PHANEROZOIC NON-MARINE ICHNOFACIES AND ATMOSPHERIC CHANGE

The array of ichnofacies pioneered by Adolf Seilacher represents marine ecosystems of varied water depth from the shore to deep ocean, but includes only one non-marine Scoyenia ichnofacies. Additional lacustrine ichnofacies have recently been recognized. Here I outline a variety of paleosol ichnofacies. The Scoyenia ichnofacies is known in red, calcareous Inceptisols as ancient as Ordovician. Root, rhizome and tree-stump traces (paleobotanical form-genus Radicites) are common in virtually all paleosols from the Silurian on. The Coprulus ichnofacies is dominated by small, fecal pellets, and found in Carboniferous Inceptisols and Histosols. The Termitichnus ichnofacies of termite nests is very common in Mesozoic and early Cenozoic calcareous, red Alfisols and Aridisols. The Coprinisphaera ichnofacies of dung beetle and bee nests is abundant and widespread in late Cenozoic brown and gray Mollisols and mollic Inceptisols. None of these ichnofacies has become extinct, but carbon-oxidizing ecosystems (Scoyenia, Skolithos, and Termitichnus ichnofacies) were abundant during times of globally high carbon dioxide and greenhouse paleoclimate, as inferred from sedimentary mass balance and stomatal index proxies. Conversely, carbon-sequestering (Coprulus and Coprinisphaera) ichnofacies were widespread at times of globally low carbon dioxide and icehouse paleoclimate, as inferred from the same proxies and from sedimentological and paleosol (Gelisol) evidence of glaciation. This can be seen on shorter time scales as well. For example, alternations between paleosols of Coprinisphaera and Scoyenia ichnofacies reflect Milankovitch-scale (40-100 ka) greenhouse-icehouse paleoclimatic fluctuation inferred from depth to paleosol calcic horizon in both the Quaternary Palouse Loess of Washington and the Oligocene John Day Formation of Oregon. In addition, abundant Skolithos in Permo-Triassic paleosols of Antarctica and Australia may represent transient greenhouses following late Permian and terminal Permian mass extinctions. Soil invertebrates and plants play an important role in cycling atmospheric gases because they are abundant and widespread at the nexus of rock and air where ecosystems are fueled. They may have been important to atmospheric gas composition in the geological past as well.