Grain size reduction and gouge formation are found to be ubiquitous in brittle faults at all scales, and most slip along mature faults is observed to have been localized within gouge zones. This fine-grain gouge is thought to control earthquake instability, and thus understanding its properties is central to an understanding of the earthquake process. Here we show that gouge from the San Andreas fault, California, with approximately 160 km slip, and the rupture zone of a recent earthquake in a South African mine with only approximately 0.4 m slip, display similar characteristics, in that ultrafine grains approach the nanometre scale, gouge surface areas approach 80 m2 g(-1), and grain size distribution is non-fractal. These observations challenge the common perception that gouge texture is fractal and that gouge surface energy is a negligible contributor to the earthquake energy budget. We propose that the observed fine-grain gouge is not related to quasi-static cumulative slip, but is instead formed by dynamic rock pulverization during the propagation of a single earthquake.

An artesian sulfide-and sulfur-rich spring in southwestern Oklahoma is shown to sustain an extremely rich and diverse microbial community. Laboratory incubations and autoradiography studies indicated that active sulfur cycling is occurring in the abundant microbial mats at Zodletone spring. Anoxygenic phototrophic bacteria oxidize sulfide to sulfate, which is reduced by sulfate-reducing bacterial populations. The microbial community at Zodletone spring was analyzed by cloning and sequencing 16S rRNA genes. A large fraction (83%) of the microbial mat clones belong to sulfur-and sulfate-reducing lineages within ␦-Proteobacteria, purple sulfur ␥-Proteobacteria, -Proteobacteria, Chloroflexi, and filamentous Cyanobacteria of the order Oscillatoria as well as a novel group within ␥-Proteobacteria. The 16S clone library constructed from hydrocarbon-exposed sediments at the source of the spring had a higher diversity than the mat clone library (Shannon-Weiner index of 3.84 compared to 2.95 for the mat), with a higher percentage of clones belonging to nonphototrophic lineages (e.g., Cytophaga, Spirochaetes, Planctomycetes, Firmicutes, and Verrucomicrobiae). Many of these clones were closely related to clones retrieved from hydrocarbon-contaminated environments and anaerobic hydrocarbondegrading enrichments. In addition, 18 of the source clones did not cluster with any of the previously described microbial divisions. These 18 clones, together with previously published or database-deposited related sequences retrieved from a wide variety of environments, could be clustered into at least four novel candidate divisions. The sulfate-reducing community at Zodletone spring was characterized by cloning and sequencing a 1.9-kb fragment of the dissimilatory sulfite reductase (DSR) gene. DSR clones belonged to the DesulfococcusDesulfosarcina-Desulfonema group, Desulfobacter group, and Desulfovibrio group as well as to a deeply branched group in the DSR tree with no representatives from cultures. Overall, this work expands the division-level diversity of the bacterial domain and highlights the complexity of microbial communities involved in sulfur cycling in mesophilic microbial mats.Within sulfur-and sulfide-rich environments (e.g., springs, hydrothermal vents, anaerobic zones of lakes, and shallow marine and intertidal systems), utilization and cycling of sulfur species play a major role in energy production and the maintenance of the microbial community (16). Since a wide array of microorganisms are able to oxidize, reduce, and disproportionate sulfur species, the microbial community structure of sulfurrich habitats is clearly influenced by the prevalent environmental conditions at a specific site, e.g., pH; temperature; sulfide, sulfur, or sulfate concentrations; redox conditions; presence of other electron acceptors; light availability; and organic content.The microbial community structure has been extensively studied in several sulfur-rich habitats, e.g., in hypersaline lakes in Sinai, Egypt (34,46,68,69), and Guerrero Negro, ...

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Mineral-water interactions under conditions of nonhydrostatic stress play a role in subjects as diverse as ductile creep in fault zones, phase relations in metamorphic rocks, mass redistribution and replacement reactions during aliagenesis, and loss of porosity in deep sedimentary basins. As a step toward understanding the fundamental geochemical processes involved, using naturally rounded St. Peter sand, we have investigated the kinetics of pore volume loss and quartz-water reactions under nonhydrostatic, hydrothermal conditions in flow-through reactors. Rate laws for creep and mineral-water reaction are derived from the time rate of change of pore volume, sand-water dissolution kinetics, and (flow rate independent) steady state silica concentrations, and reveal functional dependencies of rates on grain size, volume strain, temperature, effective pressure (confining minus pore pressure), and specific surface areas. Together the mechanical and chemical rate laws form a self-consistent model for coupled deformation and water-rock interaction of porous sands under nonhydrostatic conditions. Microstructural evidence shows a progressive widening of nominally circular and nominally flat grain-grain contacts with increasing strain or, equivalently, porosity loss, and small quartz overgrowths occurring at grain contact peripheries. The mechanical and chemical data suggest that the dominant creep mechanism is due to removal of mass from grain contacts (termed pressure solution or solution transfer), with a lesser component of time-dependent crack growth and healing. The magnitude of a stress-dependent concentration increase is too large to be accounted for by elastic or dislocation strain energy-induced supersaturations, favoring instead the normal stress dependence of molar Gibbs free energy associated with grain-grain interfaces. Introduction One of the most important, and least understood, chemical processes leading to compaction and cementation in thesubsurface is solution transfer creep [Rutter, 1983; Tada and Siever, 1989; Houseknecht, 1988; Spiers and Schutjens, 1990; Hickman and Evans, 1991, this issue]. It has been implicated, ,'often circumstantially, in subsurface porosity loss [Houseknecht, 1988], as a creep process within fault zones [Rutter and Mainprice, 1978; Chester and Higgs, 1992], in the development of stylolites [Heal& 1955], and in providing solutes for cements [McBride, 1989]. However, mechanisms governing solutiontransfer creep are a subject of debate. There are two .fundamentally different mechanisms currently in favor, which date back to classic studies by Weyl [1959] and Bathurst [1958]. The Weyl hypothesis, which is generally equated to the process of "pressure so!ution", involves grain boundary or thin film diffusion in response to chemical potential gradients induced by normal tractions across grain-grain contacts. The Bathurst model, also known as free-face pressure solution [Tada eta/; 1987] (see also Wintsch and Dunning [1985] and Engelder [1982]), involves reaction at solid-fluid interfa...

Microbiological reduction of soluble U(VI) to insoluble U(IV) is a means of preventing the migration of that element in groundwater, but the presence of nitrate in U(IV)-containing sediments leads to U(IV) oxidation and remobilizaton. Nitrite or iron(III) oxyhydroxides may oxidize U(IV) under nitrate-reducing conditions, and we determined the rate and extent of U(IV) oxidation by these compounds. Fe(III) oxidized U(IV) at a greater rate than nitrite (130 and 10 microM U(IV)/day, respectively). In aquifer sediments, Fe(III) may be produced during microbial nitrate reduction by oxidation of Fe(II) with nitrite, or by enzymatic Fe(II) oxidation coupled to nitrate reduction. To determine which of these mechanisms was dominant, we isolated a nitrate-dependent acetate- and Fe(ll)-oxidizing bacterium from a U(VI)- and nitrate-contaminated aquifer. This organism oxidized U(IV) at a greater rate and to a greater extent under acetate-oxidizing (where nitrite accumulated to 50 mM)than under Fe(II)-oxidizing conditions. We showthatthe observed differences in rate and extent of U(IV) oxidation are due to mineralogical differences between Fe(III) produced by reaction of Fe(II) with nitrite (amorphous) and Fe(III) produced enzymatically (goethite or lepidocrocite). Our results suggest the mineralogy and surface area of Fe(III) minerals produced under nitrate-reducing conditions affect the rate and extent of U(IV) oxidation. These results may be useful for predicting the stability of U(IV) in aquifers.