Luminescence dating of Romanian loess using feldspars

Abstract

This work investigates the potential of optically and infrared stimulated luminescence signals (OSL and IRSL, respectively) from polymineral silt-sized grains to establish sedimentation chronologies for Romanian loess. The incentive for the study was the observation that various grain sizes of quartz yielded significantly different OSL ages, a behaviour that remains to be understood; feldspar also has the potential to date older deposits. The samples used in this study were taken from the loess/palaeosol sequence near Mircea Vodă, and all luminescence measurements were made using a single-aliquot regenerative-dose (SAR) protocol. A conventional approach, in which stimulation with IR was at 50°C and the detection window in the blue, was found to be compromised by thermal instability of the signals and/or initial sensitivity changes. A double-SAR approach – involving successive stimulation of the polymineral fine grains with IR and blue light, and detection of the resulting IRSL and post-IR OSL signals in the UV – was also tested. Although both signals exhibited significantly different fading rates, the corrected ages are mutually consistent and in agreement with OSL ages for purified silt-sized quartz. This indicates that it may not be necessary to isolate pure quartz to obtain reliable ages for Romanian loess. Owing to the limitations of the fading-correction method, however, the approach is limited to samples from the last glacial period. Finally, a post-IR IRSL protocol was tested, in which a first stimulation at 50°C was followed by a second stimulation at 225°C, and detection was in the blue. The post-IR IR225 signal appears not to suffer from anomalous fading, and yields ages entirely consistent with both OSL ages for silt-sized quartz and independent age control over four interglacial/glacial cycles. The overall conclusion is that post-IR IR dating of polymineral fine grains is very promising. The obtained age results urge the long-established chronostratigraphical framework for Romanian loess to be revised; the two uppermost well-developped palaeosols can no longer be thought of as interstadial soils that developed during the Last Glacial, but have formed during MIS 5 and MIS7, respectively