Search

Sedimentary architecture dominated by transgression–regression cycles in the shallow Bohai shelf region contains information about global sea-level, climate and local tectonics. However, previous studies of transgression–regression cycles in this region at orbital timescales that extend back to the early Pleistocene are sparse, mainly because of the shortage of well-dated long cores. Although transgression–regression sedimentary cycles in the region have been interpreted in terms of local tectonics, sea-level, and climate change, the detailed structure of marine transgressions and their significance for Quaternary global sea-level variations remains to be examined. In this study, we present an integrated sedimentological, geochemical and paleontological study of a 212.4 m (∼1 Ma) core (BH08) recovered from the Bohai Sea, China, for which an astronomically-based age model is available. Correspondence between marine-terrestrial sedimentary cycles and global sea-level fluctuations suggests that stacking of marine and terrestrial sediments was driven mainly by glacio-eustatic sea-level fluctuations in a context in which tectonic subsidence was largely balanced by sediment supply over the last ∼1 Ma. We report a dominant 100-kyr cycle beginning at ∼650 ka, which reflects the worldwide influence of the mid-Pleistocene transition (MPT) in sea-level records. We find that neritic deposits after the MPT were relatively thicker than before the MPT, which indicates an important control of the MPT on sedimentary architecture through lengthening of the duration of sea-level highstands. [ABSTRACT FROM AUTHOR]

Abstract: Sediments from the continental shelf/coastal region bear significant signals of sea-level, climate change as well as local tectonic information. This study presents a high-resolution magnetostratigraphic and rock magnetic study of a 212.4m core (BH08, with a basal age of 1.06Ma) recovered from the shallow (<30m below sea level) Bohai Sea, China. Astronomical tuning based on the sediment redness (a*) of core BH08 allows the construction of a high-resolution chronology that is assisted by magnetostratigraphic data. Sedimentology and associated proxies (grain size and redness) indicate that the cyclic alternation of neritic/littoral sandy deposits and terrestrial silts/clayey silts was mainly controlled by changes in sea-level and associated base-level at glacial–interglacial timescales. The a* record of the core can be correlated with marine δ18O records at 40- and 100-kyr cycles, indicating that the sediments in the study area are continuous at least at orbital timescales regardless of significant base-level variations. This is likely due to continued subsidence in the basin and creation of accommodation space. The significant lower sedimentary accumulation rates since 750ka might be due to the deviation of the depositional locus from core site because of extreme low sea-level stands during the glacials since the Mid-Pleistocene transition (MPT). Especially the glacial sea-level lowstands might have resulted in bypassing or erosion of sediments on the BH08 core site during peak glacials (MIS 6, MIS 10 and MIS 16), limiting the sediment preservation potential. This study documents the longest Quaternary sedimentary succession in the Bohai Sea and provides, for the first time, a clear link between sedimentation in the area and glacial–interglacial climate-driven sea-level changes. This study also highlights the feasibility of astronomical tuning to obtain high-resolution chronology for shallow shelf deposits, which is otherwise very difficult to be achieved on the basis of magnetostratigraphic data alone. [Copyright &y& Elsevier]

Abstract: Two gravity cores collected off the modern Huanghe (Yellow River) delta in the southern Bohai Sea were analyzed for grain size, the total organic carbon (TOC)/total nitrogen (TN) ratio, color diffraction, magnetic susceptibility, 14C dating and 137Cs and 210Pb isotope contents to clarify changes in the sedimentary environment during the Holocene. In particular, the effect of natural and artificial river-course shifts of the Huanghe on the Bohai Sea sediment was investigated. A peat layer, scouring surface and sharp changes in the grain size, TOC/TN ratio, sediment color (L ⁎, a ⁎) and magnetic susceptibility were identified and are likely to be due to the early-Holocene sea-level rise resulting in environmental changes from coastal to shelf environments in the Bohai Sea. After the sea level reached its maximum at 6–7kaBP, the lateral shifts in the river course of the Huanghe formed 10 superlobes, and superlobe 7 (11–1048 AD) and superlobe 10 (1855–present) of the Huanghe delta affected the core sites. The northern site of BH-239 has been more affected by the Huanghe since the middle Holocene. Notably, in the superlobe 10 period, the reshaping of the northern Huanghe delta due to an artificial river-course shift from northward to eastward in 1976 (e.g., a∼10km shoreline retreat due to coastal erosion) was recorded in the core sediments, particularly in terms of the TOC/TN ratio, sediment color (L ⁎ and a ⁎) and magnetic susceptibility, owing to the huge sediment supply from coastal erosion of the former river mouth area. [Copyright &y& Elsevier]