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), also termed blue carbon, beyond the mere quantification of C stocks. Here, we discuss the use of the Pb dating technique to determine the rate of C accumulation in these habitats. We review the most widely used Pb 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 records in vegetated coastal ecosystems and evaluate how anomalies in Pb specific activity profiles affect sediment and C accumulation rates. Our results show that the discrepancy in sediment and derived C accumulation rates between anomalous and ideal Pb 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 accumulation rates over the last century, they may not always provide a reliable geochronology or historical reconstruction. Reliable estimates of C 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 Pbderived results. The framework provided in this study can be instrumental in reducing the uncertainties associated with estimates of C accumulation rates in vegetated coastal sediments.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: This work was funded by the CSIRO Flagship Marine & Coastal Carbon Biogeochemical Cluster (Coastal Carbon Cluster), the Spanish Ministry of Economy and Competitiveness (projects EstresX CTM2012-32603, MedShift CGL2015-71809-P), the Generalitat de Catalunya (MERS 2017 SGR – 1588), the Australian Research Council LIEF Project (LE170100219), the Edith Cowan University Faculty Research Grant Scheme and the King Abdullah University of Science and Technology (KAUST) through baseline funding to Carlos M. Duarte. This work contributes to the ICTA Unit of Excellence (MinECo, MDM2015-0552). Ariane Arias-Ortiz was funded by a PhD fellowship from Obra Social “la Caixa”. Oscar Serrano was supported by an ARC DECRA (DE170101524). Inés Mazarrasa was funded by a postdoctoral grant (Juan de la Cierva-Formación) from the Spanish Ministry of Economy, Industry and Competitiveness.We thank Sabine Schmidt and three anonymous reviewers whose comments helped improve and clarify this paper.