Mercury mobility at the Carson River Superfund Site, west-central Nevada, USA: Interpretation of mercury speciation data in mill tailings, soils, and sediments

The Carson River Superfund Site in west-central Nevada is an area of Hg-contaminated soil, sediment, water, air, and biola resulting from the amalgamation milling of Ag-Au ores of the Comstock lode worked approximately a century ago. In order to develop an understanding of the behavior, transport, and fate of Hg at this site, a technique was developed to estimate the proportions of total, elemental, exchangeable, organic, and sulfide Hg in soils, sediments, and tailings.Results of this analysis performed on active Carson River sediments indicate that Hg is selectively dissolved out of Hg-Au amalgam particles and subsequently adsorbed to fine-grained sediments which are then deposited in downstream, low-energy reaches of the Carson River and Labontan Reservoir. In the relatively more-reducing environment of the reservoir Hg appears to be converted, in large part, to relatively-insoluble HgS.The original elemental form of Hg released to the environment is the chemical form which is still dominant in most highly-contaminated soils, sediments, and tailings. Deeper, more-reducing soil horizons, however, appear to fix a significant portion of the Hg as HgS, analogous to the Lahontan Reservoir example described above. This fixation as HgS is documented to be largely limited to higher-sulfur areas where sulfide minerals from the Comstock ores increase the total sulfur concentrations of contaminated soils, sediments, and tailings.     

Colonization and evolution of lakes on the central Norwegian coast following deglaciation and land uplift 9500 to 7800 years B.P.

Invertebrate colonization of lakes following the uplift of land from the sea was studied in four lakes, currently situated between 39 and 24 m a.s.l., on the central Norwegian coast. The lakes were isolated from the sea between 9500 and 7700 years B.P. Animal and algal remains picked from core samples showed that the first colonizers preserved as fossils were usually members of the Chironomidae, Daphnidae/Chydoridae, Acarina, Porifera (Ephydatia mülleri and Spongilla lacustris), Bryozoa (Cristatella mucedo and Plumatella spp.) and Charophyta (Chara sp.). Of the chironomids, the genus Chironomus was present in the oldest lacustrine layers of all four lakes, but other genera recorded at the marine/lacustrine boundary were Dicrotendipes, Procladius (?), Einfeldia, Microtendipes, and Glyptotendipes. Remains of the caddis fly family Limnephilidae were also present in the earliest lacustrine sediments in Kvennavatnet and Kvernavatnet. The oldest invertebrate fauna is typical for mesotrophic lakes. However, chironomids and mites have been present in this area from at least about 10?500 years B.P. A diverse chironomid community was established between 300 and 800 years after isolation from the sea at Kvernavatnet on the island of Hitra, while only between 80 and 120 years passed before a comparably diverse community developed at Kvennavatnet on the mainland coast. A similar development of the invertebrate fauna occurred in Kvennavatnet, Kvernavatnet and Storkuvatnet. However, Litjvatnet deviates greatly from the ‘normal’ pattern because a tsunami disturbed the bottom sediments and fauna. The tsunami, a gigantic sea wave, was caused by a submarine slide from the Norwegian continental slope. It reached Litjvatnet, today located 24 m a.s.l., but was not traced in Storkuvatnet at 30 m a.s.l. This event happened about 7200 years B.P.     

Comment on: 'Can horizontal P waves be trapped and resonate in a shallow sedimentary basin?' by F. J. Sácnhez-Sesma and F. LuzÓ

We show that most of the arguments in the above paper are either incorrect or irrelevant to the point the authors are trying to make. We show that their results have no bearing on the model proposed by our group, as they claim. They discuss the seismic response of a valley with a 2-D trapezoidal cross-section in a vertical plane, whereas we dealt with a closed basin with a 2-D cross-section but of arbitrary geometry and in the horizontal plane. Even more significantly, the width of the valley they use is much smaller than the wavelength of the horizontal P waves that can resonate, thereby precluding any possibility of them being trapped. Therefore, their arguments do not clarify the issue posed in the title of their article.     

Geotechnical characterisation of Obudu damsite, Obudu, south-eastern Nigeria

The Obudu dam is being built across Abeb river in Obudu area of the Cross River State (Nigeria). The earthfill dam will be approximately 18 m high with a crest length of 385 m. The dam site is located within part of the Obudu crystalline basement plateau which is a region of low seismicity. The terrain is smoothly undulating and low lying and was known to be composed of unclassified basement and decomposed bedrock (overburden). The present study was carried out to assess the suitability of the chosen dam axis based on the determination of the nature and geotechnical characteristics of the overburden and bedrock. The investigation included geophysical surveys, bearing capacity tests (cone penetrometer and standard penetration), classification and grain size distribution as well as tests for compaction, consolidation and compressive strength. The results show that the bedrock is heterogeneous, including gneisses sillimanite, biotite and granite types), dolerite, quartzite and pegmatite with an overall moderate strength (about 76.04 MN m⁻² average) and fair rock mass rating (RMR). The bedrock along the dam axis is apparently lacking in major fractures which could lead to short circuiting of the future impoundment. The overburden (soil) comprises silt (MH, ML), silty clay (CL) and silty sand (SM) with a combined thickness of about 2–20 m, increasing away from the valley floor towards the shoulders. Generally the overburden is of suitable compressive strength (150–300 KN m⁻²), low to medium plasticity and swelling potential and low permeability (up to 1.41 × 10⁻⁷ ms⁻¹) which would ensure a tight reservoir. The material settlement is expected to be small and slow. Accordingly, excavation of up to 5 m, decreasing towards the valley shoulders has been suggested to expose the recommended bearing medium: fresh/competent bedrock and overburden in the valley floor and flanks, respectively. While a detailed investigation of burrow areas was not part of the present study, a few potential sites in the dam axis and reservoir areas have been suggested based on the evaluation of material properties.     

The age and thermal history of Cerro Rico de Potosi, Bolivia

 Cerro Rico de Potosi, Bolivia, is the world’s largest silver deposit and has been mined since the sixteenth century for silver, and for tin and zinc during the twentieth century, together with by-product copper and lead. The deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia. The deposit is compositionally and thermally zoned, having a core of cassiterite, wolframite, bismuthinite, and arsenopyrite surrounded by a peripheral, lower-temperature mineral assemblage consisting principally of sphalerite, galena, lead sulfosalt, and silver minerals. The low-temperature assemblage also was superimposed on the high-temperature assemblage in response to cooling of the main hydrothermal system. Both the dacite dome and the ore fluids were derived from a larger magmatic/hydrothermal source at depth. The dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement. The dome was extruded at 13.8±0.2 Ma (2σ), based on U-Th-Pb dating of zircon. Mineralization and alteration occurred within about 0.3 my of dome emplacement, as indicated by a ⁴⁰Ar/³⁹Ar date of 13.76±0.10 Ma (1σ) for sericite from the pervasive quartz-sericite-pyrite alteration associated with the main-stage, high-temperature, mineralization. The last thermal event able to reset zircon fission tracks occurred no later than 12.5±1.1 Ma (1σ), as indicated by fission-tract dating. Minor sericite, and magmatic-steam alunite veins, were episodically formed around 11 Ma and between 8.3 and 5.7 Ma; the younger episodes occurring at the time of extensional fracturing at Cerro Rico and widespread volcanism in the adjacent Los Frailes volcanic field. None of these younger events appear to be significant thermal/mineralizing events; the exceptionally flat thermal release pattern of ³⁹Ar from sericite and the results of the fission-tract dating of zircon show that none of the younger events was hot enough, and lasted long enough, to cause significant loss of Ar or annealing of zircon fission tracks. U-Th-Pb dating of zircon cores indicates a Precambrian progenitor for some zircons, and REE analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive Eu anomaly, which constrains interpretations of the origin and evolution of the magmatic/hydrothermal system. Received: 14 October 1995/Accepted: 29 January 1996     

Porewater pressure increases in soil and rock from underground chemical and nuclear explosions

A review and analysis of chemical and nuclear explosive-induced porewater pressure increases and induced rise in groundwater table elevations (groundwater mounding) is presented. Our analysis indicates that residual pore pressure increases and groundwater mounding can be induced by underground chemical and nuclear explosions to scaled distances of 879 m/(kt)^1/3. This relationship is linear over seven orders of magnitude of explosive energy ranging from a 0.01 kg chemical explosion to a 100 kt nuclear explosion and is valid for a wide variety of saturated geological profiles. Underground chemical explosions, and probably underground nuclear explosions have the potential to induce liquefaction of water-saturated soils to scaled distances of about 260 m/(kt)^1/3.     

Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil

High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser ⁴⁰Ar/³⁹Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.704 and ¹⁴³Nd/¹⁴⁴Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C.     

Ophiolite remnants at the eastern margin of the Bohemian Massif and their bearing on the tectonic evolution

Summary Amphibolites are widespread in the eastern part of the Bohemian Massif, east of the South Bohemian Pluton. Based on their geological situation, their metamorphic evolution and their geochemistry, they were separated into three genetically different units: the Rehberg ophiolite, the Buschandlwand amphibolite and the Raabs group. The metamorphic Rehberg ophiolite consists of a ultramafic to mafic plutonic sequence overlain by a gabbro/dike complex and a volcanic section with basaltic, andesitic and rhyolitic volcanics associated with pelitic to psammitic sediments. The entire ophiolite underwent amphibolite facies metamorphism. Rock/MORB normalisation plots and other element ratio plots, such as Ti vs. V or Ta/Yb vs. Th/Yb argue for a supra-subduction zone environment as site of the origin of the Rehberg ophiolite. The Letovice ophiolite in Moravia is structured in a similar way. Geochemically it shows a more MORB like composition but also a distinct tholeiitic island arc group. Although there is no age data for the protolith of both ophiolites, it is believed that they have formed in an oceanic basin separating the Brunovistulian-Moravian block to the east and the Moldanubian block to the west. Possible traces of the continuation of the ophiolites towards the north can be found in the Stare Mesto ophiolites and the ophiolites surrounding the Sowie Gory block in the Sudetes.

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Ophiolitrelikte am ostrand der Böhmischen Masse und ihre bedeutung für die tektonische entwicklung

Zusammenfassung Amphibolite sind in der Böhmischen Masse, östlich des Südböhmischen Plutons weit verbreitet. Ausgehend von ihrer geologischen Position, ihrer metamorphen Entwick lung und ihrer Geochemie lassen sie sich in drei genetisch unterschiedliche Einheiten teilen: den Rehberg Ophiolith, den Buschandlwand Amphibolit und die Raabser Gruppe. Der metamorphe Rehberg Ophiolith besteht aus einer ultramatischen bis mafischen plutonischen Abfolge, die von einem Gabbro/Gangkomplex überlagert wird und einer vulkanischen Folge mit Basalten, Andesiten and Rhyoliten, die mit tonigen und sandigen Sedimenten verknüpft sind. Der gesamte Ophiolithkörper wurde unter amphibolitfaziellen Bedingungen metamorph überprägt. Rock/MORB Diagramme und andere Elementdarstellungen, wie z.B. das Ti vs. V, oder das Ta/Yb vs. Th/Yb Diagramm sprechen für eine Entstehung des Rehberg Ophioliths in einer Suprasubduction Zone. Der Letovice Ophiolith in Mähren weist eine ähnliche Struktur auf. Geochemisch zeigt er stärkere Ähnlichkeiten mit MORB, enthält aber auch Elemente von tholeiitischen Inselbogenbasalten. Auch wenn es für beide Ophiolithe noch keine Protolith-Altersdaten gibt, scheint es dennoch wahrscheinlich, daß sie in einem Ozeanbecken, das den Brunovistulisch-Moravischen Kontinentalblock im Osten vom Moldanubischen Kontinent im Westen trennt, gebildet wurden. Die mögliche Fortsetzung beider Ophiolithe könnten im Norden der Stare Mesto Ophiolith und die Ophiolithe rund um den Sowie Gory Block in den Sudeten darstellen.

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With 7 Figures