Lead and its isotopes in the sediment of three sites on the Lebanese coast: Identification of contamination sources and mobility

Lead concentrations and isotopic composition of sediment samples collected from three sites within the Lebanese coastal zones were measured: at Akkar, Dora and Selaata. Akkar is located far from any direct source of contamination, while Dora and Selaata receive urban and industrial wastes, respectively. Low Pb concentrations (6–16 μg g⁻¹) were detected in the Akkar sediments, and high concentrations of Pb (70–101 μg g⁻¹) were detected in the Dora sediments. Measuring stable isotope ratios of Pb makes it possible to identify the principal sources of Pb in the Akkar sediments as Pb emitted from gasoline combustion and Pb originating from natural sources. On the other hand, Pb stable isotopic ratios in Dora sediments indicate that they are more highly influenced by anthropogenic sources. Isotopic Pb ratios in the Selaata deposits, where Pb concentrations range between 5 and 35 μg g⁻¹, have an exceptional radiogenic signature for marine sediments 1.25 < ²⁰⁶Pb/²⁰⁷Pb < 1.6 and 0.5 < ²⁰⁶Pb/²⁰⁸Pb < 0.67, which shows the impact of the phosphogypsum discharged by Selaata’s chemical plant. Isotopic Pb analysis applied to EDTA extracts, to test the mobility of Pb, shows that that this mobility is high (>60%) after 24 h of extraction, and that the extracted Pb is less radiogenic than the residual Pb.     

Pathways and regulation of carbon, sulfur and energy transfer in marine sediments overlying methane gas hydrates on the Opouawe Bank (New Zealand)

This study combines sediment geochemical analysis, in situ benthic lander deployments and numerical modeling to quantify the biogeochemical cycles of carbon and sulfur and the associated rates of Gibbs energy production at a novel methane seep. The benthic ecosystem is dominated by a dense population of tube-building ampharetid polychaetes and conspicuous microbial mats were unusually absent. A 1D numerical reaction-transport model, which allows for the explicit growth of sulfide and methane oxidizing microorganisms, was tuned to the geochemical data using a fluid advection velocity of 14 cm yr⁻¹. The fluids provide a deep source of dissolved hydrogen sulfide and methane to the sediment with fluxes equal to 4.1 and 18.2 mmol m⁻² d⁻¹, respectively. Chemosynthetic biomass production in the subsurface sediment is estimated to be 2.8 mmol m⁻² d⁻¹ of C biomass. However, carbon and oxygen budgets indicate that chemosynthetic organisms living directly above or on the surface sediment have the potential to produce 12.3 mmol m⁻² d⁻¹ of C biomass. This autochthonous carbon source meets the ampharetid respiratory carbon demand of 23.2 mmol m⁻² d⁻¹ to within a factor of 2. By contrast, the contribution of photosynthetically-fixed carbon sources to ampharetid nutrition is minor (3.3 mmol m⁻² d⁻¹ of C). The data strongly suggest that mixing of labile autochthonous microbial detritus below the oxic layer sustains high measured rates of sulfate reduction in the uppermost 2 cm of the sulfidic sediment (100-200 nmol cm⁻³ d⁻¹). Similar rates have been reported in the literature for other seeps, from which we conclude that autochthonous organic matter is an important substrate for sulfate reducing bacteria in these sediment layers. A system-scale energy budget based on the chemosynthetic reaction pathways reveals that up to 8.3 kJ m⁻² d⁻¹ or 96 mW m⁻² of catabolic (Gibbs) energy is dissipated at the seep through oxidation reactions. The microorganisms mediating sulfide oxidation and anaerobic oxidation of methane (AOM) produce 95% and 2% of this energy flux, respectively. The low power output by AOM is due to strong bioenergetic constraints imposed on the reaction rate by the composition of the chemical environment. These constraints provide a high potential for dissolved methane efflux from the sediment (12.0 mmol m⁻² d⁻¹) and indicates a much lower efficiency of (dissolved) methane sequestration by AOM at seeps than considered previously. Nonetheless, AOM is able to consume a third of the ascending methane flux (5.9 mmol m⁻² d⁻¹ of CH₄) with a high efficiency of energy expenditure (35 mmol CH₄ kJ⁻¹). It is further proposed that bioenergetic limitation of AOM provides an explanation for the non-zero sulfate concentrations below the AOM zone observed here and in other active and passive margin sediments.     

Geochemical anomalies in the mantle of the Pamirs and Tien Shan with applications to the deep-seated sources of ore material

The mantle metasomatites (fluidized magmatic rocks) of the Pamir-Tien Shan region show extremely high contents of lithophile and chalcophile trace elements, which often exceeds the regional average abundances of the Earth’s crust. Geochemical relations were established between mantle and crustal rocks, and it was shown that the compositions of magmatic rocks of different age and formation depth and polychronous mineralization are relatively stable. These data and some other facts indicate the possible influence of mantle fluids (melts) on the crustal rocks and processes. An alternative model implies the geochemical influence of crustal rocks on the geochemical characteristics of the regional mantle. The ore material of alkali basic rocks and some hydrothermal and rare metal deposits shows a geochemical affinity to the supposed mantle (mantle-crustal) sources. The ore-bearing fluids (melts) were presumably related to the evolution of ultradeep “hot” material of mantle plumes and “daughter” diapirs carrying alkaline, trace, and ore elements.     

Plagioclase buoyancy in basaltic liquids as determined with a centrifuge furnace

Plagioclase buoyancy experiments have been carried out in a high-temperature centrifuge furnace using seventeen basaltic liquids and plagioclase crystals of three compositions: An₈₉, An⁷⁶ and An₅₅. The results show that the floating tendency of plagioclase in basaltic liquids is at least 0.03 g/cm³ greater than indicated by the calculations. If this correction factor is applied to calculations of plagioclase buoyancy in the Skaergaard Intrusion, it is found that the plagioclase crystals in the lower and middle zones were less dense than the coexisting liquids.Other phenomena relevant to crystal transport in basaltic liquids were observed in the centrifuge experiments. These included crystal flotation by rising bubbles, plagioclase sinking because of the formation of plagioclase-magnetite composite grains, graded bedding of olivine and magnetite, and more than 60% intercumulus basaltic liquid between settled olivine crystals.     

Hydrogeochemical modelling of an active system of uranium fixation by organic soils and sediments (Needle's Eye,Scotland)

Uranium accumulation in organic-rich sediments can be closely modelled by assuming that the dominant effect of the uranium-organic matter interaction is the direct or indirect reduction of uranyl compounds to form U(IV) minerals, especially uraninite-pitchblende. Application of this model to the Needle's Eye (Scotland) site where uranium is actively accumulating in Quaternary sediments demonstrates that uranium accumulation is both effective and rapid in environments involving shallow, organic-rich, reducing horizons. The period of uranium deposit formation at Needle's Eye is estimated to be as short as 5000 years. The transport of uranium to the site of deposition by oxidizing groundwaters and the channelling of these oxidizing uraniferous groundwaters are identified as important factors involved in the rapid accumulation of uranium. The regional hydrogeological model indicates that a fault in the area appears to act as a hydraulic screen for the uraniferous groundwaters. On one side of the fault the Quaternary sediments are well drained whilst on the other the flow of groundwater seeps out creating a major flux just at the bottom of the organic-rich layers. The local hydrogeological model shows that the groundwater flow is vertical in this area. A third significant factor in the development of these uranium accumulations is the presence of a significant nearby source of leachable primary uranium. In the case of the Needle's Eye site this is in the form of some thirty 185 ±20 Ma, pitchblende-bearing veins.     

Compression characteristics of saturated re-compacted glacial tills in Tianmo Gully of Tibet,China

Glacial tills are highly compressible soils composed of loosely cemented sandy silt particles. Their role, for instance, as initial filler materials in geo-technical projects along temperate glacier regions warrant a systematic evaluation of their compressive characteristics. As such, we carry out a series of onedimensional compression tests on re-compacted glacial till samples obtained from Tianmo Gully, Tibet, with the aims of evaluating their compressive behavior under controlled initial stress conditions. The yield stress was determined from the void ratio versus consolidation pressure(e-log) planes. Its compression and swelling behaviors were observed during the primary and secondary compressions of the consolidation tests. From the compression curves, a correlation can be found between the consolidation stress and the void index. In addition, we find that the compression curves of glacial tills collapse into a single curve when normalized by the compression index. The experimental results in this study provide a basic understanding of the compression characteristics of the glacial tills in Tibet, China.     

Spatial and temporal variations in aboveground and belowground biomass of Spartina maritima (small cordgrass) in created and natural marshes

Spartina species are commonly used for salt marsh manipulative projects, where aboveground and belowground biomasses are functional traits that play important roles, showing high spatial and temporal variations. This work analyses variations in AGB and BGB of Spartina maritima and abiotic environmental parameters along a chronosequence of six marshes created from 1997 to 2003 with disparate sediment dynamics, and adjacent natural marshes and unvegetated tidal flats. S. maritima behaved as an autogenic engineer, as its colonization of bare sediments yielded abiotic environmental changes: specifically, bed level rise accompanied by higher oxygenation and salinity. These modifications of the environment were site-specific, depending mainly on sedimentary dynamics. At the same time, abiotic environmental changes determined biomass production rates of S. maritima that were higher in more-accreting marshes; however, AGB was kept constant from early in its development (2 years). The increase in BGB with elevation seemed to be related to the inhibition of subsurface tissue development in anoxic sediments. Biomass accumulation and production varied markedly, depending on the spatial scale, indicating the relevance of the plot size chosen for the analysis of biomass of cordgrasses. Our results show that managers of salt marshes should consider sedimentary dynamics carefully when setting realistic expectations for success criteria of created and restored wetlands.     

Hydrologic and sedimentologic aspects of mining aggregate from the shipsk bank (Baltic sea)

Abstract

Investigations aimed at obtaining knowledge about the environmental effects of mining aggregate from the sea bottom were carried out on the Stupsk Bank. Special attention was given to the magnitude and reach of changes of hydrologic conditions, in that to changes in concentration of matter suspended in sea water, and to dynamics of the sea bottom—change of bottom relief, resulting from both natural causes and from mining of the aggregate.