Sediment depth and accretion shape belowground mangrove carbon stocks across a range of climatic and geologic settings

Estimating the long-term sedimentary carbon sinks of mangroves and other blue carbon ecosystems has rapidly become a focus of coastal research and conservation attention. Sampling coverage, however, remains low, with sediment cores sparsely distributed across a subset of mangrove environmental settings globally. Furthermore, the ecological and geological drivers of variation in these stocks remain incompletely understood. We assessed the limits of mangrove sedimentary carbon storage by sampling sediments in diverse mangrove environments across four geographic areas: the volcanic Galapagos, the arid Baja Peninsula, and the geologically and climatically distinct Caribbean and Pacific coasts of Panama. At 80 sites across these areas, we quantified the organic carbon density, sampled with depth, of entire sediment columns. Depth-integrated carbon stocks are highly variable, from 1700 MgCorg ha(-1). The positive relationship between sediment carbon density and annual rainfall demonstrated in global studies was not observed across these areas, though some carbon density differences were evident. Variation in sediment depth, ranging from 7 to 427 cm across sites, largely explained variation in carbon stock across the areas studied. These results underscore the importance of using measurements of sediment depth to estimate carbon stock. Long-term geomorphological processes, such as the build-up of sediment deposits shaped by coastal dynamics, play a major role in shaping mangrove carbon stocks. Investigating and accounting for these processes will enable more accurate estimation of this variable and valuable carbon pool.