Wetland carbon storage controlled by millennial-scale variation in relative sea-level rise

Abstract

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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Fig. 1: Millennial-scale influence of RSL on wetland evolution and accommodation space.
Fig. 2: Influence of late-Holocene RSLR on carbon concentration.
Fig. 3: Conceptual links between mineral and organic-matter accumulation and realized accommodation space.
Fig. 4: Sediment accumulation and character related to available accommodation space.

Data availability

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.

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Acknowledgements

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.

Reviewer information

Nature thanks Andrew Ashton and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Contributions

K.R., N.S. and C.D.W. developed the premise of the study. J.J.K., N.S., J.P.M., J.B.A., J.R.H., M.L., L.S.-B. and K.R. gathered and produced input data for the global analysis. J.J.K. carried out global-scale statistical analyses. K.R., D.M. and A.Z. undertook local-study fieldwork, sample preparation, laboratory and statistical analyses. K.R., J.J.K., N.S. and C.D.W. wrote the paper. All authors contributed to editing and revising the paper.

Corresponding author

Correspondence to Kerrylee Rogers.

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Extended data figures and tables

Extended Data Fig. 1 Relationship between late-Holocene RSLR and carbon concentration for data-poor late-Holocene RSLR zones and transitional zones.

ad, Box plots of tidal-marsh soil C concentration for data-poor Holocene RSLR zones and transitional regions: zone I (a), zone II–III transition (b), zone III (c) and zone IV–V transition (d).

Extended Data Fig. 2 210Pb and 137Cs activity of submerged and mangrove sediments.

ad, Submerged-core supported and total (a) and unsupported (b) 210Pb activity, 137Cs activity (c) and CRS-based 210Pb chronology (d). eh, Mangrove-core supported and total (e) and unsupported (f) 210Pb activity, 137Cs activity (g) and CRS-based 210Pb chronology (h). The grey validation lines confirm that the 137Cs activity peak corresponds to a sediment date of 1963, in agreement with CRS-based 210Pb chronology (core dating occurred in 2014). Source Data

Extended Data Fig. 3 Relationship between sea-surface salinity and C concentration over three depth intervals.

ac, Regression analysis of C concentration and global-scale sea-surface salinity, derived from the NASA Aquarius Satellite Mission, exhibited extremely weak relationships over the depth intervals 0–20 cm (a; R2 = 0.07, P < 0.001), 20–50 cm (b; R2 = 0.07, P < 0.01) and 50–100 cm (c; R2 = 0.06, P < 0.001). Soil %C values are low for zones I and IV in part owing to the removal of roots before analysis (see Supplementary Table 1). Source Data

Extended Data Fig. 4 Relationship between late-Holocene RSLR and C density for data-rich late-Holocene RSLR zones and transitional zones.

ad, Box plots of tidal-marsh soil C density for data-rich Holocene RSLR zones and transitional regions: zone I–II transition (a), zone II (b), zone IV (c), and zone V (d). Source Data

Extended Data Table 1 Results of the linear mixed model used for the dependent variable of tidal-marsh soil carbon concentration
Extended Data Table 2 Results of linear-mixed-model pairwise comparisons among Holocene RSLR zones for the dependent variable of tidal-marsh soil carbon concentration
Extended Data Table 3 Results of global data compilation of tidal marsh soil carbon concentration and RM-ANOVA
Extended Data Table 4 Results of the linear mixed model for the dependent variable of tidal marsh soil carbon density (in grams of carbon per cubic centimetre)
Extended Data Table 5 Results of linear-mixed-model pairwise comparisons among Holocene RSLR zones for the dependent variable of tidal-marsh soil carbon density (in grams of carbon per cubic centimetre)
Extended Data Table 6 Results of global data compilation of tidal marsh soil carbon density and RM-ANOVA

Supplementary information

Supplementary Information

This file contains Supplementary Methods, Supplementary Table and Supplementary References. The Supplementary Methods includes a detailed site description of Chain Valley Bay, Lake Macquarie, Australia, and the method for analysing C density, and the influence of global scale climatic and sea surface salinity variability on carbon concentration. The Supplementary Table includes the compilation of tidal marsh soil carbon concentration used in this study and associated Supplementary References.

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Rogers, K., Kelleway, J.J., Saintilan, N. et al. Wetland carbon storage controlled by millennial-scale variation in relative sea-level rise. Nature 567, 91–95 (2019). https://doi.org/10.1038/s41586-019-0951-7

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