The blue carbon ecosystem in the Beilun River Estuary, Beibu Gulf, China, underwent severe deforestation until the national nature reserve establishment in 2000. However, the influence of the deforestation event on sedimentary organic carbon (OC) biogeochemistry has not been reported yet. Based on the ²¹⁰Pb chronology of four sediment cores sampled in the Beilun River Estuary National Nature Reserve, this study analyzed soil texture, bulk OC, stable and radiocarbon isotopes, and lignin biomarker to reconstruct historical changes in OC biogeochemistry over the past half a century. Three stages were observed. In Pre-establishment I (Pre-EST I, before 1970), the average TOC (1.24 ± 0.14 %) was mainly older (6406 ± 362 BP) terrestrial sources (72.2 ± 6.6 %) with finer particles (76.2 ± 6.2 %), relative higher carbon accumulation rate (CAR, 88.2 ± 9.2 gC m^-2 yr^-1) and mangrove soil carbon density (98.9 Mg C ha^-1). After significant changes in Pre-EST II (1970 - 2000), the average TOC became lower (0.68 ± 0.15 %) in Post-EST stage (after 2000), with mainly younger (1357 ± 147 BP) marine OC (52.8 ± 11 %) with coarser particles (59.4 ± 4.7 %), lower CAR (75.4 ± 15.6 gC m^-2 yr^-1) and mangrove soil carbon density (85.6 Mg C ha^-1). These changes were mainly caused by deforestation, which not only changed the sources of OC, but also reduced the carbon density and carbon storage by 8 % and 53 % respectively. Thus, the remnant sedimentary OC in the study indicated significant loss in OC capacity in the Beibu Gulf, China due to deforestation.
 

Beibu Gulf,blue carbon ecosystem,organic carbon,deforestation,biomarker