The Influence of Different Pretreatment Methods on the Results of Grain Size Analyses for Archaeological Sites in Chongqing

Abstract

In this study, 24 samples from sections of Shiniu Temple and Dashidong archaeological sites in Chongqing were selected to compare the effects of different pretreatment methods on grain size analysis. Each sample was pretreated using one of five methods: (A) pretreating with distilled water, (B) pretreating with (NaPO3)6, (C) pretreating with H2O2, (D) pretreating with HCl, and (E) comprehensively pretreating with H2O2,HCl, and (NaPO3)6. Pretreated samples were ultrasonically dispersed for 10 minutes before examination using a Malvern Mastersizer 2000. First, with the addition of distilled water or the 0.05 mol/L (NaPO3)6 dispersant, a dispersion effect was not obvious. Indeed, the main grain size parameters were similar to those seen without any pretreatment (method A), indicating that physical pretreatment methods can neither remove possible impurities, such as carbonates and organic matter, nor disperse the potential cementitious particles in the sample. Second, the addition of H2O2 effectively removed organic debris at the archaeological site, enabling the experimental results to accurately reflect the grain size characteristics of inorganic particles in the sediment. The average median grain size of the sediments at the Shiniu Temple archaeological site was the smallest, and the clay content was the highest after being pretreated in this way. The sand content was reduced by approximately 50% compared with that of the sample without pretreatment. The performance is very stable throughout the Shiniu Temple profile; however, for some samples rich in carbonate cement at the Dashidong archaeological site, the effect of this pretreatment method is limited. Third, after pretreatment with 10% dilute HCl, the median grain size of the sample from Shiniu Temple site increased, the sand content remained similar, and the clay content was significantly reduced. The clay content and sand content of Dashidong site decreased simultaneously, revealing that HCl reacts with intergranular carbonate cement to reduce the content of coarse-grained components and causes clay minerals to potentially flocculate in the acidic environment, resulting in content reduction. Acid pretreatment of the grain size was originally suggested from research on loess, aiming to remove a large number of authigenic carbonate minerals in sediments from arid and semi-arid areas. However, it may not be suitable for sediments in humid areas. On the one hand, the content of authigenic carbonate in the sample in such an area is extremely low, making acid treatment nonessential. On the other hand, clay minerals, such as montmorillonite and kaolinite, flocculate under the colloid action of iron oxide in an acidic environment, which makes the grain size coarser, having a counterproductive effect. In this study, archaeological samples from typical karst areas were selected and the optimal particle size pretreatment method was determined through comparative experiments, providing an important reference for follow-up research. Namely, the content of carbonate cement is low, the optimal pretreatment effect can be achieved by adding H2O2 to remove organic impurities. If the carbonate composition is significant, the iron oxide in the sample can be removed first and then treated with dilute HCl, effectively avoiding the flocculation of clay minerals. Whether better pre-treatment methods are available needs to be explored further.

Keywords

• chongqing
• environmental archaeology
• grain size
• pretreatment methods