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.

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Recognition of terrestrial dust in geological records is essential for reconstructing paleoenvironments and quantifying dust fluxes in the past. However, in contrast to eolian sands, silt-sized dust is difficult to recognize in pre-Quaternary records due to a lack of macroscopic features indicating eolian transport and mixing with alluvial sediments. Windblown dust deposits are commonly identified by comparing their sedimentological and petrological features with Quaternary examples of dust known as loess. Here, we review the characteristics of terrestrial dust deposits and conclude that most of these features are not exclusively windblown and may be formed by alluvial deposits as well. We therefore synthesize a set of criteria which enable a reliable identification and quantification of dust while acknowledging potential contributions of alluvial components. These methods include quartz-grain surface morphology analysis to distinguish eolian and alluvial transport modes, provenance studies to identify local and extrabasinal sources, grain-size-shape end-member modelling to quantify the various sedimentary contributions to the record, and a basin-scale stratigraphic approach to derive regional patterns and avoid interpretation of local phenomena. We reassess the Eocene to Pliocene records of the Chinese Loess Plateau and conclude that these strata represent both alluvial and eolian sediments deposited in extensive mudflat systems. Quaternary loess, by contrast, is almost exclusively composed of windblown dust. The early Pleistocene shift from mudflat to loess deposits is associated with a significant increase in accumulation rates, likely due to increased dust production upwind, overwhelming and blanketing the local mudflat systems in central China.

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Massive addition of isotopically-depleted carbon to the ocean and atmosphere caused a carbon isotope excursion (CIE) and global greenhouse warming during the Paleocene–Eocene Thermal Maximum (PETM) circa 56 million years ago. The body of the CIE is followed by a recovery interval that is key to understanding Earth's capacity for carbon uptake, mechanisms of carbon uptake, and biotic responses following an extreme greenhouse warming event. Expanded terrestrial stratigraphic sections in the Bighorn Basin of Wyoming provide exceptionally high-resolution records of the CIE and can be linked directly to the mammalian fossil record. Here, we provide carbon isotope records of unprecedented resolution measured on in-situ pedogenic carbonate nodules in two parallel 8-km-spaced sections of upper Paleocene and lower Eocene fluvial sediments in the northern Bighorn Basin. We find consistent precession-driven sedimentary cycles in the two sections. Cycle thicknesses show significant lateral, and thus vertical, variation, demonstrating that astronomical age models constructed for fluvial successions require detailed sedimentary facies analysis of parallel sites. Plotting the high-resolution carbon isotope records in time using our astronomical age model for the correlated sections indicates a CIE body duration of 101 ± 9 kyr. The CIE shows an initial recovery step of +2.7 ± 1.0‰. This step occurs across the single, well-developed paleosol marker bed known as Purple-4, which represents a time interval of up to 15 kyr. The rapidity of recovery at the end of the CIE body is remarkable in light of existing hypotheses for carbon removal from the ocean–atmosphere system. Concurrent mammal finds show that the transition from faunal zone Wa-0 to faunal zone Wa-1 occurred in two steps, with a transitional Wa-R fauna preceding the CIE initial recovery step and Wa-1 fauna following the step.

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The establishment and evolution of the Asian monsoons and arid interior have been linked to uplift of the Tibetan Plateau, retreat of the inland proto-Paratethys Sea and global cooling during the Cenozoic. However, the respective role of these driving mechanisms remains poorly constrained. This is partly due to a lack of continental records covering the key Eocene epoch marked by the onset of Tibetan Plateau uplift, proto-Paratethys Sea incursions and long-term global cooling. In this study, we reconstruct paleoenvironments in the Xining Basin, NE Tibet, to show a long-term drying of the Asian continental interior from the early Eocene to the Oligocene. Superimposed on this trend are three alternations between arid mudflat and wetter saline lake intervals, which are interpreted to reflect atmospheric moisture fluctuations in the basin. We date these fluctuations using magnetostratigraphy and the radiometric age of an intercalated tuff layer. The first saline lake interval is tentatively constrained to the late Paleocene–early Eocene. The other two are firmly dated between ∼46 Ma (top magnetochron C21n) and ∼41 Ma (base C18r) and between ∼40 Ma (base C18n) and ∼37 Ma (top C17n). Remarkably, these phases correlate in time with highstands of the proto-Paratethys Sea. This strongly suggests that these sea incursions enhanced westerly moisture supply as far inland as the Xining Basin. We conclude that the proto-Paratethys Sea constituted a key driver of Asian climate and should be considered in model and proxy interpretations.@en