Holocene pedogenesis along a chronotoposequence of soils from the Altiplano to the Cordillera Real, Bolivian Andes

Abstract

The high mountains of Central Andean represents one of the most diverse and heterogeneous landscape of South America, with very contrasting climates across an east-west gradient that remains poorly investigated in terms of Late Quaternary soil formation. Climatic forces within a diverse pedogeomorphological framework generates a complex geoenvironmental scenario, extremely susceptible to climatic changes. From the high aridic bolivian altiplano to moist periglacial regions at high mountain, and from these, to the tropical rainforest downslope, one finds soil chronosequences and different ecosystems. The objective of this work was to describe and interpret chronological, morphological, physical, chemical and mineralogical attributes of 18 profiles along a long and representative east-west transect through the Cordillera Real, at altitudes varying between 1800 and 5030 m a.s.l. The parent material is lithologically diverse, ranging from till, fluvio-glacial, talus and loess sediments. Most soils show a sequence of O, A, C and C/R with high organic carbon contents at surface and subsurface, forming Umbric horizons. Buried, polycyclic organic layers are common and the 14C dating reveals a sequence of processes as one moves away from the glaciated peaks, varying from 1500 to 6000 years (BP). The occurrence of paleo-peatlands and past and present-day hydromorphic areas (bofedales) result in widespread melanization trend, with histic horizons and histosols formation in the bottomlands. The relict, degraded state of bofedales indicates a reduction of waterlogging in the Altiplano in the late Holocene, resulting in increasing water shortage. Inceptisols occur at the high mountain sector, under periglacial conditions and high slope instability and solifluxion. In general soils are acid or high acid, with low base saturation and high Al saturation. High mountain Tundra soils at (5020 m) are exclusively developed on moraines of resistant granodiorite and have low clay content and less mineralogical diversity. Soils at the western slope were affected by the last glaciation and show more diverse mineralogy with cryogenic features under dry, open vegetation (Puna). On the other hand, soils at the eastern slope are associated with a greater climatic variation, varying mineralogical attributes, greater organic carbon mineralization and formation of organometallic complexes. Although most soils show influence of non-crystalline minerals, such as allophanes and amorphous Fe oxides, andic soil properties were apparently absent.