Hydrogeology, Chemical Weathering, and Soil Formation. Allen Hunt

PREFACE

      The soil is a nexus for water, chemicals, and biologically coupled nutrient cycling; it is an upper bound of the sediment and rock cycle and constitutes a lower bound for water and carbon recycling through the atmosphere. Our choice of title, “Hydrogeology, Chemical Weathering, and Soil Formation,” emphasizes the linkage of the topics of soil formation and chemical weathering through water cycling. Of course, biological processes are a foundation of all three of these topics and work on many levels, from nitrogen fixation and soil zonation through bioturbation to large‐scale ecology. The importance of the chemical weathering of silicate minerals to soil formation, the composition of the atmosphere and Earth’s biological history, has been studied for more than a century (e.g. Darwin, 1881; Dokuchaev, 1883) and extensively reviewed (e.g. Berner, 1992).

      According to the Urey reaction, taken to represent a general model of the carbonate‐silicate cycle and weathering of silicate minerals, calcium silicate (wollastonite) is converted to calcium carbonate, while simultaneously gaseous CO₂ is replaced by solid SiO₂. Thus is atmospheric carbon dioxide sequestered in the reservoir of carbonate rocks. Removal of large quantities of atmospheric CO₂ by this reaction is the basis of its relevance to climate change and the history of Earth’s atmosphere, as well as to the evolution of life (chapter 2 of this volume). In other words, without silicate weathering, the Earth would likely have become a runaway greenhouse with an unhabitable climate (Frings & Buss, 2019). No long‐term effect on atmospheric CO₂ results from subsequent weathering of carbonate rocks, which merely triggers renewed sequestration of the same quantity of carbon, even though a short‐term CO₂ drawdown may result from an increase in carbonate weathering rates.

      The silicate weathering reaction is effective only in the presence of water. While the role of water in kinetics is clarified in the expression for the reaction of plagioclase, its typically even more important role in the actual rate of weathering near the Earth’s surface is still largely hidden from view. The rate at which chemical weathering of silicate minerals actually occurs is proportional to the flux of water through Earth’s surface, meaning that the reaction rate of minerals such as plagioclase is often limited not by its reaction kinetics but by transport of either the reacting species into, or reaction products out of, the chemical weathering zone (Blättler & Higgins, 2017; Maher, 2010). Thus, the reaction rate is proportional to the water throughflow within the Earth’s skin (i.e. the soil or regolith), and quantification of the partitioning of the water at the interface between the terrestrial surface and the atmosphere becomes key to understanding the rates of weathering around the world. Chemical weathering and soil formation intersect in this way the cycles of water and of carbon at the terrestrial surface.

      The release of P as well as ions such as K⁺ and Ca²⁺ (or likewise Mg²⁺ and related species) turns out to be critical for soil formation, as such mineral nutrients are essential for the functions of vascular plants, whose respiration, for example, provides a significant part of the CO₂ required to continue the silicate weathering. The process by which plants are established and form communities with microorganisms, leading to the development of soil, is quite complex and is the subject of a great deal of research. In any case, more mature soils are differentiated in depth, with plant roots, litter, and other organic material near the surface, a primarily mineral layer rich in carbon (the A horizon) just below, and a weathered layer with much lower organic content (the B horizon) below that. The rate at which these layers form and differentiate, together with the processes by which they form, is important in a range of human activities as well as geologic processes too wide to list here. The rate at which predominantly the A horizon is lost to erosion is important in agriculture, water quality, river management, and geomorphology, as well as other areas. Most important for the present volume is that the process of chemical weathering appears to be water flow‐rate limited overall. Yet the history of the study of soil formation, tracing back to Darwin and to Dokuchaev in the 19th century, has left an imprint on the study of soils, even while modern understanding places the single process of chemical weathering at the center of examination. While Dokuchaev emphasized the soil formation factors, which can be related to silicate weathering, Darwin’s emphasis was rather on bioturbation. A contrasting approach to soil is discussed by Huggett in the introductory chapter. Here, rather than focusing on rate‐limiting processes or controlling reactions, a unifying picture of a collection of mutually interacting constituents is presented.

      Looked at from a deep‐Earth perspective, the formation of soil feeds back into the deep rock hydrologic cycles, potentially providing a limitation on the rate at which sediment can be transported from its source regions on the continents and islands to regions of deposition along rivers, in lakebeds, and especially in oceans. Variable ranges of soil formation in time and space relate to landforms and their changes, as well as to the isostatic adjustments in the Earth’s crust from erosion and deposition. The volume of sediment subducted in trenches is closely related to its water content through its porosity, described through porosity‐depth relationships, linking the subaerial and subsurface water cycles.

Why should we wish to bring out this volume on soil formation and chemical weathering now? Standard interpretations of soil formation and chemical weathering have been challenged in the 21st century. White and Brantley’s (2003) summary of the chemical weathering of silicate minerals has shown that results of laboratory experiments often bear little resemblance to field results, where silicate substrates may be weathering at a rate as much as six orders of magnitude more slowly. These authors showed that the decline in chemical weathering rates conformed to a power law for periods from weeks to about 6 Myr. The decline in weathering rates tracks rather closely the decline in soil formation rates, which have been known for a long time to slow with time, though no consensus had been reached regarding an associated time‐dependence. Field studies (e.g. Blättler & Higgins, 2017; Maher, 2010; White & Brantley, 2003) provide evidence of the relevance of water fluxes to chemical weathering rates and require development of a compatible theoretical framework. Ultimately, interpretations of the development of paleosols together with the evolution of the atmosphere need to be revised accordingly, with possible relevance to such far‐flung topics as extinctions. Partitioning of carbon between ocean, atmosphere, and biosphere has varied drastically over glacial‐interglacial cycles. The resulting swings in atmospheric CO₂ and climate are particularly instructive for learning how the weathering feedback works (Frings, 2019).

      This volume attempts to place the problem of chemical weathering and soil formation in its geological, climatological, biological, and hydrological perspective. The way in which this is approached is reflected in the organization of the book, which, following Huggett’s defining chapter, continues with a perspective based on Earth’s history (Driese and coauthors). The authors trace the evolution of soils over 3.25 billion years together with geology, paleoclimate and atmospheric composition, and the evolution of life. The following section emphasizes that several basic processes compete for importance in soil formation, namely soil production with its relationship to chemical (and physical) weathering, bioturbation, and aeolian deposition. Chemical weathering and soil formation are related to water and energy fluxes (Hunt), where theoretical approaches of the interactions between the hydrological cycle, soil formation, and net primary production are provided. Le Bayon and coauthors address some of the fundamental aspects of soil formation and development associated with bioturbation, which harks back to Charles