Activity In Surface Sediments Research Articles

Environmental Risk Assessment of Petroleum Activities in Surface Sediments, Suez Gulf, Egypt

The present study focuses on the risk assessment of heavy metal contamination in aquatic ecosystems by evaluating the current situation of heavy metals in seven locations (North Amer El Bahry, Amer, Bakr, Ras Gharib, July Water Floud, Ras Shokeir, and El Marageen) along the Suez Gulf coast that are well-known representative sites for petroleum activities in Egypt. One hundred and forty-six samples of surface sediments were carefully collected from twenty-seven profiles in the intertidal and surf zone. The hydrochemical parameters, such as pH and salinity (S‰), were measured during sample collection. The mineralogy study was carried out by an X-ray diffractometer (XRD), and the concentrations of Al, Mn, Fe, Cr, Cu, Co, Zn, Cd, and Pb were determined using inductively coupled plasma mass spectra (ICP-MS). The ecological risks of heavy metals were assessed by applying the contamination factor (CF), enrichment factor (EF), geoaccumulation index (Igeo), pollution load index (PLI), and potential ecological risk index (RI). The mineralogical composition mainly comprised quartz, dolomites, calcite, and feldspars. The average concentrations of the detected heavy metals, in descending order, were Al > Fe > Mn > Cr > Pb > Cu > Zn > Ni > Co > Cd. A non-significant or negative relationship between the heavy metal concentration in the samples and their textural grain size characteristics was observed. The coastal surface sediment samples of the Suez Gulf contained lower concentrations of heavy metals than those published for other regions in the world with petroleum activities, except for Al, Mn, and Cr. The results for the CF, EF, and Igeo showed that Cd and Pb have severe enrichment in surface sediment and are derived from anthropogenic sources, while Al, Mn, Fe, Cr, Co, Ni, Cu, and Zn originate from natural sources. By comparison, the PLI and RI results indicate that the North Amer El Bahry and July Water Floud are considered polluted areas due to their petroleum activities. The continuous monitoring and assessment of pollutants in the Suez Gulf will aid in the protection of the environment and the sustainability of resources.

Biogenic matter content in marine sediments in the vicinity of the Antarctic Peninsula: Recent sedimentary conditions under a diverse environment of production, transport, selective preservation and accumulation

Burial fluxes of organic carbon and biogenic silica were determined in 17 continental shelf sediment cores collected from the northern Weddell Sea, the Bransfield Strait, and the southern Drake Passage. Coring sites included open-shelf stations as well as slope and glacial trough environments, with water depths varying from 220 to 760 m. Apparent 210Pb accumulation rates from these cores ranged from 0.04 g m−2y−1 to 0.21 g m−2y−1 (1 to 3 mm y−1), with organic carbon burial rates ranging from 3 to 15 g OC m−2y−1 and biogenic silica accumulation rates ranging from 15 to 126 g SiO2 m−2y−1. OC contents below the surface mixed layer ranged from 0.26 to 1.51 wt. % (avg. 0.64 %). Biogenic silica contents at depth ranged from 2.3 to 11.2 wt. % (avg. 7.5%), with an average bSi/OC ratio (wt. %/wt. %) at depth of 12. Annual OC primary production rates and biogenic silica production rates in the euphotic zone were estimated from satellite chlorophyll-a data in the literature and from a seasonal model for biogenic particle export from surface waters. Based on these biogeochemical data, preservation efficiencies (i.e., mass burial rate/water column production rate) were calculated for organic carbon and biogenic silica. These preservation efficiency values ranged from 2 to 18% (avg. 9%) for OC and 8 to 106% (avg. 54%) for bSi. These relatively high preservation efficiencies resulted from extensive lateral sediment focusing (210Pb Psi (Ψ) values [burial flux/water column production rate] ranging from 2 to 33; avg. of 16), cold bottom water temperatures (2 to −2°C), and relatively high biogenic Si and OC production rates in the euphotic zone. The enhanced preservation efficiency for bSi relative to OC (i.e., 54% vs. 9%) in these Antarctic settings is consistent with the change in the phytoplankton bSi/OC (wt. %/wt. %) value of 2 for this area to the burial bSi/OC value of 12. Excess 210Pb activities in surface sediments varied from 4 to 47 dpm g−1. The surface mixed layer in the seabed varied in thickness from 0 to 4 cm. The penetration of excess 210Pb into these Antarctic Peninsula sediments varied from 6 to 28 cm (avg. 18 cm). The inventory of excess 210Pb in the seabed varied from 13 to 230 dpm cm−2 (avg. 110 dpm cm−2). Although 210Pb was the only radionuclide measured in this study, “apparent” 210Pb sediment accumulation rate (SAR) values from these 17 cores (assuming that deep bioturbation is negligible) are believed to be accurate SAR values because of good agreement between 210Pb and 14C chronologies from nearby cores reported in the literature.

Phosphate Control Effect and Water Body Remediation of Magnesium Modified Reed Biochar

Adding biochar from harvested reed to sediments is a new method to control the release of sedimentary phosphorus. Three types of Mg-modified biochars were prepared by pyrolysis of reed modified by magnesium chloride at 300, 450, and 600℃.The phosphate adsorption characteristics of the three types of biochars were analyzed by isothermal adsorption experiments. Biochar MBC-450 with good phosphate adsorption effects was selected as the material for the following experiments. Taking the sediment and overlying water of a campus river as the experimental object, we studied the adsorption of phosphate in overlying water and the control of sedimentary phosphorus by Mg-modified biochar under different dosage modes (mixing and covering). The concentration of DIP in the overlying water could be effectively reduced by biochar mixing and covering, and the accumulative phosphorus adsorption capacity increased by 17.3% (mixing) and 11.7% (covering) compared with that of the control. The control effect of sedimentary phosphorus by biochar mixing was more obvious; the DIP in sedimentary water decreased by 14.7%, 18.9%, and 35.36% from 0-2 cm to 4-6 cm compared with the control. The DIP in sedimentary water decreased by 33.3%, -28.2%, and 12.9% when covered with biochar. Compared with the control, the proportion of NH4 Cl-P in the sediment of 0-2 cm and 2-4 cm increased by 15% and 15% (mixing) and 12% and 2% (covering), respectively, while BD-P in TP decreased by 7% and 9% (mixing) and 6% and 3% (covering), respectively, and the Al-P in TP decreased by 7% and 6% (mixing) and 7% and -1% (covering), respectively. The other forms of phosphorus did not change significantly. Biochar mixing and covering can both improve the microbial activity in surface sediment, and biochar mixing can improve the microbial activity in deeper sediments more significantly.

Baseline survey of sediment contamination with 210Polonium in three peri-urban creeks of Mombasa, Kenya

210Po in marine sediment from three peri-urban creeks in Mombasa was analyzed by alpha spectrometry in order to determine its baseline contamination levels. The mean 210Po activity in marine sediment from Mtwapa, Tudor and Makupa creeks were 18.67 ± 1.8, 21.17 ± 2.2 and 13.09 ± 1.6 Bq Kg−1 dw, for surface sediment and 37.56 ± 2.14, 28.64 ± 2.86 and 30.42 ± 2.1 Bq Kg−1 dw respectively, for sediment cores. Comparison of 210Po activity in surface sediment indicated that 210Po in the creeks could be originating from the same source. However, comparison of activities in the entire cores from the three creeks indicated that 210Po sources could have been different in the past. 210Po activities in this study were relatively elevated compared to marine environments under similar anthropogenic influence and there is need to understand its source and fate.

Aryl hydrocarbon receptor (AhR) inducers and estrogen receptor (ER) activities in surface sediments of Three Gorges Reservoir, China evaluated with in vitro cell bioassays

Two types of biological tests were employed for monitoring the toxicological profile of sediment cores in the Three Gorges Reservoir (TGR), China. In the present study, sediments collected in June 2010 from TGR were analyzed for estrogen receptor (ER)- and aryl hydrocarbon receptor (AhR)-mediated activities. The estrogenic activity was assessed using a rapid yeast estrogen bioassay, based on the expression of a green fluorescent reporter protein. Weak anti-estrogenic activity was detected in sediments from an area close to the dam of the reservoir, and weak estrogenic activities ranging from 0.3 to 1 ng 17β-estradiol (E2) equivalents (EQ) g(-1) dry weight sediment (dw) were detected in sediments from the Wanzhou to Guojiaba areas. In the upstream areas Wanzhou and Wushan, sediments demonstrated additive effects in co-administration of 1 nM E2 in the yeast test system, while sediments from the downstream Badong and Guojiaba areas showed estrogenic activities which seemed to be more than additive (synergistic activity). There was an increasing tendency in estrogenic activity from upstream of TGR to downstream, while this tendency terminated and converted into anti-estrogenic activity in the area close to the dam. The AhR activity was detected employing rat hepatoma cell line (H4IIE). EROD activities were found homogenously distributed in sediments in TGR ranging from 200 to 311 pg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) EQ g(-1) dw for total AhR agonists and from 45 to 76 pg TCDD EQ g(-1) dw for more persistent AhR agonists. The known AhR agonists polycyclic aromatic hydrocarbon, polychlorinated biphenyl, and PCDD/F only explained up to 8 % of the more persistent AhR agonist activity in the samples, which suggests that unidentified AhR-active compounds represented a great proportion of the TCDD EQ in sediments from TGR. These findings of estrogenic potential and dioxin-like activity in TGR sediments provide possible weight-of-evidence of potential ecotoxicological causes for the declines in fish populations which have been observed during the past decades in TGR.

Effect of Mechanical Aeration on Nitrogen and Microbial Activity in Sediment-Water Interface from Urban Lake

Aeration was one of the common techniques which were focused in environmental engineering research. In this study, experiments were carried out to investigate the effect of aeration on nitrogen (N) fractions and its transformation in the sediment-water interface from urban lake. In addition, we measured the dehydrogenase (DHA) and Chla content in surface sediment to detect the changes of the microbial activity. The aim is to explore the regulation of N migration and transformation in the water-sediment interface during the treatment work of lake pollution. By mechanical aeration, we control the DO content of the overlying water at the range of 1.2-1.5 kg/L, 2.5-4.5 mg/L and 6.1-6.2 mg/L. Results showed that DO in overlying water play a role to the variation of N in water-sediment interface. NH4+-N was the mainly N fraction released from sediment, the lower DO concentration (1.2-1.5 mg/L) in overlying water is favorable to the release of NH4+-N from sediment. Whether under aerobic or anaerobic condition, the releasing of NH4+-N from sediment were both higher than its consumption in the overlying water. The benthic microbial activity of the surface sediment (0-2 cm) was higher than that from bottom (6-8 cm). And the microbial activity in surface sediment was in optimum range when the overlying water DO content was about 2.5-4.5 mg/L.

Determination of 137Cs activities in surface sediments and derived sediment accumulation rates in Thessaloniki Gulf, Greece

Surficial and core samples collected from the eastern Thessaloniki Gulf, located in the NW Aegean Sea, were analyzed for their sedimentation rate and inventories of 137Cs and 210Pb. The study of the spatial radionuclide dispersion in the specific region is essential for the assessment of marine pollution levels. The sedimentation rates were calculated from the vertical distribution of 137Cs and excess 210Pb in the sediment cores. The spatial distribution of 137Cs was studied with respect to sediment characteristics such as the grain size of the sample and the organic carbon content. The activity concentrations were measured by means of gamma ray spectrometry using HPGe detector for 137Cs and gross alpha counting using alpha counter for 210Pb. The average sedimentation rate along the sediment cores is characterized by a consistent pattern and varies from 0.18 ± 0.02 to 0.22 ± 0.03 cm year−1. The 137Cs inventories varied from 205 ± 15 to 602 ± 39 Bq m−2, while the 210Pb inventories of the studied cores exhibited average value of 140 ± 9 Bq m−2. Elevated 137Cs activities were observed compared to certain Mediterranean marine areas; however, they were lower than the reported values in the Black Sea.

Effects of overlying water aeration on phosphorus fractions and alkaline phosphatase activity in surface sediment

Microbial activity may influence phosphorus (P) deposit and release at the water sediment interface. The properties of DO (dissolved oxygen), pH, P fractions (TP, Ca-P, Fe-P, OP, IP), and APA (alkaline phosphatase activity) at the water sediment interface were measured to investigate microbial activity variations in surface sediment under conditions of two-month intermittent aeration in overlying water. Results showed that DO and TP of overlying water increased rapidly in the first week and then decreased gradually after 15 day of intermittent aeration. Microorganism metabolism in surface sediment increased pH and decreased DO and TP in the overlying water. After two-month intermittent aeration, APA and OP from surface sediment (0-2 cm) were both significantly higher than those from bottom sediment (6-8 cm) ( p < 0.05), and surface sediment Fe-P was transferred to OP during the course of microorganism reproduction on the surface sediment. These results suggest that microbial activity and microorganism biomass from the surface sediment were higher than those from bottom sediment after two-month intermittent aeration in the overlying water.

Experimental investigation of sulphur isotope partitioning during outgassing of hydrogen sulphide from diluted aqueous solutions and seawater

The diffusion of hydrogen sulphide across the sediment–water interface and subsequent liberation to the atmosphere may occur in iron-deficient coastal marine environments with enhanced microbial activity in surface sediments and corresponding accumulation of dissolved H2S in near-surface pore waters. The involvement of analogue processes in periods of global mass extinctions during Earth's history (e.g. at the Permian–Triassic boundary) is currently in discussion [L.R. Kump, A. Pavlov, and M. Arthur, Massive Release of Hydrogen Sulfide to the Surface Ocean and Atmosphere During Intervals of Oceanic Anoxia, Geology 33, 397 (2005)]. The outgassing of H2S is associated with a fractionation of the stable sulphur isotopes, which has so far only been investigated experimentally at selected acidic and neutral pH values, and no experiments with seawater had been carried out. In this communication, we report on sulphur isotope fractionation that takes place during the experimental degassing of H2S from aqueous solution by an inert gas (N2) at 21°C. Experiments were conducted in the pH range between 2.6 and 10.8, corresponding to the dominance fields of dissolved hydrogen sulphide (H2S(aq)), bisulphide (HS), and mixtures of both sulphide species. Overall isotope enrichment factors between−1.6 and +3.0‰ were observed, with the residual dissolved sulphide being enriched or depleted in 34S compared to the liberated H2S at low and high pH values, respectively. The difference in the low and high pH isotope fractionation effects can be explained by isotope exchange between H2S(aq) and HS [B. Fry, H. Gest, and J.M. Hayes, Sulfur Isotope Eeffects Associated with Protonation of HS− and Volatilization of H2S, Chem. Geol. (Isot. Geosci. Sec.) 58, 253 (1986); R. Geßler and K. von Gehlen, Investigation of Sulfur Isotope Fractionation Between H2S Gas and Aqueous Solutions, Fresenius J. Anal. Chem. 324, 130 (1986)] followed by the subsequent transfer of H2S(aq) to the gaseous phase. The assumption of pure physical outgassing of H2S(aq) at low pH values leads to an isotope enrichment factor of−0.9±0.4‰ (n=14) which is caused by the combined differences in dehydration and diffusion coefficients of HS(aq) and S(aq). In the pH range of natural surface and shallow pore waters, 34S will be equal to or enriched in the gaseous phase compared to the aqueous solution, therefore creating no or a slight enrichment of 32S in the aqueous solution. Experiments in seawater solution showed no significant influence of increased ionic strength and changed corresponding aqueous speciation on sulphur isotope effects.

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A large number of sediment cores collected during 2005–2010 from the Taiwan Strait were analyzed for radionuclides ( 210Pb, 137Cs and 7Be) to elucidate sedimentation dynamics in this all-important gateway linking two largest marginal seas in the western Pacific (namely, the South China Sea and the East China Sea). Apparent sediment accumulation rates derived from 210Pb and 137Cs profiles vary from <0.1 to >2 cm/yr, averaging ∼0.4 cm/yr and showing a spatial pattern closely related to hydrodynamics and sediment source-to-sink pathways. Spatial-temporal variation of 7Be activity in surface sediments off Taiwan’s west coast indicates episodic deposition of flood layers and their mobility from river estuaries toward the north. In conjunction with particle size distribution in surface sediments and the structure of sediment strata revealed by sub-bottom echo images; the radionuclide data can be used to outline three different sediment source-to-sink dispersal systems. Based on sediment loads of surrounding rivers and the distribution of sediment accumulation rates, lateral transport is required to account for the budget and size distribution of sediments in the strait.

SEDIMENTATION RATE OF MUD AREA IN THE EAST CHINA SEA INNER CONTINENTAL SHELF

Seven gravity sediment cores were collected from muddy area of inner continental shelf of the East China Sea. Measurement of 210Pb activity was performed on all cores. The results demonstrate that the sedimentation rates were between 0.79 and 3.66 cm/a,with 1.97 cm/a on average. The highest sedimentation rate was recognized in core CJ12-11,located in the high sedimentation rate area of East China Sea. Six other cores with relative low sedimentation rates were located in the sub-higher sedimentation rate area. Distribution of 210Pb activity in surface sediments is correlated closely with the types of sediments,and probably controlled by the clay mineral,but shows a positive correlation with suspended matter concentration. Furthermore,these results reveal relative stable sedimentation processes in the muddy area during the past 100 years and the northward expansion of this muddy area.

Iodine-131: A Potential Short-Lived, Wastewater-Specific Particle Tracer in an Urbanized Estuarine System

The short-lived, fission-produced radioisotope, 131I (t1/2 = 8.04 days), was detected in wastewater, surficial sediment, and suspended particulate matter (SPM) samples collected from New York Harbor (NYH) between 2001 and 2002. lodine-131 is used as a radiopharmaceutical for medical imaging, diagnostics, and treatments for conditions of the thyroid. It is introduced into the municipal waste stream by medical facilities and patients and is subsequently released into the estuary via wastewater effluent. Measured 131I activities in surface sediments were correlated with those of 7Be (t1/2 = 53.2 days), a naturally occurring radioisotope that is widely used to quantify particle dynamics, sediment focusing, and short-term sediment deposition and accumulation in aquatic systems. Surficial sediment 131I activities were also compared with measured trace metal (Cu, Pb) and organic carbon (OC(sed)) concentrations which can be linked to wastewater inputs. These preliminary results from NYH introduce 131I as a potentially valuable source-specific, shortlived biogeochemical tracer (timescales < 1 month) for particles, sediments, and wastewater-sourced contaminants in urbanized aquatic systems.

In situ fluxes and zonation of microbial activity in surface sediments of the Håkon Mosby Mud Volcano

From the Hakon Mosby Mud Volcano (HMMV) on the southwest Barents Sea shelf, gas and fluids are expelled by active mud volcanism. We studied the mass transfer phenomena and microbial conversions in the surface layers using in situ microsensor measurements and on retrieved cores. The HMMV consists of three concentric habitats: a central area with gray mud, a surrounding area covered by white mats of big sulfide oxidizing filamentous bacteria (Beggiatoa), and a peripheral area colonized by symbiontic tube worms (Pogonophora). A fourth habitat comprised gray microbial mats near gas seeps. The differences between these four methane-fueled habitats are best explained by different transport rates of sulfate into the sediments and pore- water upflow rates. The upflow velocities were estimated by two independent methods at 3-6 m yr21 in the central area and 0.3-1 m yr21 in Beggiatoa mats. In the central area no sulfide was found, indicating that the rapidly rising sulfate-free fluids caused sulfate limitation that inhibited anaerobic oxidation of methane (AOM). Under Beggiatoa mats a steep sulfide peak was found at 2 to 3 cm below the seafloor (bsf), most likely due to AOM. All sulfide was oxidized anaerobically, possibly through nitrate reduction by Beggiatoa. The Beggiatoa mats were dominated by a single filamentous morphotype with a diameter of 10 mm and abundant sulfur inclusions. A high diversity of sulfide oxidizer morphotypes was observed in a grayish microbial mat near gas vents, where aerobic sulfide oxidation was important. The sediments colonized by Pogonophora were influenced by bioventilation, allowing sulfate penetration and AOM to 70 cm bsf. The HMMV is a unique and diverse ecosystem, the structure and functioning of which is mainly controlled by pore-water flow.

Distribution of Technetium-99 in sub-tidal sediments of the Irish Sea

To date, relatively little attention has been given to the accumulation of 99Tc discharged from Sellafield in the subtidal sediments of the Irish Sea. The potential implications for secondary seafood contamination from contaminated sediment has driven the UK Food Standards Agency to commission further research into this pathway. The work reported here reviews existing data and provides new measurements of 99Tc specific activity in surface and sub-surface sediments of the Irish Sea, together with environmental K d values. The results are used to assess the spatial and temporal evolution of 99Tc in the seabed after 8 years of enhanced Sellafield discharges (between 1994 and 2002), of the aforementioned radionuclide. The information is discussed with reference to other studies, in an attempt to infer the processes controlling 99Tc uptake and release from seabed sediments. The average environmental K d value for 99Tc in the Irish Sea (1.9×10 3) was more than an order of magnitude greater than the presently recommended value of 10 2 [IAEA, 2004. Sediment distribution coefficients and concentration factors for biota in the marine environment. Technical Report Series No. 422, IAEA, Vienna]. Comparison with results from laboratory studies indicates that the observed distribution may represent metastable binding rather than thermodynamic equilibrium. Activities in surface sediments decreased with increasing distance from Sellafield but were also dependent upon the nature of the underlying substrate, being greater on muddy material. Preliminary measurements of grain-size distribution indicated that the observed variation in activities was probably not due to surface area effects. There is an emerging body of evidence from other studies that indicate the differences were most likely due to variations in redox regimes between the different substrates. Vertical profiles were significantly irregular, probably due to the effects of variable sediment mixing processes. Comparison of profiles, close to the Sellafield pipeline, with a core taken over 20 years earlier (pre-EARP) indicated that the increase in the cumulative Sellafield discharge and redissolution from surficial sediment were required to explain the temporal variation. Since the surveys reported here were completed, substantial progress has been made in reducing 99Tc discharges from Sellafield. Assuming that the rate and extent of 99Tc remobilisation follows a similar pattern to that previously observed for caesium ( 137Cs), then the half-time for redissolution of 99Tc bound to sedimentary material in the Irish Sea is likely to be of the order of several tens of years. It is probable that small but nevertheless measurable 99Tc contamination of local seafood will persist for several decades, due to this secondary source.

Extracellular Proteolytic Activity in the Surface Sediment of a Eutrophic Inlet.

The potential aminopeptidase activity (APA) in surface sediment (0-1 cm), and important ecological factors of a eutrophicated inlet were monitored for one and a half years on a weekly to biweekly basis to elucidate the fluctuations in extracellular hydrolysis of protein during an annual cycle, and main regulating factors on an annual and/or a seasonal basis. APA was estimated in sediment slurry prepared with unamended bottom water, as the Vmax of the rate of the enzymatic hydrolysis of a coumarine derivative fluorophore at in situ temperatures. APA varied 5-6 fold during the study period with highest values (225-275 nmole AMC h-1 g-1 dry weight) during September-October or early June when the temperature was 22-28°C and dissolved oxygen concentrations (DO) (2-4 mg L-1) in overlying bottom water were high, and much lower values (ca. 50 nmoles AMC h-1 g -1 dry weight) in July-August or December-February. On an annual basis, APA correlated only marginally (p > 0.05; r = 0.25) with the protein content of the surface sediment and not at all with any other variables. However, it showed highly significant correlations (p > 0.001) with temperature (r = 0.81), and protein content (0.65) in mid December-early June, when temperatures were relatively low (ca. 12-22°C) and DO was abundant (ca. 2-6 mg L-1) in overlying bottom water.

Bacterial activity in sediments of the deep Arabian Sea in relation to vertical flux

In the Arabian Sea, productivity in the surface waters and particle flux to the deep sea are controlled by monsoonal winds. The flux maxima during the South-West (June–September) and the North-East Monsoon (December–March) are some of the highest particle fluxes recorded with deep-sea sediment traps in the open ocean. Benthic microbial biomass and activities in surface sediments were measured for the first time in March 1995 subsequent to the NE-monsoon and in October 1995 subsequent to the SW-monsoon. These measurements were repeated in April/May 1997 and February/March 1998, at a total of six stations from 1920 to 4420 m water depth. This paper presents a summary on the regional and temporal variability of microbial biomass, production, enzyme activity, degradation of 14 C -labeled Synechococcus material as well as sulfate reduction in the northern, western, eastern, central and southern Arabian deep sea. We found a substantial regional variation in microbial biomass and activity, with highest values in the western Arabian Sea (station WAST), decreasing approximately threefold to the south (station SAST). Benthic microbial biomass and activity during the NE-monsoon was as high or higher than subsequent to the SW-monsoon, indicating a very rapid turnover of POC in the surface sediments. This variation in the biomass and activity of the microbial assemblages in the Arabian deep sea can largely be explained by the regional and temporal variation in POC flux. Compared to other abyssal regions, the substantially higher benthic microbial biomasses and activities in the Arabian Sea reflect the extremely high productivity of this tropical basin.

Sedimentation and mixing rates of radionuclides in Barents Sea sediments off Novaya Zemlya

Radionuclide measurements have been conducted on sediment cores collected in 1992 in the south-eastern region of the Barents Sea, known as the Pechora Sea. Cesium-137 and 239,24OPu activities in surface sediments are generally less than 30 Bq/kg, with the highest levels being measured in sediments off the southwestern coastline of the island of Novaya Zemlya. High correlations between both 137Cs and 239,24OPu and the concentration of fine (< 63 μm) particles in surface sediments indicate that much of the variance in radionuclide concentrations throughout the Pechora Sea can be explained by particle size fractionation. However, elevated activities of 137Cs (138 Bq/kg), 60Co (92 Bq/kg), 241Am (433 Bq/kg), and especially 239,24OPu (8.47 × 103 Bq/kg) were measured in one surface sediment sample from the fjord of Chernaya Bay on the southern coast of Novaya Zemlya. The source of radioactive contamination is two underwater nuclear tests conducted in Chernaya Bay in 1955 and 1957. The 238Pu 239,240Vu activity ratio of 0.0245 in Chernaya Bay is equivalent to values measured in global fallout. The 240Pu 239Pu atom ratio (0.0304), measured by mass spectrometry, is much lower than values (0.18) typical of global fallout, but is consistent with ratios measured for fallout from the early (1951–1955) series of weapons tests at the Nevada Test Site. The timing of the Chernaya Bay source term, estimated from the 241Am 241Pu ratio, is consistent with the timing of the 1955 and 1957 underwater nuclear tests. Relatively low initial yields of 241Pu ( 241Pu 239Pu atom ratio = 0.00 123) in these tests have resulted in relatively low 241Am 239,240Pu activity ratios (0.05) in recent sediments in Chernaya Bay. Radionuclide tracer profiles in cores from the Pechora Sea can be simulated using a two-layer biodiffusion model with rapid, near-homogeneous mixing in the surface mixed layer and reduced mixing in the deep layer. Lead-210 profiles are consistent with a wide range of sedimentation and mixing rates in the deep sediment layer. However, the 137Cs and 239,240Pu results further constrain the model parameters and indicate that the downward transport of radionuclides in the sediments is governed primarily by sediment mixing, with sediment burial playing a secondary role.

Nitrogenase activity in marine sediments from a temperate saltmarsh lagoon: modulation by complex polysaccharides, ammonium and oxygen

Nitrogenase activity (acetylene reduction) was examined in marine sediments associated with seagrass beds and thalassinidean prawn burrows, in a temperate saltmarsh lagoon. Nitrogenase activity in unamended sediments from different microhabitats ranged from undetectable values to ≈0.75 nmol C 2H 4 (g dry sediment) −1·hour −1. The bulk of organic material available to heterotrophic bacteria in saltmarsh lagoons is probably derived from primary production. Amendment of sediments with structural plant polysaccharides showed that xylan and alginate stimulated nitrogenase activity on average by 5 to 18-fold relative to unamended sediments, whereas cellulose and carrageenan were less effective. Amendment of sediments with storage plant polysaccharides produced the greatest stimulation of nitrogenase activity. Addition of laminarin and glycogen (amylopectin) significantly ( p <0.05) stimulated nitrogenase activity by 19 to 92-fold. In contrast to these polysaccharides of plant origin, chitin (a polymer of prawn exoskeletons) did not significantly enhance nitrogenase activity in these sediments. Aerobic conditions stimulated nitrogenase activity by 2 to 20-fold in surface sediments, but not in sub-surface sediments, indicating that aerobic or microaerophilic respiration was a significant source of energy for nitrogenase activity in surface sediments. Oxygen-stimulation of nitrogenase activity was less marked in aerobic sediments around the rim of prawn burrow openings where subsurface sediments had been displaced to the surface by bioturbation. Ammonium did not appear to play a major role in the regulation of nitrogenase activity in these sediments as in situ concentrations were ≈20-fold lower than the concentration (50 μM) which resulted in a slight (0–15%) reduction in nitrogenase activity.

Interpretation of210Pb profiles and verification of the CRS dating model in PIRLA project lake sediment cores

This paper reports results and analysis of210Pb-activity measurements in 51 lake-sediment cores from 32 lakes in the four PIRLA (Paleoecological Investigations of Recent Lake Acidification) project regions (Adirondack Mountains [New York], Northern New England, Northern Florida, and the Northern Great Lakes States). General application of the Constant Rate of Supply (Constant Flux) model for210Pb dating is valid for lakes in the PIRLA study, although application of the model is equivocal in a few lakes.210Pb inventories and profiles are replicable among closely spaced cores within a lake. Specific210Pb activity in surface sediments is negatively correlated with bulk sediment accumulation rate in seepage lakes, but not in drainage lakes. Drainage lakes with lower pH have lower unsupported210Pb inventories in sediments, but the relationship does not occur in seepage lakes.210Pb profiles in only seven of the cores, all from either the Adirondacks or the northern Great Lakes states, exhibit exponential decay curves. Deviations from an exponential profile include a flattening of the profile in the top few cm or excursions of one or a few measurements away from an exponential curve.210Pb dates typically agree with other chronostratigraphic markers, most of which are subject to greater uncertainty. Several hypotheses, including sediment mixing, hydrologic regime, sediment focusing, and acidification, are proposed to explain variation of210Pb distribution among lakes and regions. Hydrologic factors exert control on unsupported210Pb inventories in PIRLA lakes, and there is a strong focusing effect in drainage lakes but a weak focusing effect in seepage lakes.

Seasonal and Spatial Variations in Mercury Methylation and Demethylation in an Oligotrophic Lake

Microbial mercury methylation and methylmercury decomposition were examined in Lake Clara, an oligotrophic northern Wisconsin seepage lake, using radioisotopic tracers. Methylation activity was near background in the water column, was greatest in the profundal surficial sediments, and decreased with depth in sediment cores. Active demethylation occurred in the water column but was variable. Demethylation was greatest in the surficial sediments and decreased slightly with sediment depth. The methylation/demethylation ratio (M/D) was >1 in the water column, exhibited a sharp peak in surface sediments, and decreased in deeper sediments. Methylation and demethylation activity varied in surface sediments collected along a lake transect. The M/D ratio in surface sediments ranged from 1.4 to 5.8. Methylation in attached microbial communities was near background, while demethylation was high. The M/D ratios in the attached communities were all <0.20. Methylation activity in surface sediments incubated at in situ temperature increased from spring to late summer and decreased in the fall. Demethylation increased from early to midsummer and then declined. The M/D ratio in surface sediments increased from mid- to late summer, and decreased in the fall. These results indicate that the greatest potential for methylation in Lake Clara occurs in the surficial sediments and that methylation in surficial sediments is greatest from mid-July through September. In addition, the net rate of methylmercury production may be significantly affected by demethylation.