Fe Speciation Research Articles

“A spectroscopic picture paints 1000 words” mapping iron speciation in brain tissue with “full spectrum per pixel” X-ray absorption near-edge structure spectroscopy

Coordination chemistry enables a variety of vital functions in biological systems; however, characterising the chemical form of metal ions in cells and tissue is notoriously difficult. One technique that is gaining substantial momentum in this research area is X-ray absorption near-edge structure (XANES) spectroscopy. The XANES spectrum can be a rich source of information with respect to the coordination environment of metal ions. Further, XANES spectroscopy is compatible with microscopy mapping protocols as the spectra are recorded across a relatively narrow range of data points (typically 50–100). Although the potential of XANES spectroscopy to study metal ion coordination chemistry has long been known, data collection speed has only relatively recently reached the state in which maps can be collected with a full spectrum per pixel. The realisation of this capability now places XANES spectroscopic mapping among a suite of other spectroscopic imaging techniques, such as Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy, which are available to characterise biochemical composition, in situ within cells and tissue. Herein, we report a proof-of-concept application of XANES spectroscopic mapping to begin exploration of Fe speciation in brain tissue, which demonstrates the potential of this method for the biomedical sciences, and identifies important areas for consideration with respect to future protocol developments.

Equilibrium calculations of iron speciation and apparent iron solubility in the Celtic Sea at ambient seawater pH using the NICA-Donnan model

We used a combined ion pairing - organic matter speciation model (NICA-Donnan) to predict the organic complexation of iron (Fe) at ambient pH and temperature in the Celtic Sea. We optimized our model by direct comparison with Fe speciation determined by Adsorptive Cathodic Stripping Voltammetry using the added Fe-binding ligand 1-nitroso-2-naphthol (HNN) in the presence and absence of natural organic matter. We compared determined Fe speciation with simulated titrations obtained via application of the NICA-Donnan model with four different NICA parameter sets representing a range of binding site strengths and heterogeneities. We tested the assumption that binding sites scale to dissolved organic carbon (DOC) concentrations in marine waters. We found that a constant low DOC concentration resulted in an improved fit of our titration data to the simulated titrations, suggesting that inputs of autochthonous marine DOM may not increase the heterogeneity or concentrations of Fe binding sites. Using the optimal parameter set, we calculated pFe(III)´ (−log(∑Fe(OH)i3−i)) and apparent Fe(III) solubility (SFe(III)app) at ambient pH and temperature in the water column of the Celtic Sea. SFe(III)app was defined as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to DOM formed at a free Fe (Fe3+) concentration equal to the limiting solubility of Fe hydroxide (Fe(OH)3(s)). SFe(III)app was within range of the determined dissolved Fe concentrations observed after winter mixing on the shelf and in waters >1500 m depth at our most offshore stations. Our study supports the hypothesis that the ocean dissolved Fe inventory is controlled by the interplay between Fe solubility and Fe binding by organic matter, although the overall number of metal binding sites in the marine environment may not be directly scalable to DOC concentrations.

Progressive aqueous alteration and iron oxidation record in the matrix of Mukundpura CM2 chondrite, a new fall

• Mukundpura- a matrix-rich CM2, fresh fall. • Matrix shows evidences of progressive alteration. • Iron speciation record is consistent with presence of Cronstedtite. The matrices of CM chondrites are fine-grained and have undergone aqueous alteration. Based on mineral chemical composition of matrix phyllosilicate of Mukundpura CM2, the evidence of progressive aqueous alteration is understood and also reflected in gradual changes in S/SiO 2 and FeO/SiO 2 ratios. Further, we performed X-ray absorption near-edge spectroscopy at the Fe K-edge to explain the redox state of the phyllosilicate. The Iron Oxidation Index of Mukundpura yields 2.423 and the Fe +3 / Fe +2 ratio is 0.733. The Fe speciation in phyllosilicate is also sensitive to the degree of aqueous activity. Our data are consistent with the presence of Cronstedtite and Tochilinite-Cronstedtite intergrowths in the matrix suggested for different extent of alteration and recorded history of pervasive aqueous alteration in the asteroidal parent body.

Iron speciation in organic-rich shales from the Upper Triassic Yanchang Formation, Ordos Basin, Northern China: Implications for depositional environment

The iron (Fe) cycle is one of the most important marine and lacustrine geochemical cycles and is closely related to environmental redox conditions, which directly affects carbon, sulfur, phosphorus cycling. Analysis of Fe speciation (including Feox, Femag, Fecarb, and Fepy) of organic-rich shale samples provides new insights into the controversial redox conditions during the Chang 7 sedimentary period. Our results show that Fecarb and Fepy of the Chang 7 organic-rich shale are the main forms of reactive Fe pools, indicating that reactive Fe mainly existed in the form of Fe2+ in sediments. With a few exceptions, most samples fall into the ferruginous region in crossplots of Fepy/FeHR versus FeHR/FeT, indicating that pore water environments during early diagenesis were ferruginous accompanied by intermittent euxinic. Previous works examining the size of framboidal pyrite, the molar ratio of organic carbon and total phosphorus (P), and nitrogen isotope values suggest the bottom water environments conditions were oxic-suboxic accompanied by intermittent anoxic. We argue these patterns may be explained by the proximity of the redox boundary of depositional environment to the sediment–water interface during most of the Chang 7 period, with short-term fluctuations. Euxinic pore water conditions correspond to anoxic bottom water conditions and elevated total organic carbon (TOC) content, indicating that these environmental conditions would increase the flux of P diffused from sediments. As a result, lacustrine primary productivity would be elevated, creating conditions conducive to the accumulation of organic matter that is characteristic of the Chang 7 sedimentary period. This work will further our understanding of the accumulation mechanism of organic matter in the Chang 7 shale, the principal source rock for the Mesozoic oil-bearing system in the Ordos Basin.

Possible response of N2O emission to marine redox fluctuation during the Ediacaran-Cambrian transition in Nanhua Basin, South China

The redox state of the ocean and atmosphere during the Ediacaran-Cambrian (E-C) transition is characterized by significant revolutions and perturbations, which had a major effect on the biogeochemical nitrogen cycle. Nitrous oxide (N2O), a powerful greenhouse gas that drives global warming, is primarily produced in the oxygen minimum zones of modern ocean through incomplete denitrification at low oxygen concentration, and its emission to the atmosphere has been closely tied to the redox state of the ocean. To better understand the relationship between the possible variation of N2O emission and its related marine redox fluctuation, a multi geochemical proxy approach was conducted on the upper Ediacaran to lower Cambrian marine successions spanning from platform to slope facies of the Yangtze Block. The results show that δ15N features during the late Ediacaran to Fortunian interval from platform facies displayed high values (up to 8.8‰), indicating a higher degree of denitrification. The Fe speciation data along with elevated Cu/Al ratios in contemporaneous sediments suggests that the gradual depletion of dissolved Cu in water column due to expanded euxinic conditions. Because Cu is the essential element in the terminal step of denitrification pathway, the Cu deficiency in water column could result in incomplete denitrification. We infer that the possible increase of N2O discharge in this interval could contribute to a greenhouse climate and intense the anoxic degree of water column. However, during the Cambrian Stage 2 continuous consumption of NO3– and NO2– in seawater by denitrification would limit denitrification rate and reduce N2O production, and finally N2 fixation dominated the nitrogen cycle in photic zone as evidenced by low δ15N value. Both progressive oxygenation of deep water during the Cambrian Stage 3, and habitable environment temperature caused by less N2O emission could stimulate subsequent biosphere revolution. Our study highlights that N2O emission due to marine redox fluctuations possibly played an important role in regulating environmental and metazoan diversification during the E-C transition.

Phytate exudation by the roots of Pteris vittata can dissolve colloidal FePO4.

Phosphorus (P) is limiting nutrient in many soils, and P availability may often depend on iron (Fe) speciation. Colloidal iron phosphate (FePO4coll) is potentially present in soils, and we tested the hypothesis that phytate exudation by Pteris vittata might dissolve FePO4coll by growing the plant in nutrient solution to which FePO4coll was added. The omission of P and Fe increased phytate exudation by P. vittata from 434 to 2136 mg kg-1 as the FePO4coll concentration increased from 0 to 300 mM. The total P in P. vittata tissue increased from 2880 to 8280 mg kg-1, and the corresponding increases in the trichloroacetic acid (TCA) extractable P fractions were inorganic P (860-5100 mg kg-1), soluble organic P (250-870 mg kg-1), and insoluble organic P (160-2030 mg kg-1). That is, FePO4-solubilizing activity was positive correlated with TP, TCA P fractions in P. vittata, TP in growth media, and root exudates. This study shows that phytate exudation dissolved FePO4coll due to the chelation effect of phytic acid on Fe; however, the wider question of whether phytic acid excretion was prompted by deprivation of P, Fe, or both remains to be answered.

Selective Formation of α-Fe(II) Sites on Fe-Zeolites through One-Pot Synthesis.

α-Fe(II) active sites in iron zeolites catalyze N2O decomposition and form highly reactive α-O that selectively oxidizes unreactive hydrocarbons, such as methane. How these α-Fe(II) sites are formed remains unclear. Here different methods of iron introduction into zeolites are compared to derive the limiting factors of Fe speciation to α-Fe(II). Postsynthetic iron introduction procedures on small pore zeolites suffer from limited iron diffusion and dispersion leading to iron oxides. In contrast, by introducing Fe(III) in the hydrothermal synthesis mixture of the zeolite (one-pot synthesis) and the right treatment, crystalline CHA can be prepared with >1.6 wt % Fe, of which >70% is α-Fe(II). The effect of iron on the crystallization is investigated, and the intermediate Fe species are tracked using UV-vis-NIR, FT-IR, and Mössbauer spectroscopy. These data are supplemented with online mass spectrometry in each step, with reactivity tests in α-O formation and with methanol yields in stoichiometric methane activation at room temperature and pressure. We recover up to 134 μmol methanol per gram in a single cycle through H2O/CH3CN extraction and 183 μmol/g through steam desorption, a record yield for iron zeolites. A general scheme is proposed for iron speciation in zeolites through the steps of drying, calcination, and activation. The formation of two cohorts of α-Fe(II) is discovered, one before and one after high temperature activation. We propose the latter cohort depends on the reshuffling of aluminum in the zeolite lattice to accommodate thermodynamically favored α-Fe(II).

Responses of oceanic chemistry to climatic perturbations during the Ordovician-Silurian transition: Implications for geochemical proxies and organic accumulations

The organic-rich sediments deposited in the Ordovician-Silurian transition (OST) on the Yangtze Platform not only represent significant shale gas sources, but also record information on the evolution of the paleo-environment. Much effort has been made to clarify the correlations among the climate, paleo-ocean, and organic accumulations, but debates remain. In this study, we further investigate the major and trace elements and total sulfur contents of the Datianba section, which has been studied for C–Fe–S geochemistry. Our new results, combined with the published Fe speciation, show that diagenesis may have a significant impact on Fe speciation. The incorporation of Fe into carbonate may lead to mixed signals of pore water and water columns, limiting the use of Fe speciation in carbonate-rich strata. In the pre- and post-Hirnantian periods, the high productivity caused by strong upwelling and chemical weathering, combined with euxinic water conditions, facilitated organic accumulation. However, productivity may play a more important role in the pre-Hirnantian period because it not only provides organic matter, but also reforms the redox conditions. Although euxinia and upwelling also developed in the early Hirnantian, the depleted micronutrient inventories caused by euxinia and the reduced chemical weathering inhibited productivity, and thus the organic accumulation. In Hirnantian, the poor preservation conditions (suboxic water columns and non-euxinic pore waters) led to low organic accumulations. This study stresses the importance of a multi-proxy method in paleo-environmental reconstruction and reveals climate-controlled oceanic evolution and organic accumulation.

The lake as an iron sink - new insights on the role of iron speciation

The solubility and behavior of iron (Fe) in natural waters is tightly linked to Fe speciation, and Fe speciation likely influences how Fe distributes between the water column and sediments. In this study, the function of a lake as an Fe sink, with focus on the role of Fe speciation, was assessed for Lake Bolmen in southern Sweden. We found that a large fraction of the Fe flowing in to the lake was efficiently lost by sedimentation in the lake basin. Fe in inflowing water was a mix of organically complexed mononuclear Fe, Fe-(oxy)hydroxides and Fe-bearing clays, while surface sediments were composed of Fe-(oxy)hydroxides, Fe-bearing clays, Fe-bearing silicates and Fe sulfides. The absence of organically complexed Fe in the surface sediments indicates that the lake is mainly a sink for minerogenic fractions. Furthermore, while lakes are considered to be sinks of Fe, it has been suggested that this function may be impaired by increasing precipitation and consequently shorter water residence time. In this study there were large within- and between-year variations in precipitation and Fe concentrations. However, rather than smaller Fe losses to the sediments during wet years, within-lake losses tended to be larger due to higher loading of Fe from the catchment. Thus, forecasted increases in precipitation may result in enhanced catchment export and Fe loading to lakes, and subsequently enhanced Fe sequestration in sediments.

Early Cambrian oceanic oxygenation and evolution of early animals: A critical review from the South China Craton

Co-evolution of marine redox conditions and early animals during the early Cambrian was fundamental to the development of modern Earth ecosystem. Numerous Burgess Shale-type fossil Lagerstätten (i.e., the Chengjiang and Qingjiang biotas) in South China offer taphonomic windows to probe the evolutionary traits of early animals and marine redox variations, and to evaluate the relationship between the two during the Cambrian Ages 2 and 3 (~529 to 514 Ma). Here, we critically review records of various geochemical proxies and fossil assemblages from 24 Lagerstätten-bearing lower Cambrian sections representing a wide variety of depositional settings from the deep basin to inner shelf across the Nanhua Basin, South China. Geochemical redox proxies: Fe speciation, and Mo and U enrichment factors were utilized to reconstruct water depth-related redox gradients within the early Cambrian Nanhua Basin. The results show that a dynamic ‘euxinic wedge’ may have existed at the mid-depth, and its expansion/contraction may have controlled redox variations in the Nanhua Basin during the early Cambrian. The dramatic contraction of the euxinic wedge in the Cambrian Age 3 suggests that local oceanic oxygenated states coincided with the emergence of the Chengjiang and Qingjiang biotas. Meanwhile, a ~ 25‰ negative shift in pyrite sulfur isotopes deciphers a major increase in seawater sulfate availability, reflecting strongly enhanced subaerial oxidative weathering and/or a lower burial flux of pyrite. Concurrent variations in U and Mo isotopes support the global extent of this ocean oxygenation event. Accordingly, the global marine oxygenation may have facilitated the emergence of both ecologic complexity and biodiversification of early animals indicated by the Chengjiang and Qingjiang Lagerstätten in the early Cambrian (mainly Cambrian Age 3).

The impact of postdepositional alteration on iron- and molybdenum-based redox proxies

Abstract Ratios of (un)reactive iron species, authigenic molybdenum contents (Moauth), and molybdenum isotope compositions (δ98Moauth) in sedimentary rocks are geochemical proxies that are widely used to reconstruct past marine redox states, which have been calibrated in modern marine settings covering oxic to euxinic conditions. However, syn- and postdepositional processes can result in alterations and ambiguities of proxy-derived redox signals that can challenge the validity of paleoreconstructions. We present new data from modern organic-rich sediments of two oxygen minimum zone settings in the Gulf of California and the Peruvian margin. The results show that Mo is fully immobilized shortly after deposition by reaction with hydrogen sulfide (H2S) produced during organoclastic sulfate reduction. Thus, any H2S produced deeper in the sediment (e.g., by sulfate reduction coupled to anaerobic methane oxidation) leaves the initially deposited Mo concentrations and δ98Mo signatures unaltered, which supports the robustness of Mo-based redox proxies. In contrast, the Fe speciation data reveal continued pyritization due to constant exposure of Fe minerals to H2S. Importantly, both Fe bound to oxides and carbonates (highly reactive Fe) and also poorly reactive Fe (e.g., sheet silicates) undergo pyritization during early diagenesis. This process generates Fe-based proxy signatures that falsely imply ferruginous or euxinic conditions.

Influence of pH and Dissolved Organic Matter on Iron Speciation and Apparent Iron Solubility in the Peruvian Shelf and Slope Region.

The chemical speciation of iron (Fe) in oceans is influenced by ambient pH, dissolved oxygen, and the concentrations and strengths of the binding sites of dissolved organic matter (DOM). Here, we derived new nonideal competitive adsorption (NICA) constants for Fe(III) binding to marine DOM via pH-Fe titrations. We used the constants to calculate Fe(III) speciation and derive the apparent Fe(III) solubility (SFe(III)app) in the ambient water column across the Peruvian shelf and slope region. We define SFe(III)app as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to DOM at a free Fe (Fe3+) concentration equal to the limiting solubility of Fe hydroxide (Fe(OH)3(s)). A ca. twofold increase in SFe(III)app in the oxygen minimum zone (OMZ) compared to surface waters is predicted. The increase results from a one order of magnitude decrease in H+ concentration which impacts both Fe(III) hydroxide solubility and organic complexation. A correlation matrix suggests that changes in pH have a larger impact on SFe(III)app and Fe(III) speciation than DOM in this region. Using Fe(II) measurements, we calculated ambient DFe(III) and compared the value with the predicted SFe(III)app. The underlying distribution of ambient DFe(III) largely reflected the predicted SFe(III)app, indicating that decreased pH as a result of OMZ intensification and ocean acidification may increase SFe(III)app with potential impacts on surface DFe inventories.

A chemical weathering control on the delivery of particulate iron to the continental shelf

The delivery of potentially reactive iron minerals to the marine environment exerts a major control on ocean and marine sediment biogeochemistry, including the preservation and burial of organic carbon. One aspect of the global iron cycle that has received little attention concerns the phase partitioning of iron in highly chemically-weathered sediments from (sub)tropical mountainous regions, where the sediment is commonly deposited directly onto adjacent continental margins. Here, we report the phase partitioning of particulate iron in surface seafloor sediments deposited from oxic bottom waters along the east coast of China, including the Bohai Sea, Yellow Sea and East China Sea, in addition to the continental shelf region adjacent to the (sub)tropical mountainous island of Hainan in the northern South China Sea. We document highly reactive (FeHR) to total iron (FeT) ratios for the Bohai Sea, Yellow Sea and East China Sea that are typical of global marine sediments deposited under oxic water column conditions. By contrast, FeHR/FeT ratios in Hainan offshore sediments are significantly elevated above normal oxic marine sediments. These Hainan offshore sediments display a strong positive correlation between FeHR/FeT ratios and the chemical index of alteration (CIA) weathering proxy, suggesting that the Fe speciation systematics are controlled by the intensity of chemical weathering. This observation is supported by additional geochemical data and Nd isotope analyses, which suggest a common source lithology for all the studied regions. The Hainan sediments thus document a specific scenario, whereby highly chemically weathered sediments from the mountainous Hainan region bypass the inner shore settings that commonly disproportionately trap Fe (oxyhydr)oxide minerals, depositing high concentrations of FeHR on the adjacent continental margin. Such enrichments likely aid in the preservation of organic carbon in marine sediments, suggesting that organic carbon preservation and burial may be enhanced by elevated FeHR in such settings. We also establish a set of essential criteria for potential recognition of chemical weathering-induced enrichments in FeHR/FeT ratios in ancient marine sediments.

Low‐Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite

AbstractSerpentinized peridotite is reacting with groundwater in the subsurface of the Samail ophiolite in Oman. Although these rocks are partially to completely serpentinized, they interact with a groundwater aquifer containing hyperalkaline fluids rich in H2 and CH4. Since the mechanisms by which H2 production may continue at low temperatures (<50°C) are not fully understood, core recovered during the Oman Drilling Project provides an excellent opportunity to study the mineralogy and Fe speciation in highly serpentinized harzburgite recording multiple stages of water/rock interaction. In Hole BA3A, early hydration of olivine and pyroxene, which likely occurred at temperatures of ∼100°C–200°C, produced serpentine, Fe‐rich brucite, awaruite, and little magnetite. Notably, Fe‐rich brucite is only preserved at >∼100 m depth in the core. Fe‐rich brucite is nearly absent within two near‐surface reaction zones where later stages of reaction are recorded, which include replacement of Fe‐rich brucite by Fe(III)‐bearing serpentine, and increases in the proportion of other Fe(III)‐bearing phases such as magnetite and hydroandradite. Thus, Fe‐rich brucite at depth represents substantial stored capacity for H2 production that can continue at low temperature, even after primary olivine and pyroxene are exhausted, thereby sustaining habitable conditions for microbial life.

Holocene Spatiotemporal Redox Variations in the Southern Baltic Sea

Low oxygen conditions in the modern Baltic Sea are exacerbated by human activities; however, anoxic conditions also prevailed naturally over the Holocene. Few studies have characterized the specific paleoredox conditions (manganous, ferruginous, euxinic) and their frequency in southern Baltic sub-basins during these ancient events. Here, we apply a suite of isotope systems (Fe, Mo, S) and associated elemental proxies (e.g., Fe speciation, Mn) to specifically define water column redox regimes through the Baltic Holocene in a sill-proximal to sill-distal transect (Lille Belt, Bornholm Basin, Landsort Deep) using samples collected during the Integrated Ocean Drilling Program Expedition 347. At the sill-proximal Lille Belt, there is evidence for anoxic manganous/ferruginous conditions for most of the cored interval following the transition from the Ancylus Lake to Littorina Sea but with no clear excursion to more reducing or euxinic conditions associated with the Holocene Thermal Maximum (HTM) or Medieval Climate Anomaly (MCA) events. At the sill-distal southern sub-basin, Bornholm Basin, a combination of Fe speciation, pore water Fe, and solid phase Mo concentration and isotope data point to manganous/ferruginous conditions during the Ancylus Lake-to-Littorina Sea transition and HTM but with only brief excursions to intermittently or weakly euxinic conditions during this interval. At the western Baltic Proper sub-basin, Landsort Deep, new Fe and S isotope data bolster previous Mo isotope records and Fe speciation evidence for two distinct anoxic periods but also suggest that sulfide accumulation beyond transient levels was largely restricted to the sediment-water interface. Ultimately, the combined data from all three locations indicate that Fe enrichments typically indicative of euxinia may be best explained by Fe deposition as oxides following events likely analogous to the periodic incursions of oxygenated North Sea waters observed today, with subsequent pyrite formation in sulfidic pore waters. Additionally, the Mo isotope data from multiple Baltic Sea southern basins argue against restricted and widespread euxinic conditions, as has been demonstrated in the Baltic Proper and Bothnian Sea during the HTM or MCA. Instead, similar to today, each past Baltic anoxic event is characterized by redox conditions that become progressively more reducing with increasing distance from the sill.

Mercury and sulfur isotopic evidence for the linkages between the ca. 510 Ma Kalkarindji large igneous province and trilobite crisis

The first major mass extinction of trilobites occurred at the transition from Cambrian Series 2 (CS2) to Miaolingian (M) and coincided with a large marine transgression and volcanic eruptions of a large igneous province (LIP). Understanding the causal links between these events is important in deciphering environmental changes and life evolution at that time. This paper presents S-Hg-C isotopic and Fe speciation data for calcareous shales from the CS2-M Yangliugang Formation in the Dongjin section, South China. In the lower part of this section (Interval I), calcareous shales have limited S isotopic differences between carbonate-associated sulfates (δ34SCAS) and pyrites (δ34SPy) with Δ34S values from –4.2‰ to +8.3‰; they also have high FeHR/FeT ratios from 0.5 to 0.66, that are indicative of a low SO42− content in anoxic seawater. Calcareous shales from Interval I display at least two Hg/TOC peaks (up to 207 ppb/wt%), coincident with volcanic eruptions associated with the ca. 510 Ma Kalkarindji LIP in Australia. In the middle part of the Dongjin section (Interval II), calcareous shales display negative excursions of Δ199Hg values, δ34SPy values and FeHR/FeT ratios, which were likely led by a large terrestrial input into a marginal basin from where Interval II deposited. Calcareous shales in the upper part of the section (Interval III) contain pyrite framboids with a mean diameter of <4 μm and high ratios of FeHR/FeT (>0.82) and FePy/FeHR (>0.78), indicating an euxinic depositional environment. They also record negative excursions in δ13C and δ202Hg (with low values down to –3.05‰ and –1.68‰, respectively), providing evidence for ocean upwelling. The negative shift of δ13C values in the Dongjin section was temporally comparable to the C isotope excursions that coincided with the Redlichiid-Olenellid Extinction Carbon Isotope Excursion (ROECE). We conclude that volcanic eruptions of the ca. 510 Ma Kalkarindji LIP enhanced the continental erosion rate, leading to a high terrestrial SO42− input and an accumulation of H2S in deep marginal basins at the end of CS2. The subsequent ocean upwelling (>506 Ma) brought anoxic/euxinic seawaters into the continental shelf, contributing to the mass extinction of Redlichiid and Olenellid.

Mo isotope records from Lower Cambrian black shales, northwestern Tarim Basin (China): Implications for the early Cambrian ocean

AbstractThe widely developed black shales deposited during the early Cambrian recorded paleoenvironmental information about coeval seawater. Numerous studies have been conducted on these shales to reconstruct the paleomarine environment during this time period. However, most research has been conducted on stratigraphic sections in South China, and equivalent studies of sections from other cratons are relatively rare. Here, we report Mo isotopic compositions as well as redox-sensitive trace-element and iron (Fe) speciation data for black shales of the Lower Cambrian Yuertusi Formation from the Tarim block (i.e., a small craton). The Fe speciation data show high FeHR/FeT and Fepy/FeHR ratios, indicating roughly sustained euxinic bottom-water conditions during their deposition. Based on Mo isotopic compositions (δ98/95Mo), we further classified the euxinic black shales into two intervals: a lower interval (0–21.3 m) and an upper interval (21.3–32.3 m). The lower interval is characterized by variable Mo isotopic compositions (−2.12‰ to +0.57‰, mean = –0.52‰ ± 0.72‰), with an obvious negative excursion in its middle portion. The overlying upper interval has relatively heavy δ98/95Mo values up to +1.42‰ (mean = +0.62‰ ± 0.37‰). We ascribe δ98/95Mo differences in the lower and upper intervals to inadequate aqueous H2S concentrations for quantitative thiomolybdate formation under euxinic conditions. The most negative Mo isotope excursion may have been caused by upwelling hydrothermal inputs during a transgression, consistent with significantly elevated total organic carbon (TOC) contents, Mo and U enrichments, and Fe supply. Relatively positive δ98/95Mo values in the upper interval have roughly similar variations with other coeval sections, indicating such variations were common for early Cambrian euxinic deposits, and they were most likely caused by local differences in [H2S]aq. Compilation of Mo isotope data from the early Cambrian and earlier times further indicates relatively oxygenated seawater, especially the deep-marine areas during the early Cambrian before reaching a state like modern seawater.

Organic acids as complexing agents for iron and their effects on the nutrition and growth of maize and soybean

ABSTRACT Synthetic chelating agents have high cost and low biodegradation in the environment; thus, the use of natural complexing agents for iron increases the sustainability in crop production. The aim of this study was to verify the efficiency of natural organic acids added to the nutrient solution on the Fe acquisition. For that purpose, two experiments were carried out using soybean and maize. Citric acid (CA), malic acid (MA), tartaric acid (TA), and oxalic acid (OA) were used as complexing agents for iron. Fe-EDTA (ethylenediaminetetraacetic acid) and FeSO4 were also tested. Soluble Fe and chemical speciation in a nutrient solution were determined. Organic acids maintained less complexed Fe in the nutrient solution than EDTA, but the complexed Fe fraction was not associated with the efficiency of the Fe organic acids in supplying Fe to plants. Maize and soybean treated with Fe-MA and Fe-CA had reduced plant development. For maize, the TA treatment at both SRs increased Fe accumulation and promoted Fe translocation similar to EDTA. For soybean, Fe-OA at both SRs promoted biomass, Fe accumulation, and Fe translocation similar to EDTA. Therefore, Fe complexes were able to assure plant growth and nutrition and showed effectiveness depending on the plant species.

Iron speciation and annual records in black coral as new proxy for mining and environmental impacts.

Iron (Fe) is a micronutrient and plays an important role in regulating ocean primary production and consequently changing oceanic CO2 uptake. However, approaches for high-resolution of Fe records in marine environment has been a great challenge. In this study, we report for the first time an annual Fe record on black coral organic skeleton from the northern South China Sea (SCS) as an archive to study the environmental change during the past century. In situ micro-Raman, synchrotron micro X-ray absorption near edge spectroscopy (μ-XANES) and synchrotron micro X-ray fluorescence (μ-XRF) were applied to investigate the Fe speciation and the radial Fe profile in black coral. The preliminary results from micro-Raman and synchrotron micro XANES analysis demonstrated that Fe in black coral was mainly combined with 3,4-dihydroxyphenylalanine (dopa) as tris-DOPA-Fe complex. Such spatial coordination structure of complexation makes Fe have high affinity with dopa in black coral. Furthermore, elevated Fe concentration in Fe profile recorded on synchrotron μ-XRF spectra with 2.5μm resolution corresponded well to the exploitation history of the adjacent onshore Tiandu Iron Mine (Sanya, China) from 1939 to 1960. Other distinct Fe peak coincides with the war activities in 1970s. The findings presented in this work indicate that the high-resolution iron record with low annual growth rate (~17.8μmyear-1) of black coral may serve as a proxy of marine environmental record.

Does ultrafiltration kinetics bias iron isotope compositions?

Iron (Fe) isotopes are now recognized as useful tracers of Fe sources and biogeochemical processes in natural environments but many uncertainties remain regarding the mechanisms that control their isotopic fractionation. Ultrafiltration techniques applied to separate Fe species could potentially bias Fe isotopic compositions. Here, we investigated frontal centrifugal ultrafiltration. We have set up time-ultrafiltration experiments at low and high Fe concentrations both with and without organic matter (OM) at pH values of 1 and 6.5. The ultrafiltration impact was studied by monitoring the Fe isotope composition in the <30 kDa ultrafiltrates relative to the ultrafiltration time. No Fe isotopes bias resulted from the ultrafiltration technique regardless the Fe and OM concentrations and speciation. This work, therefore, validates the use of the frontal centrifugal ultrafiltration technique to study the signature of Fe isotopes in environmental samples composed of various Fe species sizes such as colloids, nanoparticles, clusters or soluble complexes.