Historical variation in sources of sedimentary organic carbon and burial fluxes in mangrove wetlands of Tieshangang Bay

Mangrove wetlands are efficient coastal blue carbon sinks and play an important role in regulating the global carbon cycle. In this study, using grain size distribution, total organic carbon/ total nitrogen (TOC/TN) ratio, δ¹³C values, and²¹⁰Pb chronological parameter, we investigated the sources of organic carbon (OC) and the temporal trends in sediment deposition and OC burial fluxes in the mangrove wetland of Tieshangang Bay, Guangxi. The results indicated that the sediment grain size was dominated by silt and sand, with TOC content and δ¹³C ranging from 0.26% to 3.96% and from −27.4‰ to −21.4‰, respectively. There was a strong correlation between TOC, TN, and δ¹³C values. The sedimentary organic matter in Tieshangang Bay had mainly a mixed signature with an average terrigenous contribution of 35.0%, marine sources 30.5% and mangrove contributions 34.5%, respectively. The average sediment OC flux over the past 100 years is 87.6 g/(m^2·a), and the carbon storage in sediments over the upper 50 cm can account for 65.3% of the total storage over the 95 cm depth. After 1961, there was a general decrease in sediment flux due to the construction of reservoir dams. Between 1961 and 1999, more terrestrial sources of OC were imported and buried due to climate and human disturbances around the watershed. Between 1999 and 2020, mangrove wetlands were destroyed and degraded due to extreme weather and natural factors. Degradation of mangrove wetlands under extreme weather and natural factors reduced the TOC content and OC burial fluxes. Since 2010, TOC content in the sediments has increased, probably due to higher water content, accumulation of litter and root growth. The OC source is dominated by the contribution from mangrove sources.