Applications of optically stimulated luminescence dating in the Chinese Loess Plateau

Abstract

Dose recovery studies with K-feldspar samples show that the electron trapping probability change of low-temperature IRSL signals caused by the first preheat treatment could result in age underestimation, with the single-aliquot regenerative-dose (SAR) dating protocol. However, the multi-elevated-temperature post-IR IRSL (MET-pIRIR) signal at 250 ºC does not have this problem. With the multiple-aliquot regenerative-dose (MAR) protocol or the modified SAR protocol with solar bleaching behind each cycle, this kind of age underestimation could also be avoided.

Standard growth curves (SGCs) for K-feldspar, plagioclase and polymineral fractions of northern China have been constructed, to improve the dating efficiency. For coarse grains (63-180 µm), the SGCs are quite similar between K-feldspar, plagioclase and polymineral fractions. But the fine-grain fraction (4-11 µm) has a distinct SGC, which is supposed to be a result of alpha irradiation. With the SGCs, the time used to measure the equivalent dose is only one third of the time needed before.

The loess-paleosol sequence in the Chines Loess Plateau (CLP) is an important paleoclimate archive. However, the assumption of continuous loess deposition has been questioned in recent years. Several studies proposed that loess was of high mobility, with frequent wind erosion and dust recycling. Huge age hiatuses of ~ 60 ka were also identified in the north margin of CLP. In this study, high-resolution OSL dating is performed from S1 layer to L4 layer in Luochuan section, central CLP. With the up-to-date dating protocol, the loess has been successfully dated back to ~350 ka. An age hiatus of ~ 10 ka is discovered at the top of L2 layer. But, it is proved to be a fake age one resulted from the wrongly estimated dose rates, due to the carbonate leaching and accumulation process. The continuity of loess deposition in Luochuan section has been confirmed for the last three glacial-interglacial cycles. And it further indicates that loess deposition should be continuous over the whole Quaternary period in the central CLP. Previous paleoclimate reconstructions based on astronomically tuned ages are of high reliability.

Quartz OSL sensitivity variation has been studied in Luochuan and Jingyuan sections. The OSL sensitivity in Luochuan section is overall higher than that in Jingyuan section. Particularly, the sensitivity in paleosol layers could be 20 times higher than that of loess layers in Luochuan section, which cannot be explained by different provenances or deposition/transportation cycles, but can be reasonably explained by wildfire heating. Quartz OSL sensitivity can be applied as a new proxy for paleo-wildfires.