Holocene sea-level history from the southern Bohai Sea coast, China: Far-field GIA processes and an associated mid-Holocene sea-level highstand

Abstract

The Bohai Sea, located in the innermost part of the East Asian marginal seas, is of particular interest in the studies of relative sea level (RSL) and glacial isostatic adjustment (GIA) due to its apparent far-field position. This study analyzed six cores and one pre-existing archaeological site from the southern Bohai Sea coast, generating 11 sea-level index points (SLIPs) from supratidal, upper tidal, and mid-lower tidal flat sediments using foraminifera tests and sedimentary analysis. All SLIPs were corrected for possible self-compaction, long-term tectonic effect, and the lowering effect due to water extraction. This enabled a high-quality reconstruction of RSL changes over the age range of 9000–3000 cal a BP. The RSL rose rapidly from about −17.19 ± 1.32 m to 1.76 ± 1.32 m MSL between around 9000 and 7000 cal a BP. However, the RSL rise rates quickly decreased from around 8–10 mm/a before 8000 cal a BP to ∼4mm/a by 7000 cal a BP. After 7000 cal a BP, the RSL continued to rise, reaching its peak of 2.44 ± 1.34 m MSL around 6000 cal a BP followed by a gradual decline to the present height at 3000–4000 cal a BP, with a maximum falling rate of 1.2 mm/a around 5000 cal a BP. Comparisons of the reconstructed RSL in this study with those from other non-deltaic regions, including the inner Hangzhou Bay and the surrounding coasts of western Bohai Sea, as well as with GIA predictions by ICE6G, ANU, and region-specific ‘final model’ reveal: (1) a significant and long-term early-to-mid Holocene continental levering effect on the southern Bohai Sea coast; (2) a close fit with the region-specific ‘final model’, albeit with SLIPs that fall slightly below predictions from 9000 to 8000 cal a BP and form a distinct mid-Holocene highstand above model projections after 7000 cal a BP; (3) these data-model misfits may result, in the former case, from the GIA model's Holocene ice melting history not embedding accelerated sea-level rise events, and in the latter, from the presence of a relatively strong upper mantle at the southern Bohai Sea coast, rather than the extremely weak upper mantle assumed for the Bohai Sea region.