鄱阳湖北部沙山一处平行层理砂层的释光信号回零分析及年代学研究

鄱阳湖作为中国最大淡水湖，其周边发育若干沙山。虽然此前研究多认为这些沙山为风力堆积的产物，但是这些沙山中上部还普遍保存有水成沉积构造的沉积层。选取江西省湖口县沙洲村沙山一处代表性的平行层理砂层（吴淞高程约40.3 m）为研究对象，首先对目标砂层的石英和钾长石开展光释光信号回零分析。研究结果显示该砂层的石英光释光信号和钾长石光释光信号均呈现明显的晒退不均。这种现象与沉积构造解译一致，即目标沉积层为水成堆积过程的产物。之后，对目标砂层开展年代学研究，通过对比鄱阳湖湖岸一处现代平行层理砂层的光释光年代结果，将沙洲村沙山目标砂层的年代限定为全新世大暖期（石英单片和单颗粒光释光年龄分别为(6.9±1.6) ka和(6.4±2.2) ka）。

The bleaching of OSL signals and the chronology for a sand layer with parallel bedding within a sand hill at north of Poyang Lake

Background, aim, and scope Poyang Lake is the largest freshwater lake in China. Currently, several sand hills were developed around Poyang Lake and such phenomenon has attracted a lot of attention from geoscientist. In previous studies, the sand hills around Poyang Lake were interpreted to be dominantly formed by aeolian process. However, by field investigation, it is found that some sand and silt layers within the sand hills exhibit clear water-lain sedimentary structures (e.g. parallel bedding and horizontal bedding). The accurate interpretation of these sedimentary layers within the sand hills is not only important for understanding the environment evolution of Poyang Lake during the Quaternary, but also crucial for the assessment of flood risks of surrounding areas. In this study, one typical sand layer with parallel bedding at an elevation of about 40.3 m from a sand hill at north of Poyang Lake was investigated. The optical dating technique was applied to samples from this sandy layer, not only to obtain its depositional age, but also to investigate the bleaching of the quartz OSL signals and K-feldspar IRSL signals. Combining the sedimentary structure with the optical dating results, the sedimentary process of the targeted sand layer was further determined and its hydrological implication was also discussed. Materials and methods An OSL sample (HGDS-OSL7) was collected from the targeted sand layer with parallel bedding within a sand hill at Shazhou Village, Hukou County. Firstly, the bleaching of both quartz OSL and K-feldspar IRSL signals were investigated by using optical dating techniques. The bleaching of quartz OSL signals was tested by measuring the single-aliquot equivalent dose (D_e) distribution and single grain D_e distribution, while the bleaching of K-feldspar IRSL signals was investigated by using multi-elevated-temperature post-IR IRSL (MET-pIRIR) protocol. Secondly, by comparing a modern sand layer with parallel bedding at the shore of Poyang Lake, the final OSL age of the targeted sand layer at Shazhou Village, Hukou County was further determined. Results (1) Although the quartz OSL signals were bright and dominated by strong fast component, the over-dispersion of single aliquot quartz OSL D_e and single grain quartz OSL D_e of HGDS-OSL7 are very large and they are about 75.2% and 101.0%, respectively. (2) The K-feldspar of HGDS-OSL7 does not exhibit D_e plateau when the average age model is used in the MET-pIRIR method. (3) For the targeted sand layer, the single aliquot quartz OSL age and single grain quartz OSL age are dated at (6.9±1.6) ka and (6.4±2.2) ka, respectively, while the K-feldspar MET-pIRIR age is measured at (165.2±11.0) ka. (4) By optical dating a modern sand layer with parallel bedding at the shore of Poyang Lake, it is found that quartz OSL ages are more appropriate for the final age determination, compared with K-feldspar IRSL ages. Thus, targeted sand layer with parallel bedding was constrained to be formed during Mid-Holocene. Discussion The large over-dispersion of quartz OSL D_e, the lack of K-feldspar D_e plateau in the MET-pIRIR method and large discrepancy between quartz OSL ages and K-feldspar IRSL age indicate that some quartz and K-feldspar grains of targeted sand layer were partially bleached before deposition and they are not formed by aeolian process. Thus, OSL dating technique can not only serve as an important chronological tool, but also can be used as an assistant tool for distinguishing sedimentary facies, i.e. aeolian deposits or water-lain deposits. The targeted sand layer suggested that the studied site experienced strong hydrodynamic conditions during Mid-Holocene. Our result is very different from the interpretations of sand hills by previous studies, as sand layers in the middle-top of sand hills around Poyang Lake were thought to be aeolian deposits and they were treated as indicators for arid environment. Conclusions For the targeted sand layer, both the quartz OSL and K-feldspar IRSL signals exhibit heterogeneously bleaching phenomenon, suggesting that the targeted sands were more likely formed by water-lain process rather than aeolian process. The quartz single aliquot OSL age and quartz single grain OSL age for the targeted sand layer were measured at (6.9±1.6 ka) and (6.4±2.2) ka, respectively, corresponding to the Holocene Optimum. Recommendations and perspectives In addition to aeolian sediments, some water-lain deposits were also preserved within sand hills around Poyang Lake and they may record some crucial information on hydrological process of Poyang Lake during the Late Quaternary. In the future, more studies should be carried out on the sand hills around Poyang Lake.