Human impacts on organic matter sedimentation in a proximal shelf setting, Hong Kong

This research examines the mean grain size characteristics, organic carbon content, C/N ratios, and δ13C and δ15N values preserved in Hong Kong marine sediments, and presents data on local sedimentation rates. Studies of piston and gravity cores from four contrasting regions demonstrate a distinct change in sedimentation from the late 15th to early 16th centuries. Particle sizes are stable in older deposits, but become coarser at discrete intervals in younger sequences. Organic matter signatures are also generally stable in the lower parts of cores, but show distinct variability in more recent deposits. The increased variability in the parameters measured commences at about the same depth within individual cores. Organic carbon percentages vary between sites and through time, increasing by up to 40% locally. C/N ratios rise toward the top of some cores, but decrease in others. δ13C values tend to be elevated in the upper sediment sequences, but often decline in the near-surface layers. δ15N shows greater contrasts between cores than δ13C, especially in the Deep Bay area. Sedimentation rates vary spatially and increased through the 20th century, with particularly fast rates having occurred in recent decades. It is suggested that these changes in marine sedimentation reflect, in part, human impacts on land. The onset of organic matter variability pre-dates urbanisation, and possibly reflects farming and/or deforestation. Later changes appear to coincide with rapid urbanisation, industrialisation, and reclamation.