Changes In Geochemical Conditions Research Articles

A record of change in oyster environment through high-resolution geochemical analysis of Late-Holocene sediments from Coastal Ghana

The near-coast environments where oysters occur are among the most impacted by humans globally, especially during the Late-Holocene. Yet, in West Africa, there is no documented historical record of change in these environments. We provide insight into the changing geochemical conditions of two oyster environments through high-resolution analysis of total organic carbon (C), total nitrogen (N), carbon and nitrogen isotope ratios (δ13C, δ15N), and trace elements, in two cores retrieved from the Densu estuary and the Anyanui (Keta) Creek in Ghana. Drastic shifts in sedimentation rate occurred in the Keta and Densu cores around 1996 CE and 960 CE respectively. At these times, comparatively, low levels of C and N were found in the Densu core. Increasing C and N levels and decreasing δ13C upcore aligned with the observed shift in sedimentation rate in the Keta core. The C/N ratios in the Keta core suggest allochthonous organic matter (OM) dominance in the creek. The Densu core showed periodic changes in C/N ratios from very high values (>20) between 1918 BCE and 1321 BCE, to values between 20 and 11 between 1321 BCE and 1977 CE and below 10 from the late 1970s CE to the present day, suggesting a varying degree of transformation in the catchment basin. Extremely high Sulfur (S) and moderate to significant Iron (Fe) increases suggest reducing conditions in the Keta sediments. Moderate Calcium (Ca), Zinc (Zn), and Strontium (Sr) concentrations in the upper part of the Densu core suggest a stronger influence of marine processes in the Densu in recent times. The findings reflect the impacts of catchment basin modification on the health of the two coastal environments, likely to impact the growth, productivity, and sustainability of the fishery of the West African mangrove Oyster.

Mineral transformation and dissolution of jarosite coprecipitated with hazardous oxyanions and their mobility changes

Jarosite coprecipitation with hazardous oxyanions can attenuate the concentrations of these elements in acid mine drainage. However, jarosite can be easily transformed to goethite with changes in geochemical conditions. Consequently, the released oxyanions can greatly affect environments. The changes in the mineralogy and mobility of five oxyanions, namely AsO4, SeO3, SeO4, MoO4, and CrO4, which were coprecipitated with jarosite, are investigated herein during the mineral transformation. Our results show that the oxyanion species and the pH values greatly affect the mineral transformation and dissolution rates of jarosite-containing oxyanions. The transformation and dissolution rates of the jarosite samples at pH 8 are noticeably higher than those at pH 4. The X-ray diffraction results show that the CrO4 and SeO4 jarosites are as effectively transformed to goethite as the jarosite without oxyanions, while the SeO3 and AsO4 jarosites are least transformed, resulting in different sulfate and oxyanion concentrations in the solution. The oxyanions in jarosite are the main controlling factor in the mineral transformation and dissolution rates. In acid mine drainage, although CrO4 is easily attenuated by the jarosite precipitation, it has the highest mobility during the goethite transformation. On the contrary, AsO4 shows the opposite case.

Rare earth elements in Uranium ore for nuclear forensic application

The contents of the Lanthanides, which are Rare Earth Elements (REE), that show consistent patterns under changing geochemical conditions in uranium ore have been studied. The objective is to determine the nuclear forensic parameters inherent to the various uranium ore samples collected from parts of northern Nigeria using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analytical technique for the purpose of baseline data generation and nuclear forensic application. The REE fingerprint obtained in this study suggests that the uranium is of uraninite/pitchblende granitic (vein-type) origin. The REE concentrations varied from 0.011 ± 0.000 ppm to 2.337 ± 0.082 ppm for Riruwai, 0.001 ± 0.000 ppm to 0.035 ± 0.002 ppm for Mika-I, 0.002 ± 0.000 ppm to 0.357 ± 0.007 ppm for Mika-II and 0.001 ± 0.001 ppm to 0.159 ± 0.005 ppm for Michika. A positive correlation between the REEs showed symmetrical chemical properties as well as their overall presence in the samples. The normalised REE/Cl-Chondrites pattern show Light Rare Earth Elements(LREE) enrichment, Heavy Rare Earth Elements (HREE) fractionation with a negative Europium (Eu) anomaly. The ΣLREE/ΣHREE ratio presented varied values for the samples while a negative value of 143Nd/144Nd ratio (0.512638) in the Chondritic Uniform Reservoir (CHUR) ε Nd(-0.64285) suggested LREE uniformity and radiogenic origin.

In-field bioreactors demonstrate dynamic shifts in microbial communities in response to geochemical perturbations.

Subsurface microbial communities mediate the transformation and fate of redox sensitive materials including organic matter, metals and radionuclides. Few studies have explored how changing geochemical conditions influence the composition of groundwater microbial communities over time. We temporally monitored alterations in abiotic forces on microbial community structure using 1L in-field bioreactors receiving background and contaminated groundwater at the Oak Ridge Reservation, TN. Planktonic and biofilm microbial communities were initialized with background water for 4 days to establish communities in triplicate control reactors and triplicate test reactors and then fed filtered water for 14 days. On day 18, three reactors were switched to receive filtered groundwater from a contaminated well, enriched in total dissolved solids relative to the background site, particularly chloride, nitrate, uranium, and sulfate. Biological and geochemical data were collected throughout the experiment, including planktonic and biofilm DNA for 16S rRNA amplicon sequencing, cell counts, total protein, anions, cations, trace metals, organic acids, bicarbonate, pH, Eh, DO, and conductivity. We observed significant shifts in both planktonic and biofilm microbial communities receiving contaminated water. This included a loss of rare taxa, especially amongst members of the Bacteroidetes, Acidobacteria, Chloroflexi, and Betaproteobacteria, but enrichment in the Fe- and nitrate- reducing Ferribacterium and parasitic Bdellovibrio. These shifted communities were more similar to the contaminated well community, suggesting that geochemical forces substantially influence microbial community diversity and structure. These influences can only be captured through such comprehensive temporal studies, which also enable more robust and accurate predictive models to be developed.

The geochemical behaviour of Be and F in historical mine tailings of Yxsjöberg, Sweden

The speciation, mobility, transport, and fate of beryllium (Be) in the terrestrial environment is poorly studied even though it is considered to be one of the most hazardous elements in the periodic table. Historical tailings containing the unusual mineral danalite [Be3(Fe4.4Mn0.95Zn0.4)(SiO4)3.2S1.4] together with Fe-sulfides and fluorite has been stored open to the atmosphere for more than 50y. Environmental mineralogy, which combines geochemical and mineralogical techniques, was used to elucidate the weathering of danalite and fluorite. Danalite is unstable in oxic conditions due to the occurrence of Fe (II) and S-(II) in the crystal lattice and has oxidized at the same pace as pyrrhotite in the tailings. The acidic conditions generated from sulfide oxidation and the release of F from fluorite weathering have most likely enhanced Be mobility in the tailings. Secondary gypsum, hydrous ferric oxides and Al-oxyhydroxides are hypothesized to have played an important role regarding the mobility of Be in the tailings. The results indicate that Be released from danalite was first scavenged by these secondary minerals through co-precipitation. However, the dissolution of secondary gypsum due to changing geochemical conditions has also released Be to the groundwater. The groundwater at the shore of the tailings revealed the highest Be concentrations measured anywhere in the world (average: 4.5 mg/L) even though the water has a circumneutral pH. This extraordinary finding can be explained by high concentrations of F (73 mg/L), as F and Be have been shown to form strong complexes. The weathering of danalite and fluorite will continue for hundreds of years if remediation measures are not taken. Re-mining the tailings could be an appropriate remediation method.

Environmental filtering determines family-level structure of sulfate-reducing microbial communities in subsurface marine sediments.

Recent work has shown that subsurface microbial communities assemble by selective survival of surface community members during sediment burial, but it remains unclear to what extent the compositions of the subsurface communities are a product of their founding population at the sediment surface or of the changing geochemical conditions during burial. Here we investigate this question for communities of sulfate-reducing microorganisms (SRMs). We collected marine sediment samples from the upper 3-5 m at four geochemically contrasting sites in the Skagerrak and Baltic Sea and measured SRM abundance (quantitative PCR of dsrB), metabolic activity (radiotracer rate measurements), and community composition (Illumina sequencing of dsrB amplicons). These data showed that SRM abundance, richness, and phylogenetic clustering as determined by the nearest taxon index peaked below the bioturbation zone and above the depth of sulfate depletion. Minimum cell-specific rates of sulfate reduction did not vary substantially between sites. SRM communities at different sites were best distinguished based on their composition of amplicon sequence variants (ASVs), while communities in different geochemical zones were best distinguished based on their composition of SRM families. This demonstrates environmental filtering of SRM communities in sediment while a site-specific fingerprint of the founding community is retained.

Modeling the Ionic Strength Effect on Diffusion in Clay. The DR-A Experiment at Mont Terri

Author(s): Soler, JM; Steefel, CI; Gimmi, T; Leupin, OX; Cloet, V | Abstract: Solute diffusion in compacted clays depends on ionic strength through its control on the thickness of the electrical double layer (EDL) on the charged clay surfaces. In the DR-A field experiment (Mont Terri, Switzerland), synthetic porewater was circulated through a borehole for 189 days, leading to the out-diffusion of a variety of tracers into the Opalinus Clay. The borehole solution was then replaced with a higher-salinity solution for an additional 540 days, leading to the diffusion of Cs+, Ca2+, Mg2+, and Sr2+ back into the borehole and to an increase in the out-diffusion of anions (I-, Br-) and 3H. The experimental results were interpreted using the CrunchClay code, which includes a mean electrostatic potential model for the EDL. The EDL corresponds to a second continuum in addition to bulk electrically neutral porewater. Species-specific diffusion (Nernst-Planck equation) occurs through both domains. A 1D radial model considered a single pore diffusion coefficient (Dp = 10-9 m2/s) for cations and 3H in the bulk porosity, and a smaller Dp (3 × 10-10 m2/s) for anions. Dp values in the EDL were smaller (10-11 m2/s), except for Cs+ and K+ (5 × 10-10 and 2 × 10-10 m2/s, respectively). The model reproduced well the experimental results and showed the capability to consider temporal changes in geochemical conditions affecting the transport and retention of potentially important radionuclide contaminants (e.g., 137Cs+, 90Sr2+, 129I-) in underground geological nuclear waste repositories. Coupled multicomponent diffusion together with the electrostatic properties of the charged surfaces are essential in the development of predictive models for ion transport in clays.

Acid mine drainage affects the diversity and metal resistance gene profile of sediment bacterial community along a river

Acid mine drainage (AMD) is one of the most hazardous byproducts of some types of mining. However, research on how AMD affects the bacterial community structure of downstream riverine ecosystems and the distribution of metal resistance genes (MRGs) along pollution gradient is limited. Comprehensive geochemical and high-throughput next-generation sequencing analyses can be integrated to characterize spatial distributions and MRG profiles of sediment bacteria communities along the AMD-contaminated Hengshi River. We found that (1) diversities of bacterial communities significantly and gradually increased along the river with decreasing contamination, suggesting community composition reflected changes in geochemical conditions; (2) relative abundances of phyla Proteobacteria and genus Halomonas and Planococcaceae that function in metal reduction decreased along the AMD gradient; (3) low levels of sediment salinity, sulfate, aquatic lead (Pb), and cadmium (Cd) were negatively correlated with bacterial diversity despite pH was in a positive manner with diversity; and (4) arsenic (As) and copper (Cu) resistance genes corresponded to sediment concentrations of As and Cu, respectively. Altogether, our findings offer initial insight into the distribution patterns of sediment bacterial community structure, diversity and MRGs along a lotic ecosystem contaminated by AMD, and the factors that affect them.

Reactive Transport Modeling of Swelling Processes in Clay‐sulfate Rocks

AbstractSwelling of clay‐sulfate rocks often causes large problems in geotechnical applications such as tunneling. The primary mechanism inducing the increase in rock volume is the chemical transformation of anhydrite to gypsum, which is triggered by the ingress of groundwater. In the present study, a novel conceptual and numerical modeling approach is developed that emphasizes the effect of groundwater flow in conjunction with the associated availability of water and changing geochemical conditions on the chemical transformation of anhydrite to gypsum. A reactive transport model was developed and hydraulic, reactive, and solute transport as well as mechanical model parameters were estimated through an inversion process, constrained by geodetic ground heave measurements from a study site in Staufen, Germany. The conceptual model of the swelling process was implemented numerically through a dual‐domain modeling approach, whereby the mobile domain accounts for solute transport along discontinuities, and the immobile reactive domain represents the matrix. A mass transfer process accounts for diffusive and/or capillary water transport into the matrix, where the rate‐limited transformation of anhydrite to gypsum takes place. The model calculates heave at the land surface depending on water inflow, the transformation of anhydrite into gypsum and the local stress conditions exerted by overburden pressure. The results show that the proposed reactive transport modeling approach is suitable to quantify the observed swelling‐induced heave at the study site with a plausible parameterization. The study also highlights that diffusion is a decisive factor for the effective rate of anhydrite dissolution and, therefore, the overall chemical transformation process.

Reactive transport model for predicting arsenic transport in groundwater system in Datong Basin

High As concentration in groundwater of Datong Basin has emerged as an issue of great concern in the past decade because of its serious impact on the health of many people. The present study is to our knowledge the first attempt model the transport of arsenic in this system using a 1D reactive transport model (PHREEQC). Based on published field data from Shanyin Field Site (Datong Basin), the importance of the (bio)geochemical processes for arsenic mobility is evaluated quantitatively using geochemical models of growing complexity. Arsenic mobility during the surface water and groundwater interactions can be explained by the reductive dissolution of iron oxyhydroxides, sulfate, nitrate and arsenate, and by surface sorption/de-sorption occurring in response to changes in geochemical conditions. Both the Fe(III) surface complexation model and the 1D reactive transport model were employed to depict the arsenic transport in shallow groundwater system during the surface water-groundwater interactions. The surface complexation model results showed that a total of 0.34 mmol and 0.56 mmol arsenic were sorbed onto Fe(III) oxyhydroxides for Well 1-2S and Well 2-2S, respectively, reflecting the important role of surface sorption process on arsenic transport. The 1D model results revealed that redox processes were also critical. The improved calibration of model simulations involving both of these components indicates that the combination of processes is critical to understanding and characterising the transport of arsenic in groundwater during groundwater surface water interaction, especially close to the surface water body.

Smart Kd-values, their uncertainties and sensitivities - Applying a new approach for realistic distribution coefficients in geochemical modeling of complex systems

One natural retardation process to be considered in risk assessment for contaminants in the environment is sorption on mineral surfaces. A realistic geochemical modeling is of high relevance in many application areas such as groundwater protection, environmental remediation, or disposal of hazardous waste. Most often concepts with constant distribution coefficients (Kd-values) are applied in geochemical modeling with the advantage to be simple and computationally fast, but not reflecting changes in geochemical conditions.In this paper, we describe an innovative and efficient method, where the smart Kd-concept, a mechanistic approach mainly based on surface complexation modeling, is used (and modified for complex geochemical models) to calculate and apply realistic distribution coefficients. Using the geochemical speciation code PHREEQC, multidimensional smart Kd-matrices are computed as a function of varying (or uncertain) environmental conditions. On the one hand, sensitivity and uncertainty statements for the distribution coefficients can be derived. On the other hand, smart Kd-matrices can be used in reactive transport (or migration) codes (not shown here). This strategy has various benefits: (1) rapid computation of Kd-values for large numbers of environmental parameter combinations; (2) variable geochemistry is taken into account more realistically; (3) efficiency in computing time is ensured, and (4) uncertainty and sensitivity analysis are accessible.Results are presented exemplarily for the sorption of uranium(VI) onto a natural sandy aquifer material and are compared to results based on the conventional Kd-concept. In general, the sorption behavior of U(VI) in dependence of changing geochemical conditions is described quite well.

Arsenic-containing soil from geogenic source in Hong Kong: Leaching characteristics and stabilization/solidification

Geogenic sources of arsenic (As) have aroused extensive environmental concerns in many countries. This study evaluated the vertical profiles, leaching characteristics, and surface characteristics of As-containing soils in Hong Kong. The results indicated that elevated levels of As (486–1985 mg kg−1) were mostly encountered in deeper layer (15–20 m below ground). Despite high concentrations, geogenic As displayed a high degree of chemical stability in the natural geochemical conditions, and there was minimal leaching of As in various leaching tests representing leachability, mobility, phytoavailability, and bioaccessibility. Microscopic/spectroscopic investigations suggested that As in the soils was predominantly present as As(V) in a coordination environment with Fe oxides. Sequential extraction indicated that the majority of As were strongly bound with crystalline Fe/Al oxides and residual phase. Yet, uncertainties may remain with potential As exposure through accidental ingestion and abiotic/biotic transformation due to changes in geochemical conditions. Hence, the effectiveness of stabilization/solidification (S/S) treatment was evaluated. Although the leached concentrations of As from the S/S treated soils increased to varying extent in different batch leaching tests due to the increase in alkalinity, the mobility of As was considered very low based on semi-dynamic leaching test. This suggested that As immobilization in the S/S treated soils was predominantly dependent on physical encapsulation by interlocking framework of hydration products, which could also prevent potential exposure and allow controlled utilization of S/S treated soils as monolithic materials. These results illustrate the importance of holistic assessment and treatment/management of As-containing soils for enabling flexible future land use.

Cell Membrane Fatty Acid Composition of Chryseobacterium frigidisoli PB4T, Isolated from Antarctic Glacier Forefield Soils, in Response to Changing Temperature and pH Conditions.

Microorganisms in Antarctic glacier forefields are directly exposed to the hostile environment of their habitat characterized by extremely low temperatures and changing geochemical conditions. To survive under those stress conditions microorganisms adapt, among others, their cell membrane fatty acid inventory. However, only little is known about the adaptation potential of microorganisms from Antarctic soil environments. In this study, we examined the adaptation of the cell membrane polar lipid fatty acid inventory of Chryseobacterium frigidisoli PB4T in response to changing temperature (0°C to 20°C) and pH (5.5 to 8.5) regimes, because this new strain isolated from an Antarctic glacier forefield showed specific adaptation mechanisms during its detailed physiological characterization. Flavobacteriaceae including Chryseobacterium species occur frequently in extreme habitats such as ice-free oases in Antarctica. C. frigidisoli shows a complex restructuring of membrane derived fatty acids in response to different stress levels. Thus, from 20°C to 10°C a change from less iso-C15:0 to more iso-C17:1ω7 is observed. Below 10°C temperature adaptation is regulated by a constant increase of anteiso-FAs and decrease of iso-FAs. An anteiso- and bis-unsaturated fatty acid, anteiso-heptadeca-9,13-dienoic acid, shows a continuous increase with decreasing cultivation temperatures underlining the particular importance of this fatty acid for temperature adaptation in C. frigidisoli. Concerning adaptation to changing pH conditions, most of the dominant fatty acids reveal constant relative proportions around neutral pH (pH 6–8). Strong variations are mainly observed at the pH extremes (pH 5.5 and 8.5). At high pH short chain saturated iso- and anteiso-FAs increase while longer chain unsaturated iso- and anteiso-FAs decrease. At low pH the opposite trend is observed. The study shows a complex interplay of different membrane components and provides, therefore, deep insights into adaptation strategies of microorganisms from extreme habitats to changing environmental conditions.

Future Impacts of Hydroelectric Power Development on Methylmercury Exposures of Canadian Indigenous Communities.

Developing Canadian hydroelectric resources is a key component of North American plans for meeting future energy demands. Microbial production of the bioaccumulative neurotoxin methylmercury (MeHg) is stimulated in newly flooded soils by degradation of labile organic carbon and associated changes in geochemical conditions. We find all 22 Canadian hydroelectric facilities being considered for near-term development are located within 100 km of indigenous communities. For a facility in Labrador, Canada (Muskrat Falls) with planned completion in 2017, we probabilistically modeled peak MeHg enrichment relative to measured baseline conditions in the river to be impounded, downstream estuary, locally harvested fish, birds and seals, and three Inuit communities. Results show a projected 10-fold increase in riverine MeHg levels and a 2.6-fold increase in estuarine surface waters. MeHg concentrations in locally caught species increase 1.3 to 10-fold depending on time spent foraging in different environments. Mean Inuit MeHg exposure is forecasted to double following flooding and over half of the women of childbearing age and young children in the most northern community are projected to exceed the U.S. EPA's reference dose. Equal or greater aqueous MeHg concentrations relative to Muskrat Falls are forecasted for 11 sites across Canada, suggesting the need for mitigation measures prior to flooding.

The usefulness of otolith chemistry to determine the life history of the honeycomb grouper around Reunion Island (SW Indian Ocean)

Due to the relative geographical isolation of Reunion Island the grouper Epinephelus merra is assumed to be self-recruited but almost nothing is known about its larval history. We used elemental composition of the otoliths (Ba, Sr, Mn, Mg, Na) of fifty-eight one year-old groupers collected from four main coral reefs on the west coast of Reunion Island, to determine environmental variations during their early life history. Hierarchical clustering analysis of the otolith chemical composition of core and early larval phase, allowed the identification of three groups of larvae. Larvae from the first group (cluster 1) were born and dispersed in Ba-enriched water bodies, while the second and third groups included larvae that had crossed Ba-poor water masses. Larvae of group 1 were primarily found in the southern sector (74%) decreasing northward, while group 3 were more abundant in the northern sector (56%) and cluster 2 showed a scattered distribution. Such opposite spatial distributions might suggest that clusters 1 and 3 originated from opposite dispersal kernels, south and north respectively. It is possible however that larvae from both groups came from the same spawning ground but represented different cohorts that experienced changing geochemical conditions either at the spawning site or within the surrounding pelagic environment, over the course of the breeding season. Both scenarios on the origin and dispersion may thus suggest significant influence of hydrographic features causing or preventing larvae from becoming displaced far from their natal area.

Aquifer Arsenic Cycling Induced by Seasonal Hydrologic Changes within the Yangtze River Basin.

Consumption of groundwater containing >10 μg L(-1) arsenic (As) adversely impacts more than 100 million people worldwide. Multiyear trends in aquifer As concentrations have been documented, but strong seasonal variations are not commonly observed. Here we report dramatic seasonal changes in As concentrations and aquifer chemistry within the Jianghan Plain of the Yangtze River, China. At some wells, concentrations fluctuate by more than an order of magnitude within a single year (100-1200 μg L(-1)). Groundwater extraction and sustained water levels of surface channels during the dry season induces a strong downward hydraulic gradient, seasonally supplying oxidizing (oxygen, nitrate) water to the otherwise anoxic aquifer. Oxygen and/or nitrate addition promotes a transient drop in As concentrations for 1-3 months. When recharge ceases, reducing, low-arsenic conditions are reestablished by reactive, endogenous organic carbon. Temporal variability in As concentrations is especially problematic because it increases the probability of false-negative well testing during low-arsenic seasons. However, periods of low As may also provide a source of less toxic water for irrigation or other uses. Our results highlight the vulnerability and variability of groundwater resources in the Jianghan Plain and other inland basins within Asia to changing geochemical conditions, both natural and anthropogenic, and reinforce that continued monitoring of wells in high-risk regions is essential.

Uranium and thorium leachability in contaminated stream sediments from a uranium minesite

The leachability and potential environmental risks of uranium (U) and thorium (Th) in aquifer sediments from a recent uranium minesite of northern Guangdong Province, China were investigated as part of ongoing environmental investigations. Data for the non-mineralogical portion of U, Th and of additional major elements (Mn, Fe, Ca, Mg, Na, Al and K) using a 0.5mol/L HCl partial leach and inductively coupled plasma optical emission spectrometry (ICP-OES) were presented for the contaminated sediments and a background sediment. The contaminated sediments reported leachable contents of U between 22.85 and 2700μg/g and those of Th between 4.800 and 102.1μg/g, which exceed by a few to several hundred times the respective content found in the background. Generally, the leachable U and Th contents in the stream sediments decrease with distance from the potential contamination sources. A special case is the runoff gathering reservoir located around 1km downstream of the U mine-site, representing the most severe metal contamination hotspot within the alpine watershed, where the highest leachable content of U and Th in the inlet sediment was observed. The highest leachability of U (68.6%) and Th (44.8%) was observed in this sediment as well, indicating a high potential of bioaccumulation and bioconcentration of U and Th under changing geochemical conditions. Correlations analysis indicates that the dilute HCl leachable fraction of U and Th bound in the sediments are strongly dependent on anthropogenic inputs from the U mining/milling activities, and Ca salts are the main vectors.

Elevated microbial CO2 production and fixation in the oxic/anoxic interface of estuarine water columns during seasonal anoxia

Gradients of dissolved oxygen concentrations in stratified estuarine water columns directly influence microbial composition and metabolic pathways, resulting in vertical and axial chemical gradients of redox-active species. Understanding such microbial responses to changing geochemical conditions and elucidating the diversity of microbially-mediated processes are needed to comprehensively identify ecosystem functions. We tested the hypothesis that microbial metabolic processes in the vicinity of the oxic/anoxic interface in Chesapeake Bay would be elevated due to the coincidence of oxidized and reduced compounds. We measured rates of microbial redox processes associated with carbon cycles and quantified geochemical constituents. The transition from oxic to anoxic to oxic conditions was correlated with the pattern of dissolved nitrogen species, with the most reducing conditions comprising the highest concentrations of dissolved chemical compounds. The vertical distribution of community respiration was measured with changes in dissolved inorganic carbon concentrations and was related to water column stratification, with the highest median and variability (1.2 ± 1.4 μmol L−1 h−1) in the most stratified conditions. Rates of CO2 fixation under dark conditions, a measure of chemoautotrophy, were elevated at the base of oxyclines and significantly correlated with a gradient of density and some reduced compounds. Although vertical interpolation must be made with caution due to high vertical variability, chemoautotrophic production averaged 0.5 mmol m−2 h−1 from May to August and added 5.8% to autochthonous organic carbon production in the mesohaline Chesapeake Bay. Overall, our results suggest that anaerobic community metabolism and chemoautotrophy in the oxic/anoxic interface exert a small impact on estuarine carbon cycles.

Coralline algae and arsenic fixation in near shore sediments

Brazilian tropical beaches are enriched with arsenic (As) for hundreds of kilometres. The high levels of As in sediments, exceeding the legislative environmental thresholds, correlate by the presence of detritus material of geniculate calcareous algae involved in diagenetic processes in the coastal zone. Due to the porous structure of calcareous algae, the precipitation of iron hydroxides and subsequent absorption of arsenic occur not only on the surface but also in the inner parts of algal particles. The subsequent diagenetic filling of the cell lumen with calcium and magnesium carbonate preserves As inside the particles. This preservation contributes to the conservative behaviour of As in the changing geochemical conditions in sediments, and greatly reduces the efficiency of surf abrasion in the purification of beach sand from the high concentrations of As. The type of As enrichment in beach sands may exist in other regions of the world in addition to Brazil.

Phosphorus and arsenic distributions in a seasonally stratified, iron- and manganese-rich lake: microbiological and geochemical controls

Environmental context Despite being present at trace concentrations, arsenic and phosphorus are among the most important of freshwater contaminants. This research highlights the biogeochemical coupling of both elements in a New Zealand lake. We find that the mineralisation of organic residues coupled to the dissolution of colloidal iron and manganese hydroxides may be an important driver of the bioavailability of phosphorus and arsenic. Abstract Seasonal stratification in temperate lakes greater than a few metres deep provides conditions amenable to pronounced vertical zonation of redox chemistry. Such changes are particularly evident in eutrophic systems where high phytoplankton biomass often leads to seasonally established anaerobic hypolimnia and profound changes in geochemical conditions. In this study, we investigated the behaviour of trace elements in the water column of a seasonally stratified, eutrophic lake. Two consecutive years of data from Lake Ngapouri, North Island, New Zealand, demonstrate the occurrence of highly correlated profiles of phosphorus, arsenic, iron and manganese, all of which increased in concentration by 1–2 orders of magnitude within the anaerobic hypolimnion. Stoichiometric and mass-balance considerations demonstrate that increases in alkalinity in hypolimnetic waters were consistent with observed changes in sulfate, Fe and Mn concentrations with depth, corresponding to dissimilatory reduction of sulfate, FeIII and MnIV hydroxides. Thermodynamic constraints on Fe, Mn and Al solubility indicate that amorphous FeIII, MnIV hydroxides most probably controlled Fe and Mn in the surface mixed layer (~0 to 8m) whereas AlIII hydroxides were supersaturated throughout the entire system. Surface complexation modelling indicated that iron hydroxides (HFO) potentially dominated As speciation in the lake. It is likely that other colloidal phases such as allophanic clays also limited HPO42– activity, reducing competition for HAsO42– adsorption to iron hydroxides. This research highlights the coupling of P, As, Fe and Mn in Lake Ngapouri, and the apparent role of multiple colloidal phases in affecting P and As activity within overarching microbiological and geochemical processes.