Controlling Factors On Heavy Mineral Assemblages In Chinese Loess And Red Clay And Provenance History Of The Chinese Loess Plateau

The loess sequence and the underlying red clay sequence on the Chinese Loess Plateau contain important paleoclimatic and paleoenvironmental information, but great debates exist about provenance of Chinese loess and red clay. Heavy-mineral analysis is a sensitive technique in constraining provenance of sandstone, but has been rarely applied to loess. Here we report a heavy-mineral study of selected loess and red clay samples from the Luochuan, Xifeng, and Caoxian sections and some nearby desert samples based on the novel QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) technique. We found that heavy mineral assemblages of loess deposited through the past500kyr are similar and unchanged by post-depositional chemical dissolution. In contrast, the ratio of stable versus unstable minerals tends to increase downsection in samples deposited from900kyr to3Ma. This pattern suggests that the heavy mineral assemblages of the older strata were changed by post-depositional chemical dissolution, and that the degree of modification increases over time. In addition, the samples from the drier Xifeng section display consistently higher contents of unstable ferromagnesian minerals than those from the wetter Luochuan section, suggesting that climate plays an important role in modification of heavy minerals. Our study demonstrates that both climate and time play key roles in determining composition of heavy-mineral assemblages contained in loess deposited500kyr ago. However, heavy mineral assemblages of loess deposited during the past500kyr have not been altered by chemical dissolution. Therefore, heavy mineral analysis is a powerful tool to reconstruct provenance history of the Chinese Loess Plateau during the past500kyr.Numerous studies suggest that the proximal deserts of China are main provenance for the Chinese Loess Plateu. To test this hypothesis, we performed the heavy mineral analysis on surface samples from the Mu Us desert, the Badain Jaran desert, the Ulan Buh desert, and the Tengger desert and compared the results with those of loess and paleosol samples deposited during the past500kyr on the Chinese Loess Plateau. We found that the western Mu Us desert, the Badain Jaran desert, the Ulan Buh desert, and the Yellow river are likely key dust contributors for the Chinese Loess Plateau. In contrast, the eastern Mu Us desert and the Tengger desert are less likely key dust sources for the Chinese Loess Plateau. In addition, our results reveal that the heavy mineral composition between loess and paleosol is similar, suggesting that no provenance shift on the Chinese Loess Plateau between glacial and interglacial timescale. This conclusion is consistent with that based on very diagnostic but time-consuming zircon U-Pb geochronology.In summary, this paper studies the controlling factors of the heavy mineral assemblages of loess for the first time. The results reveal that heavy mineral assemblages of loess deposited through the past500kyr are unchanged by post-depositional chemical dissolution. In contrast, climate and time-controlled diagenesis have altered heavy mineral composition of earlier deposited loess. This study also reveals that no apparent provenance shift on the CLP between glacial and interglacial timescale since0.4myr ago and the western Mu Us desert, the Badain Jaran desert, the Ulan Buh desert, and the Yellow river are likely key dust sources for the CLP.