Late Cenozoic paleosols as climatic proxies: the Sauce Grande River, Buenos Aires province, Argentina, as a study case

Abstract

The Late Cenozoic of the Sauce Grande River basin (Pampas plain, Argentina) is representedby fluvial, eolian and alluvial units that, in part, are pedogenically modified. The aim of thiswork is to present the different recognized paleosols to characterize the mainpaleopedogenic processes and to infer preliminary paleoclimatic conditions.Characterization of the paleosols was made based on macro- and micromorphologicalfeatures recognized in a detail outcrop study. Ten paleosols samples were analyzed usinggeochemical analysis (ICP-MS) to calculate climofunctions, and X-ray diffraction used toidentify the clay mineral composition. We used samples from Bt, Btk and Bw horizons tocalculate PWI, CIA-K and clayness, and to estimate mean annual precipitation (MAP), andmean annual temperature, (MAT).Four pedotypes were defined. Calcic Argillisols (P1) present argillic coatings, micritic nodulesand incomplete sparitic infillings (Btk, Bk horizons). Smectite predominates over illite, CIA-Kis 44, MAP(CIA-K) is 535±182 mm/yr and MAT(PWI) is 9° ±2.1°C. Calcisols (P2) arecharacterized by micritic groundmass, sparitic hypocoatings and micritic nodules (Bkhorizons). Clay minerals are smectite and illite, CIA-K is 57, MAP(CIA-K) is 674.76±182 mm/yrand MAT(PWI) is 10.3°±2.1°C. Argillisols (P3) show argillic coatings (Bt horizons). Illite is themain clay mineral. CIA-K is 68, MAP(CIA-K) is 841±182 mm/yr and MAT(PWI) is 10.6°±2.1°C.Protosols (P4) present channels and Fe and Mn nodules (Bw horizons). Clay minerals areillite and smectite. CIA-K is 62, MAP(CIA-K) is 756±182 mm/yr and MAT(Clayness) is11°±0.6°C.Macro- and micromorphology, geochemical analyses, and X-ray diffraction, indicate that themain pedogenic processes were calcification in P1, illuviation and calcification in P2,illuviation in P3, and bioturbation and hidromorphism in P4. A cyclical alternation isinterpretated between Calcic Argillisols and Calcisols that formed under seasonal semiaridtemperate conditions, while Protosols and Argillisols develop under subhumid temperateconditions.