Tropical Weathering History Recorded in the Silicon Isotopes of Lateritic Weathering Profiles

Under tropical climates, mineral assemblages composing lateritic weathering profiles offer precious records of past weathering conditions. Silicon (Si) isotope signatures in the clay fractions of a deep lateritic profile in central Amazonia, Brazil, in combination with previously determined kaolinite ages, suggest that the surrounding region underwent two major weathering episodes, distinct in their intensity. The first episode (ca. 35–20 Ma) of moderate intensity produced well‐crystallized kaolinites from the parent sediment with limited Si isotope fractionation. A more recent (8–6 Ma) and shorter phase caused the replacement, from top to bottom of the profile, of the first kaolinite generation by a new population characterized by higher crystallographic disorder and stronger Si isotope fractionation, suggesting weathering under conditions of rapid water percolation. These inferences are supported by results from an isotope‐enabled reactive transport model, and they are consistent with paleoclimatic and paleogeographic evidences recorded over the Amazon Basin. Plain Language Summary Tropical landscapes are often covered by nutrient‐depleted soils overlying lateritic weathering profiles, that result from a complex set of chemical reactions between air, water, rocks, and living organisms. Mineral phases hosted by these deep weathering profiles, such as clays, have formed from reactions between primary minerals and elements present in subsurface waters, and thus hold the potential to record local past environmental conditions. We use numerical models to show that the distribution of the various isotopes of the major clay‐forming element silicon (Si) depends on the rapidity at which rainwater percolates downward through soil porosity. We demonstrate that clays present in a lateritic profile in central Amazonia formed during two distinct episodes. The first episode occurred between 35 and 20 million years ago at a time when rain in Amazonia was much less abundant that it is today. A second, shorter episode occurred between 8 and 6 million years ago under significantly wetter conditions than at present. Our study thus shows that silicon isotope analyses of lateritic profile can reveal the past evolution of climate in the tropics, which can in turn illuminate the future changes that will inevitably affect this region of our planet. Key Points Silicon isotopes in lateritic weathering profiles combined with dating tools record past weathering conditions Silicon is