Heterogeneous distributions of black carbon in sediments from three Asian mangrove wetlands.

Black carbon (BC) is produced by the incomplete combustion of biomass and fossil fuels and plays an important role in global climate change and carbon cycling. Mangroves, seagrass beds, and salt marshes are collectively recognized as the three major blue carbon systems. However, research on BC in mangroves remains limited. We investigated mangrove sediments from three regions, Hong Kong (HK), Guangxi (GX), and Bangladesh (BD). Using Chemical Thermal Oxidation (CTO-375) and Benzene Polycarboxylic Acids (BPCA) analytical methods, along with elemental analysis, stable carbon isotope (δ13C), and MixSIAR model source apportionment, we explored the content, source, and composition of BC in sediments from these three mangrove ecosystems. The results indicate that the BC content is the highest in HK, followed by BD and GX (0.18 ± 0.02 %, 0.11 ± 0.01 % and 0.05 ± 0.01 %, respectively), while the BC/Total Organic Carbon (TOC) ratio is the highest in BD, followed by GX and HK (0.18 ± 0.01, 0.14 ± 0.02 and 0.08 ± 0.01, respectively). The contributions of fossil fuel combustion to the mangrove sediments are in a decreasing order of HK (59.0 ± 0.3 %), BD (55.1 ± 1.3 %), and GX (50.0 ± 3.4 %). These spatial variabilities are related to the development levels and energy structures in surrounding areas. In HK mangrove sediments, BC may be more influenced by long-distance transport and has undergone more pronounced photodegradation prior to burial, resulting in the lowest aromaticity, as indicated by the (B5CA + B6CA)/BPCAs ratio. We demonstrated BC as an important but heterogeneous component of the carbon pool in mangrove sediments.