Black Sea Sediments Research Articles

Ancient DNA derived from alkenone‐biosynthesizing haptophytes and other algae in Holocene sediments from the Black Sea

Holocene sea surface temperatures (SST) of the Black Sea have been reconstructed using sedimentary C37 unsaturated alkenones assumed to be derived from the coccolithophorid haptophyte Emiliania huxleyi, whose fossil coccoliths are an important constituent of the unit I sediments. However, alkenones can also be biosynthesized by haptophyte species that do not produce microscopic recognizable coccoliths. A species‐specific identification of haptophytes is important in such U37K′‐based past SST reconstructions since different species have different alkenone‐SST calibrations. We showed that 18S rDNA of E. huxleyi made up only a very small percentage (less than 0.8%) of the total eukaryotic 18S rDNA within the up to 3600‐year‐old fossil record obtained from the depocenter (>2000 m) of the Black Sea. The predominant fossil 18S rDNA was derived from dinoflagellates (Gymnodinium spp.), which are predominant members of the summer phytoplankton bloom in the modern Black Sea. Using a polymerase chain reaction/denaturing gradient gel electrophoresis method selective for haptophytes, we recovered substantial numbers of a preserved 458‐base‐pair (bp)‐long 18S rDNA fragment of E. huxleyi from the Holocene Black Sea sediments. Additional fossil haptophyte sequences were not detected, indicating that the E. huxleyi alkenone‐SST calibration can be applied for at least the last ∼3600 years. The ancient E. huxleyi DNA was well protected against degradation since the DNA/alkenone ratio did not significantly decrease throughout the whole sediment core and 20% of ∼2700‐year‐old fossil E. huxleyi DNA was still up to 23,000 base pairs long. We showed that fossil DNA offers great potential to study the Holocene paleoecology and paleoenvironment of anoxic deep‐sea settings in unprecedented detail.

Characterization of unusual alkenones and alkyl alkenoates by electron ionization gas chromatography/mass spectrometry

Unusual long-chain, diunsaturated alkenones and alkyl alkenoates exhibiting double bonds separated by three methylene units instead of the more usual five were characterized by electron ionization (EI) gas chromatography/mass spectrometry. In a first step, the positions of the double bonds of these compounds (isolated from Holocene Black Sea sediments) were confirmed after OsO4 treatment and silylation. Mass spectra of the resulting tetratrimethylsilyloxy derivatives allowed unambiguous determination of the positions of unsaturations. The EI mass spectra of the non-derivatized compounds were then compared with those of the alkenones and alkyl alkenoates having double bonds separated by five methylene units. Specific fragment ions resulting from gamma-H rearrangements were found to be prominent in EI mass spectra of these unusual 'Black Sea' diunsaturated alkenones and alkyl alkenoates. These fragment ions can be used to characterize these compounds in natural samples without the need for laborious derivatization treatments.

Geochemistry and sedimentology of shelf and upper slope sediments of the south-central Black Sea

Eighty-five surface samples from oxic, suboxic and anoxic sites of the south-central Black Sea shelf consist predominantly of clayey silts and silty clays, reflecting the moderate-to-high energy, wave-dominated hydrological regime. Grain size data and net sediment transport patterns suggest that the western part of this shelf is an area of east-directed active sediment transport. With the, exception of localized nearshore zones, the eastern shelf is by contrast an area of major sediment deposition. The sediment transport vectors are correlated with patterns of surface water circulation and storm-generated longshore currents. The sources of the shelf sediments are indicated by the total concentrations of 30 major, minor and trace elements in 59 surface samples. Factor analysis shows that 99.1% of the total variance can be explained by five factors. Factor 1 (As, Cu, Cr, Fe, Pb and Zn) reflects anthropogenic pollution by the Etibank copper mine. Factor 2 (mainly total organic carbon and S) is linked to anoxia/redox as saline conditions. Factor 3 (mainly Al, Si) represents aluminosilicates whereas Factor 4 (sand and silt abundance, total carbonate, Ca and Sr) represents the carbonate component. Factor 5 (Fe, Mn, P, Cr, Ni and V) tracks primary detrital delivery with strong overprints by redox-dependent, biogeochemical reactions within the anoxic sediments and the oxic/suboxic bottom waters. The combined data suggest that the sediments on the south-central Black Sea shelf were largely derived from the North Anatolian Mountains and supplied to the shoreline by a network of small rivers.

Concretionary methane-seep carbonates and associated microbial communities in Black Sea sediments

Gas seeps in the euxinic northwestern Black Sea provide an excellent opportunity to study anaerobic, methane-based ecosystems with minimum interference from oxygen-dependent processes. An integrated approach using fluorescence- and electron microscopy, fluorescence in situ hybridization, lipid biomarkers, stable isotopes (δ13C), and petrography revealed insight into the anatomy of concretionary methane-derived carbonates currently forming within the sediment around seeps. Some of the carbonate concretions have been found to be surrounded by microbial mats. The mats harbour colonies of sulphate-reducing bacteria (DSS-group), and archaea (ANME-1), putative players in the anaerobic oxidation of methane. Isotopically-depleted lipid biomarkers indicate an uptake of methane carbon into the biomass of the mat biota. Microbial metabolism sustains the precipitation of concretionary carbonates, significantly depleted in 13C. The concretions consist of rectangularly orientated, rod- to dumbbell-shaped crystal aggregates made of fibrous high Mg-calcite. The sulphate-reducing bacteria exhibit intracellular storage inclusions, and magnetosomes with greigite (Fe3S4), indicating that iron cycling is involved in the metabolism of the microbial population. Transfer of Fe3+ into the cells is apparently mediated by abundant extracellular vesicles resembling known bacterial siderophore vesicles (marinobactine) in size (20 to 100 nm) and structure.

Sedimentary Mo isotope record across the Holocene fresh–brackish water transition of the Black Sea

Mo isotope data on Black Sea sediments spanning the transition from Pleistocene oxic–limnic conditions to the prevailing anoxic marine conditions are presented. Samples were taken from a gravity core collected at a water depth of 396 m. Samples deposited under oxic bottom water conditions range from δ 98/95Mo MOMO − 2.2‰ to − 1.95‰ (MOMO = Mean Ocean Molybdenum) while samples deposited under anoxic bottom water conditions range from δ 98/95Mo MOMO − 1‰ to − 0.54‰. The change of sedimentary environment is also recorded in the Mo contents increasing from oxic to anoxic sediments. The Mo isotopic composition and invariably low Mo content of the oxic sediments deposited under oxic bottom water conditions are compatible with a pure detrital origin of the Mo, irrespective of whether the deposits are of limnic or brackish origin. Mo content and isotopic compositions are identical above and below a sulfidisation front, which originates from the diffusion of sulfur species and in-situ microbial activity after establishment of brackish bottom water conditions. Further, no signal of the overlaying sapropels is seen in the underlying sediments. Thus, transport of sulfur species has not mobilised Mo during diagenesis. The δ 98/95Mo MOMO values of anoxic samples indicate seawater as the dominant source of Mo. However, even the heaviest Mo value of the anoxic period recorded in this core is δ 98/95Mo MOMO = − 0.5‰, with an average of − 0.7‰ for all anoxic sediments, i.e. 0.7‰ lighter than seawater. All samples can be explained qualitatively as three component mixtures of detrital, dissolved riverine and marine Mo. For the lower units a mass balance model can be successfully applied. For the youngest unit mixing models do not yield satisfactory results given present day water fluxes. It is therefore likely that additional Mo isotope fractionation effects are involved. First order modelling suggests that the lighter δ 98/95Mo MOMO values of the most recent samples reflect the presence of some Mo remaining dissolved as MoO 4 2− in a larger part of water column above the core depth, thus allowing for a preservation of a net fractionation between MoO 4 2− and MoS 4 2−. This hypothesis is supported by the fact that the H 2S concentration critical for the MoO 4 2− ↔ MoS 4 2− chemical switch is found at about 400 m water depth in the present Black Sea, close to the depth at the sampling site. At greater depth, increased H 2S concentrations lead to almost complete Mo removal, erasing the fractionation signal. Difference between Unit I samples from this study, and those from earlier publications (with samples taken at greater depth) may thus merely reflect the fraction of Mo scavenged at different depths. The degree of Mo scavenging in fossil black shales and the continental Mo contribution are both difficult to constrain. Therefore, black shales from restricted semi-enclosed basins may not document the Mo isotopic composition of coeval ocean waters. However, oceanic Mo is a dominant Mo source in these basins, and anoxic sediments give reliable minimum values for coeval ocean water Mo isotopic compositions.

The use of steroid markers to assess sewage contamination of the Black Sea

Analyses of faecal steroids in coastal sediments taken from throughout the Black Sea indicate chronic sewage contamination at some locations. These include Sochi, where concentrations of coprostanol up to 5400 ng g −1 (dry wt) were recorded, and in the coastal areas adjacent to the Danube delta (2600 ng g −1 dry wt). Comparatively high values of 5β/(5β + 5α) isomeric ratios of cholestan-3-ol and cholestan-3-one are reported at these locations and are characteristic of sediments contaminated with sewage. Lower concentrations of coprostanol are reported for Bosphorus sediments (12–440 ng g −1 dry wt) and in the region of Odessa (130–290 ng g −1 dry wt). Isomeric ratios at these locations also confirm sewage as a significant contributor to steroids in the Bosphorus, but lower values of the ratios indicate only minor sewage inputs in Odessa. In contrast, steroid concentrations and compositions from most Ukrainian sites are characteristic of uncontaminated environments. To place these results into perspective, the range in levels of coprostanol in the Black Sea sediments compares to the lower to mid-range of concentrations reported for coastal sediments on a world-wide basis, with Black Sea sedimentary levels substantially below those previously reported for heavily impacted sites.

Anaerobic methane oxidation and a deep H 2S sink generate isotopically heavy sulfides in Black Sea sediments

The main terminal processes of organic matter mineralization in anoxic Black Sea sediments underlying the sulfidic water column are sulfate reduction in the upper 2–4 m and methanogenesis below the sulfate zone. The modern marine deposits comprise a ca. 1-m-deep layer of coccolith ooze and underlying sapropel, below which sea water ions penetrate deep down into the limnic Pleistocene deposits from >9000 years BP. Sulfate reduction rates have a subsurface maximum at the SO 4 2−-CH 4 transition where H 2S reaches maximum concentration. Because of an excess of reactive iron in the deep limnic deposits, most of the methane-derived H 2S is drawn downward to a sulfidization front where it reacts with Fe(III) and with Fe 2+ diffusing up from below. The H 2S-Fe 2+ transition is marked by a black band of amorphous iron sulfide above which distinct horizons of greigite and pyrite formation occur. The pore water gradients respond dynamically to environmental changes in the Black Sea with relatively short time constants of ca. 500 yr for SO 4 2− and 10 yr for H 2S, whereas the FeS in the black band has taken ca. 3000 yr to accumulate. The dual diffusion interfaces of SO 4 2−-CH 4 and H 2S-Fe 2+ cause the trapping of isotopically heavy iron sulfide with δ 34S = +15 to +33‰ at the sulfidization front. A diffusion model for sulfur isotopes shows that the SO 4 2− diffusing downward into the SO 4 2−-CH 4 transition has an isotopic composition of +19‰, close to the +23‰ of H 2S diffusing upward. These isotopic compositions are, however, very different from the porewater SO 4 2− (+43‰) and H 2S (−15‰) at the same depth. The model explains how methane-driven sulfate reduction combined with a deep H 2S sink leads to isotopically heavy pyrite in a sediment open to diffusion. These results have general implications for the marine sulfur cycle and for the interpretation of sulfur isotopic data in modern sediments and in sedimentary rocks throughout earth’s history.

Evidence for the submarine weathering of silicate minerals in Black Sea sediments: Possible implications for the marine Li and B cycles

The role of sediment diagenesis in the marine cycles of Li and B is poorly understood. Because Li and B are easily mobilized during burial and are consumed in authigenic clay mineral formation, their abundance in marine pore waters varies considerably. Exchange with the overlying ocean through diffusive fluxes should thus be common. Nevertheless, only a minor Li sink associated with the low‐temperature alteration of volcanic ash has been observed. We describe a low‐temperature diagenetic environment in the Black Sea dominated by the alteration of detrital plagioclase feldspars. Fluids expelled from the Odessa mud volcano in the Sorokin Trough originate from shallow (≈100–400 m deep) sediments which are poor in volcanic materials but rich in anorthite. These fluids are depleted in Na+, K+, Li+, B, and 18O and enriched in Ca2+ and Sr2+, indicating that anorthite is dissolving and authigenic clays are forming. Using a simple chemical model, we calculate the pH and the partial pressure of CO2 (PCO2) in fluids associated with this alteration process. Our results show that the pH of these fluids is up to 1.5 pH units lower than in most deep marine sediments and that PCO2 levels are up to several hundred times higher than in the atmosphere. These conditions are similar to those which favor the weathering of silicate minerals in subaerial soil environments. We propose that in Black Sea sediments enhanced organic matter preservation favors CO2 production through methanogenesis and results in a low pore water pH, compared to most deep sea sediments. As a result, silicate mineral weathering, which is a sluggish process in most marine diagenetic environments, proceeds rapidly in Black Sea sediments. There is a potential for organic matter‐rich continental shelf environments to host this type of diagenesis. Should such environments be widespread, this new Li and B sink could help balance the marine Li and Li isotope budgets but would imply an apparent imbalance in the B cycle.

Sources and mechanisms for the enrichment of highly reactive iron in euxinic Black Sea sediments

The deep basinal euxinic sediments of the Black Sea are enriched in iron that is highly susceptible to sulfidization compared to oxic/suboxic continental margin sediments in the Black Sea and oxic/dysoxic continental margin and deep-sea sediments worldwide. A mass balance treatment of iron speciation data from three deep basin sediment cores shows that this enrichment is due to a combination of (1) highly reactive iron-bearing phases (sulfides and oxides) whose ultimate source was by diagenetic mobilization from the shelf (Wijsman and others, 2001) and (2) enhanced iron reactivity in the lithogenous component of deep basinal sediments. The cause of the enhanced reactivity of lithogenous iron is problematical. Possible mechanisms include microbial oxidation of non-reactive iron silicates and the preferential deposition of a fine-grained, reactive iron-enriched lithogenous component in the deep basin by the fractionation of the lithogenous flux during transport across the shelf. The application of paleoredox indicators based on iron reactivity (Degree of Pyritization, Indicator of Anoxicity) should take into account that the availability of highly reactive iron, and hence the concentration of reactive iron phases in the sediment, is controlled by a variety of chemical, physical and biological factors, some of which are not related directly to redox conditions in the water column.

Nitrogen budget of the northwestern Black Sea shelf inferred from modeling studies and in situ benthic measurements

A 3D eddy-resolving coupled biogeochemical-hydrodynamical model and in situ obser- vations are used to investigate benthic processes on the Black Sea's NW shelf. Measurements of ben- thic fluxes (oxygen, nutrients, redox compounds) with in situ flux chambers are analyzed in regard to sediment dynamics on the shelf. The seasonal/interannual and spatial variability of benthic processes is explained in terms of 3D ecohydrodynamics. The space/time distribution of benthic fluxes depended on the position of the river plume and the associated primary production, intensity of ver- tical mixing and water depth. Model results and in situ observations reveal the presence of a region of intense benthic recycling and high benthic nutrient fluxes nearshore and in the northern part of the shelf. The model estimates that this region covers about 15% of the shelf area and is connected to the high productivity and high sedimentation caused by river input of organic matter. On the off- shore shelf, covering about 85% of the shelf area, benthic nutrient regeneration is low due to low pro- ductivity. Benthic mineralization pathways (aerobic respiration, denitrification, sulfate reduction, methanogenesis) are quantified. In the high flux region, more than 55% and in the offshore low flux region more than 80% of the organic matter is decomposed by aerobic respiration. In the high flux region, sulfate reduction is the main anaerobic pathway, whereas denitrification is more important on the low flux offshore shelf. At the shelf edge, under the influence of anoxic waters, more than 60% of organic matter is remineralized by sulfate reduction. Little organic matter is decomposed by methanogenesis. A mass balance of particulate organic carbon (POC) and particulate organic nitro- gen (PON), integrated over the whole shelf and year, shows that 3.7 to 4.2 × 10 6 t of POC reach the sediments, while model results give a value of 1.4 × 10 6 t C. The annual ammonium benthic outflux is estimated at 85 × 10 3 and 174 × 10 3 t N by in situ data and the model, respectively. The amount of nitrogen lost by burial and denitrification estimated from in situ observations is 57 × 10 3 and 324 × 10 3 t N, respectively. Therefore, NW shelf Black Sea sediments are an efficient sink for riverine nitrogen, trapping about 50% of the annual river discharge in total inorganic N.

Geochronological reconstruction of 137Cs transport from the Coruh river to the SE Black Sea: comparative assessment of radionuclide retention in the mountainous catchment area

The deposition record of 137Cs was traced in the SE Black Sea sediments adjacent to the Coruh river mouth in comparison with the earlier studied chronology of 137Cs deposition in front of the Danube delta (NW Black Sea). In both cases, the 137Cs profiles showed two subsurface peaks attributable to maximum fallout of ‘bomb’ and Chernobyl radionuclides. The Coruh profile revealed a larger contribution of ‘bomb’ 137Cs in comparison with the Chernobyl input, suggesting different coverage of NW and SE Black Sea regions with the Chernobyl fallout. The 137Cs-derived dating showed that maximum deposition of particulate bound 137Cs in sediments adjacent to the Coruh river mouth was delayed for ∼14 yr relative to date of Chernobyl accident, reflecting a buffer effect of the watershed soils. This transit time is 3 times longer than in the Danube catchment area, indicating a difference in retention processes in these mountainous (Coruh) and lowland (Danube) river basins. The 137Cs profile in Coruh sediments showed penetration of 137Cs to much greater depth than would be expected from 137Cs fallout chronology, suggesting the sediment mixing rate of 1.3 cm 2 yr −1. This value was used to evaluate deposition chronology of 137Cs, applying the model developed for pulse fallout case. Comparing the measured and modelled data has allowed differentiation of the flood-induced discharge of the 137Cs-containing suspended matter and the slower transit of eroded soil particles from the contaminated catchment areas. The obtained results may be used for the prediction of period when the pollutants, deposited over the river basins, can reach the Black Sea.

The age of microbial carbonate structures grown at methane seeps in the Black Sea with an implication of dating of the seeping methane

The 14C-dating of microbial carbonate structures growing at methane seeps in anoxic waters of the Black Sea has shown a gradual increase with depth of the age of carbonates of these buildups. Comparing the radiocarbon age of the base and middle parts of the microbial structures gives an approximate time of origin of the deepest and shallowest microbial buildups as about 5300 and 2900 years before present, respectively. These dates correspond to the first appearance of hydrogen sulfide in the deepest Black Sea waters and to the stabilization of the upper boundary of the anoxic zone around the present-day level. Stable carbon isotope analyses were used to evaluate contribution of carbon from methane and from the seawater bicarbonates to carbonate material of the microbial structures. The results have shown that 48–60% of carbon in the carbonates is derived from the seeping methane. The given percentage allows calculation of the age of methane, which is assessed to be in the range of about 8500–10,600 years before present for methane seeping from the shallowest microbial structures. These time limits correspond to the last glacial period, when the Black Sea was a freshwater lake. Accordingly, the age of methane emanating from the deepest microbial structures may be assessed as 29,200–36,500 years, suggesting an older sedimentary source of methane seeping from the abyssal Black Sea sediments.

Origin and Formational Environment of Noda-Tamagawa Manganese Ore, Northeast Japan: Constraints from Isotopic Studies

Abstract The Noda-Tamagawa mine, one of the biggest bedded manganese deposits in Japan, is located in the north of the Kitakami mountain range. The manganese ore (1–5 m in thickness) is conformably embedded in the cherty and shaly wall rocks, and is mineralogically zoned from the rhodonite ore of the wall rock side to the central ore constituted by pyrochroite-hausmannite passing by the intermediate tephroite ore. This zonal distribution of the manganese ore is the result of thermal metamorphism by the Cretaceous Tanohata granodiorite body. The presence of manganosite and pyrochroite in the metamorphosed ore suggests that the primary manganese mineral, before the metamorphism, was rhodochrosite. The sulfur and carbon content in the wall rocks and manganese ore were quantified, and the results display a pattern similar to that of Black Sea sediments. A series of sulfur isotopic data obtained from a ∼200 m stratigraphic traverse including the manganese ore horizon shows that the sulfur isotopic composition (δ 34S) varies between –42 and +10‰. The majority of values is around –25‰ suggesting that the sulfides in the rocks of the Noda-Tamagawa mine were formed during diagenesis by the intervention of sulfur reducing bacteria. The rarely found manganese carbonate in hausmannite ore has light δ 13C values ranging between –21‰ and –3‰, confirming the sedimentary origin of this primary manganese ore. Thus, rhodochrosite (the most probable precursor of the manganese minerals of the Noda-Tamagawa deposits) was metamorphosed by the intrusion of the Tanohata granodiorite body to form the several manganese silicates and oxide minerals currently found in the Noda-Tamagawa site.

Pterocysta gen. nov.: A new dinoflagellate cyst from pleistocene glacial‐stage sediments of the black and Marmara Seas

A new dinoflagellate cyst, Pterocysta cruciformis Rochon gen. et sp. nov., is described from glacial Late Pleistocene sediments of the southwestern Black Sea and the central Marmara Sea. Pterocysta cruciformis is distinguished by a S‐type paratabulation, the cruciform shape of the endocyst and the presence of a prominent narrow wing‐like periphragm on the ventral side. Similarities between Pterocysta cruciformis and other camocavate cysts are discussed. The new species is associated with cold climatic conditions and probably indicates brackish to fresh water conditions.

Distribution coefficients (Kd) and desorption rates of 137Cs and 241Am in Black Sea sediments.

The distribution coefficients (Kd) and desorption rates of 137Cs and 241Am radionuclides in bottom sediments at different locations in the Black Sea were studied under laboratory conditions. The Kd values were found to be 500 for 137Cs and 3800 for 241Am at the steady state and described exponential curves. Rapid uptake of the radionuclides occurred during the initial period and little accumulation happened after four days. The desorption rates for 137Cs in different bottom sediments were best described by a three-component exponential model. The desorption half-times of 137Cs ranged from 26 to 50 d at the slow components. However, the desorption rate of 241Am described one component for all sediment samples and desorption half-time was found to be 75 d. In general, the results showed that the 241Am radionuclide is more effectively transferred to bottom sediment and has longer turnover time than 137Cs under Black Sea conditions.

Constraints on Black Sea outflow to the Sea of Marmara during the last glacial–interglacial transition

New cores from the upper continental slope off Romania in the western Black Sea provide a continuous, high-resolution record of sedimentation rates, clay mineralogy, calcium carbonate content, and stable isotopes of oxygen and carbon over the last 20 000 yr in the western Black Sea. These records all indicate major changes occurring at 15 000, 12 800, 8400, and 7100 yr before present. These results are interpreted to reflect an evolving balance between water supplied by melting glacial ice and other river runoff and water removed by evaporation and outflow. The marked retreat of the Fennoscandian and Alpine ice between 15 000 and 14 000 yr is recorded by an increase in clays indicative of northern provenance in Black Sea sediments. A short return toward glacial values in all the measured series occurs during the Younger Dryas cold period. The timing of the first marine inflow to the Black Sea is dependent on the sill depths of the Bosporus and Dardanelles channels. The depth of the latter is known to be −80±5 m, which is consistent with first evidence of marine inundation in the Sea of Marmara around 12 000 yr. The bedrock gorge of the Bosporus reaches depths in excess of −100 m (relative to present sea level), though it is now filled with sediments to depths as shallow as −32 m. Two scenarios are developed for the connection of the Black Sea with the Sea of Marmara. One is based on a deep Bosporus sill depth (effectively equivalent to the Dardanelles), and the other is based on a shallow Bosporus sill (less than −35 m). In the deep sill scenario the Black Sea’s surface rises in tandem with the Sea of Marmara once the latter connected with the Aegean Sea, and Black Sea outflow remains continuous with inflowing marine water gradually displacing the freshwater in the deep basin. The increase in the δ 18O of mollusk shells at 12 800 yr and the simultaneous appearance of inorganic calcite with low δ 18O is compatible with such an early marine water influx causing periodic weak stratification of the water column. In the shallow sill scenario the Black Sea level is decoupled from world sea level and experiences rise and fall depending on the regional water budget until water from the rising Sea of Marmara breaches the shallow sill. In this case the oxygen isotope trend and the inorganic calcite precipitation is caused by increased evaporation in the basin, and the other changes in sediment properties reflect climate-driven river runoff variations within the Black Sea watershed. The presence of saline ponds on the Black Sea shelf circa 9600 yr support such evaporative draw-down, but a sensitive geochemical indicator of marine water, one that is not subject to temperature, salinity, or biological fractionation, is required to resolve whether the sill was deep or shallow.

Radioactive Contamination of the North-western Black Sea Sediments

The paper deals with the input and deposition of the man-made radionuclides 137Cs, 238 Pu, 239+240Pu and 241Am introduced to the north-western Black Sea, over the last few decades, as the result of atmospheric nuclear weapons testing and the Chernobyl nuclear accident. One approach taken was to retrieve the deposition records of these radionuclides in the sediments. The deposition chronology was compared with monitoring data of the post-Chernobyl input of 137Cs to the NW Black Sea sediments from the Danube River. The partitioning of 137 Cs between suspended matter and water was traced along the salinity gradient in the Danube mixing zone. In sediments deposited in the vicinity of the Danube delta and the Dniepr estuary, the activity of Chernobyl 137Cs had reached its maximum 5 and 10 years after the accident, respectively. The activity ratio of 137Cs to 239+240Pu and 241 Am revealed a higher mobility of 137Cs in the Danube River basin compared to plutonium and americium. The percentage of particulate 137Cs and its distribution coefficient vs salinity have allowed the estimation of sedimentary scavenging and desorption of caesium in the Danube mixing zone. Comparison of the post-Chernobyl 137Cs input from the Danube to the 137Cs inventory in the adjacent Black Sea sediments showed that more than 70% of this radionuclide was deposited in the Danube–Black Sea mixing zone.

The Nature of Organic Matter in the Danube River Particles and North-western Black Sea Sediments

Suspended matter and sediments from the Danube River and the adjacent north-western Black Sea were analysed for organic carbon, nitrogen and biogenic carbonate and silica, as well as for different fractions of organic matter such as amino acids and carbohydrates. In addition, the nitrogen isotopic composition of surficial sediments of the NW Black Sea was examined. Particulate organic carbon (POC) contents decrease with total suspended solid (TSS) concentration due to dilution by mineral matter derived from upstream and riverbed erosion. The contribution of labile constituents such as amino acids and carbohydrates to POC remains uniform across the measured TSS contents. This suggests that any possible decrease in labile POC (LPOC) from dilution with refractory organic matter is masked by inputs of fresh organic matter from tributaries downstream. Comparison of TSS, POC and LPOC leaving the Iron Gate I barrier at the head of the Danube delta shows that the bulk of material currently entering the Black Sea is derived from inputs from tributaries and riverbed erosion, downstream of the Iron Gate I. The nature and distribution of organic matter in the surface sediments of the Black Sea are influenced by mixing between Danube-derived material and freshly produced organic matter on the shelf and in the open sea, on the one hand, and by oxic degradation during transport from the shelf to the abyssal Black sea, on the other. Preliminary data on the stable isotopic composition of nitrogen in surface sediments of the Black Sea further suggest that organic matter production on the shelf and the adjacent north-western Black Sea is fuelled by different nitrogen sources.

Middle Proterozoic ocean chemistry: Evidence from the McArthur Basin, northern Australia

We report on the iron-sulfur systematics and S-isotopic compositions of upper Paleoproterozoic carbonaceous shales from the Wollogorang and lower Reward formations, McArthur Basin, northern Australia. Iron speciation in these rocks is similar to that of modern euxinic Black Sea sediments and distinct from sediments depositing from oxygenated water, providing persuasive evidence of euxinic depositional environments in a mid-Proterozoic marine basin. High values of Degree of Pyritization (DOP) also indicate anoxic bottom-water conditions at the time of shale deposition as does the relatively uniform isotopic composition of sedimentary sulfides. The isotopic composition of the sulfides is, however, quite enriched in 3 4 S and for the lower Reward Formation similar to contemporaneous seawater sulfate. These 3 4 S-enriched sulfides are consistent with extensive, or even complete, sulfate depletion within the euxinic basins in which the sediments were deposited. Based on analogy with modern euxinic environments and further constrained by model results, middle Proterozoic seawater sulfate concentrations were probably within the range of 0.5 to 2.4 mM. Taken together, our results indicate a relatively low-sulfate late Paleoproterozoic ocean and sulfidic deep waters in the McArthur seaway, a condition that could have encompassed much larger regions of the ocean.

Energy dispersive X-ray fluorescence and neutron activation analysis of surficial sediments of the Sea of Marmara and the Black Sea around Istanbul

Surficial sediment samples, collected from 27 sampling sites of the Sea of Marmara and the Black Sea nearby Istanbul area in 1996 and 1999 years, were analyzed quantitatively for some heavy metals and some other elements by using radioisotope excited energy dispersive X-ray fluorescence (EDXRF). The U and Th concentrations of the samples were determined by instrumental neutron activation (INAA). The results were compared with those from other contaminated and uncontaminated sedimentary materials. It is found that some elemental concentrations of the Sea of Marmara and the Black Sea sediments are comparable with those in most other sea sediments in the vicinity of densely populated regions.