Applied Geochemistry (v.19, #8)

Widespread cementation induced by inflow of continental water in the eastern part of the Paris basin: O and C isotopic study of carbonate cements by S Buschaert; S Fourcade; M Cathelineau; E Deloule; F Martineau; M Ayt Ougougdal; A Trouiller (1201-1215).
Mesozoic limestones from the eastern part of the Paris Basin display, mostly in strata rich in bioherms, abundant macro-cavities (vugs) and some connected micro-fractures almost completely filled with euhedral calcite crystals (late calcite spar). The interparticle porosity of limestones is also filled with euhedral calcite spar cements similar to those observed in vugs and fractures, and rarely displays the preservation of the structures typical of early marine cements. These calcite spars (both from vugs/fractures infillings and from cemented porosity) were studied in drilled cores cross-cutting the Kimmeridgian to Bathonian series with the main objective of determining the nature and flow regime of the fluids responsible for the strong porosity reduction observed in that part of the Paris Basin Mesozoic limestones. The approach combined mineralogical and geochemical studies (O and C stable isotopic compositions measured by conventional and SIMS methods) with temperature estimates from fluid inclusion analyses.The δ 13C values of late calcite are close to those of the host limestones and correlate with the δ 13C changes in host rocks, indicating a local source of C. By contrast, late calcites from vugs, fractures and also in the cemented interparticle porosity of limestones have δ 18O values around +21‰/SMOW, values which are distinct from those of bulk host limestones (+25‰ to +27‰). The δ 18O value of the fluid is estimated to be −6.8 to −2.5‰ for a crystallisation temperature between 32 and 42 °C determined from fluid inclusion studies. Thus, the parent fluid of cemented interparticle porosity and vug- and fracture-infillings likely contained a component of meteoric water, in contrast to the previous belief that the reduction of porosity in those series was related to early marine diagenesis. Possible situations in which such meteoric fluid could have occurred are: (i) a stage when the limestone aquifers were directly connected to surface water, and/or (ii) channelling of water, from deeper zones in the basin, through regional fault systems.The O isotopic compositions of the calcite cements in the underlying 140 m – thick Callovo-Oxfordian argillites and those of the less recrystallised Bathonian to Kimmeridgian limestones are those expected for unmodified marine calcite: δ 18O=ca. +28‰ SMOW. Therefore, from the isotopic point of view, the Callovo-Oxfordian argillites remained essentially unaffected by the paleo-fluid circulation documented in the adjacent limestones series. This indicates the argillites acted as an efficient permeability barrier in the past.

Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO2 (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m−2  h−1 at pH 2.78 in the batch experiments to 0.009 μmol Si m−2  h−1 at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H+ consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO2 demonstrated that dissolution rates were independent of CO2 within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO2 have been elevated over the last decade. This release of magmatic CO2 has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO2 may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO2 can be excluded.

Hydrogeochemical and isotopic constraints on the origins of dryland salinity, Murray Basin, Victoria, Australia by I. Cartwright; T.R. Weaver; S. Fulton; C. Nichol; M. Reid; X. Cheng (1233-1254).
Combined hydrogeological and hydrogeochemical data allow flow systems and the origins of solutes in the Honeysuckle Creek area of the southeastern Murray Basin, which is an area affected by dryland salinity, to be constrained. Recharge occurs both on the uplands that are composed of fractured Violet Town Volcanic rocks and the Riverine Plain that comprises sediments of the Shepparton and Coonambidgal Formations. Groundwater from the Violet Town Volcanics has low salinity (<20 mmol/L Cl) and major ion geochemistry that is controlled largely by dissolution of silicate minerals. Low Cl/Br ratios (as low as 281 molar) suggest that this groundwater has not dissolved halite. Groundwater that recharged through the Riverine Plain sediments has higher Cl/Br ratios (up to 1146) and Cl concentrations of <20 mmol/L, consistent with it dissolving minor halite. Higher salinity (>20 mmol/L) groundwater has intermediate Cl/Br ratios (600–1000), which indicate that the high salinities do not simply result from halite dissolution. Rather, mixing of groundwater homogenises Cl/Br ratios, and evaporation as a consequence of a shallow water table is the dominant process that increases salinity. Oxygen and H isotopes also indicate that mixing and evaporation have occurred. These results indicate that land use over the whole region, not just the uplands, needs to be considered in any salinity management plans. Additionally future development of salinity is controlled by depth to the water table on the plains and the efficiency of recharge rather than by salt stores (halite or brines) in the unsaturated zone.

Natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water: the example of West Bengal and its worldwide implications by J.M McArthur; D.M Banerjee; K.A Hudson-Edwards; R Mishra; R Purohit; P Ravenscroft; A Cronin; R.J Howarth; A Chatterjee; T Talukder; D Lowry; S Houghton; D.K Chadha (1255-1293).
In order to investigate the mechanism of As release to anoxic ground water in alluvial aquifers, the authors sampled ground waters from 3 piezometer nests, 79 shallow (<45 m) wells, and 6 deep (>80 m) wells, in an area 750 m by 450 m, just north of Barasat, near Kolkata (Calcutta), in southern West Bengal. High concentrations of As (200–1180 μg L−1) are accompanied by high concentrations of Fe (3–13.7 mg L−1) and PO4 (1–6.5 mg L−1). Ground water that is rich in Mn (1–5.3 mg L−1) contains <50 μg L−1 of As. The composition of shallow ground water varies at the 100-m scale laterally and the metre-scale vertically, with vertical gradients in As concentration reaching 200 μg L−1  m−1. The As is supplied by reductive dissolution of FeOOH and release of the sorbed As to solution. The process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit. In well waters, thermo-tolerant coliforms, a proxy for faecal contamination, are not present in high numbers (<10 cfu/100 ml in 85% of wells) showing that faecally-derived organic matter does not enter the aquifer, does not drive reduction of FeOOH, and so does not release As to ground water.Arsenic concentrations are high (≫50 μg L−1) where reduction of FeOOH is complete and its entire load of sorbed As is released to solution, at which point the aquifer sediments become grey in colour as FeOOH vanishes. Where reduction is incomplete, the sediments are brown in colour and resorption of As to residual FeOOH keeps As concentrations below 10 μg L−1 in the presence of dissolved Fe. Sorbed As released by reduction of Mn oxides does not increase As in ground water because the As resorbs to FeOOH. High concentrations of As are common in alluvial aquifers of the Bengal Basin arise because Himalayan erosion supplies immature sediments, with low surface-loadings of FeOOH on mineral grains, to a depositional environment that is rich in organic mater so that complete reduction of FeOOH is common.

Oxic–anoxic conditions in the water column of a tropical freshwater reservoir (Peña-Larga dam, NW Venezuela) by Boris Bellanger; Sylvain Huon; Philipp Steinmann; François Chabaux; Fernando Velasquez; Vincent Vallès; Kaspar Arn; Norbert Clauer; André Mariotti (1295-1314).
A freshwater reservoir (area: 122 km2, volume: 2850 × 106 m3, maximum depth: 72 m) located at the outlet of a tropical Andean watershed (Rio Boconó, 1620 km2, 08°57–09°31N, 70°02–70°34W, NW Venezuela) was studied combining: (1) high resolution monitoring of the major environmental parameters using a CTD probe; (2) dissolved (major and trace) element concentration measurements; (3) water (δ18O, δ2H) isotope measurements and; (4) organic carbon (C) concentration – δ13C measurements on suspended matter and on sediments. Composed of two sub-reservoirs with contrasted water inputs, the Peña Larga dam provides the opportunity to better constrain C budgets and source – composition of suspended organic matter within two parts of a unique freshwater system with contrasted redox conditions. The redox status is reflected by drastic differences between the two parts of the reservoir, pictured by water residence time, water column ventilation, dissolved trace element concentration and suspended organic matter composition. Seasonal renewal of water near the main inlet supports higher surface productivity, oxic water column conditions, homogenous water chemical composition and, below the thermocline, the transport of suspended organic matter mainly derived from soil erosion on the watershed. Calculated sediment and organic C storages amount to 5.6 × 106 t a−1 and 21 ± 5 × 103 t C a−1, respectively. Despite a markedly lower suspended sediment load, outflow waters exhibit POC-δ13C values similar to those in inflow waters, due to a short water residence time and a rapid recycling of primary products. In the more isolated parts of the reservoir, where water discharge is thoroughly reduced, hypoxic to anoxic conditions are found immediately below the thermocline (ca. 16 m, [O2]<30 μmol L−1). They induce a correlative increase of trace element concentrations with water depth. Suspended organic matter display 13C-depleted compositions, contrasting sharply with that of photosynthetic organisms and land-derived detritus, that either reflect the contribution of microbial biomass (chemoautotrophic and/or methanotrophic bacteria) or the selective degradation of more labile organic compounds in the water column. The hypoxic to anoxic conditions displayed in the major part of the water column extend to the first cm of lake bottom sediments. A simplified budget based on the main redox processes active in the hypolimnion and on average water residence times, supports drastic differences in mineralization rate: 83–444 μmol C L−1 a−1 for the oxic reservoir and only 43 μmol C L−1 a−1 for the hypoxic to anoxic reservoir. This study shows that, if water renewal is not sufficient, tropical freshwater lakes may be subject to severe dissolved O2 depletion conditions at shallow depths, comparable to those observed in deep sections of the water column of temperate eutrophic lakes.

This paper describes the influence of siliceous low-temperature hydrothermal fluid on the elemental concentrations and mineralogical characteristics of the late Permian anthracitic (R o,max=3.58%) coal seam (No. 30) from the Zhijin Coalfield in western Guizhou Province, SW China. Coal samples were examined using instrumental neutron activation analysis, inductively coupled plasma-mass spectroscopy, inductively coupled plasma-atomic emission spectrometry, X-ray fluorescence, powder X-ray diffraction, scanning electron microscopy equipped with energy-dispersive X-ray, and isotope analysis. The modes of occurrence of elements were determined using a sequential chemical extraction procedure. The studies indicate that the No. 30 coal seam has a very high content of veined quartz (vol. 9.4%), whose isotope values of δ30Si and δ18O are 0.6‰ and 15.4‰, respectively, indicating that the quartz originated from siliceous low-temperature hydrothermal fluid (formation temperature 160–220 °C) rather than detrital material of terrigenous origin or magmatic hydrothermal inputs. Results of scanning electron microscopy equipped with energy-dispersive X-ray and sequential chemical extraction procedure show that the veined quartz is the dominant source of Fe, Cu, U, Pd, Pt and Ir, which are as high as 2.31%, 356, 8, 2.1, 2.43, and 0.006 μg/g in this coal seam, respectively. The studies have also found that elements, such as Fe and Cu are mainly in the veined quartz and they do not occur as sulfides in this coal seam, in sharp contrast to many other coal seams in China. The geochemical and mineralogical anomalies of the coal are attributed to the siliceous low-temperature hydrothermal fluid.

Two capillary electrophoretic analysis methods are presented; one optimized for the analysis of Br and the other for determination of K+ in groundwaters of high conductivity. The water sampling was performed from the planned final disposal facility area for spent nuclear fuel at Olkiluoto in the municipality of Eurajoki, Finland. Bromide was analysed in an acidic electrolyte solution containing 5 mM formic acid and 42 mM NaCl (pH 3.5) and using direct UV detection (200 nm). Sample stacking was needed for the preconcentration. Potassium was analysed at pH 4.5 using imidazole-18-crown-6 ether solution. The accuracies of the Br and K+ methods were tested using laboratory-made reference sample mixtures with high salt concentration. In the Br analyses, the limits of detection and determination were 0.1 and 1 mg L−1, respectively. The developed CE analysis for K+ in saline water was repeatable (RSD% 14.5–18.0) and the detection and the determination limits were 0.5 and 2.0 mg L−1, respectively. The interlaboratory results showed that CE measurements of Br and K+ were compatible with those made with traditional solvent chemistry techniques.

Rare earth element partitioning between hydrous ferric oxides and acid mine water during iron oxidation by Philip L. Verplanck; D.Kirk Nordstrom; Howard E. Taylor; Briant A. Kimball (1339-1354).
Ferrous iron rapidly oxidizes to Fe (III) and precipitates as hydrous Fe (III) oxides in acid mine waters. This study examines the effect of Fe precipitation on the rare earth element (REE) geochemistry of acid mine waters to determine the pH range over which REEs behave conservatively and the range over which attenuation and fractionation occur. Two field studies were designed to investigate REE attenuation during Fe oxidation in acidic, alpine surface waters. To complement these field studies, a suite of six acid mine waters with a pH range from 1.6 to 6.1 were collected and allowed to oxidize in the laboratory at ambient conditions to determine the partitioning of REEs during Fe oxidation and precipitation. Results from field experiments document that even with substantial Fe oxidation, the REEs remain dissolved in acid, sulfate waters with pH below 5.1. Between pH 5.1 and 6.6 the REEs partitioned to the solid phases in the water column, and heavy REEs were preferentially removed compared to light REEs. Laboratory experiments corroborated field data with the most solid-phase partitioning occurring in the waters with the highest pH.