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Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications
Quaternary International xxx (2016) 1e11
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Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications Xin Wang a, *, Haitao Wei a, Farhad Khormali b, Mehdi Taheri b, Martin Kehl d, Manfred Frechen e, Tobias Lauer e, f, Fahu Chen a, c a Key Laboratory of Western China's Environmental Systems (Ministry of Education), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China b Department of Soil Science, Faculty of Water and Soil Engineering, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran c CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China d Institute of Geography, University of Cologne, Albertus Magnus Platz, 50923 Cologne, Germany e Leibniz Institute for Applied Geophysics (LIAG), Stilleweg 2, 30655 Hannover, Germany f Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, D-04103 Leipzig, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
The loess deposits in northern Iran are located in a key region connected to the European and central Asian loess belts. However, the lack of previous detailed sedimentological and palaeoclimatic studies of the Pleistocene loess in the region limits our understanding of the nature of ancient aeolian processes and loess history in the mid-latitudes of Euro-Asia as a whole. Here, we present the results of grain-size analyses of the Pleistocene loess from the so-called Iranian Loess Plateau (ILP) in northern Iran. Our results reveal that the grain-size distribution of the deposits is characterized by trimodal and bimodal distributions, comprising a dominant well-sorted coarse dust component (ca. 7e75 mm), a small poorlysorted fine dust component (ca. 2e7 mm), and a minor pedogenic clay component (<2 mm). The dominance of the coarse dust component in the samples suggests that the main part of the Pleistocene loess in northern Iran was transported predominantly by the local low-level winds from proximal source regions. The modal size of the coarse dust component is systematically coarser in the lower Pleistocene loess succession than in the lower Pleistocene loessepalaeosol sequence, indicating a progressively intensifying wind strength during the Pleistocene. The proportion of the clay fraction (<2 mm) decreases systematically from the lower Pleistocene to the upper Pleistocene loess strata, suggesting a relatively drier and colder climate in northern Iran during the late Pleistocene than during the early Pleistocene. © 2016 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Grain size Iranian loess Pleistocene Palaeoclimate Aeolian process
1. Introduction The loess deposits in the mid-latitudes of Euro-Asia provide unique long-term terrestrial archives for reconstructing the history of palaeoclimatic changes and wind-patterns in the continental interior. Extensive studies have been performed on the loess deposits in China (e.g., Heller and Liu, 1982; Liu, 1985; Kukla, 1987; Sun et al., 1998; Ding et al., 1999; An et al., 2001; Guo et al., 2002; An, 2014); central Asia (e.g., Dodonov and Baiguzina, 1995; Frechen and Dodonov, 1998; Ding et al., 2002; Yang et al., 2006); and Europe (e.g., Fink and Kukla, 1977; Smalley, 1995; Frechen et al., 2003; Haase et al., 2007; Markovi c et al., 2008, 2009, 2015; Stevens
* Corresponding author. E-mail address: [email protected] (X. Wang).
et al., 2011). However, the correlation of the loess records between these regions is relatively poor (e.g., Vasiljevic et al., 2014), mainly due to the lack of well-studied loess records from the transitional zones. The loess deposits in northern Iran are located in a critical region connecting the European and central Asian loess belts (Fig. 1, Muhs, 2007), and understanding the palaeoclimatic record of their loess deposits and the dynamics of dust mobilization, transport and deposition could facilitate correlations between the two loess belts. Previous loess studies in northern Iran mainly focused on the chronology and sedimentology of the middleeupper Pleistocene yellowish loess successions. For example, Kehl et al. (2005), Kehl (2010), and Khormali and Kehl (2011) described the lithology and stratigraphy of the middleeupper Pleistocene loessepalaeosol sequences along a climatic gradient in northern Iran; Frechen et al. (2009) and Lauer et al. (2015, 2016a,b) reported the results of luminescence dating of representative sections from the ILP and the
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Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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Fig. 1. Map showing the location of the ILP and the studied sections. The inset map shows the distribution of loess in Euro-Asia (modified from Muhs, 2007). The data for the distribution of loess in northern Iran is from the Central Office of the Golestan Natural Resources and Watershed Management.
northern foothills of the Alborz Mountains; and Vlaminck et al. (2016) presented the first high-resolution multi-proxy records from the Toshan Section from the northern slopes of the Alborz Mountains. These new results demonstrate that most of the loess deposits in northern Iran formed during the middle to late Pleistocene, and that the moderately-developed palaeosol and loess layers formed mainly during interglacial and glacial periods, respectively. Recently, a Sino-Iranian international joint program was carried out to investigate the ancient loess deposits in northern Iran. Lithological and multi-proxy evidence indicate that the widespread red-coloured sediments, unconformably underlying the upper Pleistocene loess successions, are aeolian in origin; while palaeomagnetic dating results indicate that the reddish loess accumulated during ~2.4e1.8 Ma (Wang et al., 2016). This work has extended the history of Iranian loess back to the early Pleistocene; however, the nature of aeolian processes and palaeoclimatic changes in northern Iran during the Pleistocene need further investigation. The grain-size distribution of wind-blown dust deposits is a valuable tool for reconstructing past aeolian processes and wind circulation patterns (e.g. Folk, 1966; McCave et al., 1995; Pye, 1995; Sun et al., 2002; Machalett et al., 2008; Vandenberghe, 2013). Given the specific physical properties of a given transport medium, the grain-size of the transported sediments characteristically exhibits a smooth, unimodal curve (Ashley, 1978; Bagnold and BarndorffNielsen, 1980). For example, wind-blown dust in the North Pacific Ocean is carried by the high-level Westerlies as a long-distance
suspension component, and its grain-size distribution is characterized by a unimodal distribution with a modal size generally finer than 10 mm (Rea and Hovan, 1995). Sand dune particles in arid lands are mainly transported by strong near-surface winds in a traction mode, and their grain-size distribution is characterized by a unimodal distribution with a dominant aeolian sand component; the modal sizes generally range from 70 to 250 mm (Sun et al., 2011a; Wang et al., 2013, 2014; Li et al., 2014; Wang et al., 2015). Wind can transport particles in traction, saltation and suspension modes, depending on the balance between the settling velocity of the grain and the vertical velocity component of the wind (Pye, 1995). Consequently, the aeolian traction, saltation and suspension components are represented in a sedimentary deposit by a series of overlapping grain-size distribution curves (e.g. Middleton, 1976; Sun et al., 2002); and in principle these specific components can be mathematically identified and partitioned (e.g. Sun et al., 2002; Qin et al., 2005; Weltje and Prins, 2007; Vandenberghe, 2013). In the present work, grain-size analyses were conducted on the upper Pleistocene yellowish loess successions as well as on the lower Pleistocene reddish loessepalaeosol sequences from the ILP. By using the grain-size distribution function approach developed by Sun et al. (2002), we determined that the grain size of the Pleistocene Iranian loess is characterized by trimodal and bimodal distributions, and that it demonstrates systematic variations in modal size and in the proportions of the specific components through time. This new dataset provides valuable information about the ancient dust dynamics and palaeoclimatic changes in northern Iran during the Pleistocene.
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
X. Wang et al. / Quaternary International xxx (2016) 1e11
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Fig. 2. Stratigraphy of the upper Pleistocene yellowish loess profile at the Agh Band profile (A) and the lower Pleistocene reddish loessepalaeosol sequence at the AB1 section (B). The yellow line indicates the sampling intervals. The chronological results for the Agh Band loess profile and the AB1 section were cited from Laur et al. (2016) and Wang et al. (2016), respectively. The section photography in Fig. B is cited from Wang et al. (2016). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. Geological setting, stratigraphy and field sampling The ILP in northern Iran is flanked by the Alborz Mountains to the south, the Caspian Sea and the Arid Caspian Lowland to the west, and the Kopet Dag Range to the north (Fig. 1B). The present
day climate is semi-arid, with a mean annual temperature of about 17 C and with mean annual precipitation of less than 300 mm, with more than 85% falling during the boreal winter months. The high-level wind regime is closely related to the zonal Westerlies, which carry moisture from the Mediterranean and Caspian Sea to
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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Fig. 3. Results of grain-size analysis of loess samples from the surface and the upper Pleistocene Agh Band loess profile. Grain-size distribution of representative surface loess samples (AeB) collected from the ILP; and of loess samples collected from the late Pleistocene yellowish loess succession (CeD); topmost palaeosol (S1) from the Agh Band profile (E); and the aeolian sand dune deposits from the Arid Caspian lowland (F). Frequency of the modal sizes (G), frequency of the proportion (H), and size-proportion plot of all of the grain-size components (I) from the Agh Band section. Abbreviations: CD e coarse dust component, FD e fine dust component, UF e ultrafine component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the ILP (Ballato et al., 2010). In contrast, the near-surface wind regime is dominated by northeasterly to northwesterly winds during the boreal summer, and by the opposite wind directions during the boreal winter season. This seasonal reversal in wind direction is mainly driven by pressure differences between the Caspian Sea and the Central Iranian highlands (Ganji, 1963). The middleeupper Pleistocene loessepalaeosol profiles (Fig. 2A) in northern Iran are distributed on the ILP and along the
northern foothills of the Alborz Mountains (Fig. 1). The reported profiles consist of alternations of dull yellowish loess layers and moderately well-developed brown palaeosols, which were deposited and formed during glacial and interglacial periods, respectively (Kehl et al., 2005; Frechen et al., 2009; Lauer et al., 2015; Vlaminck et al., 2016). The lower Pleistocene reddish loessepalaeosol sequences (Fig. 2B) are exposed across a broad area of the ILP (Fig. 1). They unconformably underlie the middleeupper Pleistocene
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
X. Wang et al. / Quaternary International xxx (2016) 1e11
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Fig. 4. Variations of the mean grain-size of bulk samples and modal sizes, and proportions of each specific component in the Agh Band yellowish loess profile. Abbreviations: CD e coarse dust component, FD e fine dust component, UF e ultrafine component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
yellowish loess successions, and are composed of alternations of reddish-yellow loess and well-developed brownish-red palaeosols (Fig. 2B, Wang et al., 2016; Taheri et al., 2016). The basal and topmost ages of the reddish loess sediments on the ILP were dated at ~2.4 and ~1.8 Ma, respectively; and there is a large sedimentary hiatus between the lower Pleistocene reddish loess and the middleeupper Pleistocene yellowish loess (Wang et al., 2016). For grain-size analyses, 395 bulk samples were collected from the reddish loessepalaeosol sequence at the AB1 section (Fig. 2B), with a sampling interval of about 5 cm. Samples from layers containing gypsum were excluded from grain-size measurements since the traditional pretreatment method (see below) cannot remove gypsum crystals. Thirty bulk surface samples were collected from the neighboring Agh Band loess profile, described in detail in Lauer et al. (2016a,b) (Fig. 2A), at an approximate 1-m-resolution. In addition, two bulk samples were collected from the uppermost palaeosol layers in the Agh Band loess profile, and two bulk samples were collected from the late Pleistocene sand dune deposits in the arid Caspian Lowland (see Fig. 1 for sampling locations). 3. Methods A laser diffraction method was used for measuring the grainsize of the collected samples. Standard methods were used for pretreatment (Konert and Vandenberghe, 1997): 1) 3e4 g of sample were treated with 10 ml of 30% H2O2 to remove organic matter; 2) 10 ml of 10% HCl were added to remove carbonates; 3) about 100 ml of distilled water were added to the samples and left for 12 h to remove acidic ions; and 4) the sample residues were dispersed with 10 ml of 0.5 N (NaPO3)6 on an ultrasonic vibrator for 7 min. Subsequently the samples were measured using a Malvern Mastersizer 2000 laser grain-size analyzer, which has a measurement range of 0.02e2000 mm, with a 0.1F interval
resolution. It should be noted that the laser diffraction method may potentially underestimate the content of the clay fraction in comparison with the pipette method (Konert and Vandenberghe, 1997). However, this potential source of error would be unlikely to have a significant effect on the interpretation of long-term trends of the grain-size parameters. All of the measured grain-size data were analyzed using the grain-size distribution function method developed by Sun et al. (2002). The Weibull or Normal function was selected to partition the original grain-size data and to calculate the parameters (i.e. modal size and proportion) on the basis of goodness-of-fit criteria. The origin of each specific sedimentary component was identified on the basis of the frequencies of modal sizes and proportions of all the measured samples from the studied section, and via comparison with the previously-determined grain-size distributions of characteristic loess sediments determined by laser granulometry (e.g. Sun et al., 2002, 2006; Vandenberghe, 2013; An, 2014). Ages of the measured samples from the AB1 section were obtained by the linear interpolation between the ages of geomagnetic polarity boundaries (Wang et al., 2016), whilst, the approximate ages of each sample collected from the Agh Band loess profile were established by comparison with the luminescence dating results of Laur et al. (2016). 4. Results 4.1. Grain-size distribution of the surface samples and upper Pleistocene yellowish loess Representative grain-size distribution curves of the surface and upper Pleistocene loess samples from the ILP are illustrated in Fig. 3(AeD). The frequency distribution of the modal sizes of all of
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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Fig. 5. Results of grain-size analysis of loess samples from the lower Pleistocene loessepalaeosol sequence from the AB1 section. Grain-size distribution of representative samples from reddish loess strata and palaeosols (AeD); frequency of the modal sizes (E), frequency of the proportions (F), and (G) size-proportion plot of all of the grain-size components from the AB1 section. Abbreviations: CD e coarse dust component, FD e fine dust component, UF e ultrafine component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the measured samples from the Agh Band section and the modern loess samples exhibits three distinct ranges, spanning 0.7e2 mm, 2e7 mm, and 7e70 mm (Fig. 3G). The frequency of the proportion of each component has two distinct ranges with boundaries at approximately 70e91% and 7e30% (Fig. 3H). On the basis of the observed differences in the frequencies of the modal sizes and the components, the following specific sedimentary components were identified: an ultrafine component, a fine component, and a coarse component. Based on comparison with typical grain-size distributions of Chinese loess (Sun et al., 2002, 2004, 2006, 2010; Vandenberghe, 2013; An, 2014) and on observations of aeolian particle dynamics (e.g. Pye, 1987; Pye and Tsoar, 2009), these components were interpreted as pedogenic clay that formed during post-depositional pedogenesis; fine dust transported mainly in long-distance suspension; and coarse dust transported in shortdistance suspension mode (Fig. 3I).
The grain-size distributions of the surface and upper Pleistocene loess in the ILP are generally consistent. They are characterized by a trimodal distribution (Fig. 3AeD), with a large proportion (ca. 81%) of well-sorted coarse dust (30e70 mm), a small proportion (ca. 16%) of fine dust (7e10 mm), and a minor proportion (ca. 3%) of an ultrafine pedogenic component (<2 mm). This grain-size distribution is comparable to that of the last glacial loess (L1) from the Chinese Loess Plateau (Sun, 2004). 4.2. Modal grain-size and variation of the proportion of each grainsize component in the upper Pleistocene loess succession For the upper Pleistocene loess strata on the ILP, the coarse dust component comprises the main part of the grain-size distribution (Fig. 4B). The proportion of the coarse dust component ranges from 73.4% to 87.8%, and maintains high values (ca. 85.2%) within the
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
X. Wang et al. / Quaternary International xxx (2016) 1e11
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Fig. 6. Variations of the mean grain-size of bulk samples and modal sizes and the proportions of each individual component for the AB1 reddish loessepalaeosol sequence. Abbreviations: CD e coarse dust component, FD e fine dust component, UF e ultrafine component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
11e22 m interval of the studied section (Fig. 4B). The modal size of the coarse dust component exhibits an overall increasing trend from the bottom to the top of the section (Fig. 4C), which is similar to that of the mean size of the bulk samples and the modal size of the fine dust component (Fig. 4A, E). The proportion of the fine dust component varies from 10.4% to 23.9%, and exhibits an overall antiphased relationship with that of the coarse dust component (Fig. 4D). The modal size of the ultrafine component is generally consistent (Fig. 4G), and its proportional contribution decreases from the bottom to the top of the section (Fig. 4F), which is similar to that of the clay (<2 mm) content (Fig. 4H). 4.3. Grain-size distribution of the lower Pleistocene reddish loess Typical grain-size distributions of the lower Pleistocene reddish loess are illustrated in Fig. 5(AeD). The frequency of the modal sizes of all of the measured samples from the AB1 section exhibits three distribution ranges, spanning 0.7e2 mm, 2e7 mm, and 7e50 mm (Fig. 5E). The frequency of the proportions of all samples exhibits two distinct ranges with a boundary at approximately 50% (Fig. 5F). These specific sedimentary components in the reddish loess samples are interpreted as representing pedogenic clay, fine dust, and coarse dust components, respectively (Fig. 5G). For the lower Pleistocene reddish loess, the modal size of the coarse component is significantly finer, and the proportion of the ultrafine component is systematically greater, than in the surface and upper Pleistocene loess. The grain-size distributions of the lower Pleistocene loess are divided into two groups. The first group is characterized by a bimodal distribution with a short-distance suspension component (7e70 mm) and a minor proportion of the pedogenic component
(<2 mm) (Fig. 5A), which is comparable to the uppermost palaeosols (Fig. 3E) from the Agh Band section. The second group is characterized by a trimodal distribution with an additional overlapping poorly-sorted fine dust component (2e7 mm) (Fig. 5B, C, D). 4.4. Modal grain-size and variation of the proportion of each grainsize component in the lower Pleistocene loessepalaeosol sequence The modal sizes and variations of the proportions of each specific component are illustrated in Fig. 6. The coarse dust component comprises the main part of the grain-size distribution of the lower Pleistocene reddish loess strata (Fig. 6B). However, in the welldeveloped palaeosol layers, the proportion of the fine dust component increases dramatically (Fig. 6D). The modal size of the coarse dust component exhibits an overall increasing trend from the bottom to the top of the section (Fig. 6C), which is similar to the long-term trend of the mean grain-size of the bulk sample (Fig. 6A) and is independent of that of the fine dust component (Fig. 6E). The modal size and proportion of the ultrafine component exhibit large fluctuations, with overall decreasing trends from the bottom to the top of the section (Fig. 6F, G). 5. Discussion 5.1. Trimodal grain-size distribution of the Pleistocene loess in the ILP and its implications for ancient dust dynamics in northern Iran Systematic grain-size analyses of the Pleistocene loess in the ILP reveals that the grain-size distribution of the Pleistocene loess is characterized by trimodal and bimodal distributions, and these
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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findings provide new evidence for reconstructing the ancient dust dynamic in northern Iran during the Pleistocene. The sources and aeolian processes involved in modern-day dust transport and deposition in northern Iran are unclear (e.g. Okhravi and Amini, 2001; Karimi et al., 2009; Kehl, 2010; Asadi et al., 2013). Generally, there are two types of dust process operating in the region, as indicated by our field observations and the evidence from large-scale satellite images (http://earthobservatory.nasa.gov/). Locally, the near-surface winds frequently transport silt-sized materials from the neighboring arid Caspian Lowland and the Karakum Desert to the ILP in short-distance suspension mode (Fig. 7A), and this process carries coarse silts to the ILP. The process also results in the formation of linear or barchans dunes in the arid Caspian Lowland (Fig. 7B), which grain-size distribution is dominated by an aeolian traction component (Fig. 3F). Regionally, large-scale high-level westerly winds, associated with the zonal Westerlies, carry fine dust from distant sources (i.e. the vast area of arid land in the Middle East) in long-distance suspension mode to the ILP (Fig. 7C, D); and this process carries fine dust particles to northern Iran. Generally, the combination of these two aeolian processes dominates the dust input to the ILP at the present day. The results for the Pleistocene loess in the ILP reveal that the grain-size distribution of the Pleistocene loess is characterized by trimodal and bimodal distributions. The coarse component, with a
modal size of 7e70 mm, is mainly transported by near-surface winds in short-distance suspension mode (Pye, 1987). This coarse dust component dominates the Pleistocene loess samples (Figs. 4 and 6), suggesting that the main component of the Pleistocene loess on the ILP is mainly transported by near-surface winds from near-source regions, such as the arid Caspian Lowland and the Karakum Desert (see Fig. 1B for location). The origin of the fine-grained component in loess sediments is relatively complex. Typically, there are three sources of fine dust in loess samples (Pye, 1987; Sun et al., 2011b): 1) from distant source regions, transported by large-scale high-level winds in long-term suspension mode; 2) from near-source regions adhering to coarse silt grains in short-distance suspension mode; and 3) produced during post-depositional pedogenesis, which results in the reduction in size of coarse grains. For the upper Pleistocene loess successions, the fine dust component has narrow modal size ranges and specific proportions (Fig. 3). The modal size is independent of the coarse dust component, suggesting that these grains are mainly derived from distant source regions. However, the grain-size boundary in the early Pleistocene loess is indistinct (Fig. 5), suggesting a complex origin of the fine dust particles; and we infer that these grains may be influenced significantly by pedogenic processes, and are derived from the comminution of coarse silt particles.
Fig. 7. Photographs and satellite image (http://earthobservatory.nasa.gov/) illustrating present-day aeolian activity in the ILP and adjacent areas. (A) Frequently-occurring local dust storm on the ILP; (B) aeolian sand dunes in the arid Caspian lowland; (C, D) large-scale dust storm in the ILP and surrounding regions.
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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This suggests an overall intensification of wind strength during the Pleistocene. The clay fraction in a loess sample is mainly formed during postdepositional weathering and pedogenesis (Bronger and Heinkele, 1990), and is sensitive to regional climate change (Chamley, 1989). For the lower Pleistocene loessepalaeosol sequence in the AB1 section, the grain-size boundary between the ultrafine and the fine dust components is indistinct (Fig. 5E) and this results in uncertainties in differentiating the ultrafine and the fine dust components. Consequently, both the modal size and the content of the ultrafine component in the lower Pleistocene loessepalaeosol sequences exhibit large fluctuations which are not obviously related to the lithology (Fig. 6F, G). Under these circumstance the clay fraction (<2 mm) content, despite the fact that it may be systematically underestimated by the laser granulometry method, is a more reliable indicator of variations in the clay fraction. The clay content in the lower Pleistocene loess strata is higher than in the upper Pleistocene loess successions, and this long-term trend is consistent with that of the records of redness and Rb/Sr ratio (Wang et al., 2016). This suggests that a relatively drier and possibly colder climate characterized the ILP during the late Pleistocene. The longterm aridification trend in the ILP, since ~2.4 Ma, is probably driving by the stepwise expansion of northern hemisphere ice sheets (Wang et al., 2016).
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Fig. 8. Time series of mean grain-size, modal size of the coarse dust component, and proportion of the ultrafine component in the Pleistocene loess strata. The basal age of the upper Pleistocene loess succession in the Agh Band section was determined by pIRIR290 dating (unpublished data). Note that the content of the clay fraction may by systematically underestimated by the laser diffraction method (Konert and Vandenberghe, 1997).
The coarse dust component dominates the Pleistocene loess from the ILP, suggesting that this component is mainly transported by near-surface winds in short-distance suspension mode. The common occurrence of fine dust components in the samples suggest the high-level large-scale wind regim, likely associated with the zonal Westerlies, also contributed to the dust input in north Iran during the late Pleistocene. 5.2. Palaeoclimatic implications of the variations in grain-size and related parameters The grain-size distribution, especially the mean grain-size, of loess deposits is traditionally used as a proxy for palaeo-wind strength and/or the distance from the studied sites and the dust sources (Ding et al., 1994, 2002; Xiao et al., 1995; Porter and An, 1995; Chen et al., 1997; Sun et al., 2002, 2010; Vandenberghe, 2013), despite it may potentially be influenced by aridity in the source regions. Throughout the Pleistocene loessepalaeosol sequences in the ILP, the mean grain-size maintained low values during the early Pleistocene and then increased systematically during the late Pleistocene (Fig. 8). This long-term trend suggests an intensified palaeo-wind strength and/or decreased distance from the dust sources. The coarse dust in loess deposits, with modal size ranges from 7 to 70 mm, is mainly transported by near-surface winds in short-distance suspension episodes (Pye, 1995); and thus it reflects near-surface wind intensity, despite the fact that it may also be influenced by the distance from the source areas (Sun et al., 2008). The modal size of the coarse dust component in the upper Pleistocene yellowish loess strata is systematically coarser than that of the early Pleistocene loessepalaeosol sequences, and is similar to the long-term trend of the U-ratio (Wang et al., 2016).
6. Conclusions 1) The grain size of the Pleistocene loess from the ILP is characterized by trimodal and bimodal distributions, with a wellsorted coarse dust component (ca. 7e75 mm), a poorly-sorted fine component (ca. 2e7 mm), and a minor ultrafine component (<2 mm). 2) The dominance of coarse dust components in the samples suggests that the Pleistocene loess on the ILP is mainly transported in short-distance suspension mode by near-surface winds; however, the common occurrence of a fine dust component indicates that the large-scale high-level winds also contribute to the dust input on the ILP. 3) The modal size of the coarse dust component in the upper Pleistocene yellowish loess strata is systematically coarser than in the early Pleistocene loessepalaeosol sequences, suggesting a strengthening of the wind intensity during the Pleistocene. Acknowledgements We are grateful to Prof. D.H. Sun for analyzes grain size data, Z.W. Ma and B.Q. Liang for assistance with the laboratory work, and Prof. J. Bloemendal for improve the English usage. Financial support for this research was provided by the National Natural Science Foundation of China Grant (grant no. 41302144, 41130102), the Chinese 111 Project (grant no. B06026), the Fundamental Research Funds for the Central Universities (grant no. lzujbky-2015-129), and the Open Foundation of MOE Key Laboratory of Western China's Environmental System, Lanzhou University (grant no. lzujbky2013-bt01). References An, Z.S., 2014. Late Cenozoic Climate Change in Asia. Springer, Dordrecht. An, Z.S., Kutzbach, J.E., Prell, W.L., Porter, S.C., 2001. Evolution of Asian monsoons and phased uplift of the HimalayaeTibetan plateau since Late Miocene times. Nature 411, 62e66. Asadi, S., Moore, F., Keshavarzi, B., 2013. The nature and provenance of Golestan loess deposits in northeast Iran. Geological Journal 48, 646e660. Ashley, G.M., 1978. Interpretation of polymodal sediments. The Journal of Geology 86, 411e421.
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications Xin Wang a, *, Haitao Wei a, Farhad Khormali b, Mehdi Taheri b, Martin Kehl d, Manfred Frechen e, Tobias Lauer e, f, Fahu Chen a, c a Key Laboratory of Western China's Environmental Systems (Ministry of Education), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China b Department of Soil Science, Faculty of Water and Soil Engineering, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran c CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China d Institute of Geography, University of Cologne, Albertus Magnus Platz, 50923 Cologne, Germany e Leibniz Institute for Applied Geophysics (LIAG), Stilleweg 2, 30655 Hannover, Germany f Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, D-04103 Leipzig, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
The loess deposits in northern Iran are located in a key region connected to the European and central Asian loess belts. However, the lack of previous detailed sedimentological and palaeoclimatic studies of the Pleistocene loess in the region limits our understanding of the nature of ancient aeolian processes and loess history in the mid-latitudes of Euro-Asia as a whole. Here, we present the results of grain-size analyses of the Pleistocene loess from the so-called Iranian Loess Plateau (ILP) in northern Iran. Our results reveal that the grain-size distribution of the deposits is characterized by trimodal and bimodal distributions, comprising a dominant well-sorted coarse dust component (ca. 7e75 mm), a small poorlysorted fine dust component (ca. 2e7 mm), and a minor pedogenic clay component (<2 mm). The dominance of the coarse dust component in the samples suggests that the main part of the Pleistocene loess in northern Iran was transported predominantly by the local low-level winds from proximal source regions. The modal size of the coarse dust component is systematically coarser in the lower Pleistocene loess succession than in the lower Pleistocene loessepalaeosol sequence, indicating a progressively intensifying wind strength during the Pleistocene. The proportion of the clay fraction (<2 mm) decreases systematically from the lower Pleistocene to the upper Pleistocene loess strata, suggesting a relatively drier and colder climate in northern Iran during the late Pleistocene than during the early Pleistocene. © 2016 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Grain size Iranian loess Pleistocene Palaeoclimate Aeolian process
1. Introduction The loess deposits in the mid-latitudes of Euro-Asia provide unique long-term terrestrial archives for reconstructing the history of palaeoclimatic changes and wind-patterns in the continental interior. Extensive studies have been performed on the loess deposits in China (e.g., Heller and Liu, 1982; Liu, 1985; Kukla, 1987; Sun et al., 1998; Ding et al., 1999; An et al., 2001; Guo et al., 2002; An, 2014); central Asia (e.g., Dodonov and Baiguzina, 1995; Frechen and Dodonov, 1998; Ding et al., 2002; Yang et al., 2006); and Europe (e.g., Fink and Kukla, 1977; Smalley, 1995; Frechen et al., 2003; Haase et al., 2007; Markovi c et al., 2008, 2009, 2015; Stevens
* Corresponding author. E-mail address: [email protected] (X. Wang).
et al., 2011). However, the correlation of the loess records between these regions is relatively poor (e.g., Vasiljevic et al., 2014), mainly due to the lack of well-studied loess records from the transitional zones. The loess deposits in northern Iran are located in a critical region connecting the European and central Asian loess belts (Fig. 1, Muhs, 2007), and understanding the palaeoclimatic record of their loess deposits and the dynamics of dust mobilization, transport and deposition could facilitate correlations between the two loess belts. Previous loess studies in northern Iran mainly focused on the chronology and sedimentology of the middleeupper Pleistocene yellowish loess successions. For example, Kehl et al. (2005), Kehl (2010), and Khormali and Kehl (2011) described the lithology and stratigraphy of the middleeupper Pleistocene loessepalaeosol sequences along a climatic gradient in northern Iran; Frechen et al. (2009) and Lauer et al. (2015, 2016a,b) reported the results of luminescence dating of representative sections from the ILP and the
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Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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Fig. 1. Map showing the location of the ILP and the studied sections. The inset map shows the distribution of loess in Euro-Asia (modified from Muhs, 2007). The data for the distribution of loess in northern Iran is from the Central Office of the Golestan Natural Resources and Watershed Management.
northern foothills of the Alborz Mountains; and Vlaminck et al. (2016) presented the first high-resolution multi-proxy records from the Toshan Section from the northern slopes of the Alborz Mountains. These new results demonstrate that most of the loess deposits in northern Iran formed during the middle to late Pleistocene, and that the moderately-developed palaeosol and loess layers formed mainly during interglacial and glacial periods, respectively. Recently, a Sino-Iranian international joint program was carried out to investigate the ancient loess deposits in northern Iran. Lithological and multi-proxy evidence indicate that the widespread red-coloured sediments, unconformably underlying the upper Pleistocene loess successions, are aeolian in origin; while palaeomagnetic dating results indicate that the reddish loess accumulated during ~2.4e1.8 Ma (Wang et al., 2016). This work has extended the history of Iranian loess back to the early Pleistocene; however, the nature of aeolian processes and palaeoclimatic changes in northern Iran during the Pleistocene need further investigation. The grain-size distribution of wind-blown dust deposits is a valuable tool for reconstructing past aeolian processes and wind circulation patterns (e.g. Folk, 1966; McCave et al., 1995; Pye, 1995; Sun et al., 2002; Machalett et al., 2008; Vandenberghe, 2013). Given the specific physical properties of a given transport medium, the grain-size of the transported sediments characteristically exhibits a smooth, unimodal curve (Ashley, 1978; Bagnold and BarndorffNielsen, 1980). For example, wind-blown dust in the North Pacific Ocean is carried by the high-level Westerlies as a long-distance
suspension component, and its grain-size distribution is characterized by a unimodal distribution with a modal size generally finer than 10 mm (Rea and Hovan, 1995). Sand dune particles in arid lands are mainly transported by strong near-surface winds in a traction mode, and their grain-size distribution is characterized by a unimodal distribution with a dominant aeolian sand component; the modal sizes generally range from 70 to 250 mm (Sun et al., 2011a; Wang et al., 2013, 2014; Li et al., 2014; Wang et al., 2015). Wind can transport particles in traction, saltation and suspension modes, depending on the balance between the settling velocity of the grain and the vertical velocity component of the wind (Pye, 1995). Consequently, the aeolian traction, saltation and suspension components are represented in a sedimentary deposit by a series of overlapping grain-size distribution curves (e.g. Middleton, 1976; Sun et al., 2002); and in principle these specific components can be mathematically identified and partitioned (e.g. Sun et al., 2002; Qin et al., 2005; Weltje and Prins, 2007; Vandenberghe, 2013). In the present work, grain-size analyses were conducted on the upper Pleistocene yellowish loess successions as well as on the lower Pleistocene reddish loessepalaeosol sequences from the ILP. By using the grain-size distribution function approach developed by Sun et al. (2002), we determined that the grain size of the Pleistocene Iranian loess is characterized by trimodal and bimodal distributions, and that it demonstrates systematic variations in modal size and in the proportions of the specific components through time. This new dataset provides valuable information about the ancient dust dynamics and palaeoclimatic changes in northern Iran during the Pleistocene.
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
X. Wang et al. / Quaternary International xxx (2016) 1e11
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Fig. 2. Stratigraphy of the upper Pleistocene yellowish loess profile at the Agh Band profile (A) and the lower Pleistocene reddish loessepalaeosol sequence at the AB1 section (B). The yellow line indicates the sampling intervals. The chronological results for the Agh Band loess profile and the AB1 section were cited from Laur et al. (2016) and Wang et al. (2016), respectively. The section photography in Fig. B is cited from Wang et al. (2016). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. Geological setting, stratigraphy and field sampling The ILP in northern Iran is flanked by the Alborz Mountains to the south, the Caspian Sea and the Arid Caspian Lowland to the west, and the Kopet Dag Range to the north (Fig. 1B). The present
day climate is semi-arid, with a mean annual temperature of about 17 C and with mean annual precipitation of less than 300 mm, with more than 85% falling during the boreal winter months. The high-level wind regime is closely related to the zonal Westerlies, which carry moisture from the Mediterranean and Caspian Sea to
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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Fig. 3. Results of grain-size analysis of loess samples from the surface and the upper Pleistocene Agh Band loess profile. Grain-size distribution of representative surface loess samples (AeB) collected from the ILP; and of loess samples collected from the late Pleistocene yellowish loess succession (CeD); topmost palaeosol (S1) from the Agh Band profile (E); and the aeolian sand dune deposits from the Arid Caspian lowland (F). Frequency of the modal sizes (G), frequency of the proportion (H), and size-proportion plot of all of the grain-size components (I) from the Agh Band section. Abbreviations: CD e coarse dust component, FD e fine dust component, UF e ultrafine component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the ILP (Ballato et al., 2010). In contrast, the near-surface wind regime is dominated by northeasterly to northwesterly winds during the boreal summer, and by the opposite wind directions during the boreal winter season. This seasonal reversal in wind direction is mainly driven by pressure differences between the Caspian Sea and the Central Iranian highlands (Ganji, 1963). The middleeupper Pleistocene loessepalaeosol profiles (Fig. 2A) in northern Iran are distributed on the ILP and along the
northern foothills of the Alborz Mountains (Fig. 1). The reported profiles consist of alternations of dull yellowish loess layers and moderately well-developed brown palaeosols, which were deposited and formed during glacial and interglacial periods, respectively (Kehl et al., 2005; Frechen et al., 2009; Lauer et al., 2015; Vlaminck et al., 2016). The lower Pleistocene reddish loessepalaeosol sequences (Fig. 2B) are exposed across a broad area of the ILP (Fig. 1). They unconformably underlie the middleeupper Pleistocene
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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Fig. 4. Variations of the mean grain-size of bulk samples and modal sizes, and proportions of each specific component in the Agh Band yellowish loess profile. Abbreviations: CD e coarse dust component, FD e fine dust component, UF e ultrafine component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
yellowish loess successions, and are composed of alternations of reddish-yellow loess and well-developed brownish-red palaeosols (Fig. 2B, Wang et al., 2016; Taheri et al., 2016). The basal and topmost ages of the reddish loess sediments on the ILP were dated at ~2.4 and ~1.8 Ma, respectively; and there is a large sedimentary hiatus between the lower Pleistocene reddish loess and the middleeupper Pleistocene yellowish loess (Wang et al., 2016). For grain-size analyses, 395 bulk samples were collected from the reddish loessepalaeosol sequence at the AB1 section (Fig. 2B), with a sampling interval of about 5 cm. Samples from layers containing gypsum were excluded from grain-size measurements since the traditional pretreatment method (see below) cannot remove gypsum crystals. Thirty bulk surface samples were collected from the neighboring Agh Band loess profile, described in detail in Lauer et al. (2016a,b) (Fig. 2A), at an approximate 1-m-resolution. In addition, two bulk samples were collected from the uppermost palaeosol layers in the Agh Band loess profile, and two bulk samples were collected from the late Pleistocene sand dune deposits in the arid Caspian Lowland (see Fig. 1 for sampling locations). 3. Methods A laser diffraction method was used for measuring the grainsize of the collected samples. Standard methods were used for pretreatment (Konert and Vandenberghe, 1997): 1) 3e4 g of sample were treated with 10 ml of 30% H2O2 to remove organic matter; 2) 10 ml of 10% HCl were added to remove carbonates; 3) about 100 ml of distilled water were added to the samples and left for 12 h to remove acidic ions; and 4) the sample residues were dispersed with 10 ml of 0.5 N (NaPO3)6 on an ultrasonic vibrator for 7 min. Subsequently the samples were measured using a Malvern Mastersizer 2000 laser grain-size analyzer, which has a measurement range of 0.02e2000 mm, with a 0.1F interval
resolution. It should be noted that the laser diffraction method may potentially underestimate the content of the clay fraction in comparison with the pipette method (Konert and Vandenberghe, 1997). However, this potential source of error would be unlikely to have a significant effect on the interpretation of long-term trends of the grain-size parameters. All of the measured grain-size data were analyzed using the grain-size distribution function method developed by Sun et al. (2002). The Weibull or Normal function was selected to partition the original grain-size data and to calculate the parameters (i.e. modal size and proportion) on the basis of goodness-of-fit criteria. The origin of each specific sedimentary component was identified on the basis of the frequencies of modal sizes and proportions of all the measured samples from the studied section, and via comparison with the previously-determined grain-size distributions of characteristic loess sediments determined by laser granulometry (e.g. Sun et al., 2002, 2006; Vandenberghe, 2013; An, 2014). Ages of the measured samples from the AB1 section were obtained by the linear interpolation between the ages of geomagnetic polarity boundaries (Wang et al., 2016), whilst, the approximate ages of each sample collected from the Agh Band loess profile were established by comparison with the luminescence dating results of Laur et al. (2016). 4. Results 4.1. Grain-size distribution of the surface samples and upper Pleistocene yellowish loess Representative grain-size distribution curves of the surface and upper Pleistocene loess samples from the ILP are illustrated in Fig. 3(AeD). The frequency distribution of the modal sizes of all of
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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Fig. 5. Results of grain-size analysis of loess samples from the lower Pleistocene loessepalaeosol sequence from the AB1 section. Grain-size distribution of representative samples from reddish loess strata and palaeosols (AeD); frequency of the modal sizes (E), frequency of the proportions (F), and (G) size-proportion plot of all of the grain-size components from the AB1 section. Abbreviations: CD e coarse dust component, FD e fine dust component, UF e ultrafine component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the measured samples from the Agh Band section and the modern loess samples exhibits three distinct ranges, spanning 0.7e2 mm, 2e7 mm, and 7e70 mm (Fig. 3G). The frequency of the proportion of each component has two distinct ranges with boundaries at approximately 70e91% and 7e30% (Fig. 3H). On the basis of the observed differences in the frequencies of the modal sizes and the components, the following specific sedimentary components were identified: an ultrafine component, a fine component, and a coarse component. Based on comparison with typical grain-size distributions of Chinese loess (Sun et al., 2002, 2004, 2006, 2010; Vandenberghe, 2013; An, 2014) and on observations of aeolian particle dynamics (e.g. Pye, 1987; Pye and Tsoar, 2009), these components were interpreted as pedogenic clay that formed during post-depositional pedogenesis; fine dust transported mainly in long-distance suspension; and coarse dust transported in shortdistance suspension mode (Fig. 3I).
The grain-size distributions of the surface and upper Pleistocene loess in the ILP are generally consistent. They are characterized by a trimodal distribution (Fig. 3AeD), with a large proportion (ca. 81%) of well-sorted coarse dust (30e70 mm), a small proportion (ca. 16%) of fine dust (7e10 mm), and a minor proportion (ca. 3%) of an ultrafine pedogenic component (<2 mm). This grain-size distribution is comparable to that of the last glacial loess (L1) from the Chinese Loess Plateau (Sun, 2004). 4.2. Modal grain-size and variation of the proportion of each grainsize component in the upper Pleistocene loess succession For the upper Pleistocene loess strata on the ILP, the coarse dust component comprises the main part of the grain-size distribution (Fig. 4B). The proportion of the coarse dust component ranges from 73.4% to 87.8%, and maintains high values (ca. 85.2%) within the
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
X. Wang et al. / Quaternary International xxx (2016) 1e11
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Fig. 6. Variations of the mean grain-size of bulk samples and modal sizes and the proportions of each individual component for the AB1 reddish loessepalaeosol sequence. Abbreviations: CD e coarse dust component, FD e fine dust component, UF e ultrafine component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
11e22 m interval of the studied section (Fig. 4B). The modal size of the coarse dust component exhibits an overall increasing trend from the bottom to the top of the section (Fig. 4C), which is similar to that of the mean size of the bulk samples and the modal size of the fine dust component (Fig. 4A, E). The proportion of the fine dust component varies from 10.4% to 23.9%, and exhibits an overall antiphased relationship with that of the coarse dust component (Fig. 4D). The modal size of the ultrafine component is generally consistent (Fig. 4G), and its proportional contribution decreases from the bottom to the top of the section (Fig. 4F), which is similar to that of the clay (<2 mm) content (Fig. 4H). 4.3. Grain-size distribution of the lower Pleistocene reddish loess Typical grain-size distributions of the lower Pleistocene reddish loess are illustrated in Fig. 5(AeD). The frequency of the modal sizes of all of the measured samples from the AB1 section exhibits three distribution ranges, spanning 0.7e2 mm, 2e7 mm, and 7e50 mm (Fig. 5E). The frequency of the proportions of all samples exhibits two distinct ranges with a boundary at approximately 50% (Fig. 5F). These specific sedimentary components in the reddish loess samples are interpreted as representing pedogenic clay, fine dust, and coarse dust components, respectively (Fig. 5G). For the lower Pleistocene reddish loess, the modal size of the coarse component is significantly finer, and the proportion of the ultrafine component is systematically greater, than in the surface and upper Pleistocene loess. The grain-size distributions of the lower Pleistocene loess are divided into two groups. The first group is characterized by a bimodal distribution with a short-distance suspension component (7e70 mm) and a minor proportion of the pedogenic component
(<2 mm) (Fig. 5A), which is comparable to the uppermost palaeosols (Fig. 3E) from the Agh Band section. The second group is characterized by a trimodal distribution with an additional overlapping poorly-sorted fine dust component (2e7 mm) (Fig. 5B, C, D). 4.4. Modal grain-size and variation of the proportion of each grainsize component in the lower Pleistocene loessepalaeosol sequence The modal sizes and variations of the proportions of each specific component are illustrated in Fig. 6. The coarse dust component comprises the main part of the grain-size distribution of the lower Pleistocene reddish loess strata (Fig. 6B). However, in the welldeveloped palaeosol layers, the proportion of the fine dust component increases dramatically (Fig. 6D). The modal size of the coarse dust component exhibits an overall increasing trend from the bottom to the top of the section (Fig. 6C), which is similar to the long-term trend of the mean grain-size of the bulk sample (Fig. 6A) and is independent of that of the fine dust component (Fig. 6E). The modal size and proportion of the ultrafine component exhibit large fluctuations, with overall decreasing trends from the bottom to the top of the section (Fig. 6F, G). 5. Discussion 5.1. Trimodal grain-size distribution of the Pleistocene loess in the ILP and its implications for ancient dust dynamics in northern Iran Systematic grain-size analyses of the Pleistocene loess in the ILP reveals that the grain-size distribution of the Pleistocene loess is characterized by trimodal and bimodal distributions, and these
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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findings provide new evidence for reconstructing the ancient dust dynamic in northern Iran during the Pleistocene. The sources and aeolian processes involved in modern-day dust transport and deposition in northern Iran are unclear (e.g. Okhravi and Amini, 2001; Karimi et al., 2009; Kehl, 2010; Asadi et al., 2013). Generally, there are two types of dust process operating in the region, as indicated by our field observations and the evidence from large-scale satellite images (http://earthobservatory.nasa.gov/). Locally, the near-surface winds frequently transport silt-sized materials from the neighboring arid Caspian Lowland and the Karakum Desert to the ILP in short-distance suspension mode (Fig. 7A), and this process carries coarse silts to the ILP. The process also results in the formation of linear or barchans dunes in the arid Caspian Lowland (Fig. 7B), which grain-size distribution is dominated by an aeolian traction component (Fig. 3F). Regionally, large-scale high-level westerly winds, associated with the zonal Westerlies, carry fine dust from distant sources (i.e. the vast area of arid land in the Middle East) in long-distance suspension mode to the ILP (Fig. 7C, D); and this process carries fine dust particles to northern Iran. Generally, the combination of these two aeolian processes dominates the dust input to the ILP at the present day. The results for the Pleistocene loess in the ILP reveal that the grain-size distribution of the Pleistocene loess is characterized by trimodal and bimodal distributions. The coarse component, with a
modal size of 7e70 mm, is mainly transported by near-surface winds in short-distance suspension mode (Pye, 1987). This coarse dust component dominates the Pleistocene loess samples (Figs. 4 and 6), suggesting that the main component of the Pleistocene loess on the ILP is mainly transported by near-surface winds from near-source regions, such as the arid Caspian Lowland and the Karakum Desert (see Fig. 1B for location). The origin of the fine-grained component in loess sediments is relatively complex. Typically, there are three sources of fine dust in loess samples (Pye, 1987; Sun et al., 2011b): 1) from distant source regions, transported by large-scale high-level winds in long-term suspension mode; 2) from near-source regions adhering to coarse silt grains in short-distance suspension mode; and 3) produced during post-depositional pedogenesis, which results in the reduction in size of coarse grains. For the upper Pleistocene loess successions, the fine dust component has narrow modal size ranges and specific proportions (Fig. 3). The modal size is independent of the coarse dust component, suggesting that these grains are mainly derived from distant source regions. However, the grain-size boundary in the early Pleistocene loess is indistinct (Fig. 5), suggesting a complex origin of the fine dust particles; and we infer that these grains may be influenced significantly by pedogenic processes, and are derived from the comminution of coarse silt particles.
Fig. 7. Photographs and satellite image (http://earthobservatory.nasa.gov/) illustrating present-day aeolian activity in the ILP and adjacent areas. (A) Frequently-occurring local dust storm on the ILP; (B) aeolian sand dunes in the arid Caspian lowland; (C, D) large-scale dust storm in the ILP and surrounding regions.
Please cite this article in press as: Wang, X., et al., Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.058
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This suggests an overall intensification of wind strength during the Pleistocene. The clay fraction in a loess sample is mainly formed during postdepositional weathering and pedogenesis (Bronger and Heinkele, 1990), and is sensitive to regional climate change (Chamley, 1989). For the lower Pleistocene loessepalaeosol sequence in the AB1 section, the grain-size boundary between the ultrafine and the fine dust components is indistinct (Fig. 5E) and this results in uncertainties in differentiating the ultrafine and the fine dust components. Consequently, both the modal size and the content of the ultrafine component in the lower Pleistocene loessepalaeosol sequences exhibit large fluctuations which are not obviously related to the lithology (Fig. 6F, G). Under these circumstance the clay fraction (<2 mm) content, despite the fact that it may be systematically underestimated by the laser granulometry method, is a more reliable indicator of variations in the clay fraction. The clay content in the lower Pleistocene loess strata is higher than in the upper Pleistocene loess successions, and this long-term trend is consistent with that of the records of redness and Rb/Sr ratio (Wang et al., 2016). This suggests that a relatively drier and possibly colder climate characterized the ILP during the late Pleistocene. The longterm aridification trend in the ILP, since ~2.4 Ma, is probably driving by the stepwise expansion of northern hemisphere ice sheets (Wang et al., 2016).
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Fig. 8. Time series of mean grain-size, modal size of the coarse dust component, and proportion of the ultrafine component in the Pleistocene loess strata. The basal age of the upper Pleistocene loess succession in the Agh Band section was determined by pIRIR290 dating (unpublished data). Note that the content of the clay fraction may by systematically underestimated by the laser diffraction method (Konert and Vandenberghe, 1997).
The coarse dust component dominates the Pleistocene loess from the ILP, suggesting that this component is mainly transported by near-surface winds in short-distance suspension mode. The common occurrence of fine dust components in the samples suggest the high-level large-scale wind regim, likely associated with the zonal Westerlies, also contributed to the dust input in north Iran during the late Pleistocene. 5.2. Palaeoclimatic implications of the variations in grain-size and related parameters The grain-size distribution, especially the mean grain-size, of loess deposits is traditionally used as a proxy for palaeo-wind strength and/or the distance from the studied sites and the dust sources (Ding et al., 1994, 2002; Xiao et al., 1995; Porter and An, 1995; Chen et al., 1997; Sun et al., 2002, 2010; Vandenberghe, 2013), despite it may potentially be influenced by aridity in the source regions. Throughout the Pleistocene loessepalaeosol sequences in the ILP, the mean grain-size maintained low values during the early Pleistocene and then increased systematically during the late Pleistocene (Fig. 8). This long-term trend suggests an intensified palaeo-wind strength and/or decreased distance from the dust sources. The coarse dust in loess deposits, with modal size ranges from 7 to 70 mm, is mainly transported by near-surface winds in short-distance suspension episodes (Pye, 1995); and thus it reflects near-surface wind intensity, despite the fact that it may also be influenced by the distance from the source areas (Sun et al., 2008). The modal size of the coarse dust component in the upper Pleistocene yellowish loess strata is systematically coarser than that of the early Pleistocene loessepalaeosol sequences, and is similar to the long-term trend of the U-ratio (Wang et al., 2016).
6. Conclusions 1) The grain size of the Pleistocene loess from the ILP is characterized by trimodal and bimodal distributions, with a wellsorted coarse dust component (ca. 7e75 mm), a poorly-sorted fine component (ca. 2e7 mm), and a minor ultrafine component (<2 mm). 2) The dominance of coarse dust components in the samples suggests that the Pleistocene loess on the ILP is mainly transported in short-distance suspension mode by near-surface winds; however, the common occurrence of a fine dust component indicates that the large-scale high-level winds also contribute to the dust input on the ILP. 3) The modal size of the coarse dust component in the upper Pleistocene yellowish loess strata is systematically coarser than in the early Pleistocene loessepalaeosol sequences, suggesting a strengthening of the wind intensity during the Pleistocene. Acknowledgements We are grateful to Prof. D.H. Sun for analyzes grain size data, Z.W. Ma and B.Q. Liang for assistance with the laboratory work, and Prof. J. Bloemendal for improve the English usage. Financial support for this research was provided by the National Natural Science Foundation of China Grant (grant no. 41302144, 41130102), the Chinese 111 Project (grant no. B06026), the Fundamental Research Funds for the Central Universities (grant no. lzujbky-2015-129), and the Open Foundation of MOE Key Laboratory of Western China's Environmental System, Lanzhou University (grant no. lzujbky2013-bt01). References An, Z.S., 2014. Late Cenozoic Climate Change in Asia. Springer, Dordrecht. An, Z.S., Kutzbach, J.E., Prell, W.L., Porter, S.C., 2001. Evolution of Asian monsoons and phased uplift of the HimalayaeTibetan plateau since Late Miocene times. Nature 411, 62e66. Asadi, S., Moore, F., Keshavarzi, B., 2013. The nature and provenance of Golestan loess deposits in northeast Iran. Geological Journal 48, 646e660. Ashley, G.M., 1978. Interpretation of polymodal sediments. The Journal of Geology 86, 411e421.
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