CARBON EXPORTS FROM THE CRITICAL ZONE

In the terrestrial environment vascular plants photosynthesize and respire to produce organic matter and CO₂. These products dissolve and are processed belowground to become dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) for export in catchment drainage. This is a synthesis study gathering recent developments from Earth and allied sciences to assemble a simple framework explaining the interplay among these processes as water moves through the heterogeneous Critical Zone (CZ), the skin of Earth from the top of the canopy to the base of active groundwater circulation. In this framework chemical weathering is a keystone process fueling energy flow through the system: its shallow-focused “ecologic” function dynamically generates nutrients positively feeding back into biosynthesis (and DOC generation), supporting heterotrophy in the CZ and receiving waters. This in turn drives the steadier, more spatially diffuse “geologic” function of weathering, entraining and stabilizing CO₂ in solution (DIC generation) for storage in the hydrosphere and lithosphere. At the planetary scale, carbon exports are plate-tectonically and thermostatically capped and floored by volcanic CO₂ production and carbonate chemistry in Earth’s crust. The CZ framework prompts straightforward hypotheses to explain temporal and spatial patterns of exports from catchments, including how DOC and DIC exports differentially respond to geologic setting, hydrologic dynamics, and ecosystem development.