Iron Oxide-rich Sediments Research Articles

Pyrolysis of Carboxylic Acids in the Presence of Iron Oxides: Implications for Life Detection on Missions to Mars.

The search for, and characterization of, organic matter on Mars is central to efforts in identifying habitable environments and detecting evidence of life in the martian surface and near surface. Iron oxides are ubiquitous in the martian regolith and are known to be associated with the deposition and preservation of organic matter in certain terrestrial environments, thus iron oxide-rich sediments are potential targets for life-detection missions. The most frequently used protocol for martian organic matter characterization (also planned for use on ExoMars) has been thermal extraction for the transfer of organic matter to gas chromatography-mass spectrometry (GC-MS) detectors. For the effective use of thermal extraction for martian samples, it is necessary to explore how potential biomarker organic molecules evolve during this process in the presence of iron oxides. We have thermally decomposed iron oxides simultaneously with (z)-octadec-9-enoic and n-octadecanoic acids and analyzed the products through pyrolysis-GC-MS. We found that the thermally driven dehydration, reduction, and recrystallization of iron oxides transformed fatty acids. Overall detectability of products greatly reduced, molecular diversity decreased, unsaturated products decreased, and aromatization increased. The severity of this effect increased as reduction potential of the iron oxide and inferred free radical formation increased. Of the iron oxides tested hematite showed the least transformative effects, followed by magnetite, goethite, then ferrihydrite. It was possible to identify the saturation state of the parent carboxylic acid at high (0.5 wt %) concentrations by the distribution of n-alkylbenzenes in the pyrolysis products. When selecting life-detection targets on Mars, localities where hematite is the dominant iron oxide could be targeted preferentially, otherwise thermal analysis of carboxylic acids, or similar biomarker molecules, will lead to enhanced polymerization, aromatization, and breakdown, which will in turn reduce the fidelity of the original biomarker, similar to changes normally observed during thermal maturation.

An alternative sequential extraction scheme for the determination of trace elements in ferrihydrite rich sediments

Sequential extraction schemes (SES) were evaluated to investigate the fractionation of Al, As, Cd, Cs, Cr, Co, Cu, Fe, Mn, Mo, Pb, Sr, U and Zn between the different mineral phases in iron oxide rich deposits of a former uranium mining site. Ineffective dissolution of iron oxide was observed when applying the BCR sequential extraction scheme. The hydroxylamine hydrochloride reagent in nitric acid could not effectively dissolve the iron oxide phase, even after several consecutive extractions. The use of sodium pyrophosphate to remove organic matter prior to the dissolution of the iron oxides and the use of oxalic acid buffer and dithionite citrate buffer to dissolve the amorphous and crystalline iron oxides was evaluated. An alternative six step SES for iron oxide rich sediments is proposed consisting of F1) Exchangeable-Carbonate fraction using acetic acid/ sodium acetate buffer (pH 5) F2) Labile organic fraction using sodium pyrophosphate, F3) Easily reducible amorphous oxides using an oxalic acid buffer, F4) Organic and Sulfide fraction using hydrogen peroxide, followed by ammonium acetate extraction, F5) Poorly reducible crystalline oxides using sodium dithionite and F6) residual fraction using a microwave digestion step with hydrofluoric acid, nitric acid and hydrochloride acid mixture. A good correspondence between the sum of the sequential extraction steps and total metal concentrations was found. Both the BCR method and new method were also applied to the CRM BCR 701 providing indicative values for non-certified elements with the BCR method and indicative values for the method presented.

On the P–T– $$fO_{2}$$ f O 2 stability of Fe4O5, Fe5O6 and Fe4O5-rich solid solutions

The high-pressure phases Fe4O5 and Fe5O6 have recently been added to the list of known iron oxides. As mixed-valence phases, it has been suggested that they could form in the Earth’s mantle once the dominant minerals become saturated in ferric iron. The possibility that Fe4O5 could exist in the mantle is also supported by the fact that it forms extensive solid solutions with both Mg2+ and Cr3+. In this study, we present the results of high-pressure and high-temperature multi-anvil experiments performed between 5 and 24 GPa at 1000–1400 °C aimed at constraining the stability field of the Fe4O5 phase. We combine these results with published phase equilibria, equation of state and Fe–Mg partitioning data to estimate the thermodynamic properties of Fe4O5, Fe5O6 and the (Mg,Fe)2Fe2O5 solid solution. Using our thermodynamic model, the oxygen fugacity at which the high-pressure iron oxides become stable is calculated and the redox stability of (Mg,Fe)2Fe2O5 in an assemblage of olivine and pyroxene is calculated as a function of the bulk Fe/(Fe + Mg) ratio. Fe4O5 and (Mg,Fe)2Fe2O5 are stable at oxygen fugacities higher than the diamond stability field and are, therefore, unlikely to be found as inclusions in diamonds. The stability field of Fe5O6, on the other hand, extends to oxygen fugacities compatible with diamond formation. Using the Mg–Fe solid solution model, we show that Fe4O5-structured phases would be restricted to aluminium-poor environments in the mantle such as dunites or silica–iron oxide-rich sediments transported into the mantle via subduction.

Microbial and geochemical features suggest iron redox cycling within bacteriogenic iron oxide-rich sediments

The geochemistry, mineralogy and microbiology of bacteriogenic iron oxides (BIOS) deposited at a circumneutral groundwater seep were examined to evaluate the biogeochemical controls on the redox cycle of iron in such environments. X-ray diffraction and extended X-ray absorption fine structure (EXAFS) spectroscopy showed that 2-line ferrihydrite dominated (60–70%) the BIOS mineralogy, with more crystalline lepidocrocite and goethite phases comprising the remainder, likely the product of Fe(II)-induced dissolution–reprecipitation pathways. Subsurface porewater profiles displayed an increase in dissolved (<0.2μm) equilibrium concentrations of Fe(II), also reflected in peaks in HCl-extractable sedimentary Fe(II) at depth, indicative of biological iron reduction. Interestingly EXAFS analysis of sediments collected from such depths revealed little mineralogical change from the surficial BIOS. Dissolved concentrations of Fe(III), likely present as nanoparticulates or organic complexes, also rose with depth. Scanning electron microscopy images of the surface BIOS samples showed a proliferation of sheath structures characteristic of Fe(II)-oxidizing Leptothrix spp. A 16S rRNA clone library obtained from the surficial BIOS included microorganisms closely associated to known Fe(II)-oxidizing (Gallionella spp., Sideroxydans spp.) and Fe(III)-reducing (Rhodoferax ferrireducens and Geothrix fermentans) bacteria. Analysis of subsurface sediment collected from an Fe(III)-reducing horizon revealed a drop in the proportion of β-Proteobacteria, which hosts numerous Fe(II)-oxidizing genera, and a rise in the δ-Proteobacteria, home to anaerobic metal-reducing prokaryotes. The recognition of clones with close identity to bacteria intimately involved in complementary iron redox transformations in the same surficial BIOS sediment suggests that microbial oxidation and reduction of iron may be tightly coupled in this, and other, shallow sedimentary environments.

Combining digital elevation data (SRTM/ASTER), high resolution satellite imagery (Quickbird) and GIS for geomorphological mapping: A multi-component case study on Mediterranean karst in Central Crete

Remote sensing data have become more and more popular for geomorphological investigations because their steadily increasing level of detail and accessibility opens up new potentials. In this context, this paper examines the application and quality of digital elevation models (SRTM and ASTER DEMs), high resolution satellite imagery (Quickbird) and GIS techniques for the detection and mapping of karst landforms (mainly enclosed depressions) at different scales in the Ida Mountains of Central Crete. Besides discussing methodological issues and evaluating suitability potentials, we conducted an exemplary case study based on spatial analysis of the regional karst morphology. Different input datasets and processing methods are applied (GIS-based analysis, land cover classification, raster calculations, etc.) in order to carry out an area-wide surveying and mapping of karst depressions. The findings are supported and validated by auxiliary field studies. Due to the level of detail and occasional data errors, an exclusive use of satellite imagery or digital elevation models for automatic karst landform detection performs insufficiently. Our results demonstrate that mapping karst features through remote sensing is significantly dependent on scale of interest, existing environmental conditions and data quality. A semi-automatic data integration approach on the basis of digital datasets generated by combined satellite image processing and DEM analysis yields the best results, especially when indirect karst-indicating variables like iron oxide-rich sediments are included as detection criteria. The multi-component application presented in this paper provides a time-saving and effective tool for meso- to macro-scale object detection and extensive study areas. However, the potential of fully automated karst feature mapping still needs to be explored in future work. Concerning the spatial dimension of karstification in Central Crete, the GIS-based results allow differentiating further geomorphological characteristics, e.g. by indicating a significant altitudinal change of karst forms within the study area. Size and shape of depressions (dolines, uvalas, and poljes) vary considerably due to the geological setting, climatic impacts, neotectonics and elevation.

Metal sequential extraction procedure optimized for heavily polluted and iron oxide rich sediments

A sequential extraction procedure has been optimized in relation with the characteristics of the sediments found in the Odiel Marshes Natural Park. They presented high percentages of iron oxides (up to 17%) and a total metal content, in the range of hundreds of μg g −1 and higher, for As, Cu, Pb and Zn. The metals studied included As, Cr, Cd, Cu, Fe, Hg, Mn, Ni, Pb and Zn. NH 4OAc was superior to MgCl 2 and NaOAc in relation to the metal determination using AAS. However, it was not possible to distinguish between exchangeable and bound-to-carbonate metals. For the evaluation of the sum of the metal concentration associated with both these phases, an optimal volume of 35 ml and a concentration of 1 M was selected for the extractant NH 4OAc. No readsorption problems were observed for any metal except for Hg. Several repetitive extractions with NH 2OH·HCl and H 2O 2–NH 4OAc were necessary for the complete extraction of metals bound to oxides and organic matter, respectively. Percentages of iron oxide coated with organic matter were found insignificant for the low organic matter content sediments considered in the study. Finally, a comparison between the optimized and a low selectivity (LS) sequential extraction scheme for metal mobility assessment showed an important overestimation of As, Cr and Ni released by oxidant mechanism and an underestimation of As, Cd, Cr and Ni by ion exchangeable+acid–base and reductant mechanism using the LS scheme.

Optimization of a Sequential Extraction Scheme for the Characterization of Heavy Metal Mobility in Iron Oxide Rich Sediments

Several parameters governing the extraction of metals from iron oxide rich sediments, using sequential extraction schemes were optimized. The mode of shaking, soil/extractant ratio and concentrations of MgCl2 and NH2OH·HCl, for samples collected in the Odiel Marshes Natural Park (SW Spain), were considered. The concentration of NH2OH·HCl deserved particular attention due to the nature of the studied sediments. A 0.4 M concentration of this extractant was needed to avoid readsorption of Cu and As in the samples. In addition, readsorption processes were studied using a candidate to reference material with a high organic matter content that was previously analyzed in an interlaboratory study.

Selective extraction of iron oxide associated arsenic species from sediments for speciation with coupled HPLC-HG-AAS

J. L. Gómez-Ariza, D. Sánchez-Rodas and I. Giráldez, J. Anal. At. Spectrom., 1998, 13, 1375 DOI: 10.1039/A806111D

Examination of hydrothermal influences on oceanic beryllium using fluids, plume particles and sediments from the TAG hydrothermal field

We have carried out a comprehensive study of the distributions of 9Be and the cosmogenic radionuclide 10Be in fluids, nearfield sediments, and neutrally buoyant plume particles associated with the TAG hydrothermal field. These results, coupled with previously published uranium decay series radionuclide distributions [1,2], suggest that the level of MnO 2 in iron oxide rich sediments and particles may play a role in Be scavenging. In addition, the data are used to quantify the net effects of nearfield coprecipitation with FeS and of scavenging by iron oxyhydroxide plume particles on the oceanic balance of the Be isotopes. These calculations suggest that hydrothermal input of 9Be is nearly balanced by nearfield and plume removal processes, and that hydrothermally induced scavenging has a negligible effect on the oceanic 10Be budget.