Carbonates in the Critical Zone.

Earth's Critical Zone (CZ), the near‐surface layer where rock is weathered and landscapes co‐evolve with life, is profoundly influenced by the type of underlying bedrock. Previous studies employing the CZ framework have focused primarily on landscapes dominated by silicate rocks. However, carbonate rocks crop out on approximately 15% of Earth's ice‐free continental surface and provide important water resources and ecosystem services to ∼1.2 billion people. Unlike silicates, carbonate minerals weather congruently and have high solubilities and rapid dissolution kinetics, enabling the development of large, interconnected pore spaces and preferential flow paths that restructure the CZ. Here we review the state of knowledge of the carbonate CZ, exploring parameters that produce contrasts in the CZ in different carbonate settings and identifying important open questions about carbonate CZ processes. We introduce the concept of a carbonate‐silicate CZ spectrum and examine whether current conceptual models of the CZ, such as the conveyor model, can be applied to carbonate landscapes. We argue that, to advance beyond site‐specific understanding and develop a more general conceptual framework for the role of carbonates in the CZ, we need integrative studies spanning both the carbonate‐silicate spectrum and a range of carbonate settings. Plain Language Summary: Carbonate landscapes, which cover ∼15% of Earth's land surface and provide critical water resources and other services to ∼1.2 billion people, require focused studies to understand how life and rocks interact. Most integrated studies of this "critical zone (CZ)" focus on landscapes underlain by silicate minerals instead of considering the full spectrum of the minerals that make up bedrock. Weathering extends to greater depths in carbonate landscapes compared with silicate landscapes, leading to the development of interconnected subsurface flow systems that transport both water and sediments. As a result, the flow of water and the movement of materials left behind by weathering rock may be disconnected from streams, unlike in silicate landscapes. Furthermore, responses of the carbonate CZ to changes in land use and climate may be rapid because carbonate rocks dissolve faster than silicate rocks. Integrative studies of silicate, carbonate, and mixed silicate‐carbonate landscapes will be required to construct a holistic understanding of Earth's CZ. Key Points: A holistic understanding of Earth's critical zone (CZ) requires integrative studies spanning the spectrum of carbonate and silicate landscapesPorosity developed by congruent dissolution of carbonates decouples hillslopes from stream channels, altering topographic equilibriumShifts in carbonate CZ structure from changing ecology, land use, and climate may be rapid because of fast dissolution kinetics [ABSTRACT FROM AUTHOR]

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