Zhisheng An

Millennial- and orbital-scale changes in the East Asian monsoon over the past 224,000 years

High-resolution speleothem records from China have provided insights into the factors that control the strength of the East Asian monsoon. Our understanding of these factors remains incomplete, however, owing to gaps in the record of monsoon history over the past two interglacial–glacial cycles. In particular, missing sections have hampered our ability to test ideas about orbital-scale controls on the monsoon, the causes of millennial-scale events and relationships between changes in the monsoon and climate in other regions. Here we present an absolute-dated oxygen isotope record from Sanbao cave, central China, that completes a Chinese-cave-based record of the strength of the East Asian monsoon that covers the past 224,000 years. The record is dominated by 23,000-year-long cycles that are synchronous within dating errors with summer insolation at 65° N (ref. 10), supporting the idea that tropical/subtropical monsoons respond dominantly and directly to changes in Northern Hemisphere summer insolation on orbital timescales. The cycles are punctuated by millennial-scale strong-summer-monsoon events (Chinese interstadials), and the new record allows us to identify the complete series of these events over the past two interglacial–glacial cycles. Their duration decreases and their frequency increases during glacial build-up in both the last and penultimate glacial periods, indicating that ice sheet size affects their character and pacing. The ages of the events are exceptionally well constrained and may thus serve as benchmarks for correlating and calibrating climate records.

Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of central China during the last 130,000 years

The magnetic susceptibility of loess and paleosols in central China represents a proxy climate index closely related to past changes of precipitation and vegetation, and thus to summer monsoon intensity. Time series of magnetic susceptibility constructed for three loess-paleosol sequences in the southern part of the Chinese Loess Plateau document the history of summer monsoon variation during the last 130,000 yr. They correlate closely with the oxygen isotope record of stages 1 to 5 in deep-sea sediments. Soils were forming during intervals of strong summer monsoon, whereas loess units were deposited at times of reduced monsoon intensity. The Chinese loess-paleosol sequence can thus be viewed as a proxy record of Asian monsoon variability extending over the last 2.5 myr.

Loess stratigraphy in Central China

Abstract Loess sequences in central China provide a detailed record of climate changes during the last 2.5 million years. At least 44 major shifts from glacial to interglacial conditions occurred during this time in China. Prolonged loess deposition lasting more than 40 millennia occurred eleven times with only four of these intervals in Brunhes Chron. The climate oscillations took place with increased frequency between 2.4 and 0.5 Ma compared to the last half million years. Major shifts toward cooler and drier climates occurred at about 2.4 Ma, 1.2 Ma, and 0.5 Ma, while less pronounced shifts are dated to 1.65 Ma, 0.8 Ma and 0.2 Ma. Three of the last four reversals of the earths magnetic field whose lithostratigraphic position has been detail, are recorded in a glacial deposit. The low field magnetic susceptibility of loess and soils which reflects the concentration of ultrafine magnetite is related to the degree of pedogenesis and serves as a proxy measure of climate. The accumulation rate of the magnetite was fairly constant throughout the last 1.2 million years and can serve as a measure of time. Susceptibility records from localities near Xifeng and Luochuan, located 160 km apart are compared and used to define informal lithostratigraphic units in the Loess Plateau of China.

Pliocene uplift of the northern Tibetan Plateau

Neogene redbeds passing upward into upward-coarsening conglomerate and debris-flow deposits at the foot of the Kunlun Mountains record the change in paleoslope related to uplift of the surface of the northern Tibetan Plateau. Detailed magnetostratigraphy of a 4.5 km section near Yecheng in the western Kunlun Mountains shows that the change from deposition on distal alluvial plains to proximal alluvial fans occurred during the Gilbert reversed chron (4.5–3.5 Ma). The change in depositional facies was accompanied by an increase in sedimentation rate from an average ∼0.15 mm/yr between the earliest Oligocene and the earliest Pliocene to 1.4 mm/yr in the Gauss normal chron (3.6–2.6 Ma). We interpret the change in depositional facies and increase in sedimentation rate as indicating that the main uplift of the northwestern Tibetan Plateau began ca. 4.5 Ma.

Grain-size distribution function of polymodal sediments in hydraulic and aeolian environments, and numerical partitioning of the sedimentary components

Most continental sediments are polymodal, composed of overlapping components of which the grain size generally obeys some type of natural distribution. The grain-size components and their function types can be determined from frequency and cumulative curve plots in order to define the function formula of the grain-size distribution. The function parameters can be estimated by fitting a defined function formula to the measured grain-size data of the sample, which simultaneously achieves numerical partitioning of the sedimentary components. Genetic analysis of grain-size components of hydraulic and aeolian sediments demonstrates the following environmental implications: Fluvial sediment is composed of isolated saltation and suspension components. The sediments in closed lake basins are dominated by a suspension silt-clay component with a small proportion of saltation sand. The fine sand component makes up the majority of desert sand, overlapping with a small proportion of fine dust. Aeolian loess is composed of two overlapping components: a short suspension-time silt component and a long suspension-time fine component. Aeolian material in the North Pacific deep-sea sediments is dominated by long suspension-time fine dust. The fine component in aeolian sediments shows a consistent grain-size distribution and genetic connection from the desert sand, loess of northern China to the North Pacific Ocean, which is mainly transported by westerly winds and is dispersed in the atmosphere, forming a background dust

Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka

Two atmospheric circulation systems, the mid-latitude Westerlies and the Asian summer monsoon (ASM), play key roles in northern-hemisphere climatic changes. However, the variability of the Westerlies in Asia and their relationship to the ASM remain unclear. Here, we present the longest and highest-resolution drill core from Lake Qinghai on the northeastern Tibetan Plateau (TP), which uniquely records the variability of both the Westerlies and the ASM since 32 ka, reflecting the interplay of these two systems. These records document the anti-phase relationship of the Westerlies and the ASM for both glacial-interglacial and glacial millennial timescales. During the last glaciation, the influence of the Westerlies dominated; prominent dust-rich intervals, correlated with Heinrich events, reflect intensified Westerlies linked to northern high-latitude climate. During the Holocene, the dominant ASM circulation, punctuated by weak events, indicates linkages of the ASM to orbital forcing, North Atlantic abrupt events, and perhaps solar activity changes.

Late quaternary dust flow on the chinese Loess Plateau

Abstract The dust flux of over 25 g/cm2/103yr was reached in the central part of the chinese Loess Plateau during the last glacial maximum. This is more than three times higher than during the last interglacial and early glacial time. The grain size of the pleniglacial dust reached a peak shortly after 20,000 years ago and testifies to the occurrence of strong winds and frequent dust storms.

Magnetostratigraphy and paleoclimatic interpretation of a continuous 7.2 Ma Late Cenozoic Eolian sediments from the Chinese Loess Plateau

An almost 300m thick eolian sequence of Late Cenozoic sediments, which includes 162.5m of Quaternary loess-paleosols and 126m of Late Tertiary eolian Red Clay from the central part of the Chinese Loess Plateau, was investigated to determine the magnetostratigraphy. The results show that eolian dust accumulation, and by inference the related East Asia paleomonsoon, had begun by 7.2Ma. As paleomonsoon are largely controlled by the Tibetan Plateau, this implies that the Plateau had reached some critical elevation by 7.2Ma. The section also documents a rapid increase in eolian dust accumulation in the Late Cenozoic at 3.2Ma that is probably due to the influence of global ice volume on the East Asian monsoon.

Persistent sulfate formation from London Fog to Chinese haze

Significance Exceedingly high levels of fine particulate matter (PM) occur frequently in China, but the mechanism of severe haze formation remains unclear. From atmospheric measurements in two Chinese megacities and laboratory experiments, we show that the oxidation of SO2 by NO2 occurs efficiently in aqueous media under two polluted conditions: first, during the formation of the 1952 London Fog via in-cloud oxidation; and second, on fine PM with NH3 neutralization during severe haze in China. We suggest that effective haze mitigation is achievable by intervening in the sulfate formation process with NH3 and NO2 emission control measures. Hence, our results explain the outstanding sulfur problem during the historic London Fog formation and elucidate the chemical mechanism of severe haze in China. Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atmospheric models consistently underpredict sulfate levels under diverse environmental conditions. From atmospheric measurements in two Chinese megacities and complementary laboratory experiments, we show that the aqueous oxidation of SO2 by NO2 is key to efficient sulfate formation but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH3 neutralization or under cloud conditions. Under polluted environments, this SO2 oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH3 and NO2 control measures. In addition to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate production mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.

Magnetic susceptibility record of Chinese Loess

Records of the low field magnetic susceptibility at three sites in the Chinese Loess Plateau, each with sediments covering the last 2·5 Ma, were averaged and combined into a single stacked file whose dating is independent of astronomic chronology. The stack reveals orbital frequencies, which are in general agreement with the astronomically tuned oxygen isotope deep sea chronology in the upper part of the Brunhes epoch, but disagree prior to 0·5 Ma ago.