Measuring Carbon stock and accumulation rates prior to a beneficial use restoration project, Deal Island, MD, USA

Global climate change has inspired fervent investigation into salt marshes as blue carbon sinks and their potential role in mitigating climate change through CO 2 drawdown and long-term storage. Conservation and restoration of existing marsh complexes can preserve the valuable carbon stock already in place while reducing marsh vulnerability to erosion and future sea-level rise. One such restoration method is the beneficial use or re-use of dredge material to raise a marsh platform. While this method has shown success in minimizing vulnerability by enhancing vegetation growth, the blue carbon implications are largely unknown. This work aims to address the gaps in research around blue carbon in conservation and restoration by establishing a baseline carbon stock prior to the large-scale restoration of a marsh site in Deal Island, MD, USA through the beneficial use of dredged sediments. We measured carbon accumulation rates since marsh formation (integrated over an average basal age of approximately 670 y) and showed similar rates to those found in the literature. Carbon accumulation rates since 1900 were faster than the long-term rates (since approximately 1343 AD) but still similar to those found in the literature integrated over comparable time periods (344 ± 9 g C m -2 y -1 ). Total soil carbon stock was estimated within the proposed Deal Island restoration site to be 15,500 tons. Surface sediment cores taken 4 months to 1 year post placement suggest that organic matter in the placed sediments is increasing across the platform since placement. These long timescale carbon accumulation rates and soil carbon stock measurements within the Deal Island site offer an example of a high-resolution baseline for projects looking to potentially move forward with carbon crediting through avoided loss. Additionally, the combination of long and short timescale carbon measurements (10-cm push cores, 1-m push cores, and 1.5 m peat cores) provide insight to a point of comparison once the restoration project has matured and monitoring commences. This data coupled with some early post-placement carbon measurements give a valuable snapshot into how a restored marsh complex functions from a carbon perspective before to early post placement. • Established a baseline for carbon stock and accumulation rates prior to a large-scale beneficial use marsh restoration project. • Demonstrated that the marsh’s pre-restoration sediment accumulation rates lagged behind local sea-level rise, justifying the intervention to enhance resilience. • Provided early evidence that surface organic matter is increasing across the platform within one-year post-placement, offering initial insight into the fate of capped carbon.