Significance of different modes of rhizolith preservation to interpreting paleoenvironmental and paleohydrologic settings: Examples from Paleogene paleosols, Bighorn Basin, Wyoming, USA

Different modes of preservation of root traces (rhizoliths) provide information on soil-moisture regimes in alluvial paleosols in the Paleogene Fort Union and Willwood formations of the Bighorn Basin, Wyoming, U.S.A. This paper links different styles of rhizolith preservation to paleosols whose other pedogenic attributes provide information on ancient soil drainage. Consequently, the paleodrainage significance of different rhizolith preservation patterns is established more confidently. Rhizoliths consisting of elongate gray mottles (rhizohaloes) with red rims are common in relatively well-drained red paleosols. The gray zones are depleted of iron (Fe) whereas the color of the rims indicates hematite accumulation. These features are typical of surface-water gley processes that caused Fe and manganese (Mn) to move from the root channel outward to the soil matrix. Calcareous rhizocretions-either calcareous, tubular concretions or micro-accumulations of carbonate within gray rhizotubules-are also common in moderately well-drained red paleosols. More poorly drained purple paleosols also have rhizoiiths consisting of Fe depletion zones; however, the rhizohaloes are surrounded by yellow-brown (goethite) rims rather than red hematitic rims. In many paleosols, the red and yellow-brown accumulation rims were partly removed by continued depletion, producing abundant irregular-shaped mottles. The nature of the rhizohalo and the color of the Fe-oxide rim provide information about the intensity of gleying that a paleosol underwent. The poorly drained paleosols also show rhizoliths preserved as goethite accumulations contained within gray depletion zones, indicating movement of reduced Fe from the matrix towards the root channel, probably as a result of groundwater-gley processes. The goethite accumulations are intermingled locally with lignite, representing preservation of the root organic material. Some rhizoliths are preserved as tube-shaped concentrations of small (0.1-1.2 mm diameter) black spheres whose color indicates a mix of Mn and Fe oxides. These are most common in paleosols with low-chroma (gray) matrix colors, indicating very poor drainage. The very poorly drained paleosols also show rhizoliths preserved in jarosite, which is an oxidation product of pyrite. Careful observational and geochemical analysis of rhizoliths, which are common in continental deposits, can help produce a clearer and more thorough interpretation of ancient drainage conditions. Information about degree of ancient soil wetness or moisture is important for understanding past climatic conditions and for reconstructing terrestrial paleolandscapes.