4.1. Grain-Size Distribution of the Modeled Loess Components
“Typical” loess sediments are characterized by fine-skewed GSDs, with modal grain sizes in the silt to very-fine-sand range [e.g., Pye, 1995]. In general, one can state that there is consensus on the significance of sand- and silt-sized particles in loess sediments, namely that they represent primary dust particles supplied to the deposition site primarily by saltation and suspension processes. However, various interpretations of the origin of clay-size particles and the significance of the clay content in loess sediments can be found in the literature. Post-depositional weathering and pedogenesis is often held responsible for alteration of the loess GSDs [e.g., Xiao et al., 1995; Porter and An, 1995; Kemp, 2001; Sun et al., 2006]. Chemical weathering of unstable minerals (e.g., feldspar) will result in the transfer of grains from the silt and sand fractions to the clay fraction, as a result of breakdown of these minerals and in-situ clay mineral formation, eventually resulting in the enrichment of clay-size particles. Following this line of reasoning, various studies have used the clay content of loess deposits as a proxy for summer-monsoon rainfall intensity, which is thought to be the main driving mechanism of post-depositional weathering and pedogenesis [e.g., Feng and Wang, 2006, and references therein]. Others suggested therefore to use the GSD of the weathering-resistant quartz fraction as a more reliable proxy indicator of winter monsoon strength [e.g., Xiao et al., 1995; Porter and An, 1995; Sun et al., 2006]. Sun et al.  determined the degree of pedogenic modification of two loess-paleosol and red clay sequences located on the Loess Plateau by comparing the bulk-sample GSDs and the quartz GSDs. Their results indicate that the pedogenic modification of the glacial loess deposits (e.g., L1 and L2) is negligible, and that the effect of pedogenic modification is only significant in highly weathered interglacial paleosol units (e.g., S1 and S2) found at the southern Loess Plateau. In general, however, their results suggest that the loess-paleosol sequences discussed here have been subjected to only minor pedogenic alteration and that the loess deposits are predominantly of primary eolian origin.
Others assumed that clay-size particles in loess sediments represent primary dust grains, often citing studies which showed that the dust component in Pacific marine sediments exists of very-fine silty to clay-size particles [e.g., Ono and Irino, 2004, and references therein]. Recently, Sun et al. [2002, 2004], Sun  and Qin et al.  proposed a decomposition of loess GSDs into two or three end-member (lognormal or Weibull) distributions by parametric curve-fitting procedures. Parametric decomposition assumes that the end-members that make up an observed GSD are continuous unimodal distributions, which can be adequately described by analytical functions (e.g., lognormal, Weibull distributions) with a small number of parameters. Standard curve-fitting techniques may be used to decompose a single observed GSD into proportional contributions of analytical distribution functions belonging to a predefined class. This method cannot be used to simultaneously decompose a series of GSDs. The latter studies distinguished a silty loess component (with a mode at generally >20 μm) and one or two loess components within the clay fraction. Sun  distinguished one clay component referred to as the fine component, with a modal grain size varying between 2 and 10 μm. Qin et al.  recognized two components within the fine fraction referred to as the medium and fine modes, which have modal grain sizes in general between ∼3.5–5.5 μm and ∼0.5–1 μm, respectively. Sun  interpreted the fine component as the background dust load of the atmosphere which is thought to be mainly transported by high-altitude westerly airstreams, and used the modal grain size of the fine component as a proxy for high-level westerly air stream intensity. The coarse, most abundant component is interpreted to be the product of dust storms generated by low-altitude northwesterly winds. Qin et al.  proposed a very complex model in which they relate the median grain size and the proportional contribution of the three modes to variations in aerodynamic forcing (lift force related to vertical wind and turbulence) during dust entrainment in the source area and turbulence intensity in the depositional area (in order to reveal the aerodynamic patterns and evolution of the dust source area and the dust depositional area).
As there is no compelling reason why end-members should fit into any particular class of parametric models, Vriend and Prins , Prins et al.  and Vriend et al. (submitted manuscript, 2007) applied a non-parametric inversion technique (the end-member modeling algorithm EMMA [Weltje, 1997]) to obtain mixing models of their loess grain-size records from the loess region in China. In contrast to curve-fitting algorithms, EMMA does not require any case-specific assumptions, i.e., the number of end-member GSDs and their shapes do not have to be specified. EMMA does not make use of parametric GSD models corresponding to continuous functions because it was designed to process categorical data (the order in which one places grain-size classes does not influence the modeling results). Non-parametric decomposition regards GSDs as spectra which record a combination of some initial state modified by various processes, notably mixing and selective transport. This method of decomposition needs an array of GSDs (it cannot operate on a single GSD), as EMMA has been designed to provide the simplest possible explanation of the observed variation among a set of compositions in terms of (un)mixing. EMMA exploits the covariance structure of grain-size classes across a series of GSDs which contains information on the mixing structure of the data set. End-member GSDs determined with EMMA are thought to represent an assemblage of grains referred to as “dynamic populations” that are likely to occur together because they respond in a similar way to the dynamics of sediment production and dispersal within the system [Weltje and Prins, 2003, 2007].
Not surprisingly, the non-parametric decompositions of loess GSDs presented by Vriend and Prins , Prins et al.  and Vriend et al. (submitted manuscript, 2007) resulted in very different end-member GSDs compared with the parametric decomposition methods described above. The mixing models presented in these studies have three end-members which are characterized by unimodal, fine-skewed GSDs with a pronounced tail of fine silt and clay particles. Pye  already indicated that care must be taken in interpreting the clay component of loess sediments, since a significant proportion of the finest grains are transported and deposited as silt- or sand-size aggregates held together by electrostatic forces, salts, or organic matter. Hence it is likely that the fine silt and clay particles recorded in the tail of the fine-skewed loess GSDs (and modeled loess components) have been originally present partly as aggregates and partly as individual particles, some of which adhered to the surfaces of larger grains. These aggregates have been disintegrated during pre-treatment of the loess samples and during subsequent grain-size analysis. The observations made by Pye  clearly indicate that care should also be taken when loess GSDs are decomposed by a parametric curve-fitting method.
4.2. Interpretation of Modeled Loess Components
Prins et al.  compared their modeled end-members with modern dust samples (data taken from Sun et al. ) in terms of their grain-size distributions and flux rates. This comparison strongly suggests that the modeled end-members, with their characteristic tails of fine particles, may indeed be regarded as primary, unaltered loess components. The clayey loess component (EM-3) appears to be very similar to the fine dust component supplied over the entire loess region, partly during major dust outbreaks in spring and early summer, but mainly as part of a background supply system active throughout the year. The silty loess component (EM-2) was found to be very similar to the dominant dust fraction supplied over the proximal parts of the Loess Plateau during major dust outbreaks in spring and early summer. No modern analogue was found for the sandy loess component (EM-1). However, from the composition of EM-1 and its temporal and spatial distribution pattern in the loess deposits it was concluded that this component reflects the dust fraction supplied during very strong dust outbreaks in the glacial periods and that it might have been partly transported in saltation, rather than in suspension as is the case for end-members EM-2 and EM-3. The spatial distribution patterns of the sandy loess component in the full glacial loess units L1-1 and L2-1, reflected by the contoured [EM1:(EM1 + EM2)] ratio plots presented here, indicate that it occurs in significant volumes only in a narrow zone on the northern Loess Plateau (ZJC, HX and YN sections), just south of the present-day desert margin of the Mu Us and Tengger Desert, and within the Huang Shui river valley on the Tibetan Plateau (TXD and LD sections). The proximity of the sandy deserts and the Huang Shui river valley, the two likely source areas of the sandy loess component (EM-1) found in the Loess Plateau and Tibetan Plateau sections, respectively, supports the idea that the transport mode of this component has been saltation, or more likely, a combination of saltation and short-term suspension.
4.3. Spatial and Temporal Distribution Patterns of the Modeled Loess Components
The grain-size records presented in this study are obtained from the loess region of the NE Tibetan Plateau and the Loess Plateau. The similarities between the mixing models presented by Prins et al.  and Vriend et al. (submitted manuscript, 2007), which are based on two subsets from the series of loess grain-size records, indicate that the “average” mixing model presented here might be regarded as representative for the vast loess region in northern China. Despite the highly variable grain-size characteristics of the studied loess and paleosol sediments, both in space and through geological time, the grain-size distributions are well described by this relatively simple mixing model. The mixing model describes the observed spatiotemporal grain-size variations by variations in the mixing coefficients of three loess components (end-members). The loess deposited at the proximal sites (ZJC, HX, YN) are mixtures of predominant the sandy and silty loess components (EM-1 and EM-2), whereas the intermediate (XF, LC) and especially the distal sites (XY, WB, DJ) are dominated by binary mixing of the silty and clayey loess components (EM-2 and EM-3). The overall spatial trends in end-member contributions thus clearly mirrors the transition from sandy loess to clayey loess across the Loess Plateau [e.g., Liu, 1985; Nugteren and Vandenberghe, 2004; Yang and Ding, 2004].
The unmixing results presented by Prins et al.  and Vriend et al. (submitted manuscript, 2007) in conjunction with loess accumulation rate estimates revealed that two contrasting dust supply patterns were active over the loess region of northern China during the last glacial-interglacial cycle (EM-1 and EM-2 versus EM-3). The fractionated dust flux results presented here support the existence of two dust-supply patterns as very similar results have now been obtained also for three additional loess-paleosol sequences (XF, WB, DJ) and for the penultimate glacial-interglacial cycle (recorded in HX, XF, XY, YN and LC sections). An episodic, highly variable sediment input pattern, dominant during glacial periods throughout the region and noticeable during interglacial periods only over the northern Loess Plateau, is reflected in the admixture of the silty and the sandy loess components (EM-1, EM-2). The distribution patterns of these two loess components, which mirror the spatial thinning and fining trends of the loess deposits, and to a lesser extent of the intercalated paleosols [e.g., Liu, 1985; Pye and Zhou, 1989; Lu and Sun, 2000; Nugteren and Vandenberghe, 2004; Yang and Ding, 2004; Ding et al., 2005], strongly suggests that a northwesterly wind system has been the dominant supplier. In northern China, prevailing near-surface winter monsoon winds from the northwest, generated by the Siberian anti-cyclone, play a crucial role in transporting dust from the desert areas of Mongolia and Gansu toward the Loess Plateau at present-day [e.g., Derbyshire et al., 1998; Sun et al., 2003; Ta et al., 2004]. End-members EM-1 and EM-2 thus represent the dust components which are transported by the northwesterly winter monsoon during major dust outbreaks in spring and early summer.
The coarse-grained loess components represent the main part of the dust supplied by the low-level winter monsoon during major dust outbreaks in spring and early summer. The proportional contribution of the coarse dust fraction to the loess sediment appears to be a first-order approximation of the flux rate of event dust, as the flux rate of the fine “background dust” fraction is approximately constant. The grain size of the coarse dust fraction is thought to be mainly a function of the strength of the winter monsoon circulation system and of the transport distance, i.e., the distance to the eolian dust source area. Ding et al. [1999, 2005] proposed to use the sand fraction content (>63 μm) of loess deposits as a proxy-indicator of the location of the southern margin of the sandy desert north of the Loess Plateau, i.e., an indicator of source-area proximity. The end-member modeling results suggest that this is largely correct, as EM-1 consist of ∼46% sand (>63 μm), and therefore is the dominant “sand carrier.” However, the results also indicate that a substantial part of the fine-sand grains (EM-2 consists of 16% “sand”) are transported in suspension, which may occur over a distance of several hundred kilometers. The EM distribution patterns indicate that the [EM-1:(EM-1 + EM-2)] ratio can be used on the Loess Plateau as a potentially more precise proxy-indicator of the proximity of the sandy-desert margin, i.e., can be used to reconstruct changes in the location of the boundary (“transitional zone”) between source (desert) and sink (loess deposits). Ding et al.  suggested that the migration of the desert margin is essentially controlled by the amount of summer-monsoon precipitation in the desert. However, it should be realized that variations in winter-monsoon strength might also play a role in the variable input of the sandy loess component across the Loess Plateau.
According to Prins et al.  and Vriend et al. (submitted manuscript, 2007), the relatively constant and low flux of the clayey loess component (EM-3) represents a background sedimentation pattern which has been dominant during interglacial periods, especially over the central and southern parts of the Loess Plateau. As mentioned earlier, the clayey loess component (EM-3) resembles the fine dust fraction which is supplied over the entire loess region in modern times, partly during major dust outbreaks in spring and early summer, but mainly as part of a background supply system active throughout the year. The “background character” of EM-3 is clearly seen in the relatively uniform flux-distribution patterns (Figure 11) reconstructed for the interstadial (e.g., L1-2) and interglacial periods (e.g., S1). These observations suggest that non-dust storm processes may have been the dominant supplier during these time periods [Zhang et al., 1999; Prins et al., 2007]. Ding et al.  noticed that the Tertiary fine-grained red clay deposits of the Chinese Loess Plateau, which underlie the Quaternary loess sequences, do not show a southward decrease in grain size along north–south transects. The relatively uniform sedimentation pattern of the red clay let them suggest that the fine dust may have been transported and deposited by westerly air streams. Similarly, the uniform flux pattern recorded in the interstadials/interglacials MIS 3 and 5 suggest that the high-level subtropical jet stream (westerly winds) might, at least partly, be responsible for the input of the fine-grained loess component.
The “background character” of EM-3 is less clear during the glacial periods (e.g., L1-1 and L1-3), where a systematic north-to-south increase in the flux of the clayey loess component is observed. This suggests that during glacial periods a significant portion of this component has been supplied by major dust outbreaks, which dominantly deposited the EM-1 and EM-2 components on the northern and central parts of the Loess Plateau, and the finer-grained EM-3 component to the southern part of the loess region (thereby largely by-passing the northern part of the Loess Plateau). However, the “exceptional high” flux rates of EM-3 on the southern part of the Loess Plateau may have been favored by some additional geological factors which are explained below.
A factor which may have enhanced the deposition of the clayey loess component is the effect of the Qin Ling and Luliang Shan mountains, acting as a topographic barrier enhancing loess accumulation on the windward, northward side of the mountain range [Pye, 1995]. Moreover, local dust sources may also have contributed in the increased dust fluxes recorded at the southern sites. Exposed fluvial sediments in the floodplains of the Wei He and its tributaries may have significantly enhanced dust flux rates at sites located proximal to these river systems, like for instance the WB site which is located on a fluvial terrace of the Wei He [Nugteren and Vandenberghe, 2004]. Exceptional high flux rates at this site of the silty loess component during MIS 2 (L1-1), and to a lesser degree during MIS 3 (L1-2), support this hypothesis. Additional sediment supply from local dust sources may also partly explain the enhanced flux rates of the clayey component during MIS 2 and 4 (L1-1 and L1-3, respectively) at the southern sites (XY, WB, DJ). Finally, deposition of the fine dust particles may have been enhanced by a dense vegetation cover on the southern plateau which acted as an effective dust trap. The transition from a sparse (or even lack of) vegetation cover in the north toward a denser cover in the south is the result of the northwest-to-southeast transition from (semi-) arid to more humid conditions. At this stage, however, it is very difficult to quantify the relative importance of these factors. Independent dust provenance data of the fine loess fraction are needed to determine the exact origin and further underpin the paleoclimatic significance of the clayey loess component.