Extractable Fe, Al and Mn in paleosols of late Quaternary age in the Virguna Mountains, Northwestern Rwanda

Abstract Ground soils and buried paleosols in sediments of late Quaternary age were studied to determine if Fe, Al and Mn concentrations might provide information on relative age as well as past and present soil-forming environments. Extractable Fe, Al, and Mn distributions indicate leaching was more aggressive in older interstadial paleosols of Wurm age. The ratio of ammonium oxalate-extractable Fe (Fe o ) to dithionite-extractable Fe (Fe d ) (the Fe o /Fe d activity ratio) is often used to determine the degree of conversion of amorphous to crystalline Fe. This ratio could not be used with confidence in this study because magnetite (which is abundant in these samples) is susceptible to attack by acid ammonium oxalate. Oxalate-extractable Al (Al o ) is often higher than dithionite-extractable Al (Al d ) which appears to be a function of the pH of the buffer solutions. Extractable Al and Fe provide a reasonable assessment of relative age, while Mn yields little useful information. The ratios Fe p +Al p /Fe d +Al d and Fe d /Fe t provide useful information regarding movement of pyrophosphate-extractable Fe (Fe p ) and Al (Al p ) as well as development of organically-bound and extractable crystalline Fe over discrete intervals of time. The most important paleoenvironmental assessment is that the Kalambo interstadial paleosol of middle Wurm age appears to have formed under a wetter and warmer climate than the postglacial soil.