Sediment-seawater Interface Research Articles

High-Throughput Sequencing Reveals a Potentially Novel Sulfurovum Species Dominating the Microbial Communities of the Seawater-Sediment Interface of a Deep-Sea Cold Seep in South China Sea.

In the Formosa cold seep of the South China Sea (SCS), large amounts of methane and sulfide hydrogen are released from the subseafloor. In this study, we systematically investigated the microbial communities in the seawater–sediment interface of Formosa cold seep using high-throughput sequencing techniques including amplicon sequencing based on next-generation sequencing and Pacbio amplicon sequencing platforms, and metagenomics. We found that Sulfurovum dominated the microbial communities in the sediment–seawater interface, including the seawater close to the seepage, the surface sediments, and the gills of the dominant animal inhabitant (Shinkaia crosnieri). A nearly complete 16S rRNA gene sequence of the dominant operational taxonomic units (OTUs) was obtained from the Pacbio sequencing platforms and classified as OTU-L1, which belonged to Sulfurovum. This OTU was potentially novel as it shared relatively low similarity percentages (<97%) of the gene sequence with its close phylogenetic species. Further, a draft genome of Sulfurovum was assembled using the binning technique based on metagenomic data. Genome analysis suggested that Sulfurovum sp. in this region may fix carbon by the reductive tricarboxylic acid (rTCA) pathway, obtain energy by oxidizing reduced sulfur through sulfur oxidizing (Sox) pathway, and utilize nitrate as electron acceptors. These results demonstrated that Sulfurovum probably plays an important role in the carbon, sulfur, and nitrogen cycles of the Formosa cold seep of the SCS. This study improves our understanding of the diversity, distribution, and function of sulfur-oxidizing bacteria in deep-sea cold seep.

Aerobic biodegradation of bio-based plastics in the seawater/sediment interface (sublittoral) marine environment of the coastal zone – Test method under controlled laboratory conditions

Pollution of the seas due to plastic litter is a rapidly growing environmental problem. Among several actions, legal and technological, undertaken to alleviate this problem, included are the control of single use conventional plastics and the replacement of conventional non-biodegradable plastics with innovative biodegradable-in-the-sea polymers, both aiming at the mitigation of marine litter accumulation. Laboratory tests have been used to characterize plastics regarding their biodegradation in various environments. Biodegradation of plastics depends on the inherent characteristics of the polymer and the particular marine habitat conditions. In the present work, the international standard test method ISO 19679 (2016) for determining the aerobic biodegradation of non-floating plastic materials in a seawater-sediment interface of the coastal marine zone under laboratory simulated conditions is evaluated. Modifications are proposed to improve the reliability of this test method in some aspects. Agitation of the seawater surface in the bioreactor was found to enhance the continuous availability of oxygen at the seawater-sediment interface, thus assuring aerobic biodegradation conditions throughout the test simulating real sublittoral conditions. Additional recommended improvements include the use of larger samples and supplementary nutrients to optimise biodegradation conditions. The laboratory measurements were validated by field experiments. The proposed modifications and refinements optimise the robustness and reliability of the test method for determining aerobic biodegradation of plastic materials in a simulated seawater-sediment interface of the coastal zone.

Reactive iron isotope signatures of the East Asian dust particles: Implications for iron cycling in the deep North Pacific

Iron released from wind-blown dust has been widely suggested to be an important nutrient source for oceanic primary productivity. Once introduced into the seawater, the reactive Fe might get dissolved and impart its isotope signature into the water column. Characterization of the reactive iron isotope signatures of eolian dust will thus help to understand the budget of Fe in the global ocean. Yet, there are few systematic studies on the Fe isotope systematics of the dust sources across large spatial scale. Here, we report Fe isotope data from HCl leaching experiment on surficial samples from deserts and loess deposits of different locations spanning >3000 km spatially in east Asia, one of the most important dust source regions. Our data suggest most of the samples have relatively similar leaching signatures (δ56Fe = –0.09±0.07‰, n=9, 2 SD). To understand the kinetics of Fe release during proton promoted leaching of natural particles, a synthetic model taking into account of different dissolution dynamics of easily dissolvable Fe and silicate Fe was developed. The model was tested by continuous leaching of two representative pelagic sediment samples recovered from core LL44-GPC3 in the North Pacific. When introducing only one tunable parameter (i.e., the initial dissolution rate of Fe from hydro(oxides), with other parameters constrained by the dissolution kinetics), the model successfully reproduces leachate δ56Fe evolution and agrees reasonably well with Fe speciation in the studied marine sediment samples. We thus suggest that the leaching process can be well characterized by our kinetic model, while Fe isotopes in the initial leachates of the samples are dominated by endmember composition of the easily dissolvable Fe species. The small but distinct fractionation of Fe isotopes in the desert/loess samples from the parental upper continental silicate materials are suggested to record signatures of the secondary Fe species produced during weathering in the oxic environment. The significantly lighter Fe isotope compositions in the initial leachates of the North Pacific pelagic sediment samples compared to the east Asian desert/loess deposits indicate apparent isotope exchange of the dust reactive components with seawater dissolved species. The sedimentary light reactive Fe isotope signatures in turn point toward the contribution of sources characterized by low δ56Fe to the deep North Pacific and/or preferential uptake of 56Fe by seawater ligands from the dust reactive components particularly at the seawater-sediment interface.

Microbial processing of sedimentary organic matter at a shallow LTER site in the northern Adriatic Sea: an 8-year case study

Benthic prokaryotes are the key-players in C-cycling at the sediment-seawater interface, one of the largest biologically active interfaces on Earth. Here, microbial-mediated processes, such as the degradation of organic matter and the incorporation of mobilized C into microbial biomass, depend on several factors such as environmental temperature and substrate availability, especially in shallow sediments at mid-high latitudes where seasonal fluctuations of these variables occur. In the present study, four degradative activities (β-glucosidase, lipase, chitinase and aminopeptidase), Heterotrophic C Production (HCP), Total Organic C (TOC), Total Nitrogen (TN) and Biopolymeric C (BPC) were investigated seasonally from April 2010 to April 2018 in the surface sediments of a shallow Long-Term Ecological Research (LTER) station of the northern Adriatic Sea. Significant temperature-dependences were described by Arrhenius-type equations for HCP and each of the degradative activities tested with the exception of aminopeptidase. The relatively low apparent Activation Energies suggested that these microbial-mediated processes were enhanced by the availability of palatable substrates over the study period. Nevertheless, a clear and tight dependence from such substrates was detected only for aminopeptidase, the most pronounced degradative activity observed. TN was identified by the stepwise multiple regression analysis as the environmental variable that mainly drove this exoenzymatic activity. Enhanced aminopeptidase rates mirrored peaks of TN that seemed, in turn, linked to the seasonal proliferation of benthic microalgae. By supplying prokaryotes with promptly available substrates, these autotrophs, represented mainly by diatoms, seemed to play an important role in the C-cycling regulation at the studied LTER station.

Adsorption of Quinolone Antibiotics to Goethite under Seawater Conditions: Application of a Surface Complexation Model.

The assessment of antibiotics mobility under seawater conditions has been rarely studied, as an accurate description of such multicomponent systems is quite challenging. In this study, the adsorption of a widely used quinolone antibiotic in aquaculture, Oxolinic acid (OA), to a synthetic goethite (α-FeOOH) was examined in the presence of major (e.g., Mg2+, SO42-) and trace (e.g., Cu2+) ions naturally occurring in seawater. The OA adsorption can be successfully predicted using a charge distribution multisite complexation model (CD-MUSIC) coupled with the three plane model (TPM). This modeling approach allowed a quantification of the competitive and synergetic effects of different ions in seawater over a large range of environmentally relevant conditions. In addition, the transport of OA in flow-through columns can be well predicted through coupling hydrodynamic parameters and surface complexation constants obtained under seawater conditions. These results may have strong implications for assessment and prediction of the fate of quinolones in sediment-seawater interface systems.

Low temperature stable mineral recrystallization of foraminiferal tests and implications for the fidelity of geochemical proxies

Fossilized foraminiferal tests are widely used as proxies to constrain paleoenvironments, including past sea surface and bottom water temperatures and the chemical composition of ancient seawater. However, relatively little is known about the extent to which calcareous foraminiferal tests exchange with seawater prior to burial. In this study, we used a 45Ca radiotracer approach to quantify the extent and rate of foraminiferal calcite–fluid exchange over a three-month period. Modern foraminiferal tests exhibited faster exchange with a 45Ca-spiked solution (2.6⋅10−4 to 2.0⋅10−5 mol/m2/d) than fossil foraminiferal tests (7.6⋅10−5 to 5.3⋅10−6 mol/m2/d) from ODP Site 807. The impact of dissolved Si on the exchange rate of modern foraminifera was minimal (2.6⋅10−4 to 9.2⋅10−6 mol/m2/d). Unlike foraminiferal calcite, inorganic calcite exchanged at remarkably slow rates (6.6⋅10−6 to 5.9⋅10−7 mol/m2/d) suggesting that biogenic calcite is more reactive than inorganic calcite. Increased Ca2+, Mg2+, and Sr2+ concentrations in fluids from parallel tracer-free reactors indicated preferential dissolution of tests, though there was no overt change in test morphology during experiments (as imaged by scanning electron microscopy and micro-CT scanning). Time-dependent box model simulations of 45Ca, as well as aqueous elemental chemistry, support the hypothesis that calcite–fluid exchange occurred via dissolution–reprecipitation of foraminiferal tests that was induced by intra-test chemical heterogeneity. The short-term exchange reactions investigated in this study are comparable to the reactions that can occur in the seawater column and at the sediment–seawater interface. This work suggests that dissolution–reprecipitation in such settings has the potential to overprint the chemistry of foraminiferal tests without overt physical alteration, which can lead to a potential bias in paleoenvironmental reconstructions. Model-based estimates suggest a maximum of 2–3°C alteration of Mg/Ca-based temperatures over experimental time scales. Model efforts also suggest that the diagenetic trajectories in Mg/Ca–δ18O space, for instance, are impacted by the partition coefficient of Mg in calcite (λMg) as well as by the mechanism of alteration. This is demonstrated for (i) the ratio of exchange to dissolution and (ii) the relative rate of high-Mg-calcite to low-Mg-calcite reaction. We have illustrated that the mechanism of alteration, in addition to water-rock ratios, determine diagenetic trajectories. Finally, surface reaction has implications for our ability to constrain fractionation factors in experimental and natural systems. The experimental observations suggest that exchange between tests and solution is nominal over ∼100 days. Experiments aimed at deducing fractionation factors rely on the equilibration of bulk solid and fluid, since these are the readily analyzed reservoirs. We quantify the fraction of carbonate (>5%) that must react so that surface reaction does not impact the interpretation of Ca isotopic fractionation factors, and discuss the implications for more “fluid-dominated” elements such as Mg and O.

“Live” (stained) benthic foraminiferal living depths, stable isotopes, and taxonomy offshore South Georgia, Southern Ocean: implications for calcification depths

Abstract. It is widely held that benthic foraminifera exhibit species-specific calcification depth preferences, with their tests recording sediment pore water chemistry at that depth (i.e. stable isotope and trace metal compositions). This assumed depth-habitat-specific pore water chemistry relationship has been used to reconstruct various palaeoenvironmental parameters, such as bottom water oxygenation. However, many deep-water foraminiferal studies show wide intra-species variation in sediment living depth but relatively narrow intra-species variation in stable isotope composition. To investigate this depth-habitat–stable-isotope relationship on the shelf, we analysed depth distribution and stable isotopes of living (Rose Bengal stained) benthic foraminifera from two box cores collected on the South Georgia shelf (ranging from 250 to 300 m water depth). We provide a comprehensive taxonomic analysis of the benthic fauna, comprising 79 taxonomic groupings. The fauna shows close affinities with shelf assemblages from around Antarctica. We find live specimens of a number of calcareous species from a range of depths in the sediment column. Stable isotope ratios (δ13C and δ18O) were measured on stained specimens of three species, Astrononion echolsi, Cassidulinoides porrectus, and Buccella sp. 1, at 1 cm depth intervals within the downcore sediment sequences. In agreement with studies in deep-water settings, we find no significant intra-species variability in either δ13Cforam or δ18Oforam with sediment living depth on the South Georgia shelf. Our findings add to the growing evidence that infaunal benthic foraminiferal species calcify at a fixed depth. Given the wide range of depths at which we find living, infaunal species, we speculate that they may actually calcify predominantly at the sediment–seawater interface, where carbonate ion concentration and organic carbon availability is at a maximum.

Nutrient Distribution in Coral Reef Degraded Areas within Sanya Bay, South China Sea

ABSTRACT Guo, J.; Yu, K.; Wang, Y.; Xu, D.; Huang, X.; Zhao, M.; Yang, H., and Zhang, R., 2017. Nutrient distribution in Coral Reef degraded areas within Sanya Bay, South China Sea. An investigation was conducted in August 2014 to explore the spatial distribution of dissolved inorganic nitrogen (DIN) and phosphate ( ) in overlying water, pore water, and surface sediment from Sanya Bay and living coral cover on Luhuitou fringing reef. Generally, nutrient contents gradually decreased from the estuary to the central bay, which confirmed that terrigenous inputs from Sanya River and Sanya Harbor were the primary nutrient sources. The results of K-means cluster analysis suggested the Luhuitou fringing reef is in danger of being affected by the pollution source. The results of nutrient diffusive fluxes at the sediment-seawater interface indicated diffusion from pore water to overlying water. Compared with other coral reef areas around the world, nutrient levels in Luhuitou fringing reef were at the medium level....

Microbial Sulfide Filter along a Benthic Redox Gradient in the Eastern Gotland Basin, Baltic Sea

The sediment-water interface is an important site for material exchange in marine systems and harbor unique microbial habitats. The flux of nutrients, metals, and greenhouse gases at this interface may be severely dampened by the activity of microorganisms and abiotic redox processes, leading to the “benthic filter” concept. In this study, we investigate the spatial variability, mechanisms and quantitative importance of a microbially-dominated benthic filter for dissolved sulfide in the Eastern Gotland Basin (Baltic Sea) that is located along a dynamic redox gradient between 65 and 173 m water depth. In August-September 2013, high resolution (0.25 mm minimum) vertical microprofiles of redox-sensitive species were measured in surface sediments with solid-state gold-amalgam voltammetric microelectrodes. The highest sulfide consumption (2.73–3.38 mmol m−2 day−1) occurred within the top 5 mm in sediments beneath a pelagic hypoxic transition zone (HTZ, 80–120 m water depth) covered by conspicuous white bacterial mats of genus Beggiatoa. A distinct voltammetric signal for polysulfides, a transient sulfur oxidation intermediate, was consistently observed within the mats. In sediments under anoxic waters (>140 m depth), signals for Fe(II) and aqueous FeS appeared below a subsurface maximum in dissolved sulfide, indicating a Fe(II) flux originating from older sediments presumably deposited during the freshwater Ancylus Lake that preceded the modern Baltic Sea. Our results point to a dynamic benthic sulfur cycling in Gotland Basin where benthic sulfide accumulation is moderated by microbial sulfide oxidation at the sediment surface and FeS precipitation in deeper sediment layers. Upscaling our fluxes to the Baltic Proper; we find that up to 70% of the sulfide flux (2281 kton yr−1) toward the sediment-seawater interface in the entire basin can be consumed at the microbial mats under the HTZ (80–120 m water depth) while only about 30% the sulfide flux effuses to the bottom waters (>120 m depth). This newly described benthic filter for the Gotland Basin must play a major role in limiting the accumulation of sulfide in and around the deep basins of the Baltic Sea.

The role of Fe on the formation and diagenesis of interstratified glauconite-smectite and illite-smectite: A case study of Upper Cretaceous shallow-water carbonates

The widespread formation of interstratified glauconite-smectite (Gl-S) and illite-smectite (I-S) in modern and ancient diagenetic settings records the physicochemical conditions prevailing during clay mineral authigenesis. To date, however, significant gaps in our knowledge persist in respect to the influence of interstitial solution chemistry, temperature and reaction kinetics on the evolution of composition, mineralogy and microstructure of Gl-S and I-S. Herein, we present a study on the reaction mechanisms and the physicochemical conditions that led to the precipitation of early diagenetic Gl-S and late diagenetic I-S on a stable carbonate platform during the Cenomanian at Langenstein in the Northern German Basin.The texture and the K-Ar age (95.0±1.8Ma) of the green glauconitized grains revealed that green-clay authigenesis progressed in initially organic-rich, semi-confined micromilieus, i.e., in fecal pellets and in foraminifera, close to the sediment-seawater interface. The composition of Gl-S varied in the range (K+0.20–0.74Na+0–0.10Ca2+0–0.05)0.28–0.75 (Fe3+0.63–1.20Fe2+0.08–0.24Al3+0.19–0.97Mg2+0.29–0.52)2.01–2.12 [Al3+0.09–0.35Si4+3.65–3.91O10](OH2), and depended on the rate of aqueous Fe2+ and K+ ion diffusion, the micromilieu of glauconitization and on the bulk sedimentation rate. The mineralogical, microstructural and chemical changes of the ongoing Gl-S products revealed the following reaction for green-clay authigenesis at Langenstein: Fe(III)-smectite reacted with monosilicic acid, goethite and aqueous K+, Mg2+ and Fe2+ to form glauconite and aqueous Na+, Ca2+ and H+ ions. This process considers complex mineral transformations commonly associated with glauconitization, such as early diagenetic oxidation of organic matter and microbial-catalyzed dissolution of Fe-(oxy)hydroxides, carbonates and detrital silicates. In contrast, the K-Ar age of I-S (68.0±1.6Ma) and its compositional variability, (K+0.29–0.45Na+0–0.10Ca2+0–0.06)0.30–0.55 (Fe3+0.16–0.29Fe2+0–0.10Al3+1.37–1.68Mg2+0.18–0.43)2.00–2.12 [Al3+0.17–0.39Si4+3.61–3.83O10](OH2), indicate a burial diagenetic origin for this mineral phase, rather than transformation of illitic clays into I-S during weathering under warm and humid climatic conditions. The results from kinetic modelling support a diagenetic origin of I-S (50–60%I layers and 50–40%S layers) and imply its formation by the replacement of pre-existing K-feldspar at high pore-fluid activity K/Na ratios and at low Fe2+ concentrations.We propose that the substitution of Al3+ for Fe3+, Fe2+ and Mg2+ in the octahedral sheet shifts the stability field of the kaolinite–Fe-Al-Mg-smectite–Fe-Al-Mg-illite (or glauconite) triple point to much lower monosilicic acid activities, and stabilizes the I-S (or Gl-S) structure. This reaction supports the idea that the (bio)availability of Fe is the rate-limiting factor for glauconitization, which is not the case for the diagenetic growth of I-S, whereby the pore water Fe2+ concentration may be limited by the competing formation of Fe-(oxy)hydroxides and/or Fe-sulfides.

Methane in shallow subsurface sediments at the landward limit of the gas hydrate stability zone offshore western Svalbard

Offshore western Svalbard plumes of gas bubbles rise from the seafloor at the landward limit of the gas hydrate stability zone (LLGHSZ; ∼400m water depth). It is hypothesized that this methane may, in part, come from dissociation of gas hydrate in the underlying sediments in response to recent warming of ocean bottom waters. To evaluate the potential role of gas hydrate in the supply of methane to the shallow subsurface sediments, and the role of anaerobic oxidation in regulating methane fluxes across the sediment–seawater interface, we have characterised the chemical and isotopic compositions of the gases and sediment pore waters. The molecular and isotopic signatures of gas in the bubble plumes (C1/C2+=1×104; δ13C-CH4=−55 to −51‰; δD-CH4=−187 to −184‰) are similar to gas hydrate recovered from within sediments ∼30km away from the LLGHSZ. Modelling of pore water sulphate profiles indicates that subsurface methane fluxes are largely at steady state in the vicinity of the LLGHSZ, providing no evidence for any recent change in methane supply due to gas hydrate dissociation. However, at greater water depths, within the GHSZ, there is some evidence that the supply of methane to the shallow sediments has recently increased, which is consistent with downslope retreat of the GHSZ due to bottom water warming although other explanations are possible. We estimate that the upward diffusive methane flux into shallow subsurface sediments close to the LLGHSZ is 30,550mmolm−2yr−1, but it is <20mmolm−2yr−1 in sediments further away from the seafloor bubble plumes. While anaerobic oxidation within the sediments prevents significant transport of dissolved methane into ocean bottom waters this amounts to less than 10% of the total methane flux (dissolved+gas) into the shallow subsurface sediments, most of which escapes AOM as it is transported in the gas phase.

Diagenetic Mg-calcite overgrowths on foraminiferal tests in the vicinity of methane seeps

Methane is a potent greenhouse gas and some episodes of past global warming appear to coincide with its massive release from seafloor sediments as suggested by carbon isotope records of foraminifera. Here, we present structural, geochemical, and stable carbon isotope data from single foraminiferal calcite tests and authigenic Mg-calcite overgrowths in a sediment core recovered from an area of active methane seepage in western Svalbard at ca. 340 m water depth. The foraminifera are from intervals in the core where conventional bulk foraminiferal δ13C values are as low as −11.3 ‰. Mg/Ca analyses of the foraminiferal tests reveal that even tests for which there is no morphological evidence for secondary authigenic carbonate can contain Mg-rich interlayers with Mg/Ca up to 220 mmol/mol. Transmission electron microscopy (TEM) of the contact point between the biogenic calcite and authigenic Mg-calcite layers shows that the two phases are structurally indistinguishable and they have the same crystallographic orientation. Secondary ion mass spectrometry (SIMS) analyses reveal that the Mg-rich layers are strongly depleted in 13C (δ13C as low as −34.1 ‰). These very low δ13C values indicate that the authigenic Mg-calcite precipitated from pore waters containing methane-derived dissolved inorganic carbon at the depth of the sulfate–methane transition zone (SMTZ). As the depth of the SMTZ can be located several meters below the sediment-seawater interface, interpretation of low foraminiferal δ13C values in ancient sediments in terms of the history of methane seepage at the seafloor must be undertaken with care.

Fluxes and the mass balance of mercury in Augusta Bay (Sicily, southern Italy)

The flux (Φ) of mercury (Hg) at the sediment-seawater interface was investigated in Augusta Bay (southern Italy) where uncontrolled industrial discharge from one of the most important chlor-alkali plant in Europe has caused significant negative effects on the environment. Hg fluxes were measured by the deployment of in-situ benthic chamber. The obtained value of 1.3 kmol y−1 clearly emphasizes the role of the sediments as source of Hg for the overlying water column. Moreover, Hg concentrations in the outflowing bottom waters were measured to estimate the export of this pollutant from Augusta Bay to the open sea. The calculated value of 0.54 kmol y−1, corresponding to ∼4% of the anthropogenic input of Hg from coastal point/diffuse sources to the Mediterranean Sea (12.5 kmol y−1; Rajar et al., 2007; UNEP-MAP, 2001), assigns this area a crucial role in the Hg inventory of the entire Mediterranean basin. Finally, a consistent and robust mass balance for Hg in Augusta Bay was provided by combining the obtained data with Hg fluxes at seawater-atmosphere interface.

Effect of a controlled sub-seabed release of CO2 on the biogeochemistry of shallow marine sediments, their pore waters, and the overlying water column

The potential for leakage of CO2 from a storage reservoir into the overlying marine sediments and into the water column and the impacts on benthic ecosystems are major challenges associated with carbon capture and storage (CCS) in subseafloor reservoirs. We have conducted a field-scale controlled CO2 release experiment in shallow, unconsolidated marine sediments, and documented the changes to the chemical composition of the sediments, their pore waters and overlying water column before, during and up to 1 year after the 37-day long CO2 release. Increased levels of dissolved inorganic carbon (DIC) were detected in the pore waters close to the sediment-seawater interface in sediments sampled closest to the subsurface injection point within 5 weeks of the start of the CO2 release. Highest DIC concentrations (28.8mmolL−1, compared to background levels of 2.4mmolL−1) were observed 6 days after the injection had stopped. The high DIC pore waters have high total alkalinity, and low δ13CDIC values (−20‰, compared to a background value of −2‰), due to the dissolution of the injected CO2 (δ13C=−26.6‰). The high DIC pore waters have enhanced concentrations of metals (including Ca, Fe, Mn) and dissolved silicon, relative to non-DIC enriched pore waters, indicating that dissolution of injected CO2 promotes dissolution of carbonate and silicate minerals. However, in this experiment, the pore water metal concentrations did not exceed levels considered to be harmful to the environment. The spatial extent of the impact of the injected CO2 in the sediments and pore waters was restricted to an area within 25m of the injection point, and no impact was observed in the overlying water column. Concentrations of all pore water constituents returned to background values within 18 days after the CO2 injection was stopped.

Orbisiana linearis from the early Ediacaran Lantian Formation of South China and its taphonomic and ecological implications

Orbisiana is an Ediacaran fossil characterized by linearly arranged circular or cylindrical units. The genus was first described from the late Ediacaran Redkino Series in the Moscow Syncline of Russia where it is preserved as pyritic molds in siltstone. Here we report Orbisiana linearis, originally described as Seirisphaera lineare, from black shale of the early Ediacaran Lantian Formation in South China. Unlike the type species (O. simplex), O. linearis is preserved as carbonaceous compressions on the bedding surface, with rare specimens preserved three-dimensionally, allowing morphological characterization using X-ray microCT techniques. Morphological, paleoecological and taphonomic analysis suggests that O. linearis was a chain-like modular organism characterized by uniserially arranged, millimeter-sized, and tangentially contacting circular or cylindrical units. Ontogenetic growth of O. linearis was likely achieved through both accretionary addition of new modules and inflational expansion of existing units, and it appears that inflational growth was determinate. Although its phylogenetic affinity remains uncertain, O. linearis was likely a procumbent epibenthic or semi-endobenthic organism that lived on the seawater-sediment interface or partly buried in sediments. O. linearis in the Lantian Formation was preserved in situ, although some specimens are disarticulated and many are tectonically deformed. Orbisiana joins a growing list of Ediacaran genera that have a remarkable range of stratigraphic, environmental, ecological, and taphonomic distribution.

Trace element bias in the use of CO2 vents as analogues for low pH environments: Implications for contamination levels in acidified oceans

Research into the effects of ocean acidification on marine ecosystems has increasingly focused on natural CO2 vents, although their intrinsic environmental complexity means observations from these areas may not relate exclusively to pH gradients. In order to assess trace element levels and distribution in the Levante Bay (Vulcano Island, NE Sicily, Italy) and its suitability for studying biological effects of pH decline, Ba, Fe and trace elements (As, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, V and Zn) in sediment were analysed from 7 transects. Where present, Cymodocea nodosa leaves and epiphytes were also analysed. At the spatial scale of the bay, trace element concentrations in sediments and biota showed wide variability, possibly related to both input from fluid emissions and seawater physico-chemical variables (i.e. pH and Eh), which may considerably affect the solubility and bioavailability of potentially harmful trace elements. According to two pollution indices (MSPI: Marine Sediment Pollution Index and SQG-Q: Sediment Quality Guideline Quotient), the bay can be considered to be affected by low contamination with moderate potential for adverse biological effects, especially in the area between about 150 and 350 m from the primary vent, where localized detrimental effects on biota may occur. Generally, biological samples showed concentrations that were comparable with the lower values of seagrass ranges. The overall results of this study support the complex spatial dynamics of trace elements in the CO2 vent studied, which are constrained by both direct input from the vent and/or biogeochemical processes affecting element precipitation at the sediment–seawater interface. Consequently, great caution should be used when relating biological changes along pH gradients to the unifactorial effect of pH only, as interactions with concurrent, multiple stressors, including trace element enrichments, may occur. This finding has implications for the use of CO2 vents as analogues in ocean acidification research. They should be considered more appropriately as analogues for low pH environments with non-negligible trace element contamination which, in a scenario of continuous increase in anthropogenic pollution, may be very common.

Bacterial diversity of the sediments transiting through the gut of Holothuria scabra (Holothuroidea; Echinodermata)

This work analyzes bacterial diversity of sediments transiting through the gut of Holothuria scabra which is an important bioturbator in tropical shallow waters. This edible holothurian species has a social and economic importance for coastal populations in many developing countries. Bacterial biodiversity was analyzed by sequencing the 16S rRNA of bacterial cultures and clones. DAPI and FISH methods were used to determine and compare the number of bacteria found in the various gut compartments. A total of 116 phylotypes belonging to the γ-Proteobacteria (60.5 %), α-Proteobacteria (24.5 %), Bacteroidetes (6 %), Actinobacteria (2.75 %), Fusobacteria (1.75 %), Firmicutes (1.75 %), Cyanobacteria (1.75 %) and δ-Proteobacteria (1 %) were identified. The number of bacteria is significantly greater (1.5×) in the foregut than in the ambient sediments. The number of bacteria significantly decreases in the midgut and remains stable until defecation. Some γ-Proteobacteria, especially Vibrio, are less affected by digestion than other bacterial taxa. The season has an impact on the bacterial diversity found in the sediments transiting through the gut: in the dry season, γ-Proteobacteria are the most abundant taxon, while α-Proteobacteria dominate in the rainy season. Vibrio is the most frequent genus with some well-known opportunistic pathogens like V. harveyi, V. alginolyticus and V. proteolyticus. Findings show that sediment-associated microbial communities are significantly modified by H. scabra during their transit through the gut which supports the view that holothurians play a substantial role in the structuring of bacterial communities at the sediment–seawater interface.

The Rate and Mechanism of Deep-Sea Glauconite Formation at the Ivory Coast-Ghana Marginal Ridge

AbstractThe environmental conditions and reaction paths of shallow-water glauconitization (&lt;500 m water depth, ~15°C) close to the sediment-seawater interface are generally considered to be well understood. In contrast, the key factors controlling deep-sea glauconite formation are still poorly constrained. In the present study, green grains formed in the recent deep-sea environment of the ODP Site 959, Ivory Coast-Ghana Marginal Ridge, (~2100 m water depth, 3-6°C) were investigated by X-ray diffraction and electron microscopic methods in order to determine the rate and mechanism of glauconitization.Green clay authigenesis at Hole 959C occurred mainly in the tests of calcareous foraminifera which provided post-depositional conditions ideal for glauconitization. Within this organic-rich microenvironment, Fe-smectite developed &lt;10 ky after deposition of the sediments by precipitation from precursor gels containing Fe, Mg, Al, and silica. This gel formation was supported by microbial activity and cation supply from the interstitial solution by diffusion. At a later stage of early marine diagenesis (900 ky), the Fe-smectites reacted to form mixed-layer glauconite-smectite. Further down (~2500 ky), almost pure glauconite with no compositional gaps between the Fe-smectite and glauconite end members formed. This burial-related Fe-smectite-to-glauconite reaction indicates that the glauconitization process was controlled mainly by the chemistry of the interstitial solutions. The composition of the interstitial solution depends heavily on micro-environmental changes related to early diagenetic oxidation of biodegradable (marine) organic matter, microbial sulfate reduction, silicate mineral alteration, carbonate dissolution, and Fe redox reactions. The availability of Fe is suggested as the probable limiting factor for glauconitization, explaining the various states of green-grain maturity within the samples, and this cation may be the most important rate-determining element.The rate of glauconite formation at ODP Site 959 is given by %GlSed = 22.6·log(ageSed) + 1.6 (R2 = 0.97) where %GlSed is the state of glauconitization in the sediment and ageSed is the sediment age (in ky). This glauconitization rate depends mainly on continuous cation supply (in particular Fe) and is about five times less than that in shallow-shelf regions, suggesting significantly slower reaction at the lower temperature of deep-sea environments.

Glauconites from the Bahariya Oasis: An evidence for Cenomanian marine transgression in Egypt

Although the Cenomanian sediments at the Bahariya Oasis, Egypt have been extensively studied, the origin of glauconites in these sediments is still questionable. In addition the paleoenvironmental significance of these glauconites was not discussed in these studies. In this paper, the glauconites that occur in the upper most part of the Bahariya Formation were subjected to petrographical, mineralogical, and geochemical studies to assess their origin and depositional environment.Glauconite appears as oval, rounded, fine to medium-grained (100–200μm), moderately- to well-sorted, pale green to yellowish green pellets in clayey and iron oxides matrix. X-ray diffraction analysis indicated that bulk samples are composed of glauconite, while clay fractions (<2μm) are mainly smectite. K2O contents (4.8–6.1%) as well as the distance between (001) and (020) reflections (11.3–11.8cm) suggested that the studied glauconite can be classified as slightly evolved or of low to moderate maturity. The occurrence of clayey matrix and patches of diffuse green glauconitic pigmentation in the matrix may support the autochthonous origin of the studied glauconite. On the other hand, the occurrence of glauconite as rounded, moderately- to well-sorted and well-defined grains along with the abundance of detrital quartz grains may suggest a detrital or extrasequential origin of these glauconites. This apparent controversy can be solved by considering the studied glauconite as parautochthonous glauconite in which the autochthonous glauconites have been transported for short distance from their place of origin. The abundance of smectite may also suggest the formation of these glauconites from detrital smectite precursor through diffusion of K from the seawater–sediment interface. Average ΣREE content in the studied glauconites is 484ppm and chondrite-normalized REEs patterns exhibit LREE enrichment relative to HREE as shown by (La/Yb)N ratios, that vary from 5 to 8 and slightly negative Eu anomalies are pronounced with Eu/Eu∗ from 0.7 to 0.8. The difference in the REE concentrations and patterns between the Cenomanian glauconites of the Bahariya Formation and the typical seawater can be attributed to the overprint of the smectite precursor on the REE contents and patterns in the studied glauconites. These glauconites are considered to represent a Transgressive System Tract (TST) in the upper part of the Cenomanian Bahariya Formation.

Formation processes of methane-derived authigenic carbonates from the Gulf of Cadiz

The Gulf of Cadiz, NE Atlantic, represents an area of extensive formation of methane-derived authigenic carbonates (MDAC), indicative of fluid seepage. These MDAC, that reach extraordinary length and thickness, were geophysically mapped and sampled and the recovered carbonate-cemented material has δ 13C values as low as − 56.2‰ VPDB, indicating methane as the major carbon source. The MDAC form two main lithologic groups, one mainly comprising dolomite and the second dominated by aragonite. The dolomite-dominated samples were found along fault-controlled diapiric ridges, on some mud volcanoes and mud diapirs, all on the pathway of the Mediterranean Outflow Water, and along fault scarps. Aragonite pavements were found associated with mud volcanoes and along fault scarps, but are otherwise not restricted to the pathways of the Mediterranean Outflow Water. Based on the results from this study, we propose that the two lithologic groups reflect different geochemical formation environments associated with a formation model based on their morphology, mineralogy and geochemistry. The aragonite-dominated samples represent precipitation of authigenic carbonates at the sediment–seawater interface or close to it, in a high alkalinity environment resulting from anaerobic oxidation of methane-rich fluids venting into sulphate-bearing porewaters. In contrast, the dolomite-dominated samples result from cementation along fluid conduits inside the sedimentary column with a somewhat restricted seawater ventilation. The dolomite chimneys form in places presently swept by the strong flow of the Mediterranean undercurrent so that the unconsolidated sediments are eroded and the chimneys are exposed at the seafloor. The widespread and large abundance of MDAC is a direct evidence of extensive methane seepage episodes in the Gulf of Cadiz. The coincidence of the different lithologic types in close spatial and temporal association indicates a persistence of seepage episodes in some structures over large periods of time.