Late Oligocene–Early Miocene paleosols of distal fluvial systems, Ebro Basin, Spain

The study of paleosols can be a vital tool in the paleoenvironmental interpretation of continental deposits because their physical and chemical characteristics are a reflection of variations in aspects of soil formation. Changes in paleosol characteristics can be used to determine the architecture and nature of parent material, paleosol maturity, topography, climate and soil-forming organisms (including vegetation cover) on the ancient floodplain, and can be utilized to recreate a palaeocatena. Furthermore, paleosols can provide quantitative data on paleoclimate where other sources, such as organic matter and pedogenic carbonate are absent. Late Oligocene–Early Miocene strata exposed in the Ebro Basin, Spain, have been identified as proximal alluvial fan, fluvial and lacustrine continental facies. In parts of the distal fluvial system pedogenically altered channel and overbank deposits interfinger with ephemeral lacustrine facies. Three main paleosol types developed in these sediments and have been described and compared to modern soils: Entisol-like (early successional soils), Inceptisol-like (young soils) and Alfisol-like (open woodland soils). Paleoenvironmental reconstructions suggest a mosaic of ecotypes with areas of open woodland composed of shrubs, herbs and small trees and other plants of low stature, and with each ecotype being controlled primarily by fluctuating paleohydrological conditions. From field observations and geochemical analysis the climate was found to be humid continental (mean annual temperature (MAT) 10–14 °C±4 °C, mean annual precipitation (MAP) 450–830 mm yr±200 mm yr). These results indicate that climatic conditions were wetter than the present-day Ebro Basin (MAT 14 °C, MAP 320 mm yr), and differ from previous interpretations of an arid to semi-arid environment based on sedimentological criteria alone. © 2006 Elsevier B.V. All rights reserved.