Late Quaternary paleosol–sediment-sequences and landscape evolution along the Andean piedmont, Bolivian Chaco

As part of the Bolivian Chaco, the Andean piedmont is situated in the transitional zone between the tropical-humid climate regime to the north (Amazonia) and the subtropical-semiarid regime to the south (Gran Chaco). In this zone, which is sensitive to climatic changes, late Quaternary landscape and climate history was reconstructed based on detailed sedimentological, pedological and geochemical analyses of paleosol–sediment-sequences along the Andean piedmont. The determination of characteristic lithofacies allowed correlation between the investigated profiles and their interpretation in terms of paleoenvironmental conditions, i.e. geomorphic stability (soil formation) or geomorphic activity (fluvial or fluvio–aeolian sedimentation, erosion, etc.). Coarse fluvial deposits (gravels, sands) form the base of the investigated profiles and have likely been deposited in a braided-river environment with low vegetation cover and intensive discharge events before and during the Last Glacial Maximum. Well-developed paleosols formed in the upper part of these sediments during the Lateglacial, documenting geomorphic stability (high vegetation cover) and wet conditions. Overlying palustrine sediments, dated to the Early Holocene, indicate a dramatic change in seasonality, i.e. increased winter precipitation and decreased summer precipitation. Subsequent erosion, followed by accumulation of fluvial and fluvio–aeolian sands, points to much drier conditions during the Mid-Holocene. Since ∼ 2.9 cal ka BP soil formation indicates a return to geomorphic stability and wetter conditions (characteristic of the present day climate) interrupted by short arid intervals.