Inorganic carbon export from intertidal salt marshes revealed from 224Ra:228Th disequilibria

Intertidal salt marsh respiration generates dissolved inorganic carbon (DIC) that is exported to the coastal ocean by tidal drainage. The relative magnitude and spatial variability of carbon export remains poorly understood, despite recognition that these ecosystems are an important "blue carbon" sink. We constrained the temporal and spatial variability of DIC export from an intertidal salt marsh of the U.S. northeast region (Sage Lot Pond; Winter 2018, Spring 2019, Summer 2019). Temporal variability was constrained from a surface water tidal time-series that combines tidal creek DIC concentrations, high-frequency surface water flux measurements and a hydrodynamic model to capture the net, lateral DIC export through the tidal channel over time. DIC contributions from seawater, marsh porewaters and terrestrial groundwaters were constrained using both a δ¹³DIC and a ²²⁸Ra/²²⁶Ra three-endmember mixing model. To constrain spatial variability in DIC export, we collected a transect of sediment cores and porewater profiles from the tidal creek towards the interior of the marsh platform, for two separate marsh systems. Disequilibrium between ²²⁴Ra and its sediment-bound parent ²²⁸Th indicate significant porewater exchange within the upper few centimeters of each marsh platform (vertical exchange) followed by variable equilibrium, disequilibrium and excess ratios at depth, suggesting heterogeneous subsurface water transport. Integrating ²²⁴Ra fluxes from the marsh surface to the peat-sand interface (~150 cm depth) results in water exchange rates on the order of tens of L m^-2 d^-1. Water exchange rates combined with shallow porewater DIC concentrations constrain the magnitude of lateral DIC export from ~100 to >1000 g C m^-2 y^-1, depending upon distance from the tidal creek and season. Results will be compared between the tidal time-series and ²²⁴Ra:²²⁸Th disequilibria to elucidate spatial variability in DIC flux over winter, spring and summer seasons.