Blue carbon sources and carbon accumulation rates in restored versus historic marshes in southern Puget Sound, Washington, USA

Little is known about carbon accumulation processes in restoring tidal marshes, particularly those that are subsided and have extended restoration trajectories. Here we quantified carbon sources and carbon accumulation rates (CARs) in the sediment sinks of two tidal marshes in the Nisqually River Delta (NRD) of Washington, USA: a restoring, sparsely vegetated tidal marsh (Six Gill Slough, SG-R) and a closely situated historic (reference) marsh (Animal Slough, AS-H). Three sediment cores were collected at both inland and seaward locations at each site approximately six years after restoration. Benthic diatoms, C3 plants, C4 plants, and particulate organic matter (POM) were collected throughout the NRD. δ¹³C and δ¹⁵N values of sources and mixtures were used in a Bayesian stable isotope mixing model to determine the contribution of each carbon source to restoring and historic marsh sediments. Mean CARs at SG were somewhat greater than at AS, but the difference was not significant due to high variability (SG: 81–210 g C m⁻² yr⁻¹; AS: 115–168 g C m⁻² yr⁻¹). Autochthonous marsh C3 plants contributed 73 ±10% (98 g C m^-2 yr^-1) and 89 ±11% (119 g C m^-2 yr^-1) to AS-Inland and AS-Seaward sediment carbon sinks, respectively. At SG-R, allochthonous sources accounted for almost all carbon accumulation. Marsh POM contributed the most at SG-Seaward (42 ±34%) (69 g C m^-2 yr^-1), while Riverine POM contributed the most at SG-Inland (32 ±40%) (52 g C m^-2 yr^-1). Sediment carbon sink composition at both AS-H and SG-R was closely related to carbon source abundance and proximity. Our study demonstrates that sparsely vegetated, restoring tidal marshes represent novel blue carbon ecosystems, which, in storing high amounts of allochthonous carbon, provide greenhouse gas benefits that are similar in magnitude to historic, vegetated tidal marshes.