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

PASQUINI, ANDREA; ROUZAUT SABRINA; CAMPODONICO, VERENA

GEODERMA

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2019 p. 235 - 249

0016-7061

A loess-paleosol sequence in the Pampean Plain, central Argentina was analyzed geochemically, in order toevaluate the weathering signature and sedimentary recycling as well as to analyze the origin of these loessicsediments. Corralito I sequence is composed of three paleosols and a buried soil separated by loess layers. Theloess-paleosol sequence shows a similar geochemical composition throughout, suggesting that pedogenesis hasnot 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 indicateincipient 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 balancecalculations reveal the losses of some major oxides (i.e., CaO, Na2O) and Sr in the buried soil and paleosolscompared to its loess mantles, reflecting the weathering of plagioclase. Besides, the losses of K2O and Ba in theburied soil and paleosols I and II, when compared to its loess layers, suggest the chemical alteration of Kfeldspar.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 areconcentrated in paleosols and the buried soil compared to its loess mantles. The geochemical approach used toconstrain the origin of these Pampean Plain loessic sediments indicates they derive mainly from the Andeanregion. Conversely, the Pampean ranges, the Paraná River basin and Uruguayan Precambrian outcrops, whichwere pointed out as minor local sources, do not contribute significantly to the loess deposits. This work alsoshows that Corralito I loess-paleosol sequence and other loess samples from Argentina do not evidence sedimentaryrecycling, suggesting that these Pampean Plain loess deposits are mainly composed by young materialsderived from undifferentiated volcanic rocks.