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Using Petrography and Geochemistry to Determine the Origin and Formation Mechanism of Calcitic Plant Molds; Rhizolith or Tufa?
Rhizoliths, root casts, and rhizocretions are prolific in the geologic record, and their morphology and isotopic signatures can be important indicators of paleoenvironment and paleoclimate. In a semiarid playa in Nevada, the calcite plant encrustations preserved at the playa surface resemble rhizoliths (morphologically) and would seem to indicate pedogenesis and root activity. The features are preserved as short, thin, upright and/or overturned tubes that often exhibit an apron at the base. Branching is rare, and the tubes can be filled with sediment or calcite, or left open. However, detailed macroscopic, microscopic, and geochemical analyses of these features reveal that they are, instead, macrophyte (stem) encrustations that form in standing water. While these features form in a manner similar to paludal tufa, they do not resemble typical tufa deposits (e.g., barrages, mounds, cascades); instead, they mimic rhizoliths. This study describes the macroscopic and microscopic characteristics of these features and distinguishes them from rhizoliths so that they can be accurately identified in the geologic record. Furthermore, this study identifies a more appropriate method of interpreting the stable-isotope values of these tufa-like plant molds. These types of carbonates are faithful recorders of ambient aquatic conditions (e.g., geochemistry) as well as local hydrologic parameters (e.g., water depth) when the playa surface is flooded, but their geological application is restricted to a partial annual signal (e.g., plant growing season, presence of standing water on the playa). It is important to correctly identify these types of plant encrustations in the geologic record, since (1) their presence indicates a shallow standing body of water (rather than a paleosol) and (2) their isotopic composition records not only an ambient, aquatic signal but an extremely restricted climatic signal that should not be interpreted with models for pedogenic carbonates.
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