[Delta]¹³C as a palaeo-environmental indicator in a sediment core fromHong Kong

Abstract

A study of marine palaeo-landscape development through a sediment core in the western Hong Kong waters has been carried out in this project as the sedimentation record could reflect the environmental changes during the Holocene period. The objectives of this study was to reveal the influences of Holocene post-glacial sea-level fluctuations and the monsoonal freshwater discharge changes to the sedimentation processes in the mouth region of the Pearl River Estuary. These are the two important factors which shape the palaeo-landscape development. Data has been obtained from a ground-truthing vibrocore to 35m below the seafloor at the south of the Shek Pik Reservoir, south Lantau Island. The sediment core was subsampled for elemental analysis to study the sources of the organic carbon by use of stable carbon isotope ratio (Ϭ13C) and for particle size analysis to reveal the energy state of the sedimentary environment. Shell fragments were chosen for radiocarbon analysis to construct the core chronology and marine seismic profiles were obtained to present the sub-bottom geological layers. The data is presented in the form of graphs and tables in showing the sediment changes along the core in different period of time. The data has been interpreted in relation to the knowledge of post-glacial sea-level rise and the proposed model of the Pearl River Estuary evolution from previous studies. Comparison is also made with other sediment cores around the Lantau Island to reveal the regional palaeo-environmental changes in the Holocene period. It is concluded that the sediment core has revealed four major phases of environmental change which is in general agreement with the previous studies except this data set has also shown a few localized climatic events in the mid-Holocene period. The sea-level low stand in the late Pleistocene had incised a network of drainage system on the exposed continental shelf which is the north South China Sea in present time. The rapid rising sea-level by Melt Water Pulse 1B filled the palaeo-valleys with alluvium deposits from 10600 to 9000 cal. yr BP at high sedimentation rate and the East Asian Monsoon was strengthened from 8800 to 5500 cal. yr BP as shown by the freshwater organic carbon deposited in the marine environment. Unconfined deposition occurred when the sediments spilled out of the palaeo-valleys and slowly covered the basin in stable sea-level and low hydrodynamic conditions.