Quartz OSL dating and formation mechanism study for a sand layer with parallel bedding within the Shaling sand hill at west bank of Poyang Lake

The Shaling sand hill is located in Lushan City, the west bank of Poyang Lake. In previous studies, most of the sand and silt layers within the sand hill were interpreted as aeolian deposits. However, during field investigations, it was found that some sand layers within the Shaling sand hill exhibit clear parallel bedding. In this study, a sand layer with parallel bedding within a sedimentary profile(29°21′17.12″N, 116°01′12.72″E)was targeted and studied. The strike of profile is from east to west and it includes two sedimentary parts(the sand layer with parallel bedding at the top and the sand layer with massive bedding at the bottom). The sand layer with parallel bedding at the top changes along the horizontal direction. From east to west, the thickness of the sand layer with parallel bedding changes from 0 m to above 0.5 m. The thickness of the sand layer with massive bedding at the bottom is approximately 1 m. Currently, the Wusong elevation of the sand with parallel bedding can reach 36.5 m, while the highest water level of Poyang Lake in Xingzi Hydrological Station was only 22.63 m in 2020. To further clarify the cause of the sand layer with parallel bedding and its significance on environment, two OSL samples were collected from the studied profile. For these two OSL samples, one(XZ220120-1)was collected from the sand layer with parallel bedding at the top, while the other one(XZ220120-2)is collected from the sand layer with massive bedding at the bottom. XZ220120-1 was collected at the depth of 0.4 m, while XZ220120-2 was collected at the depth of 0.7 m. The optical dating results show that the quartz single grain equivalent dose(De)values of XZ220120-1 are very scattered, with over dispersion(OD)of 87% ±1%. In comparison, quartz single grain(De)values of XZ220120-2 are homogeneously distributed, with over dispersion(OD)of 4% ±3%. Through analysis, it is found that the abnormally high OD value of XZ220120-1 was mainly caused by heterogeneous or partial luminescence signal resetting for quartz OSL signals during their deposition. Such results suggest that the targeted sand layer with parallel bedding at ca. 36.5 m(Wusong elevation)was not formed by aeolian process. The quartz single grain OSL age of XZ220120-1 is 230±66 a, corresponding to the Yongzheng-Xianfeng period of the Qing Dynasty. In comparison, while the quartz single grain OSL age of XZ220120-2 is 25.9±1.9 ka, corresponding to the MIS 2 stage. In addition to OSL dating, the ranges of Poyang Lake were simulated using ArcGIS software and the relevant local chronicles in Jiangxi Province were collected. Based on the above studies, it is concluded that the parallel bedding sand layer in the LSSL-1 profile is not caused by tectonic uplift, although previous studies suggested that the sand hills around Poyang Lake suffered from significant tectonic uplift during the Late Quaternary. In addition, the parallel bedding sand layer in the LSSL-1 profile can't be used as the indicator for paleo-water level of Poyang Lake, as the simulated range of Poyang Lake using the elevation of 36.5 m is highly different from the range of Poyang Lake during Qing Dynasty. Instead, the parallel bedding sand layer in the LSSL-1 profile is interpreted to be caused by heavy rain or extreme rainfall events during the Yongzheng-Xianfeng period of the Qing Dynasty, which led to water flow in some areas of the Shaling sand hill.