Geochemistry of a Late Quaternary loess-paleosol sequence in central Argentina: Implications for weathering, sedimentary recycling and provenance

Abstract

A loess-paleosol sequence in the Pampean Plain, central Argentina was analyzed geochemically, in order to evaluate the weathering signature and sedimentary recycling as well as to analyze the origin of these loessic sediments. Corralito I sequence is composed of three paleosols and a buried soil separated by loess layers. The loess-paleosol sequence shows a similar geochemical composition throughout, suggesting that pedogenesis has not been intense enough to mask the chemical signature of the parent material. Weathering indices (CIA, CIW, PIA and LWI), elemental ratios (Rb/Sr, CaO/TiO2 and Na2O/TiO2), and the A-CN-K ternary diagram indicate incipient chemical alteration for this sequence, compatible with a weathering-limited regime. As expected, paleosols (CIAs ~60)exhibit a slightly higher chemical alteration than loess layers (CIAs ~58). Mass balance calculations reveal the losses of some major oxides (i.e., CaO, Na2O)and Sr in the buried soil and paleosols compared to its loess mantles, reflecting the weathering of plagioclase. Besides, the losses of K2O and Ba in the buried soil and paleosols I and II, when compared to its loess layers, suggest the chemical alteration of K-feldspar. The chemical alteration of volcanic glass is evidenced by the relative losses of As in almost all levels. Conversely, the relative gains of Fe2O3 and MgO reveal that pyroxenes and amphiboles or Fe oxy-hydroxides are concentrated in paleosols and the buried soil compared to its loess mantles. The geochemical approach used to constrain the origin of these Pampean Plain loessic sediments indicates they derive mainly from the Andean region. Conversely, the Pampean ranges, the Paraná River basin and Uruguayan Precambrian outcrops, which were pointed out as minor local sources, do not contribute significantly to the loess deposits. This work also shows that Corralito I loess-paleosol sequence and other loess samples from Argentina do not evidence sedimentary recycling, suggesting that these Pampean Plain loess deposits are mainly composed by young materials derived from undifferentiated volcanic rocks.