Cuantificación de la tasa de denudación de los últimos 20 Ka utilizando nucleídos cosmogénicos (10Be) en el suroeste de los Andes Peruanos: Laguna Aricota

Existe un debate actual sobre el principal factor que controla los procesos de erosión en los Andes occidentales: McPhillips et al., 2014 proponen que la erosión depende fuertemente de la ocurrencia de terremotos mientras que otros estudios, reportan correlaciones positivas entre las variaciones climáticas y la erosión de la cuenca (Carretier et al., 2014; Reber et al., 2017) basadas en dataciones con nucleídos cosmogénicos (10Be). En el sector de Curibaya el deslizamiento Aricota, rellenó el valle y formó un gran dique que dio origen a la laguna Aricota, utilizando nucleídos cosmogénicos (10Be), Delgado et al., (2016) fecharon el deslizamiento (dique) en ~17 ka. Nosotros en este estudio tenemos la oportunidad de comparar (1) la tasa de erosión a partir del volumen de sedimentos acumulados en la laguna desde su formación, y (2) la tasa de denudación a partir de la concentración de isotopos de 10Be. Los resultados obtenidos en general muestran una importante correspondencia, indicando de esta manera que no hay influencia de productos volcánicos y de las superficies cuaternarias heredadas (morrenas)

Controls on pebbles size and shape in streams of the Swiss Alps

Although the size and shape of pebbles reflect the ensemble of sediment supply and transport processes along a stream, neither positive nor negative correlations have been found between the grain size of gravel bars, sediment flux, water discharge, and river flow strengths in Swiss streams. The relative frequency of 207–307 hillslope angles per catchment is the only variable that positively correlates with the size of the largest clasts (D96 percentile). We relate these observations to the detachment-limited states of the Swiss streams where ongoing fluvial dissection steepens the bordering hillslopes, thereby promoting the supply of material through lithology-controlled failure

Chronology of alluvial terrace sediment accumulation and incision in the Pativilca Valley, western Peruvian Andes

The incision and aggradation of the Pativilca alluvial fan delta system in the western Peruvian Andes through Quaternary time can be traced in detail using well-exposed fill terraces studied by a combination of cosmogenic nuclide dating, terrace mapping and paleo-erosion rate calculations. Two alluvial terraces have been dated through depth-profile exposure dating using in-situ 10Be. The dating results return an age for the abandonment of the terrace at 200 ± 90 ka in Pativilca and 1.2 Ma ± 0.3 Ma in Barranca. These new ages complete the database of previously dated terrace fills in the valley. Together with the results of the terrace mapping and the absolute ages of the terraces, we show that the valley fills are made up of at least four terraces; two terraces near the city of Pativilca and two terraces in the city of Barranca. While previous studies have shown two periods of sed- iment aggradation, one period around 100 ka (Barranca) and another period around 30 ka (Pativilca), our new results show two additional periods of sediment aggradation and subsequent incision that have not been re- ported before. Finally, paleo-erosion rates at the time of the deposition of the terrace material were calculated and compared to the available modern estimates. The paleo-erosion rates vary from 140 ± 12 m Ma−1 to 390 ± 40 m Ma−1. The period of sediment accumulation prior to the abandonment of the terrace at 200 ka corre- sponds to a wet phase and a pulse of erosion. In contrast, the period of sediment accumulation prior to the aban- donment of the terrace at 1.2 Ma does not correspond to a pulse of erosion and could rather correspond to a change of the base level possibly induced by a sea-level rise

On the potential for regolith control of fluvial terrace formation in semi-arid escarpments

Cut–fill terraces occur throughout the western Andes, where they have been associated with pluvial episodes on the Altiplano. The mechanism relating increased rainfall to sedimentation is, however, not well understood. Here, we apply a hillslope sediment model and reported cosmogenic nuclide concentrations in terraces to examine terrace formation in semi-arid escarpment environments. We focus on the Pisco river system in western Peru in order to determine probable hillslope processes and sediment transport conditions during phases of terrace formation. Specifically, we model steady-state and transient hillslope responses to increased precipitation rates. The measured terrace distribution and sediment agree with the transient predictions, suggesting strong climatic control on the cut–fill sequences in western Peru primarily through large variations in sediment load. Our model suggests that the ultimate control for these terraces is the availability of sediment on the hillslopes, with hillslope stripping supplying large sediment loads early in wet periods. At the Pisco river, this is manifest as an approximately 4-fold increase in erosion rates during pluvial periods. We suggest that this mechanism may also control terrace occurrence other semi-arid escarpment settings

Spatial variability of Quaternary denudation rates across a volcanic ocean island (Santo Antão, Cape Verde) from cosmogenic 3 He

International audienceDenudation of volcanic ocean islands creates remarkable landscapes and contributes to Earth's carbon cycle, since the chemical alteration of basalts is a CO2 sink. Because many volcanic islands have large climate gradients and relatively low variations in lithology and tectonic history, they represent excellent natural laboratories for studying climatic effects on landscape evolution. However, little is known about the control of denudation rates in ocean islands and the respective influences of climatic gradients and morphological parameters. Here, we present new measurements of long-term denudation rates from Santo Antão Island, Cape Verde (17°N). In this 779 km 2 island, mean annual precipitation ranges from 40 mm.yr-1 in the southwest to 1100 mm.yr-1 in the north. To constrain the spatial distribution of denudation rates, we measured the cosmogenic 3 He concentration in river-transported pyroxene grains from 23 river bedload samples. We obtained basin wide denudation rates ranging from 2.7 ± 0.1 to 57.5 ± 0.3 m/Ma. The denudation rates display a significant spatial variability, with the highest rates in catchments located in the northeast side of the island where modern precipitation are the highest and low denudation rates in the southern and western dry basins. Our study shows that precipitation is the main control on denudation and landscape development of the Santo Antão volcanic island. This study provides for the first time the spatial distribution of denudation rates across a volcanic island located in a tropical zone

Possible climatic controls on the accumulation of Peru's most prominent alluvial fan: The Lima Conglomerate

Sediment accumulation and the formation of terrace levels can occur in response to a change in either a tectonic or climatic driving force. This is particular the case for mountain ranges adjacent to plate boundaries such as the Peruvian Andes, where tectonics and an active seismicity have been shown to drive surface mass fluxes but where modern erosion has also been related to precipitation rates and patterns thereof. Here, we explore these controls on the deposition of the Lima Conglomerate, situated in Lima, Peru, on the western Andean margin adjacent to the Pacific coast, and we relate these results to the broader context of controls on erosion in the Andes. We use a combination of quantitative methods to explore the age of sediment accumulation, the provenance of the material and the paleo-erosion rates recorded by these deposits. Isochron burial dating with cosmogenic 10Be and 26Al yield an age of c. 500 ka for the base (490 ± 70 ka) and the uppermost sample situated c. 30 m higher upsection (490 ± 80 ka). Results of paleo-erosion rate estimates with concentrations of in-situ 10Be, which were measured on sand embedded within the conglomerates, show a c. 60% increase from 105 ± 10 mm ka-1 for a sample of the lowermost level, to 169 ± 14 mm ka-1 for a sample of the highermost level. Finally, provenance tracing with in-situ U/Pb ages on detrital zircon imply that the material has been derived from the entire drainage basin including the low-relief region in the headwaters. The combination of these results suggests that sediment accumulation occurred in response to an erosional pulse, which affected the entire basin within a short time interval. Because 10Be data represents a large spatial record of erosion, we exclude the possibility where a breakout of a lake, dammed by e.g. a large landslide, was the cause for the material pulse. For the same reasons, we exclude the possibility of a focused release of material in response to earthquakes to be responsible for the large material fluxes. Instead, the inferred erosional pulse was likely to have occurred at the scale of nearly the entire basin, supporting the idea of a larger-scale, most likely climate driven control. In this context, the accumulation age of c. 500 ka falls into an orbital cycle (eccentricity), thus fostering the emerging picture in the literature that erosion and sediment routing in the Andes has most likely been driven by climate and cyclic changes thereof. We thus suggest that the Andean mountain range offers an ideal laboratory to explore the erosional history in relation to climate patterns, at least in Peru

Environmental controls on 10 Be-based catchment-averaged denudation rates along the western margin of the Peruvian Andes

We present 10Be-based basin-averaged denudation rates for the entire western margin of the Peruvian Andes. Denudation rates range from c. 9 mm/ka to 190 mm/ka and are related neither to the subduction of the Nazca plate, nor to the current seismicity along the Pacific coast and the occurrence of raised Quaternary marine terraces. Therefore, we exclude a tectonic control on denudation on a millennial timescale. Instead, we explain >60% of the observed denudation rates with a model where erosion rates increase either with mean basin slope angles or with mean annual water discharge. These relationships suggest a strong environmental control on denudation

Cosmogenic <SUP>3</SUP>He and <SUP>10</SUP>Be denudation rates in the Central Andes: Comparison with a natural sediment trap over the last 18 ka

International audienceIt is of major importance for Earth surface sciences to reconstruct denudation rates in the most precise and accurate way. For this, it can be useful to test on the same setting methods based on different assumptions, such as those relying on geomorphological and geochemical observations. Here, we use an exceptionally suited setting in the Locumba catchment (southwestern Peruvian Andes) that offers the unique opportunity to compare denudation rates derived from in situ cosmogenic 3He and 10Be with a geomorphological sediment budget integrated over the last 18 ka. The sediment budget is estimated by determining the volume of sediment trapped in the Aricota lake that formed 18 ka ago after the occurrence of a giant rockslide dam. We reconstructed the topography of the Locumba valley before the dam emplacement and established that the captured sediment volume is 0.8 ± 0.1 km3. Considering that the lake-water output is restricted to seepage through the dam and that overflow above the dam never occurred, this volume correctly represents the sediment flux integrated over the last 18 ka. Integrating this volume over the upstream catchment area (∼1500 km2), we derived a corresponding mean erosion rate of 30 ± 9 mm.ka-1. Fluvial sediments feeding the Aricota lake were sampled to derive denudation rates from in-situ cosmogenic 10Be in the silicates and from in-situ cosmogenic 3He in the ferromagnesian minerals. Cosmogenic nuclide denudation rates from the main stream are 30 ± 2, 33 ± 2, 21 ± 1 and 82 ± 5 mm.ka-1 for the 10Be-quartz, the 10Be-feldspar, the 3He-amphibole and 3He-pyroxene, respectively. The consistency between the cosmogenic nuclide denudation rates derived from 10Be in the silicates and the erosion rate derived from our sediment budget shows that the 10Be accurately estimates of the sediment flux. Additionally, this work provides the first successful application of 10Be-feldspar nuclide-mineral pair to derive catchment-mean denudation rate and demonstrate that 10Be-feldspar can thus be a good alternative in catchments dominated by volcanic rocks with no quartz. The discrepancies observed between the denudation rates derived from the 3He-amphibole and 3He-pyroxene couples require further studies