Pore water exchange-driven inorganic carbon export from intertidal salt marshes

Respiration in intertidal salt marshes generates dissolved inorganic carbon (DIC) that is exported to the coastal ocean by tidal exchange with the marsh platform. Understanding the link between physical drivers of water exchange and chemical flux is a key to constraining coastal wetland contributions to regional carbon budgets. The spatial and temporal (seasonal, annual) variability of marsh pore water exchange and DIC export was assessed from a microtidal salt marsh (Sage Lot Pond, Massachusetts). Spatial variability was constrained from ²²⁴Ra : ²²⁸Th disequilibria across two hydrologic units within the marsh sediments. Disequilibrium between the more soluble ²²⁴Ra and its sediment-bound parent ²²⁸Th reveals significant pore water exchange in the upper 5 cm of the marsh surface (0–36 L m⁻² d⁻¹) that is most intense in low marsh elevation zones, driven by tidal overtopping. Surficial sediment DIC transport ranges from 0.0 to 0.7 g C m⁻² d⁻¹. The sub-surface sediment horizon intersected by mean low tide was disproportionately impacted by tidal pumping (20–80 L m⁻² d⁻¹) and supplied a seasonal DIC flux of 1.7–5.4 g C m⁻² d⁻¹. Export exceeded 10 g C m⁻² d⁻¹ for another marsh unit, demonstrating that fluxes can vary substantially across salt marshes under similar conditions within the same estuary. Seasonal and annual variability in marsh pore water exchange, constrained from tidal time-series of radium isotopes, was driven in part by variability in mean sea level. Rising sea levels will further inundate high marsh elevation zones, which may lead to greater DIC export.