Environmental impact of heavy-metal dispersion in the Huerva River (Iberian Range,NE Spain)

In this study, the methodology of geochemical prospecting has been applied to the study of cadmium, lead, copper and zinc dispersion in the fine sediments of the upper course of the Huerva River (Iberian Range, NE Spain). The survey was organised in two stages: (1) general sampling along the river and its tributary channels, aimed at selecting the sections with the highest contents of the target elements, and (2) detailed study of those sections, aimed at an interpretation of possible positive geochemical anomalies and identification of their sources. The results of this study prove the existence of multiple and complex sources, with a significant influence of sources related to anthropogenic activity. However, the system shows a good ability to depurate itself under current conditions.     

Surface-water quality for the Salí River watershed in NW Argentina

Spatial and temporal variations of Na, Mg, Si, K, Ca, SO₄^2-, Cl^-, HCO₃^-, Ag, Al, As, Au, B, Ba, Be, Bi, Br, Ce, Cd, Co, Cr, Cs, Cu, Dy, Er, Eu, F, Fe, Ga, Ge, Gd, Hf, Hg, Ho, I, La, Li, Mn, Mo, Nb, Nd, Ni, P, Pb, Pr, Rb, Sb, Sm, Sn, Sr, Ta, Tb, Te, Th, Ti, Tl, U, V, W, Y, Yb, Zn, and Zr were monitored through 37 sampling stations to determine the main aspects influencing the surface-water quality in the Salí River watershed (Tucumán Province, NW Argentina). The influence of the regional geological setting on water chemistry allows to distinguish three sub-basins. The interaction with sedimentary rocks was found to be dominant in the northern and central-eastern sub-basin as well as in the southern sub-basin, whereas the metamorphic-granitic basement of the Pampean Ranges was noted in the central sub-basin. In addition, anthropogenic activities affect the spatial variation of K, P, Mn, Rb, and Pb as well as dissolved oxygen concentrations and Eh. Temporal water-quality variation is related to the spatial distribution of precipitation and to the seasonal character of the main local industries (sugar cane, alcohol, citrus), increasing P and K concentrations and decreasing dissolved oxygen concentration and Eh in winter. Cl^-, Na, SO₄^2-, Al, As, B, Fe, Mn, Se and U concentrations exceed the regulated drinking-water thresholds at several sampling stations.     

Separate lead isotope analyses of leachate and residue rock fractions: implications for metal source tracing in ore deposit studies

Separate lead isotope analyses of leachate and residue fractions are applied to a broad spectrum of rocks commonly investigated in metallogenic studies. Resulting data highlight a systematic behavior of leachate and residue fractions with respect to lead isotope compositions, which essentially depends on the mineralogical composition of the rock. Granitoid and high-grade metamorphic rocks have residue compositions virtually identical to common lead. In contrast, low-grade metasedimentary rocks may have residue compositions swamped by radiogenic lead of leach-resistant zircons. Mafic magmatic rocks have residues that are often more radiogenic than leachates, depending on the ratio of leach-refractory zircons to common lead in the residual fraction of these rocks. Separate leachate and residue analyses of source rocks provide two lead isotope end members whose mixture may represent lead with the appropriate ore fluid composition. Our leaching experiments indicate that hot acid solutions (and by inference hydrothermal fluids) preferentially leach radiogenic lead from medium- to high-grade metamorphic and granitoid rocks, whereas they preferentially leach common lead from low-grade metasedimentary and mafic magmatic rocks. The method presented in this study provides a reliable alternative to other methods (i.e., age-correction of bulk-rock compositions) for the determination of the common lead signature of felsic to intermediate magmatic rocks. This may be preferable to age-corrected bulk-rock analyses, where ages to apply for corrections of bulk-rock data are not known or where moderately to highly altered rocks must be used. Case studies of orogenic gold and MVT districts of Peru (Pataz and San Vicente, respectively) show that separate leachate and residue lead isotope analyses carried out systematically on whole rocks allow a more thorough evaluation of metal source reservoirs than does the standard method of age-corrected or uncorrected bulk-rock analyses.     

Predictive GIS-Based Model of Rockfall Activity in Mountain Cliffs

Rockfall susceptibility has been analysed in mountain cliffs of the Cantabrian Range, North Spain. The main aim of this analysis has been to build a predictive model of rockfall activity from a low number of environmental and geological variables. The rockfall activity has been quantified in a GIS. The cartographic information used shows the spatial distribution of all the recent talus screes as well as their associated source areas in the rock-slopes. The area relation At/Ar (recent talus scree polygon/source basins) in the rock slopes has been used as the rockfall activity indicator. This relation has been validated in 50 pilot rock-slopes and compared with the relation number of recent rock fragments/source basin, obtained from field work. The environmental factors causing rockfall depend on the rock slope situation, and these are: altitude and sun radiation on the rock cliff. The geological factors considered are: lithology, relative position of the main discontinuities with respect to the topographic surface and two morphologic parameters: the roughness and slope gradient. A logistic regression analysis has been applied to a population of 442 limestone and quartzite rock cliffs. The dependent variable is the rockfall activity indicator, which allows the definition of two classes of rock cliff units: low and high activity. The independent variables are altitude, sun radiation (equinox radiation, summer solstice radiation, winter solstice radiation), slope roughness, slope gradient,anisotropy and lithology. Results suggest that it is possible tobuild a valid cartographic predictive model for rockfall activity in mountain rock cliffs from a limited number of easily obtainable variables. The method is especially applicable in massive rock slopes or in regions with uniform rock mass characteristics.     

The red bed controversy revisited: shape analysis of Colorado Plateau units suggests long magnetization times

Several Triassic and earliest Jurassic sedimentary units from the Colorado Plateau region have distributions of virtual geomagnetic poles (VGPs) that are highly elongate along the path of apparent polar wander (APW). This suggests that the remanent magnetizations measured in these units were acquired over an extended period of time, possibly approaching 35 m.y., and are not precisely coeval with the stratigraphic age of the rock. Comparison with other paleomagnetic studies shows that the observed elongation is not a general attribute of the age of the rock, nor is it related to paleolatitude. The rocks that yield elongate VGP distributions are dominantly red to brown mudstones, and it is possible that their remanence is dominated by a slowly acquired chemical remanent magnetization, as suggested by Larson et al. [J. Geophys. Res. 87 (1982) 1081] and other authors. However, several superficially similar units from the Colorado Plateau have nearly circular VGP distributions. The process by which remanence is acquired in clastic sedimentary rocks merits further study.     

Absolute velocity of North America during the Mesozoic from paleomagnetic data

We use paleomagnetic data to map Mesozoic absolute motion of North America, using paleomagnetic Euler poles (PEP). First, we address two important questions: (1) How much clockwise rotation has been experienced by crustal blocks within and adjacent to the Colorado Plateau? (2) Why is there disagreement between the apparent polar wander (APW) path constructed using poles from southwestern North America and the alternative path based on poles from eastern North America? Regarding (1), a 10.5° clockwise rotation of the Colorado Plateau about a pole located near 35°N, 102°W seems to fit the evidence best. Regarding (2), it appears that some rock units from the Appalachian region retain a hard overprint acquired during the mid-Cretaceous, when the geomagnetic field had constant normal polarity and APW was negligible.We found three well-defined small-circle APW tracks: 245–200 Ma (PEP at 39.2°N, 245.2°E, R=81.1°, root mean square error (RMS)=1.82°), 200–160 Ma (38.5°N, 270.1°E, R=80.4°, RMS=1.06°), 160 to 125 Ma (45.1°N, 48.5°E, R=60.7°, RMS=1.84°). Intersections of these tracks (the “cusps” of Gordon et al. [Tectonics 3 (1984) 499]) are located at 59.6°N, 69.5°E (the 200 Ma or “J1” cusp) and 48.9°N, 144.0°E (the 160 Ma or “J2” cusp). At these times, the absolute velocity of North America appears to have changed abruptly.North America absolute motion also changed abruptly at the beginning and end of the Cretaceous APW stillstand, currently dated at about 125 and 88 Ma (J. Geophys. Res. 97 (1992b) 19651). During this interval, the APW path degenerates into a single point, implying rotation about an Euler pole coincident with the spin axis.Using our PEP and cusp locations, we calculate the absolute motion of seven points on the North American continent. Our intention is to provide a chronological framework for the analysis of Mesozoic tectonics. Clearly, if APW is caused by plate motion, abrupt changes in absolute motion should correlate with major tectonic events. This follows because large accelerations reflect important changes in the balance of forces acting on the plate, the most important of which are edge effects (subduction, terrane accretion, etc.). Some tectonic interpretations: (1) The J1 cusp may be associated with the inception of rifting of North America away from land masses to the east; the J2 cusp seems to mark the beginning of rapid spreading in the North Atlantic. (2) The J2 cusp signals the beginning of a period of rapid northwestward absolute motion of western North America; motion of tectonostratigraphic terranes in the westernmost Cordillera seems likely to have been directed toward the south during this interval. (3) The interval 88 to 80 Ma saw a rapid decrease in the paleolatitude of North America; unless this represents a period of true polar wander, terrane motion during this time should have been relatively northward.     

Rivers, chemical weathering and Earth's climate

We detail the results of recent studies describing and quantifying the large-scale chemical weathering of the main types of continental silicate rocks: granites and basalts. These studies aim at establishing chemical weathering laws for these two lithologies, describing the dependence of chemical weathering on environmental parameters, such as climate and mechanical erosion. As shown within this contribution, such mathematical laws are of primary importance for numerical models calculating the evolution of the partial pressure of atmospheric CO₂ and the Earth climate at geological timescales. The major results can be summarized as follow: (1) weathering of continental basaltic lithologies accounts for about 30% of the total consumption of atmospheric CO₂ through weathering of continental silicate rocks. This is related to their high weatherability (about eight times greater than the granite weatherability); (2) a simple weathering law has been established for basaltic lithologies, giving the consumption of atmospheric CO₂ as a function of regional continental runoff, and mean annual regional temperature; (3) no such simple weathering law can be proposed for granitic lithologies, since the effect of temperature can only be identified for regions displaying high continental runoff; (4) a general law relating mechanical erosion and chemical weathering has been validated on small and large catchments. The consequences of these major advances on the climatic evolution of the Earth are discussed. Particularly, the impacts of the onset of the Deccan trapps and the Himalayan orogeny on the global carbon cycle are reinvestigated. To cite this article: B. Dupré et al., C. R. Geoscience 335 (2003).     

In-situ Formation of Light-Absorbing Organic Matter in Cloud Water

Current climate models seem to underestimate the flux of solar energy absorbed by the global troposphere. All of these models are constrained with the assumption that cloud droplets consist of pure water. Here we demonstrate in a simple laboratory experiment that aromatic hydroxy-acids which are found in continental fine aerosol can react with hydroxyl radicals under typical conditions prevalent in cloud water influenced by biomass burning. The reactions yield colored organic species which do absorb solar radiation. We also suggest that the products of such reactions may be humic-like substances whose presence in continental aerosol has been confirmed but their source mechanisms are still much sought after. We also attempt to give a first order estimate of the enhancement of water absorption at a visible wavelength under atmospheric conditions.     

THE RETREAT OF THE FOREST IN SOUTHEAST ASIA: A CARTOGRAPHIC ASSESSMENT

Rapid deforestation is a major problem throughout the tropical world. The conditions and the pace under which societies and economies of the Third World are currently evolving and growing, combined with the specificities of tropical forests, render the latter increasingly vulnerable. Among the major tropical areas of the world, Southeast Asia is perhaps the one where these conditions have had the most impact on the retreat of the forest cover over the last quarter of this century. This is illustrated through the presentation of two maps of the distribution of five basic forest formations in Southeast Asia circa 1970 and circa 1990. The maps are examined and compared, as well as confronted with statistical assessments of deforestation. Finally, the complex causes behind the retreat of the tropical forests as well as the implications of this retreat are briefly discussed.