Pedogenesis in a fluvial terrace chronosequence in the Pacific Northwest, USA

We evaluated pedogenic properties in a river terrace chronosequence in the Oregon Coast Range with the purpose of identifying pathways and rates of soil genesis. Knowledge of time-dependent changes in soil properties enables approximations of residence times to be made for soils of unknown ages. We also used a mass balance analysis to determine how rates and forms of mass losses change over the course of soil development. Changes in soil properties observed in the soil chronosequence include an increase in profile thickness, soil redness, and accumulation of secondary minerals. Losses of weatherable minerals and total Si, Na, Ca, and K increase with terrace age. Fe is conserved in secondary crystalline oxides, and elemental Al is conserved in Al-Si clay minerals. The largest changes in soil physical and chemical properties occur within the first five terraces. The advancement of the weathering front to depths of nine meters in the two oldest terraces contributes to the large mass loss of Si.