C. H. Van der Weijden

Composition of anoxic hypersaline brines in the Tyro and Bannock Basins, eastern Mediterranean

At present two brine-filled depressions have been identified in the eastern Mediterranean - The Tyro Basin (33°53′N, 26°02′E) and the Bannock Basin (34°20′N, 20°02′E). In both areas, the main basin is surrounded by satellite basins that used to have or still have hypersaline and anoxic conditions. The brines are thought to originate from dissolution of outcropping or underlying (Messinian) evaporitic salt layers. The transition from normal seawater to brine water occurs at a slightly different water depth in the two basins; at 3383 dbar in the Tyro Basin and at 3330 dbar in the Bannock Basin. At this transition, salinity increases sharply, whereas the concentration of dissolved oxygen drops rapidly to zero across a depth interval of only a few metres. The brines in the anoxic Tyro and Bannock Basins are of almost identical salinity (10 times that of seawater). However, between the Tyro and Bannock brines differences occur in the major (and trace) element composition. The Tyro brine is characterized by a relatively high Na content, whereas the Bannock brine is more enriched in K, Mg and SO4. These differences are related to the composition of the (Messinian) evaporitic salt that serves as a source for each of these brines. The differences in evaporite composition result from differences in the depositional ‘stage’ of the two salt deposits. The Bannock brine results from dissolution of a late-stage evaporitic salt deposit, whereas the Tyro brine originates from an earlier-stage evaporite. In contrast to the homogeneous brine in the Tyro Basin, a double-layered brine is observed in the Bannock Basin. The lower brine (II) has a slightly higher salinity than the upper brine (I) and the brines also differ in composition. Brine I has a higher concentration of Ca and Sr, and a lower concentration of Mg, S and SO4. The concentrations of these ions in the Scirocco brine deviate even more from those in brine II, which is thought to indicate a higher degree of dolomitization in the Scirocco Basin.

Hydrogeochemistry of the River Rhine: Long term and seasonal variability, elemental budgets, base levels and pollution

Abstract The long-term and seasonal variations in the composition of water and suspended matter in the River Rhine are discussed. The study is based on the analysis of 240 water samples and of 79 suspended matter samples collected during a 10-year period from 1975 to 1984. The discharge is dominated by two hydrological regimes, namely a winter flow derived mainly from surface flow and groundwater flow in the German/French part of the drainage basin, and by a summer flow derived mainly from snowmelt and precipitation in the Alps. Principle components analysis of the data for the dissolved constituents reveals three factors: (1) heavy metals, (2) major ions, and (3) nutrients. Time series analysis of these factors demonstrates a perfect seasonality for the nutrients, a reduction of the loads of heavy metals and a more complicated pattern for the major dissolved ions. The latter pattern is due to pollution. Principle components analysis of the data for suspended matter reveals four factors: (1) heavy metals, (2) clays, (3) calcium carbonate, and (4) manganese oxides. In time series analysis the scores on the first factor decrease in the period considered, those on the second factor correlate positively with the discharge regime, and those on the third and fourth factor are at their highest during summer flow due to a combination of source area, biological activity and temperature. On the basis of recent and historical data on the chemistry of the Rhine, one can estimate the natural levels of the major element load of the river. This natural load is derived from denudation of various rock types in the drainage basin. A simple steady-state model is used to estimate the contributions of aluminosilicate rocks and of limestones to the total denudation. Data for trace element contents of common rocks are available in geochemical tables. This then allows one to estimate the base levels of the heavy element load of the river. Comparison of the actual level loads with the base level loads makes it possible to calculate a pollution factor for each of the heavy metals. The following factors (measured levels/base levels) were calculated for the pollution of the Rhine between Basel and the German-Dutch border: Cr (5), Co (

Sorption of cadmium on suspended matter under estuarine conditions; competition and complexation with major sea-water ions

Sorption of Cd at low concentrations onto river Rhine suspended matter was examined in terms of sorption rate, reversibility and factors such as competition and complexation with major inorganic sea-water ions. More than 90% of the final amount of Cd is sorbed within the first few hours. Desorption experiments show that the process is virtually reversible. In experiments with diluted sea water the sorption of Cd strongly decreases even at low salinity. Sorption isotherms show that the sorption of Cd in NaNO3 solutions is regulated by the free Cd2+ activity. In a Ca(NO3)2 environment the Cd sorption decreases with increasing Ca2+ concentrations, which implies competition between Ca2+ and Cd2+ for the different sorption sites. In different electrolyte solutions of similar ionic strength the sorption of Cd decreases in the solution order NaNO3 > NaCl > NaCl+MgCl2+CaCl2 > diluted sea water. Although inorganic speciation calculations show that even at low salinities dissolved Cd is dominated by Cd-chloro complexes, chloride accounts for only about one third of the increased mobility of Cd. As a result of addition of Ca2+ and Mg2+ the sorption capacity of suspended matter for Cd is further reduced by a factor three.

Mobility of major, minor and some redox-sensitive trace elements and rare earth elements during weathering of four granitoids in central Portugal

Abstract Element mobility was studied in four different granitoids in central Portugal. Samples of fresh and weathered rock parts were collected from each type of granitoid. An increase in the degree of weathering is accompanied by reductions in Na 2 O, CaO and SiO 2 (plagioclase), P 2 O 5 (apatite), and in most cases also by reduction in K 2 O (biotite, potash feldspar). Losses as well as gains are observed for MgO. Relative changes in the Al 2 O 3 contents are attributed to physical transport or accumulation of fine weathering products. Mass-balance calculations indicate that kaolinite and in some cases also gibbsite are the dominant weathering products; this is in agreement with X-ray diffraction analyses. Manganese is found to decrease but not Fe. There is a high correlation between V and Fe; this is preserved during weathering. There were both increases and decreases in the trace elements Cr, As and Sb; U had become slightly enriched in the weathered parts. No systematic trends in REE behaviour were observed. Differences in the behaviour of these elements and in their ratios can be explained by a combination of factors such as differential dissolution of the host minerals, redox conditions and redox kinetics, complexation by organic and inorganic ligands, and sorption. The interpretation of the results is complicated by apparent inhomogeneities in the granites on the mesoscale of an outcrop.

Organic-rich layers in the Metochia section (Gavdos, Greece): evidence for a single mechanism of sapropel formation during the past 10 My

Abstract Three well-developed 10 Ma old sapropels from the island of Gavdos (Greece) were selected for a detailed geochemical, micropaleontological and sedimentological study. The sapropels are characterized by a high organic carbon and a low carbonate content. Productivity was higher during sapropel formation, as indicated by increased Ba contents and higher percentages of the planktonic foraminiferal species Neogloboquadrina acostaensis . Enrichment of Mo and V in the sapropels indicates less oxygenated bottom water conditions, which is supported by the flourishing of low oxygen tolerant benthic foraminifera. Variation in grainsize and the Ti/Al ratio point to an alternating dominance of fluvial (sapropels) and eolian-derived (homogeneous intervals) terrigenous clastics. Evidence for a local origin of the enhanced fluvial input comes from the gradual replacement of the sapropels by cyclic turbidite sequences which were fed by bed-load type rivers. The increased fluvial input during sapropel formation increased the sedimentation rate and caused the observed carbonate depletion. The results obtained for the Gavdos sapropels are consistent with those of previous studies of younger sapropels and related CaCO 3 cycles in the Mediterranean. The consistency in most of the geochemical and micropaleontological characteristics indicates that all sapropels and related layers are the result of a single mechanism for at least the past 10 My, namely precession induced dry–wet oscillations in the Mediterranean climate. Still, the effect of this mechanism on the intensity of surface water productivity, dilution and diagenetic signals may differ from site to site, depending on location, depositional environment and intensity of the astronomical forcing.

Chromium(III) -- chromium(VI) interconversions in seawater

The stable form of dissolved chromium in oxygenated seawater is Cr(VI). But Cr(III)-species are also present at an analytically significant level. It is shown that Cr(III) is oxidized only slowly by dissolved oxygen, and that manganese oxide is a strong catalyst for such oxidation. However, the low oceanic concentration of suspended MnO2 impedes the conclusion that the former process is quantitatively less important than the latter one. The distribution of dissolved chromium among Cr(VI)- and Cr(III)-species is probably kinetically controlled.

Desorption of metals from suspended material in the rhine estuary

Abstract Aliquots of suspended matter collected from Rhine river water were resuspended in distilled water, in diluted artificial sea water (1 : 1), in artificial sea water, and in ‘nitrate sea water’, respectively at pHs 7.5 and 8.0. The desorption of heavy metals into these solutions was calculated in relation to the concentrations as determined by extraction in 4 M hydrochloric acid. The heavy metal concentrations after resuspension and the relative desorption percentages were compared with calculated inorganic metal ion speciation. These comparisons, though quantitatively not quite satisfactory, demonstrated that complex formation is important in desorption processes during estuarine mixing. The order of decreasing desorption of metals into 1 : 1 diluted sea water and sea water in the experiments is : Cd > Zn > Mn > Ni > Co > Cu > Cr; fo for Fe and Pb no desorption was found. Analyses of samples collected at a tidal station at Hoek van Holland show that both suspended matter and filtered water are depleted in most heavy metals. Certainly, in this tidal area suspended matter and adsorbed and dissolved heavy metals do not show a conservative mixing behaviour. In order to correlate experimental results and field obsevartions, large scale flocculation and sedimentation of suspended matter must be accepted at an early stage of estuarine mixing. It is not certain whether this can be proved; future research should include more chemical indicators for testing these estuarine processes.

Sub-surface iodide maxima: evidence for biologically catalyzed redox cycling in Arabian Sea OMZ during the SW intermonsoon

Abstract Sub-surface I− maxima (200–600 nM) were found in five of the six profiles from the Somali and Arabian basins of the Northwest Indian Ocean. In addition to these maxima, dissolved I− exhibited normally high (100 nM or greater) values in the surface and values ranging from 3 to 40 nM at depth, which are higher than other open oceanic systems. Sulfide was generally found to be less than 2 nM in the water column, indicating that the chemical reduction of iodate by sulfide is not important in regulating iodine speciation in the Northwest Indian Ocean. These novel high iodide values below the euphotic zone do not appear to be related to other bulk chemical or hydrographic features (σθ) but may be the result of two distinct biologically mediated remineralization processes: (1) the direct reduction of I03− to I− as seen in the sub-surface maxima, and (2) release of I− from CI bonds during the decomposition of organic matter. Iodine normalized to salinity, specific iodine, is not found to be conservative in this system. Overall, our specific iodine data support the incorporation of iodine into organic material in the surface. Iodide, when present below the euphotic zone, is a product of the decomposition of that exported organic material. These data and processes are consistent with those found previously in the Black Sea and the Chesapeake Bay. However, in the Northwest Arabian Sea, iodide and oxygen are measurable throughout the water column, indicating that the system is not at equilibrium with the prevailing redox condition and that traditional thermodynamic considerations of pE do not appear to be applicable. Porewater iodide in the top 150 cm increased with depth to approximately 19 μM as a result of the release of I− during decomposition of organic matter.

Some geochemical characteristics of sediments from the North Atlantic Ocean

Abstract During the NAVADO-III expedition, Eh- and pH-measurements were made in a number of North Atlantic deep-sea sediments from different stations. Ten cores with marked changes in redox potential were analysed for Si, Al, Na, K, Cl, Fe, Mn, Co, Sc, Cu, and carbonate. A significant correlation was found between Eh and Mn-content. Fe, Co, and Sc are correlated with the (illitic) clay-content, while no relation was found between Cu and any other main constituent. The examined sediments chiefly consist of calcium carbonate, quartz, feldspars, and illitic clays.

Trace metals in suspended matter from the Rhine/Meuse estuary

Abstract The contents of trace metals in suspended matter of the river Rhine have decreased during the last few decades but still exceed background contents. Contamination factors for suspended matter in the river Rhine at Lobith are found to be: Cd (7), Cu (5), Zn (9), Pb (3.5), Ni (2.5) and Cr (3). In the Rhine/Meuse estuary geochemical trace-metal behaviour is influenced by industrial discharges. Budget calculations show that the additional enrichment due to industry is a few percent (0 to 3%) for Cr, Cu, Hg, Ni, Pb and Zn, but is much higher for As (11.5%) and Cd (48.6%). Suspended-matter samples collected in the estuary along a salinity gradient were analysed for major elements and trace metals. The results show a decrease in the suspended-matter contents of Cd, Co, Cu, Cr, Ni, Pb and Zn as a function of salinity. The Sr content shows an increase with salinity. By using Cr as a conservative tracer, the differences in trace-metal contents between freshwater and seawater can be attributed to the physical mixing of relatively contaminated Rhine/Meuse suspended matter with relatively uncontaminated North Sea suspended matter. The results show an enrichment in Mn, Co, Ni and Pb in the samples collected in May 1988 in comparison to the samples collected in March 1989. This may be due to the enhanced oxidation of reduced Mn at higher temperatures. As a result of the formation of Mn-oxyhydroxides the trace metals Ni, Co and Pb are scavenged from the solution.