Thematic Articles

Soils are mixtures of material derived from substrate weathering, plant decomposition, and solute and particulate deposition from the atmosphere. The relative contribution from each source varies widely among soil types and environments. Atmospheric deposition of marine and mineral aerosols can have a major impact on the geochemistry and biogeochemistry of the Critical Zone. Some of the best-studied examples are from ocean islands because of the strong geochemical contrast between bedrock and atmospheric sources, but for the most part continental areas are more severely impacted by atmospheric deposition. With dust flux greater than 10% of the global river sediment flux, deposition from the atmosphere plays an important role in the biogeochemistry of soils worldwide.

The surface of our planet is the result of billions of years of feedback between biota and Earth materials. The chemical weathering of soils and the resulting stream and ocean chemistry bear the signature of the biological world. Physical shaping of the Earth’s surface in many regions is a biologically mediated process. Given the pervasiveness of life, it is challenging to disentangle abiotic from biotic processes during field observations, yet it is of paramount importance to quantify these interactions and their feedbacks as the human impact on climate and ecosystems becomes more profound. Here we briefly review the fascinating connection between rocks and life and highlight its significance to science and society.

Many processes that affect soil and water quality occur at the water wetted interface of weathering products such as clays, oxides, and organic matter. Especially near the sunlit surface of the Critical Zone, these interfaces associate with plant roots and soil organisms to form porous, aggregated structures. Soil aggregates and intervening pore networks give rise to a patchwork of interconnected microenvironments. The ensuing steep geochemical gradients affect weathering processes, fuel the activities of microbes, and drive interfacial reactions that retain and transform rock- or ecosystem-derived chemicals and anthropogenic pollutants.

Geochemists have long recognized a correlation between rates of physical denudation and chemical weathering. What underlies this correlation? The Critical Zone can be considered as a feed-through reactor. Downward advance of the weathering front brings unweathered rock into the reactor. Fluids are supplied through precipitation. The reactor is stirred at the top by biological and physical processes. The balance between advance of the weathering front by mechanical and chemical processes and mass loss by denudation fixes the thickness of the Critical Zone reactor. The internal structure of this reactor is controlled by physical processes that create surface area, determine flow paths, and set the residence time of material in the Critical Zone. All of these impact chemical weathering flux.

The Critical Zone (CZ) is the system of coupled chemical, biological, physical, and geological processes operating together to support life at the Earth’s surface. While our understanding of this zone has increased over the last hundred years, further advance requires scientists to cross disciplines and scales to integrate understanding of processes in the CZ, ranging in scale from the mineral–water interface to the globe. Despite the extreme heterogeneities manifest in the CZ, patterns are observed at all scales. Explanations require the use of new computational and analytical tools, inventive interdisciplinary approaches, and growing networks of sites and people.