Paleosols and Andean uplift in Venezuela: assessing competing hypotheses of relict tropical soils versus paleohydrogeochemical variations

Abstract

Paleosols in northern Venezuela, which have formed in residual regoliths of metaargillite rock complexes, have been considered to have developed in tropical, humid climates at low elevation. Their reddish colors and clay-enriched horizons have been used to support the hypothesis of significant recent uplift of the Andes Mountains in Late Quaternary time. We suggest an alternative hypothesis of long-term (+5 Myr) pedogenesis in a paleohydrogeochemical regime, characterized by fluctuating redox conditions at high elevation (2000–2500 m a.s.l.); the evidence for this redox history is the presence of Mn–Fe nodules in the surface epipedon. The sites, though relatively dry now, are interpreted to have been much wetter in the past, with soil moisture levels capable of translocating clay to form argillic horizons. The presence of thick Ah horizons, complete with large (1–2 mm) Mn–Fe nodules and approximately 30 cm thick argillic horizons, suggests long-term pedogenesis. The previous classification of these paleosols as Oxisols is untenable because of the presence of a mollic epipedon and the concentration of Mn–Fe nodules in the epipedon. The paleosols bear some resemblance to Placosols of the Canadian taxonomy, previously described for the maritime cool, moist climates of the Pacific and North Atlantic areas of Canada, but in Venezuela they occur without an impermeable subsurface Fe oxide layer. In the US taxonomy, they are classified as Typic or Aridic Argiustolls.