Applying the Critical Zone Concept to Reconstruction of Ancient Terrestrial Systems

The pedosphere (active soil zone) is a biogeochemical mediator of the Earth surface system. Recent studies have advanced our ability to reconstruct ancient pedospheres with development of refined geochemical proxies for climate and biological interpretations and pedotransfer functions for biogeochemical foundation. In 2006 the US National Science Foundation (NSF) Earth Sciences Directorate (EAR) funded creation of a national Critical Zone Observatory (CZO) program. As defined in the National Research Council and NSF reports leading to establishment of this program, the “Critical Zone” (CZ) extends from the top of the vegetation canopy to the groundwater table, and integrates interactions of the atmosphere, lithosphere, pedosphere, biosphere, and hydrosphere. We propose to extend the CZ concept to include deep-time Critical Zones (DTCZs), where deep-time is arbitrarily defined as pre-Quaternary (>2 million years ago). Because of recent advances in the study of fossil soils, which are known as “paleosols”, it is now possible to reconstruct CZs and associated biogeochemical cycles from paleosols preserved in the sedimentary record in deep-time. We present a case study of a DTCZ as investigated within the conceptual framework of a deep-time Critical Zone Observatory (DTCZO). Additional advances in interpretations derived from systematic mining of a modern soil geochemical data base will improve our understanding of the geochemistry of weathering, and the veracity of the records of the paleo-atmosphere, biosphere, and hydrosphere. Studies of past terrestrial systems, and especially soil systems, are crucial to making predictions of the state of the future Earth system in response to climate change.