ALBERT

Description: Asian mineral dust has been studied extensively for its role in affecting regional‐to global‐scale climate and for its deposits, which enable reconstructing Asian atmospheric circulation in the past. However, the timing and origin of the dust deposits remain debated. Numerous loess records have been reported across the Asian continent with ages varying from the Miocene to the Eocene and linked to various mechanisms including global cooling, Tibetan Plateau uplift and retreat of the inland proto‐Paratethys Sea. Here, we study the Eocene terrestrial mudrocks of the Xining Basin in central China and use nonparametric end‐member analysis of grain‐size distributions to identify a loess‐like dust component appearing in the record at 40 Ma. This is coeval with the onset of high‐latitude orbital cycles and a shift to predominant steppe‐desert vegetation as recognized by previous studies in the same record. Furthermore, we derive wind directions from eolian dune deposits which suggest northwesterly winds, similar to the modern‐day winter monsoon which is driven by a high pressure system developing over Siberia. We propose that the observed shifts at 40 Ma reflect the onset of the Siberian High interacting with westerly derived moisture at obliquity timescales and promoting dust storms and aridification in central China. The timing suggests that the onset may have been triggered by increased continentality due to the retreating proto‐Paratethys Sea.

Description: The Cenozoic strata of the Xining Basin, NE Tibet, have provided crucial records for understanding the tectonic and palaeo-environmental evolution of the region. Yet, the age of the lower part of the sedimentary stratigraphy and, consequently, the early tectonic evolution of the basin remain debated. Here, we present the litho- and magnetostratigraphy of various early Eocene sections throughout the Xining Basin independently constrained by the U–Pb radiometric age of a carbonate bed. Our study extends the dated stratigraphy down to 53.0 Ma (C24n.1r) and reveals highly variable accumulation rates during the early Eocene ranging from 0.5 to 8 cm/ka. This is in stark contrast to the low but stable accumulation rates (2–3 cm/ka) observed throughout the overlying Palaeogene and Neogene strata. Such a pattern of basin infill is not characteristic of flexural subsidence as previously proposed, but rather supports an extensional origin of the Xining Basin with multiple depocentres, which subsequently coalesced into a more stable and slowly subsiding basin. Whether this extension was related to the far-field effects of the subducting Pacific Plate or the India–Asia collision remains to be confirmed by future studies.

Description: The dry continental interior of Asia has remained arid throughout most of its geological history, yet the future of this unique ecosystem remains unclear. Here we use palynological and isotopic records to track vegetation and moisture throughout the warm early Eocene (57 to 44 million years ago) as an analogue for extreme atmospheric CO2 scenarios. We show that rainfall temporarily doubled and replaced the regional steppe by forested ecosystems. By reconstructing the season of pedogenic carbonate growth, we constrain the soil hydrologic regime and show that most of this rainfall occurred during the summer season. This humid event is therefore attributed to an inland expansion of monsoonal moisture following the massive greenhouse gas release of the Palaeocene–Eocene Thermal Maximum as identified by a negative carbon isotope excursion. The resulting abrupt greening of the Central Asian steppe-desert would have enabled mammal dispersal and could have played a role in carbon cycle feedbacks by enhancing soil organic carbon burial and silicate weathering. These extreme Eocene proto-monsoons, albeit different from the topography-driven Asian monsoon today, highlight the potential for abrupt shifts in Central Asian rainfall and ecosystems under future global warming.