Optical dating of Romanian loess: A comparison between silt-sized and sand-sized quartz

Abstract

The loess-palaeosol sequence near Mircea Vodă (Dobrogea, SE Romania) is thought to comprise at least five glacial/interglacial cycles. It is considered as one of the most significant records of Late Quaternary environmental and climatic change in the region. Age information for the sequence was previously obtained through time–depth modeling of magnetic susceptibility data and optically stimulated luminescence dating (OSL) of fine silt-sized (4–11 μm) quartz. This study documents the granulometric properties of the sequence, and compares the OSL characteristics and age of fine sand-sized (63–90 μm) quartz to those of silt-sized quartz. OSL signals from both sand and silt-sized quartz are dominated by a thermally-stable fast decaying component. A suite of laboratory test confirms that the single-aliquot regenerative dose (SAR) protocol is suitable for these samples. Each grain-size fraction yields a set of optical ages that is consistent with the stratigraphic position of the samples. The ages obtained using sand-sized grains, however, are up to 70% higher than those obtained using silt-sized quartz. The discrepancy in age results is not understood as, according to procedural tests, the OSL signal from both fractions should be suitable for dating the deposits. As such, it cannot be established whether the uppermost weakly-developed palaeosol was formed during MIS3 or MIS5 and if the rate of loess accumulation varied during the Last Glacial, only by using standard SAR-OSL techniques. Both sets of ages do confirm, however, that the first well-developed palaeosol (S1) is of Last Interglacial age.