[Show abstract][Hide abstract] ABSTRACT: Extensive lacustrine deposits in the eastern Tarim Basin provide records of climate change influenced by the westerly winds and the Asian monsoon. To characterize the evolution of climate change in this region, we analyze elemental concentrations of barium (Ba) from the Ls2 drill core of Lop Nor, a paleo-lakebed located in the eastern Tarim Basin. Biogenic Ba concentrations from this drill core display a large-amplitude oscillation that generally follows a pattern similar to that of Artemisia content and ostracod assemblages, suggesting that is may serve as an index for climate change experienced in the basin. Our results indicate that biogenic Ba is especially sensitive to precipitation. All climatic proxies served in this study vary significantly over late Miocene to early Pleistocene time period. Strong aridification of eastern Tarim in the late Miocene to the early Pliocene may be attributed to a latitudinal shift in the westerly winds, which would have resulted in more moisture transported to southern and eastern Tibet. The growth of the Himalaya and Tibetan Plateau may have acted as an orographic barrier that blocked moisture sourced in the south from the northern margins of the plateau. We link weaker aridification in the late Pliocene to an increased intensity of the Indian Monsoon.
Chinese Science Bulletin 10/2014; 59(28). · 1.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Rapid industrialization and urbanization in developing countries has led to an increase in air pollution, along a similar trajectory to that previously experienced by the developed nations. In China, particulate pollution is a serious environmental problem that is influencing air quality, regional and global climates, and human health. In response to the extremely severe and persistent haze pollution experienced by about 800 million people during the first quarter of 2013 (refs 4, 5), the Chinese State Council announced its aim to reduce concentrations of PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 micrometres) by up to 25 per cent relative to 2012 levels by 2017 (ref. 6). Such efforts however require elucidation of the factors governing the abundance and composition of PM2.5, which remain poorly constrained in China. Here we combine a comprehensive set of novel and state-of-the-art offline analytical approaches and statistical techniques to investigate the chemical nature and sources of particulate matter at urban locations in Beijing, Shanghai, Guangzhou and Xi'an during January 2013. We find that the severe haze pollution event was driven to a large extent by secondary aerosol formation, which contributed 30-77 per cent and 44-71 per cent (average for all four cities) of PM2.5 and of organic aerosol, respectively. On average, the contribution of secondary organic aerosol (SOA) and secondary inorganic aerosol (SIA) are found to be of similar importance (SOA/SIA ratios range from 0.6 to 1.4). Our results suggest that, in addition to mitigating primary particulate emissions, reducing the emissions of secondary aerosol precursors from, for example, fossil fuel combustion and biomass burning is likely to be important for controlling China's PM2.5 levels and for reducing the environmental, economic and health impacts resulting from particulate pollution.
[Show abstract][Hide abstract] ABSTRACT: Precipitation variation on the Loess Plateau (LP) of China is not only important for rain-fed agriculture in this environmentally sensitive region, but also critical for the water and life securities over the whole Yellow River basin. Here we reconstruct high resolution precipitation variation on the western LP during the past 370 years by using two replicated, annually-laminated stalagmites. Spatial analysis suggests that the reconstruction can be also representative for the whole LP region. The precipitation variations show a significant quasi-50 year periodicity during the last 370 years, and have an important role in determining the runoff of the middle Yellow River. The main factor controlling the decadal scale variations and long-term trend in precipitation over this region is southerly water vapour transport associated with the Asian summer monsoon. The Pacific Decadal Oscillation is also an important influence on precipitation variation in this region, as it can affect the East Asian summer monsoon and the West Pacific Subtropical High.
[Show abstract][Hide abstract] ABSTRACT: Proxy records of summer monsoon moisture at Lake Qinghai on the northeastern Tibetan Plateau reveal a late Quaternary climate history that is subtly different from that of speleothems from southern and eastern China. Total organic carbon and authigenic carbonate in two independently analyzed and dated cores indicate (1) relative stability and aridity during the glacial interval, (2) small variations during the Bølling–Allerød and the Younger Dryas intervals, (3) comparatively abrupt change at the late Pleistocene/Holocene transition, and (4) relatively high variability during a wet early Holocene. Taken together, the data suggest that a climate threshold exists for penetration of Asian monsoon rainfall onto the Tibetan Plateau, a threshold that was crossed at the beginning of the Holocene. Conceptually, the threshold simply may be related to the topographic barrier that the eastern margin of the Tibetan Plateau presents to the landward penetration of the monsoon, or it may be related to non-linearities in the climate system itself, such as sudden shifts in the configuration of the Westerly jet stream. Different mechanisms for producing a threshold are not mutually exclusive and may have combined to affect the dynamics of the Asian monsoon. In any case, the threshold is related to the presence of the Tibetan Plateau, which has a profound influence on the Asia monsoon system.
Geophysical Research Letters 07/2014; · 3.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper provides a comprehensive review of the Global Monsoon (GM) that encompasses findings from both modern and paleo monsoon studies. A definition for the GM is introduced that incorporates its three dimensional distribution and ultimate causes, emphasizing on the direct drive of seasonal pressure system changes on monsoon circulation, and depicting the monsoon intensity with both circulation and precipitation. The interface between time and space in monsoon formation and climate change are explored across a wide range of timescales, including: tectonic, orbital, suborbital, centennial, interdecadal, interannual, intraseasonal and diurnal. Common features of the GM are shown to be global homogeneity, regional diversity, seasonality, quasi-periodicity, irregularity, instability and asynchroneity. The physical nature of monsoon dynamics is elucidated, with emphasis on the influence exerted on monsoon variability by solar insolation, earth orbital parameters, underlying surface properties and land-air-sea interactions. The primary driving force of monsoon variability on each timescale and the relationship of multiscale dynamics, as well as the concurrent challenges in these issues, have been discussed. The modern monsoon is considered in terms of natural processes and anthropogenic impacts, and prospects for using our current knowledge of the monsoon as a means of understanding future behavior are discussed.
Annual Review of Earth and Planetary Sciences 05/2014; 42(1). · 8.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We analyzed variations in the Sr/Ca, Ba/Ca, REE/Ca (REE: rare earth element), Zn/Ca, and Pb/Ca ratios preserved in an annually layered stalagmite, XL21, from central China. The stalagmite record spans the 95 year period AD 1914–2008. The Sr/Ca and Ba/Ca ratios have a significant positive correlation with the stalagmite’s growth rate, suggesting they were primarily controlled by growth-rate variations. Variations in REE/Ca ratios are consistent with local temperature changes, suggesting temperature influenced REE concentrations in the stalagmite over decadal to annual timescales. Higher temperature in this humid area can increase vegetation cover, microbial activity, and organic decomposition in the soil, resulting in enhanced pCO2, organic matter concentration and reduced pH, and consequently increased REE mobilization from the overlying soil layer and host rock. Higher temperatures may also increase the natural Zn mobilization from the overlying soil mediated by organic matter and consequently may have led to increased Zn retention in XL21. An increasing trend is seen in the Pb/Ca ratios from XL21 since 1985, which is consistent with increased lead production in this area, and indicates an increase in mine-derived lead pollution in the local environment over the past 30 years.
Quaternary Research 03/2014; 81:181-188. · 2.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The atmospheric nitrogen cycle is a key process driving the earth's environmental evolution. Current model studies require knowledge of NOx soil emissions from various land types, but desert emissions remain unquantified or are not addressed with high confidence. Our measurements at two observatories in Taklimakan desert during a dust episode showed an approximately stable and dust-independent nitrate in the air. Its concentration estimated from PM2.5, PM10 and TSP samples under non-dust, floating dust and dust storm conditions was 3.81 ± 1.24 μg m−3, 2.95 ± 0.69 μg m−3, 4.99 ± 1.71 μg m−3, respectively, despite the more-than-one-order difference of dust loading. This concentration was much larger than that in remote marine and tropical forest air. Comprehensive investigation revealed a similar presence of nitrate in other desert air. The nitrate was hypothesized to be the consequence of the conversion of NOx released from desert soils. These results indicate a background-like nitrate and active reactions of nitrogen compounds in desert air.
[Show abstract][Hide abstract] ABSTRACT: Aragonite, a mineralogical constituent of speleothems in cave environments, is unstable and susceptible to inversion to calcite, a diagenetic process that involves changes in the mineralogy, texture and geochemistry of speleothems. However, the exact alterations of stable isotope compositions during such diagenesis have not been fully investigated. In this study, two aragonite stalagmites (SN3 and SN15) from Shennong Cave, southeast China, were found partially inverted to calcite, as determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses, and thin-section inspections under microscope. The fibre relics and textural ghosts of aragonite preserved in coarse and equant mosaics calcite crystals clearly indicate that the calcite in these two stalagmites were inverted from aragonite. The stable isotope compositions (δ13C and δ18O, given in per mil versus VPDB standard) of primary aragonite and secondary calcite were analysed and compared, along both growth layers and growth axes. The results show that, along growth layers, differences of δ13C values between aragonite and calcite are negligible (0.1‰–0.2‰), whereas differences of δ18O values between aragonite and calcite are significant (0.63‰–0.87‰). Comparisons along growth axes show similar results: i.e., differences of δ13C values are negligible (0.06‰ ± 0.22‰) whereas differences of δ18O values are significant (0.85‰ ± 0.29‰). Most likely, the aragonite in SN3 and SN15 were internally inverted by interactions of trace calcite crystallites and pore water within intercrystalline pore spaces, by a dissolution–reprecipitation process occurring in trapped pore water. In the case of the inversion of aragonite to calcite in speleothems, such as that observed in SN3 and SN15, the δ13C values could be used in paleoclimate and paleoenvironment reconstructions because they are inherited from those of primary aragonite. Although the δ18O values might be cross-calibrated to those of primary aragonite if the aragonite–calcite fractionation offset is known (e.g., 0.85‰ ± 0.29‰ in this study), however, the δ18O values of secondary calcite should be used with caution in such reconstructions as the δ18O offset value is not consistently invariable.
[Show abstract][Hide abstract] ABSTRACT: This book is the first of its kind on environmental change research devoted to monsoon-arid environment evolution history and its mechanism involved. Capturing the most prominent features of Asian climate and environmental changes, it gives a comprehensive review of the Asian Monsoon records providing evidence for spatial and temporal climatic and environmental changes across the Asian continent since the Late Cenozoic. The dynamics underlying these changes are explored based on various bio-geological records and in particular based on the evidence of loess, speleothems as well as on mammal fossils. The Asian monsoon-arid climate system which quantifies the controlling mechanisms of climate change and the way it operates in different time scales is described. Attempts to differentiate between natural change and human-induced effects, which will help guide policies and countermeasures designed to support sustainable development on the Chinese Loess Plateau and the arid west.
[Show abstract][Hide abstract] ABSTRACT: Growth anomaly of trees in some regions was detected under current episode of rapid warming. This raises a dilemma for temperature reconstructions by using tree-ring width which is believed to be the most important proxy on inter-annual temperature reconstruction during the past millenniums. Here we employed the tree-ring δ13C to reconstruct temperature variations for exploring their potential on capturing signals of rapid warming, and to test how its difference with the tree-ring width based reconstruction. In this study the mean May–July temperature (TM–J) was reconstructed over the past century by tree-ring δ13C of Chinese pine trees growing in the Nanwutai region. The explained variance of the reconstruction was 43.3% (42.1% after adjusting the degrees of freedom). Compared to a ring-width temperature reconstruction (May–July) from the same site, the tree-ring δ13C-based temperature reconstruction offered two distinct advantages: 1) it captured a wider range of temperature variability, i.e., at least May–July, even over a longer part of the year, January–September; and 2) the reconstruction preserved more low-frequency climate information than that of ring width did.
The 20th century warming was well represented in the Nanwutai tree-ring δ13C temperature reconstruction, which implied that stable carbon isotope of tree rings potentially represents temperature variations during historical episodes of rapid warming. A spatial correlation analysis showed that our temperature reconstruction represented climate variations over the entire Loess Plateau in north-central China. Significant positive correlations (p < 0.1) were found between the temperature reconstruction and ENSO, as well as SSTs in the Pacific and Indian Oceans. The reconstruction showed the periodicities of 22.78-, 4.16-, 3.45–3.97- and 2.04–2.83-year quasi-cycles at a 95% confidence level. Our results suggested that temperature variability in the Nanwutai region may be linked to Pacific and Indian Ocean SST variations and solar activity.
[Show abstract][Hide abstract] ABSTRACT: Loess deposits in Central Asia provide an important record of regional climate and environmental change. However, in contrast to the intensively investigated loess deposits on the Chinese Loess Plateau (CLP), loess sediments in the Ili Basin of eastern Central Asia are poorly understood. Based on field investigation and existing literature, this paper presents a preliminary study of the distribution, strata and composition of the Ili loess. The distribution of the Ili loess is clearly controlled by topographic and geomorphic conditions, mainly in the river terraces, low uplands, and slopes of the Tianshan Mountains. The thickness of the loess varies from several meters to over two hundred meters. To characterize the Ili loess composition, the authors analyzed its grain size, geochemistry, X-ray diffraction pattern and heavy mineral assemblage. Grain size analyses reveal that the Ili loess consists predominantly of silt (4–63 μm) with a minor proportion of sand, which is coarser than the loess of the CLP and suggests a nearby provenance. The bulk mineral components of the Ili loess are dominated by quartz and feldspar with minor amounts of calcite, chlorite, mica, dolomite and hornblende. More than 20 types of heavy minerals were observed with major components of amphibole, magnetite and epidote. The major elements of the Ili loess are characterized by high abundance of SiO2, Al2O3 and CaO and minor amounts of Fe2O3, MgO, Na2O and K2O. Compared to the CLP loess, the Ili loess is relatively rich in Na2O and displays higher Na2O/Al2O3 ratios, suggesting weak weathering. The distribution patterns of REEs indicate that both the Ili loess and CLP loess are of typical upper crustal composition, with enrichment in LREE, negative Eu anomalies, and depletion of HREE. Both the heavy mineral assemblage and geochemistry indicate that the local bedrock of the Ili Basin may have contributed only a little to the loess sediments.
Quaternary International 01/2014; · 1.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A high-resolution rock magnetic investigation has been carried out on the Chaona Quaternary loess/paleosol sequence in Central Chinese Loess Plateau (CLP). Statistical analyses of magnetic parameters show that the loess layers are characterized by relatively higher coercivity (Bc), remanent coercivity (Bcr), and lower magnetic susceptibility (X), whereas the paleosol layers are characterized by relatively higher Xlf, saturation magnetization (Ms), and remanent saturation magnetization (Mr). We suggest that Mr, Ms in addition to Xlf can be regarded as summer monsoon intensity proxies, whereas Bc and Bcr can be regarded as winter monsoon intensity proxies, although there are some differences between Bc (Bcr) and traditional coarse grain size winter monsoon proxies. On the basis of a newly developed independent time scale (not orbitally-tuned), we reconstruct the history of the East Asia summer and winter Monsoons inferred from magnetic records during the last 3Ma. Spectrum analysis of our records, unlike most previous studies, indicates that there are various periods in addition to orbital periodicities, but no significant periodic signals observed during the interval 880-530ka. The results suggest that the forcing mechanism for the Pleistocene glacial cycles may be more complicated than the Milankovitch theory predicted, or global ice volume change. Wavelet power spectrum analyses indicate that the appearance of the 100ka period occurred at the middle of L15 (about 1260ka), not at the beginning of L15. The time interval 880~530ka indicates a nonlinear response of the global climate system to solar insolation. After 530ka, the ~100 ka period recorded in most magnetic parameters becomes strong and stable. If the 100 ka period appearance is the onset of the Middle Pleistocene Transition (MPT), then this climate transition in the loess paleosol sequence occurred at L15 (~1260ka) and ended at the beginning of S5-1 (~530ka), which differs with the duration obtained by previous investigations based on orbitally-tuned timescales.
[Show abstract][Hide abstract] ABSTRACT: There is an acid deposition conundrum in China: contrary to conventional wisdom, extremely high ambient sulfate concentrations in northeastern China are not always accompanied by correspondingly high acidities. To investigate this discrepancy, data from two independent sets of in situ field measurements were analyzed along with Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) satellite observations and Model for Ozone and Related chemical Tracers (MOZART) chemical transport model calculations. The field measurements included soluble aerosol ion concentrations and pH and particulate data from 11 cities, as well as pH measurement data from 74 sites in China. This study explores the basis for and the impacts of the large discrepancy in northeastern China between the major acidity precursors (SO2 and NOx) and measured acidity levels as indicated by pH values. There are extremely high SO2 emissions and ambient concentrations in northeastern China, while the corresponding acidity is unusually low (high pH) in this region. This is inconsistent with the usual situation where high‐acidity precursor pollutants result in low pH (high acidity) values and acid rain conditions. In other regions, such as southern China and the United States, high SO2 concentrations are typically well correlated with high acidities. Using measured soluble particle measurements (including both positively and negatively charged ions), it is seen that there are high values of alkaline ions in northeastern China that play an important role in neutralizing acidity in this region. This result strongly suggests that the high alkaline concentrations, especially Ca2+, increase warm season pH values by about 0.5 in northern China, partially explaining the inconsistency between sulfate concentrations and acidity. This has a very important implication for acid rain mitigation—especially in northeastern China. However, there are additional issues pertaining to the precursor‐acidity relationship that need further investigation. Why is it that the reduction in acidity due to the alkaline ions is only significant in summer? During winter, the measured alkaline ions play a much smaller role in explaining the discrepancy. The measured alkaline ions in this study were mostly obtained from particles in the PM2.5 range. However, the size of calcium particles is typically much larger—extending well beyond 2.5 µm—and so a significant amount of calcium may be underestimated by PM2.5 measurements alone. The under‐sampling of calcium particles is further exacerbated in that the sampling protocol excluded particle (and soluble ion and pH) measurements during dust storms. This all leads to the need for an improved understanding of pollutant‐ion‐particulate interactions in China, and their role in explaining the counter‐intuitive conclusion that dust mitigation strategies in China could have the unintended consequence of exacerbating acid rain conditions.
Journal of Geophysical Research: Atmospheres. 05/2013; 118(10).