Holocene sea level change in the western North Pacific marginal seas and coastal responses to recent sea level change in the Deep Bay Wetlands

Abstract

The coast of East and Southeast Asia is a far-field region and thus a scientifically valuable site for sea level and coastal study. The high population density in low-lying coastal areas of this region means a high vulnerability to human society if sea level is going to rise in the future. This research is designed to tackle two scientific issues: (1) Analyze the past local land level change including the levering effect and tectonic movements; and (2) Investigate how coastal system have responded to past sea level change.

In order to tackle the first issue, three glacial isostatic adjustment (GIA) models were simulated to produce a series of predictions for the study area, and 560 sea level data from published sources were re-analyzed and re-calibrated to convert them into useable sea level index points for sea level reconstructions. After a comparison between the model predictions and the reconstructed sea level histories, the effects of GIA, levering effects and tectonic movements are identified and assessed. The results show the three coastal sectors have had different responses to sea-level change. The Sundaland Block as a teconically stable region is more sensitive to GIA intensity changes. The Cathaysian Block is also sensitive to GIA intensity changes. But the coast was affected by small magnitude, localized uplift and subsidence. These tectonic movements can be clearly distinguished from the GIA processes, paving the way for eventual quantification of land movements. The Yangtze Block and North China Craton are located in low to middle latitudes, but still show characteristics of far-field GIA. Tectonic movement in this sector is relatively negligible, except the west Bohai Bay where subsidence is more obvious.

For the second issue, the sedimentary sequence of a protected coastal wetland (Mai Po) in Hong Kong was investigated. Its developmental history and how it has responded to the past sea level change were examined. The results show, before 1000 years ago, the Mai Po area was a subtidal area, with sandy silt and abundant marine shells deposited in seabed implying an environment under deeper water and with better sea water exchange. A change took place about 800 years ago, and the area became a low energy environment with finer sediment and no marine shell. Soon after this, the sedimentation slowed down markedly possibly due to a fall in sea level. By about 200 years ago, mangrove vegetation colonized the site and kept expanding seawards progressively. In the past 100 years, due to human activity in the Deep Bay area, sedimentary supply from both Hong Kong and Shenzhen has increased. As sea level has also risen, another phase of rapid sedimentation takes place in the lower part of the mangrove forest and current mudflats.