COMPARATIVE STUDY OF PRECIPITATION PROXIES FROM MODERN NORTH AMERICAN SOILS AND PALEOSOLS FROM RUSINGA AND MFANGANO ISLANDS, LAKE VICTORIA, KENYA

One of the most frequently utilized proxies for mean annual paleoprecipitation (MAP) estimates is measurement of the chemical index of alteration minus potassium (CIA-K). A common approach to measurement of CIA-K values includes chemical pretreatments to remove calcite and soluble cations, producing CIA-K measurements that differ significantly from untreated samples whether calcite is present or not in the sample. Therefore, chemical pretreatments may bias MAP estimates derived from the CIA-K proxy. Early Miocene Calcisols from Rusinga and Mfangano Islands, Lake Victoria, Kenya yield CIA-K values that result in MAP estimates of 222-606 mm for untreated samples, whereas the same samples, chemically pretreated to remove calcite, yield CIA-K values that correspond to higher MAP estimates of 731-881 mm. Protosols on the islands yield CIA-K values that yield MAP estimates of 225-642 mm for untreated samples, whereas pretreated samples produce CIA-K values which correspond to MAP estimates of 283-444 mm. In order to gain insight into the significance of chemical pretreatment and its effects on MAP estimates, CIA-K values were calculated for a suite of modern Aridisols, Vertisols, Mollisols, Alfisols, Inceptisols and Entisols from California before and after chemical treatments to remove calcite. CIA-K values among decalcified soil samples correspond to MAP estimates as much as 339 mm/yr greater than their untreated counterparts; a difference that is much greater than the reported uncertainty of the CIA-K proxy (±182 mm). The magnitude of the MAP increase from decalcification depends on the weight percent calcite in the soil, with the largest shift (339 mm/yr) seen in an Entisol with ~10% carbonate. The smallest increase in MAP estimates (34 mm/yr) results from a noncalcareous Inceptisol. Comparison of estimated and measured MAP values reveals that all CIA-K-based estimates of MAP overestimate actual MAP among modern soils, and decalcifying these samples typically exacerbates the overestimation of MAP. Given the consistent overestimation of MAP in the modern soils tested here, researchers should be cautious before estimating paleoprecipitation by applying bulk soil geochemical proxies to paleosols with calcareous matrix.