Water Redox Conditions Research Articles

Distribution of tetraether lipids in agricultural soils – differentiation between paddy and upland management

Abstract. Rice paddies constitute almost a fifth of global cropland and provide more than half of the world's population with staple food. At the same time, they are a major source of methane and therewith significantly contribute to the current warming of Earth's atmosphere. Despite their apparent importance in the cycling of carbon and other elements, however, the microorganisms thriving in rice paddies are insufficiently characterized with respect to their biomolecules. Hardly any information exists on human-induced alteration of biomolecules from natural microbial communities in paddy soils through varying management types (affecting, e.g., soil or water redox conditions, cultivated plants). Here, we determined the influence of different land use types on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs), which serve as molecular indicators for microbial community structures, in rice paddy (periodically flooded) and adjacent upland (non-flooded) soils and, for further comparison, forest, bushland and marsh soils. To differentiate local effects on GDGT distribution patterns, we collected soil samples in locations from tropical (Indonesia, Vietnam and Philippines) and subtropical (China and Italy) sites. We found that differences in the distribution of isoprenoid GDGTs (iGDGTs) as well as of branched GDGTs (brGDGTs) are predominantly controlled by management type and only secondarily by climatic exposition. In general, upland soil had higher crenarchaeol contents than paddy soil, which by contrast was more enriched in GDGT-0. The GDGT-0 ∕ crenarchaeol ratio, indicating the enhanced presence of methanogenic archaea, was 3–27 times higher in paddy soils compared to other soils and increased with the number of rice cultivation cycles per year. The index of tetraethers consisting of 86 carbons (TEX86) values were 1.3 times higher in upland, bushland and forest soils than in paddy soils, potentially due to differences in soil temperature. In all soils brGDGT predominated over iGDGTs with the relative abundance of brGDGTs increasing from subtropical to tropical soils. Higher branched vs. isoprenoid tetraether (BIT) values in paddy soils compared to upland soils together with higher BIT values in soils from subtropical climates indicated effects on the amounts of brGDGT induced by differences in management as well as climate. In acidic soils cyclization ratio of branched tetraethers (CBT) values correlated well with soil pH. In neutral to alkaline soils, however, no correlation but an offset in CBT between paddy and upland managed soils was detected. This is interpreted as indicating soil moisture exerting an additional control on the CBT in these soils. Lower modified methylation index of branched tetraether (MBT′) values and temperatures calculated from this (TMC) in paddy soils compared to upland soils are attributed to a management-induced (e.g. enhanced soil moisture via flooding) effect on mean annual soil temperature (MST).

Water level fluctuations in a tropical reservoir: the impact of sediment drying, aquatic macrophyte dieback, and oxygen availability on phosphorus mobilization.

Reservoirs in semi-arid areas are subject to water level fluctuations (WLF) that alter biogeochemical processes in the sediment. We hypothesized that wet-dry cycles may cause internal eutrophication in such systems when they affect densely vegetated shallow areas. To assess the impact of WLF on phosphorus (P) mobilization and benthic P cycling of iron-rich sediments, we tested the effects of (i) sediment drying and rewetting, (ii) the impact of organic matter availability in the form of dried Brazilian Waterweed (Egeria densa), and (iii) alternating redox conditions in the surface water. In principle, drying led to increased P release after rewetting both in plant-free and in plant-amended sediments. Highest P mobilization was recorded in plant amendments under oxygen-free conditions. After re-establishment of aerobic conditions, P concentrations in surface water decreased substantially owing to P retention by sediments. In desiccated and re-inundated sediments, P retention decreased by up to 30% compared to constantly inundated sediments. We showed that WLF may trigger biochemical interactions conducive to anaerobic P release. Thereby, E. densa showed high P release and even P uptake that was redox-controlled and superimposed sedimentary P cycling. Macrophytes play an important role in the uptake of P from the water but may be also a significant source of P in wet-dry cycles. We estimated a potential for the abrupt release of soluble reactive phosphorus (SRP) by E. densa of 0.09-0.13 g SRP per m(2) after each wet-dry cycle. Released SRP may exceed critical P limits for eutrophication, provoking usage restrictions. Our results have implications for management of reservoirs in semi-arid regions affected by WLF.

Benthic-pelagic coupling of nutrients and dissolved organic matter composition in an intertidal sandy beach

Subterranean estuaries (STEs) are important biogeochemical land–sea interfaces, where fresh groundwater mixes with seawater in coastal aquifers. However, the sources of dissolved organic matter (DOM) and the connection of its molecular-level processing to pore water chemistry and redox conditions in these ecosystems are still not well understood. We studied the cycling of DOM in the STE of an intertidal sandy beach of the North Sea on spatial and seasonal scales. Ultrahigh-resolution mass spectrometry was used to identify thousands of DOM molecular formulae. These data were interpreted in the context of inorganic pore water chemistry, stable carbon isotope composition of solid-phase extracted (SPE) DOM and chemical tracers for bioavailable (dissolved carbohydrates, DCHOs) as well as biorefractory DOM (dissolved black carbon, DBC). Numerical modelling was used to estimate pore water residence times indicating relatively young pore water in the upper saline plume (USP, <4 years) and decades-old groundwater in the freshwater discharge tube. The detected levels of dissolved Fe and ammonium in the USP at sediment depths exceeding 50cm demonstrated suboxic conditions. Statistical analyses revealed complex biotic and abiotic DOM processing apart from conservative mixing of marine and terrestrial endmembers. We propose that the input of bioavailable marine and terrestrial DOM, such as DCHOs, by percolating seawater and meteoric groundwater and its degradation by microbes caused oxygen depletion favoring Fe oxide/hydroxide reduction. In the freshwater discharge tube, the presence of highly aromatic compounds, DBC, and 13C-depleted SPE-DOM indicated the intrusion of meteoric groundwater containing terrestrial DOM. The discharge of this groundwater appears to be a significant source of nutrients (e.g., ammonium) and biorefractory, e.g. combustion-derived, DOM to the adjacent water column.

Environmental changes in the catchment of the Jarosławianka Stream recorded in mill pond deposits

Abstract Interdisciplinary studies on environmental evolution during the last 650 years based on the analysis of mill pond deposits have been conducted in the valley of the Jarosławianka Stream (left-bank tributary of the River Wieprza). According to historical data, confirmed also by radiocarbon dating, the water mill was located in the valley in 1351 and operated until the 1960s. The sequence of stratified deposits with a thickness of up to about 2 metres was accumulated in mill pond basin during this period. The vertical variability in the physical and chemical properties of these deposits reflects temporal environmental changes in the catchment, particularly the intensity of chemical and mechanical denudation and human activity. Analysis of subfossil Cladocera suggests four phases in the development of the mill pond. Changing along with the intensity of the pond basin water flow, redox and nutritional conditions are reflected in the spatial variability of deposits, especially their textural parameters, pH and abundance in organic carbon, nitrogen and carbonates.

Geochemical changes in the Early Cambrian interval of the Yangtze Platform, South China: Implications for hydrothermal influences and paleocean redox conditions

The possible triggering mechanisms of the Ediacaran–Cambrian transition in South China have been debated for many years. In this study, 80 samples from Liuchapo, Niutitang and Mingxinsi Formations were collected from Well YK-1, located in southeastern Chongqing. A high-resolution chemostratigraphic study, including evaluation of rare earth elements (REEs) and trace elements, was used to elucidate the paleoenvironments of the Upper Yangtze Platform margin. The Mingxinsi gray shale and the Liuchapo Formation exhibit flat Post-Archean Australian-normalized REE patterns with weakly negative Ce anomalies, which is typical of detritus. Obvious positive Eu anomalies in chert samples of the basal Niutitang Formation imply a short-term influence of hydrothermal fluid. The REE patterns of the basal Niutitang phosphatic rocks show significant enrichment of heavy REEs, significantly negative Ce anomalies, and positive Y anomalies, similar to the REE-distribution pattern in seawater, demonstrating that these sediments were produced under oxic seawater conditions. Moderately positive Eu anomalies and positive Y anomalies of the lower Niutitang shales may indicate mixing of hydrothermal fluids and seawater. The flat REE patterns with weak Eu anomalies of the upper Niutitang shale may have been generated by hydrothermal inputs diluted by detrital materials. Dramatic increases of Ce anomalies in the basal Niutitang Formation reveal a significant decrease of the oxygen level in seawater. Trace elements, such as Ba, U, V, U, Cu, Cr, Zn, Ni and Sr, are commonly enriched in these formations compared to the upper continental crust. Authigenic abundances of Al-normalized U and Al-normalized V as well as ratios of Th/U and V/Sc are used as indicators of redox conditions. These distributions demonstrate that the redox conditions of the bottom water during the Ediacaran–Cambrian transition abruptly changed from oxic in the Nemakit–Daldynian age to anoxic in the early Tommotian age, and gradually became less reducing in the late Tommotian age and the Atdabanian age. A strong correlation between total organic carbon (TOC) concentrations and ratios of U/Al and V/Al in the entire section (except in the phosphatic rocks) implies non-sulfidic conditions. Most trace elements cannot be used as proxies for redox conditions associated with the phosphatic rocks in the basal Niutitang Formation because abundant trace elements were especially absorbed in apatite. The TOC concentrations correlated well with the redox conditions of bottom water. We infer that the section deposited in the platform margin was influenced by episodic hydrothermal activities that began at the beginning of the Tommotian age, and these activities may have terminated at the beginning of the Atdabanian age. Enrichments of organic matter that were mainly derived from the photic zone were controlled by the redox conditions of the bottom water.

Carbon isotopes, sulfur isotopes, and trace elements of the dolomites from the Dengying Formation in Zhenba area, southern Shaanxi: Implications for shallow water redox conditions during the terminal Ediacaran

Carbon isotope, sulfur isotope, and trace element (including Rare Earth Elements, REE) analyses were conducted on the carbonates of the Dengying Formation at Lianghekou section in southern Shaanxi to reconstruct the terminal Ediacaran shallow-water environment on the northwestern margin of the Yangtze Platform. At Lianghekou section, samples in the middle 50-m of the Beiwan Member show characteristics of low ΣREE concentrations, no MREE-enriched REE distribution patterns, high Ce/Ce* values close to 1, and enriched redox-sensitive elements, whereas samples in the lower 30-m and upper 10-m show opposite characteristics of high ΣREE concentrations, MREE-enriched REE distribution patterns, low Ce/Ce* values around 0.6, and no redox-sensitive elements enriched, indicating that oxygenation did occur in the shallow water on the northwestern margin of the Yangtze Platform and redox conditions of the shallow water fluctuated from relatively oxygenated to anoxic and then back to oxygenated again. We propose that the anoxia appeared in middle of the Beiwan time may associate with the anoxic upwelled water. On one hand, abundant nutrients were brought in by this upwelling event, which stimulated the photosynthetic carbon fixation and increased the organic carbon burial under this anoxic condition, causing a peak of 3.6‰ in γ 13 C. On the other hand, because the anoxic upwelled water replaced the oxic shallow water, together with the increasing organic matter in the water column, bacterial sulfate reduction was enhanced and therefore quickly reduced the sulfate concentration, which eventually caused γ 34 S increasing to 50‰. However, as the upwelling gradually disappeared, γ 13 C and γ 34 SCAS values decreased as well in the late Beiwan time, indicating the shallow water went back to suboxic or oxic again.

Bio-Electrochemical Systems in the Subalpine Lake Idro (Italy)

The subalpine Lake Idro in the north of Italy is here described from a physico-chemical and electrochemical point of view. The glacial origin of the lake, together with the steep walls of the basin and the maximum depth of 120 m, creates the conditions for the evolution of a stable meromictic condition during the year. The presence of a superficial layer of aerobic water (a mixolimion of about 50 m) and of a deep constant layer under anaerobiosis (monimolimion) makes it a unique environment for electrochemical studies. A peculiarity of this lake is the presence of a cloudy white layer at the chemocline, detected at 40-60 m of depth by scuba divers and never reported in literature. The hydrographic basin is mostly composed of dolomite, the typical rock of the Alps. The general chemical composition is Mg(Ca)CO3, but it could include impurities such as Fe and Mn.Many are the peculiarities converging to produce a unique environment for Mn cycling in the mixolimion, chemocline, and monimolimion characterizing the lake. On the basis of home-built sensors and a standard multiparametric probe, oxygen, pH, sulphide, conductivity, red-ox and temperature profiles were determined. Strict correlations were found between these parameters and the different oxidation states of Mn (Mn+2, Mn+3 and Mn+4) detected in the water and in the particulate. Equilibria with Fe and other elements have been considered. A chain of copper alloy (CuNi 70/30) and carbon steel was immersed at different depths in the water down to the bottom of the lake, in a point where the lake depth is the maximum (120 m), to study electrochemical and biological characteristics of the lake. Unique correlations between the chemical-physical parameters and electrochemical characteristics of copper alloys specimens, exposed at different depth in the lake, were demonstrated. The open circuit potential of copper alloys, measured versus the same Ag/AgCl reference electrode used for the sulphide measurements, varied as a function of the oxygen concentration in the different water layers and stabilized in time in each layer (from +200 mV down to -600 mV vs. Ag/AgCl). The results indicated that, in different ways, handmade sensors can be cost effective tools, able to give insight into the redox conditions of water. The stratification of the lake poses a new interesting studies for MFCs. In this environment, microbial fuel cells (MFC) might produce power by utilizing the microbial capacity of catalyzing the oxygen reduction at the cathode, and the Mn(OH)2 oxidation at the anode. Another type of MFC might develop power by reduction of Mn2O3. The natural scenario of the Lake Idro suggests studies of new bio-electrochemical technologies that could be applied for environment-depollution purposes.

Cadmium-isotopic evidence for increasing primary productivity during the Late Permian anoxic event

Earth's most extreme extinction event near the end of the Late Permian decimated more than 90% of all extant marine species. Widespread and intensive oceanic anoxia almost certainly contributed to the catastrophe, though the driving mechanisms that sustained such conditions are still debated. Of particular interest is whether water column anoxia was a consequence of a ‘stagnant ocean’, or if it was controlled by increases in nutrient supply, primary productivity, and subsequent heterotrophic respiration. Testing these competing hypotheses requires deconvolving sedimentary/bottom water redox conditions from changes in surface water productivity in marine sediments. We address this issue by studying marine shales from East Greenland and the mid-Norwegian shelf and combining sedimentary redox proxies with cadmium-isotopic analyses. Sedimentary nitrogen-isotopic data, pyrite framboid analyses, and organic and inorganic shale geochemistry reveal sulfidic conditions with vigorous upwelling, and increasingly anoxic conditions with a strengthening upwelling in the Greenland and Norwegian sections, respectively. Detailed analysis of sedimentary metal budgets illustrates that Cd is primarily associated with organic carbon and records primary geochemical signatures, thus enabling reconstruction of surface water nutrient utilization. Cadmium-isotopic analyses of the authigenic shale fraction released by inverse aqua regia digestion yield an average δCd114/110 of +0.15±0.01‰ (2 SE, n=12; rel. NIST SRM 3108), indicative of incomplete surface water nutrient utilization up-section. The constant degree of nutrient utilization combined with strong upwelling requires increasing primary productivity – and not oceanic stagnation – to balance the larger nutrient fluxes to both study sites during the development of the Late Permian water column anoxia. Overall, our data illustrate that if bottom water redox and upwelling can be adequately constrained, Cd-isotopic analyses of organic-rich sediments can be used to provide valuable information on nutrient utilization and therefore past productivity.

Projecting changes in Everglades soil biogeochemistry for carbon and other key elements, to possible 2060 climate and hydrologic scenarios.

Based on previously published studies of elemental cycling in Everglades soils, we projected how soil biogeochemistry, specifically carbon, nitrogen, phosphorus, sulfur, and mercury might respond to climate change scenarios projected for 2060 by the South Florida Water Management Model. Water budgets and stage hydrographs from this model with future scenarios of a 10% increased or decreased rainfall, a 1.5 °C rise in temperature and associated increase in evapotranspiration (ET) and a 0.5 m rise in sea level were used to predict resulting effects on soil biogeochemistry. Precipitation is a much stronger driver of soil biogeochemical processes than temperature, because of links among water cover, redox conditions, and organic carbon accumulation in soils. Under the 10% reduced rainfall scenario, large portions of the Everglades will experience dry down, organic soil oxidation, and shifts in soil redox that may dramatically alter biogeochemical processes. Lowering organic soil surface elevation may make portions of the Everglades more vulnerable to sea level rise. The 10% increased rainfall scenario, while potentially increasing phosphorus, sulfur, and mercury loading to the ecosystem, would maintain organic soil integrity and redox conditions conducive to normal wetland biogeochemical element cycling. Effects of increased ET will be similar to those of decreased precipitation. Temperature increases would have the effect of increasing microbial processes driving biogeochemical element cycling, but the effect would be much less than that of precipitation. The combined effects of decreased rainfall and increased ET suggest catastrophic losses in carbon- and organic-associated elements throughout the peat-based Everglades.

Paleoenvironmental implications of uranium concentrations in lacustrine calcareous clastic-evaporite deposits in the western Qaidam Basin

Uranium (U) provides especially useful information for reconstructing paleoredox conditions due to its unique geochemical properties and behavior in supergene environments. Its proxy potential under suboxic–oxic conditions associated with salinization of surface water in closed hydrologic systems, however, has not been adequately explored. This study examined the abundance and distribution of U in a sedimentary sequence of calcareous lacustrine clastics and salt layers from a high quality (938.5m-long) core drilled in the western Qaidam Basin, NE Tibetan Plateau. This study examined U concentrations in calcareous clastic sediments (including carbonates, acid insoluble fractions, as well as the amounts of iron and manganese oxides/oxyhydroxides) and salt layers. In the calcareous clastic sediments, variations in U concentrations within the acid insoluble fraction follow the fluctuations of reconstructed lake levels based on lithologic investigations, providing a tool for tracing redox conditions in paleolake bottom water. Correlations between the variations in carbonate content with U and Sr concentrations in the carbonate fractions demonstrate that the types and abundance of carbonate minerals control the U distribution in carbonates. In the salt layers, U and Al concentrations correlate closely, implying that both derived mainly from the detrital fraction in the salt layers. However, enrichments of U and Mg are systematically higher in salt minerals formed in more concentrated brine and lower in those formed in less concentrated brine. These results indicate that during the process of carbonate precipitation, carbonate might effectively remove U from lake water. During the subsequent stage of salt mineral precipitation, U behavior is conservative, and U resides for a long period in dissolved forms within the saline water, giving rise to enhanced U enrichment in the more concentrated brines and the associated precipitated salt minerals. Our study suggests, however, that U-related redox proxies should be interpreted with caution in suboxic–oxic environments during the long-term paleolake evolution.

Processes controlling the fate of chloroethenes emanating from DNAPL aged sources in river–aquifer contexts

This work dealt with the physical and biogeochemical processes that favored the natural attenuation of chloroethene plumes of aged sources located close to influent rivers in the presence of co-contaminants, such as nitrate and sulfate. Two working hypotheses were proposed: i) Reductive dechlorination is increased in areas where the river–aquifer relationship results in the groundwater dilution of electron acceptors, the reduction potential of which exceeds that of specific chloroethenes; ii) zones where silts predominate or where textural changes occur are zones in which biodegradation preferentially takes place. A field site on a Quaternary alluvial aquifer at Torelló, Catalonia (Spain) was selected to validate these hypotheses. This aquifer is adjacent to an influent river, and its redox conditions favor reductive dechlorination. The main findings showed that the low concentrations of nitrate and sulfate due to dilution caused by the input of surface water diminish the competition for electrons between microorganisms that reduce co-contaminants and chloroethenes. Under these conditions, the most bioavailable electron acceptors were PCE and metabolites, which meant that their biodegradation was favored. This led to the possibility of devising remediation strategies based on bioenhancing natural attenuation. The artificial recharge with water that is low in nitrates and sulfates may favor dechlorinating microorganisms if the redox conditions in the mixing water are sufficiently maintained as reducing and if there are nutrients, electron donors and carbon sources necessary for these microorganisms.

Characterising the nickel isotopic composition of organic-rich marine sediments

New Ni stable isotope data (δ60Ni) determined by double-spike MC-ICP-MS for two geologically distinct suites of organic-rich marine sediments from the Sinemurian–Pliensbachian (S–P) Global Stratotype Section and Point (GSSP; Robin Hood's Bay, UK) and the Devonian–Mississippian Exshaw Formation (West Canada Sedimentary Basin) is presented herein. These sediments yield δ60Ni values of between 0.2‰ and 2.5‰, and predominantly have Ni isotopic compositions that are heavier than those of abiotic terrestrial and extraterrestrial samples (0.15‰ and 0.27‰), and in some cases present-day seawater (1.44‰) and dissolved Ni from riverine input (0.80‰). In addition, the observed degree of isotopic fractionation in the marine sediments is far greater than that of these other sample matrices. However, a strong similarity is exhibited between the δ60Ni values of the organic-rich sediments studied here and those of ferromanganese crusts (0.9–2.5‰), suggesting that factors ubiquitous to the marine environment are likely to play a key role in the heightened level of isotopic fractionation in these sample matrices.A lack of correlation between the Ni stable isotope compositions of the organic-rich sediments and Ni abundance suggests that isotopic fractionation in these sediments is not controlled by incorporation or enrichment of Ni during sediment accumulation. Further, no relationship is observed between δ60Ni and TOC concentrations or bottom–water redox conditions, indicating that the organic carbon reservoir and levels of oxygenation at the sediment–water interface do not exert a primary control on Ni isotope fractionation in marine sediments. Following examination of these relationships, it is therefore more likely that the heavy Ni isotope compositions of marine sediments are controlled by the weathering environment and the dominant sources of dissolved Ni into the global ocean reservoir.

Biogeochemistry of the North Atlantic during oceanic anoxic event 2: role of changes in ocean circulation and phosphorus input

Abstract. The geological record provides evidence for the periodic occurrence of water column anoxia and formation of organic-rich deposits in the North Atlantic Ocean during the mid-Cretaceous (hereafter called the proto-North Atlantic). Both changes in primary productivity and oceanic circulation likely played a role in the development of the low-oxygen conditions. Several studies suggest that an increased input of phosphorus from land initiated oceanic anoxic events (OAEs). Other proposed mechanisms invoke a vigorous upwelling system and an ocean circulation pattern that acted as a trap for nutrients from the Pacific Ocean. Here, we use a detailed biogeochemical box model for the proto-North Atlantic to analyse under what conditions anoxia could have developed during OAE2 (94 Ma). The model explicitly describes the coupled water, carbon, oxygen and phosphorus cycles for the deep basin and continental shelves. In our simulations, we assume the vigorous water circulation from a recent regional ocean model study. Our model results for pre-OAE2 and OAE2 conditions are compared to sediment records of organic carbon and proxies for photic zone euxinia and bottom water redox conditions (e.g. isorenieratane, carbon/phosphorus ratios). Our results show that a strongly elevated input of phosphorus from rivers and the Pacific Ocean relative to pre-OAE2 conditions is a requirement for the widespread development of low oxygen in the proto-North Atlantic during OAE2. Moreover, anoxia in the proto-North Atlantic is shown to be greatly influenced by the oxygen concentration of Pacific bottom waters. In our model, primary productivity increased significantly upon the transition from pre-OAE2 to OAE2 conditions. Our model captures the regional trends in anoxia as deduced from observations, with euxinia spreading to the northern and eastern shelves but with the most intense euxinia occurring along the southern coast. However, anoxia in the central deep basin is difficult to achieve in the model. This suggests that the ocean circulation used in the model may be too vigorous and/or that anoxia in the proto-North Atlantic was less widespread than previously thought.

Uranium concentrations and 238U/235U isotope ratios in modern carbonates from the Bahamas: Assessing a novel paleoredox proxy

In order to validate the use of 238U/235U as a paleoredox proxy in carbonates, we examined the incorporation and early diagenetic evolution of U isotopes in shallow Bahamian carbonate sediments. Our sample set consists of a variety of primary precipitates that represent a range of carbonate producing organisms and components that were important in the past (scleractinian corals, calcareous green and red algae, ooids, and mollusks). In addition, four short push cores were taken in different depositional environments to assess the impact of early diagenesis and pore water chemistry on the U isotopic composition of bulk carbonates. We find that U concentrations are much higher in bulk carbonate sediments (avg. 4.1ppm) than in primary precipitates (avg. 1.5ppm). In almost all cases, the lowest bulk sediment U concentrations were as high as or higher than the highest concentrations found in primary precipitates. This is consistent with authigenic accumulation of reduced U(IV) during early diagenesis. The extent of this process appears sensitive to pore water H2S, and thus indirectly to organic matter content. δ238/235U values were very close to seawater values in all of the primary precipitates, suggesting that these carbonate components could be used to reconstruct changes in seawater U geochemistry. However, δ238/235U of bulk sediments from the push cores was 0.2–0.4‰ heavier than seawater (and primary precipitates). These results indicate that authigenic accumulation of U under open-system sulfidic pore water conditions commonly found in carbonate sediments strongly affects the bulk U concentrations and 238U/235U ratios. We also report the occurrence of dolomite in a tidal pond core which contains low 234U/238U and 238U/235U ratios and discuss the possibility that the dolomitization process may result in sediments depleted in 238U. From this initial exploration, it is clear that 238U/235U variations in ancient carbonate sediments could be driven by changes in global average seawater, by spatial and temporal variations in the local deposition environment, or subsequent diagenesis. To cope with such effects, proxies for syndepositional pore water redox conditions (e.g., organic matter content, iron speciation, and trace metal distributions) and careful consideration of possible post-deposition alteration will be required to avoid spurious interpretation of 238U/235U data from ancient carbonate sediments.

Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic

The global variability in nickel (Ni) isotope compositions in ferromanganese crusts is investigated by analysing surface samples of 24 crusts from various ocean basins by MC–ICPMS, using a double-spike for mass bias correction. Ferromanganese crusts have δNi60 isotopic compositions that are significantly heavier than any other samples thus far reported (−0.1‰ to 0.3‰), with surface scrapings ranging between 0.9‰ and 2.5‰ (relative to NIST SRM986). There is no well resolved difference between ocean basins, although the data indicate somewhat lighter values in the Atlantic than in the Pacific, nor is there any evidence that the variations are related to biological fractionation, presence of different water masses, or bottom water redox conditions. Preliminary data for laterite samples demonstrate that weathering is accompanied by isotopic fractionation of Ni, which should lead to rivers and seawater being isotopically heavy. This is consistent with the slightly heavier than average isotopic compositions recorded in crusts that are sampled close to continental regions. Furthermore, the isotopic compositions of crusts growing close to a hydrothermal source are clustered around ∼1.5‰, suggesting that hydrothermal fluids entering the ocean may have a Ni isotopic composition similar to this value. Based on these data, the heavy Ni isotopic compositions of ferromanganese crusts are likely due to input of isotopically heavy Ni to the ocean from continental weathering and possibly also from hydrothermal fluids. A depth profile through one crust, CD29-2, from the north central Pacific Ocean displays large variations in Ni isotope composition (1.1–2.3‰) through the last 76Myr. Although there may have been some redistribution of Ni associated with phosphatisation, there is no systematic difference in Ni isotopic composition between deeper, older parts and shallower, younger parts of the crust, which may suggest that oceanic sources and sinks of Ni have largely remained in steady state over the Cenozoic. Additionally, the isotope profile is in agreement with a profile of Mn concentration through the same crust. This implies a link between the Ni isotopic composition recorded in ferromanganese crusts and the release of Ni into the ocean through hydrothermal activity. This supports the conclusions drawn from surface data, that Ni isotope ratios in ferromanganese crusts are largely controlled by the isotopic compositions of the Ni oceanic input sources.

Shallow water redox conditions from the Permian–Triassic boundary microbialite: The rare earth element and iodine geochemistry of carbonates from Turkey and South China

Redox sensitive elements serve as useful proxies of the oxygenation state of ancient environments, but their interpretation may not always be straightforward. To evaluate the inherent complexities, rare earths and yttrium (REY), iodine, and major element concentrations are determined in two carbonate sections spanning the Permian–Triassic (P–Tr) transition in Demirtas, Turkey and Cili, South China. We use major oxides to identify non-seawater REY sources such as siliciclastics, Fe-oxides, phosphates and diagenetic fluids. Additionally, we employ Y/Ho ratio, La anomaly, and light rare earth element depletion to identify which samples preserve a seawater-like REYSN distribution. In contrast to past interpretations, we find that the P–Tr boundary microbialites in both sections contain REY signatures indicative of deposition in an oxic environment. These boundary microbialites have their base at the extinction horizon and are widespread within the Tethyan region. In the Cili section, the underlying Permian limestone also preserves a seawater-like REY signature with a negative Ce anomaly. This indicates that the water column was oxygenated both before and after the extinction event. The Permian limestone in the Demirtas section does not preserve a seawater-like REYSN distribution, so the absence of a Ce anomaly cannot be used to distinguish prevailing redox conditions during deposition. However, in these samples, we find the presence of a diverse Permian benthic community sufficient to identify deposition in an oxic environment. The geochemical evidence for a continuously oxic environment during the deposition of the boundary microbialite presented in this study strongly supports work done using ostracods as redox indicators within the boundary microbialite in South China. The microbialite has been proposed as a disaster facies, in part resulting from the exclusion of grazers. Although anoxia is one of the suggested mechanisms for the exclusion of grazers, we find that it is not supported by geochemical and biotic evidence.

Gradual onset of anoxia across the Permian–Triassic Boundary in Svalbard, Norway

The Permian–Triassic extinction event is considered to be the most devastating environmental crisis of the Phanerozoic. With many coinciding factors involved, the role of marine anoxia during the extinction is still poorly understood. In this study a boreal Permian–Triassic Boundary (PTB) section from Svalbard, Norway has been investigated with the aim of better understanding the timing and nature of local marine anoxia onset and extinction events across the PTB. The section comprises the Kapp Starostin and Vikinghøgda formations; δ13Corg values indicate the PTB is located within the lower part of the Vikinghøgda Formation. Lag deposits at the top of the Kapp Starostin Formation indicate a marine hiatus and ensuing transgression during the Late Permian shortly before the extinction event, implying concurrence of major ecological changes and sea-level rise. Pyrite framboid size distributions and total organic carbon (TOC) were used to evaluate bottom water oxygen conditions, and show that changes in bottom water redox conditions and extinction are clearly linked. Oxic to dysoxic bottom water conditions prevailed during deposition of Kapp Starostin Formation sediments and changed to anoxic to euxinic conditions above the formation boundary. The onset of anoxia is not abrupt but rather shows a gradual increase within the Kapp Starostin Formation during the Late Permian. The tipping point where bottom waters reach a long-term state of anoxia to euxinia coincides with the final extinction event, though changes in biotic assemblages at the top of the Kapp Starostin Formation indicate a marine ecosystem crisis prior to this. Oxygen depletion in the boreal region, as seen in our study section and correlating to Greenland and Arctic Canada, seems to be consistently more severe than in lower latitude settings.

An oxygenation event occurred in deep shelf settings immediately after the end-Permian mass extinction in South China

Widespread oceanic anoxia has been consistently considered as a main characteristic of the oceanic environment after the end-Permian mass extinction. However, newly obtained pyrite framboid data suggest otherwise from a deep shelf setting (Changtanhe section) of northwestern Hunan province in South China. Our results reveal that an oxygenation event occurred immediately after the end-Permian mass extinction in this section, where the redox conditions of bottom water changed from lower dysoxic to upper dysoxic during the Permian–Triassic (P/Tr) transition. The oxygenation event likely resulted from mixing of deep dysoxic bottom waters with shallow, oxygenated waters triggered by enhanced upwelling and seawater circulation as well as the large regression during the P/Tr transition. These may also be the cause of the partial remission of dysoxic conditions immediately after the end-Permian mass extinction in other deep shelf settings, especially in South China.

Reconstruction of environmental changes using a multi‐proxy approach in the Ulleung Basin (Sea of Japan) over the last 48 ka

AbstractBased on elemental geochemical data, we reconstructed the sediment provenance, surface productivity and bottom water redox conditions for the last 48 ka in the Ulleung Basin (Sea of Japan) and inferred the factors controlling them. Al2O3/TiO2 ratio and chemical index of alteration (CIA) suggest that sediment provenance changed during the glacial period (48–18 ka) compared to the deglacial (ca. 18–11 ka) and Holocene. Mass accumulation rates of total organic carbon (TOC), CaCO3, phosphorus, cadmium and excess barium reveal low paleoproductivity during low sea stand. During 18–11 ka, productivity increased due to increasing inflow of nutrient‐rich water masses – the Oyashio and the East China Sea coastal water – in tandem with the rising sea level. Maximum productivity occurred during Younger Dryas and Pre‐boreal periods when sea level was at ∼ −60 m and then gradually decreased as the Tsushima Warm Current inflow kicked off at ca. 9.3 ka, consistent with other paleoredox proxies, which reveal the presence of anoxic bottom water during ca. 12–9 ka. With the changes in paleoredox proxies and their ratios (TOC, Mo, U, Mn, C/S ratio and Uauthigenic and Mo contents), we hypothesized that the redox changes were mainly ventilation driven and were superimposed on the influence of circulation‐induced productivity changes. The global climate and sea‐level changes on a millennial timescale play a major role in enhancing paleoproductivity and restrict bottom water advection, subsequently driving the oxygenation of bottom water in the Ulleung Basin. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Pyrite morphology in the first member of the Late Cretaceous Qingshankou Formation, Songliao Basin, Northeast China

The study of sedimentary pyrite morphological characteristics provides useful information on depositional environments and early diagenic processes and can be used as an indicator of redox conditions in ancient lake bottom waters. The results of this study in the Songliao basin in northeastern China show that euhedral crystals and framboids are the dominant pyrite forms in the mudstones of the first member of the Qingshankou Formation (K2qn1). The framboidal size distribution indicates that during the deposition of K2qn1, redox conditions in the bottom water fluctuated from oxic–dysoxic to euxinic–anoxic to oxic–dysoxic. The presence of euxinic–anoxic bottom water suggests that an anoxic event developed in the Songliao ancient-lake and continued for 196ka. The development of an anoxic bottom environment was the most important factor in hydrocarbon source rock formation in the Songliao Basin.