Quartz single grain OSL dating for two sand layers with mud laminae and mud clasts within a sand hill at north of Poyang Lake

There are several sand hills around Poyang Lake, and previous studies show that, in addition to the aeolian sediments, some water-lain sediments with clear hydraulic sedimentary structures were also preserved within the sand hills around Poyang Lake. In this study, two new profiles were targeted within a sand hill(29° 49' 52.03″~29° 50' 25.03″ N, 116° 22' 26.47″~116° 23' 3.25″ E)at Shazhou Village, Hukou County, Jiangxi Province and the two profiles contain yellow sand layers with mud laminae and mud clasts. In this study, the two sand profiles were termed as HKSZ-3 (29° 50'16.3082″ N, 116° 22'48.9375″ E)and HKSZ-4 (29° 50' 7.20″ N, 116° 22' 50.95″ E), respectively. The thickness of HKSZ-3 is 1.15 m, while the top sand layer with mud laminae and mud clasts is 0.8 m in thickness. The thickness of HKSZ-4 is 1.0 m and the profile is all composed of sand layer with mud laminae and mud clasts. Two OSL samples(20231231-OSL-1 and 20240106-OSL-2)were collected from HKSZ-3 and their depths are 0.1 m and 0.7 m, respectively. One OSL sample(20230304-OSL-8)was collected from HKSZ-4 and its depth is 0.5 m. The three OSL samples were then studied using quartz OSL dating techniques. For comparison with equivalent dose(De)distribution characteristic, another OSL sample(20240106-OSL-9)was collected from the sand layer with massive bedding at the top of sand hill at Shazhou Village. 20240106-OSL-9 was collected at 29° 50' 17.14″ N, 116° 22' 49.62″ E and the depth is at 0.6 m. The results show that the quartz single aliquot OSL exhibit bright luminescence signal and good OSL dose response curves. In the single grain scale, quite a few quartz grains also exhibit bright luminescence signal and good OSL dose response curves. It is found that quartz single grain De values of targeted sand layers with mud laminae and mud clasts are very scattered. The over-dispersion(OD)values of three samples(20231231-OSL-1, 20240106-OSL-2 and 20230304-OSL-8)are calculated at 110% ±2%, 110% ±1% and 75% ±1%, respectively. In comparison, the OD value of quartz single grain De values of 20240106-OSL-9 is only 21% ±2%. Through analysis, it is found that the anomalous high OD values of the three OSL samples(20231231-OSL-1, 20240106-OSL-2 and 20230304-OSL-8)can't be explained by heterogeneous environmental dose rate alone and it should be also due to the different bleaching of quartz OSL signal during the deposition. The observation of insufficient bleaching of quartz OSL for some quartz grains is consistent with the sedimentary characteristics of the sand layers. The quartz single grain OSL ages results show that the targeted sand layers was mainly formed during the Middle and Late Holocene. The quartz single grain OSL ages of three samples were dated at 1.0±0.3 ka, 2.3±0.6 ka and 6.2±1.3 ka, respectively. Combined with bleaching of quartz OSL signals, quartz single grain OSL ages, sedimentary characteristics and spatial distribution of targeted sand layers, it is found that the new identified sand layers with mud laminae and mud clasts were not formed by aeolian process from the local low flood plain of Yangtze River and its tributary Ganjiang River to the relative high sand hills. They can neither be used as the evidence for regional tectonic uplift, nor be used as the indicator for past water level of Poyang Lake. It is interpreted that the sand layers with mud laminae and mud clasts of HKSZ-3 and HKSZ-4 were as the results of heavy rain or extreme rainfalls during the Middle Holocene(6.2±1.3 ka)and Late Holocene(from 2.3±0.6 ka to 1.0±0.3 ka). The heavy rain or extreme rainfalls led to the formation of obvious runoff at the surface of sand hills, the erosion of the sand and mud at the surrounding, the transportation of sands and mud, as well as the deposition of them in the low-lying areas of sand hills.