Insights from CLP palaeoclimate records. The grain size of the Quaternary CLP loess-palaeosol sequences reflect dust transport by winter monsoon winds and is generally interpreted as a winter monsoon intensity indicator20,23,30. We measured the grain size distribution of the Luochuan (109°24′E, 35°48′N) and Chaona (107°12′E, 35°7′N) loess-palaeosol sections on the central CLP (Fig. 1) to investigate winter monsoon dust transport evolution over the last 1.6 Myr. Similar to the global chronology for benthic foraminiferal δ18O records from marine sediment cores31, the loess-palaeosol chronology has been established based on different sections/cores across the CLP using orbital tuning, land-sea correlation, and/or grain-size age models, which result in similar ages and the same correlations of loess and palaeosol layers to glacial and interglacial periods defined by the marine benthic δ18O record18,20,23,28,32. Chronological uncertainties do not result in major differences in loess-to-marine correlations across a glacial-interglacial cyclicity. Our CLP loess-palaeosol chronology was established by combined palaeomagnetic and pedostratigraphic constrains and correlations of Luochuan and Chaona median grain size and magnetic susceptibility (χ) records to the marine benthic foraminiferal δ18O record (see Methods and Supplementary Fig. 1 for details). Other CLP loess-palaeosol sections were also synchronized to this chronology (Figs. 2 and 3). Our new median grain size records from the Chaona and Luochuan sections have consistent glacial-interglacial variabilities that are also comparable with those from other sections across the CLP (Fig. 2). To reduce the effects of local changes, and to better reveal large-scale glacial-interglacial winter monsoon dust transport across the CLP, we compile a median grain size stack based on our new records from Chaona and Luochuan sections and existing records from the Lingtai18, Jingchuan18, and Baicaoyuan33 (Fig. 2; Methods).
Summer monsoon precipitation contributes 60–75% of annual precipitation on the CLP and its variations have major impacts on regional environmental conditions19. CLP loess-palaeosol χ is a much-used proxy for summer monsoon precipitation because stronger pedogenesis during periods of increased precipitation accelerates fine magnetite/maghemite and hematite formation that causes higher χ values20,21,34. To assess summer monsoon variability on the CLP coeval to our grain-size-based winter monsoon record, we compose our new loess-palaeosol χ stack by compiling existing χ records from Luochuan32, Chaona27, Jingchuan35, Zhaojiachuan23, Lantian36, and Lingtai23 (Fig. 3). Like median grain size, temporal χ variability matches well among sections and generally correlates cycle-to-cycle with glacial-interglacial cycles in the benthic foraminiferal δ18O record (Figs. 2–3). Nevertheless, the records contain subtle differences in their details, largely because they represent distinct climate variables (i.e., summer monsoon precipitation for χ, winter monsoon intensity for grain size, and deep-sea temperature and global ice volume for benthic foraminiferal δ18O).
Consistent with previous studies18,20,23,24,28, glacial loess layers have larger median grain sizes (stronger winter monsoons) and lower χ values (weaker summer monsoons and lower precipitation) than interglacial palaeosol layers (Figs. 2–4). Notably, loess layers L15 (correlating to MIS 38 at ~ 1.25 Ma) and L9 − 1 (correlating to MIS 22 at ~ 0.9 Ma) have notably lower χ values and exceptionally larger median grain sizes than other loess layers (Fig. 4a–b). The distinct grain size increases in L15 and L9 − 1 are observed in all loess-palaeosol sequences from both the eastern and western CLP (Fig. 2), which indicates dominant and widespread dust transport changes over a large areal extent during MIS 38 and MIS 22 across the entire CLP. We infer that winter monsoon conditions over Asia during these periods were amplified (i.e., cooler, drier, and windier) compared to preceding and succeeding glacials.
To investigate the main (orbital) periodicities of the Asian summer and winter monsoon, and to compare these to the global (i.e., high-latitude) climatic response, we present time-evolutive spectral analyses of the χ, grain size, and marine benthic foraminiferal δ18O stacks. Spectral analyses of the median grain size and χ stacks suggest a major transition from a predominant ~ 40-kyr to ~ 100-kyr periodicity across the MPT, albeit with subtle differences in the exact expression of the transition (Fig. 5a–b). These differences likely reflect more nuanced regional response differences to MPT climate change. CLP precipitation (indicated by χ) is dominated by moisture transport from the West Pacific and Indian Oceans to inland Asia by the summer monsoon, which is a low-latitude process, whereas transport of cold and dry air from high-latitude Eurasia toward the tropical oceans by the winter monsoon (indicated by median grain size) represents a high- to mid-latitude process. Spectral analyses of the benthic foraminiferal δ18O record31 reveal a prominent switch from predominant ~ 40-kyr to ~ 100-kyr cycles across the MPT, with combined ~ 40-kyr and ~ 100-kyr cycles between ~ 1.2 Ma and ~ 0.6 Ma (Fig. 5c). Weakened continuation of obliquity (~ 40-kyr) cyclicity until ~ 0.6 Ma suggests that the major MPT periodicity shift was more gradual and delayed in the global mean glacial cycle pattern reflected in the benthic foraminiferal δ18O relative to the CLP precipitation (χ) and winter monsoon (median grain size) records (Fig. 5).
Mechanism of distinct loess coarsening throughout the CLP across the MPT. Our new grain size records reveal broadly consistent orbital-scale variability and extreme pulses across the MPT as documented in previous records18,23−25. Prominent loess grain size anomalies at ~ 1.25 Ma (L15) and ~ 0.9 Ma (L9 − 1) have been explained previously in terms of phased Tibetan Plateau uplift24. However, evidence for major plateau uplift during the MPT is tenuous. The Tibetan Plateau was already close to its present-day elevation and configuration at least in the late Miocene, with only limited and more regional Quaternary adjustments37–42. Thus, plateau uplift cannot explain the distinct coarsening of L15 and L9 − 1 across the CLP, nor explain their astronomical pacing. This leaves the cause(s) of L15 and L9 − 1, and their palaeoclimatic significance, open to further investigation18.
In contrast to the Tibetan Plateau uplift interpretation24 and other CLP studies that focused on the MPT shift in orbital periodicities26–29, we assess the distinct loess coarsening of L15 and L9 − 1 across the CLP from first principles, and within a global context. Fundamentally, these exceptionally coarse loess layers must reflect a combination of (i) widespread wind strength increase, (ii) transport pathway shortening due to enhanced and expanded central Asian aridity, (iii) enhanced coarse dust production through increased aridity and sediment availability, and/or (iv) reduced vegetation cover with lower soil stability and greater soil erosion by wind during glacials MIS 38 and MIS 22, at the onset (~ 1.25 Ma) and halfway (~ 0.9 Ma) through the MPT. We find that these distinctly-amplified glacial Asian climate and environmental conditions coincided with Northern Hemisphere ice sheet expansion at the onset and middle of the MPT, when expression of ~ 100-kyr glacial cyclicity initiated and enhanced, respectively6, 8–10,13,43. Both marine and terrestrial data suggest that glacial Northern Hemisphere ice sheets expanded substantially at the beginning of, and halfway through, the MPT. For example, various sea level reconstructions suggest notable lowstands during MIS 38 and MIS 22 relative to preceding glacials, albeit with subtle amplitude differences among reconstructions that relate to variable uncertainties in different methods6,8−10 (Fig. 4c–f). In addition, 26Al-10Be burial dating of tills suggests that the Laurentide Ice Sheet advanced to its extreme southern limit (~ 40°N) at ~ 1.3 Ma43. The ODP Site 887 magnetic susceptibility and Deep Sea Drilling Project (DSDP) Site 607 carbonate concentration records suggest that Northern Hemisphere ice sheets expanded and shed more ice-rafted debris into the Gulf of Alaska at ~ 1.3 Ma and central North Atlantic Ocean at ~ 0.9 Ma, respectively44,45. We infer that the marked glacial climate intensification in Asia at ~ 1.25 Ma and ~ 0.9 Ma indicated by CLP loess coarsening may be linked to concomitant shifts to greater glacial Northern Hemisphere ice sheet expansion. Here, we subsequently use climate modelling simulations to assess whether and how Northern Hemisphere ice sheet expansion may have amplified the MPT Asian glacial conditions.
We used the Community Earth System Model (CESM 1.2) to simulate Asian climate responses to Northern Hemisphere ice sheet expansion from pre-MPT (1.6–1.3 Ma) to mid-MPT (~ 0.9 Ma) glacial maximum conditions, aiming to provide a better mechanistic understanding of the observed MPT extreme glacial events from the CLP grain size records in a global context. We note that the respective Northern Hemisphere ice sheet distributions remain poorly constrained across the MPT. To obtain reasonable configurations for the Northern Hemisphere ice sheet expansion across the MPT, our pre-MPT and mid-MPT experiments use the well-reconstructed Northern Hemisphere ice sheet distributions at 13 ka and Last Glacial Maximum (LGM, ~ 20 ka), respectively5 (Fig. 6a). We argue that these configurations are broadly realistic because sea levels6,8−10 and benthic foraminiferal δ18O values31 are comparable for these time slices. Except for ice volume difference, we keep other boundary conditions the same in our pre-MPT and mid-MPT experiments, including well-established LGM orography, vegetation, lakes, aerosol conditions, orbital parameters, and solar constant. To better represent the early Pleistocene greenhouse gas conditions under which the MPT occurred, both experiments include CO2 and CH4 concentrations that are fixed to their full glacial values at ~ 1.5 Ma (220 ppm CO2 and 450 ppb CH4), as reconstructed from Antarctic ice cores46. Note that our simulations are designed as sensitivity experiments to examine Asian climate responses to Northern Hemisphere ice sheet expansion and are not meant to reproduce exactly the full range of changing boundary conditions across the MPT.
Our simulations suggest that Northern Hemisphere glacial ice sheet expansion from the pre-MPT to mid-MPT experiments led to lowering of Asian mean annual temperature, precipitation, and net surface moisture (precipitation minus evaporation) (Fig. 6b–d), which facilitated intensification and expansion of central Asian aridity and increased dust production. Mean annual precipitation in the mid-MPT experiment decreased by ~ 14% in arid inland regions (60–100°E, 30–60°N) and by ~ 9% in East Asian monsoon regions (100–120°E, 20–40°N) relative to the pre-MPT experiment. Furthermore, ice sheet expansion strengthened Asian high-pressure cells and winter monsoon circulation (Fig. 6e–f), which enhanced winter monsoon dust transportability toward the CLP. Overall, annual dust fluxes emitted from arid regions north and east of the Tibetan Plateau increased by up to an order of magnitude from the pre-MPT to mid-MPT experiments (Fig. 6g). This was associated with a broadly doubled annual atmospheric dust loading over East Asian down-wind regions (Fig. 6h), which is comparable to the largely doubled median grain sizes of L15 and L9 − 1 relative to adjacent loess layers observed across the CLP (Figs. 2 and 4b). Dust changes are substantially larger than precipitation changes in the pre-MPT to mid-MPT experiments (Fig. 6), which is consistent with the significantly larger grain size changes compared to χ changes at ~ 1.25 Ma and ~ 0.9 Ma (Fig. 2a–b).
Our general circulation model does not include dynamic vegetation responses as vegetation is fixed. Hence, our dust inferences likely are minimum estimates because temperature and precipitation lowering across the MPT (with arid zone expansion) would have also decreased the vegetation cover36,47, which in turn would have reduced soil stabilization, facilitating erosion and dust production and availability. Other potential dust producing processes not included in the model, such as enhanced physical weathering and rock fracturing through intensified frost wedging and/or glacial grinding under colder MPT conditions, would also produce more dust material for ablation48. Regardless, our minimum estimates from model results strongly support the hypothesis that intensification and southeastward expansion of Asian aridity, increased coarse dust availability, and winter monsoon wind strengthening caused increased coarse dust transport and loess coarsening across the CLP in response to Northern Hemisphere ice sheet expansion across the MPT. Our model output is broadly consistent with previous post-MPT simulations of marked ice sheet impacts on the Asian climate49–53, although those simulations used different boundary conditions.
Consistent with the above scenario, new sandy deserts (e.g., Badain Jaran Desert, Tengger Desert) formed at ~ 1.2–0.9 Ma to the north of the CLP16,54, while existing sandy deserts (e.g., Mu Us Desert) expanded southward at ~ 1.25 Ma17 (Fig. 4i). Sandy desert environments first appeared in the Hobq Desert at ~ 1.3–1.2 Ma, replacing preceding fluvio-lacustrine environments55. The Tarim and Qaidam Basins also aridified from ~ 1.25 Ma onward48,56,57 (Fig. 4i). Loess coarsening and dust flux increases on the West Kunlun Shan are consistent with expanded central Asian arid regions across the MPT58. These Asian arid regions, especially the neighbouring Badain Jaran, Tengger, Mu Us, and Hobq Deserts (see Fig. 1a for locations), provided important coarse dust sources for the CLP16. Southeastward desert condition expansion to the west and north of the CLP together with synchronous winter monsoon wind strengthening could readily lead to loess coarsening on the CLP. The amplified expression of grain size pulses during MIS 38 and MIS 22 relative to their preceding glacials suggests nonlinear responses of dust accumulation on the CLP to Northern Hemisphere ice sheet expansion (Fig. 4). The CLP grain size responses appear to have been more vigorous at the onset and middle of the MPT when a hypothesized threshold of Asian aridification and winter monsoon intensity was passed for the first and second time, allowing increased coarser particles to be transported by stronger winds. For example, a change from fluvio-lacustrine to desert environments when the first (particularly) or second threshold was passed in different Asian interior arid regions, would offer more abundant dust material to be transported to the CLP than the later sustained sandy deserts without fluvial-lacustrine processes, because fluvial-lacustrine conditions generally produce abundant fine dust that can be readily transported atmospherically by the stronger winter monsoon once water bodies dried and sediments exposed59,60. We argue that the CLP loess coarsening at ~ 1.25 Ma and ~ 0.9 Ma was related to a combination of both winter monsoon intensity and the supply of newly erodible and deflatable material in source regions. In addition to the onset and middle of the MPT, these loess coarsening events also coincided broadly with 400-kyr eccentricity minimum nodes61 (Fig. 4h), which were associated with distinct cooling events in tropical sea surface temperature records62. Under such eccentricity node and cooling conditions, Northern Hemisphere ice sheet expansion could have more easily driven anomalous Asian glacial climate and environment changes at ~ 1.25 Ma and ~ 0.9 Ma. It appears that the studied events across the MPT are consistent with non-linear responses that broadly exist in astronomical climate dynamics63–65.
Synthesizing observations, land-sea correlations, and simulations, we propose that Northern Hemisphere ice sheet expansion drove large-scale amplification of Asian glacial conditions through hitherto unknown non-linear threshold-style responses of the Asian winter monsoon and aridification at the onset (~ 1.25 Ma) and halfway (~ 0.9 Ma) through the MPT, when expression of ~ 100-kyr glacial cyclicity initiated and enhanced, respectively. These greatly amplified regional glacial excursions were marked by a combination of intensified and expanded Asian aridity, winter monsoon strengthening, and summer monsoon weakening, with distinct coarsening of the L15 and L9 − 1 layers across the CLP. Our combined palaeoclimate and simulation results offer a new perspective on the exceptional coarsening of the L15 and L9 − 1 loess layers in a globally significant context. Our findings also portray a systematic manifestation of the MPT across Asia in association with high-latitude Northern Hemisphere ice sheet expansion, shedding light on extreme climate variability across the MPT. The MPT reflects not only the well-known shift from predominantly ~ 40-kyr to ~ 100-kyr orbital cycles, but also contains distinct anomalies in terrestrial climate and environmental conditions.