Biogeochemical characterization of a lithified paleosol: Implications for the interpretation of ancient Critical Zones

Abstract

Modern soils are characterized by an array of physical, chemical, mineralogical, and biological laboratory analyses of samples taken from horizons of pedogenic profiles. In contrast, fossil soils (paleosols) are typically characterized from assays of whole-rock molecular oxides because of sample lithification where element sources are unconstrained. Here we for the first time subject a lithified paleosol to an array of modern soil analytical techniques and new pedotransfer functions, providing a glimpse into the colloidal world of an ancient Critical Zone as an analog to research conducted on a network of modern Critical Zones. This methodology provides a framework for decoding a previously unknown archive of terrestrial biogeochemical information at multiple temporal and spatial scales. Application to a paleosol within an early Paleocene Critical Zone reveals that many biogeochemical properties have been preserved since burial that are similar to modern clay-rich, Vertisols. The measured and calculated physical properties of this paleosol include clay content and mineralogy, bulk density and water retention, available water capacity, and coefficient of linear extensibility (shrink–swell potential). The chemical properties include cation exchange capacity, exchangeable cations, base saturation, and exchangeable sodium percentage. The solution properties electrical conductivity and pH seem reasonable, but are interpreted with less confidence because of their greater vulnerability to alteration from fluid flow. New pedotransfer functions to reconstruct pre-burial organic carbon and nitrogen contents provide invaluable information of organically-derived nutrient content. The sum of the measured properties of the early Paleocene paleosol in context of reconstructed regional environmental conditions indicate the presence of a mid-successional hardwood forest in a humid climate with high water holding capacity, high nutrient retention, and rapid flux of elements through biogeochemical cycling.