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 of the CZ have focused almost exclusively 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 and examine whether current conceptual models of the CZ, such as the conveyor model, can be applied to carbonate landscapes. We introduce the concept of a carbonate-silicate CZ spectrum. To obtain a holistic understanding of Earth’s CZ we must understand CZ processes and architecture along the entire spectrum between the carbonate and silicate endmembers. We explore parameters that produce contrasts in the CZ in different carbonate settings and identify important open questions about carbonate CZ processes. 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.

Large rivers can retain a substantial amount of nitrogen (N), particularly in submerged aquatic vegetation (SAV) meadows that may act as disproportionate control points for N retention in rivers. However, the temporal variation of N retention remains unknown since past measurements were snapshots in time. Using high frequency measurements over the summers 2012-2017, we investigated how climate variation influenced N retention in a SAV meadow at the confluence zone of two agricultural tributaries entering the St. Lawrence River. Distinctive combinations of water temperature and level were recorded between years, ranging from extreme hot-low (2012) and cold-high (2017) summers (2 ˚C and 1.4 m interannual range). Using an indicator of SAV biomass, we found that these extreme hot-low and cold-high years had reduced biomass compared to hot summers with intermediate levels. In addition, change in main stem water levels were asynchronous with the tributary discharges that controlled NO3- inputs at the confluence. We estimated daily N uptake rates from a moored NO3- sensor, and partitioned these into assimilatory and dissimilatory pathways. Measured rates were variable but among the highest reported in rivers (median 576 mg N m-2 d-1;, range 60 – 3893 mg N m-2 d-1) and SAV biomass promoted greater proportional retention and permanent N loss through denitrification. We estimated that the SAV meadow could retain up to 0.8 kt N per year and 87% of N inputs, but this valuable ecosystem service is contingent on how climate variations modulate both N loads and SAV biomass.