Development of texture contrast soils by a combination of bioturbation and translocation
Introduction
Regoliths throughout the world exhibit vertical textural contrasts, whereby coarser surface material overlies finer subsurface layers. Erosion and deposition can account for some of these profiles, as can the inheritance of textural contrasts from sedimentary layering or lithological boundaries. The ubiquity of such texture contrasts where inheritance or erosion/deposition are not viable explanations, however, suggests other, more general causes. The interpretation of such textural boundaries is important in a number of geomorphological contexts, such as distinguishing pedological vs. geological layering, the identification of colluvial and other geomorphic processes, and the interpretation of weathering profiles. The formation of texture contrast soils (TCS) is important beyond the study of soil geomorphology and pedogenic processes in specific soils precisely because TCS are so widespread. Explaining the occurrence of TCS in a variety of environmental settings has been an important goal for pedologists and geomorphologists because such an explanation might elucidate general principles of soil development. Conversely, the common occurrence of TCS might be a case of equifinality, whereby different processes and/or histories lead to the same result.
Several authors have pointed out the likelihood of multiple causality in the evolution of TCS (e.g., Kemp and McIntosh, 1989, Nooren et al., 1995, Paton et al., 1995, Schaetzl, 2001), and I have previously used the development of vertical texture contrasts as an illustration of the importance of local historical contingencies in pedology (Phillips, 2001a). Building from these notions, the purpose of this paper is to develop an explanatory model for the formation of TCS that combines bioturbation and translocation of fines as critical elements (thereby explicitly incorporating multiple causality). This represents a specific application of a more general model recently developed and illustrated with several brief case studies, including the study area of this paper (Phillips, 2004).
Section snippets
General considerations
In some regoliths coarse-over-fine vertical textural contrasts may simply be inherited from the parent material; for example in a weathering profile produced from a sandstone layer overlying shale. In other cases surface deposition of coarser material may account for the contrast. In all other situations a texturally-homogeneous, slightly contrasting, or finer-over-coarser vertical sequence must somehow be transformed such that coarser material overlies finer. In this study a TCS is arbitrarily
Texture contrasts in coastal plain soils
The most strongly developed soils on stable uplands of the southeastern U.S. coastal plain are dominantly Ultisols (Nitosols or Acrisols in the FAO-UNESCO system). These are acid soils with low base saturation and an argillic horizon. Typically surficial (A and E) horizons are in the loamy sand or sandy loam textural class, while the argillic (Bt) horizons are sandy clay loam. The parent materials are coastal plain sediments which are typically sandy, but which often include bedding planes or
Soil inventory
At the Redneck Beach site (Pamlico terrace), upland soils are entirely classified in the State series (typic Hapludult). These have loamy sand A and E horizons 25 to 50 cm thick, overlying strong brown (7.5 YR 5/6) sandy clay loam Bt horizons 50 to 70 cm thick, sometimes with a BC horizon transitional to the C horizon. The latter is sand with thin lenses of finer material, giving the C horizon a loamy sand textural class.
At the Talbot terrace sites, a wide variety of soils was found. The
Discussion
Revisiting the multiple causality model (Section 3.1), we can evaluate the terms of Eq. (1) based on the field evidence. TCo is apparently low, as the parent sediments are loamy sand resulting from sandy material with small amounts of fines, or single-grained sand with thin clay lenses or lamellae. Coarse surface additions (Ca+) are negligible, with possible localized exceptions of aeolian sand. Coarse surface losses (Ca−) are also generally negligible at the non-eroding study sites. Fine
Conclusions
In some soils of the lower coastal plain of North Carolina, U.S.A., no single existing explanation–such as eluviation–illuviation or bioturbation–is sufficient to explain the observed vertical texture contrasts. 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 explanatory model are coastal plain sediments which include
Acknowledgements
Conversations on the formation of texture contrast soils with Carsten Lorz (University of Leipzig) stimulated me to revisit the soils of the lower coastal plain of North Carolina. Lee and Bob Roche of Croatan, N.C. provided accommodations during fieldwork.
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