PEDOGENIC INDEX OF PALEO-PRECIPITATION IN TROPICAL RAIN FORESTS

The greater mobility of base cations relative to aluminum oxides during pedogenesis has been exploited to develop chemical weathering indices based on oxide concentrations in soil profiles. Some indices, such as the Chemical Index of Alteration without Potash (CIA-K), have been used to estimate paleoprecipitation in paleosols. However, this relationship has only been established for temperate climates with mean annual precipitation (MAP) <1500 mm/yr. To assess this for tropical soils under MAP >1500 mm/yr, we sampled a series of modern soils along a strong rainfall gradient (1400–3500 mm/year) in the Republic of Panama. The soils supported undisturbed forests and were developed on basalts of similar geochemical composition. At each site, we collected samples to a maximum of 120 cm depth from duplicate profiles on local summits. Major and trace (Ti, Zr, Nb) element concentrations were measured as oxides by X-ray fluorescence. Modern mean annual precipitation was not correlated significantly with CIA-K (R²= 0.26), but was correlated strongly with Al₂O₃/(Al₂O₃+TiO₂) (R²=0.59, p<0.01). When these relationships were applied to a series of 15 paleosols from a Paleocene–Eocene section in the Bogota Basin, Colombia, the estimated range in MAP was 1231–1559 mm/yr based on CIA-K and 2600–3139 mm/yr based on Al₂O₃ and TiO₂. The latter is more reasonable given the calibration to high MAP, the tropical paleolatitude of the samples (<5° N), and other proxies indicating increased weathering across the Paleocene–Eocene boundary in the region. This new proxy will allow a more accurate estimation of paleo-precipitation from humid tropical paleosols.