Substantial biologically mediated phosphorus depletion from the surface of a Middle Cambrian paleosol

Abstract

Despite recent work, a comprehensive understanding of pre-Devonian terrestrial ecosystems remains elusive, resulting in frequent speculation about their composition. Terrestrial microbial mats are known from the Precambrian paleosol record, and bryophytes are known from the Middle Ordovician palynological record. The intervening time, however, remains enigmatic, although recent studies indicate a progressive greening of the Precambrian prior to the first unambiguous fossil evidence for land plants. In this study, an older than 503 Ma, heavily weathered, clay-rich soil was identified and provides a window to a probable Cambrian terrestrial landscape. The paleosol developed on the ancient continent of Laurentia in a dry, subtropical climate. Mobilization of Si, Al, and P indicate the influence of organic acids. Apatite, a recalcitrant mineral present throughout the majority of the weathered section, is notably absent from the top 30 cm of the paleosol. Few secondary phosphates and a complete absence of oxide- or carbonate-bound P indicate that once biologically weathered, P remained in an ecosystem-available form during the lifetime of this paleosol. The zone of apatite dissolution coincides with intense argillan development, indicating clay flocculation in the presence of soil solutions rich in labile Ca 2+ . Enhanced P weathering coupled with high Ca 2+ concentrations as a result of in situ apatite dissolution is a biosignature previously linked to the activities of mycorrhizal fungi. Biosignatures of mycorrhizal fungi in a Middle Cambrian paleosol add further evidence to the notion that there was an important terrestrial fungal presence prior to the first unequivocal evidence for terrestrial land plants.