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عنوان انگلیسی مقاله:
The origin of bimodal grain-size distribution for aeolian deposits
سال انتشار : 2016
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مقدمه انگلیسی مقاله:
1. Introduction
Loess and other aeolian deposits often present bimodal grain size distribution (BGSD) with a coarse (the modal size is of >20 lm) and a fine component (the modal size is of <20 lm). Many researchers regarded that the coarse component (>20 lm) can only be transported through surface wind by saltation and/or shortrange suspension (Tsoar and Pye, 1987; Pye, 1987, 1995), and that the fine component (<20 lm) can be widely dispersed and longrange transported (Windom, 1975; Glaccum and Prospero, 1980; Tsoar and Pye, 1987; Pye, 1987, 1995). Generally, different peaks of grain size distribution were suggested to delegate different sedimentary process (Middleton, 1976; Ashley, 1978; Bagnold and Barndorff-Nielsen, 1980). So, it has been regarded that two sub-components of the aeolian deposits were from different sources (Pye and Zhou, 1989; Sun et al., 2004, 2008b; Muhs and Benedict, 2006; Vandenberghe et al., 2006; Lim and Matsumoto, 2006). Granularity is an excellent proxy for reconstructing paleoenvironment changes. The motion feature of detrital particles in the airflow has been described since 1930s (Bagnold, 1941; Gillette et al., 1974; Tsoar and Pye, 1987; Pye, 1995). Based on the motion laws of aeolian dust, some researchers have long noted that there existed certain relationship between granularity and East Asia winter monsoon. So, many researchers have tried to trace the material sources of the loess on Chinese Loess Plateau (CLP) since 1960s (Liu, 1965; Liu, 1966). And later, based the granularity analysis of high resolution loess profiles, some researchers regarded the granularity has completely and systematically recorded the evolutionary of the East Asia winter monsoon since 2.6 million (Ding et al., 1994), which made it as one of three best archives of paleo-environmental changes (the other two are ice core and deep-sea sediment core). In Northern China, two types of atmospheric circulations influence the regional climate during dust storm seasons, i.e., the East-Asia winter monsoon prevailing at low altitude and the westerlies prevailing at high altitude (Chen, 1991; Qiao and Zhang, 1994). Sun et al. (2002) and Sun et al. (2004) presented bimodal grain size distribution (BGSD) (with a coarse modal size of20–40 lm and a fine modal size of 2–8 lm) of loess on Chinese Loess Plateau (CLP) by mathematical fitting. They argued that the coarse component was transported by low-altitude East-Asia winter monsoon, and the fine component was transported by highaltitude westerlies (Sun et al., 2004). Similar results have also been found in Cheju Island (Korea) and Central Asian loess (Lim and Matsumoto, 2006; Vandenberghe et al., 2006). So, the fine component was used as an intensity indicator of the westerly circulation (Sun, 2004; Lim and Matsumoto, 2006). However, some other researchers suggested that most of the dusts (including both the coarse and fine components) on CLP were from the proximal sources, which were transported by the low-altitude East Asia winter monsoon (Sun et al., 2001; Sun, 2002; Prins et al., 2007). This denied the viewpoint that fine component of the loess on CLP was from a significantly different source. To the sources of the loess on CLP, Sun et al. (2001) and Sun (2002) found that the loess was mainly from the Gobi deserts of southern Mongolia and northern China (including the Badain Jaran Desert, Tengger Desert, Ulan Buh Desert, Hobq Desert and Mu Us Desert), which were transported by the low level atmosphere circulation (Prins et al., 2007). Moreover, Sun et al. (2008a) inferred that fine quartz particles (<16 lm) on CLP were also from the Gobi deserts of southern Mongolia and northern China based on the analysis of crystallinity index of fine-grained (<16 lm) quartz and electron spin resonance signal intensity. Shi and Liu (2011) further concluded that more than 90% of fine particles of modern aeolian deposits on CLP came from the southern Mongolia and northern China being transported via the low-level atmosphere, whereas the dust from Taklimakan Desert were mainly transported further to the pacific via the upper-level westerlies. In addition, the phenomena of aggregation and/or fine particles adhering to larger ones in modern dust have often been found under the scanning electron microscope (SEM) (Pye, 1987, 1995; Falkovich et al., 2001; Derbyshire et al., 1998). So, Pye (1987) and Qiang et al. (2010) argued that the fine particles of aeolian deposits could be caused by aggregation and/or fine particles adhering to larger ones, and dispersed when measurement was taken. Moreover, the post-depositional pedogenesis could also make the coarse particles smaller which increases the content of fine silt (Dixon et al., 1984; Sun et al., 2000). All of these also undermine the multi-sources origin of the aeolian deposits with BGSD or even the validity of the fine particle components as an intensity indicator of westerlies. From the aforementioned debate, some problems could be raised: How does the fine component (<20 lm) of BGSD in aeolian deposits produce? Whether or not the two sub-peak components delegate different sources? Or can the fine sub-peak component be as an intensity indicator of the high-level westerly circulation? At present, the sedimentary process (including the origin of BGSD) of the aeolian deposits has not been completely understood yet, and this makes us hard to exactly interpret the information from the specific size fraction of aeolian deposits in reconstructing the palaeo-environmental changes. Modern dust deposition is the continuation of dust deposition in geological history. This study aims to reveal the origin of BGSD in modern dust deposition based on the observation of their temporal and spatial variation characteristics. The origin of BGSD may provide a theoretic thinking for palaeoenvironmental changes study using the indicator of grain size.
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کلمات کلیدی:
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