Enhancing understanding of breakdown and collapse in the Yorkshire Dales using ground penetrating radar on cave sediments

Historical evidence shows block breakdown and collapse are actively occurring in large fault aligned caverns in the Yorkshire Dales karst. Deployment of ground penetrating radar at two such sites has provided detailed images of the sedimentary sequences below the present day cavern floor but no large blocks are imaged within the sediments. Solutional processes must be removing limestone from the sediment to allow continued cavern growth. Possible mechanisms to account for the lack of large blocks within the sediment fill are discussed.     

Assessing soil erosion in Mediterranean karst landscapes of Lebanon using remote sensing and GIS

Maps showing the potential for soil erosion at 1:100,000 scale are produced in a study area within Lebanon that can be used for evaluating erosion of Mediterranean karstic terrain with two different sets of impact factors built into an erosion model. The first set of factors is: soil erodibility, morphology, land cover/use and rainfall erosivity. The second is obtained by the first adding a fifth factor, rock infiltration. High infiltration can reflect high recharge, therefore decreasing the potential of surface runoff and hence the quantity of transported materials. Infiltration is derived as a function of lithology, lineament density, karstification and drainage density, all of which can be easily extracted from satellite imagery. The influence of these factors is assessed by a weight/rate approach sharing similarities between quantitative and qualitative methods and depending on pair-wise comparison matrix.The main outcome was the production of factorial maps and erosion susceptibility maps (scale 1:100,000). Spatial and attribute comparison of erosion maps indicates that the model that includes a measure of rock infiltration better represents erosion potential. Field investigation of rills and gullies shows 87.5% precision of the model with rock infiltration. This is 17.5% greater than the precision of the model without rock infiltration. These results indicate the necessity and importance of integrating information on infiltration of rock outcrops to assess soil erosion in Mediterranean karst landscapes.     

Spectroscopic investigation of uranyl(VI) and citrate coadsorption to Al2O3

ATR-FTIR spectroscopy is used to understand the adsorption of uranyl-citrate complexes to Al₂O₃. Spectral data indicate that uranyl-citrate complexes partially dissociate upon adsorption, allowing full or partial hydrolysis of the uranyl ion. K_ads values determined for free citrate adsorption are similar to those for citrate in uranyl-citrate complexes, indicating that the complexation of uranyl by citrate does not significantly affect the ability of citrate to bond with the surface. The isotherm data also indicate enhanced citrate adsorption to Al₂O₃ in the presence of uranyl, suggesting that uranyl may be the central link between two citrate ligands, and that uranyl is associated with the surface through a bridging citrate ligand. Finally, uranyl-citrate complexes interact with citrate adsorbed to Al₂O₃ through outer sphere interactions.     

An absolute paleotemperature record from 10 to 6 Ka inferred from fluid inclusion D/H ratios of a stalagmite from Vancouver Island, British Columbia, Canada

By using continuous helium flow during the crushing of calcite speleothem samples, we are able to recover liberated inclusion waters without isotopic fractionation. A paleotemperature record for the Jacklah Jill Cave locality, Vancouver Island, BC, was obtained from a 30-cm tall stalagmite that grew 10.3-6.3 Ka ago, using δ¹⁸O values of the crushed calcite and of the inclusion water as inferred from its δD. It is found that the locality experienced mean annual temperature variations up to 11 °C over a 4-Ka period in the early Holocene. At the beginning of the period, local temperature quickly increased from a minimum of ∼1 °C to around 10 °C, but this early climate optimum, about 3 °C warmer than today, only lasted for ∼1200 years. About 8.6 Ka ago, temperature had declined to ∼7 °C, approximately the same as the modern cave temperature. Since then, the study area has experienced only minor temperature fluctuations, but there was a brief fall to ∼4 °C at around 7 Ka ago, which might be caused by a short lived expansion of local alpine glaciers. The long-term T-dependence of δD was 1.47‰/°C, identical to the value in modern precipitation.     

Silicon isotope fractionation in bamboo and its significance to the biogeochemical cycle of silicon

A systematic investigation on silica contents and silicon isotope compositions of bamboos was undertaken. Seven bamboo plants and related soils were collected from seven locations in China. The roots, stem, branch and leaves for each plant were sampled and their silica contents and silicon isotope compositions were determined. The silica contents and silicon isotope compositions of bulk and water-soluble fraction of soils were also measured. The silica contents of studied bamboo organs vary from 0.30% to 9.95%. Within bamboo plant the silica contents show an increasing trend from stem, through branch, to leaves. In bamboo roots the silica is exclusively in the endodermis cells, but in stem, branch and leaves, the silica is accumulated mainly in epidermal cells. The silicon isotope compositions of bamboos exhibit significant variation, from −2.3‰ to 1.8‰, and large and systematic silicon isotope fractionation was observed within each bamboo. The δ³⁰Si values decrease from roots to stem, but then increase from stem, through branch, to leaves. The ranges of δ³⁰Si values within each bamboo vary from 1.0‰ to 3.3‰. Considering the total range of silicon isotope composition in terrestrial samples is only 7‰, the observed silicon isotope variation in single bamboo is significant and remarkable. This kind of silicon isotope variation might be caused by isotope fractionation in a Rayleigh process when SiO₂ precipitated in stem, branches and leaves gradually from plant fluid. In this process the Si isotope fractionation factor between dissolved Si and precipitated Si in bamboo (α_pre-sol) is estimated to be 0.9981. However, other factors should be considered to explain the decrease of δ³⁰Si value from roots to stem, including larger ratio of dissolved H₄SiO₄ to precipitated SiO₂ in roots than in stem. There is a positive correlation between the δ³⁰Si values of water-soluble fractions in soils and those of bulk bamboos, indicating that the dissolved silicon in pore water and phytoliths in soil is the direct sources of silicon taken up by bamboo roots. A biochemical silicon isotope fractionation exists in process of silicon uptake by bamboo roots. Its silicon isotope fractionation factor (α_bam-wa) is estimated to be 0.9988. Considering the distribution patterns of SiO₂ contents and δ³⁰Si values among different bamboo organs, evapotranspiration may be the driving force for an upward flow of a silicon-bearing fluid and silica precipitation. Passive silicon uptake and transportation may be important for bamboo, although the role of active uptake of silicic acid by roots may not be neglected. The samples with relatively high δ³⁰Si values all grew in soils showing high content of organic materials. In contrast, the samples with relatively low δ³⁰Si values all grew in soil showing low content of organic materials. The silicon isotope composition of bamboo may reflect the local soil type and growth conditions. Our study suggests that bamboos may play an important role in global silicon cycle.     

Evidence for crustal degassing of CF4 and SF6 in Mojave Desert groundwaters

Dissolved tetrafluoromethane (CF₄) and sulfur hexafluoride (SF₆) concentrations were measured in groundwater samples from the Eastern Morongo Basin (EMB) and Mojave River Basin (MRB) located in the southern Mojave Desert, California. Both CF₄ and SF₆ are supersaturated with respect to equilibrium with the preindustrial atmosphere at the recharge temperatures and elevations of the Mojave Desert. These observations provide the first in situ evidence for a flux of CF₄ from the lithosphere. A gradual basin-wide enhancement in dissolved CF₄ and SF₆ concentrations with groundwater age is consistent with release of these gases during weathering of the surrounding granitic alluvium. Dissolved CF₄ and SF₆ concentrations in these groundwaters also contain a deeper crustal component associated with a lithospheric flux entering the EMB and MRB through the underlying basement. The crustal flux of CF₄, but not of SF₆, is enhanced in the vicinity of local active fault systems due to release of crustal fluids during episodic fracture events driven by local tectonic activity. When fluxes of CF₄ and SF₆ into Mojave Desert groundwaters are extrapolated to the global scale they are consistent, within large uncertainties, with the fluxes required to sustain the preindustrial atmospheric abundances of CF₄ and SF₆.     

Association of dissolved aluminum with silica: Connecting molecular structure to surface reactivity using NMR

We studied uptake mechanisms for dissolved Al on amorphous silica by combining bulk-solution chemistry experiments with solid-state Nuclear Magnetic Resonance techniques (²⁷Al magic-angle spinning (MAS) NMR, ²⁷Al{¹H} cross-polarization (CP) MAS NMR and ²⁹Si{¹H} CP-MAS NMR). We find that reaction of Al (1 mM) with amorphous silica consists of at least three reaction pathways; (1) adsorption of Al to surface silanol sites, (2) surface-enhanced precipitation of an aluminum hydroxide, and (3) bulk precipitation of an aluminosilicate phase. From the NMR speciation and water chemistry data, we calculate that 0.20 (±0.04) tetrahedral Al atoms nm⁻² sorb to the silica surface. Once the surface has sorbed roughly half of the total dissolved Al (∼8% site coverage), aluminum hydroxides and aluminosilicates precipitate from solution. These precipitation reactions are dependent upon solution pH and total dissolved silica concentration. We find that the Si:Al stoichiometry of the aluminosilicate precipitate is roughly 1:1 and suggest a chemical formula of NaAlSiO₄ in which Na⁺ acts as the charge compensating cation. For the adsorption of Al, we propose a surface-controlled reaction mechanism where Al sorbs as an inner-sphere coordination complex at the silica surface. Analogous to the hydrolysis of , we suggest that rapid deprotonation by surface hydroxyls followed by dehydration of ligated waters results in four-coordinate (>SiOH)₂Al(OH)₂ sites at the surface of amorphous silica.     

Low-oxygen and chemical kinetic constraints on the geochemical niche of neutrophilic iron(II) oxidizing microorganisms

Neutrophilic iron oxidizing bacteria (FeOB) must actively compete with rapid abiotic processes governing Fe(II) oxidation and as a result have adapted to primarily inhabit low-O₂ environments where they can more successfully compete with abiotic Fe(II) oxidation. The spatial distribution of these microorganisms can be observed through the chemical gradients they affect, as measured using in situ voltammetric analysis for dissolved Fe(II), Fe(III), O₂, and FeS_(aq). Field and laboratory determination of the chemical environments inhabited by the FeOB were coupled with detailed kinetic competition studies for abiotic and biotic oxidation processes using a pure culture of FeOB to quantify the geochemical niche these organisms inhabit. In gradient culture tubes, the maximum oxygen levels, which were associated with growth bands of Sideroxydans lithotrophicus (ES-1, a novel FeOB), were 15-50 μM. Kinetic measurements made on S. lithotrophicus compared biotic/abiotic (killed control) Fe oxidation rates. The biotic rate can be a significant and measurable fraction of the total Fe oxidation rate below O₂ concentrations of approximately 50 μM, but biotic Fe(II) oxidation (via the biotic/abiotic rate comparison) becomes difficult to detect at higher O₂ levels. These results are further supported by observations of conditions supporting FeOB communities in field settings. Variablity in cell densities and cellular activity as well as variations in hydrous ferrous oxide mineral quantities significantly affect the laboratory kinetic rates. The microbial habitat (or geochemical niche) where FeOB are active is thus largely controlled by the competition between abiotic and biotic kinetics, which are dependent on Fe(II) concentration, P_O2, temperature and pH in addition to the surface area of hydrous ferric oxide minerals and the cell density/activity of FeOB. Additional field and lab culture observations suggest a potentially important role for the iron-sulfide aqueous molecular cluster, FeS_(aq), in the overall cycling of iron associated with the environments these microorganisms inhabit.     

Anaerobic oxidation of methane (AOM) in marine sediments from the Skagerrak (Denmark): II. Reaction-transport modeling

A steady-state reaction-transport model is applied to sediments retrieved by gravity core from two stations (S10 and S13) in the Skagerrak to determine the main kinetic and thermodynamic controls on anaerobic oxidation of methane (AOM). The model considers an extended biomass-implicit reaction network for organic carbon degradation, which includes extracellular hydrolysis of macromolecular organic matter, fermentation, sulfate reduction, methanogenesis, AOM, acetogenesis and acetotrophy. Catabolic reaction rates are determined using a modified Monod rate expression that explicitly accounts for limitation by the in situ catabolic energy yields. The fraction of total sulfate reduction due to AOM in the sulfate-methane transition zone (SMTZ) at each site is calculated. The model provides an explanation for the methane tailing phenomenon which is observed here and in other marine sediments, whereby methane diffuses up from the SMTZ to the top of the core without being consumed. The tailing is due to bioenergetic limitation of AOM in the sulfate reduction zone, because the methane concentration is too low to engender favorable thermodynamic drive. AOM is also bioenergetically inhibited below the SMTZ at both sites because of high hydrogen concentrations (∼3-6 nM). The model results imply there is no straightforward relationship between pore water concentrations and the minimum catabolic energy needed to support life because of the highly coupled nature of the reaction network. Best model fits are obtained with a minimum energy for AOM of ∼11 kJ mol⁻¹, which is within the range reported in the literature for anaerobic processes.     

The effect of species diversity on metal adsorption onto bacteria

In this study, we measure proton, Pb, and Cd adsorption onto the bacteria Deinococcus radiodurans, Thermus thermophilus, Acidiphlium angustum, Flavobacterium aquatile, and Flavobacterium hibernum, and we calculate the thermodynamic stability constants for the important surface complexes. These bacterial species represent a wide genetic diversity of bacteria, and they occupy a wide range of habitats. All of the species, except for A. angustum, exhibit similar proton and metal uptake. The only species tested that exhibits significantly different protonation behavior is A. angustum, an acidophile that grows at significantly lower pH than the other species of this study. We demonstrate that a single, metal-specific, surface complexation model can be used to reasonably account for the acid/base and metal adsorption behaviors of each species. We use a four discrete site non-electrostatic model to describe the protonation of the bacterial functional groups, with averaged pK_a values of 3.1 ± 0.3, 4.8 ± 0.2, 6.7 ± 0.1, and 9.2 ± 0.3, and site concentrations of (1.0 ± 0.17) × 10⁻⁴, (9.0 ± 3.0) × 10⁻⁵, (4.6 ± 1.8) × 10⁻⁵, and (6.1 ± 2.3) × 10⁻⁵ mol of sites per gram wet mass of bacteria, respectively. Adsorption of Cd and Pb onto the bacteria can be accounted for by the formation of complexes with each of the bacterial surface sites. The average log stability constants for Cd complexes with Sites 1-4 are 2.4 ± 0.4, 3.2 ± 0.1, 4.4 ± 0.1, and 5.3 ± 0.1, respectively. The average log stability constants for Pb complexes with Sites 1-4 are 3.3 ± 0.2, 4.5 ± 0.3, 6.5 ± 0.1, and 7.9 ± 0.5, respectively. This study demonstrates that a wide range of bacteria exhibit similar proton and metal adsorption behaviors, and that a single set of averaged acidity constants, site concentrations, and stability constants for metal-bacterial surface complexes yields a reasonable model for the adsorption behavior of many of these species. The differences in adsorption behavior that we observed for A. angustum demonstrate that genetic differences do exist between the cell wall functional group chemistries of some bacterial species, and that significant exceptions to the typical bacterial adsorption behavior do exist.