Pedro J. Depetris

Material Sources, Chemical Weathering, and Physical Denudation in the Chubut River Basin (Patagonia, Argentina): Implications for Andean Rivers

The Chubut is a medium‐size (42,000 km2) river basin that drains the arid‐to‐semiarid Patagonian seaboard and pours its waters into the southwestern Atlantic Ocean (ca. lat 43°20′S, long 65°04′W). The materials eroded from the continent and deposited in the sea are scarcely affected by chemical weathering (the chemical index of alteration of riverbed sediments is ∼55) and bear a typical chemical and mineralogical signature characteristic of volcanic arcs. Clearly, flowing toward a passive margin, the river carries the mineralogical and chemical signature of an active margin. Physically weathered andesites and basalts occupy only about 25% of the drainage area, and therefore most exported material must be supplied by outcropping sedimentary beds of variable age. The Chubut River headwaters are placed in a tectonically active region, soil formation is incipient (“weathering‐limited regime”), and the rate of denudation (24.6 t km−2 yr−1) is much lower than the rates exhibited by similar rivers in other parts of the world. The depleted dissolved and particulate load is determined by scarce atmospheric precipitations (i.e., the drainage basin is in the Andean rain shadow) and by the protective effect of Cenozoic lava flows that often shield sedimentary formations from denudation. Although the index of chemical variability suggests that materials exported are products of the first denudational cycle, the geological history supports the view that most materials may have passed two or even three times through the exogenous cycle without acquiring a chemical or mineralogical signature indicative of repeated weathering. This is probably also true for other basins in temperate Andean climates.

Weathering and the Riverine Denudation of Continents

Denudation, volcanism, and tectonics are intertwined Earth system processes that constitute the main driving forces intervening in shaping the Earth’s landscape. Clearly, the wearing away of the Earth’s surface cannot occur unless a series of synergistic processes, collectively known as ‘‘weathering,’’ are initiated. This term, in use for a long time, promotes the idea that climate (weather) always plays a major role in rock breakdown; since this is not the case in every instance the change for ‘‘rock decay’’ has been proposed recently. At any rate, the linkage between weathering and denudation is not straightforward because the latter may be limited by the former (‘‘weathering-limited denudation’’) or, in contrast, it may be restricted by the hindered transport of the weathering-produced debris (‘‘transport-limited denudation’’). In addition to these possible scenarios, two new approaches have been gaining growth in the recent past: one is the study of the ‘‘regolith’’ as a convenient research framework, and the other is the notion of ‘‘the critical zone,’’ where the dynamic interaction with the atmosphere and vegetation is emphasized and added to the materials collectively defined as regolith.

Hydrochemical considerations and heavy metal variability in the middle Paraná River

The Paraná River is one of the largest drainage systems in the Americas. Its hydrology is characterized by an active teleconnection with the ENSO, and by a significant discharge increase trend, evident since the mid-1970s. An Eh–pH data set collected in the Paraná’s middle stretch suggests that large flood events, such as the one triggered by the 1982–1983 ENSO, are discernible in the plot, probably due to the influx of water draining flood plain water bodies. The total (particulatexa0+xa0dissolved) concentration of a set of heavy metals (Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb) was determined in a downriver survey of the middle stretch. With the exception of Cu, Cd, and Pb, the metals exhibit a significantly increasing concentration trend towards the river mouth. The slopes of the regression lines imply that Zn and Ni, on one hand, and Mn and Cr, on the other would have common controlling sources. Another set of analyses were performed during the 1982–1983 flooding event; besides an increased variability observable during the flood arrival, most elements, with the only exception of Pb, did not show a variability coherent with the discharge series.

Ra and Rn isotopes as natural tracers of submarine groundwater discharge in the patagonian coastal zone (Argentina): an initial assessment

Submarine groundwater discharge (SGD) is herein recognized as a significant pathway of material transport from land to the coastal SW Atlantic Ocean and thus, it can be a relevant factor affecting the marine biogeochemical cycles in the region. This paper focuses on the initial measurements of 226Ra, 228Ra and 222Rn made in Patagonia’s coastal zone of Chubut and Santa Cruz provinces (42°S–48°S, Argentina). 226Ra activity ranged from 2.9 to 73.5xa0dpm 100xa0L−1, and 228Ra activity ranged from 11.9 to 311.0xa0dpm 100xa0L−1 in groundwater wells. The radium activities found in Patagonia’s marine coastal regions and adjacent shelf indicate significant enrichment throughout the coastal waters. Groundwater samples presented the largest 222Rn activity and ranged from 2.66 to 1083xa0dpmxa0L−1. Conversely, in the coastal marine environment, the 222Rn activity ranged from 1.03 to 6.23xa0dpmxa0L−1. The Patagonian coastal aquifer showed a larger enrichment in 228Ra than in 226Ra, which is a typical feature for sites where SGD is dominant, probably playing a significant role in the biogeochemistry of these coastal waters.

Viscosity improvement of an Argentine kaolin by ionic treatment

Abstract Although kaolins from the provinces of Chubut and Santa Cruz (in Argentinas Patagonia) exhibited some promising characteristics for their use as paper-coating pigments, the rheological properties of their high solids suspensions preclude their industrial utilization. Verwey-Overbeeks theory provides the information necessary to improve the flow properties of suspensions by means of ionic treatment. Saturation with a monovalent ion — such as Na + — generates an energy barrier which prevents flocculation and significantly improves the viscosity of suspensions so that they compare favorably with coating clays from Georgia in the U.S.A.

Factors affecting the physical and chemical properties of Argentine kaolins

Abstract A number of physical and chemical parameters determined on a set of kaolin samples from the provinces of Chubut and Santa Cruz (Argentina) have been evaluated with a multivariate statistical method, to interpret the factors controlling the variation in their physical and chemical properties. Q-mode factor analysis showed that two factors account for almost 100% of the variance in the analysis. They represent brightness and particle size which classifies the samples between these two end members of an ideal series. R-mode, on the other hand, demonstrates that four factors account for over 82% of the total variance. They represent the control of the flow properties of suspensions by the chemical properties (Factor 1, with 33%); the control of additional flow properties by means of particle size distribution (Factor 2, with 18.8%); the factor affecting brightness (Factor 3, with 14.4%); and abrasiveness (Factor 4, with 15.8%). These factors indicate the main characteristics affecting their paper coating properties and suggest, as well, the main parameters to be measured in future surveys of the deposits.

Foreword to the special issue on I Reunión Argentina de Geoquímica de la Superficie (I RAGSU) (First Argentine meeting on geochemistry of the earth’s surface)

Geochemical understanding can be traced as far back as prehistory with the development of bronze and iron tools. This initiated a path that eventually led to the current knowledge on metal identification and processing. The geochemical discipline, however, has its origin hand-in-hand with geological science; first, during the 19th century when analytical chemistry and later physical chemistry promoted the superlative geochemical knowledge growth seen today. As is generally known, there are a number of diverse specialties within the wide field encompassed by geochemistry. One of them is the geochemistry of earth’s surface processes, which is mainly oriented to understand the chemical mechanisms that take place in the ‘‘critical zone’’, the relatively thin layer where the interaction of the atmosphere, biosphere, hydrosphere, and lithosphere, takes place. On the other hand, the current environmental crisis has triggered a large number of scientific studies worldwide, which mainly seek to unveil anthropogenic environmental problems and find corrective measures to minimize undesirable impacts. The first meeting, I RAGSU, was envisioned to probe into the geochemical nature of the processes occurring on the surface of the earth and took place at the city of Cordoba (Argentina), between the 27th and the 30th of September, 2009. The meeting was hosted by Argentina’s Academia Nacional de Ciencias (http://www.acad.uncor. edu/) and was jointly organized by the Academy and the Centro de Investigaciones en Ciencias de la Tierra (CICTERRA, http://www.cicterra-conicet.gov.ar/) and the Centro de Investigaciones Geoquimicas y de Procesos de la Superfice (CIGeS, http://www.efn.unc.edu.ar/investigacion/ ciges/). Although organized by earth scientists, the goal was to assemble as many environmental scientists as possible (e.g., biologists, agronomists, chemists, geologists). Almost 90 scientists and advanced students attended the meeting at Argentina’s Academia Nacional de Ciencias. Sixty-two research papers were presented, either as oral presentations (27) or as posters (35). Initially conceived as a national get-together of Argentine surface geochemists, it was expanded to include attendees from neighboring nations (e.g., Brazil, Chile), Europe (France, Switzerland), and the Americas (e.g., USA, Venezuela). Daniel Ariztegui (University of Geneva, Switzerland); Jay A. Brandes and Herbert L. Windom (Skidaway Institute of Oceanography, Savannah, Georgia, USA); Luis Felipe Niencheski (FURG, Rio Grande, Brazil); Sandor Muslow (Universidad Austral de Chile, Concepcion, Chile); and Christine Vallet-Coulomb (Aix-Marseille Universite, Aix en Provence, France) were in charge of six key presentations. Seven oral and poster presentations covered the following topics: (1) environmental geochemistry, (2) hydrogeochemistry, (3) biogeochemistry, (4) marine geochemistry, (5) isotopic geochemistry, (6) sedimentary and soil geochemistry, (7) analytical geochemistry. Within the framework of the meeting, three short actualization courses were given: ‘‘Introduction to weathering processes’’, ‘‘Geochemistry of the solid–liquid interface’’, and ‘‘Biogeochemistry: origins and global cycles’’. Discussions were interesting and lively, and the initial icebreaker as well as the final dinner gave the opportunity to exchange views, socialize, and program future jointventures. I RAGSU was the initial step in what is visualized as the beginning of a long sequence of regional P. J. Depetris A. I. Pasquini (&) Centro de Investigaciones en Ciencias de la Tierra (CICTERRA-CIGeS, CONICET-UNC), Av. Velez Sarsfield 1611, X5016GCA Cordoba, Argentina e-mail: apasquini@com.uncor.edu