Interpreting Redoximorphic Features In Paleosols: What They Can and Cannot Tell Us about Ancient Climates

Redoximorphic features form by movement of Fe and Mn oxides under saturated and reduced conditions. They can be identified by Munsell color, and their relationship to root channels and structural voids. Interpreting redoximorphic features in paleosols presents certain challenges because of changes to soil colors that may have occurred post formation. In addition, the relationship of redoximorphic features to hydrology and climate is difficult to know with certainty.

Redoximorphic features form when four conditions are met in the soil that cause the reduction of Fe or Mn to occur: organic matter must be present, bacteria must be oxidizing the organic matter, the soil must be saturated, and the soil must be anaerobic. When these conditions are met, the redoximorphic features usually begin to form around the source of organic matter. In cases where root tissues are the organic source, redox depletions form around the root channels by loss of Fe. The Fe may diffuse into the matrix and may oxidize when the soils drain. Color patterns diagnostic of redoximorphic features are those with both the gray colors (chromas of 2 or less) where Fe was lost, and also higher (>3) chroma colors where Fe has oxidized.

Four questions that researchers must ask when evaluating redoximorphic features in paleosols will be addressed in the presentation using case studies: 1) are the color patterns seen in the soil truly redoximorphic features, 2) what processes change soil colors over time that could alter the color of the redoximorphic features, 3) what can be inferred from the features about a soil's hydrology, and 4) did soil conditions become drier after the redox features formed? We will use results from contemporary studies on redoximorphic features to interpret similar features in paleosols and answer the four questions posed earlier.