William J. Ullman

Laboratory evidence for microbially mediated silicate mineral dissolution in nature

Abstract Bacteria may potentially enhance or inhibit silicate mineral dissolution in nature by a variety of mechanisms. In the laboratory, some microbial metabolites enhanced dissolution rates by a factor of ten above the expected proton-promoted rate by an additional ligand-promoted mechanism focussed principally at Al sites at the mineral surface. In investigations with bacteria, it was found that organic acids are produced in organic-rich/nutrient-poor cultures, resulting in increased mineral dissolution rates compared to abiotic controls. Alginate and poly-aspartate inhibited dissolution rates either by a reduction in surface reactivity or reactive surface area (or both). Bacteria may also influence dissolution rates by creating and maintaining microenvironments where metabolite concentrations are higher than in the bulk solution.

FELDSPAR DISSOLUTION IN ACIDIC AND ORGANIC SOLUTIONS : COMPOSITIONAL AND PH DEPENDENCE OF DISSOLUTION RATE

The steady-state dissolution rates of plagioclase feldspars into inorganic acid solutions in a flow-through reactor increased with Al content of the mineral from 1.4 · 10−11 mol Si/m2/s for albite to 5.6 · 10−9 mol Si/m2/s for bytownite. A similar trend was observed for minerals dissolved in neutral solutions although the rates were lower. The results of these experiments are used to develop a simple empirical equation to describe the dissolution of tectosilicates (quartz + feldspars): RH=kHaH+nH where RH is the dissolution rate of tectosilicates in acid solution, aH+ is the activity of H+ ion, and kH and nH are dependent on the aluminum fraction in the tectosilicate framework [AlAl+Si]:logkH=−11.24+25.98*[AlAl+Si]2andnH=−0.052+4.23*[AlAl+Si]2. This model, with its strong dependence on Al fraction, suggests that tectosilicate dissolution in acid solution results primarily from attack at Al sites at the mineral surface. In acidic oxalate solutions the steady-state dissolution rates were, in some cases, up to a factor of 10 higher than dissolution rates in inorganic solutions at the same pH and appeared to have a similar dependence on pH and mineral composition, at least away from the extremes in aluminum fraction (quartz and bytownite). On the basis of the results of the experiments with acidic oxalate and previous experiments showing a linear dependence of feldspar dissolution rate on organic ligand concentration, an empirical expression for the ligand-promoted component of tectosilicate dissolution rates as measured by silica release (RL) is proposed: RL = (κHL [L] − kH)aH+n + ℝHL(Si) where the first term describes the effect of competitive proton and ligand attack at Al sites at the mineral surface leading to silica release to solution and RHL(Si) reflects the smaller rate of attack at Si sites (κHL is a factor depending on the ligand, [L] is the ligand concentration, kH and aH+ are as given above, and n describes the pH dependence of ligand- and proton-promoted dissolution and is taken to be equal to nH away from the extremes of aluminum fraction). The strong dependence of dissolution rate in acidic organic solutions on aluminum fraction indicates that both protons and ligands attack the mineral surface at the same, presumably Al, sites.

Enhanced dissolution of silicate minerals by bacteria at near-neutral pH

Previous studies have shown that various microorganisms can enhance the dissolution of silicate minerals at low (<5) or high (>8) pH. However, it was not known if they can have an effect at near-neutral pH. Almost half of 17 isolates examined in this study stimulated bytownite dissolution at near-neutral pH while in a resting state in buffered glucose. Most of the isolates found to stimulate dissolution also oxidized glucose to gluconic acid. More detailed analysis with one of these isolates suggested that this partial oxidation was the predominant, if not sole, mechanism of enhanced dissolution. Enhanced dissolution did not require direct contact between the dissolving mineral and the bacteria. Gluconate-promoted dissolution was also observed with other silicate minerals such as albite, quartz, and kaolinite.

Early chemical diagenesis, sediment-water solute exchange, and storage of reactive organic matter near the mouth of the Changjiang, East China Sea

Abstract A substantial proportion of the material delivered to the modern oceans is supplied by a few large rivers such as the Changjiang. Early diagenetic reactions in surficial bottom sediments determine in large part both the eventual influence of these rivers on the sea and the nature of sedimentary deposits formed. The region off the mouth of the Changjiang exemplifies the interplay between physical, chemical, and biological factors which can produce particular spatial patterns of diagenesis and sediment-water exchange. To examine these patterns measurement of pore water solute profiles, sediment-water solute fluxes, and solute reaction rates in the upper few decimeters of sediment were made at 27 stations near the Changjiang in the East China Sea. Direct measurements of dissolved Si(OH)4, NH4+, and NO3−, fluxes from or into bottom sediments made during summer and autumn periods (15 to 24°C) range from 0.13 to 13.2, −2.6 to 3.4, and −1.4 to 3.2mmol m−2 day−1, respectively. Net solute flux from the sea floor is often lowest from deposits having the highest interstitial solute concentrations. In addition, bottom regions having the highest build up of reaction products or depletion of reactants in pore waters (with respect to overlying water) actually have the lowest rates of reaction. These same areas of elevated (products) or depleted (reactants) pore water solute concentrations, low reaction rates, and low net rate of solute exchange which are located near the mouth of the Changjiang are sites of high sedimentation rates and depauperate benthic communities. High water turbidity and resuspension apparently hinder water column production and input of reactive organic matter or other biogenic material which drive many diagenetic reactions. Rapid sedimentation or disturbance hinders benthic community development, lowers biogenic reworking, and allows build up or depletion of reaction products or reactants in bottom sediments. Offshore areas of lower sedimentation, higher productivity, and active bottom communities are sites of high initial reaction rates and increased sediment-water solute exchange compared with rapid sedimentation regions. A diagenetic paradox resulting from the interaction between benthic communities and the physical environment of sedimentation is that proportionally the greatest storage of diagenetic products related to organic matter decomposition can occur in sediments that are initially the least diagenetically reactive.

Late quaternary environments of the Carpentaria Basin, Australia

Abstract Quaternary variations in the level of the sea have exposed the shallow ( 29,000 km2) but shallow (maximum depth s 4 0 kyr. The pollen assemblages closely resemble the black soil plains presently found along the southern Gulf. Thus, the biogeographic and climatic barrier across the Australia Papua New Guinea land bridge has remained largely intact in spite of the range of environmental conditions that have occured during the last

Dissolved nutrient fluxes from the nearshore sediments of Bowling Green Bay, central Great Barrier Reef Lagoon (Australia)☆

The fluxes of dissolved inorganic N, P, and Si from the nearshore sediments of the Great Barrier Reef Lagoon are significantly lower than those reported from sediments in temperate regions at similar temperatures. The directly measured fluxes range from −23 to +28, −154 to +890, and −990 to +1750 μmol m−2 day−1 for PO43−, ΣN(=NH4+ + NO2− + NO3−) and Si, respectively. Estimates suggest that sediments are the major source of dissolved N to near-shore waters of the Lagoon greatly exceeding the dissolved flux from rivers. Resuspension of up to 1 cm of sediment during storms would have a very small effect on the PO43− or Si(OH)4 concentration of the overlying water, but would significantly raise the ΣN concentration. The productivity of these waters may be controlled at various times by the balance between the steady-state sedimentary flux of nutrients, the fluvial input, and storm resuspension.

Dissolved iodine flux from estuarine sediments and implications for the enrichment of iodine at the sediment water interface

Abstract During the anaerobic decomposition of organic matter in sediments iodine is released into solution. Three techniques have been applied to independently estimate the resulting flux of soluble I from the sediments to the overlying water of Mud Bay, Georgetown, South Carolina. Flux estimates (summer) range between ~ 5 and 41 μmol/m 2 /day. The estimates predicted from either the pore water I concentration gradient across the sediment-water interface or the dissolved I production rate are higher than the apparent flux measured directly at the same site. This suggests that I which is released to the pore water under the anoxic conditions below the sediment surface reacts with a sedimentary component at or near the sediment water interface and is lost from solution.

The effect of microbial glucose metabolism on bytownite feldspar dissolution rates between 5° and 35°C

Abstract The rate of Si release from dissolving bytownite feldspar in abiotic batch reactors increased as temperatures increased from 5° to 35°C. Metabolically inert subsurface bacteria (bacteria in solution with no organic substrate) had no apparent effect on dissolution rates over this temperature range. When glucose was added to the microbial cultures, the bacteria responded by producing gluconic acid, which catalyzed the dissolution reaction by both proton- and ligand-promoted mechanisms. The metabolic production, excretion, and consumption of gluconic acid in the course of glucose oxidation, and therefore, the degree of microbial enhancement of mineral dissolution, depend on temperature. There was little accumulation of gluconic acid and therefore, no significant enhancement of mineral dissolution rates at 35°C compared to the abiotic controls. At 20°C, gluconate accumulated in the experimental solutions only at the beginning of the experiment and led to a twofold increase in dissolved Si release compared to the controls, primarily by the ligand-promoted dissolution mechanism. There was significant accumulation of gluconic acid in the 5°C experiment, which is reflected in a significant reduction in pH, leading to 20-fold increase in Si release, primarily attributable to the proton-promoted dissolution mechanism. These results indicate that bacteria and microbial metabolism can affect mineral dissolution rates in organic-rich, nutrient-poor environments; the impact of microbial metabolism on aluminum silicate dissolution rates may be greater at lower rather than at higher temperatures due to the metabolic accumulation of dissolution-enhancing protons and ligands in solution.

SEAFLOOR STABILITY IN CENTRAL LONG ISLAND SOUND: Part I. Temporal Changes In Erodibility of Fine-Grained Sediment

Abstract: This preliminary study documents temporal changes in seafloor erodibility at a 14-meter-deep mud bottom station in central Long Island Sound over a 29-month period. The mean critical rolling and saltation velocities of the bottom were determined in the laboratory with a specially constructed flume containing salt water. A removable 45-cm length of the 10 cm by 10 cm closed flume channel was used as a box core. A removable bottom on the core box allowed scuba divers to obtain relatively undisturbed samples of the seafloor. Samples were returned to the laboratory in thermally insulated containers and critical erosion velocities were determined within a few hours. In 1974 and 1975, minimum annual mean rolling velocities were measured in July (16–19 cm sec -1 at z = 100 cm). Maximum mean rolling velocities were recorded in November 1974 and October 1975 (respectively 28 cm sec -1 and 23 cm sec -1 at z = 100 cm). The observed change in mean critical velocities in 1976 was different from that observed in 1974 and 1975. The minimum mean rolling velocity in 1976 was recorded in October (19 cm sec -1 at z = 100 cm) and the peak threshold velocity was measured in July (28 cm sec -1 at z = 100 cm). We propose tentatively that two opposing biogenic processes influence the observed changes in critical velocities; stabilization by sediment binding and destabilizaton by bioturbation. An estimate of the influence of microbial growth and mucus binding on bottom erodibility was obtained by culturing microorganisms on beds of glass beads of various sizes in the flume. Critical rolling velocities of glass beads increased 25% to 60% after 3–15 days related to mucus production and binding of the beads. Seafloor stabilization by sedentary polychaetes was also studied in the laboratory by comparing mean critical rolling velocities of natural sediment without tube-forming polychaetes with mean rolling velocities of the same sediment after introducing dense aggregations of Heteromastus filiformis. Critical rolling velocities increased by 80% over a period of 11 days.

Ground waters with unradiogenic 87Sr/86Sr ratios in the Great Artesian Basin, Australia

Ground water from the major Jurassic aquifer in the Great Artesian Basin, Australia, shows significant variation in calcium and strontium concentration and strontium isotopic composition with age and distance from the recharge area. Hydrologically young bore waters in the eastern part of the basin exhibit surprisingly unradiogenic 87 Sr/ 86 Sr ratios (0.7045 to 0.7054). By contrast, hydrologically older waters in the central and discharge regions of the basin have significantly more radiogenic isotopic compositions (0.7060 to 0.7118). Waters with unradiogenic 87 Sr/ 86 Sr ratios are interpreted to be the result of reaction between recharge water in the aquifer and lithologies that have juvenile 87 Sr/ 86 Sr compositions. These juvenile strontium isotope signatures most likely reflect interaction with Cenozoic mafic igneous rocks which occur on the eastern side of the basin. More radiogenic waters in the basin are the result of addition of 87 Sr derived by in situ dissolution of aquifer silicates.