Vesselin M. Dekov

Chemical composition of sediments, suspended matter, river water and ground water of the Nile (Aswan-Sohag traverse)

Sediment, suspended matter, river water and ground water samples were collected at twelve sites in the drainage valley of the Nile River, around Sohag (Central Egypt) and close to the Aswan High Dam. Elemental composition of the river water (27 elements), ground water (eight elements), suspended matter (12 elements) and sediments (12 elements) was studied. Aswan High Dam construction, agricultural and industrial human activities have led to dramatic changes in the Nile River chemistry. Nowadays, the Nile River has the highest dissolved salt content among the major African rivers. Dissolved transport is a major process for Ca, K, Sr, Zn, Cu, Ni and V. Manganese, Fe and Cr are mainly carried by suspended matter. The Nile suspended matter is exhausted in almost all elements studied (except for Mn) compared to the world average river suspended matter. Along the course of the river, the distribution of elements in the suspended matter and sediments is generally controlled by natural processes: the relative importance of elemental transport phases; and the oxidation, precipitation and sedimentation of mineral species through the varying physico-chemical conditions of the environment. Pollution input in the Nile particulate load is not major, as compared to the natural inputs. Eight genetic particle types describe the composition of the Nile suspended matter and sediments: (1) biogenous-aeolian (or silica); (2) terrigenous (Fe-aluminosilicate); (3) authigenic (calcium carbonate); (4) biogenous (apatite); (5) authigenous-terrigenous (Fe-oxyhydroxide-montmorillonite); (6) diagenetic (iron-sulfide); (7) terrigenous (titanium oxide); (8) authigenous (Mn-Fe-oxyhydroxide).

Hydrothermal activity in the SE Tyrrhenian Sea: an overview of 30 years of research

Subduction-related volcanic activity in the Southeast Tyrrhenian back-arc basin has resulted in a large variety of calcalkaline rocks (ranging from arc tholeiitic to shoshonitic and rhyolitic composition), which were emplaced in both oceanic and continental crust. Hydrothermal systems in the SE Tyrrhenian Sea have leached these magmatic rocks, generating a wide range of oxyhydroxide and sulfide deposits. All the hydrothermal occurrences formed at much shallower depths than those from the mid-ocean ridges. Oxyhydroxide deposits (metalliferous sediments, crusts, and nodules) are almost pure hydrothermal formations with insignificant hydrogenetic and diagenetic contributions. They exhibit strong separation of Mn-rich and Fe-rich end members with Mn-rich occurrences dominant. Their geochemistry suggests a relative enrichment of Mn in the hydrothermal systems of the SE Tyrrhenian. The sulfides are Zn- and Pb-rich, but poor in Cu and relatively poor in Fe. This suggests either deep-seated magma chambers or boiling of the hydrothermal fluids beneath the seafloor. As a consequence: (1) Cu and a large proportion of the Fe are deposited as sulfides in the basement rocks and sediment cover; (2) the hydrothermal fluids become relatively enriched in Mn, Zn, and Pb. A number of accessory sulfosalts and selenides not typical of mid-ocean ridge massive sulfides are present.

Chemical composition of sediments and suspended matter from the Cauvery and Brahmaputra rivers (India)

Abstract Suspended matter, surface and core sediments from the Cauvery River and core sediments from the Brahmaputra River, India were analysed (15 elements) by the energy-dispersive X-ray fluorescence technique. The Cauvery River load indicates a relative predominance of solute vs. suspended transport. Cauvery River suspended matter is depleted in Al and Si, and enriched in Ca, Mn, Cr, Ni, Cu, Zn and Sr relative to the worlds average river suspended matter. As the sampling web covered mainly the Cauvery Delta, the enhanced heavy metal concentrations in the suspended matter are explainable in terms of chemical–physical processes in the zone of mixing. Flocculation of the dissolved metal species causes their transformation in suspended state. Consequently, deltaic suspended matter is one–two orders of magnitude richer in heavy metals than the main river course background. Cauvery surface sediments reflect the average Indian river sediment composition. They have a homogenous composition along the river course and a heavy metal content lower than that of the Cauvery suspended matter. Sedimentation rates in the Cauvery and Brahmaputra are comparable with those published for other Indian rivers. Sediment flux to the Brahmaputra bed is higher than for the Cauvery. Mass accumulation rates for heavy metals in the sediments were determined. It is supposed that the large and synchronous fluctuations in the temporal distribution of the heavy metal mass accumulation rates have been caused by cyclic climatic changes over the Indian sub-continent. An additional supply of transition elements, due to urbanisation and industrial activities, could be assumed to cause the enrichment in the uppermost core layers.

Heavy metals in sediments from the Paraguay river: a preliminary study

The first results ever obtained on heavy metal concentrations (Fe, Mn, Cr, Cu, Zn and Pb) in the Paraguay River surface sediments are presented. Samples collected at 11 locations, along a distance of 570 km, between the cities of Bahia Negra and Alberdi in Paraguay, for six different periods between November 1991 and 1993, were analyzed. The Paraguay sediments appeared to have features of an unpolluted river even though significant amounts of domestic and industrial effluents are discharged near the river channel. The relative heavy metal enrichment in sediments between Bella Vista and Asunción, caused by local domestic sewage and industrial outfalls, is less than for the shale standard values. The heavy metal content of the sediments exhibited seasonal variations. Enhanced organic matter content and biochemical oxygen demand of the river load in winter caused most likely a retainment of the heavy metals in a dissolved state. Consequently, the sediments deposited in the winter were relatively depleted in these elements.

SMECTITE FORMATION IN SUBMARINE HYDROTHERMAL SEDIMENTS: SAMPLES FROM THE HMS CHALLENGER EXPEDITION (1872–1876)

Clay processes, mineral reactions, and element budgets in oceans continue to be important topics for scientific investigation, particularly with respect to understanding better the roles of chemistry, formation mechanism, and input from hydrothermal fluids, seawater, and non-hydrothermal mineral phases.To that end, the present study was undertaken.Thre e samples of submarine metalliferous sediments of hydrothermal origin were studied to investigate the formation of smectite, usually Fe-rich, which takes place in such environments.The samples are from the historical collection returned by the British HMS Challenger expedition (1872–1876) and kept at the Natural History Museum in London.The samples were collected from the vicinity of the Pacific–Antarctic Ridge and the Chile Ridge.The samples were analyzed by means of X-ray diffraction (XRD), chemical analysis, scanning electron microscopyenergy dispersive X-ray spectroscopy (SEM-EDX), infrared (IR), and transmission electron microscopyanalytical electron microscopy (TEM-AEM).After removal of biogenic calcite the samples appeared to consist mainly of two low-crystallinity phases mixed intimately: Fe/Mn (oxyhydr)oxides and a Si-Al-Mg- Fe phase of similar chemical characteristics to smectite and with variable proportions of the above elements, as indicated by XRD, IR, and SEM-EDX.In particular, analysis by XRD revealed the presence of highly disordered δ-MnO2.The TEM-AEM analysis showed that Fe/MnOOH particles have Fe/Mn ratios in the range 25–0.2 and textures changing from granular to veil-like as the proportion ofMn increased. The smectite-like material has the morphology and chemistry of smectite, as well as 10–15 Å lattice fringes. Selected area electron diffraction (SAED) patterns indicated a very poorly crystalline material: in some cases distances between diffraction rings corresponded to d values of smectite.The smectite composition indicated a main Fe-rich dioctahedral component with a substantialMg-rich trioctahedral component (total octahedral occupancy between 2.02 and 2.51 atoms per O10[OH]2). The (proto-) smectite is interpreted to have formed within the metalliferous sediment, as a slow reaction between Fe/MnOOH, seawater (providing Mg), detrital silicates from the continent (providing Si and Al), and X-ray amorphous silica of hydrothermal origin that adsorbed on Fe/MnOOH phases and deposited with them.This material is possibly in the process of maturation into well crystallized smectite.

Environmental conditions controlling the chemical weathering of the Madara Horseman monument, NE Bulgaria

Abstract The conditions that led to the deterioration of the Madara Horseman sculpture, a relief carved in a sandy limestone cliff in north-east Bulgaria, were studied, with most emphasis on the chemical weathering at the stone–atmosphere interface. Total deposition sampled close to the monument contains high concentrations of NH 4 + and K + , which shows the influence of the dense vegetation in the area, and Ca 2+ , which reflects the cliffˈs weathering products. Ca 2+ is also one of the most important constituents (next to aluminosilicates) of the atmospheric particulate collected at the monument. S-rich particles are the most abundant particles in the size range – , K + and NO 3 – , is formed in the rain-sheltered areas of the monument. Secondly, karstic dissolution in the interior of the rock substrate leads to the formation of a hard carbonate crust at the surface of the sculpture. This natural surface hardening leads to a state of ‘auto-conservationˈ of the monument. The atmospheric composition seems to have only a limited influence on the deterioration of the Madara Horseman. Far more important are the micro-climatic conditions and the action of lichens.

Single particle characterisation of inorganic and organic North Sea suspension

Abstract Automated electron probe X-ray micro-analysis and scanning transmission electron microscopy were used to characterise the chemical and mineral composition of individual suspended particles, collected in the Southern Bight of the North Sea. The suspended matter contains 13 particle types, which can be classified in seven genetic groups: (i) terrigenous (aluminosilicate and calcium carbonate particle types); (ii) terrigenous–biogenic (silica type); (iii) terrigenous–hydrogenous (Fe-rich type); (iv) biogenic–authogenic (S-containing type); (v) terrigenous–anthropogenic (Ti- and Al-rich types); (vi) biogenic–anthropogenic (organic type); and (vii) terrigenous–anthropogenic–authogenous (heavy metal rich type). Staining the suspended matter with RuO4, revealed extra information about their organic matter content. Organic matter coatings were detected on all of the suspended particles. A high concentration of organic material was detected in all Mn-rich, Cr-rich, Zn-rich and Ni-rich particles. A strong correlation between organic- and heavy metal-rich particles which was revealed by factor analysis, may indicate that organic matter is complexing heavy metals. Organic material in the southern part of the North Sea is least abundant in fall and winter, and most common, but also very variable, during primary bioproduction boom in spring. The amount of suspended matter was relatively constant throughout the year close to the Westerschelde estuary.

Fe-Mn micronodules born in the metalliferous sediments of two spreading centres: the East Pacific Rise and Mid-Atlantic Ridge

Abstract A geochemical study of Fe–Mn micronodules associated with the metalliferous sediments at two spreading centres has shown that their composition depends on the site of micronodule formation. Close to the hydrothermal mounds they exhibit significant variation in elemental content related to the type of hydrothermal discharge (low- or high-temperature), the nature of primary hydrothermal matter (plume fall-out, oxidised sulfides), and the extent of diagenesis. In this environment three types of micronodules can be distinguished although not observed as pure end-members: (1) diagenetic micronodules; (2) micronodules formed generally from the plume fall-out of oxyhydroxide matter; and (3) micronodules grown on the oxidised sulfide grains supplied to the sediments by slumping or fall-out of nearby buoyant plume. Away from the active spreading centre, the hydrothermal signatures of primary precipitates are gradually masked and hydrogenous/diagenetic processes lead the micronodule formation. Composition of micronodules becomes less variable. Well-pronounced, deep rift valleys confine the hydrothermal plume, which brings the hydrothermal suspension into contact with restricted volumes of seawater and, consequently, weakens the hydrogenous influence on the primary hydrothermal matter. Shallow rift valleys do not confine hydrothermal plumes, which are scattered over hundreds of kilometres by bottom currents. This brings the hydrothermal suspended matter into contact with large volumes of seawater. Extensive scavenging occurs, which masks the hydrothermal signal away from the spreading axis and enhances the hydrogenous one. Thus, the ridge crest morphology, defined by the spreading rate, is supposed to play a certain role, though indirect, in the chemical composition of the primary precipitates and, consequently, in the composition of the micronodules formed.

Crystal chemistry of the mixed-layer sequence talc–talc-smectite–smectite from submarine hydrothermal vents

Abstract Clay samples of hydrothermal origin from several oceanic spreading centers were studied using XRD, microprobe, infrared, and thermal analysis. They are talc, smectite, and mixed-layer talc-smectite (T-S) where the talc layers have several degrees of crystalline order. The smectite is trioctahedral in most cases, but there is also dioctahedral smectite both as mixed-layer and as a separate phase. All specimens contain Fe3+, some of them in moderate amounts (up to 17% Fe2O3) distributed between the tetrahedral and octahedral sheets (maximum values: IVFe = 0.32, VIFe = 0.68, per O10[OH]2). Octahedral Fe abundance correlates with the presence of molecular water that is lost in a well-defined dehydration event above 200 °C. This water does not cause layer expansion and is interpreted to be present within the pseudo-hexagonal cavity, next to Fe3+ ions that generate a local charge imbalance. The presence of octahedral Fe3+ is accompanied by vacancies in the octahedral sheet to balance the excess positive charge. An infrared band at ~790 cm-1 is assigned to OH bending in the group Fe-Mg-□-OH. Analysis of this band suggests a range of short-range Fe-□ distributions, from random to ordered. Our sequece talc, T-S, trioctahedral smectite is defined by an increasing Al for Si substitution in the tetrahedra and increasing crystal disorder. The presence of Fe also causes crystal defects. This mixed-layer series can be considered as a continuum generated by the combination of chemical and crystal defect variability. Kerolite was used to designate disordered, hydrated talc. We find that there is no clear line delimiting talc from kerolite as a single phase or in mixed-layer minerals and that it is better to use a descriptive term for the latter such as “disordered talc.” Dioctahedral smectite is also a possible end-member of the mixed-layer sequence, which implies an Al + □ for Mg substitution in the octahedral sheet. If T-S consists of polar TOT layers, the existence of dioctahedral smectite in T-S raises the question of the actual composition of the octahedral sheets within polar TOT layers.

Single particle and inorganic characterization of rainwater collected above the North Sea

Suspended matter and the dissolved fraction of rainwater collected above the North Sea were characterized using electron probe X-ray microanalysis (EPXMA), inductively coupled plasma-optical emission spectrometry (ICP-OES) and ion chromatography (IC), respectively. Suspended particulate matter was dominated by aluminosilicates and organic particles. Fifteen particle types describe the composition of the North Sea rainwater suspended matter. Factor analysis, particle size distributions and manual EPXMA measurements illustrated the complex genesis of different particle types: terrigenous; biogenic (both marine and continental); and anthropogenic. It was demonstrated that at the beginning of a shower of rain the coarse particles that are present in the air under the cloud are washed out, while during the second phase rainout particles, formed in the cloud, become more important due to the absence of new coarse particles under the cloud. Above the sea, the total amount of suspended matter (TSM) is much smaller and more variable than above the land and also the decrease in particle diameter is less visible. Approximately 10% of the studied particles contained trace heavy metals. The dissolved compounds in the North Sea rainwater were also variable in time and space. In general, over a short period of time, the concentrations of all dissolved compounds seem to decrease during a shower, but this decrease is much larger above land than above sea. The concentrations of dissolved trace metals present in rainwater above the southern North Sea has decreased over the last 15 years.