Tracking the Provenance of Aeolian Loess in Northeastern China by Uranium Isotopes

The aeolian loess deposits in northeastern China have substantial potential for reconstructing the paleoclimate in the East Asian monsoon boundary zone. Identifying the source of the aeolian dust is crucial to interpret the paleo-proxies recorded in the loess deposits in northeastern China; however, traditional geochemical tracers such as Nd and Sr isotopes cannot discriminate between the potential source areas, that is, the Otindag Sandy Land, Horqin Sandy Land, and Hulun Buir Sandy Land regions. In this study, the (U-234/U-238) activity ratio is used to constrain the provenance of the loess deposits in northeastern China. The (234U/238U) of the studied loess-paleosol sequences (from 0.935 to 0.910) decreases with increasing depositional age (from 10 to similar to 1,000 ka), which is consistent with predictions from uranium isotope comminution age model. A derived (U-234/U-238) of 0.935 +/- 0.003 for the newly deposited loess (zero depositional age) is most similar to that of the Otindag Sandy Land positioned at a proximal upwind direction, suggesting that the Otindag Sandy Land is the most likely source of the aeolian dust that formed the loess deposits in northeastern China. A transportation time of 432 +/- 65 ka was calculated for these deposits, indicating extensive mixing of the fine dust in the source regions, which could partially explain the relative homogeneity of loess deposits in northeastern China. The transportation time longer than several glacial-interglacial cycles suggests that reconstructions of regional paleoclimate on orbital timescales should be viewed with caution because the potential influence of climatic changes could largely be buffered. Plain Language Summary The activity ratio of the uranium isotopes (U-234 and U-238) in fine-grained particles can be used to determine the comminution age, or the amount of time since the production of fine sediment particles (<= 50 mu m) from the breakdown of bedrock by weathering. The (U-234/U-238) activity ratio provides a new approach to track the age and origin of dust particles. In this study, we measure the (U-234/U-238) composition of the loess deposits in northeastern China and surface sand samples in potential source areas to better understand the transport of aeolian dust in the region. We find that the Otindag Sandy Land, which is in the proximal upwind direction, is the main source region for the loess deposits in northeastern China and that the dust materials have been resided in the source region for around 432 +/- 65 thousand years before aeolian transport and deposition. Our results shed light on the transport and depositional history of loess in northeastern China and provide valuable information for reconstructing paleoclimate using the loess deposits in this region.

Automated summary

This study uses the (U-234/U-238) activity ratio to determine the provenance and depositional age of loess deposits in northeastern China. The results indicate that the Otindag Sandy Land is the most likely source of the aeolian dust, and the transportation time for the dust is approximately 432 thousand years. These findings are important for reconstructing the paleoclimate in this region.