Sulfate reduction and the rate of deposition of marine sediments

The initial gradient of dissolved sulfate in the pore waters of anoxic marine sediments, representing a wide variety of environments from the deep-sea to estuaries, has been found to be directly proportional, within a factor of two, to the rate of sedimentation. Data from all areas considered, except the Mississippi Delta, fall along the same straight line. The linear proportionality can be explained on the basis of a theoretical model which assumes that organic matter decomposition by sulfate-reducing bacteria, plus associated fermentative micro-organisms, is first order with respect to the concentration of metabolizable organic matter. The model also assumes that the reactivity of metabolizable organic matter varies considerably from sediment to sediment while its concentration remains essentially constant. It is likely that the proportionality observed here also applies to other sediments and, thus, the initial sulfate gradient may be a useful parameter for estimating the rate of deposition of anoxic sediments.     

Rate control in dissolution of alkali feldspars—I. Study of residual feldspar grains by X-ray photoelectron spectroscopy

Sanidine grains (100–600 μm in diameter) were subjected to dissolution at 82°C in aqueous electrolyte solutions of pH ranging from 4 to 8 for 293 or 377 hr. Dissolution equivalent to the removal of silica from the outer 300–900 A of these grains was accomplished. The shallow subsurfaces of feldspar grains were then analyzed for K, Al, and Si by X-ray photoelectron spectroscopy. The results rule out any continuous precipitate layer; if an alkali-depleted subsurface zone (leached layer) was present in the feldspar, the thickness of such a zone approximated by linear increase of alkali concentration with depth was not more than about 17 Å.It is concluded that in the absence of a compact precipitate layer, dissolution of feldspars in the temperature range corresponding to deep diagenesis is controlled by the processes at the feldspar-solution interface and a leached layer more than one feldspar unit cell thick does not form. Whether the same applies at the temperatures of shallow diagenesis and weathering cannot be judged with certainty, but parallels with leached layers on alkali silicate glasses suggest that it does.     

X-ray photoelectron studies of the mechanism of iron silicate dissolution during weathering

Iron silicate minerals (bronzite, fayalite), exposed to aqueous dissolution in the laboratory for up to 60 days at room temperature and pH 1, 1.5, and 6, have been studied for evidence of changes in surface composition, using XPS, and these results compared with those obtained from solution chemical analysis. In the absence of dissolved O₂ or at low pH (1–1.5) dissolution proceeds congruently after the initial formation of a thin (<10 Å) protonated surface layer depleted in Fe relative to Si. This layer is unstable and does not grow with time as attested to by long term congruent dissolution and by the formation of an amorphous silica surficial breakdown product at pH 1 and 1.5. In bronzite the layer is also slightly depleted in Mg but much less than it is in Fe due to the preferential occupation by Fe⁺² of more weakly bonded M₂ sites. The behavior of the layer is similar to that found earlier on iron-free pyroxene (Schottet al., 1981); in other words, because of its thinness and instability it is not diffusion-inhibiting or protective toward dissolution.In the presence of dissolved O₂, as would be the case in most weathering solutions, dissolution of bronzite and fayalite results in the formation of two surface layers whose compositions were deduced by measurements of XPS binding energies. The outer layer, consisting of hydrous ferric oxide, is readily removed by ultrasonic cleaning and, most likely, is not protective toward dissolution. The inner layer consists of Fe⁺³ in a protonated or hydroxylated silicate (Mg-silicate in the case of bronzite) matrix. This layer appears to impede dissolution over the time scale of the experiment as attested to by parabolic dissolution rates. However, the layer does not continue to grow on the time scale of weathering because ultrasonically cleaned soil grains (Berner and Schott, 1982) exhibit surface compositions similar to those found in the present month-long laboratory experiments. In other words, a thick, highly altered, diffusion-inhibiting, protective surface layer does not form at the acidic pH of most soils.     

Burial of organic carbon and pyrite sulfur in sediments over phanerozoic time: a new theory

In present day marine sediments, almost all of which are deposited in normal oxygenated seawater, rates of burial of organic carbon (C) and pyrite sulfur (S) correlate positively and bear a constant ratio to one another (C/S ～- 3 on a weight basis). By contrast, calculations, based on the isotopic model of Garrels and Lerman (1981), indicate that at various times during the Phanerozoic the worldwide burial ratio must have been considerably different than the present day value. This ratio change is caused by the requirement that, increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur, in order to maintain a roughly constant level of O₂ in the atmosphere. Such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments, as at present, to non-marine freshwater, or to euxinic environments, in the geologic past. A shift to predominantly freshwater burial can help explain predicted high C/S ratios in Permo-Carboniferous sediments, and a shift to euxinic environments can help explain predicted low C/S ratios during the early Paleozoic. It is demonstrated that the three environments today exhibit distinguishably different average C/S ratios.     

Mobility of arsenic in West Bengal aquifers conducting low and high groundwater arsenic. Part II: Comparative geochemical profile and leaching study

Aquifer sediments from areas of low- and high-As groundwater were characterized mineralogically and geochemically at a field site in the Nadia district of West Bengal, India. Leaching experiments and selective extraction of the sediments were also carried out to understand the release mechanism of As in the sub-surface. The correlation between measured elements (major, minor and trace) from low- and high-As groundwater areas are only significant for As, Fe and Mn. The borehole lithology and percentage of silt and clay fraction demonstrates the dominance of finer sediments in the high-As aquifer. Multivariate analysis of the geochemical parameters showed the presence of four different mineral phases (heavy-mineral fraction, phyllosilicates/biotite/Fe-oxyhydroxides, carbonates and sulphides) in the sediments. Selective extraction of sediment reveals that amorphous Fe-oxyhydroxide acts as a potential sink for As in the sub-surface. The result is consistent with microbially mediated redox reactions, which are controlled in part by the presence of natural organic matter within the aquifer sediments. The occurrences of As-bearing redox traps, primarily formed of Fe- and Mn-oxides/hydroxides, are also important factors that control the release of As into groundwater at the study site.     

Gas and fluid venting at the Makran accretionary wedge off Pakistan

The Makran accretionary complex shows a distinct bottom-simulating reflector, indicating a thick gas-hydrate-bearing horizon between the deformational front and about 1350 m water depth which seals off the upward flow of gas-charged fluids. A field of presently inactive mud diapirs with elevations up to 65 m was discovered in the abyssal plain seawards of the deformation front, suggesting that in the past conditions were favorable for periodic but localized vigorous mud diapirism. Regional destabilization of the gas hydrate leading to focused flow was observed where deep-penetrating, active faults reach the base of the gas-hydrate layer, as in a deeply incised submarine canyon (2100–2500 m water depth). At this location we discovered seeps of methane and H₂S-rich fluids associated with chemoautotrophic vent faunas (e.g., Calyptogena sp.). Driven by the accretionary wedge dynamics, the landward part of the gas-hydrate layer below the Makran margin is being progressively uplifted. Due to reduced hydrostatic pressure and rising ocean bottom-water temperatures, gas hydrates are progressively destabilized and dissociated into hydrate water, methane and H₂S. Sediment temperatures lie outside the methane stability field wherever water depth is less than 800 m. Above this depth, upward migration of fluids to the seafloor is unimpeded, thus explaining the abundance of randomly distributed gas seeps observed at water depths of 350 to 800 m. Received: 14 June 1999 / Revision accepted: 6 February 2000