Susan L. Brantley

The size distribution of framboidal pyrite in modern sediments: An indicator of redox conditions

Abstract Pyrite framboids are densely packed, generally spherical aggregates of submicron-sized pyrite crystals. In this study, a survey was made of framboid size distributions in recently deposited sediments from euxinic (Black Sea; Framvaren Fjord, Norway; Pettaquamscutt River Estuary, Rhode Island, USA), dysoxic (Peru Margin), and oxic (Wallops Island, Virginia, USA; Great Salt Marsh, Delaware, USA) environments. Pyrite framboids in sediments of modern euxinic basins are on average smaller and less variable in size than those of sediments underlying dysoxic or oxic water columns. Down-core trends indicate framboid size distribution is a sediment property fixed very early during anoxic diagenesis, generally within the top few centimeters of burial. Size distributions in modern sediments are comparable with those in ancient sedimentary rocks, evidence that framboid size is preserved through advanced stages of diagenesis and lithification. It is proposed that where secondary pyrite growth is limited, as to preserve primary pyrite textures, framboid size distribution may be used to indicate whether fine-grained sedimentary rocks were deposited under oxic or anoxic conditions. The Crystal Size Distribution Theory relates framboid size to growth time and rate. On the basis of this theory, the characteristic smaller sizes of framboids in sediments of modern euxinic basins reflect shorter average growth times relative to oxic or dysoxic environments. In euxinic environments, framboid nucleation and growth occurs within anoxic water columns, and growth times are, on average, shorter because of hydrodynamic effects than when framboid nucleation and growth occurs within anoxic sediment porewaters underlying oxic water columns. A maximum framboid growth time of 0.4 years is indicated for framboids forming in the water columns of euxinic basins.

Characterization of an electron conduit between bacteria and the extracellular environment

A number of species of Gram-negative bacteria can use insoluble minerals of Fe(III) and Mn(IV) as extracellular respiratory electron acceptors. In some species of Shewanella, deca-heme electron transfer proteins lie at the extracellular face of the outer membrane (OM), where they can interact with insoluble substrates. To reduce extracellular substrates, these redox proteins must be charged by the inner membrane/periplasmic electron transfer system. Here, we present a spectro-potentiometric characterization of a trans-OM icosa-heme complex, MtrCAB, and demonstrate its capacity to move electrons across a lipid bilayer after incorporation into proteoliposomes. We also show that a stable MtrAB subcomplex can assemble in the absence of MtrC; an MtrBC subcomplex is not assembled in the absence of MtrA; and MtrA is only associated to the membrane in cells when MtrB is present. We propose a model for the modular organization of the MtrCAB complex in which MtrC is an extracellular element that mediates electron transfer to extracellular substrates and MtrB is a trans-OM spanning β-barrel protein that serves as a sheath, within which MtrA and MtrC exchange electrons. We have identified the MtrAB module in a range of bacterial phyla, suggesting that it is widely used in electron exchange with the extracellular environment.

Kinetics of Water-Rock Interaction

Analysis of Rates of Geochemical Reactions.- Transition State Theory and Molecular Orbital Calculations Applied to Rates and Reaction Mechanisms in Geochemical Kinetics.- The Mineral-Water Interface.- Kinetics of Sorption-Desorption.- Kinetics of Mineral Dissolution.- Data Fitting Techniques with Applications to Mineral Dissolution Kinetics.- Nucleation, Growth, and Aggregation of Mineral Phases: Mechanisms and Kinetic Controls.- Microbiological Controls on Geochemical Kinetics 1: Fundamentals and Case Study on Microbial Fe(III) Oxide Reduction.- Microbiological Controls on Geochemical Kinetics 2: Case Study on Microbial Oxidation of Metal Sulfide Minerals and Future Prospects.- Quantitative Approaches to Characterizing Natural Chemical Weathering Rates.- Geochemical Kinetics and Transport.- Isotope Geochemistry as a Tool for Deciphering Kinetics of Water-Rock Interaction.- Kinetics of Global Geochemical Cycles.

Feldspar dissolution at 25°C and pH 3: Reaction stoichiometry and the effect of cations

Abstract Feldspar powders, An 0 An 76 , were dissolved in flow-through reactors at 25°C, pH 3, to investigate the effect of feldspar composition, electrolyte concentration, and cation identity upon dissolution rates. BET surface area increased 1.5–7 times over the approximately 2000 hour reaction times; we, therefore, calculated dissolution rates with the final, rather than the initial surface area. This correction resulted in calculated rates which were, correspondingly, 1.5–7 times lower than several previously published rate estimates. Dissolution rates increase linearly with increasing anorthite content over the composition range studied. Rates decreased with increasing NaCl, and to a lesser extent, increasing (CH 3 ) 4 NCl concentrations. We interpret our rate data with a surface-controlled rate model: rate = k · [SOH ex ] n , where [z.tbnd;SOH ex ] is the concentration of H + which reacts with the feldspar surface through proton-cation exchange reactions. Previous workers have used [z.tbnd;SOH ex ] to represent protons adsorbed to surface hydroxyl sites. We express [z.tbnd;SOH ex ] with a Langmuir competitive adsorption isotherm, and fit our rates to the model: rate = kN s K H {H + } 1 + K H {H + } + K Na {Na + } 0.5 , where k = the rate constan, N s = the surface site density, K H = the H + constant for adsorption at the exchange site, K Na = the Na + constant for adsorption at the exchange site, and { i } denotes the activity of species i . Aluminum and the network-modifiers, Na, K, and Ca, were preferentially released compared to Si during the initial phase of dissolution. After 500–1000 hours in H 2 O-HCl, dissolution became stoichiometric for the microcline, albite, and bytownite compositions. Oligoclase and labradorite continued to exhibit preferential Ca and Al release even after 3000 hours of dissolution. Exsolution texture, observed in labradorite, may provide a structural control for preferential Ca and Al release. Apparent nonstoichiometric dissolution in oligoclase is due to the presence of Ca- and Al-rich accessory phases, present in the original feldspar samples. This work suggests that in the absence of accessory phases and mineral defects, steady-state feldspar dissolution is stoichiometric for all compositions.

Models of quartz overgrowth and vein formation: Deformation and episodic fluid flow in an ancient subduction zone

Steady state models of overgrowth and vein formation are developed using kinetic data for quartz dissolution and precipitation and estimates of fluid advection, pore-fluid and grain-boundary diffusion. Application of these models to overgrowths and veins in the Kodiak accretionary complex suggests that the Kodiak Formation deformed continuously by a grain-boundary diffusion-limited mechanism, accompanied by episodic pore fluid diffusion of quartz from the matrix to vertical fluid-filled fractures near the base of the accretionary wedge. These processes produced two types of syntectonic crystal textures within the Kodiak Formation: overgrowths containing displacement-controlled fibers, and throughgoing veins composed of face-controlled elongate blocky quartz crystals. Based on textural observations, displacement-controlled quartz growth in overgrowths is rate-limited by either diffusion along a cohesive interface or the rate of matrix strain. The magfiitude of elongation recorded by displacement-controlled crystal growth varies smoothly (elongation of 1 to 3) from the shallowest to the deepest structural levels of the Kodiak Formation, suggesting that the diffusional component of deformation in the accretionary wedge increases with depth. In contrast, face-controlled quartz growth is largely restricted to veins within the deepest level, where the cleavage is subhorizontal and deformation involves a component of simple shear, suggesting proximity to a decollement. The facecontrolled quartz veins represent mode I cracks which seal periodically and contain continuous planar solid inclusion bands, cracks which partially seal periodically and contain discontinuous solid inclusion bands, or cracks that remain open and contain euhedral quartz crystals with no solid inclusions. The initial crack aperture, inferred from spacing of inclusion bands, varies from 8 gm in

Surface area and porosity of primary silicate minerals

Abstract Surface area is important in quantifying mineral-water reaction rates. Specific surface area (SSA) was measured to investigate controls on this parameter for several primary silicate minerals (PSM) used to estimate rates of weathering. The SSA measured by gas adsorption for a given particle size of relatively impurity-free, laboratory-ground samples generally increases in the order: quartz ≈ olivine ≈ albite < oligoclase ≈ bytownite < hornblende ≈ diopside. Reproducibility of BET SSA values range from ±70% (SSA < 1000 cm2/g) to ± 5% (SSA > 4000 cm2/g) and values measured with N2 were observed to be up to 50% larger than values measured with Kr. For laboratory-ground Amelia albite and San Carlos olivine, SSA can be calculated using log (SSA, cm2/g) = b + m log (d), where d = grain diameter (μm), b = 5.2 ± 0.2 and m = -1.0 ± 0.1. A similar equation was previously published for laboratory-ground quartz. Some other samples showed SSA higher than predicted by these equations. In some cases, high SSA is attributed to significant second phase particulate content, but for other laboratory-ground samples, high SSA increased with observed hysteresis in the adsorption-desorption isotherms. Such hysteresis is consistent with the presence of pores with diameters in the range 2 to 50 nm (mesopores). In particular, porosity that contributes to BET-measured SSA is inferred for examples of laboratory-ground diopside, hornblende, and all compositions of plagioclase except albite, plus naturally weathered quartz, plagioclase, and potassium feldspar. Previous workers documented similar porosity in laboratory-ground potassium feldspar. Surface area measured by gas adsorption may not be appropriate for extrapolation of interfacelimited rates of dissolution of many silicates if internal surface is present and if it does not dissolve equivalently to external surface. In addition, the large errors associated in measuring SSA of coarse and/or impurity-containing silicates suggest that surface area-normalized kinetics in both field and laboratory systems will be difficult to estimate precisely. Quantification of the porosity in laboratory- ground and naturally weathered samples may help to alleviate some of the discrepancy between laboratory- and field-based estimates of weathering rate

Role of bacterial siderophores in dissolution of hornblende

Hornblende, a common mineral in granitic soils, may act as a source for a variety of metals needed by bacterial species for enzyme function (e.g., Fe, Zn, Mn, Cu, Co, Mo, V, Ni). A species of the bacterial genus Streptomyces was cultured from an Adirondack soil and isolated because of its ability to grow robustly in low Fe medium with hornblende present. Studies with unbuffered culture medium, to discover whether Streptomyces sp. cultures affected solution pH, showed a decrease of 2.0 pH units in 21 d, then an increase of 3.0 pH units at 56 d. Cells that adhered to the hornblende surface at 56 days were difficult to remove, presumably because of mycelial growth deep into pits and cracks. Decreases and increases in pH may have been due to production of organic acids and ammonia respectively. Increases in pH could also have been related to release of components during death of organisms. In a buffered medium, Streptomyces sp. increased the initial Fe release rate from hornblende approximately fivefold over that of an abiotic control. A catechol derivative, produced by the Streptomyces sp. and characterized by chromatography and mass spectrometry, is presumed to cause this Fe release enhancement. Hornblende dissolution was also analyzed in the presence of a commercially available hydroxamate siderophore, desferrioxamine mesylate (DFAM). DFAM is the methane sulfonate form of one of many siderophores known to be a product of streptomycetes. The rate of Fe release obtained when incubating the hornblende with 24 μm of DFAM was similar to the rate observed in the presence of the Streptomyces sp. isolate. Higher concentrations of DFAM increased the dissolution rate nonlinearly, described by the rate equation R = (7.6 × 10−13)C0.47, where R is the release rate of Fe (mol/m2s), and C is the concentration (mol/l) of DFAM. The DFAM also increased release of Al and Si from hornblende into solution; however, these release rates were not increased by addition of the Streptomyces sp. alone. Preferential release of Al and Si in the presence of DFAM, but not in the presence of bacteria alone, may be related to the difference in selectivity of catechol vs. hydroxamate siderophores. Addition of Streptomyces sp. in the presence of DFAM at three concentrations consistently enhanced Fe release approximately two to threefold the rate with siderophore alone. Recycling of siderophore molecules or enhanced production of one siderophore by microorganisms in the presence of other siderophores makes it difficult to predict a priori release rates when both siderophore and bacteria are present, as would be the case in natural soils.

The role of dislocations and surface morphology in calcite dissolution

Abstract We have measured the dissolution rate for undeformed (ρ ~ 10 3 dislocations · cm −2 ) calcite to be 3.1 × 10 −10 mol · cm −2 · s −1 in free-drift rotating disk experiments at 1160 rpm, 25°C and pH 8.6 in 0.7 M KCl solution far from equilibrium. The rate increased by a factor of ~2.3 for a strained sample (ρ = 6 × 10 8 · cm −2 ) . Dissolution rates of calcite far from equilibrium were observed to depend on surface preparation and surface morphology resulting from defects outcropping at the crystal surface, but large rate increases due to surface roughness were not observed. The high dissolution rate for mechanically polished surfaces is attributed to enhanced dissolution at cracks and dislocation loops produced in the grinding process. The initial dissolution rate for cleaved surfaces depends on the surface morphology, but reaches a reproducible steady state value when a constant bimodal size distribution of intersecting pits with time-independent wall slope (θ ~ 3°) is achieved (t > 3 h) . Steady state is also characterized by a constant ratio of sloped to relatively flat surface. The two populations of etch pits consist of abundant, short-lived, small etch pits (attributed to nucleation at impurity or point defect clusters) and long-lived, larger point-bottomed pits (attributed to dislocations). Consistent with this interpretation, significant dissolution at an abundance of nondislocation nucleation sites in undeformed calcite explains the relatively small increase in dissolution rate for strained samples. Simulation of bulk crystal dissolution based on etch pit growth rates is in reasonable agreement with observed dissolution rates and surface morphology and indicates that discontinuous dissolution at dislocations is necessary to explain the observed steady state surface morphology. Activation energies for pit deepening and widening were measured between 5 and 50°C as 27 ± 5 and 37 ± 3 kJ · mol −1 , respectively. These values are lower than the measured activation energy for bulk dissolution (59 ± 12 kJ · mol −1 ) .

Iron isotope fractionation during microbial reduction of iron: The importance of adsorption

In experiments investigating the causes of Fe isotope fractionation, the d 56/54 Fe value of Fe(II) remaining in solution (Fe(II)(aq)) after reduction of Fe(III) (goethite) by Shewanella putrefaciens is ;21.2‰ relative to the goethite, in agreement with previous research. The addition of an electron shuttle did not affect fractionation, suggesting that Fe isotope fractionation may not be related to the kinetics of the electron transfer. Furthermore, in abiotic, anaerobic FeCl2(aq) experiments in which approximately one-third of Fe(II)(aq) is lost from solution due to adsorption of Fe(II) onto goethite, the d 56/54 Fe value of Fe(II)(aq) remaining in solution is shifted by 20.8‰ relative to FeCl 2. This finding demonstrates that anaerobic nonbiological interaction between Fe(II) and goethite can generate signif- icant Fe isotope fractionation. Acid extraction of sorbed Fe(II) from goethite in experi- ments reveals that heavy Fe preferentially sorbs to goethite. Simple mass-balance modeling indicates that the isotopic composition of the sorbed Fe(II) pool is ;11.5‰ to 12.5‰ heavier than Fe in the goethite (;2.7‰-3.7‰ heavier than aqueous Fe(II)). Mass balance is also consistent with a pool of heavy Fe that is not released to solution during acid extraction.

Dissolution at dislocation etch pits in quartz

Abstract Several samples of quartz were etched hydrothermally at 300°C in etchams of controlled dissolved silica concentration in order to measure the critical concentration, Ccrit, above which dislocation etch pits would not nucleate on the quartz surface. Ccrit for 300°C was theoretically predicted to be 0.6C0 and the measured Ccrit, was 0.75 ± 0.15C0 (C0 is the equilibrium concentration). Above this value, some dislocation etch pits formed, but the rate of formation significantly decreased. These results are the first experimental validation of etch pit formation theory under hydrothermal conditions. Dune sands showed a generally angular and pitted surface when etched in dilute solutions, while sands etched at C ~ Ccrit showed less angular pitting. Analysis of a soil profile developed in situ on the Parguaza granite, Venezuela, revealed a gradual change from angular, pitted grain surfaces at the top of the profile to rounded surfaces on grains sampled just above bedrock. Since quartz dissolution without surface pitting continues deep in the profile, the Si concentration must exceed Ccrit, at depth. These results indicate that for C >Ccrit, dissolution occurs at edges and kinks on the surface of quartz and very few pits form; in contrast, at C ⪡ Ccrit, dislocation etch pits grow rapidly, contributing to the overall dissolution rate.