Soil morphology of an alluvial chronosequence from the Little River, North Carolina Coastal Plain, USA

Abstract

A soil chronosequence on terraces of the Little River in the North Carolina Coastal Plain was characterized to evaluate age-related trends in pedogenesis. Five representative pedons per landform were studied from six late Quaternary fluvial surfaces, and temporal trends were assessed by regression of morphological parameters against previously reported terrace optical age estimates. Solum, B horizon, and Bt horizon thickness, subsoil clay content, and Buntley-Westin Index rubification have strong, positive correlations with age and together distinguish between soils of the floodplain (≤200yr BP), the first terrace (9.9±2.0 ka), intermediate surfaces (T2, T3b, T4; 17.4±4.2–74.6±10.4 ka), and the oldest terrace (T5b, 94.0±15.9 ka). Over time, soils develop from Entisols with respective A-C and A-E-Bw-C profiles on the floodplain and first terrace to Ultisols with increasing argillic horizon thickness, rubification, and clay content on T2-T5b. Solum thickness, Bt horizon thickness, and clay content display linear increases through time, with soil thickening reflective of downward pedogenesis through permeable, coarse-grained alluvium. Increasing argillic horizon clay content is attributable to illuviation, and possibly also to a combined bioturbation-translocation process, whereby clays scattered throughout sandy parent sediments are delivered to the surface by bioturbation, then concentrated in B horizons by eluviation-illuviation. Rubification increases with age and reflects transformation and subsoil concentration of inherited free iron, rather than iron oxide formation from primary mineral weathering. T5b pedons display morphologies comparable to those of 200 ka to >1 Ma soils along Coastal Plain rivers that also drain the Appalachian Piedmont, probably because sandy, Coastal Plain-derived Little River alluvium has a higher hydraulic conductivity that promotes faster rates of downward weathering, clay translocation, and rubification than the more finely-grained sediments of Piedmont-draining rivers. Thus, textural differences related to sedimentary provenance should be recognized when interpreting chronosequences from the region. Although rates of pedogenesis differ among Coastal Plain alluvial soils, linear trends in the ≤100 ka Little River chronosequence may nonetheless correspond to the early, more linear phases of development in studies from the region that span much longer intervals of time.