Characteristics of sea level changes in the northern South China Sea since the Holocene and prediction of the future trends

The northern South China Sea is one of the key front zones for the transition from the South China Sea to land and one of the sensitive areas of global climate change. Affected by sea level changes, sea-level indicators are widely developed in this area, and offers an ideal area for the study of past sea-level changes. However, there are still some differences in the understanding of the Holocene sea-level change history in the northern South China Sea. Therefore, we first added 6 new and high-quality sea-level index points and reviewed and re-corrected the age and elevation of the published sea-level data obtained from the northern South China Sea, followed by mutual verification and reliability analysis. At the same time, the modern sea level observation data from 12 tide-gauge stations with relatively continuous monitoring records are collated, and the reliability of the reconstructed results are further verified. Finally, based on 679 sea level data compiled after correction and reliability assessment, we reconstructed the history and characteristics of sea level changes in the northern South China Sea since the Holocene, especially in the last 2 000 years. The results show that during the early Holocene, the sea level in the northern South China Sea rose rapidly from −16.16 m at (8 211±128) cal a BP to 1.5～2.5 m during 6 000～7 000 cal a BP, and maintained a high sea-level oscillation of 2 600 years in the Mid-Holocene, and at last began to fluctuate and drop to the current sea level in the late Holocene. The sea level in the northern South China Sea showed a phased change process in the past 2 000 years. First, it showed a downward trend from 0 to 350 CE, and then followed by a rapid sea level rise in 350-850 CE. In 880 CE, the sea level was at its highest point in the past 2 000 years (1.05 ± 0.35) m, and the sea-level continued to decline to (−0.18 ± 0.35) m in 1850 CE, then the sea level basically kept a narrow horizontal fluctuation until 1897 CE (−0.19 ± 0.05) m. Since then, the sea level kept continuous oscillation and gradually rises to 0.076 m in 2 020 CE. If the sea-level in the northern South China Sea in 2100 A.D. is estimated based on the rising rate in the past 100 years, it would be 0.19～0.28 m higher than the current sea level. Therefore, when carrying out large-scale engineering construction in the coastal low-lying areas in this area, it is necessary to consider the adverse effects of future sea-level rise. In addition, it is further found that different sea-level indicators and different reconstruction methods are the main reason for the regional differences of sea-level reconstruction results in the northern South China Sea since the Holocene. On the whole, the sea-level in northern South China Sea has shown a fluctuating downward trend since the Mid-Holocene. Despite the differences in the time and height of sea-level in different regions, the sea-level records in the northern South China Sea are consistent with those in the surrounding coast, suggesting the high sea-level during the Mid-Holocene in the South China Sea, and it is likely to have a global background.