Abstract
AbstractRespiration 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.
Highlights
Carbon outwelling is intimately linked to marsh hydrogeology (Santos et al 2019); external drivers of marsh hydrogeology should be well characterized to estimate carbon flux (Guimond et al 2020b)
Estimated dissolved inorganic carbon (DIC) fluxes are in relative agreement with tidal creek-based modeling approaches
The 224Ra : 228Th disequilibria approach demonstrates that the majority of lateral DIC exports are derived from low marsh sediment horizons intersected by mean low tide, driven by tidal pumping
Summary
The terrestrial groundwater table and sea-level interact with the marsh surface elevation gradient to drive variability in subsurface flow (Guimond et al 2020b), with notable differences between shallow (surficial) and deep flow paths across the marsh. Shallow flow paths are influenced by topographic and tidal gradients (Harvey et al 1987; Nuttle and Hemmond 1988; Xin et al 2011), macro-pore structures (Harvey and Nuttle 1995), bioirrigation and bioturbation (Guimond et al 2020a; Xiao et al 2019), evapotranspiration (Xin et al 2013), vegetation type, and depth of the active root zone (Moffett et al 2012). The 224Ra : 228Th technique may be a valuable tool to characterize key depth horizons of DIC export from intertidal salt marshes
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
