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

Vegetated coastal ecosystems, including tidal marshes, mangroves and seagrass meadows, are being increasingly assessed in terms of their potential for carbon dioxide sequestration worldwide. However, there is a paucity of studies that have effectively estimated the accumulation rates of sediment organic carbon (C-org), also termed blue carbon, beyond the mere quantification of C-org stocks. Here, we discuss the use of the Pb-210 dating technique to determine the rate of C-org accumulation in these habitats. We review the most widely used Pb-210 dating models to assess their limitations in these ecosystems, often composed of heterogeneous sediments with varying inputs of organic material, that are disturbed by natural and anthropogenic processes resulting in sediment mixing and changes in sedimentation rates or erosion. Through a range of simulations, we consider the most relevant processes that impact the Pb-210 records in vegetated coastal ecosystems and evaluate how anomalies in Pb-210 specific activity profiles affect sediment and C-org accumulation rates. Our results show that the discrepancy in sediment and derived C-org accumulation rates between anomalous and ideal Pb-210 profiles is within 20% if the process causing such anomalies is well understood. While these discrepancies might be acceptable for the determination of mean sediment and C-org accumulation rates over the last century, they may not always provide a reliable geochronology or historical reconstruction. Reliable estimates of C-org accumulation rates might be difficult at sites with slow sedimentation, intense mixing and/or that are affected by multiple sedimentary processes. Additional tracers or geochemical, ecological or historical data need to be used to validate the Pb-210-derived results. The framework provided in this study can be instrumental in reducing the uncertainties associated with estimates of C-org accumulation rates in vegetated coastal sediments.