Sediment depth and accumulation constrain belowground carbon stocks across northern New World mangroves.

Much recent effort has been spent estimating mangrove carbon stocks, but ecological and geological drivers of the variation in these stocks remain poorly understood. The long-term sedimentary carbon sinks of mangroves and other blue carbon ecosystems have rapidly become a focus of research and conservation attention. However, sampling coverage remains very low, with sediment cores sparsely distributed across a subset of mangrove environmental settings. We cored diverse mangrove environments to assess the limits of mangrove sedimentary carbon storage: the volcanic Galapagos, the arid lagoons of the Baja Peninsula, and the geologically and climatically distinct Caribbean and Pacific coasts of Panama. We cored entire sediment columns, subsampled with depth, and dried, weighed, and analyzed the samples for their organic carbon content. Depth-integrated carbon stocks are highly variable, from < 17 to > 1,700 MgC_org/ha. Contrary to global models, we did not find a positive relationship between sediment carbon density and annual rainfall, though some carbon density differences across areas emerged. Variation in sediment depth, ranging from 7 to 427 cm across sites, dominated variation in carbon stock locally and across the four areas studied. Sediment depth positively covaried with regional variation in recent rates of relative sea-level rise. This research emphasizes the role that long-term ecosystem processes play in shaping mangrove peat deposits, allowing more accurate estimation of this variable and valuable carbon pool.