MIMA AND OTHER ANIMAL MOUNDS AS POINT-CENTERED BIOMANTLES

The biomantle was conceptualized and named in 1975 in the first edition of Soil Taxonomy. It constitutes the upper part of soil produced by biota, dominantly by bioturbation, and is often equivalent to the topsoil, but it may include deeper levels in some soils. If a stone-line is present, the biomantle extends to its base. The biomantle is the most biologically dynamic part of the soil in terms of species diversity and numbers of soil organisms, and of its largely biologically-produced and/or mediated general character. By definition it consists of at least 50 percent biofabric, and may range in thickness from a few centimeters to many meters. Biomantles form above finer textured illuvial and other subsoils, above pans, above bedrock, above saprolite and/or other weathered material.

The main processes and conditions that produce biomantles include: 1) sorting of upper soil profiles by animals and other organisms, which includes various surface moundings; and 2) rainwash erosional-eluvial removals of fines from these sortings and surface moundings. If large clasts are present, a biomantle is formed with two biostratified horizons: a bioturbated upper layer, and a subsurface layer of accumulated clasts too large for animals to bioturbate -- the so-called stonelayer, or stone-line.

All soil organisms move and wriggle, and thus bioturbate -- some far more than others. Many make moundlets and mounds, which can range from those barely visible produced by solitary insects, to megamounds produced by social insects (termites and ants) and by some vertebrates. More than half of all terrestrial mammals, for example, make mounds as a consequence of their foraging, denning, predation, and other lifestyle behaviors. Because organisms bioturbate differently, and myriad species are involved, the formational pathways of biomantles are invariably complex, and vary from place to place -- sometimes significantly. We conclude that animal mounds are simply point-centered biomantles produced by such bioturbations. Mima-type mounds are morphologically exaggerated versions and form where thin soil over dense substrate occurs, and/or where high water table-wetness situations exist. Evidence for these conclusions is wide ranging and compelling.