Abstract:

：The grain size of sediment is an important tool for the extraction of paleo-climate and paleo-environment change information. This article attempts to study the influence of sample pretreatment methods on particle size analysis result. Loess and paleosol sample were chosen from the Qianfangcun profile in the upper reaches of the Hanjiang river valley as the representative material. They are measured using a laser particle analyzer (LS1320) made by Beckman Company, U.S.A. These samples were performed using 6 different pretreatment methods before their grain-size distribution were measured, respectively. The experimental results show that the pretreatment methods and steps for the grain size measurement procedures have certainly influenced on results of grain- size distribution. Different samples should be used different retreatment methods and steps. In the process of sample pretreatment, ultrasonic vibration and oscillation time significantly influence the results of particle size measurement. Aiming at samples from the upper reaches of the Hanjiang river valley, pretreatment method A（0.8 g sample put into 500 mL beaker, then to pour 10 mL10% H₂O₂ and 10 mL 10% hydrochloric acid in the beaker, respectively. The beaker is filled with distilled water and static for 72 h, slow pumping to distilled water, putting into 5 mL 0.05 mol/L dispersing agent((NaPO₃)₆, then to measure particle size using a laser particle analyzer) is more suitable to weak cementation sediment samples, such as loess, modern topsoil and river sediments et al. Using pretreatment method A could obtain better effect of the particle size measurement because its particles can be sufficiently dispersed. The caking property of paleosol sample in the Hanjiang River valley is very high because of its strong pedogenesis. It is difficult to be completely dispersed particles of paleosol because of closely cemented between particles. There exists great differences in grain-size distribution of the palaeosol sample while it was pretreated by the pretreatment method A, B, C, D, E and F（ Method B: In the method based on A, then ultrasonic vibration with 10 min. Method C: 0.8 g sample was put into 500 mL beaker, then to pour 10 mL10% H₂O₂ and 10 mL 10% hydrochloric acid in the beaker, respectively. The beaker is filled with distilled water and static for 72 h, slow pumping to distilled water, putting into 5 mL 0.05 mol/L dispersing agent( (NaPO₃)₆), at the same time with using ultrasonic vibration 20 min, then to measure particle size using a laser particle analyzer. Method D: The samples were digested with H₂O₂ and hydrochloric acid, beaker was filled with distilled water and static for 72 h, putting into 10 mL 0.05 mol/L dispersing agent((NaPO₃)₆), then ultrasonic vibration with 20 min. Method E: The samples were digested with H₂O₂ and hydrochloric acid, stirring 15 min, beaker was filled with distilled water and static for 72 h, putting into 10 mL dispersant, ultrasonic vibration with 20 min. Method F: The samples were digested with H₂O₂ and hydrochloric acid, stirring 30 min, beaker was filled with distilled water and static for 72 h, plus 10 mL dispersant, ultrasonic vibration with 20 min）, respectively. The experimental results show that differences of the grain-size index such as the average particle size can be several times fluctuation. The pretreatment method C is more suitable for palaeosol samples from the Hanjing river valley. Dispersion effect of the pretreatment method C for palaeosol sample is better than those of other pretreatment method, including method A, B, D, E and F.

Key words: grain size analysis, pretreatment methods, palaeosol, the Hanjiang river