Testing the natural limits of infrared radiofluorescence dating of the Luochuan loess-palaeosol sequence, Chinese Loess Plateau

Abstract

Testing the upper limit of infrared radiofluorescence (IR-RF) dating in nature is a critical step in developing our understanding of the signal and its potential. The Luochuan loess-palaeosol sequence on the Chinese Loess Plateau is a well-documented sequence spanning over 2.5 Ma, that has served as a proving ground for many trapped charge dating techniques, for example: feldspar post-infrared infrared stimulated luminescence (pIRIR), quartz electron spin resonance (ESR), and quartz violet stimulated luminescence (VSL). This study evaluates the IR-RF signal from coarse-grained feldspar on 10 samples from the loess-palaeosol sequence with depositional ages ranging from ̴̴25 ka to ̴900 ka. Initial work tested 6 samples using the RF70 protocol with a bleaching duration of 1500 s using UV-LEDs between the natural and regenerated IR-RF measurements which resulted in consistent and significant underestimation across all but the youngest sample. The bleaching duration was increased to 20 000 s and tested on 10 samples. The IR-RF ages of 5 samples younger than 300 ka (̴1100 Gy) were consistent with the reference ages while the IR-RF ages for samples older than 300 ka were still significantly underestimated. Natural and laboratory dose response curves were constructed, and they revealed significantly different curves in the case of the shorter bleaching duration, but consistent curves in the case of the longer bleaching duration, confirming the importance of the selected bleaching duration. Furthermore, our study suggests that while the IR-RF signal of feldspar can be used successfully to date samples up to 1100 Gy (∼300 ka at our site), it may not be possible to reach the theoretical laboratory-generated dating limit of 3500 Gy.