Spatiotemporal characteristics and anthropogenic signals of aerosol optical thickness (AOT) distributions over Korea are investigated in this study using AOT time-series data from the Moderate Resolution Imaging Spectroradiometer (MODIS). AOT observations, a quantitative measure of the atmospheric quality, had significant geographical variations during the study period (2000–2010). Comparing metro cities or counties with similar populations, western regions showed higher AOT values than eastern regions. Particular matter with a diameter < 10 µm (PM10), including aeolian dust or yellow dust, is the primary component of atmospheric aerosols, and their transport into the region has shown a strong seasonal pattern with its peak from March to April and lows from July to September. These seasonal dust patterns, however, did not correspond well to temporal AOT records, which typically reached the maximum level in June nationwide. Rather, widespread regional fire events and humidity showed significant correlations with AOT time-series. This correlation rapidly increased as the range of fire occurrence was extended to the west as far as 115°E. The relative humidity also had a significant correlation with AOT during the month of June. In addition to urban emission of anthropogenic aerosols, regional biomass burning and secondary growth of hygroscopic aerosols are considered important contributors to the degradation of the atmospheric environment during the non-Asian Dust season over the Korean Peninsula. 相似文献
The regional climate correlation within the Northern Hemisphere in the cold/dry mid-Younger Dryas event (YD) remains elusive. A key to unraveling this issue is sufficient knowledge of the detailed climate variability at the low latitudes. Here we present a high-resolution (3-yr) δ18O record of an annually laminated stalagmite from central China that reveals a detailed Asian monsoon (AM) history from 13.36 to 10.99 ka. The YD in this record is expressed as three phases, characterized by gradual onsets but rapid ends. During the mid-YD, the AM variability exhibited an increasing trend superimposed by three centennial oscillations, well-correlated to changes in Greenland temperatures. These warming/wetting fluctuations show a periodicity of ~ 200 yr, generally in agreement with centennial changes in cosmogenic nuclides indicated by the 10Be flux from the Greenland ice. This relationship implies that centennial-scale climate changes during the mid-YD are probably caused by solar output and rapidly transported over broad regions through atmosphere reorganization. 相似文献
Voluminous granitic intrusions are distributed in the West Junggar, NW China, and they can be classified as the dioritic rocks, charnockite and alkali-feldspar granite groups. The dioritic rocks (SiO2 = 50.4–63.8 wt.%) are calc-alkaline and Mg enriched (average MgO = 4.54 wt.%, Mg# = 0.39–0.64), with high Sr/Y ratios (average = 21.2), weak negative Eu (average Eu/Eu = 0.80) and pronounced negative Nb–Ta anomalies. Their Sr–Nd and zircon Hf isotopic compositions ((87Sr/86Sr)i = 0.7035–0.7042, εNd(t) = 4.5–7.9, εHf(t) = 14.1–14.5) show a depleted mantle-like signature. These features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials. The charnockites (SiO2 = 60.0–65.3 wt.%) show transitional geochemical characteristics from calc-alkaline to alkaline, with weak negative Eu (average Eu/Eu = 0.75) but pronounced negative Nb–Ta anomalies. Sr–Nd and zircon Hf isotopic compositions ((87Sr/86Sr)i = 0.7037–0.7039, εNd(t) = 5.2–8.0, εHf(t) = 13.9–14.7) also indicate a depleted source, suggesting melts from a hot, juvenile lower crust. Alkali-feldspar granites (SiO2 = 70.0–78.4 wt.%) are alkali and Fe-enriched, and have distinct negative Eu and Nb–Ta anomalies (average Eu/Eu = 0.26), low Sr/Y ratios (average = 2.11), and depleted Sr–Nd and zircon Hf isotopic compositions ((87Sr/86Sr)i = 0.7024–0.7045, εNd(t) = 5.1–8.9, εHf(t) = 13.7–14.2). These characteristics are also comparable with those of rocks derived from juvenile lower crust. Despite of the differences in petrology, geochemistry and possibly different origins, zircon ages indicate that these three groups of rocks were coevally emplaced at ~ 305 Ma.A ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks. As the “slab window” opened, upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms: partial melting of the subducting slab formed the dioritic rocks; partial melting of the hot juvenile lower crust produced charnockite and alkali-feldspar granite, and partial melting in the mantle wedge generated mafic rocks in the region. These results suggest that subduction was ongoing in the Late Carboniferous and, thus support that the accretion and collision in the Central Asian Orogenic Belt took place in North Xinjiang after 305 Ma, and possibly in the Permian. 相似文献
With the analysis of the sources and formation mechanism of the clay minerals in the sediment core from the Dalianhai lake in the Gonghe Basin,northeastern Tibet-Qinghai Plateau,clay mineral composition proxies,grain-size and carbonate contents have been employed for high-resolution study in order to reconstruct East Asian Summer Monsoon (EASM) over the northeastern Tibet-Qinghai Plateau during the lastdeglacial.The study also extended to establish a relationship between vegetation cover changes and erosion during the last 14.5 ka with pollen record and clay mineral proxies.Smectite/kaolinite and smectite/(illite+chlorite) ratios allow us to assess hydrolysis conditions in lowlands and/or physical erosion process in highlands of the Gonghe Basin.Before 12.9 Cal ka BP,both mineralogical ratios show low values indicative of strong physical erosion in the basin with a dominant cold and dry phase.After 12.9 Cal ka BP,an increase in both mineralogical ratios indicates enhanced chemical weathering in the basin associated with a warm and humid climate.The beginning of Holocene is characterized by high smectite/(illite+chlorite) and smectite/kaolinite ratios that is synchronous as with deposition of many peat laminae,implying the best warm and humid conditions specifically between 8.0 to 5.5 Cal ka BP.The time interval after 5.0 Cai ka BP is characterized by a return to high physical erosion and low chemical weathering with dry climate conditions in the basin.Comparing variations of clay mineral assemblages with previous pollen results,we observe a rapid response in terms of chemical weathering and physical erosion intensity to a modification of the vegetation cover in the basin. 相似文献
NE China is the easternmost part of the Central Asian Orogenic Belt (CAOB). The area is distinguished by widespread occurrence of Phanerozoic granitic rocks. In the companion paper (Part I), we established the Jurassic ages (184–137 Ma) for three granitic plutons: Xinhuatun, Lamashan and Yiershi. We also used geochemical data to argue that these rocks are highly fractionated I-type granites. In this paper, we present Sr–Nd–O isotope data of the three plutons and 32 additional samples to delineate the nature of their source, to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the Phanerozoic.
Despite their difference in emplacement age, Sr–Nd isotopic analyses reveal that these Jurassic granites have common isotopic characteristics. They all have low initial 87Sr/86Sr ratios (0.7045±0.0015), positive Nd(T) values (+1.3 to +2.8), and young Sm–Nd model ages (720–840 Ma). These characteristics are indicative of juvenile nature for these granites. Other Late Paleozoic to Mesozoic granites in this region also show the same features. Sr–Nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two-stage processes: (1) formation of parental magmas by melting of a relatively juvenile crust, which is probably a mixed lithology formed by pre-existing lower crust intruded or underplated by mantle-derived basaltic magma, and (2) extensive magmatic differentiation of the parental magmas in a slow cooling environment.
The widespread distribution of juvenile granitoids in NE China indicates a massive transfer of mantle material to the crust in a post-orogenic tectonic setting. Several recent studies have documented that juvenile granitoids of Paleozoic to Mesozoic ages are ubiquitous in the Central Asian Orogenic Belt, hence suggesting a significant growth of the continental crust in the Phanerozoic. 相似文献