Oxygen and hydrogen isotopes in pedogenic minerals — Implications for paleoclimate evolution in Amazonia during the Cenozoic

Stable isotopes in gibbsite and kaolinite were analyzed to reconstruct the Amazonia palaeoclimate during the Cenozoic and to investigate the environmental conditions of the weathering process and the origin of the Oxisol that cover the lateritic/bauxite crust. At least two types of kaolinite were identified in the Oxisol; near the surface, the kaolinite is less fractionated and was developed at a higher temperature (35 °C). This is probably due to the effect of vegetation and/or evapotranspiration or is a consequence of surface microenvironmental conditions or allochthonous inputs. The other kaolinite samples ranged in formation temperatures from 21 °C to 26 °C and developed under conditions similar to the Pleistocene and the modern climate of Amazonia. At least two types of gibbsite were found. One type developed in the modern Amazonia climate, in a way similar to most kaolinite samples, whereas the samples of the other type are more fractionated. This is because they may have been formed in low recycling rainwater and/or under a low amount of precipitation. These two conditions indicate a seasonal climate for bauxite development that can be linked to the low exhumation of the Andes cordillera and its geographical position relative to the Trombetas region during the period ranging from the Late Oligocene to the Early Miocene. At the end of the Cenozoic, the climate was less seasonal and wetter and the temperature was lower In addition, there was the probable inland position of the Trombetas region, which allowed Oxisol development. The gibbsite developed earlier than the kaolinite of the Oxisol, but there is a partial inheritance, or temporal equivalence, among the bauxite horizon and the Oxisol, as well as multiple generations of gibbsite and kaolinite, which is to be expected for materials developing over long periods in hard leaching conditions.