Distinct Regulation of Riverine Inorganic Carbon by Soil CO2 and Climate
Abstract
The evasion of CO2 from inland waters, a major carbon source to the atmosphere, depends on dissolved inorganic carbon (DIC) concentrations. Despite extensive research on carbon cycling, we have a limited understanding of stream DIC dynamics across gradients of climate, geology, and vegetation conditions. To understand large-scale patterns and drivers of DIC, we collated instantaneous and mean (multi-year average) DIC concentrations (C and Cm) from about 100 rivers draining minimally impacted watersheds in the contiguous United States. Within individual sites, instantaneous concentrations (C) measured at daily to seasonal scales exhibit a near-universal response to changes in river discharge (Q) in a negative power law form, also known as dilution behavior. High concentrations occur at low discharge when DIC-enriched groundwater dominates river discharge; low concentrations occur under high flow when relatively DIC-poor shallow soil water dominates discharge. Such patterns echo the widely observed increase of soil CO2 and DIC with depth and the shallow-and-deep flow path hypothesis that emphasizes the importance of source water chemistry in driving solute export behavior. A parsimonious model reveals that high mean DIC concentrations arise from soil CO2 accumulation in arid climates when DIC export fluxes are lower than rates of DIC-generating reactions. Although instantaneous and mean DIC concentrations decrease with instantaneous and mean discharge similarly, results here highlight their distinct drivers: daily to seasonal-scale instantaneous variations (C) are controlled by subsurface CO2 distribution over depth (from below), whereas long-term baseline concentrations (Cm) are regulated by climate (from above). The results underscore the significance of land-river connectivity via subsurface flow paths and the importance of characterizing subsurface structure and collecting subsurface functioning data to illuminate below-ground processes. It is crucial to continue collecting such data, as understanding subsurface processes and carbon cycling is essential to projecting the future of water and carbon dynamics in a warming climate.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFM.H12G..03S