Transcontinental retroarc sediment routing controlled by subduction geometry and climate change (Central and Southern Andes, Argentina)

AbstractCentral Argentina from the Pampean flat‐slab segment to northern Patagonia (27°–41°S) represents a classic example of a broken retroarc basin with strong tectonic and climatic control on fluvial sediment transport. Combined with previous research focused on coastal sediments, this actualistic provenance study uses framework petrography and heavy‐mineral data to trace multistep dispersal of volcaniclastic detritus first eastwards across central Argentina for up to ca. 1,500 km and next northwards for another 760 km along the Atlantic coast. Although detritus generated in the Andes is largely derived from mesosilicic volcanic rocks of the cordillera, its compositional signatures reflect different tectono‐stratigraphic levels of the orogen uplifted along strike in response to varying subduction geometry as well as different character and crystallization condition of arc magmas through time and space. River sand, thus, changes from feldspatho‐litho‐quartzose or litho‐feldspatho‐quartzose in the north, where sedimentary detritus is more common, to mostly quartzo‐feldspatho‐lithic in the centre and to feldspatho‐lithic in the south, where volcanic detritus is dominant. The transparent‐heavy‐mineral suite changes markedly from amphibole ≫ clinopyroxene > orthopyroxene in the north, to amphibole ≈ clinopyroxene ≈ orthopyroxene in the centre and to orthopyroxene ≥ clinopyroxene ≫ amphibole in the south. In the presently dry climate, fluvial discharge is drastically reduced to the point that even the Desaguadero trunk river has become endorheic and orogenic detritus is dumped in the retroarc basin, reworked by winds and temporarily accumulated in dune fields. During the Quaternary, instead, much larger amounts of water were released by melting of the Cordilleran ice sheet or during pluvial events. The sediment‐laden waters of the Desaguadero and Colorado rivers then rushed from the tract of the Andes with greatest topographic and structural elevation, fostering alluvial fans inland and flowing in much larger valleys than today towards the Atlantic Ocean. Sand and gravel supply to the coast was high enough not only to promote rapid progradation of large deltaic lobes but also to feed a cell of littoral sediment transport extending as far north as the Río de la Plata estuary

Dinámica sedimentaria Neógena y Cuaternaria continental en la cuenca del arroyo Claromecó, Argentina

El registro sedimentario continental del Neógeno y Cuaternario de la cuenca interserrana bonaerense ha sido objeto de numerosos trabajos estratigráficos y bioestratigráficos. Sin embargo, aún mucho se desconoce sobre la evolución y la dinámica sedimentaria dentro de este área. A tal fin, se tomó como caso de estudio la cuenca del arroyo Claromecó, ubicada en el centro de la llanura interserrana y se realizó por primera vez un estudio que integra los aspectos geomorfológicos y sedimentológicos de las unidades aflorantes. Se establecieron así dos subciclos depositacionales. El primero, que tuvo su desarrollo entre el Mioceno tardío y el Plioceno medio-tardío, está compuesto por depósitos loéssicos y por depósitos fluviales asociados a flujos en manto y canales gravosos amalgamados. El segundo subciclo se habría desarrollado a partir del Pleistoceno tardío hasta la actualidad y está integrado por facies de canales arenosos mantiformes y facies de llanura de inundación que culminan en depósitos eólicos tipo loess asociados a las últimas glaciaciones del Cuaternario. Los dos subciclos finalizan con una etapa de estabilización (paleosuelos) que se relaciona con una disminución del espacio de acomodación. Sin embargo, es luego del primer subciclo cuando tiene lugar una etapa de erosión y no depositación cuya duración estimada sería no menor a 500 ka. Estos resultados muestran diferencias en la jerarquía y en los factores de control de cada uno de los subciclos sedimentarios

Occurrence of Arsenic in Soils and Paleosols of the Claromecó River Basin, Southern Pampean Plain (Argentina)

The Chacopampean plain is an extended flatland characterized by high As within sediments anddrinking waters. The volcanic glass shards, normally present as a major constituent of the Chacopampean plain sediments, are classically considered the main source of As. Nevertheless, thick volcanic ash layers and Fe-Mn (Al) oxy-hydroxides are also contemplated as a reasonable source of As in the sediments. In order to understand the main source of As in the Claromecó river basin (southern Chacopampean plain) sediments, paleosols, present-day soils and groundwaters were sampled. Three sedimentological units were identified: a deeper early Pliocene fluvial unit (mean sediment As~ 4.6 mg/kg), a shallower late Pleistocene fluvial unit (mean As ~ 11.6mg/kg) and Holocene Loess (mean As~ 2.5 mg/kg). Two types of paleosols were characterized: a Pliocene pedogenic calcrete (mean As∿ 3.6 mg/kg) and a Late Pleistocene hydromorphic paleosol (mean As∿ 16.5 mg/kg). The present-day soils were taken into account where mean As is ∿ 10.5 mg/kg. Although mean values for each unit are quite different, statistical analyses (ANOVA) reveal no statistically significant difference between As concentrations within the various sedimentological units. However, the hydromorphic paleosols and present day soils show a statistically significant difference from the rest of the units. In these pedogenetic units, the highest As concentrations are located in the Fe-Mn(Al) oxy-hydroxide phases (nodules and rhizo-concretions) as shown by sequential extractions and μ-XRF analysis. From this context, Pliocene pedogenic calcrete shows low content of oxy-hydroxides and therefore low As. Additionally, As concentrations in waters (mean As∿ 78.42 μg/L) shows higher values in shallower groundwaters coinciding with Pleistocene paleosols. This preliminary study highlights that the sediment deposition may not necessarily represent a systematic control in the As concentrations even though an increment from the Pliocene to the Holocene in sediment’s mean As values is noticed. However, the late Pleistocene hydromorphic soils, most likely linked to a shift in the climatic conditions, favored the concentration of As within Fe-Mn (Al) oxy-hydroxides phases. These pedogenetic features could represent the principal source of As enrichment in groundwaters

Global effects of land use on local terrestrial biodiversity

Human activities, especially conversion and degradation of habitats, are causing global biodiversity declines. How local ecological assemblages are responding is less clear--a concern given their importance for many ecosystem functions and services. We analysed a terrestrial assemblage database of unprecedented geographic and taxonomic coverage to quantify local biodiversity responses to land use and related changes. Here we show that in the worst-affected habitats, these pressures reduce within-sample species richness by an average of 76.5%, total abundance by 39.5% and rarefaction-based richness by 40.3%. We estimate that, globally, these pressures have already slightly reduced average within-sample richness (by 13.6%), total abundance (10.7%) and rarefaction-based richness (8.1%), with changes showing marked spatial variation. Rapid further losses are predicted under a business-as-usual land-use scenario; within-sample richness is projected to fall by a further 3.4% globally by 2100, with losses concentrated in biodiverse but economically poor countries. Strong mitigation can deliver much more positive biodiversity changes (up to a 1.9% average increase) that are less strongly related to countries' socioeconomic status

The database of the PREDICTS (Projecting Responses of Ecological Diversity In Changing Terrestrial Systems) project

The PREDICTS project-Projecting Responses of Ecological Diversity In Changing Terrestrial Systems (www.predicts.org.uk)-has collated from published studies a large, reasonably representative database of comparable samples of biodiversity from multiple sites that differ in the nature or intensity of human impacts relating to land use. We have used this evidence base to develop global and regional statistical models of how local biodiversity responds to these measures. We describe and make freely available this 2016 release of the database, containing more than 3.2 million records sampled at over 26,000 locations and representing over 47,000 species. We outline how the database can help in answering a range of questions in ecology and conservation biology. To our knowledge, this is the largest and most geographically and taxonomically representative database of spatial comparisons of biodiversity that has been collated to date; it will be useful to researchers and international efforts wishing to model and understand the global status of biodiversity

The database of the PREDICTS (Projecting Responses of Ecological Diversity In Changing Terrestrial Systems) project

The PREDICTS project—Projecting Responses of Ecological Diversity In Changing Terrestrial Systems (www.predicts.org.uk)—has collated from published studies a large, reasonably representative database of comparable samples of biodiversity from multiple sites that differ in the nature or intensity of human impacts relating to land use. We have used this evidence base to develop global and regional statistical models of how local biodiversity responds to these measures. We describe and make freely available this 2016 release of the database, containing more than 3.2 million records sampled at over 26,000 locations and representing over 47,000 species. We outline how the database can help in answering a range of questions in ecology and conservation biology. To our knowledge, this is the largest and most geographically and taxonomically representative database of spatial comparisons of biodiversity that has been collated to date; it will be useful to researchers and international efforts wishing to model and understand the global status of biodiversity