Fe Mn Oxyhydroxides Research Articles

Persistently strong Indonesian Throughflow during marine isotope stage 3: evidence from radiogenic isotopes

The Indonesian Throughflow (ITF) connects the western Pacific Ocean with the eastern Indian Ocean, thus forming one of the major near surface current systems of the global thermohaline circulation. The intensity of the ITF has been found to be sensitive to changes in global ocean circulation, fluctuations in sea level, as well as to the prevailing monsoonal conditions of the Indonesian Archipelago and NW Australia. This study presents the first reconstruction of ITF dynamics combining radiogenic isotope compositions of neodymium (Nd), strontium (Sr), and lead (Pb) of the clay-size detrital fraction to investigate changes in sediment provenance, and paleo seawater Nd signatures extracted from the planktonic foraminifera and authigenic Fe–Mn oxyhydroxide coatings of the marine sediments focussing on marine isotope stage 3 (MIS3). Sediment core MD01-2378 was recovered within the framework of the International Marine Global Change Study (IMAGES) and is located in the area of the ITF outflow in the western Timor Sea (Scott Plateau, 13° 04.95′ S and 121° 47.27′ E, 1783 m water depth). In order to produce reliable seawater signatures, several extraction methods were tested against each other. The results of the study show that at this core location the extraction of surface water Nd isotope compositions from planktonic foraminifera is complicated by incomplete removal of contributions from Fe–Mn oxyhydroxides carrying ambient bottom water signatures. The bottom water Nd isotope signatures reliably obtained from the sediment coatings (average εNd = −5.0) document an essentially invariable water mass composition similar to today throughout the entire MIS3. The radiogenic Nd, Sr, and Pb isotope records of the clay-sized detrital fraction suggest that the Indonesian Archipelago rather than NW Australia was the main particle source at the location of core MD01-2378, and thus indicating a persistently strong ITF during MIS3. Furthermore, the variations of the detrital radiogenic isotopes are shown to be more sensitive to changes in circulation and document a somewhat enhanced ITF intensity during the early part of MIS3 until 47.4ka compared with the remaining MIS3.

Silicate Minerals as a Source of Arsenic Contamination in Groundwater

Mineral dissolution plays an essential role in controlling geogenic arsenic (As) contamination in groundwater. Although reductive dissolution of Fe oxyhydroxides is generally considered a key As release mechanism in many aquifers, some recent studies argue that silicate minerals, normally considered “inert” in As release, are the primary source of As contamination under certain conditions. The objective of this study is to determine As distribution in different minerals in a natural sediment and identify As release mechanisms and the role of silicate minerals in As release. A sediment sample was collected, characterized, and tested using leaching experiments at a range of pH and redox potentials. Our results showed that silicate minerals, which make up the bulk of the sediment, are the main As reservoir, containing 75 % of As. Fe–Mn oxyhydroxides, which are minor components in the sediment, are the second largest As reservoir and hold 16 % of As. Leaching experiments showed that silicate mineral dissolution is an important As-releasing mechanism and that high pH and low redox potential promoted silicate mineral dissolution and As release.

Geochemical and diatom records of recent changes in depositional environment of a tropical wetland, central west coast of India

Two mudflat sediment cores collected from a sub-channel (S-61) and the main channel (S-60) of a tropical wetland, along central west coast of India, were investigated for recent changes in depositional environment using geochemical (sediment grain size, total organic carbon, total nitrogen, carbon/nitrogen ratio (TOC/TN), selected metals and pH) and diatom records. The sub-channel (S-61) represents river environment which opens into main channel (S-60) which represents marine environment being close to the sea. We report a transition from river-dominated to marine-dominated depositional environment over the years. The relatively higher sand percentage together with elevated carbon/nitrogen ratio and predominance of freshwater diatoms suggested greater river runoff in the past while marine influence is supported by decrease in TOC/TN ratio and increased dominance of marine diatoms in recent years. In the main channel (S-60), distribution of metals is regulated by organic matter (total nitrogen and total organic carbon) while in the sub-channel (S-61), Fe–Mn oxyhydroxides play a significant role in trace metal distribution. The highest numbers of diatoms were recorded in the sub-channel (S-61) which is also characterized by higher total nitrogen concentration. Geochemical and diatom signatures thus have helped to infer spatial and temporal variations in depositional environment.

Rare Earth Elements Geochemistry and Provenance Discrimination of Sediments in Tolo Harbour, Hong Kong

Rare earth elements (REE) concentrations have been analyzed in 13 sediment samples from Tolo Harbour. The highest concentrations of ∑REE, ∑LREE (light rare earth elements) and ∑HREE (heavy rare earth elements) are all at site G as the enrichment of Mn–Fe oxyhydroxides and the highest pH of site G relative to its nearby areas. The relatively higher ∑LREE/∑HREE ratio at site G is probably due to the stronger REE fractionation. During the fractionation process, LREE are preferentially scavenged on particle surfaces such as Mn–Fe oxyhydroxides and clay minerals. Concentrations of ∑REE, ∑LREE and ∑HREE at sites Y and Z are much lower than those at other sites as calcareous biological debris dilute the REE abundances of the sediments in Tolo Harbour. REE are released from sediments into waters due to the high salinity in Tolo Harbour and REE fractionation occurs in this release process. The provenance of sediments in Tolo Harbour is terrigenous and the North American shale composite (NASC)-normalized REE patterns of the 13 samples in Tolo Harbour all display strongly negative Eu anomalies. ∑REE is poorly correlated with Zr and Hf, which suggests that Tolo Harbour is heavily polluted by human activities.

Rare Earth Elements As Biogeochemical Indicators In Mangrove Ecosystems (Pichavaram, Tamilnadu, India)

Abstract Mangrove forests are coastal wetland that can be sinks for material derived from terrestrial and marine settings. This study aims to establish the abundance, distribution, and enrichment of rare earth elements (REE) to track the sediment sources and infer biogeochemical processes occurring in the mangrove wetlands. Five sediment cores were collected throughout the Pichavaram mangroves in November 2010. The sampling sites were chosen based on vegetation cover, land-use pattern, and human interference. The vertical profile of the retrieved sediment cores were sub-sectioned, air dried, and homogenized into a powder before analysis. Higher REE concentration in Pichavaram sediments indicates dominance of input from the natural sources like weathering. Preferential REE enrichment was attributed to the alkaline pH (7–8.5) and reducing conditions prevailing in the mangrove environment allowing removal of MREE and LREE by adsorption and precipitation as Fe-Mn oxyhydroxides in sediments. Post Archean Australian Shale (PAAS)-normalized plots depicted convex subparallel shale-like patterns with similar enrichment. Pichavaram sediments showed strongly positive Eu anomalies, similar to that observed in Kaveri River sediments, validating the origin of REE in Pichavaram sediments as an outcome of natural weathering of post-Archean charnockitic and gneissic terrane. Also positive Eu anomalies indicated the prevalence of reducing conditions in Pichavaram. Cores near the estuarine mouth showed highly mixed vertical profile as compared to the undisturbed mangrove sites. Principal-component analysis delineated three main processes controlling REE distribution in Pichavaram, namely natural weathering, inherent physicochemical processes, and in situ biogeochemical processes occurring in this mangrove environment.

Nd and Sr isotope compositions of different phases of surface sediments in the South Pacific: Extraction of seawater signatures, boundary exchange, and detrital/dust provenance

The radiogenic isotope composition of neodymium (Nd) and strontium (Sr) are useful tools to investigate present and past oceanic circulation or input of terrigenous material. We present Nd and Sr isotope compositions extracted from different sedimentary phases, including early diagenetic Fe-Mn coatings, “unclean” foraminiferal shells, fossil fish teeth, and detritus of marine surface sediments (core-tops) covering the entire midlatitude South Pacific. Comparison of detrital Nd isotope compositions to deep water values from the same locations suggests that “boundary exchange” has little influence on the Nd isotope composition of western South Pacific seawater. Concentrations of Rare Earth Elements (REE) and Al/Ca ratios of “unclean” planktonic foraminifera suggest that this phase is a reliable recorder of seawater Nd isotope composition. The signatures obtained from fish teeth and “nondecarbonated” leachates of bulk sediment Fe-Mn oxyhydroxide coatings also agree with “unclean” foraminifera. Direct comparison of Nd isotope compositions extracted using these methods with seawater Nd isotope compositions is complicated by the low accumulation rates yielding radiocarbon ages of up to 24 kyr, thus mixing the signal of different ocean circulation modes. This suggests that different past seawater Nd isotope compositions have been integrated in authigenic sediments from regions with low sedimentation rates. Combined detrital Nd and Sr isotope signatures indicate a dominant role of the Westerly winds transporting lithogenic material from South New Zealand and Southeastern Australia to the open South Pacific. The proportion of this material decreases toward the east, where supply from the Andes increases and contributions from Antarctica cannot be ruled out.

Molybdenum isotopes in two Indian estuaries: Mixing characteristics and input to oceans

The distributions of dissolved and particulate Mo and their isotope composition (δ98Mo) have been measured in the Narmada and the Tapi estuaries draining into the Arabian Sea. During monsoon, the δ98Mo of dissolved Mo in the Narmada estuary ranges from 0.49‰ to 2.19‰ in the salinity range 0–17.2 practical salinity unit (psu) quite similar to that in the Tapi estuary, 0.99–2.36‰, in the salinity range 0–20.3psu. Mo concentration in suspended sediments of the Narmada estuary collected during monsoon average 512±44ng/g (range 459–602ng/g) similar to that measured in one sample from the Tapi estuary 560ng/g Mo. δ98Mo of particulate Mo in the Narmada ranges from −0.21‰ to 0.48‰ with an average −0.03±0.2‰. Dissolved Mo in the Narmada and the Tapi rivers display isotopically heavier Mο compared to that in basalts, the major lithology of their drainage. This could result from a variety of processes, preferential weathering of Mo rich sulphide minerals dispersed in the basalts, preferential removal of isotopically lighter Mo during transport or contribution from marine cyclic salts supplied via rain or chemical weathering of organic rich shales in the basins.The distribution of δ98Mo in the Narmada and the Tapi estuaries with salinity does not follow the theoretical mixing line between river and seawater endmembers suggesting its non-conservative behavior. Particulate Mo and δ98Mo show concomitant increase with salinity in the Narmada estuary indicating loss of dissolved Mo by adsorption onto Fe–Mn oxyhydroxide. Balancing the Mo budget along the course of these estuaries using inverse model suggests that in the Narmada estuary there could be loss up to 8% of the dissolved Mo and that in the Tapi supply from anthropogenic sources could be up to 27%. The results obtained in this study bring out the processes modifying riverine input of Mo and its δ98Mo in the estuaries, oxic sink in the Narmada and anthropogenic input in the Tapi. Repetitive adsorption and desorption of Mo in the Narmada estuary can modify the supply of dissolved Mo and its δ98Mo relative to riverine supply by up to 40%, this can significantly impact the Mo isotope budget of the oceans. In contrast, in the Tapi estuary there is enhancement in the dissolved supply of Mo relative to that from river due to anthropogenic input of Mo. The investigations in these two estuaries underscore the importance of solute particle interactions and anthropogenic input in determining the Mo flux and its δ98Mo to the open Arabian Sea.

Changes in trace metal sedimentation during freshening of a coastal basin

Holocene freshening has turned the Bothnian Bay, northern Baltic Sea into an oligotrophic basin. Sequestering of trace elements has changed significantly during the oligotrophication process. In principle, trace metals have been transferred from permanently buried sulfides to Fe–Mn-oxyhydroxides in the top layers of the sediment. The oxyhydroxide layers restrict the flux of trace metals from the sediment to the oxic bottom water. Hence, Fe–Mn cycling in the suboxic sediment enriches a number of trace metals in the surface sediment. Arsenic, Sn, Ge and Bi show enrichment in the Fe-oxyhydroxide layer, whereas Mo, Cd, Ni, Co, Cu, and Sb are enriched in the uppermost Mn-oxyhydroxide layer. This natural redox cycling in the sediment obscures pollution effects.The oligotrophication process started approximately 3500years ago, reflected in decreasing deposition of Zn, a proxy for phytoplankton production, and formation of Mn oxyhydroxide layers. Similarly, Ba/Al data indicate a decrease in the pelagic input of plankton. Barium data also suggest that dissolved sulfide in the sediment never reached high concentrations. Germanium is closely related to Ba, suggesting that Ge can be used as a proxy for phytoplankton production. Vanadium, U, Re, and Mo all indicate that the bottom water never has been significantly sulfidic during the last 5500years. Rhenium data indicate that the organic carbon oxidation rate has decreased during the last 5500years. Cadmium follows the organic matter distribution, but started to increase 1000YBP (years before present). The reason for this enhanced input of Cd is unclear.

Peak Last Glacial weathering intensity on the North American continent recorded by the authigenic Hf isotope composition of North Atlantic deep-sea sediments

We have retrieved radiogenic hafnium (Hf) isotope compositions (ɛHf) from authigenic Fe–Mn oxyhydroxides of deep northwest Atlantic sediments deposited over the past 26 ka to investigate the oceanic evidence of changes in dissolved weathering inputs from NE America during the last deglaciation. The extraction of seawater-derived Hf isotopic compositions from Fe–Mn oxyhydroxides is not a standard procedure. Comparisons between the Al/Hf ratios and Hf isotopic compositions of the chemically extracted authigenic phase on the one hand, and those of the corresponding detrital fractions on the other, provide evidence that the composition of past seawater has been reliably obtained for most sampled depths with our leaching procedures. This is endorsed most strongly by data for a sediment core from 4250 m water depth at the deeper Blake Ridge, for which consistent replicates were produced throughout. The Hf isotopic composition of the most recent sample in this core also closely matches that of nearby present day central North Atlantic seawater. Comparison with previously published seawater Nd and Pb isotope compositions obtained on the same cores shows that both Hf and Pb were released incongruently during incipient chemical weathering, but responded differently to the deglacial retreat of the Laurentide Ice Sheet. Hafnium was released more congruently during peak glacial conditions of the Last Glacial Maximum (LGM) and changed to typical incongruent interglacial ɛHf signatures either during or shortly after the LGM. This indicates that some zircon-derived Hf was released to seawater during the LGM. Conversely, there is no clear evidence for an increase in the influence of weathering of Lu-rich mineral phases during deglaciation, possibly since relatively unradiogenic Hf contributions from feldspar weathering were superimposed. While the authigenic Pb isotope signal in the same marine sediment samples traced peak chemical weathering rates on continental North America during the transition to the Holocene a similar incongruent excursion is notably absent in the Hf isotope record. The early change towards more radiogenic ɛHf in relation to the LGM may provide direct evidence for the transition from a cold-based to a warm-based Laurentide Ice Sheet on the Atlantic sector of North America.

Seasonal variations of arsenic at the sediment–water interface of Poyang Lake, China

Arsenic species including arsenite, arsenate, and organic arsenic were measured in the porewaters collected from Poyang Lake, the largest freshwater lake of China. The vertical distributions of dissolved arsenic species and some diagenetic constituents [Fe(II), Mn(II), S(−II)] were also obtained in the same porewater samples in summer and winter. In sediments the concentration profiles of total As and As species bound to Fe–Mn oxyhydroxides and to organic matter were also determined along with the concentrations of Fe, Mn and S in different extractable fractions. Results indicate that, in the summer season, the concentrations of total dissolved As varying from 3.9 to 55.8μg/L in sediments were higher than those (5.3–15.7μg/L) measured in the winter season, while the concentrations of total As species in the solid phase varied between 10.97 and 25.32mg/kg and between 7.84 and 30.52mg/kg on a dry weight basis in summer and winter, respectively. Seasonal profiles of dissolved As suggest downward and upward diffusion, and the flux of dissolved As across the sediment–water interface (SWI) in summer and winter were estimated at 3.88mg/m2a and 0.79mg/m2a, respectively. Based on porewater profiles and sediment phase data, the main geochemical behavior of As was controlled by adsorption/desorption, precipitation and molecular diffusion. The solubility and migration of inorganic As are controlled by Fe–Mn oxyhydroxides in summer whereas they appear to be more likely controlled by both amorphous Fe–Mn oxyhydroxides and sulfides in winter. A better knowledge of the cycle of As in Poyang Lake is essential to a better management of its hydrology and for the environmental protection of biota in the lake.

Evaluation of foraminiferal trace element cleaning protocols on the Mg/Ca of marine ostracod genus Krithe

The Mg/Ca of calcite from the marine ostracod genus Krithe may be an important tool for reconstructing past changes in oceanic bottom water temperature (150–4000m water depth). Rigorous cleaning procedures, routinely used to remove clays, organic matter and Fe–Mn oxyhydroxide coatings in trace element studies of foraminifera, are not regularly applied to marine ostracods despite the potential for Mg contamination. Here we apply standard oxidative and reductive foraminiferal cleaning procedures to core top Krithe pernoides valves from boxcore OCE205-50BC (26.23°N, 77.7°W, 817m water depth) to evaluate the effects of contamination on Mg/Ca ratios and assess the impact of cleaning techniques on contaminant removal and ostracod valve chemistry. Our results show that clays and Fe–Mn oxyhydroxides influence the Mg/Ca of Krithe. Following sonication in methanol/ultrapure water, there is a 1.6mmol/mol (11%) decrease in Mg/Ca (equivalent to a reduction in reconstructed temperature of 1.5°C), indicating that this is a critical step in the preparation of Krithe valves for Mg/Ca analyses. Oxidation with buffered hydrogen peroxide has little effect on the Mg/Ca of valves from our site. Reductive cleaning reduces inter-valve variability from 12% to 5%, resulting in an equivalent temperature precision of ±0.6°C. However, reductive cleaning also decreases Mg/Ca ratios due to the partial dissolution of the valve surface. Reductive cleaning offers the potential to improve Krithe Mg/Ca paleotemperature reconstructions and should be utilised in future Krithe Mg/Ca studies. Future work should also aim to constrain the effects of partial dissolution of the valve surface.

Field application of electrokinetic remediation for multi-metal contaminated paddy soil using two-dimensional electrode configuration

In this study, we evaluated the feasibility of in situ electrokinetic remediation for arsenic (As)-, copper (Cu)-, and lead (Pb)-contaminated soil, in a pilot-scale field application with two-dimensional electrode configurations. Square and hexagonal configurations with different electrode spacing, 1 m and 2 m, were investigated under a constant 100 V. A square configuration with electrode spacing of 2 m removed 61.5 % of As, 11.4 % of Cu, and 0.9 % of Pb, respectively, and a hexagonal configuration with the same spacing showed a higher removal efficiency in top (59 % of As, 0-0.5 m) and middle (53 % of As, 0.5-1.0 m) layers, but much lower removal efficiency in the bottom layer (1-1.5 m), which was thought to be due to groundwater flow through periodic rise and fall of tides. Fractionation analysis showed that As bound to Fe-Mn oxyhydroxide was the main form of As removed by the electrokinetic process. The two-dimensional configuration wasted less electrical energy by Joule heating, and required fewer electrode installations, compared to the one-dimensional electrode configuration.

A Pb isotope tracer of ocean-ice sheet interaction: the record from the NE Atlantic during the Last Glacial/Interglacial cycle

Ice sheet-ocean interactions are both a response to climate forcing and a source of climate feedback, releasing freshwater to the surface ocean and influencing climate and atmospheric CO2 through changes in ocean circulation. Documenting the outcomes of these interactions for recent glacial cycles is important given current and future scenarios of polar ice retreat. However, this is currently hampered by lack of accurate constraints on ice sheet development and demise. Marine sedimentary Pb isotope records have potential to investigate these aspects of ice sheet feedbacks at high temporal resolution because of the sensitivity of the Pb isotope composition to continental weathering intensity and solute flux. Here we present a Pb isotope record sourced from the FeMn oxyhydroxide fraction in marine sediments from ODP Site 980 on Feni Drift (2168 mbsl, Rockall Trough, NE Atlantic), spanning the last 43 ka. The location of Site 980 at the northern edge of the British–Irish Ice Sheet (BIIS) makes it well-placed to monitor changes in BIIS development as it responded to migration of the Polar Front during the Last Glacial/Interglacial cycle. The data reveal millennial-scale cyclicity in Pb isotope composition, reminiscent of Dansgaard–Oeschger cycles, from the start of the record until Heinrich event 2 (43–24 ka), characterised by extreme shifts to radiogenic compositions (i.e. variation in 206Pb/204Pb from ∼18.9 to 20.5). The period 24–17.5 ka is also characterised by exceptionally radiogenic and highly variable Pb isotope compositions, associated with the rapid and repeated expansion and collapse of the BIIS. The presence of such radiogenic Pb isotope compositions during periods of maximum ice sheet activity support interpretation of the subglacial environment as an active weathering environment, contributing to biogeochemical cycles through the transport vectors of meltwater release and debris-laden ice calving.

Evaluation of Pleistocene groundwater flow through fractured tuffs using a U-series disequilibrium approach, Pahute Mesa, Nevada, USA

Groundwater flow through fractured felsic tuffs and lavas at the Nevada National Security Site represents the most likely mechanism for transport of radionuclides away from underground nuclear tests at Pahute Mesa. To help evaluate fracture flow and matrix–water exchange, we have determined U-series isotopic compositions on more than 40 drill core samples from 5 boreholes that represent discrete fracture surfaces, breccia zones, and interiors of unfractured core. The U-series approach relies on the disruption of radioactive secular equilibrium between isotopes in the uranium-series decay chain due to preferential mobilization of 234U relative to 238U, and U relative to Th. Samples from discrete fractures were obtained by milling fracture surfaces containing thin secondary mineral coatings of clays, silica, Fe–Mn oxyhydroxides, and zeolite. Intact core interiors and breccia fragments were sampled in bulk. In addition, profiles of rock matrix extending 15 to 44mm away from several fractures that show evidence of recent flow were analyzed to investigate the extent of fracture/matrix water exchange.Samples of rock matrix have 234U/238U and 230Th/238U activity ratios (AR) closest to radioactive secular equilibrium indicating only small amounts of groundwater penetrated unfractured matrix. Greater U mobility was observed in welded-tuff matrix with elevated porosity and in zeolitized bedded tuff. Samples of brecciated core were also in secular equilibrium implying a lack of long-range hydraulic connectivity in these cases. Samples of discrete fracture surfaces typically, but not always, were in radioactive disequilibrium. Many fractures had isotopic compositions plotting near the 230Th-234U 1:1 line indicating a steady-state balance between U input and removal along with radioactive decay. Numerical simulations of U-series isotope evolution indicate that 0.5 to 1 million years are required to reach steady-state compositions. Once attained, disequilibrium 234U/238U and 230Th/238U AR values can be maintained indefinitely as long as hydrological and geochemical processes remain stable. Therefore, many Pahute Mesa fractures represent stable hydrologic pathways over million-year timescales. A smaller number of samples have non-steady-state compositions indicating transient conditions in the last several hundred thousand years. In these cases, U mobility is dominated by overall gains rather than losses of U.

Spatio-temporal dynamics of heavy metals in sediments of the river estuarine system: Mahanadi basin (India)

Dynamics of heavy metals in the surface sediments of Mahanadi river estuarine system were studied for three different seasons. This study demonstrates that the relative abundance of these metals follows in the order of Fe > Mn > Zn > Pb > Cr > Ni ≥ Co > Cu > Cd. The spatial pattern of heavy metals supported by enrichment ratio data, suggests their anthropogenic sources possibly from various industrial wastes and municipal wastes as well as agricultural runoff. The metal concentrations in estuarine sediments are relatively higher than in the river due to adsorption/accumulation of metals on sediments during saline mixing, while there is a decreasing trend of heavy metal concentrations towards the marine side. The temporal variations for metals, such as Fe, Mn, Zn, Ni and Pb exhibit higher values during monsoon season, which are related to agricultural runoff. Higher elemental concentrations are observed during pre-monsoon season for these above metals (except Ni) at the polluted stations and for metals, such as Cr, Co and Cd at all sites, which demonstrate the intensity of anthropogenic contribution. R-mode factor analysis reveals that “Fe–Mn oxy hydroxide”, “organic matter”, “CaCO3”, and “textural variables” factors are the major controlling geochemical factors for the enrichment of heavy metals in river estuarine sediment and their seasonal variations, though their intensities were different for different seasons. The relationships among the stations are highlighted by cluster analysis, represented in dendrograms to categorize different contributing sites for the enrichment of heavy metals in the river estuarine system.

Geochemistry of trace and minor elements in sediments and manganese micronodules from the Angola Basin

Indicator role of trace elements in sedimentation and ore formation is considered for sediments from Station 2182 in the Angola Basin. It is shown that pelagic sediments were formed from two main sources: biogenic calcium carbonate and lithogenic sediment component compositionally similar to the miopelagic clay. Increase of the Mn/Al ratio, Ce anomaly in the REE composition, Co/Ni and Mo/W ratios, and anom� alous accumulation of Tl, Pb, Bi and other microelements indicate that sediments from horizons 15-20 and 30-35 cm contain significant amounts of hydrogenic material as Fe-Mn oxyhydroxides. Manganese micronodules (MN) were extracted from different horizons (10-15, 15-20, and 30-35 cm) and analyzed to study the hydrogenic component. Their development is related to retardation of biogenic and lithogenic sed� imentation. The studied manganese micronodules are represented by the hydrogenicdiagenetic formations >100 μm in size with Mn/Fe = 2.0-2.8, Co/Ni = 0.2-0.4, Ce an = 4.2-5.7, and Mo/W = 5.2-7.9. The MN content is too low to affect the major and trace element composition of sediments. The main part of Fe and Mn is confined to fraction <10 μm.

Molybdenum geochemistry in a seasonally dysoxic Mo-limited lacustrine ecosystem

Lakes are important for storage of the essential micronutrient molybdenum (Mo) during its transfer from the continents to the oceans, but little is known about the major sources and sinks for Mo in lacustrine ecosystems. We studied Mo cycling in Castle Lake, a small subalpine lake in the Klamath-Siskiyou Mountains of Northern California underlain primarily by mafic and ultramafic rocks where primary productivity is limited by Mo bioavailability. The deeper water of the lake becomes dysoxic (9–90μM dissolved oxygen) during the summer. This study was undertaken to identify the sources of Mo to Castle Lake and establish a Mo budget. We measured Mo concentrations in a suite of bulk solids (lake sediments, soils and bedrock) and aqueous samples (sediment porewaters, soil runoff, spring waters, snow and ice) from Castle Lake and its watershed. Lake sediments have elevated Mo (7–36ppm) compared to soils and bedrock (0.2–2ppm) and Mo/Al values were nearly two orders of magnitude higher than the crustal abundance. Sediment porewaters had higher Mo (4–15nM) than lake water (2–4nM), soil runoff (0.1–6.2nM), snowmelt (⩽0.1nM), lake ice (0.3–2.2nM) and local spring waters (0.03–2.72nM). Bulk lake sediments had negative δ98/95Mo values, ranging from −0.5 to −1.0‰ (±0.1). We used the numerical model PROFILE to estimate the net reaction rate of Mo in the porewater. Model calculations ruled out diagenesis as a source of Mo to lake sediments; diagenetic Mo always represented ⩽5% of the total Mo content in sediment. We also ruled out dissolved Mo inputs from groundwater and watershed inflow as important sources of Mo. Two whole-lake experimental Mo additions in the 1960’s could have contributed a sizeable amount of Mo to the lake sediments, but only over a short period. Atmospheric deposition of anthropogenic Mo from extensive copper smelting that occurred south of Castle Lake from 1896 to 1919 and from major Californian urban centers today were negligible Mo sources. Mo flux from the sediments (0.4–0.5nmolcm−2yr−1) was roughly equal to Mo fluxes from surface inflow and outflow, whereas Mo burial fluxes were significantly higher (11.5nmolcm−2yr−1). Because dissolved Mo fluxes were minimal, and atmospheric Mo deposition was estimated to be a minor source of Mo (<1nmolcm−2yr−1), the largest source of Mo is non-detrital particulate matter (∼12nmolcm−2yr−1), likely a mixture of organic matter and Fe–Mn oxyhydroxides as supported by Mo isotopic data.

전기동력학적 정화기술 적용을 위한 최적의 전해질 선택 및 전극간의 거리 평가

The influence of processing fluids and electrode spacing on the electrokinetic process was evaluated to remediate As-, Cu-, Pb-contaminated soil. Single and mixture of sodium citrate, EDTA and NaOH was used to investigate the metal extraction. EDTA for washing reagent showed the highest removal efficiency. Based on the extraction result, the electrode spacing (20, 40, 60 cm) on the electrokinetic process was investigated to remove the multi-metals from soil. The highest removal was observed at the experiment with 60 cm of electrode spacing, however, the correlation between electrode spacing and removal of metals was not clear. The electrode spacing influenced the amount of accumulated electro-osmotic flow. BCR sequential extraction showed that electrokinetic process removed the fractionation of metals bound to Fe-Mn oxyhydroxide.

Understanding distribution and abundance of metals with space and time in estuarine mudflat sedimentary environment

Spatial and depth-wise distribution of sediment components, organic carbon and selected metals (Fe, Mn, Al, Ni, Cr, Co, Zn and Pb) is studied across upper and middle tidal flats from lower and middle estuarine regions of Kundalika Estuary, central west coast of India. Silt and clay form the major components in lower and middle estuary, respectively. Sand, silt, clay and organic carbon showed band-type distribution along the estuary. The sediment deposition over the years took place in varying hydrodynamic conditions in lower and middle estuarine regions. Upper flats of the lower estuary represent mud while middle flats of the lower estuary facilitated the deposition of sandy mud. Correlation results indicated the importance of clay and organic carbon in removal and trapping of metals at lower and middle estuary, respectively. Factor analysis indicated that the distribution of metals is largely controlled by Fe–Mn oxyhydroxides and organic carbon. The middle flats of the lower estuary showed an anthropogenic source for Ni, Cr and Co while middle flats of the middle estuary showed a mainly lithogenic source.

Tracing the Nd isotope evolution of North Pacific Intermediate and Deep Waters through the last deglaciation from South China Sea sediments

The intermediate and deep waters of the Pacific Ocean play a crucial role for regulating global climate changes on the millennium timescale. However, due to poor preservation of carbonate sediments in the deep Pacific, little attention has been received to better understand deep ocean circulation and its relationship with abrupt climate events. Here we present the first authigenic Nd isotope (εNd) record extracted from Fe–Mn oxyhydroxides of the South China Sea (SCS) sediments (SO17940-2, 1727m water depth) in order to investigate changes of intermediate and deep-water circulations in the Pacific Ocean since the Last Glacial Maximum (LGM).In general, the seawater εNd record extracted from the SCS sediments paralleled the existing Nd isotopes of fish teeth/debris from the eastern North Pacific core during the last deglaciation, demonstrating that our authigenic εNd can faithfully record the seawater εNd evolution in the deep Pacific Ocean. At the onset of the deglacial period, our data exhibit a clear negative shift in εNd towards a gradually increased component of Antarctica Intermediate Water (AAIW) and reached a maximal influx during the mid Heinrich Stadial 1 (HS1, ∼16kyr BP). Between the HS1 and Younger Dryas, deep-water circulation gradually shifted back to the modern condition during the Bølling-Allerød warm interval. A pronounced negative εNd excursion had occurred during the Pre-Boreal (PB), suggesting a rapid reorganization of deep-water ventilation in the Pacific Ocean and was predominated by AAIW. This is most likely due to poor ventilation in the western North Pacific associated with an intensified summer monsoon during the PB-early Holocene warm period. After the PB period, the seawater εNd values were once again dominated by Pacific Deep Water at the study site.