Exploring the sedimentary, pedogenic, and hydrologic factors that control the occurrence and role of bioturbation in soil formation and horizonation in continental deposits: An integrative approach

Interpretation of paleosols involves deciphering the complex interplay between multiple biotic and abiotic processes. Previous ichnological research has shown that trace fossils, which record biotic influences on pedogenesis, are particularly useful for reconstructing physicochemical conditions during soil formation, which, in turn, can yield important data about paleoenvironment, paleohydrology, and paleoclimate. Our goal is to integrate ichnology with the substantial body of research that exists in the areas of sedimentology and pedology to present an integrative framework with which to interpret paleosols in the continental rock record. Tiering of traces is particularly prominent in terrestrial settings because the vertical distribution of soil biota is controlled largely by the groundwater profile. Interpretations of trace fossils are therefore facilitated by assigning traces to one of four moisture regimes: epiterraphilic, terraphilic, hygrophilic, and hydrophilic. The balance between deposition and pedogenesis is expressed by the paleosol profile, which can range from simple to compound, composite, or cumulative. The combination of sedimentation and pedogenesis, including groundwater-influenced bioturbation, can act to enhance or destroy horizonation within soils; these processes ultimately determine the paleosol characteristics that are preserved in the stratigraphic record. We illustrate our conceptual model with examples of multiple paleosol types that contain evidence of varying amounts of bioturbation attributable to crayfish. INTRODUCTION Trace fossils, studied via their architectural and surficial morphologies and the material that fills them, record information about the tracemaker and the physicochemical conditions of its surroundings (Bromley, 1996; Hasiotis, 2002). Deciphering the occurrence, depth, and tiering of plant and animal trace fossils in paleosols is paramount to interpreting the sedimentological, pedological, and hydrological conditions under which paleosols formed in continental deposits (Bown and Kraus, 1983; Hasiotis, 2002, 2007). This is particularly important for reconstructing the postdepositional histories of landscapes and the physicochemical conditions experienced by the organisms and soils, and recorded by sedimentary and pedogenic (i.e., biotic and abiotic) fabrics (Jenny, 1941; Hasiotis, 2004, 2008; Schaetzl and Anderson, 2005; Hembree and Hasiotis, 2007; Smith et al., 2008). Bioturbation in modern soils, particularly by animals, is known to be extremely effective at mixing sediment at and within the subsurface, and helping to build and destroy pedogenic structures and voids while playing a major role in nutrient cycling (Wallwork, 1970; Hole, 1981). Herein we explore the hydrological, sedimentological, and pedological factors that control the spatial and temporal distribution of terrestrial and aquatic bioturbation that results in pedogenic fabrics and horizonation, and the physicochemical expression of organism-sediment interactions. Our objective is to demonstrate that trace fossils can be equally as powerful in reconstructing paleoenvironments and the depositional histories of continental sedimentary successions as marine trace fossils based on their occurrence, tiering, depth, and relation to the sedimentary facies (Figure 1). Despite the similarities between continental and marine trace fossils and their resultant bioturbation, their genesis and significance are distinctly different from each other because the specific organism behaviors and biophysicochemical conditions under which the traces formed are exclusive to each realm of deposition (e.g., Hasiotis and Bown, 1992; Hasiotis, 2002, 2008). MATERIAL AND METHODS Actualistic studies of the spatial and temporal distribution of terrestrial and aquatic organisms in continental environments provide the dataset from which we explore ichnopedologic fabrics—sedimentary fabrics that result from bioturbation and pedoturbation (Appendix A)(Hasiotis, 2007; Hasiotis et al., 2007, in press, and references therein). These and other actualistic studies were synthesized to understand how organisms interact with sediment to produce bioturbation that results in soils and horizonation through organism activity. Tracemakers of ichnofossils in paleosols are inferred from the trace fossil and its relationship to the sedimentary and pedogenic fabric. These interpretations are also made via comparison to similar structures found in modern depositional systems, continental environments, and pedogenically modified sediment that are analogous to the studied geologic deposits.