Development of texture contrast soils by a combination of bioturbation and translocation

Soils and weathering profiles in a wide variety of parent materials and environmental settings exhibit coarse-over-fine vertical textural contrasts. Where these cannot be attributed to inherited texture contrasts or erosion–deposition, the most common explanations are based on translocation (eluviation–illuviation) which removes clays from surface layers and deposits them in the subsoil; or bioturbation, where preferentially fine material is delivered to the surface by organisms, from whence erosional winnowing creates a coarse surface layer. In some soils of the lower coastal plain of North Carolina, U.S.A., neither explanation is sufficient to explain the observed texture contrasts. A heuristic model based on a combination of translocation of fine material from surface to subsoil, and bioturbation-driven delivery and recycling of material to the surface can explain the observed vertical textural contrasts. The key elements in the model are coastal plain sediments which include some fine material; eluviation–illuviation by percolating water; delivery of additional fine and mixed grain size material to surface by bioturbation, making it available for translocation; concentration of fine material originally scattered throughout the parent material in a B horizon; and maintenance of vertical moisture fluxes by bioturbation. The model is supported by morphological evidence of the key mechanisms, argillic horizons that are finer than both the surface layers and underlying parent material, evidence that argillic horizon formation is not limited by the rate of clay synthesis, and the absence of texture contrasts in nearby soils formed from dune sands which lack fines.