Mima Mounds as Upper Soil or Whole Soil Biomantles: What Happens Morphologically When the Bioturbators Depart?.
Donald L. Johnson, Univ of Illinois, 713 S Lynn St., Champaign, IL 61820, Diana N. Johnson, Geosciences Consultants, 713 So. Lynn St., Champaign, IL 61820, and Jennifer Horwath, Dept of Earth and Space Sciences, Univ. of Washington, Box 351310, Seattle, WA 98195-1310.
A biomantle is the zone in the upper part of soils produced dominantly via bioturbation, impacted by subsidiary processes (shrink-swell, leaching-precipitation, eluviation-illuviation, biochemical processes, etc.). Many organisms on Earth bioturbate at all scales and intensities. They include most members of the five kingdoms of life (bacteria, protoctists, fungi, plants, animals). Most obvious to human observers are animals (faunalturbation), plants (floralturbation), and fungi (fungiturbation) that account for millions of bioturbators. Members of the three kingdoms likely account for most of the significant bioturbation signatures expressed in soils. They produce a set of complex processes that have been largely ignored in soil studies. We focus on animals (faunalturbation) and their effects on the mounded biomantles discussed. All life-forms bioturbate in one or more of four non-mutually exclusive process styles, that include upward biotransfers, biomixing, cratering, and volume increases (cf. Reference). These styles produce one-layered biomantles that are morphologically isotropic with regard to particle size (bioturbated soil particles form a single surface horizon), or two-layered biomantles that are morphologically anisotropic with regard to particle size (bioturbated particles form two layers, the lower being a stonelayer whose base marks the bottom of the biomantle). “Gravels” can be natural materials (stones, metallic and non-metallic nodules-concretions, caliche rubble, shells, gastroliths), or human cultural materials (brick- or concrete-rubble, arrow-points, hand-axes, coins, pottery, bullets). These items bioturbationally function as gravels, and if gravels are present the following three categories of biomantles result that depend on the dominant bioturbator and the size of particles it can move: 1). If the dominant bioturbator is a large vertebrate (badger, armadillo, wombat, etc.) a one-layered biomantle forms, where fine fractions and gravels are mixed throughout. 2). If the bioturbator is a small invertebrate (nematodes, ants, termites, worms, etc.) a two-layered biomantle forms, where fine fraction forms the upper layer, and coarse fraction forms as a lag in the lower layer. 3). If the bioturbator is a large invertebrate (land crab, crayfish) or small vertebrate (moles, mole-rats, pocket gophers, tuco tucos), a two-layered biomantle forms, where fine fraction and small gravels are scattered throughout both layers, and coarser gravels accumulate as a lag in the lower. For many midlatitude soils the biomantle includes their bioturbated upper part (A horizon of grassland soils, A-E horizons of forest soils) and stonelayer if present. The stonelayer would normally lie above any subsoil illuvial horizon that might be present. On relatively young surfaces, soil depth may be shallow (e.g., several meters or less) and its biomantle comparatively thin (± 1 meter). But on old surfaces, like some in the humid subtropics and tropics, soils may be many meters deep, and biomantles several meters thick. Aside from catenary processes, biomantle thickness is controlled by the upward biotransfers of fine fraction by generations of termites, ants, etc. balanced against erosion. Such tropical biomantles are invariably two-layered, and sometimes so thick that only exceptional exposures reveal their basal stonelayer, which itself may be a meter or more thick. On young geomorphic surfaces with shallow soils, the biomantle may comprise the whole soil (entire soil is bioturbated). Mounds at Mima Prairie, Washington, are examples. Conversely, mounds at Diamond Grove Prairie, Missouri, are formed on old surfaces with deep soils, and are examples of upper soil biomantles. Strong evidence indicates that mounds in both areas were dominantly produced by pocket gophers (Geomyidae). The mounds in both tracts are gravelly, best described as point-centered, locally thickened, two-layered biomantles, and both exemplify the third category of biomantles (see 3. above). What happens to mounded soils, like those at Mima and Diamond Grove prairies, when the dominant bioturbator abandons them? Do subsidiary processes, like eluviation-illuviation processes -- long masked by active bioturbation but nevertheless evidently still operating -- then become morphologically expressed? Our data indicate that for long abandoned Diamond Grove Prairie mounds the answer is, “Yes”, but for the very recently abandoned Mima Prairie mounds the answer is, “Not yet”. Reference: Johnson, D.L., J. Domier, and D.N. Johnson. 2005. Reflections on the nature of soil and its biomantle. Annals, Association of American Geographers 95 (1): 11-31.