Subsurface Sediments Research Articles

A field study of nuclear magnetic resonance logging to quantify petroleum contamination in subsurface sediments

In a field study conducted in Pine Ridge, South Dakota, nuclear magnetic resonance (NMR) logging measurements were used to investigate an area of hydrocarbon contamination from leaking underground storage tanks. The NMR logging measurements are directly sensitive to hydrogen-bearing fluids in the sediments surrounding a well and can be used to estimate in situ fluid volumes. The relaxation time [Formula: see text] and diffusion coefficient [Formula: see text] of the fluids may be used to differentiate between signal from water and signal from contaminant, enabling the estimation of the hydrocarbon volume. In this study, NMR measurements were collected in two PVC-cased monitoring wells, with [Formula: see text] and [Formula: see text] measurements used together to detect a contaminant smear zone at both the wells. Although the contrast in [Formula: see text] between the fluids was found to be inadequate for fluid typing, the [Formula: see text] contrast between the contaminant and water in silt enabled the estimation of contaminant volumes. Using this technique, the vertical extent of the smear zone was found to be more than 3 m with up to 5 vol% contaminant in the sediments at one well and up to 9.5 vol% at the other well. Our work reveals that NMR logging can, under certain circumstances, be used to detect and quantify in situ contamination, but it also highlights the significant limitation that sediment and contaminant properties at many sites may result in insufficient contrast in [Formula: see text] and [Formula: see text].

Impaired Short-Term Functioning of a Benthic Community from a Deep Norwegian Fjord Following Deposition of Mine Tailings and Sediments

The extraction of minerals from land-based mines necessitates the disposal of large amounts of mine tailings. Dumping and storage of tailings into the marine environment, such as fjords, is currently being performed without knowing the potential ecological consequences. This study investigated the effect of short-term exposure to different deposition depths of inert iron ore tailings (0.1, 0.5 and 3 cm) and dead subsurface sediment (0.5 and 3 cm) on a deep water (200 m) fjord benthic assemblage in a microcosm experiment. Biotic and abiotic variables were measured to determine structural and functional changes of the benthic community following an 11 and 16 day exposure with tailings and dead sediment, respectively. Structural changes of macrofauna, meiofauna and bacteria were measured in terms of biomass, density, community composition and mortality while measures of oxygen penetration depth, sediment community oxygen consumption and 13C-uptake and processing by biota revealed changes in the functioning of the system. Burial with mine tailings and natural sediments modified the structure and functioning of the benthic community albeit in a different way. Mine tailings deposition of 0.1 cm and more resulted in a reduced capacity of the benthic community to remineralize fresh 13C-labelled algal material, as evidenced by the reduced sediment community oxygen consumption and uptake rates in all biological compartments. At 3 cm of tailings deposition, it was evident that nematode mortality was higher inside the tailings layer, likely caused by reduced food availability. In contrast, dead sediment addition led to an increase in oxygen consumption and bacterial carbon uptake comparable to control conditions, thereby leaving deeper sediment layers anoxic and in turn causing nematode mortality at 3 cm deposition. This study clearly shows that even small levels (0.1 cm) of instantaneous burial by mine tailings may significantly reduce benthic ecosystem functioning on the short term. Furthermore, it reveals the importance of substrate characteristics and origin when studying the effects of substrate addition on marine benthic fauna. Our findings should alert decision makers when considering approval of new deep-sea tailings placement sites as this technique will have major negative impacts on benthic ecosystem functioning over large areas.

High-throughput sequencing of microbial eukaryotes in Lake Baikal reveals ecologically differentiated communities and novel evolutionary radiations.

We performed high-throughput 18S rDNA V9 region sequencing analyses of microeukaryote (protist) communities at seven sites with depths ranging from 0 to 1450 m in the southern part of Lake Baikal. We show that microeukaryotic diversity differed according to water column depth and sediment depth. Chrysophytes and perkinsids were diverse in subsurface samples, novel radiations of petalomonads and Ichthyobodo relatives were found in benthic samples, and a broad range of divergent OTUs were detected in deep subbenthic samples. Members of clades usually associated with marine habitats were also detected, including syndineans for the first time in freshwater systems. Fungal- and cercozoan-specific c. 1200 bp amplicon clone libraries also revealed many novel lineages in both planktonic and sediment samples at all depths, a novel radiation of aphelids in shallower benthic samples, and partitioning of sarcomonad lineages in shallow vs deep benthic samples. Putative parasitic lineages accounted for 12.4% of overall reads, including a novel radiation of Ichthyobodo (fish parasite) relatives. Micrometazoans were also analysed, including crustaceans, rotifers and nematodes. The deepest (>1000m) subsurface sediment samples harboured some highly divergent sequence types, including heterotrophic flagellates, parasites, putative metazoans and sequences likely representing organisms originating from higher up in the water column.

Interaction between Physical Heterogeneity and Microbial Processes in Subsurface Sediments: A Laboratory-Scale Column Experiment.

Physical heterogeneity determines interstitial fluxes in porous media. Nutrients and organic matter distribution in depth influence physicochemical and microbial processes occurring in subsurface. Columns 50 cm long were filled with sterile silica sand following five different setups combining fine and coarse sands or a mixture of both mimicking potential water treatment barriers. Water was supplied continuously to all columns during 33 days. Hydraulic conductivity, nutrients and organic matter, biofilm biomass, and activity were analyzed in order to study the effect of spatial grain size heterogeneity on physicochemical and microbial processes and their mutual interaction. Coarse sediments showed higher biomass and activity in deeper areas compared to the others; however, they resulted in incomplete denitrification, large proportion of dead bacteria in depth, and low functional diversity. Treatments with fine sediment in the upper 20 cm of the columns showed high phosphorus retention. However, low hydraulic conductivity values reported in these sediments seemed to constraint biofilm activity and biomass. On the other hand, sudden transition from coarse-to-fine grain sizes promoted a hot-spot of organic matter degradation and biomass growth at the interface. Our results reinforce the idea that grain-size disposition in subsurface sandy sediments drives the interstitial fluxes, influencing microbial processes.

Pyrosequencing reveals benthic bacteria changes responsing to heavy deposition of Microcystis scum in lab — searching bacteria for bloom control

Bacteria capable of degrading cyanobacteria Microcystis are crucial for determining the ecological consequences of Microcystis blooms in freshwater lakes. Scum derived from Microcystis blooms tends to accumulate in bays of large lakes and then sink to the sediments where it is finally consumed by benthic bacteria. Understanding the response of benthic bacterial communities to massive Microcystis deposition events may help identify the bacteria best suited to Microcystis hydrolyzation and even bloom control. For that purpose, an experimental system was set up in which intact sediment cores were incubated in the laboratory with normal and heavy deposits of Microcystis detritus. Pyrosequencing was performed in order to describe a phylogenetic inventory of bacterial communities in samples taken at 0–1, 1–2 and 2–3 cm depths in incubated sediments and in original untreated sediment. A hierarchical cluster tree was constructed expose differences between sediments. Similarity percentage calculations were also performed to identify the bacterial species contributing to variation. The results of this study suggest that: (1) deposition of Microcystis scums exerts a strong effect on the bacterial community composition in the surface (0–1 cm) and sub-surface (1–2 cm) sediment layers; (2) bacterial community responses to Microcystis detritus deposition vary across vertical gradients. A list of bacteria with potential roles in Microcystis degradation was compiled. These findings may inform the development of future measures for Microcystis bloom control in lakes.

Microbial acceleration of aerobic pyrite oxidation at circumneutral pH.

Pyrite (FeS2 ) is the most abundant sulfide mineral on Earth and represents a significant reservoir of reduced iron and sulfur both today and in the geologic past. In modern environments, oxidative transformations of pyrite and other metal sulfides play a key role in terrestrial element partitioning with broad impacts to contaminant mobility and the formation of acid mine drainage systems. Although the role of aerobic micro-organisms in pyrite oxidation under acidic-pH conditions is well known, to date there is very little known about the capacity for aerobic micro-organisms to oxidize pyrite at circumneutral pH. Here, we describe two enrichment cultures, obtained from pyrite-bearing subsurface sediments, that were capable of sustained cell growth linked to pyrite oxidation and sulfate generation at neutral pH. The cultures were dominated by two Rhizobiales species (Bradyrhizobium sp. and Mesorhizobium sp.) and a Ralstonia species. Shotgun metagenomic sequencing and genome reconstruction indicated the presence of Fe and S oxidation pathways in these organisms, and the presence of a complete Calvin-Benson-Bassham CO2 fixation system in the Bradyrhizobium sp. Oxidation of pyrite resulted in thin (30-50nm) coatings of amorphous Fe(III) oxide on the pyrite surface, with no other secondary Fe or S phases detected by electron microscopy or X-ray absorption spectroscopy. Rates of microbial pyrite oxidation were approximately one order of magnitude higher than abiotic rates. These results demonstrate the ability of aerobic microbial activity to accelerate pyrite oxidation and expand the potential contribution of micro-organisms to continental sulfide mineral weathering around the time of the Great Oxidation Event to include neutral-pH environments. In addition, our findings have direct implications for the geochemistry of modern sedimentary environments, including stimulation of the early stages of acid mine drainage formation and mobilization of pyrite-associated metals.

Sorption and speciation of iodine in groundwater system: The roles of organic matter and organic-mineral complexes

Characterizing the properties of main host of iodine in soil/sediment and the geochemical behaviors of iodine species are critical to understand the mechanisms of iodine mobilization in groundwater systems. Four surface soil and six subsurface sediment samples were collected from the iodine-affected area of Datong basin in northern China to conduct batch experiments and to evaluate the effects of NOM and/or organic-mineral complexes on iodide/iodate geochemical behaviors. The results showed that both iodine contents and kf-iodate values had positive correlations with solid TOC contents, implying the potential host of NOM for iodine in soil/sediment samples. The results of chemical removal of easily extracted NOM indicated that the NOM of surface soils is mainly composed of surface embedded organic matter, while sediment NOM mainly occurs in the form of organic-mineral complexes. After the removal of surface sorbed NOM, the decrease in kf-iodate value of treated surface soils indicates that surface sorbed NOM enhances iodate adsorption onto surface soil. By contrast, kf-iodate value increases in several H2O2-treated sediment samples, which was considered to result from exposed rod-like minerals rich in Fe/Al oxyhydroxide/oxides. After chemical removal of organic-mineral complexes, the lowest kf-iodate value for both treated surface soils and sediments suggests the dominant role of organic-mineral complexes on controlling the iodate geochemical behavior. In comparison with iodate, iodide exhibited lower affinities on all (un)treated soil/sediment samples. The understanding of different geochemical behaviors of iodine species helps to explain the occurrence of high iodine groundwater with iodate and iodide as the main species in shallow (oxidizing conditions) and deep (reducing conditions) groundwater.

Bioactive Metabolites from the Deep Subseafloor Fungus Oidiodendron griseum UBOCC-A-114129.

Four bioactive compounds have been isolated from the fungus Oidiodendron griseum UBOCC-A-114129 cultivated from deep subsurface sediment. They were structurally characterized using a combination of LC–MS/MS and NMR analyses as fuscin and its derivatives (dihydrofuscin, dihydrosecofuscin, and secofuscin) and identified as polyketides. Albeit those compounds were already obtained from terrestrial fungi, this is the first report of their production by an Oidiodendron species and by the deepest subseafloor isolate ever studied for biological activities. We report a weak antibacterial activity of dihydrosecofuscin and secofuscin mainly directed against Gram-positive bacteria (Minimum Inhibitory Concentration (MIC) equal to Minimum Bactericidal Concentration (MBC), in the range of 100 μg/mL). The activity on various protein kinases was also analyzed and revealed a significant inhibition of CDC2-like kinase-1 (CLK1) by dihysecofuscin.

Potential for microbial H2 and metal transformations associated with novel bacteria and archaea in deep terrestrial subsurface sediments

Geological sequestration in deep underground repositories is the prevailing proposed route for radioactive waste disposal. After the disposal of radioactive waste in the subsurface, H2 may be produced by corrosion of steel and, ultimately, radionuclides will be exposed to the surrounding environment. To evaluate the potential for microbial activities to impact disposal systems, we explored the microbial community structure and metabolic functions of a sediment-hosted ecosystem at the Horonobe Underground Research Laboratory, Hokkaido, Japan. Overall, we found that the ecosystem hosted organisms from diverse lineages, including many from the phyla that lack isolated representatives. The majority of organisms can metabolize H2, often via oxidative [NiFe] hydrogenases or electron-bifurcating [FeFe] hydrogenases that enable ferredoxin-based pathways, including the ion motive Rnf complex. Many organisms implicated in H2 metabolism are also predicted to catalyze carbon, nitrogen, iron and sulfur transformations. Notably, iron-based metabolism is predicted in a novel lineage of Actinobacteria and in a putative methane-oxidizing ANME-2d archaeon. We infer an ecological model that links microorganisms to sediment-derived resources and predict potential impacts of microbial activity on H2 consumption and retardation of radionuclide migration.

Organically bound iodine as a bottom-water redox proxy: Preliminary validation and application

Carbonate-associated iodine (I/Ca) has been used as a proxy of local, upper-ocean redox conditions, and has successfully demonstrated highly dynamic spatial and temporal patterns across different time scales of Earth history. To further explore the utility of iodine as a paleo-environmental proxy, we present here a new method of extracting organically bound iodine (Iorg) from shale using volumes of samples on the order of tens of milligrams, thus offering the potential for high-resolution work across thin shale beds. The ratio of Iorg to total organic carbon (I/TOC) in modern surface and subsurface sediments decreases with decreasing bottom-water oxygen, which may be used to reconstruct paleo-redox changes.As a proof of concept, we evaluate the I/TOC proxy in Holocene sediments from the Baltic Sea, Landsort Deep (IODP 347) and discuss those data within a framework of additional independent redox proxies, e.g., iron speciation and [Mo]. The results imply that I/TOC may be sensitive to hypoxic–suboxic conditions, complementary to proxies sensitive to more reducing, anoxic–euxinic conditions. Then, we test the usage of I/TOC in sediments deposited during Late Cretaceous, Cenomanian–Turonian Oceanic Anoxic Event (OAE) 2 from ~94millionyears ago (Ma). We generated I/TOC and Iorg records from six OAE 2 sections: Tarfaya (Morocco), Furlo (central Italy), Demerara Rise (western equatorial Atlantic), Cape Verde Basin (eastern equatorial Atlantic), South Ferriby (UK), and Kerguelen Plateau (southern Indian Ocean), which provide a broad spatial coverage. Generally, I/TOC decreases over the interval recorded by the positive carbon-isotope excursion, the global signature of OAE 2, suggesting an expansion of more reducing bottom-water conditions and consistent with independent constraints from iron speciation and redox-sensitive trace-metals (e.g., Mo). Relatively higher I/TOC values (thus more oxic conditions) are recorded at two high latitude sites for OAE 2, supporting previous model simulations (cGENIE) that indicated higher bottom water oxygen concentrations in these regions. Our results also indicate that organic-rich and oxygenated seafloors are likely a major sink of iodine and correspondingly influence its global seawater inventory.

Subseafloor Investigation of Sediments at Southern Tauranga Harbour, New Zealand, before Capital Dredging

ABSTRACT Jorat, M.E.; Moon, V.G.; Hepp, D.A.; Kreiter, S.; de Lange, W.P.; Feldmann, S., and Morz, T., 2017. Subseafloor investigation of sediments at southern Tauranga Harbour, New Zealand, before capital dredging. The Port of Tauranga plays a key role in New Zealand's export–import industry, and capital dredging commenced in October 2015 to extend the shipping channels to accommodate larger container vessels. This study investigated two-dimensional and three-dimensional subsurface estuarine sediment stratigraphy to predict the sedimentological conditions encountered during dredging operations to ensure that appropriate dredging methodologies were used to minimise the generation of turbidity. Eight cone penetration tests (CPTs), 14 core descriptions, and a high-resolution seismic investigation in Stella Passage (the main shipping channel of Tauranga Harbour) provided the basis for this research. Six major units comprise the stratigraphy; in ascending order they are lower pumiceous sand and silt (UNIT6), ...

Impact of hydrogeomorphic processes on ecological functions of brown trout habits

In recent decades, due to river regulations and their impact on river morphology, brown trout populations have been declining along Lar River downstream of Lar Dam located near Tehran, Iran. Considering the recent water scarcity in the country, development of river habitat assessment models seems necessary. Therefore, in this research, an analytically applied approach is adopted to evaluate brown trout habitat by creating a relation among the hydrologic, hydraulic, geomorphic and ecologic processes. After field survey, dimensionless shear stress of the stream flow thresholds including environmental flow, bankfull flow, surface and subsurface sediment flow thresholds was calculated for Lar, Dalichay and Sefidab Rivers using Shields formula. Then, by considering the dimensionless shear stress ranges of the stream flow thresholds, functional flows ranges and duration were calculated together with ecological efficiency of the cross sections. In addition, effects of annual water yield and entrenchment ratio of the cross sections on habitat functionality were also worked out as a result of which an exponential interaction was developed between the dimensionless shear stress and discharge. Results show that an increase in functional flows ranges and duration, together with rising of ecological efficiency, is directly proportional to an increase in median bed sediment size, entrenchment ratio and annual water yield. Therefore, flow regime, cross-sectional geometry, water-surface slope and bed sediment size could be effective on the ecological functions of the brown trout’s life cycle and functionality of river flow.

Seasonal Variation of Mercury Methylation Potential in Pristine Coastal Marine Sediment from the Gulf of Trieste (Northern Adriatic Sea)

ABSTRACTThe Gulf of Trieste (northern Adriatic Sea) is considered as an important “natural bioreactor” that can be stimulated to increase sedimentary methylmercury (MeHg) production by mobilizing its extensive mercury (Hg) pool. Sediment samples, collected in the southern pristine, less contaminated, part of the gulf, were amended with radioactive 197Hg in incubation experiments to determine the seasonal variation of Hg methylation potential at two depth layers (surface 0–2 cm and subsurface 2–4 cm). In parallel, Hg and MeHg were determined in the sediment solid phase and pore water. Total Hg and MeHg in the sediment solid phase ranged between 151 and 223 ng g−1 and between 0.17 and 0.68 ng g−1, and in pore water between 3.1 and 25.6 ng L−1 and between 0.03 and 6.16 ng L−1. Distribution coefficients (Kd) suggested that Hg species are strongly bound to the sediment (log Kd 3.86–4.87 and 1.67–3.39 for Hg and MeHg, respectively). Hg methylation potentials showed that MeHg is continuously produced, albeit not at a constant rate. The methylation potential in the sediment surface layer was higher and more variable in autumn, in parallel to higher bottom water temperature and O2 content and sediment sulfate reduction, than in late spring and summer except in July in the presence of higher labile sedimentary organic matter (OM) level. Surface sediment showed rapid methylation of added traced Hg (0–0.66% day−1 Hg methylated). However, a significant fraction of the radiolabeled Hg remained available. In the subsurface sediment, methylation was lower (0–0.22% day−1 Hg methylated). Hg methylation potentials suggest that MeHg is continuously produced, albeit not at a constant rate, in the studied sediment of the Gulf of Trieste.

Improved Measurement of Extracellular Enzymatic Activities in Subsurface Sediments Using Competitive Desorption Treatment

Extracellular enzymatic activities initiate microbially-driven heterotrophic carbon cycling in subsurface sediments. While measurement of hydrolytic activities in sediments is fundamental to our understanding of carbon cycling, these measurements are often technically difficult due to sorption of organic substrates to the sediment matrix. Most methods that measure hydrolysis of organic substrates in sediments rely on recovery of a fluorophore or fluorescently-labeled target substrate from a sediment incubation. The tendency for substrates to sorb to sediments results in lower recovery of an added substrate, and can result in data that are unusable or difficult to interpret. We developed a treatment using competitive desorption of a fluorescently-labeled, high molecular weight organic substrate that improves recovery of the labeled substrate from sediment subsamples. Competitive desorption treatment improved recovery of the fluorescent substrate by a median of 66%, expanded the range of sediments for which activity measurements could be made, and was effective in sediments from a broad range of geochemical contexts. More reliable measurements of hydrolytic activities in sediments will yield usable and more easily interpretable data from a wider range of sedimentary environments, enabling better understanding of microbially-catalyzed carbon cycling in subsurface environments.

First Complete Genome Sequence of Marinilactibacilluspiezotolerans Strain 15R, a Marine Lactobacillus Isolated from Coal-Bearing Sediment 2.0 Kilometers below the Seafloor, Determined by PacBio Single-Molecule Real-Time Technology.

ABSTRACT Marinilactibacillus piezotolerans strain 15R is a facultatively anaerobic heterotrophic lactobacillus isolated from deep marine subsurface sediment nearly 2 km below the seafloor in the northwestern Pacific. We report here the first whole-genome sequence of strain 15R. The identified genome sequence has 2,767,908 bp, 35.4% G+C content, and predicted 2,552 candidate protein-coding sequences, with no identified plasmids.

Predominance of Viable Spore-Forming Piezophilic Bacteria in High-Pressure Enrichment Cultures from ~1.5 to 2.4 km-Deep Coal-Bearing Sediments below the Ocean Floor.

Phylogenetically diverse microorganisms have been observed in marine subsurface sediments down to ~2.5 km below the seafloor (kmbsf). However, very little is known about the pressure-adapted and/or pressure-loving microorganisms, the so called piezophiles, in the deep subseafloor biosphere, despite that pressure directly affects microbial physiology, metabolism, and biogeochemical processes of carbon and other elements in situ. In this study, we studied taxonomic compositions of microbial communities in high-pressure incubated sediment, obtained during the Integrated Ocean Drilling Program (IODP) Expedition 337 off the Shimokita Peninsula, Japan. Analysis of 16S rRNA gene-tagged sequences showed that members of spore-forming bacteria within Firmicutes and Actinobacteria were predominantly detected in all enrichment cultures from ~1.5 to 2.4 km-deep sediment samples, followed by members of Proteobacteria, Acidobacteria, and Bacteroidetes according to the sequence frequency. To further study the physiology of the deep subseafloor sedimentary piezophilic bacteria, we isolated and characterized two bacterial strains, 19R1-5 and 29R7-12, from 1.9 and 2.4 km-deep sediment samples, respectively. The isolates were both low G+C content, gram-positive, endospore-forming and facultative anaerobic piezophilic bacteria, closely related to Virgibacillus pantothenticus and Bacillus subtilis within the phylum Firmicutes, respectively. The optimal pressure and temperature conditions for growth were 20 MPa and 42°C for strain 19R1-5, and 10 MPa and 43°C for strain 29R7-12. Bacterial (endo)spores were observed in both the enrichment and pure cultures examined, suggesting that these piezophilic members were derived from microbial communities buried in the ~20 million-year-old coal-bearing sediments after the long-term survival as spores and that the deep biosphere may host more abundant gram-positive spore-forming bacteria and their spores than hitherto recognized.

Viral Induced Microbial Mortality in Arctic Hypersaline Spring Sediments.

Viruses are a primary influence on microbial mortality in the global ocean. The impacts of viruses on their microbial hosts in low-energy environments are poorly explored and are the focus of this study. To investigate the role of viruses in mediating mortality in low-energy environments where contacts between viruses and microbes are infrequent, we conducted a set of in situ time series incubations in the outlet and channel sediments of two cold, hypersaline springs of the Canadian High Arctic. We found microbial and viral populations in dynamic equilibrium, indicating approximately equal birth and death rates for each population. In situ rates of microbial growth were low (0.5–50 × 103 cells cm-3 h-1) as were rates of viral decay (0.09–170 × 104 virions cm-3 h-1). A large fraction of the springs’ viral communities (49–100%) were refractory to decay over the timescales of our experiments. Microcosms amended with lactate or acetate exhibited increased microbial growth rates (up to three-fold) indicating organic carbon as one limiting resource for the microbial communities in these environments. A substantial fraction (15–71%) of the microbial populations contained inducible proviruses that were released- occasionally in multiple pulses- over the eight monitored days following chemical induction. Our findings indicate that viruses in low-energy systems maintain low rates of production and activity, have a small but notable impact on microbial mortality (8–29% attenuation of growth) and that successful viral replication may primarily proceed by non-lethal strategies. In cold, low biomass marine systems of similar character (e.g., subsurface sediments), viruses may be a relatively minor driver of community mortality compared to less energy-limited environments such as the marine water column or surface sediments.

THE STUDY OF HEAVY METAL CONCENTRATIONS IN THE RED MUD DEPOSITS AT THE GULF OF CORINTH, USING MULTIVARIATE TECHNIQUES

Detailed geochemical analyses were performed on the recent sedimentary cover (0 – 15 cm) of the northern flank of the Gulf of Corinth, in western Greece. Man-made metalliferous deposits have been formed on the sea-bed of the Gulf due to the discharge of red mud tailings on the shelf of the Antikyra Bay. The metalliferous deposits have been incorporated in the sedimentation regime of the area. A large grid of sediment samples collected from surface and subsurface red mud deposits and the surrounding natural sediments were analysed for Ag, Al, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Ni, Pb, Si, Ti, V, and Zn concentrations, using Atomic Absorption Spectrometry (AAS). Also, Total Organic Carbon and Calcium Carbonateswere determined. Additionally, correlation between heavy metal concentrations and grain-size data was examined. High heavy metal content values were measured on the floor of the Corinth basin, but the highest heavy metal concentrations occur near the discharge site. In subsurface sediments the heavy metal content (Fe, Ni, Cr, Pb, V, Ag, Co, Al, Cd, Ti, Hg) is significantly higher in the red and reddish mud sediments than in the natural mud sediments. The heavy metal content values decrease from top to base within red-mud deposits. Multivariate statistical methods (Cluster and Facto analysis) were applied on the heavy metal composition and grain-size dataset in order to investigate the inter-element relationships as well as the relationships between the geochemical and sedimentological data.

Spatial Distribution of Heavy Metals in Surface and Sub Surface Sediments of the Coastal Area of Kutubdia Island, Cox's Bazar, Bangladesh

Concentrations of selected heavy metals (Zn, Cu, Pb, Cd, Fe, Mn, and Ni) in surface and sub-surface sediments were studied with an attempt to establish a base-line data of their concentration in the sediments of the coastal area of Kutubdia Island, Cox’s Bazar, Bangladesh. Sediment sample of twelve different sites were collected and taken for open plate digestion technique (HNO3). The concentration of heavy metals was analyzed using Flame Atomic Absorption Spectrophotometer (FAAS). The concentration of Cu, Fe, Mn, Pb and Ni in the sediment samples was higher than the proposed Threshold Effect Level (TEL). The observed concentrations of the metals varied in different sampling sites and also in surface to sub-surface. The decreasing trend of metals were observed in surface and sub-surface sediments as Fe>Mn>Ni>Cu>Pb>Zn>Cd. Geoaccumulation Index (I-geo) indicates that the sediment in the studied stations were unpolluted (grade 0) with respect to Cu, Pb, Fe, Zn and Cd whereas concentration of Mn indicated slight pollution in three stations. Contamination Factor (CF) values showed low to moderately pollution in studied stations. The values of Pollution Load Index (PLI) for the samples collected from all the stations were found less than one, indicating perfection except one study area was the hot spot. The Enrichment Factor (EF) indicates that the metal in the sediment of studied stations were entirely from crustal minerals whereas the sources of Mn are may be anthropogenic. However, the elevated level of some toxic elements was found in this investigation alarming about the natural balance of the area. Constant monitoring of the coastal area of Kutubdia Island, Cox’s Bazar, Bangladesh is needed with a view to minimize the health risk of the population as well as the detrimental impacts on the aquatic ecosystem.

Carbon and Phosphorus Cycling in Arabian Sea Sediments across the Oxygen Minimum Zone

Several studies have focused on carbon, oxygen, and phosphorus dynamics across the modern oxygen minimum zone (OMZ) to constrain how signals of modern systems get “locked in” upon burial. In this study, a sequential phosphorus fractionation technique was applied to surficial and sub-surface sediments from stations at depths spanning the OMZ on the Pakistan margin of the Arabian Sea in order to test the oxygen-carbon-phosphorus connection in modern marine sediments. Some early diagenetic loss of phosphorus compared to organic carbon was observed, but a significant portion of the released phosphorus was retained by uptake on oxyhydroxides and by the formation of an authigenic phosphorus-bearing phase. This process is unaffected by station location relative to the OMZ, and results in an effective organic carbon-to-reactive-phosphorus sediment ratio that is close to the average observed for open-ocean sediments, regardless of bottom water oxygen content.