Transgressive wave- and tide-dominated barrier-lagoon system and sea-level rise since 8.2 ka recorded in sediments in northern Bohai Bay, China

Abstract

Two sediment boreholes, DQ3 and DQ4, were drilled in northern Bohai Bay to investigate the genetic type and sedimentary evolution process of the Caofeidian barrier-lagoon system and the impact of relative sea level (RSL) rise. The core sediment was tested for grain size, microfossils (foraminifera and ostracods), and 14C dating. The Holocene formation of the delta-coast morphology and sedimentary types was closely associated with different phases of RSL rise. Fluvial facies, palaeosols and limnetic facies formed successively during the latest deglaciation but did not exhibit a direct relationship with RSL rise. These sediment facies were immediately followed by the littoral limnetic facies that formed during 9.5–8.2 cal. ka BP, when the Holocene sea level reached the study area. Then, offshore sand bars formed on the southern side, and tidal sand ridges formed on the eastern and western sides, forming a barrier island, which suggest that the Caofeidian barrier-lagoon system began to form under wave- and tide-dominated ocean dynamics at 8.2–7.8 cal. ka BP as the sea level rose at a rate of ca. 11.5 mm a−1. The Caofeidian barrier-lagoon system reached the maximum extent and was mainly influenced by wave-dominated ocean dynamics at 7.8–7.5 cal. ka BP, during which the sea level rose at a rate of ca. 9.0 mm a−1. The Caofeidian barrier-lagoon system began to erode and deteriorate at 7.5–6.7 cal. ka BP with a sea-level rise rate of ca. 2.6 mm a−1. Tidal sand ridges as barrier islands formed on the shore side under the influence of tide-dominated ocean dynamics. RSL rise was estimated at 2.3 m over the past 6.7 ka. Over this period, the sandy barrier islands were eroded by tides and the lagoon areas transitioned into an intertidal zone. Our reconstruction of RSL changes showed that the RSL rose abruptly in four stages, increasing by 1.7 m, 11.0 m, 1.7 m and 2.3 m at 9.5–8.7, 8.7–7.5, 7.5–6.7 and 6.7–0 cal. ka BP, respectively.