Abstract: Background,aim,and scope Sanyangzhuang archaeological site of the Han Dynasty was deeply buried and well preserved by the Yellow River alluvium. It has the ruins of ancient villages from the late Western Han Dynasty to the early Eastern Han Dynasty, and provides ideal material for studying the historical climate and river channel changes in the lower reach of the Yellow River. Most of the existing studies on Sanyangzhuang site focus on the division of stratigraphic sequence and the description of site villages. Except for some absolute age data measured by accelerator mass spectrometry (AMS) 14C method, there is still a lack of support for high resolution age data. Quartz optically stimulated luminescence(OSL) dating protocol (OSL) has been successfully applied to the dating of various aeolian sediments, such as loess, sand dunes, etc. The precise dating of hydatogen sediments and some human cultural sites often use 14C dating method. In recent years, OSL dating protocol has been widely applied to determine the ages of alluvial/fluvial sediments. In this study, we tried to obtain an OSL chronology of the 10.40 m Sanyangzhuang archaeological profile and to compare it with AMS 14C chronology. Materials and methodsWe collected OSL dating samples near a courtyard in the northeast corner of Sanyangzhuang archaeological site. The luminescence sample tubes were processed under subdued red light conditions in the luminescence laboratory. The samples (~100 g) were first treated with 30% w.w. H2O2 and 30% v.v. HCl to remove organic materials and carbonates, respectively. The samples were washed with distilled water until reaching pH neutral, and then 4-11 μm diameter polymineral grains were separated according to Stokes’ law. These grains were immersed in 30% hydrofluorosilicic (H2SiF6) for 3-5 days to extract the fine-grained quartz component. The resultant fluoride was removed using 30% v.v. HCl. Finally, the purified quartz was deposited on 9.7-mm-diameter stainless steel discs using ethanol and dried prior to measurement. All of the OSL measurements were performed using an automated Daybreak 2200 OSL reader equipped with infrared (880±60 nm) and blue (470±5 nm) LED units and a 90Sr/90Y beta source for irradiation. For dose rate determination, U and Th concentration was measured using inductively coupled plasma mass spectrometry (ICP-MS), and inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to determine the K concentration. The fine-grained quartz single aliquot regenerative dose(SAR) OSL dating protocol is used for obtaining the eight ages of the upper 10.40 m sediments at Sanyangzhuang (SYZ) archaeological site. Conventional tests in fine-grained quartz SAR protocol and the OSL ages variation with depth indicate the reliability of quartz OSL dating in this study. Results (1) The equivalent dose distribution of the 8 samples in Sanyangzhuang profile is about 37.7-3.5 Gy, and the dose rate fluctuates greatly, which is 2.9-4.8 Gy/ka. According to the values of equivalent dose and dose rate, the ages of eight samples can be obtained. Quartz OSL dating results show that, the profile deposited between 12.43-1.21 ka, covering almost the entire Holocene, with sediment accumulation rate fluctuated rapidly. Except for the samples at 8.60 m, the age of the remaining seven samples basically conforms to the stratigraphic sequence. The cause of age reversal at 8.6 m is unknown, and this study will treat it as an abnormal age point for the time being. During the period of about 3.91-3.15 ka (depth of 9.60-5.00 m), the deposition rate is extremely high, while that before about 3.91 ka and after about 3.15 ka is low. (2) The 14C ages of depth between 8.60-5.00 m is significantly overestimated, the 14C age at 8.50-4.96 m of Sanyangzhuang section is 10.2-5.16 ka BP, and when compared with the OSL ages the difference between them generally becomes larger as depth increasing, varaying from about 2 ka to 7 ka. Discussion In the measurement of hydrous sediments, organic matter samples such as peat used in 14C dating could be mixed with samples from older strata before joining the closed watershed system and are polluted by “old carbon” from surrounding rocks, which makes the samples older. It is suggested that the discrepancy between OSL ages and 14C ages mainly results from radiocarbon reservoir effect. The OSL dating results of Sanyangzhuang section indicate the great potential of fine-grained quartz dating into fluvial-alluvial sediments. Conclusions Fine quartz OSL dating results show that, the profile deposited between about 12.43-1.21 ka, covering almost the entire Holocene, with sediment accumulation rate fluctuated rapidly, the accumulation rate of about 3.91-3.15 ka is very fast. It is suggested that the discrepancy between OSL ages and 14C ages mainly results from radiocarbon reservoir effect. Recommmendations and perspectives In the future, more efforts need to be put on the high-resolution construction of Sanyangzhuang archaeological site. Fine-grained quartz OSL dating into fluvial-alluvial sediments has great potential. Meanwile, radiocarbon reservoir effect of hydrogenic sediments needs to be studied further.
Keywords: Holocene; OSL dating; 14C dating; radiocarbon reservoir effect; Sanyangzhuang