IN PRAISE OF THE COARSE FRACTION AND BIOTURBATION: GRAVELLY MIMA MOUNDS AS TWO-LAYERED BIOMANTLES

Understanding the biodynamics of the soil coarse fraction provides insights on how Mima-type mounds form.

Particle size analysis of soils, with a focus on the fine fraction -- the sand, silt, and clay -- has a long and time-honored history in pedology and soil science, particularly in soil genesis studies. Likewise the legacy of the fine fraction in agronomic assessments of soils is legend. In fact, every student of soil science, pedology, and geomorphology is early sensitized to the agronomic and pedologic importance of the fine fraction. The coarse fraction -- those troublesome gravels, on the other hand, has a much less lofty legacy, and in fact for many decades was a poor and often ignored particle size stepchild, an ugly duckling in the agronomic scheme of things, a more or less neutral entity in pedology and soil genesis studies.

Further, in the early days of pedology, the set of processes termed bioturbation was unrecognized as a pedogenic force. In fact, the term didn't exist before 1952 (bioturbation was coined to aid in ichnological assessments). Bioturbation effectively appeared in the soil and geomorphic literature in the early 1980's, and is now a key element of our pedogenic lexicon. It became central to the biomantle concept that was formulated in 1990. The biomantle is the upper part of soil produced largely by biota, dominantly by bioturbation. Biomantles are invariably one-layered when formed in fine fraction materials, and two-layered when formed in mixed fine-and-coarse materials.

If Mima-type mounds are of fossorial rodent bioturbational origin, as many believe, biomantle theory predicts that those formed in gravels are simply two-layered biomantles. Evidence is compelling that they are.