Relative sea-level changes induced by glacial isostatic adjustment and sediment loads in the Beibu Gulf, South China Sea

Abstract

We modelled changes in relative sea level (RSL) in the Beibu Gulf region, South China Sea, caused by the melting of the Pleistocene ice sheets, i.e., glacial isostatic adjustment (GIA). Both the temporal evolution of RSL between the last glacial maximum and present day as well as present-day rates were derived. The interplay of changing ocean water volume, gravitational induced water redistribution and solid Earth deformations due to the varying surface loads is accounted for by means of the sea-level equation. The modelled RSL curves for the Beibu Gulf region reveal the shape typical for regions far away from the former centres of glaciation, exhibiting a pronounced Holocene sea-level highstand between 4 and 5 kyr before the present. A general good agreement was found between modelled and empirical RSL curves, although differences in the timing and magnitude of the highstand were revealed. In addition to GIA-induced RSL, we also modelled the effect of RSL variations due to sediment accumulation during marine isotopic stages MIS 4 to MIS 1 descending mainly from Hainan Island, but also other terrestrial sources from the Chinese (and Vietnamese) mainland. Estimates for the sediment loading originate from seismic surveys and dated sediment cores. We found that RSL changes caused by sediment loading are at least three orders of magnitude smaller than those induced by GIA. The present-day rates of both RSL effects are too small to be detectable by geodetic observations and cannot be discriminated from prominent tectonic signals.