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A catenary sequence of soils formed on reworked loess under semiarid climatic conditions was studied. Four soil profiles located on the piedmont of Sierra Chica de Córdoba, central Argentina, were described, classified and geochemically analyzed. All soils, developed on summit, shoulder, backslope, and toeslope positions, were classified as Mollisols. Decarbonatation-carbonatation, melanization, and argilluviation are the main pedogenic processes recognized in these soils, which appear to control the differentiation of genetic horizons along the catena . Different geochemical approaches indicate that there are not substantial variations in the chemical composition of the studied soils along the catena , with the exception of the soil in the toeslope position, which exhibits slightly differences. In general, all profiles show weak depletions of mobile elements (Ca, Na, Mg, Sr, U) in the upper continental crust UCC-normalized diagrams which are attributed to a slight chemical alteration. Other elements, such as Fe, Cr, Co and Ni, also exhibit depletions compared to UCC, which can be explained by the alteration of ferromagnesian silicates, but can also be an inherited feature from the parent material. The significant enrichment in As compared to UCC, evident in all profiles along the catena , is also a typical feature of the pampean plains' loess of Argentina. So, the chemical differences in the profile located on the toeslope are mainly attributed to the supply of materials from local sources, i.e., crystalline basement and sedimentary rocks, due to its position in the catena . Statistical correlations and multivariate cluster analyses reinforce the assumption that the geochemistry of the studied soils is inherited from the parent material. In addition, chemical indices (CIA, ICV), elemental ratios (Ba/Sr, Rb/Sr) and the A-CN-K ternary diagram indicate an incipient degree of chemical alteration for these soils, compatible with the weathering regime prevailing in the region. Thus, the differentiation of genetic horizons along the catena is the result of weak weathering and pedogenic processes, which have not been strong enough to mask the chemical imprint of the parent material. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Paraná River basin is one of the largest hydrological systems in South America. The present study focuses on the mineralogical and chemical composition of the suspended sediments exported by the Paraná River to the Atlantic Ocean, with the aim of analyzing their provenance, the chemical weathering signature and the likelihood of sedimentary recycling. The particulate matter of the Middle Paraná River and its main tributaries (i.e., the Paraguay and the Upper Paraná rivers) is mostly composed of quartz, plagioclase, K-feldspar, and clays, such as illite and kaolinite. Different geochemical approaches indicate that the suspended sediments transported by the Paraná River preserve the chemical signature of its sources and its composition is not significantly modified during transport. These sediments are mainly supplied by acidic arc sources located in the Andean headwaters of the Bermejo and Pilcomayo rivers (and transported by the Paraguay River); and by tholeiitic basalts outcropping in the headwaters of the Upper Paraná River. The incomplete mixing of both main tributaries produces a transverse geochemical asymmetry in the particulate material of the Middle Paraná River, which was detected ~ 32 km downflow the confluence. The suspended load transported by the Paraguay River (which includes the contributions from the Bermejo and Pilcomayo rivers) indicates incipiently to moderately weathering, whereas the particulate matter exported by the Upper Paraná River reveals a higher degree of chemical alteration. This is the result of the different lithology and climatic regimes that prevail in the headwaters of both tributaries. The weathering signature of the Middle Paraná River's suspended load resembles that of the Paraguay River, which in turns supplies most of the particulate matter through the Bermejo River. This work also shows that the suspended load exported by the Paraná River basin has a mixed origin, where the chemical signatures from young materials derived from undifferentiated volcanic rocks, and from recycled materials affected by intracrustal differentiation can be distinguished. [ABSTRACT FROM AUTHOR]