DEFINING THE NEED FOR CARBONATE CRITICAL ZONE RESEARCH

Often critical zone (CZ) dynamics in silicate terrains are conceptualized as a conveyor belt, meaning unweathered bedrock is brought to the surface and converted to regolith and soil, with insoluble components being transported down hillslopes into the stream network. Feedbacks between hillslope transport, weathering, and streams equilibrate topography and soil thickness to tectonic and climate forcing. However, this model breaks down in the critical zone of pure carbonate terrains in part because congruent weathering of carbonate minerals leaves voids rather than mineral alteration products. In addition, weathering rates are orders of magnitude faster for carbonate than silicate minerals. Consequently, the CZ has fundamental differences in physical characteristics and processes in carbonate compared with silicate terrains. Because of these differences, we define two end members of the CZ: the silicate CZ and the carbonate CZ. Rapid carbonate reaction rates allow voids to form more rapidly in the carbonate CZ than regolith in the silicate critical zone. The carbonate CZ may thus exhibit responses to short term perturbations such as floods, fires, and anthropogenic climate change unobservable in the silicate CZ over short time periods. Voids enable transfer of water and sediment through the subsurface, potentially decoupling hillslopes from stream networks. Whereas the conveyor model of the silicate CZ predicts planar weathering surfaces, carbonate weathering is focused along preferential flow paths, creating an irregular weathering surface that spans a wide range of depths. As a result, the carbonate CZ may extend hundreds or thousands of meters below the surface, to depths greater than in silicate terrains. In addition to end-member cases of pure carbonate and silicate terrains, carbonate and silicate minerals occur in mixtures from the grain scale to the stratigraphic scale. Therefore a better understanding of the impacts of carbonate minerals on CZ processes across the entire carbonate-silicate spectrum is crucial to obtaining holistic knowledge of Earth’s CZ. Consequently, the Carbonate Critical Zone Research Coordination Network (CCZ RCN) was established to advance the understanding of how carbonate minerals alter critical zone characteristics and has led to this Topical Session.