Infrared Stimulated Luminescence and Thermoluminescence Dating of Loess from Lanzhou, China

Abstract

Loess samples from the 22-m-thick Shajinping section on the second-lowest terrace of the Yellow River in Lanzhou, Gansu Province, China, were dated by infrared stimulated luminescence (IRSL) and by thermoluminescence (TL).
Isothermal preheating tests for IRSL dating were made at two different temperatures, 120 and 160°C, to determine the preheating condition most appropriate for isolating the stable IRSL signal. The irradiated/non-irradiated IRSL ratio reaches a plateau after preheating for 24 to 144 hours at 120°C, or for 1.5 to 5 hours at 160°C. The IRSL signal with preheating at 120°C was enhanced by increasing the artificial gamma irradiation, but the IRSL signal with preheating at 160°C was saturated at a low dose, so it was difficult to use the dose-response curve to calculate an equivalent dose (D_E). The 120°C preheating was determined to be most appropriate; therefore, all samples were preheated at 120°C for 96 hours before IRSL measurement.
Except for one sample, the IRSL and TL dates for samples from the upper 12m of the section agreed with each other, within the error range, but the IRSL dates for samples from the lower 9m of the section were younger than the TL dates. This discrepancy between IRSL and TL dates might have been caused by underestimation of the IRSL dates, due to the samples' low saturation value of IRSL D_Es at about 200Gy. The IRSL and TL dates indicate that the loess deposition rate during the last 60ka has changed several times. A very high loess deposition rate was obtained for the late marine isotope stage (MIS) 3, whereas the deposition rate seems to have been lower during MIS 2.