Deciphering spatio-temporal variations in Holocene relative sea level in the South China Sea : from local reconstructions to a regional database

Abstract

This thesis advances our understanding of late Holocene relative sea-level (RSL) change along the South China Sea (SCS) coast through the development and application of multiproxy paleoenvironmental approaches that enhance the accuracy and precision of RSL reconstructions and inform projections of future coastal vulnerability.

Using integrated analyses of foraminifera, diatoms, and δ¹³C and C/N geochemistry in the Pearl River Delta (PRD) mangroves, I identified ten vertically zoned foraminiferal assemblages and two distinct diatom groups. Tidal elevation primarily controlled foraminiferal distributions, while salinity influenced diatom assemblages. I developed Bayesian transfer functions (BTFs) incorporating geochemical or BTF-derived priors, which successfully captured all tide gauge observations within 1σ uncertainty. The combined-proxy approach, using diatom-derived priors for the foraminiferal BTF, reduced mean squared error (MSE) by 121% (MSE: 0.014 m²).

A re-examination of mid-Holocene Saccostrea cuccullata oyster reef remains confirmed their paleoenvironmental significance as above-present-sea-level indicators. I established the species’ modern indicative meaning in Hong Kong, with oyster encrusting on rock faces at 111 ± 73 SWLI units and in tidal pools at 153 ± 115 SWLI units (2σ). Morphological, petrographic, and mineralogical analyses confirmed in situ formation of a filled frame reef under wave influence, excluding redeposition by storm events. Two new radiocarbon dates provided additional sea-level index points (SLIPs), indicating a fall in RSL from 3.0 m at 5.4 ka to 1.5 m at 3.7 ka, aligning with regional highstand estimates of RSL in the PRD.

To contextualise these local findings within broader regional patterns, I compiled and analysed a database of 533 sea-level indicators derived from sedimentary, geomorphic, and coral archives across 12 regions of the SCS coast (14°N–26°N, 104°E–120°E), incorporating palaeotidal modelling and a spatio-temporal empirical hierarchical modelling to reveal early Holocene RSL rise, driven by glacial meltwater input, peaked at ~10.2–11.3 m/ka and slowed to 1.1–2.7 m/ka by the mid-Holocene. Above-present RSL was observed across multiple regions, reaching a maximum of 4.5 ± 2.6 m around 5 ka in the Leizhou Peninsula and northern Hainan. The spatial variability in RSL reflects the interplay of glacial isostatic adjustment, tectonic uplift near the Taiwan subduction zone, sediment-driven subsidence in deltaic regions, and mantle plume upwelling beneath Hainan.

Collectively, these findings demonstrate that multiproxy approaches significantly enhance the precision of RSL reconstructions and illuminate the diverse processes shaping spatio-temporal patterns of Holocene sea-level change, offering critical insights for assessing future coastal vulnerability in this densely populated and economically vital region.