1158 / 2024-09-20 15:33:57

Far-field GIA Process and An Associated Sea-level Highstand for the Mid-Holocene

Holocene; Sea-level change; GIA; Sea-level Highstand; Foraminifera; Bohai Sea

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 subsidence, and the lowering effect due to water extraction. This enabled a high-quality reconstruction of RSL changes over the age range of 9.0–2.9 kyr cal BP. The RSL rose rapidly from about –17.2 m to 1.9 m between 9 and 7 kyr cal BP amid a global rise in ice-volume equivalent sea level (ESL). However, the RSL rise rates quickly decreased from around 10 mm/yr at 8 kyr cal BP to 4.5 mm/yr by 7 kyr cal BP. After 7 kyr cal BP, the RSL continued to rise, reaching its peak of 2.48 ± 1.36 m around 6 kyr cal BP, followed by a gradual decline to the present height at 3–4 kyr cal BP, with a maximum falling rate of 1.2 mm/yr around 5 kyr cal BP. Comparisons of the reconstructed RSL in this study with those from other non-deltaic regions, including the inner Hangzhou Bay and the western Bohai Sea coast, as well as with GIA predictions by ICE6G, ANU, and region-specific ‘final model’ reveal: (1) a more significant and long-term early-to-mid Holocene continental levering effect on the southern Bohai Sea coast; (2) while the reconstructed RSL closely matches the region-specific ‘final model’, there are some discrepancies, with RSL levels being slightly lower than projected before 8 kyr BP and forming a distinct mid-Holocene highstand, which is higher than projected after 7 kyr BP; (3) these misfits may be due to an underestimation of upper mantle viscosity and uncertainties in the Holocene ice melting history.