CONTINENTAL MEGA-ICHNOLOGY—TRACE FOSSILS AS PROXIES FOR THE ROLES OF ORGANISMS AS A MAJOR SOIL-FORMING FACTOR AND AS ECOSYSTEM ENGINEERS OF THE CRITICAL ZONE THROUGH DEEP TIME

As one of the five soil-forming factors (Climate, Organisms, Relief, Parent material, Time; ClORPT) and one of the two major engines (Climate, Organisms) of pedogenesis (soil formation), organisms shape the soil body through diffusive and advective particle mixing that directly or indirectly mediate the creation, destruction, and/or modification of pedogenic fabrics, voids, horizons, and soil atmospheric and hydrologic conditions. Soils have formed on the surface of the Earth for ~4 byr, recording linkages between the lithosphere, atmosphere, hydrosphere, and biosphere, and changes through time. Since the Late Ordovician (~450 myr ago), pedoturbation by plants and especially animals is most important to soil formation, as together they shape the character of the soils we see preserved as paleosols and sediments within the modern Critical Zone (CZ). This deep-time record of terrestrial bioturbation, in a sense, is mega-ichnology in that the traces of continental organisms are proxies for the role in space and time of organisms as a major soil-forming factor and as ecosystem engineers that shaped the evolution of landscapes and the CZ. Here we focus on the work of macrofauna and megafauna (invertebrates and vertebrates > 2 mm in size) that interact with the soil environment, whether they belong to the epigeon (all stages of life above ground), or are transient (brief periods spent belowground by adults), geophilic (juvenile stages of life cycle and pupation belowground, emerging as adults), or permanent (whole lifecycle including adults belowground) in terms of their presence in soils. The combination of trackmaking and trampling, and advective and diffusive bioturbation from the sediment-air interface of the vadose zone to the sediment-water-air interface (water table) delineating the top of the phreatic zone as well as the upper part of that zone has ever-increasingly shaped the landscape and the degree of translation, transformation, additions, and losses of biophysicochemical constituents throughout the evolution of continental biota. This ultimately resulted in a variety of pedogenic fabrics and horizonation of soils, interconnected with the detrivore-dominated nutrient cycling system of above- and belowground communities expressed as epigeon, transient, geophilic, permanent faunas.