Chronologies of sedimentary changes in the south Bohai Sea, China: constraints from luminescence and radiocarbon dating

The alternation of terrestrial and marine deposits is an indicator of past environmental and sea-level changes. The age of deposition is usually dated by means of radiocarbon. However, radiocarbon dates of molluscan shells from coastal areas may be complicated by various sources of carbon, and problematic for deposits of 4050ka or older. Herein, we apply the Optically Stimulated Luminescence (OSL) dating method to date samples from terrestrial and marine/coastal sediments extracted from three cores in the south Bohai Sea, China. Multiple- and single-aliquot regenerative-dose procedures using OSL signals from fine-silt (411m), coarse-silt (3863m) and fine-sand (6390 or 90125m) quartz were employed to determine the equivalent dose (D e). The results showed that: (i) OSL ages from quartz of different grain sizes and different protocols are consistent with each other; (ii) for Holocene samples, most of the radiocarbon dates agree well with OSL ages; (iii) for pre-Holocene samples, radiocarbon dates cluster at 4050 14 C ka BP, whereas OSL ages are in stratigraphic order from 11ka to 176ka. Because of the self-consistency of the quartz OSL ages, the stratigraphic agreement in the three cores, and the clustering of the radiocarbon dates, we suggest that the quartz OSL ages are more reliable with respect to dating the samples from the south Bohai Sea. Finally, the four marine strata identified in the south Bohai Sea are likely to have formed during the Holocene, Marine Isotopic Stage (MIS) 35, MIS 6 and probably MIS 7, respectively.</p

Sedimentary history of the western Bohai coastal plain since the late Pliocene: Implications on tectonic, climatic and sea-level changes

Thick Cenozoic deposits from the western Bohai coastal plain, a tectonic-subsiding region, provide the potential to study the relations between sedimentary environments and tectonic, climatic and sea-level changes. However, sedimentary history of this region extending to the whole Quaternary, as well as their links to tectonic, climatic and sea-level changes are still poorly understood, mainly because of the lack of long-term records with well-constrained chronology. In this study, we present an integrated record based on sedimentology and proxies (grain-size and color reflectance) of a 203.6 m core recovered from the western Bohai coastal plain near Tianjin. The core was chronologically well constrained using paleomagnetic and optically stimulated luminescence dating methods. The results show that from the late Pliocene (similar to 3.3 Ma) to the late Pleistocene (similar to 0.10 Ma), the study region was mainly dominated by fluvial setting, and the extensive incursion of sea water into this region began during the last interglacial period (similar to 0.10 Ma). The sedimentology and the color index suggest that tectonic subsidence of the Bohai Basin during the Plio-Quaternary must have played a significant role in controlling the sedimentary environments in this region. The changes in base-level because of sea-level fluctuations during the Quaternary influenced the fluvial development greatly and led to the alternations of coarse crevasse splay/channel and finer floodplain deposits in the core sequence. In addition, climatic changes since the late Pliocene also have had significant effects on the sedimentary settings in the Bohai coastal plain by influencing the fluvial process with a series of mechanisms.</p

A reconstruction of late Pleistocene relative sea level in the south Bohai Sea, China, based on sediment grain-size analysis

Future anthropogenic sea-level rise and its impact on coastal regions is an important issue facing human civilizations. Due to the short nature of the instrumental record of sea-level change, development of proxies for sea-level change prior to the advent of instrumental records is essential to reconstruct long-term background sea-level changes on local, regional and global scales. Here, we employ numerical methods to partition sediment grain size using a combined database of marine surface and core samples, and to quantitatively reconstruct sea-level variation since the late Pleistocene in the south Bohai Sea, China. Our sea-level reconstruction indicates that relative sea-level changes in the southern Bohai Sea track global sea-level variation for the duration of the record. The results also indicate substantial regression from 70 to 30. cal. kyr. BP, and potentially subarial exposure from 38 to 20. cal. kyr. BP. Our results document the feasibility of reconstructing relative sea-level change by numerical partitioning of sediment grain size data, demonstrating the potential for future applications.</p