PALEOSOL FEATURES MEDIATED BY ANCIENT SOIL BIOTA

Lower Eocene Willwood paleosols in the Bighorn Basin, Wyoming show various paleopedogenic features including carbonate nodules and complicated paleosol color patterns. Our work shows that these features are intimately related to activities of the ancient, above and below ground soil biota. As in other studies of soils and paleosols, many of the features are attributed to roots and root channels; however, the Willwood paleosols also show that animal burrows were an important mediator of both carbonate nodules and paleosol color patterns.

Carbonate nodules appear to have precipitated in and around burrows and roots. Spherical to elliptical carbonate nodules, ranging from 2 to 30 mm in diameter, are composed of solid to boxwork carbonate surrounded by gray zones depleted of iron oxides. Also present are distinct burrows and roots cemented by carbonate. These grade into burrows and roots that have been segmented and the segments offset from the main trace by paleosol slickenside shear planes. These, in turn, grade into carbonate nodules that retain various features attributed to burrows and roots but that are dispersed through the paleosol. The original shape and surficial morphology of the burrow and root pieces are absent, probably due to continued Eocene pedogenesis.

Red, yellow-brown, and purple mottles are common and attributed not only to roots but also to the activities of other soil organisms such as bacteria, insects and other arthropods, and crustaceans. The mottles range in size from 0.1 to several tens of mm and in shape from crescents and spheres to ellipses that overlap and crosscut one another. Immature paleosols contain discrete burrows with distinct mottle patterns. More mature paleosols have complex mottles that, on close inspection and by comparison to the immature paleosol mottles, are amalgamations of burrow remnants of various sizes and shapes.