A biogeochemical perspective on pedogenesis from soils to paleosols

Soils form at the surface of the Earth under the influence of processes governed by an upper boundary with the atmosphere. These processes alter mineral, elemental and isotopic compositions of preexisting materials resulting in vertical biogeochemical gradients that are visually manifested as soil horizons. Processes and the factors that control them are thus linked with geochemical compositions. Therefore, as the factors controlling soil development change, so do soil compositions. Some compositions respond rapidly to change in controlling factors whereas others are more resistant to change. These differing stabilities result in partial memories of environmental change encoded in soil biogeochemical properties. A small fraction of soils are buried by younger sediments, with a strong bias toward those forming in sediments deposited in distal basins. Some soils are buried beneath the soil-forming zone and some are buried deeply enough to become lithified. The biogeochemical properties of these buried paleosols can, with varying degrees of certainty and specificity, be linked back to the processes that formed them, helping us understand paleoenvironments on Earth. Some biogeochemical properties preserve well during diagenesis, especially certain aspects of the mineralogy. Some biogeochemical properties do not preserve well, especially those related to organic matter and surface chemistry such as pH and cation exchange capacity. There also are some biogeochemical properties which exist on a continuum of preservation and may preserve well in spite of diagenetic conditions such as the stable isotope ratios of pedogenic minerals and bulk elemental compositions. We have a poor understanding of the degree to which some biogeochemical properties are preserved, especially those related to nutrient cycling by vegetation. In this chapter we review the biogeochemistry of soils through the lens of processes associated with an upper boundary with the atmosphere and with a focus on attributes that have been, or we believe in the future should be, studied in paleosols. We then discuss diagenesis and provide a brief review of paleoenvironmental reconstructions using paleosol biogeochemistry. We also summarize the current state of knowledge regarding what processes might and might not result in paleosol geochemical signals that survive diagenesis. At the end of the chapter we describe some frontiers that the authors think are particularly interesting and promising.