated for arsenates, and are easily distinguishable from those for arsenites at 1.78 Å. The As-S distances in our standard materials with 2.17 Å are clearly shorter than those published for arsenite-sulfide minerals with 2.24 to 2.34 Å. As expected, no As-O bonding was determined in tetrahedral tetrathioarsenate, which is fully coordinated to S. As the extent of thiol complexation increases, the position of the absorption-edge shifts systematically and linearly towards lower energies compared to that of arsenate. The structural data for the individual solid and liquid samples, measured at room temperature or at 15 K, did not show obvious differences, suggesting that the aqueous complexes have similar structures as the XRD-identified solids and are stable in natural waters. An interesting observation was made during the titration of the liquid tetrathioarsenate from pH 6 to 3. Below neutral pH, the absorption edge shifted to lower energies by ~2 eV concomitant with an increase of the As-S bond length to 2.28 Å at pH 3, comparable with those of orpiment and indicative of either thioarsenites (which have been reported to exist in arsenite-sulfide-containing solutions) or colloidal orpiment. As for thioarsenates, the observed complexes also show a linear trend together with arsenite and orpiment, clearly distinguishable from the arsenate-based line. The present data demonstrate that it is possible to differentiate thioarsenates from thioarsenites by XAS. Combined with other recent studies, these data indicate that thioarsenates can be formed in sulfidic solutions under a broad range of conditions.
B21B-0361
Electrochemical Fractionation of Molybdenum Stable Isotopes
Stable isotope signatures were measured from Molybdenum (Mo) electrodeposited from aqueous solution. As potential varied from -1.35 V to -2.00 V (relative to Ag/AgCl), fractionation decreases from Δ97/95Mo = -1.3 ‰ to -0.9 ‰ (Δ97/95Mo defined as the difference in the 97Mo/95Mo ratio of deposited Mo relative to aqueous Mo). Natural variations of δ97/95Mo span a range of ~ 3 ‰ [Barling, J. and Anbar, A. D., EPSL. 2004, 217: 315], therefore, charge transfer driven fractionation may be responsible for some of the observed variation in Mo stable isotope geochemistry. Following previous approaches with Fe and Zn [Kavner, A. et al. Geochim. Cosmochim. Acta. 2005, 69: 2971; 2008, 72: 1731], Mo was plated in a three-electrode cell from a neutral to slightly alkaline solution (pH ~ 8.7). Voltage was held constant during electrodeposition using an Autolab Potentiostat. In all experiments, less than 0.5 % of the Mo was deposited, which insures that the plating reservoir remains at an approximately constant isotopic composition. Plated Mo was then recovered in acid, and the isotopic composition of samples and stock solutions were measured using a Thermo Scientific Neptune MC-ICP-MS. These experiments show that the redox process induces an isotopic signature with respect to the starting material, with a trend showing that fractionation decreases as a function of applied voltage.
B21B-0362
Integrating Mechanisms that Control the Concentration and Distribution of Groundwater Arsenic in Cambodia
Arsenic contamination in the groundwater of the deltaic regions of southeastern Asia affects as many as one hundred million people. In these areas, the dominant mechanism for arsenic release into solution is the reductive dissolution of arsenic-bearing iron (hydr)oxides. What continues to confound researchers, however, is the heterogeneous nature of arsenic in groundwater. Recent evidence suggests that specific geomorphic environments are linked to regions of elevated groundwater arsenic. Moreover, waters from actively cycling surficial environments represent important sources of groundwater As in many environments. Here, we examine the mechanistic link between the localized expression of iron reduction and the large scale distribution of arsenic in groundwater along the Mekong River in Cambodia in a region impacted by widespread but heterogeneous arsenic contamination. In this region, arsenic levels in groundwater were correlated to sulfate levels, and both were strongly influenced by the extent of local surface water flooding (as quantified by remote sensing). Concentrations of arsenic ranged from 0 to 2100 micrograms/L, and were always found in groundwaters with significant dissolved iron and a sulfate concentration less than 1 mg/L. This indicates that the delivery of As and S was influenced by active redox processes in near-surface environments. In many, but not all, high As regions, groundwater concentrations of conservative halide anions also were similar to those of surficial environments, indicating that they were likely derived from the same locations. Low As regions, however, had widely variable halide concentrations in groundwater that may reflect additional groundwater sources. Thus, the expression of As contamination is strongly influenced by the hydrological connectivity of the aquifer with the surface. The isotopic composition of groundwater (hydrogen and oxygen) of high As areas also is similar to that of surface waters. Dissolved inorganic carbon isotopic composition of waters is more complex, with As-impacted groundwaters having distinct isotopic signatures characteristic of either extensive or relatively limited organic matter decomposition. Regional correlations of As with dissolved organic carbon are also not uniform, suggesting that sedimentary carbon is also important in generating reducing conditions. These data indicate that the distribution of organic carbon helps determine the distribution of arsenic in the environment. Moreover, these data indicate that heterogeneity in arsenic concentrations results from the interplay of variable organic matter content and reactivity within complex hydrological systems that can at least in part be explained regionally based on depositional environment.
B21B-0363
Reproducible Crystallite Size of Mono-Dispersed and Scalable Biologically Produced Metal-Substituted Nanometer-Sized Magnetites
Our previous research demonstrated that biosynthesized magnetite (biomagnetite) exhibited similar properties as chemically synthesized magnetite. To complement uses of the traditional chemically synthesized magnetite (chem-magnetite) biomagnetite must be exhibit highly reproducible sizes and be available in scalable qualities. Here we emphasize potentially advantageous properties of biomagnetite regarding size, reproducibility and scaling availability. Average crystallite size (ACS) of biomagnetites ranging from 10-100 nm was determined after varied 1) incubation times, 2) substitution of metal and lanthanide species, 3) degrees of congruent incorporation or retardation of substitution elements, 4) bacterial species with their varied ability to substitute elemental species, and 6) incubation temperature that can influence coalescence. The microbial production of biomagnetite has demonstrated capacity to make highly crystalline nanoscale particles of metal-substituted ferrites including compounds of Co, Ni, Cr, Mn, Zn and the rare earths in large quantity. Selected Zn-substituted magnetite (nominal composition of Zn0.6Fe2.4O4) has been recovered at over 1 kg (wet weight) in batches from 30 L fermentations. The massively produced extracellular magnetites were confirmed to exhibit good mono- dispersity via transmission electron microscopy (TEM). TEM also validated highly reproducible ACS of 13.1±0.8 nm size as determined through X-ray diffraction (N=7) at a 99 % confidence level. Based on the scale-up experiments performed using the 35 L reactor, the reduction in ACS variability and shorted incubation times of several days may be attributed to increases of electron donor input, and availability of divalent ions of the substitution metal with less ferrous ions in the case of doped magnetite, or a combination of the above. While costs of commercial nanometer sized magnetite (25-50 nm) may vary from $500/kg to > $1,000/kg, microbial mass production is likely capable of producing 13-90 nm magnetite or doped magnetites at a fraction of the cost of traditional chemical synthesis. While there are numerous approaches for the synthesis of nanoparticles, bacterial fermentation of magnetite or metal-substituted magnetite may represent a disruptive manufacturing technology with respect to yield, reproducibility and scalability.
B21B-0364
High-precision 40Ar/39Ar age for the Jehol Biota
Abundant fossils of the terrestrial Jehol Biota, including plants, insects, dinosaurs, birds, mammals and freshwater invertebrates, were discovered from the Yixian Formation and the overlying Jiufotang Formation in Inner Mongolia, Hebei Province and Liaoning Province, northeastern China. Because of the exceptional preservation of fossils, the Jehol Biota is one of the most important Mesozoic fossil outcrops and referred to as a Mesozoic Pompeii. The Jehol Biota has provided a rare opportunity to address questions about the origin of birds, the evolution of feathers and flight, the early diversification of angiosperms and the timing of the radiation of placental mammals. The Tuchengzi Formation, which lies unconformably just below the Yixian Formation and consists mainly of variegated sandstones, is less fossiliferous than the two overlying formations. However, dinosaur tracks, silicified wood and compressed plants are found in this formation. A systematic 40Ar/39Ar dating of the Yixian and the Jiufotang formations was undertaken to provide a framework for understanding the timing and duration of the Jehol Biota and evolutionary events represented within it. Furthermore, determining the absolute age of the Tuchengzi Formation provides information to interpret abundant dinosaur tracks within and provide better age constrains for the beginning of the Jehol Biota. Here we present robust high-precision 40Ar/39Ar data for six tuff samples and two basalt samples collected from the Tuchengzi, the Yixian and the Jiufotang formations near the classic outcrops in western Liaoning, NE China. We obtain an age of 139.5 ± 1.0 Ma for the uppermost Tuchengzi Formation, an age of 129.7 ± 0.5 Ma for a basaltic lava from the bottom of the Yixian Formation and an age of 122.1 ± 0.3 Ma for a tuff from the base of the overlying Jiufotang Formation. Our data indicate that the Yixian Formation was deposited during the Early Cretaceous, the Barremian to early Aptian, within a time span of 7 Ma. Because of the systematic sampling and the high quality of our data, these results contribute the most accurate age calibration yet of the Jehol Biota within the Yixian Formation and the overlying Jiufotang Formation, providing significant calibration for the evolution of early angiosperms, primitive birds and feathered dinosaurs.
B21B-0365
The analyze of structure of vaterite and which factors will induce vaterite formation
Calcium carbonate (CaCO3) is the most abundant mineral form of a biomineral and has important applications in industry such as nano-materials. Calcite is its most stable polymorph at ambient conditions. However, vaterite, the least stable polymorph of CaCO3, has recently been found to act as a precursor to calcite during the addition of certain organic acids. Therefore, it is increasingly important to understand the formation and properties of vaterite. In this study, we show that both the inorganic additives and the experimental setup itself may promote the formation and stabilization of vaterite. Powders of muscovite and orthoclase, but not quartz, have the ability to induce the formation of vaterite, rather than calcite, a shown by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) of the crystals formed. Empirical force-field calculations suggest that the charge distribution on the powder surfaces stabilize vaterite. Also, without any additions, polystyrene Petri dishes induce more vaterite formation than glass Petri dishes. In addition, ammonia that was used to increase solution pH and hence CaCO3 precipitation, promotes formation of vaterite rather than calcite. This shows that minor changes in experimental design can have a significant influence on the minerals precipitated. The crystal structure of the precipitating vaterite and its morphology was investigated using TEM and SEM showing that the major crystal planes of vaterite are (001) growth planes. XRD data confirmed the results of molecular dynamics simulations showing that the carbonate ions of freshly precipitated vaterite are disordered but develop long-range order after one hour of heating at 250 °C followed by quenching.