Abstract: Two sediment cores, covering histories of ca. one hundred years and located in coastal waters off Guangdong province, were analyzed for branched and isoprenoid tetraether (BIT) index based on glycerol dialkyl glycerol tetraether (GDGT) biomarkers. The index is an indicator of relative contribution of terrestrial soil organic carbon (SOC) to the total organic carbon (TOC) in the sediment. Our results revealed that BIT values were lower than 0.26 in the core sediments, indicating major contributions of aquatic organic carbon (AOC) to the TOC. BIT time series from the two cores showed similar patterns with stable and higher values before the 1940s, a gradual decreasing trend after the 1940s and a prominent decreasing trend since 1990. The decreasing trend suggests either a decrease in SOC or an increase in AOC. Based on the time series of the sedimentation flux of SOC and AOC, increasing contribution of AOC was inferred to be the main cause. The increasing sedimentation flux of AOC showed a similar pattern to that of the economic rise in Guangdong province, implying that the suggested rise of nutrient levels of coastal waters off Guangdong province could have been caused by the enhanced human activities.

Key words: BIT index, sediment core, organic carbon, sedimentation flux