资源环境科技发展态势分析平台

Eolian deposition in China is intimately related to Asian desertification. The fine-grained sediments in the Cenozoic basins along the northeastern Tibetan Plateau are increasingly reported as eolian deposits due to their loess-like characteristics, thus extending the onset time of Asian desertification from the late Miocene to the early Miocene. However, the basin-scale stratigraphic correlation and geographic origin of these fine-grained sediments remain poorly investigated, limiting any interpretation of the transportation and paleoclimatic conditions responsible for their emplacement. This study combines analyses of detrital zircon U-Pb geochronology and heavy-mineral assemblages to present the first comprehensive stratigraphic and provenance study of Miocene sediments in the Tianshui Basin in northeastern Tibet. Comparison of results from 64 sand-sized and silt-sized samples from various sedimentary beds across the basin reveals that the main stratigraphic packages can be correlated basin-wide, confirming that the silt-sized sediments were a natural part of the basin fill. Statistical visualizations of the large data set from northern and western China clearly shows that the West Qinling Mountains are the main source of the Tianshui Basin silts. Contribution estimates corroborate this conclusion and eliminate the likelihood of substantial contributions from other mountains and drylands. Moreover, the significant differences between the Tianshui Basin silts and the Chinese Loess Plateau/Red Clay are reflected by their heavy-mineral compositions and euhedral zircon U-Pb ages. These provenance analyses strongly contradict the previous assumption of desert origin for the Tianshui Basin silts. Moreover, the northward transport of the Tianshui Basin silts conflicts with the prevailing dust pathway driven by the Asian westerlies and winter monsoons, which do not support the eolian interpretation for their formation. Consequently, we suggest that the Miocene Tianshui Basin silts cannot be used as an indicator of extensive desertification, in contrast with the suggestions of previous studies. This proposal undermines the corresponding hypothesis of an early Miocene onset of Asian interior desertification, and further raises questions about the validity of prevalent paleoclimatic interpretations drawn from fine-grained basin sediments.