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Patagonia is a key place to analyze the climatic trends (waves and winds) of the Southern Hemisphere. The northern coast of the San Matías Gulf (" Bahía Sin fondo " for the original Spanish descriptions) is characterized by an attached sand barrier developed on top of former cliffs. Morphological features are indicating changes in the regional slope when the sea level drowned the depression of the gulf. An original eastern slope was progressively replaced by slopes towards the south. The availability of sand during the transgression and maximum sea level was directed to the east due to the dominant westerly winds. Sand dunes accumulated on top of former cliffs and today –due to coastal erosion-, remain as cliff-top dunes. A conceptual model for the formation of these cliff-top barriers is described in relation to the fluctuation of the sea level and the sand migration, which was studied through mineralogical analysis comparing the different environments: cliff, beach, and dunes. Their composition explains the sediment transport shift and is locally associated to the Río Negro Formation deposits and regionally to Andean Cordillera volcanic sources. • Clifftop dunes are dominant along the northern shore of San Matías Gulf, Río Negro, Argentina. • Mineral provenance indicates the erosion of the cliffs and the westerlies transport. • Although the gulf was flooded during the Holocene transgression, the successions preserved indicate a sea-level drop since then. [ABSTRACT FROM AUTHOR]

The Paraná River is one of the world's largest fluvial systems due to its discharge. During the last 30 years, water discharge has averaged 18,389 m3 s−1 downstream of the Paraguay-Paraná confluence, where its most important tributaries, the Paraguay and the upper Paraná rivers contribute 22% and 78% of its total discharge, respectively. However, the sedimentological dynamics of this system behaves peculiarly due to the influence of the Bermejo River, a tributary of the Paraguay River. Even though the contribution of the Bermejo River discharge represents only 2% of the Paraná River discharge, its input of suspended sediments is significant due to the erosive processes in the upper Bermejo basin. Most earlier studies have primarily focused on the contributions and dynamics of the wash load (silt and clay) from the Bermejo River through the Paraguay River to the Paraná River, and neglected the relevance of fine and very fine sands (coarser than 63 μm). These sediment fractions play a key role in bed sediment composition, roughness, and bed forms, ultimately modifying the flow resistance and the morphodynamic processes, among other environmental and biological parameters. This study seeks to quantify the suspended sediment contribution of the main tributaries of the middle Paraná River during the last three decades. In particular, water discharge, sediment transport and grain size data measured at gage stations located at the Paraguay River upper reach, the Bermejo River, and the upper and middle reaches of the Paraná River are analyzed. Focus is placed on the coarse suspended sediment fraction. Additionally, the authors collected sediment samples at different river cross-sections and water discharges. The temporal analysis revealed the dynamic behavior of the sediments along the system. The results show different behaviors of the wash load and suspended sand transport provided by the Bermejo River. Along the middle Paraná reach, results suggest a significant suspended coarse fraction deposition during the highest sediment input period. Bed sediments, in turn, are re-suspended during the months with low contributions from the Bermejo and/or during the high water levels stages of the Paraná. This sediment dynamics produces a complex pattern of coarse sediment transport in the Paraná River at its middle reach, also modifying the sedimentological characteristics of the river bed. • 95% of suspended sand of the middle Paraná river are provided by the Bermejo river. • 93% of wash load of the middle Paraná river are provided by the Bermejo river. • Sediment deposition and resuspension occurs along the hydrological year. • Sediment load from Bermejo have a strong impact in Paraná river morphodynamic. [ABSTRACT FROM AUTHOR]

Abstract: The erosion and transport of sediments allow us to understand many activities of significance, such as crust evolution, climate change, uplift rates, continental processes, the biogeochemical cycling of pollutants and nutrients. The Amazon basin of Peru has contrasting physiographic and climatic characteristics between the Andean piedmont and the plains and between the north and south of the basin which is why there are 8 gauging stations located along the principal rivers of the Andean piedmont (Marañón, Huallaga, Ucayali) and the plain (Marañón, Tigre, Napo, Ucayali and Amazon rivers). Since 2003, the ORE-Hybam (IRD-SENAMHI-UNALM) observatory has performed out regular measurements at strategic points of the Amazon basin to understand and model the systems, behavior and long-term dynamics. On the Andean piedmont, the suspended yields are governed by a simple model with a relationship between the river discharge and the sediment concentration. In the plain, the dilution effect of the concentrations can create hysteresis in this relationship on a monthly basis. The Amazon basin of Peru has a sediment yield of 541 *106 t year−1, 70% comes from the southern basin. [Copyright &y& Elsevier]

Abstract: Cosmogenic nuclide-based denudation rates and published erosion rates from recent river gauging in the Napo River basin (Peruvian Amazonia) are used to decipher erosion and sedimentation processes along a 600 km long transect from the headwaters to the lowlands. The sediment-producing headwaters to the Napo floodplain are the volcanically active Ecuadorian Andes, which discharge sediment at a cosmogenic nuclide-based denudation rate of 0.49 ± 0.12 mm/yr. This denudation rate was calculated from an average 10Be nuclide concentration of 2.2 ± 0.5 × 104 at/g(Qz) that was measured in bedload-derived quartz. Within the Napo lowlands, a significant drop in trunk stream 10Be nuclide concentrations relative to the Andean hinterland is recorded, with an average concentration of 1.2 ± 0.5 × 104 at/g(Qz). This nuclide concentration represents a mixture between the 10Be nuclide concentration of eroded floodplain deposits, and that of sediment eroded from the Andean hinterland that is now carried in the trunk stream. Evidence for addition of sediment from the floodplain to the trunk stream is provided by published decadal-scale sediment flux measurements from gauging stations operated in the Napo basin, from which an increase from 12 × 106 t/yr at the outflow of the Andes to ∼47 × 106 t/yr at the confluence with the Solimões (upper Amazon River) is recorded. Therefore, approximately 35 × 106 t of floodplain sediment are added annually to the active Napo trunk stream. Combined with our nuclide concentration measurements, we can estimate that the eroded floodplain deposits yield a nuclide concentration of ∼0.9 × 104 at/g(Qz) only. Under steady state surface erosion conditions, this nuclide concentration would translate to a denudation rate of the floodplain of ∼0.47 mm/yr. However, we have no geomorphologic explanation for this high denudation rate within the low relief floodplain and thus suggest that this low-nuclide concentrated sediment is Andean-derived and would have been deposited in the floodplain at a time when erosion rates of the Andes were elevated. Therefore, the recently eroded floodplain sediment provides an Andean “paleo denudation rate” of 1.2 mm/yr that was calculated for high Andean production rates. A likely period for elevated erosion rates is the LGM, where climate and vegetation cover of the Andes differed from that of the Holocene. A possible cause for the erosion of the floodplain is the tectonic uplift of the Eastern Andes, which progressively shifts the Napo River northwards. Hence, the river cuts into ancient lowland sediment, which is admixed to the Andean sediment carried in the main Napo River. [Copyright &y& Elsevier]

This article presents the results of a research on paleosols formation conditions in the Aguascalientes Valley, northern Mesoamerica, where paleoenvironmental reconstructions have been scarce. Records of environmental and climatic changes, as well as the alteration of the landscape are expressed in the memory of the soil. Soil micromorphology is the most efficient method to identify these changes. In paleosols, micromorphological features reflecting a certain soil forming process can be related to environments in which those processes took place. Paleoenvironmental reconstruction of the region consisted in identifying pedofeatures and other present soil attributes, grouping features into pedogenic facies under constant environmental conditions, examining the groundmass in order to detect features caused by erosion or sedimentation, and finally establishing a hierarchy between the different pedogenic facies, including erosional and sedimentary features if necessary. Specific statistical data analysis disclosed stratigraphical relationships. The results we obtained, such as illuvial clay coatings, redoximorphic nodules, and hypocoatings, revealed the most representative pedofeatures that developed under humid climate. Under a semiarid climate, representative pedofeatures that were revealed included micritic coatings and hypocoatings. A paleosol with ferruginous nodules and hypocoatings in groundmass, and at least one phase of dusty and high-birefringence yellow clay illuviation is proposed as stratigraphic marker for this area of the Aguascalientes Valley. Evidence indicates the sequence of events in the area was: (i) a large period of humid climate; (ii) a short period of climate change, from humid to semiarid, followed by subhumid; and (iii) a large period of semiarid climate, including the current one. The paleosols of the Aguascalientes Valley to a large extent preserve major trends in history and provide further evidence to define the 11 phases of stability (pedogenesis) in 8 cycles and to detect the interlayer phases of instability (sediment transport/deposition). • Northern Mesoamerican paleosols provide useful indicators for paleoenvironmental reconstructions. • Main pedofeatures are illuviation (humid conditions) and erosional (landscape instability). • Multivariate ordination strengthens observed pedofeatures relationships. • Paleoclimate change played several roles in driving pedogenesis in Quaternary. • Humid and arid climates cycles have prevailed in the Aguascalientes Valley. [ABSTRACT FROM AUTHOR]