Vertisol genesis in a humid climate of the coastal plain of Texas, U.S.A.

Abstract

Numerous genetic studies of Vertisols have been conducted using qualitative and semi-quantitative methods across a broad range of climates. However, no detailed investigations exist on Vertisols formed in humid climates using multiple pedogenic techniques on both a microlow (ML) and microhigh (MH). Hence, we sampled and characterized a ML and MH from a Lake Charles pedon (mean annual rainfall=1090 mm, mean annual temperature=20 °C) in the late Pleistocene coastal plain Beaumont Formation of east Texas. According to the official series description the Lake Charles classifies as a fine, smectitic, hyperthermic Typic Hapludert with mollic and calcic diagnostic horizons. Coupled with field morphology and characterization data, micromorphology and constituent mass balance interpretations were used to reconstruct the major pathway of pedogenesis. In response to greater water recharge, the ML contains darker colors, more organic carbon, greater depth of carbonate leaching, and lower pH. The proportion of oxalate-extractable iron increases toward the surface and is in greater abundance in the ML than MH. In contrast, dithionite-extractable iron increases with depth and has similar quantities in both the ML and MH. The parent material and lower profiles contain asepic microfabric dominated by micaceous-rich coarse clay and silt. The asepic zone deep contains an abundance of soft carbonate masses and iron manganese segregations that appear to be forming contemporaneously, with the presence of argillans substantiating clay translocation during the early history of soil formation. Sepic microfabric dominates the upper profiles of both the ML and MH in part from the transformation of biotite to smectite, which is more pronounced in the microlow because of more intense weathering. Hard carbonate nodules and iron manganese concretions are abundant in the upper sepic zone and contain skelsepic microfabric, suggesting that they are no longer growing. Dilation was the dominant strain force during pedogenesis from volume expansion in response to crack infills and shrink–swell processes. Mass flux calculations indicate that total clay gains occurred in both the ML and MH during pedogenesis. Late Pleistocene Vertisols appear to approach steady state when forming in humid climates. The soil mechanics model best explains the observed depth distribution of properties, including slickensides, with pedoturbation playing a secondary role in the genesis of these humid-climate Vertisols.