Evolution of Holocene ebb-tidal clinoform off the Shandong Peninsula on East China Sea shelf

Studies of mud clinoforms in marine environments have documented a wide diversity in the behavior of depositional systems on continental margins with different sedimentary dynamics and spatiotemporal characteristics. The mud clinoform associated with the Bohia Strait off the Shandong Peninsula has attracted the attention of marine geologists and sedimentologists. After systematically reviewing the climatology, oceanography, and sedimentology as well as postglacial sea level change on the East China shelf and the changes in the strength of the East Asian winter monsoon, the aim of this paper is to provide a comprehensive explanation for the evolution of the clinoform and to establish a model of a tidal current-controlled mud clinoform on the seaward side of a tidal inlet. The Bohai Strait is a large tidal inlet linking the Bohai Sea (a large bay) and the Yellow Sea (an open sea). The Yellow River delivers large amounts of fine-grained sediments to the southern Bohai Sea. Under the action of the East Asian monsoon, the speed and duration of ebb currents are higher and longer than those of the flood currents in the southern Bohai Strait. The stronger NNW wind in winter causes more powerful waves on the southern Bohai Sea, inducing high suspended sediment concentrations (SSC). These suspended sediments are transported by ebb currents from the Bohai Sea to the Yellow Sea through the southern Bohai Strait, particularly in winter, and then deposited on the seaward side of the strait, forming the ebb-tidal clinoform off the Shandong Peninsula. The evolution of the ebb-tidal clinoform is controlled mainly by historical changes in the strength of the East Asian winter monsoon and the spatial extent of the Bohai Sea. The clinoform has been accumulating since 9600 cal BP; it formed mainly from 8200 to 5000 cal BP, when a strong East Asian winter monsoon produced relatively strong ebb currents with high SSC in the southern Bohai Strait, whilst the extent of the Bohai Sea was large. The ebb currents from 9600 to 8200 cal a BP and during the last 5000 years were comparatively weak, primarily as a result of the small area of the Bohai Sea in the former time interval and a weak East Asian winter monsoon in the latter time interval. These changes in the area of the Bohai Sea and monsoon strength also caused the deposition rates within this clinoform to differ from the rates during the period 8200 to 5000 cal BP, i.e. when the area of the Bohai Sea was large and the East Asian winter monsoon was relatively strong. From 8200 to 5000 cal BP, the deposition rates were 4.4-5.8 times the rates during the past 5000 years. Whilst it has been argued that the development of the clinoform could be associated with the evolution of the Yellow River delta, this hypothesis is not consistent with the ages of these features. A significant portion of the clinoform formed before 7000 cal BP, the point at which delta development was initiated. The Yellow River delta on the west coast of the Bohai Sea and the clinoform off the Shandong Peninsula are not coeval and do not belong to a compound-clinoform system. It is concluded that the clinoform off the Shandong Peninsula is substantially a large mud ebb-tidal delta on the seaward side of the Bohai Strait. The clinoform on the seaward side of the Bohai Strait and the flood tidal sand ridges and sand sheet on the landward side of the Bohai Strait constitute a complete tidal depositional system on both sides of this large inlet. This new insight may enhance the understanding of the formation of mud clinoforms on other shelf environments and facilitate the analysis of tidal zone palaeofacies.