Abstract:
[Objective] Laizhou Bay is located at the southern part of the Bohai Sea, where the Quaternary history of regional environmental changes during the transition from land to sea has been continuously recorded in coastal sediments. Studying these geological records can provide critical insights into geographical and environmental reorganization of the Northeast Asia during the Quaternary. Previous studies have reported that there was a major transition from a lake environment to a shelf environment in the middle Pleistocene, and three transgression events have been dated to marine isotope stage (MIS) 7-6, MIS 5-3, and the Holocene, respectively. However, the evolution of the “Bohai Paleolake” is not clear, since the framework of these lacustrine deposits has not been established yet. [Methods] To reveal the history of how the “Bohai Paleolake/Sea” evolved, 11 drilling cores from the Laizhou Bay were employed to establish a preliminary framework by analyzing the sedimentary strata in the Quaternary. The main analyses include magnetostratigraphy, sediment grain size, and stratigraphic correlation. [Results] The main results are listed as follows: (1) Three new cores were analyzed in terms of magentostratigraphy, and the new obtained magnetozones can be generally correlated to the Brunhes, Matuyama, and Gauss chrons in the geological polarity timescale, which is consistent with previous studies in the Laizhou Bay; (2) Three major types of the sediments in the Quaternary were identified, including fluvial-alluvial, lacustrine, and continental-shelf, and the sedimentary properties and the relationships of sediment grain-size parameters are distinct among them, illustrating the potential of paleoenvironmental inferences; (3) Based on stratigraphic analyses and isochronous strata (Brunhes and Matuyama chrons) correlation, it is found that the Laizhou Bay may have experienced significant subsidence in the early Pleistocene; (4) By establishing a stratigraphic framework of Quaternary sediments in the Laizhou Bay, 17 alternations between sea/lake water extension and shrinking were identified, including 1 in Holocene, 2 in the late Pleistocene and the late part of the middle Pleistocene, 7 in the middle Pleistocene, and 7 in the early Pleistocene; (5) The preliminary framework indicates the predominant periodicities in the upper transgression-related strata and the lower lacustrine-fluvial strata are 100 kyr and 200 kyr, respectively, inferring a dominant role of orbital eccentricity in regional paleoenvironmental processes; (6) Correlating the Quaternary strata around the Bohai basin and correlating isochronous strata based on the Jaramillo subchron, it is found that the eastern and northwestern margins of the basin significantly subsided during the Middle Pleistocene. [Conclusions and Prospects] Based on these results, it is speculated that the land-sea transition of the Bohai basin has gone through the following processes: the alluvial and diluvial system in the early Pliocene or earlier, the lake-group environment prior to the early part of the early Pleistocene, the latest stage of “Bohai Paleolake” in the early Pleistocene, the shrinking lake and weak transgression in the Middle Pleistocene, and the shelf depositional environment since the late Pleistocene. These identified stages of the Bohai basin can be preliminary correlated to the evolution of a series of paleolakes in the middle and upper reaches of the Yellow River, possibly inferring a potential covariation. However, the main findings in the work are lack of fossil evidence, which are worthy of further investigation in future.
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