Oxic Marine Sediments Research Articles

An abundant bacterial phylum with nitrite-oxidizing potential in oligotrophic marine sediments

Nitrite-oxidizing bacteria (NOB) are important nitrifiers whose activity regulates the availability of nitrite and dictates the magnitude of nitrogen loss in ecosystems. In oxic marine sediments, ammonia-oxidizing archaea (AOA) and NOB together catalyze the oxidation of ammonium to nitrate, but the abundance ratios of AOA to canonical NOB in some cores are significantly higher than the theoretical ratio range predicted from physiological traits of AOA and NOB characterized under realistic ocean conditions, indicating that some NOBs are yet to be discovered. Here we report a bacterial phylum Candidatus Nitrosediminicolota, members of which are more abundant than canonical NOBs and are widespread across global oligotrophic sediments. Ca. Nitrosediminicolota members have the functional potential to oxidize nitrite, in addition to other accessory functions such as urea hydrolysis and thiosulfate reduction. While one recovered species (Ca. Nitrosediminicola aerophilus) is generally confined within the oxic zone, another (Ca. Nitrosediminicola anaerotolerans) additionally appears in anoxic sediments. Counting Ca. Nitrosediminicolota as a nitrite-oxidizer helps to resolve the apparent abundance imbalance between AOA and NOB in oxic marine sediments, and thus its activity may exert controls on the nitrite budget.

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.

Non-conservative removal of dissolved rhenium from a coastal lagoon: Clay adsorption versus biological uptake

The abundance of rhenium (Re) in sedimentary records can inform us about the redox conditions and inputs of organic matter prevailing in modern and ancient oceans. In this contribution, spatial distributions of dissolved rhenium concentrations along the salinity gradient of a large tropical lagoon (Chilika lagoon (India), an estuary-like feature in the Mahanadi river delta) and their possible source waters have been investigated for three different seasons (pre-monsoon (April–May 2017); monsoon (July–August 2017) and post-monsoon (January 2018)) to quantify sources and internal cycling of Re in coastal settings. Additionally, Re concentrations of macrophytes, and bed and suspended sediments have also been analysed. Biweekly analyses of dissolved Re concentrations of one of the Mahanadi distributaries (Paika river) show significant seasonal variations (1.7 to 12.0 pmol/kg), with an average Re value of 4.5 ± 2.5 pmol/kg (n = 30). Average salinities of the lagoon were different for the pre-monsoon (11 ± 6; n = 17), monsoon (8 ± 7; n = 17) and post-monsoon (4 ± 3; n = 10) seasons. The salinity-weighted Re concentrations of the Chilika also show variation during the pre-monsoon (~15.5 pmol/kg), monsoon (~14.1 pmol/kg) and post-monsoon (~6.4 pmol/kg) seasons. Co-variation between water salinity and rhenium concentrations show non-conservative Re behaviour in the lagoon. Water samples from all three seasons show the removal of Re from the high-saline regions of the lagoon. Significant correlations of sedimentary Re with Mg and Al concentrations point to adsorptive rhenium removal onto MgAl rich clay (montmorillonite and chlorite) surfaces. This proposition also draws support from higher exchangeable-Re content in bulk (~141 pg/g) and clay (~303 pg/g) fractions of Chilika sediments than that of the riverine sediments (~33 pg/g). Further, huge occurrence of biomass in the lagoon, appreciable Re concentrations in macrophytes (428 ± 259 pg/g, n = 11) and a significant Re-TN (total nitrogen) correlation indicate possible biological uptake of Re by amino acids during cellular membrane formation. Mass balance calculations show about 60% of sedimentary Re are accumulated through clay adsorption, whereas the remaining 40% is scavenged through biological activities. This authigenic Re removal from the Chilika accounts for a burial rate of 5.95 × 10−3 ng/cm2/yr, which is about four times higher than its accumulation onto oxic marine sediments (1.6 × 10−3 ng/cm2/yr) globally. Outcomes of this study identify this new and significant coastal sink for rhenium and hence, warrant the need to revisit the oceanic Re budget to incorporate these removal pathways.

A lattice-automaton bioturbation simulator with coupled physics, chemistry, and biology in marine sediments (eLABS v0.2)

Abstract. Seawater–sediment interaction is a crucial factor in carbon and nutrient cycling on a wide range of spatial and temporal scales. This interaction is mediated not just through geochemistry but also via biology. Infauna vigorously mix sediment particles, enhance porewater–seawater exchange, and consequently, facilitate chemical reactions. In turn, the ecology and activity of benthic fauna are impacted by their environment, amplifying the sensitivity of seawater–sediment interaction to environmental change. However, numerical representation of the bioturbation of sediment has often been treated simply as an enhanced diffusion of solutes and solids. Whilst reasonably successful in representing the mixing of bulk and predominantly oxic marine sediments, the diffusional approach to bioturbation is limited by a lack of environmental sensitivity. To better capture the mechanics and effects of sediment bioturbation, we extend a published bioturbation model (Lattice-Automaton Bioturbation Simulator; LABS) by adopting a novel method to simulate realistic infaunal behavior that drives sediment mixing. In this new model (extended LABS – eLABS), simulated benthic organism action is combined with a deterministic calculation of water flow and oxygen and organic matter concentration fields to better reflect the physicochemical evolution of sediment in response to bioturbation. The predicted burrow geometry and mixing intensity thus attain a dependence on physicochemical sedimentary conditions. This interplay between biology, chemistry, and physics is important to mechanistically explain empirical observations of bioturbation and to account for the impact of environmental changes. As an illustrative example, we show how higher organic rain can drive more intense sediment mixing by “luring” benthic organisms deeper into sediments, while lower ambient dissolved oxygen restricts the oxic habitat depth and hence tends to reduce bulk mixing rates. Our model, with its oxygen and food availability controls, is a new tool to interpret the trace fossil record, e.g., burrows, as well as to explore biological engineering of past marine environments.

Fe-Mn (oxyhydr)oxides as an indicator of REY enrichment in deep-sea sediments from the central North Pacific

Nowadays, pelagic sediments are globally explored as a potential resource of rare earth elements and yttrium (termed as REY), and have received unprecedented attention. However, the formation mechanism of REY-rich deep-sea mud remains controversial. The Fe-Mn (oxyhydr)oxides are ubiquitous in oxic marine sediments and potentially beneficial for the REY enrichment. The research of them is important for understanding the ore-formation of REY-rich mud, and thus can help to evaluate the role of marine sediments in global REY cycles. In this contribution, using advanced in situ analytical techniques, we provide new results on the nanoscale mineralogy and geochemistry of Fe-Mn (oxyhydr)oxides that are recovered from REY-rich deep-sea sediments in central North Pacific Ocean. In situ X-ray diffraction analyzer and scanning electron microscope reveal that the Fe-Mn minerals (e.g., vernadite, birnessite, and todorokite) occur as overgrowths on the surface of nuclei to form Fe-Mn micronodules. Transmission electron microscope (TEM) imaging reveals that these Fe-Mn minerals are composed of nanocrystals with high intercrystalline porosity, indicating a potential strong adsorption capacity. The TEM observations also imply a mineral phase transformation from birnessite (vernadite) to todorokite during the oxic diagenesis, accompanied by corresponding changes in geochemical compositions. Laser ablation-inductively coupled plasma-mass spectrometry demonstrates that Fe-Mn (oxyhydr)oxides contain high REY concentrations (varying from 439 to 1,654 ppm, with an average of 790 ppm) and can be regarded as an important host of REY. Furthermore, the PAAS-normalized and seawater-normalized REY patterns both infer that the REY contents of Fe-Mn (oxyhydr)oxides are ultimately derived from seawater and characterized by preferential scavenging of Ce. Overall, Fe-Mn (oxyhydr)oxides not only contribute a moderate proportion of REY, but also provide specific information for understanding geochemical processes related to REY enrichment in deep-sea sediments.

Marine ferromanganese oxide: A potentially important sink of light chromium isotopes?

Marine ferromanganese oxides are widely distributed in oxic marine sediments and are a sink for a range of metal elements derived from seawater. Their potential as a sink of chromium (Cr) isotopes has yet to be investigated and may have important implications for our understanding of the Cr-isotope system. In this study, we reported Cr-isotope data of modern oceanic Fe–Mn crusts for the first time. The δ53Cr values of the surface scrapings of the Fe–Mn crusts from the central North Pacific seamounts, range from −0.85 to −0.15‰, with an average of −0.42 ± 0.34‰ (2σ, n = 11), which are lower than published data from Pacific seawater (0.53–1.43‰ with an average of 0.79‰). These results reveal preferential removal of light Cr isotopes from seawater into Fe–Mn crusts, with an isotopic fractionation (△53Cr = δ53CrFe-Mn crust − δ53Crseawater) estimated from −2.3 to −0.7‰, averaging at ca. −1.2‰. We suggest that such significant negative isotopic fractionation may have been reached through adsorption of dissolved Cr (VI) oxyanions in the oxic water column onto the surface of Mn-oxide phases during the precipitation of Fe–Mn crusts. However, our modelling results imply that the deposition of Mn-oxide sediments would make insignificant contribution to elevation of the global seawater average δ53Cr value.

Release of Ni from birnessite during transformation of birnessite to todorokite: Implications for Ni cycling in marine sediments

The phyllomanganate birnessite is the main Mn-bearing phase in oxic marine sediments where it exerts a primary control on the concentration of micronutrient trace metals in seawater. However, during sediment diagenesis and under mild hydrothermal conditions birnessite transforms into the tectomanganate todorokite. We have recently shown that the transformation of birnessite to todorokite proceeds via a four-stage nucleation and growth mechanism, beginning with todorokite nucleation, then crystal growth from solution to form todorokite primary particles, followed by their self-assembly and oriented growth via oriented attachment to form crystalline todorokite laths, culminating in traditional crystal ripening (Atkins et al., 2014). Here we determine the fate and mobility of Ni sorbed by birnessite during this transformation process. Specifically, in our recent work we predict that the presence of Ni within the phyllomanganate matrix will disrupt the formation of todorokite primary particles. As such, contrary to current understanding, we suggest that Ni sorbed by birnessite will slow the transformation of birnessite to todorokite and/or be released to marine porewaters during sediment diagenesis. Here we transform a synthetic, poorly crystalline, Ni-sorbed (∼1wt% Ni) hexagonal birnessite, analogous to marine birnessite, into todorokite under a mild reflux procedure, developed to mimic marine diagenesis and mild hydrothermal conditions. We characterise our birnessite and reflux products as a time series, including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy. In addition we determine Ni speciation and mineral phase associations in a suite of natural marine ferromanganese precipitates, containing intermixed phyllomanganate and todorokite. Our work shows for the first time that Ni significantly slows the transformation of birnessite to todorokite and reduces the crystallinity of the neo-formed todorokite phase, but does not alter the mechanism and pathway of todorokite formation, compared to a Ni-free system. Furthermore, in systems tending towards todorokite as the final diagenetic product, we see that up to 50% of the Ni originally sequestered by birnessite is released to solution during the transformation. Our work indicates that the transformation of birnessite to todorokite in oxic marine sediments likely provides a significant source of Ni to marine sedimentary porewaters and potentially a hitherto unrecognized benthic flux of Ni to seawater.

The chromium isotope composition of reducing and oxic marine sediments

The chromium (Cr) isotope composition of marine sediments has the potential to provide new insights into the evolution of Earth-surface redox conditions. There are significant but poorly constrained isotope fractionations associated with oxidative subaerial weathering and riverine transport, the major source of seawater Cr, and with partial Cr reduction during burial in marine sediments, the major sink for seawater Cr. A more comprehensive understanding of these processes is needed to establish global Cr isotope mass balance and to gauge the utility of Cr isotopes as a paleoredox proxy. For these purposes, we investigated the Cr isotope composition of reducing sediments from the upwelling zone of the Peru Margin and the deep Cariaco Basin. Chromium is present in marine sediments in both detrital and authigenic phases, and to estimate the isotopic composition of the authigenic fraction, we measured δ53Cr on a weakly acid-leached fraction in addition to the bulk sediment. In an effort to examine potential variability in the Cr isotope composition of the detrital fraction, we also measured δ53Cr on a variety of oxic marine sediments that contain minimal authigenic Cr. The average δ53Cr value of the oxic sediments examined here is −0.05±0.10‰ (2σ, n=25), which is within the range of δ53Cr values characteristic of the bulk silicate Earth. This implies that uncertainty in estimates of authigenic δ53Cr values based on bulk sediment analyses is mainly linked to estimation of the ratio of Cr in detrital versus authigenic phases, rather than to the Cr-isotopic composition of the detrital pool. Leaches of Cariaco Basin sediments have an average δ53Cr value of +0.38±0.10‰ (2σ, n=7), which shows no dependency on sample location within the basin and is close to that of Atlantic deepwater Cr (∼+0.5‰). This suggests that authigenic Cr in anoxic sediments may reliably reflect the first-order Cr isotope composition of deepwaters. For Peru Margin samples, the average δ53Cr values of bulk sediments (+0.59±0.06‰; 2σ) and leach fractions (+0.61±0.06‰; 2σ) are also comparable with those of Cariaco Basin samples and modern deepwater values. Finally, we found that the δ53Cr of Peru Margin samples correlates with δ15N on glacial–interglacial timescales, which we attribute to secular variation in basinal or global-ocean redox conditions. Thus, the δ53Cr stratigraphic record of Peru Margin sediments indicates that Cr isotopes may be suited to tracking geologically short-term changes in ocean oxygenation.

Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments

The phyllomanganate birnessite is the main Mn-bearing phase in oxic marine sediments, and through coupled sorption and redox exerts a strong control on the oceanic concentration of micronutrient trace metals. However, under diagenesis and mild hydrothermal conditions, birnessite undergoes transformation to the tectomanganate todorokite. The mechanistic details of this transformation are important for the speciation and mobility of metals sequestered by birnessite, and are necessary in order to quantify the role of marine sediments in global trace element cycles. Here we transform a synthetic, poorly crystalline, hexagonal birnessite, analogous to marine birnessite, into todorokite under a mild reflux procedure, developed to mimic marine diagenesis and mild hydrothermal conditions. We characterize our birnessite and reflux products as a time series, employing X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), BET surface area analysis, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and extended X-ray absorption fine structure spectroscopy (EXAFS). We provide new insight into the crystallization pathway and mechanism of todorokite formation from birnessite under conditions analogous to those found in marine diagenetic and hydrothermal settings. Specifically we propose a new four-stage process for the transformation of birnessite to todorokite, beginning with todorokite nucleation, then crystal growth from solution to form todorokite primary particles, followed by their self-assembly and oriented growth via oriented attachment to form crystalline todorokite laths, culminating in traditional crystal ripening. We suggest that, contrary to current understanding, trace metals like Ni might retard the transformation of birnessite to todorokite and be released to marine sedimentary pore-waters during this diagenetic process, thus potentially providing a benthic flux of these micronutrients to seawater.

Geochemistry of deep sea sediments at cold seep sites in the Nankai Trough: Insights into the effect of anaerobic oxidation of methane

Two types of sediment were recovered from “methane seep” sites in the Nankai Trough: “Cold seep” sediment affected by anaerobic oxidation of methane (AOM), and unaffected “Typical oxic marine sediment,” Concentrations of carbon, sulfur, iron, and trace metals were analyzed in these sediments. The results showed that the sulfur contents of “cold seep” sediments are higher than those of typical oxic marine sediments, although the organic carbon contents of the two types of sediment are identical. The high sulfur contents of seep sediments suggest that AOM was the dominant process at the “cold seep” sites; H2S produced as a result of AOM was fixed as reduced sulfur such as iron sulfide.The highest manganese concentrations (approx. 1000ppm) were found in surface sediments at non-seep sites, whereas a strong enrichment of Mn was not observed in the “cold seep” sediments. Manganese might be released from sediments as soluble Mn2+ ion in the reduced environment produced by methane-bearing seepage. The molybdenum content in “cold seep” sediments (up to 28.3ppm) is about 30 times higher than that in the typical oxic marine sediments. However, a correlation between Mo and organic carbon contents, which is characteristic of anoxic sediment, was not observed in our samples. “Cold seep” sediments are a potential sink in the global geochemical cycle of Mo.

Molybdenum isotope, multiple sulfur isotope, and redox-sensitive element behavior in early Pleistocene Mediterranean sapropels

Organic-rich sediments (sapropels) deposited in the Mediterranean are presumed to have formed during periods of increased productivity, and/or deep water oxygen depletion, possibly including the development of sulfidic conditions (euxinia). Geochemical redox proxies (Re, Mo, Mo isotopes, V, Fe/Al, and multiple S isotopes) in 8 sapropels from the Pleistocene confirm water column euxinic conditions of varying intensity during sapropel deposition. These same proxies indicate an oxic origin for hemipelagic sediments deposited between sapropel-forming episodes. In one intensively sampled sapropel, deposited between 1.450 and 1.458 Ma, changing concentrations of organic carbon, Ba, Re, Mo, V, and Fe/Al track one another closely, reflecting coupling between water column euxinia and biological productivity. Multiple S isotope data from this sapropel suggest that the redox interface where oxidative sulfur cycling occurred was present in the sediments during hemipelagic sedimentation, but moved into the water column during sapropel deposition. Molybdenum isotopes of these 8 sapropels encompass a range of values (δ 98Mo = + 0.2 to + 1.7), but are all 98Mo-depleted relative to seawater (δ 98Mo = + 2.3‰), suggesting that quantitative removal of Mo did not occur. This finding contrasts with modern Black Sea sediments. In general, Re/Mo ratios in sapropels are greater than in modern seawater, implying that the water column was not sufficiently sulfidic during sapropel-forming episodes to induce complete removal of both these elements. Surprisingly, the heaviest δ 98Mo values are found within hemipelagic sediments. Very few of the hemipelagic samples preserve the negative δ 98Mo values commonly associated with modern oxic marine sediments. Many of the hemipelagic samples also contained higher concentrations of Re and Mo than are common in oxic sediments. These features may be attributable to diffusion from the sapropels of a 98Mo-enriched component into the hemipelagic sediments.

Responses of the marine bacterium Pseudomonas fluorescens to an excess of heavy metals: Physiological and biochemical aspects

A Pseudomonas fluorescens strain was isolated from oxic marine sediments obtained from the strand zone of the St Anne Bay (a moderately metal-contaminated site to the west of Cherbourg harbour). The strain, which exhibited a high tolerance to metal contamination when cultivated (minimal inhibitory concentration = 950 μM [62 mg L − 1 ] for Zn, 660 μM [42 mg L − 1 ] for Cu, and 505 μM [57 mg L − 1 ] for Cd), was further characterized by its physiological and biochemical responses to metal additions to the culture medium. Bacterial growth was significantly disturbed by 380 μM Zn (25 mg L − 1 ), 315 μM Cu (20 mg L − 1 ) and 90 μM Cd (10 mg L − 1 ). The Zn-containing alkaline phosphatase was studied as an intoxication biomarker. Its activity was stimulated (+ 9%) by an excess of Zn, but inhibited by Cd (− 55%) and Cu (− 10%), these two elements could displace the native Zn or/and disturb the enzyme 3D-structure. Bacterial O 2 consumption was recorded as a global physiological response to metal stress. This parameter dropped with increasing Cd and Cu contamination (− 49% and − 45%, respectively, at 20 mg L − 1 ). By contrast, Zn increased O 2 consumption (approximately + 40% for the different tested concentrations). The proteomes of bacteria grown in the presence or absence of 20 mg metal L − 1 were characterized by 2D-gel electrophoresis. The number of spots exhibiting a difference in intensity between the contaminated sample and the control was 65, 68, and 103, for Zn, Cu and Cd, respectively. Among them, 45, 61 and 82 spots respectively appeared de novo or increased in intensity, indicative of metal-stimulated synthesis, particularly for Cu and Cd. In summary, whereas Cd and Cu treatments both stressed cells and slowed down primary metabolism to differing extents, Zn has a stimulating action on several physiological and biochemical parameters.

Iron isotope variations in Holocene sediments of the Gotland Deep, Baltic Sea

Holocene sediments from the Gotland Deep basin in the Baltic Sea were investigated for their Fe isotopic composition in order to assess the impact of changes in redox conditions and a transition from freshwater to brackish water on the isotope signature of iron. The sediments display variations in δ 56Fe (differences in the 56Fe/ 54Fe ratio relative to the IRMM-14 standard) from −0.27 ± 0.09‰ to +0.21 ± 0.08‰. Samples deposited in a mainly limnic environment with oxygenated bottom water have a mean δ 56Fe of +0.08 ± 0.13‰, which is identical to the mean Fe isotopic composition of igneous rocks and oxic marine sediments. In contrast, sediments that formed in brackish water under periodically euxinic conditions display significantly lighter Fe isotope signatures with a mean δ 56Fe of −0.14 ± 0.19‰. Negative correlations of the δ 56Fe values with the Fe/Al ratio and S content of the samples suggest that the isotopically light Fe in the periodically euxinic samples is associated with reactive Fe enrichments and sulfides. This is supported by analyses of pyrite separates from this unit that have a mean Fe isotopic composition of −1.06 ± 0.20‰ for δ 56Fe. The supply of additional Fe with a light Fe isotopic signature can be explained with the shelf to basin Fe shuttle model. According to the Fe shuttle model, oxides and benthic ferrous Fe that is derived from dissimilatory iron reduction from shelves is transported and accumulated in euxinic basins. The data furthermore suggest that the euxinic water has a negative dissolved δ 56Fe value of about −1.4‰ to −0.9‰. If negative Fe isotopic signatures are characteristic for euxinic sediment formation, widespread euxinia in the past might have shifted the Fe isotopic composition of dissolved Fe in the ocean towards more positive δ 56Fe values.

Density Functional Theory Analysis of Molybdenum Isotope Fractionation

Analytical studies have found an enrichment of the lighter Mo isotopes in oxic marine sediments compared to seawater, with isotope fractionation factors of -1.7 to -2.0 per thousand for Delta97/95Mosediment-seawater. These data place constraints on the possible identities of dissolved and adsorbed species because the equilibrium isotope fractionation depends on the energy differences between the isotopomers of the adsorbed species, minor dissolved species, and the dominant solution species, MoO42-. Adsorption likely involves molybdic acid, whose structure is indicated by previous studies to be MoO3(H2O)3. Here we used DFT calculations of vibrational frequencies to determine the isotope fractionation factors versus MoO42-. The results indicate that isotope equilibration of MoO42- with MoO3(H2O)3, yielding Delta97/95Momolybdic acid-molybdate=-1.33 per thousand, is most likely responsible for the isotope fractionation of Mo between oxic sediments and seawater. The difference between the calculated value of Delta97/95Momolybdic acid-molybdate for MoO3(H2O)3 and the value observed in natural sediments and experiments is probably due to effects of solvation and adsorption onto the manganese oxyhydroxide surface.

Glacial-interglacial changes in organic carbon, nitrogen and sulfur accumulation in Lake Baikal sediment over the past 250kyr

The waters of modern Lake Baikal circulate well and maintain oxic conditions for the entire lake in spite of its great depth. Past oxic/anoxic conditions in the water column are not known. Because reconstruction of paleo-redox conditions involved with climate changes provides information on dynamics of lake water circulation and biological activity, high-resolution analyses of total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS) concentrations, and gamma ray density (GRD) were carried out using 492 sediment samples over the past 250 kyr (every other 1 cm from the 10 m core Ver98-1 St.5 taken from the Academician Ridge at 325 m water depth). Fifteen events of high TS/TOC ratio (>0.2 atomic) are observed; these are much larger than TS/TOC ratio of typical freshwater sediment (0.001–0.070) and also normal oxic marine sediments (0.13 in average). The sulfur occurs as pyrite by X-ray diffraction (XRD) analysis, probably being produced by sulfate-reducing bacteria (SRB). Such high TS/TOC ratios indicate high SRB activity in the lake and a high input of sulfate to the lake. A high TS/TOC layer often accompanies a decrease of TOC/TN ratio to ca. 5 (atomic). In particular, during a rapid cooling such as the Younger Dryas event, the TOC/TN ratio decreases steeply from 10.4 to 7.8 just prior to the increase of TS/TOC ratio from 0.02 to 0.55. Because the low TOC/TN ratio suggests low terrestrial organic matter contribution to the lake, saline water inflow from rivers could diminish to result in decreased water circulation in the lake. The rapid cooling event may restrict deep-water ventilation and create less oxic conditions in Lake Baikal.

Geochemical partitioning of heavy metals in sediments of three estuaries along the coast of Rio de Janeiro (Brazil)

The geochemical partitioning of Cu, Cd, Cr and Pb was studied in sediments of three estuaries along the coast of Rio de Janeiro. The results show that, under contaminated conditions, the lithogeneous fraction of the total metal concentration decreases in importance, and mobile fractions dominate. Dilution with less contaminated marine sediments and the effect of salinity on metal behaviour could only be observed in the uncontaminated estuary. Underoxic conditions the reducible fraction is the more important sink for metal among the mobile phases. However, in the area with high organic sewage inputs, the oxidizable fraction can account for the greater fraction of total metal content. This latter result suggests a synergistic effect between organic pollution and metal contamination, since metals in oxidizable forms may be remobilized during mixture with oxic marine sediments.