Mid-Holocene sea-level change in the northern South China Sea inferred from coral microatolls: constraints on glacial isostatic adjustment and neotectonic activity

Mid-Holocene far-field relative sea-level records characterize natural sea-level variability and ice-sheet dynamics under pre-industrial conditions, providing critical constraints for projecting future sea-level change in the anthropogenic era. Existing early mid-Holocene (7–6 ka BP) RSL reconstructions exhibit pronounced spatial variability across far-field locations, with limited high-resolution data in the northern South China Sea hindering comprehensive understanding of regional patterns and forcing mechanisms. This study addresses this knowledge gap through 12 new sea-level index points derived from fossil coral microatolls at Luhuitou Peninsula, constrained by high-precision U-Th dating and geodetic elevation measurements. Our results reveal stabilized RSL at 0.56–0.88 ± 0.34 m (2σ) above present mean sea level between 6.8 and 6.4 ka BP, exhibiting insignificant centennial-scale rates of change at −0.13 ± 1.20 mm/yr (2σ). This period of RSL stabilization is temporally consistent with the hypothesized cessation of meltwater contributions from the Laurentide Ice Sheet at approximately 7 ka BP. However, it shows limited agreement with hypotheses suggesting a pronounced mid-Holocene highstand > 1m or centennial-scale climate-driven RSL oscillations with half-metre amplitude inferred from certain far-field coral-based reconstructions. A marked >1.5 m RSL offset between our record and coeval microatoll-based data from Leizhou Peninsula suggests compound influences of glacial isostatic adjustment (GIA)-induced continental levering and regional neotectonic uplift. These findings provide critical chronological constraints for refining GIA model parameterization of ice sheet melting cessation timing and highlight the significant influence of local tectonics on RSL spatial variability and associated uncertainties in GIA predictions. To better understand the interplay among glacial isostatic adjustment, tectonic processes, and climate, we advocate for coordinated multi-proxy studies across tectonically stable sites to address limitations in temporal coverage and spatial representativeness. Additionally, integrating multi-scenario GIA modeling frameworks with heterogeneous mantle viscosity structures and explicit ice-sheet evolution parameters is essential for resolving these complex dynamics.