Coastal wetlands (mangrove, tidal marsh and seagrass) sustain the highest rates of carbon sequestration per unit area of all natural systems1,2, primarily because of their comparatively high productivity and preservation of organic carbon within sedimentary substrates3. Climate change and associated relative sea-level rise (RSLR) have been proposed to increase the rate of organic-carbon burial in coastal wetlands in the first half of the twenty-first century4, but these carbon–climate feedback effects have been modelled to diminish over time as wetlands are increasingly submerged and carbon stores become compromised by erosion4,5. Here we show that tidal marshes on coastlines that experienced rapid RSLR over the past few millennia (in the late Holocene, from about 4,200 years ago to the present) have on average 1.7 to 3.7 times higher soil carbon concentrations within 20 centimetres of the surface than those subject to a long period of sea-level stability. This disparity increases with depth, with soil carbon concentrations reduced by a factor of 4.9 to 9.1 at depths of 50 to 100 centimetres. We analyse the response of a wetland exposed to recent rapid RSLR following subsidence associated with pillar collapse in an underlying mine and demonstrate that the gain in carbon accumulation and elevation is proportional to the accommodation space (that is, the space available for mineral and organic material accumulation) created by RSLR. Our results suggest that coastal wetlands characteristic of tectonically stable coastlines have lower carbon storage owing to a lack of accommodation space and that carbon sequestration increases according to the vertical and lateral accommodation space6 created by RSLR. Such wetlands will provide long-term mitigating feedback effects that are relevant to global climate–carbon modelling.
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The data used in the global analysis of carbon concentration with respect to Holocene RSLR are provided in the Supplementary Information. The data that support the Chain Valley Bay study site analysis are available from the corresponding author upon reasonable request.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This research was supported by the Australian Research Council (FT130100532), AINSE (ALNGRA13046) and the UOW Global Challenges Program. The Smithsonian Environmental Research Center supported J.P.M., J.R.H, M.L. and L.S.-B. The data curation efforts of J.R.H. and data collection and synthesis efforts of J.P.M. were supported by United States National Science Foundation grants to the Coastal Carbon Research Coordination Network (DEB-1655622) and the Global Change Research Wetland (DEB-0950080, DEB-1457100 and DEB-1557009) and by a NASA Carbon Monitoring System programme grant (NNH14AY67I). L.S.-B. was supported by a Smithsonian Institution MarineGEO Postdoctoral Fellowship. This is contribution number 32 of the Smithsonian’s MarineGEO Network. The authors acknowledge J. Curran for assistance with sample preparation, S. Rasel, S. Oyston and M. Rupic for assistance with data collation and the students who undertook fieldwork as part of this research.
Nature thanks Andrew Ashton and the other anonymous reviewer(s) for their contribution to the peer review of this work.