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Xu H.,CAS Institute of Earth Environment | Hong Y.,Chinese Academy of science | Hong B.,Chinese Academy of science
Climate Dynamics | Year: 2012

The trend of the Indian summer monsoon (ISM) intensity and its nature during the past 100 and 200 years still remain unclear. In this study we reconstructed the ISM intensity during the past 270 years from tree ring δ18O at Hongyuan, eastern edge of the Tibet Plateau. The monsoon failures inferred from δ18Otree ring correlate well with those recorded in ice cores, speleothem, and historical literature sources. 22.6, 59.0, and 110.9-years frequency components in the Hongyuan δ18Otree ring series, which may be the responses to solar activities, synchronize well with those recorded in other ISM indices. A notable feature of the reconstructed ISM intensity is the gradually decreasing trend from about 1860 to the present, which is inversely related to the increasing temperature trend contemporaneously. Such "decreasing ISM intensity-increasing temperature" tendency can also be supported by ice core records and meteorological records over a wide geographic extension. The decrease in sea surface temperature gradient between tropical and north Indian Ocean, and the decrease in land-sea thermal contrast between tropical Indian Ocean and "Indian sub-continent-western Himalaya" are possibly responsible for the observed decreasing ISM trend. © 2012 Springer-Verlag. Source

Xiao J.,CAS Institute of Earth Environment
Environmental Earth Sciences | Year: 2016

Major ions, pH and electric conductivity (EC) in rainwater samples collected from June 2011 to July 2012 were analyzed to evaluate chemical characteristics and source apportionment of rainwater in Xi’an, a city located on the loess deposits. The rainwater was slightly alkaline with a daily volume-weighted mean (VWM) pH of 7.7, which ranged from 7.4 to 8.4. The VWM concentrations of major ions followed the order of Ca2+ > SO4 2− > NH4 + > NO3 − > Cl− > Na+ > K+ > Mg2+ > F−. Ca2+ and SO4 2− dominated in cations and anions, respectively. Investigations of neutralization factors indicated that the acidity of rainwater in Xi’an was neutralized by NH4 + and Ca2+, which presented relatively higher neutralization capability than cities in southern China. The back trajectory analysis showed that the chemistry of rainwater in Xi’an is influenced by local pollutions sources and soil dust in local and remote area. Large inputs of soil dust and coal combustion during the winter and spring led to higher values of major ions and pH values during dry seasons than wet seasons. The principal factor analysis, correlation analysis, and source apportionment indicated that SO4 2− and NO3 − were from anthropogenic sources, while Ca2+, K+, and Mg2+ were from crust fractions. The estimated annual wet depositions for sulfur, nitrogen, and calcium were 3.9, 1.5, and 4.2 t/km2, respectively. Although rainwater acidity in Xi’an was mainly caused by H2SO4, the contribution of HNO3 will be enhanced due to increasing NOx and relatively stable SO2 emissions in the future. The high concentrations of EC, SO4 2−, and NO3 − in rainwater indicated Xi’an is severely polluted. To improve air quality in Xi’an, simultaneous reduction of particle emissions and fossil fuel and vehicles emissions are worthy of advocating. The results have implications for identification of potential sources in rainwater and for evaluation of air quality improvement in Xi’an and other similar cities in China. © 2015, Springer-Verlag Berlin Heidelberg. Source

Yin J.,China Earthquake Administration | Yin J.,CAS Institute of Earth Environment | Yang X.,China Earthquake Administration | Zheng Y.,China Earthquake Administration
Scientific Reports | Year: 2014

Several attempts have been made to directly date phytoliths, but most 14 C results are not consistent with other independent chronologies. Due to the limited dataset, there is not a clear explanation for these discrepancies. Herein, we report the 14 C ages of phytolith-occluded carbon (PhytOC) from contemporary rice and millet crops that were combusted at different temperatures to investigate the relationship between the combustion temperature and resulting 14 C age. Our results show that the 14 C age of PhytOC increases directly with combustion temperature (up to 1100°C) and results in age overestimations of hundreds of years. Considerably older ages are observed at higher temperatures, suggesting that it may be possible to distinguish between two fractions of organic carbon in phytoliths: labile and recalcitrant carbon. These findings challenge the assumption that PhytOC is homogeneous, an assumption made by those who have previously attempted to directly date phytoliths using 14 C. Source

Sun Y.,CAS Institute of Earth Environment | Clemens S.C.,Brown University | Morrill C.,University of Colorado at Boulder | Lin X.,Ocean University of China | And 2 more authors.
Nature Geoscience | Year: 2012

The last glacial period was characterized by abrupt, millennial-scale climate change. These climate fluctuations are particularly pronounced in records of the East Asian monsoon system, and seem to be linked to changes in North Atlantic circulation. Here we present records of grain size variations from the northwestern Chinese Loess Plateau, dated using optically stmulated luminescence. We reconstruct changes in the strength of the East Asian winter monsoon over the past 60,000 years and find reconstructed millennial-scale variations that are broadly correlated with temperature variations over Greenland, suggesting a common forcing. We investigate the effect of a slow-down of Atlantic meridional overturning circulation on the monsoon system using a coupled climate model simulation with added freshwater flux into the northern North Atlantic, and find a strengthening winter monsoon circulation over the regions that supply dust to the Loess Plateau and a reduction in summer monsoon precipitation over East Asia. We conclude that Atlantic meridional overturning circulation is a driver of abrupt change in the East Asian winter and summer monsoon systems, and that the northern westerlies play a role in transmitting this signal from the North Atlantic to the Asian monsoon regions. Source

Sun Y.,CAS Institute of Earth Environment | Wang X.,CAS Institute of Earth Environment | Liu Q.,CAS Institute of Geology and Geophysics | Clemens S.C.,Brown University
Earth and Planetary Science Letters | Year: 2010

Rapid East Asian Monsoon oscillations recorded by Chinese loess are thought to be dynamically linked to north Atlantic climate. However, few efforts have been made to assess the effects of post-depositional processes (e.g., surface mixing and pedogenesis) on loess paleoclimatic records. Here a detailed optically stimulated luminescence dating of a thick loess sequence from the western Loess Plateau is presented, offering a reliable chronology for last glacial deposits. Magnetic susceptibility and mean grain size records from three loess-paleosol sequences along a northwest-southeast transect are investigated to evaluate impacts of post-depositional processes on these loess-based proxy records. Our results indicate that: (1) loess sequences developed within the flat tableland of the central and western Loess Plateau are nearly continuous during the last glaciation; and (2) post-depositional processes have distinct impacts on rapid monsoon signals recorded in loess sequences from different regions. In the central Loess Plateau, rapid monsoon signals have been attenuated to various degrees depending on the sedimentation rate and pedogenic intensity. In the northwestern Loess Plateau, however, due to high sedimentation rate and relatively weak pedogenesis, high-resolution grain size oscillations reliably record rapid monsoon changes and can be well correlated to rapid climate changes recorded in the Greenland ice core and Hulu cave stalagmite. © 2009 Elsevier B.V. All rights reserved. Source

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