Temporal Variations of Mangrove-Derived Organic Carbon Storage in Two Tropical Estuaries in Hainan, China Since 1960 CE

Mangrove wetlands are important sites of carbon burial amounting to 15% of the global total for coastal environments even though they comprise only 0.5% of the coastal area. Carbon fixation and burial in mangrove wetlands are sensitive to environmental changes but difficult to quantify. In this paper, records of mangrove-derived organic carbon (OC) burial were analyzed for two mangrove wetlands, Bamen Bay estuary (BMB) and Danzhou Bay estuary (DZB), in Hainan Province, China, over timescales of the last few decades. The mangrove-specific biomarker taraxerol and the more established proxies of C/N ratio and δ13C of bulk OC were used to investigate OC sources and burial history. A mixing model based on C/N ratios, bulk OC δ13C values and taraxerol concentrations indicates that mangrove-derived OC accounts for 25 ± 14% (mean ± SD) of sedimentary OC burial in (BMB and 16 ± 7% in DZB since 1960 CE. The estimated top 100 cm organic carbon storage was 242 Mg/ha in BMB and 89 Mg/ha in DZB. The carbon sequestration rate in BMB increased from 0.29 Mg/ha/yr to 6.54 Mg/ha/yr after 2012 CE, likely as a response to local mangrove protection since 1981 CE. Compared to the less-protected DZB, mangroves in BMB contributed 31% more to the sedimentary OC pool after mangrove protection, suggesting that mangrove protection has the potential to increase ecosystem health and to increase carbon burial in coastal wetlands.