Post-glacial sequence and sedimentation in the western Bohai Sea, China, and its linkage to global sea-level changes

Abstract

Post-glacial sedimentary sequences provide a unique window for observing depositional and environmental responses to rapid changes in global climate and sea level. Here, we integrate seismic records and the lithological variations of sediment cores from western Bohai Bay, China, in order to reveal the history of sedimentation and its linkages to global sea-level changes since marine isotope stage 2 (MIS 2). The post-glacial succession consists of six seismic units (SU1–SU6, in descending order) which are bounded by six key reflection surfaces (T0, R1, R2, R3, T1 and T2), respectively corresponding to six depositional units (DU1–DU6) identified in sediment cores. Together they comprise the latest sequence of infilling of western Bohai Bay, where SU6 is associated with the Lowstand Systems Tract (LST), SU5 is associated with the Transgressive Systems Tract (TST), and the last four units of SU4–SU1 comprise the complex Highstand Systems Tract (HST). During the interval of the MIS 2 (22–11.6kyrBP) sea level lowstand, the LST was formed in a long-term environment of sub-aerial exposure or fluvial floodplain. The TST is composed of transgressive deposits formed during the interval of global sea-level rise, and is characterized by a retrogradational feature, from a swamp to a tidal flat environment, and then to an estuarine subtidal environment. A series of sedimentary facies transitions represent abrupt sea-level rises in the early Holocene, which may correspond to the events MWP-1C, “8.2-ka event” and MWP-2. Erosional trenches were well developed within SU5 in the central-eastern region, and were probably part of a transgressive waterway system rooted from Bohai Strait. They demonstrate that Bohai Bay experienced strong tidal erosion in the early Holocene, as a result of its morphology which consisted of a long narrow embayment. The complex HST was initially formed at ca. 7calkyrBP when sea-level essentially stabilized in a highstand, and developed rapidly after ca. 6calkyrBP when the Yellow River supplied large amounts of sediment to the western or southwestern margin of Bohai Bay. Its complex internal structure is related to the configuration of three flooding surfaces caused by changes in sediment supply, and which in historical times separated four stages of river discharge, mainly from the Yellow River. During ca. 7.0–3.4calkyrBP, the Yellow River at one time emptied into the southwestern margin of Bohai Bay, forcing its subaqueous delta to extend over the southern area of western Bohai Bay, while open shelf sedimentation in the central and northern area of Bohai Bay consisted of the accumulation of a shelly layer subject to intensive storm-reworking. During ca. 3.4–1calkyrBP, the Yellow River once discharged into the central and northern parts of western Bohai Bay, forming two detached subaqueous deltas. During ca. 1–0.8calkyrBP, the Yellow River discharged into the central region, stacking a subaqueous delta confined by the two older underlying subaqueous deltas. During the last ~800years, the discharge of the Yellow River out of Bohai Bay has dominated coastal marine sedimentation which has a complex provenance.