Search

Despite decades of study, the factors that controlled the formation and evolution of theupper reaches of the Yellow River, including uplift of the northeastern Tibetan Plateau, Pliocene-Pleistocene climate change, and autogenetic processes are still poorly constrained. The stratigraphicrecord of such paleogeographic evolution is recorded in the sequence of nine terraces formed duringprogressive incision of the Yellow River in the last 1.7 Ma. This article investigates in detail forsediment provenance in terraces of the Lanzhou area, based on heavy-mineral and geochemical(REE) signatures. Two main provenance changes are identified, pointing each to a majorpaleogeographic reorganization coupled with expansion of the upper Yellow River catchment andenhanced sediment fluxes. The first change took place between the deposition of terrace T9 (formedaround 1.7 Ma) and terrace T8 (formed around 1.5 Ma), when rapid fluvial incision point to tectoniccontrol and active uplift of northeastern Tibetan Plateau. The second change took place betweendeposition of terrace T4 (formed around 0.86 Ma) and terrace T3 (formed around 0.14 Ma), duringa period of low incision rates and notably enhanced sediment fluxes as a response to enhanced EastAsian Summer Monsoon and consequently increased precipitations, pointing instead chiefly toclimatic control.

QEMSCAN-based (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) heavy mineral analysis has recently been demonstrated an efficient way to allow a rapid extraction of provenance information from sediments. However, one key issue to correctly obtain a provenance signal using this technique is to clearly separate effects of diagenetic alteration on heavy minerals in sediments, especially in fine-grained loess. Here we compare heavy mineral assemblages of bottom Quaternary loess (L33) and upper Pliocene Red Clay of three sites on the Chinese Loess Plateau (CLP). Two sites (Chaona and Luochuan) with similar modern climate conditions show similar heavy mineral assemblages but contain much less of the unstable heavy mineral amphibole than the drier Xifeng site. This result provides strong evidence supporting that climate-caused diagenesis is an important factor controlling heavy mineral assemblages of fine-grained loess. However, heavy mineral assemblages are similar for loess and paleosol layers deposited after 0.5 Ma on the Chinese Loess Plateau regardless of climate differences, suggesting that time is also a factor controlling heavy mineral assemblages of loess and Red Clay. Our high resolution sampling of the upper Miocene-Pliocene Chaona Red Clay sequence reveals similar heavy mineral compositions with a minor amphibole content, different from the drier Xifeng site results of the same age. This result indicates that the monsoonal climate pattern might have been maintained since the late Miocene. Furthermore, it indicates that the heavy mineral method is promising in tracing provenance for sites northwest of the Xifeng site on the Loess Plateau.

The upper Yellow River drains the central and northeastern Tibetan Plateau. Understanding the origin of this river is essential for unraveling the interplay between fluvial incision, basement uplift, and climate change. However, the formation age of the upper Yellow River is highly debated, with estimates ranging from Eocene to late Pleistocene. In order to clarify the history of the upper Yellow River, we present a heavy-mineral dataset from drill core in the Yinchuan Basin, a depositional sink at the end of the upper Yellow River course. Our results reveal that the drainage area of the upper Yellow River (i.e., northeastern Tibetan Plateau) has been serving as a major sediment source region for the Yinchuan Basin since at least 3.3 Ma. A late Pleistocene formation of the upper Yellow River, and erosion of neighboring deserts or bounding mountains as an explanation for thick sediment accumulations in this area, is thus ruled out.

Understanding provenance of glacial loess and interbedded interglacial paleosol deposits on the Chinese Loess Plateau (CLP) is important to understanding paleo-atmospheric circulation patterns. However, great debate exists about whether Chinese loess and paleosol deposits have similar provenance both spatially and temporally. Here we report heavy mineral assemblage results of two sets of loess–paleosol couplets (L1 and S1; L4 and S4) from western CLP based on scanning electron microscopy with linked energy dispersive spectrometers (SEM–EDS) analysis (QEMSCAN@). The results demonstrate no apparent provenance shift on the CLP between glacial and interglacial periods. Heavy mineral assemblage comparison between the western and the central CLP reveals no provenance shift spatially, either. Finally, a comparison of heavy mineral assemblage between loess–paleosol and potential source regions reveals that the western Mu Us desert, the Badain Jaran desert, the Ulan Buh desert as well as the Yellow river are likely key dust contributors for the CLP. In contrast, the northeastern Mu Us desert are less likely key dust sources for the CLP. The ultimate provenance history of the CLP relies no doubt on integrated provenance studies. However, due to its rapidity and robustness, integrated provenance analysis should incorporate the QEMSCAN-based heavy mineral analysis.