Reviews and syntheses: 210Pb-derived sediment and carbon accumulation rates in vegetated coastal ecosystems: setting the record straight

Vegetated coastal ecosystems, including tidal marsh, mangrove and seagrass, are being increasingly assessed for their potential in carbon dioxide sequestration worldwide. However, there is a paucity of studies that have effectively estimated the accumulation rates of sediment organic carbon (Corg) beyond the mere quantification of Corg stocks. Here, we discuss the use of the 210Pb dating technique as a practical tool to measure the rate of Corg accumulation in vegetated coastal ecosystems. We critically review the status of 210 Pb dating methods of vegetated coastal sediments and assess the limitations that apply to these ecosystems, which are often composed by heterogeneous sediments, abundant in coarse particles, with varying inputs of organic material, and are disturbed by natural and anthropogenic processes causing sediment mixing, changes in sedimentation rates or erosion. Through a range of simulations, we discuss the most relevant processes that impact the 210Pb record in vegetated coastal ecosystems and evaluate the deviations in sediment and Corg accumulation rates produced by anomalies in 210Pb profiles. Our results show that the deviation in the determination of sediment and derived Corg accumulation rates is within 20% confirming that the 210Pb dating technique is secure. However, while these uncertainties might be acceptable for the determination of mean sediment and Corg accumulation rates over the last century, they may not always allow the determination of a detailed geochronology, historical reconstruction, or to ascertain rates of change and fluxes. Additional tracers or geochemical data need to be used in concert to constrain the 210Pb-derived results and to properly interpret the processes recorded in vegetated coastal sediments. The framework provided in this study can be instrumental in reducing the uncertainties associated to the estimates of Corg accumulation rates in vegetated coastal sediments.This work was funded by the CSIRO Flagship Marine & Coastal Carbon Biogeochemical Cluster (Coastal Carbon Cluster), the Spanish Ministry of Economy and Competitiveness (Project EstresX, CTM2012- 32603), the Generalitat de Catalunya (MERS 2014 SGR – 1356), the Edith Cowan University Faculty Research Grant Scheme and King Abdullah University of Science and Technology (KAUST) through baseline funding to CMD. This work contributes to the ICTA ‘Unit of Excellence’ (MinECo, MDM2015-0552). AAO was funded by a PhD grant from Obra Social “laCaixa”. OS was supported by an ARC DECRA (DE170101524). IM was funded by a post-doctoral grant (Juan de la Cierva-Formación) from the Spanish Ministry of Economy, Industry and Competitiveness.Peer reviewe