From dust to dust: Quaternary wind erosion of the Mu Us Desert and Loess Plateau, China Paul Kapp1, Alex Pullen1,2, Jon D. Pelletier1, Joellen Russell1, Paul Goodman1, and Fulong Cai1* Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA 1 2 ABSTRACT The Ordos Basin of China encompasses the Mu Us Desert in the northwest and the Chinese Loess Plateau to the south and east. The boundary between the mostly internally drained Mu Us Desert and fluvially incised Loess Plateau is an erosional escarpment, up to 400 m in relief, composed of Quaternary loess. Linear ridges, with lengths of ~102–103 m, are formed in Cretaceous–Quaternary strata throughout the basin. Ridge orientations are generally parallel to near-surface wind vectors in the Ordos Basin during modern winter and spring dust storms. Our observations suggest that the Loess Plateau previously extended farther to the north and west of its modern windward escarpment margin and has been partially reworked by eolian processes. The linear topography, Mu Us Desert internal drainage, and escarpment retreat are all attributed to wind erosion, the aerial extent of which expanded southeastward in China in response to Quaternary amplification of Northern Hemisphere glaciation. INTRODUCTION The ~750 km (north-south) by ~450 km (east-west) Ordos Basin in China is bound by late Cenozoic rift-flank mountain ranges (Zhang et al., 1998) and encompasses the Mu Us Desert in the northwest and a large portion of the Chinese Loess Plateau in the south and east (Fig. 1). The Yellow River flows northward into the basin along the Yinchuan graben, eastward along the Hetao graben, southward through the eastern Loess Plateau to where it joins the Wei River, and then exits the basin to the east (Fig. 1); it may have followed this course since at least 2 Ma (Craddock et al., 2010; Pan et al., 2011). Much of the deflationary Mu Us Desert exhibits internal drainage and closed topographic depressions (Fig. 2). It exposes mostly Mesozoic bedrock in its western part and variably active and stabilized dune fields above Mesozoic bedrock in its eastern and southern parts (Fig. 1; Li, 2006). In stark contrast, the Chinese Loess Plateau is composed of Earth’s largest accumulation of Quaternary loess and is strongly incised by the Yellow River and its tributaries (Figs. 1 and 2). The loess strata are as much as several hundreds of meters thick and commonly interlayered with paleosols. Loess accumulation occurred primarily during glacial periods when Central Asia was colder and drier, whereas paleosols developed during interglacial periods when the East Asian Monsoon penetrated farther inland (Tungsheng and Zhongli, 1993; Porter, 2007). There are several motivating questions for this study. What is the nature of the geomorphic boundary between the Mu Us Desert and the Loess Plateau? How did this boundary evolve during the Quaternary? How was the Loess Plateau built? Central Asia became more arid during the Quaternary, concomitant with the increase in Northern Hemisphere ice volume (Tungsheng and Zhongli, 1993), and likely resulted in net desert expansion. Based on an upsection increase in the size and amount of sand in loess along the northern margin of the central Loess Plateau, Ding et al. (2005) proposed that the desert region was located ~200 km farther windward (inland) at the onset of the Quaternary compared to its position during the last glacial period. If correct, this implies a spatial migration in regions characterized by net eolian erosion versus accumulation and a retreating windward margin of the Loess Plateau. To test this hypothesis, we investigated the geology and geomorphology of the Ordos Basin (Fig. 1), with emphasis on mapping landforms in the field and with satellite imagery. We also compared wind patterns resolved from the geomorphology with modern near-surface wind vectors observed seasonally and during dust storms to evaluate our observations within a climatologic context. Our findings demonstrate the importance of wind erosion in sculpting local and regional topography, generating internal drainage, and simultaneously building and reworking a loess plateau. LOESS PLATEAU WINDWARD ESCARPMENT The spatial transition in the Ordos Basin from bedrock erosion to dunes to loess in the windward (and increased-precipitation gradient) direction (Fig. 1) is observed globally and is an intuitive pattern. Remaining enigmatic, but also widely documented, are abrupt transitions between regions of wind erosion and thick loess accumulation (e.g., Mason et al., 1999). The boundary between the Mu Us Desert and Loess Plateau provides an impressive example. Locally along the western margin of the eastern Loess Plateau, tributaries of the Yellow River mark boundaries between sand dunes of the Mu Us Desert and Loess Plateau strata, and by forming a barrier to sand transport, may contribute to proximal thick loess accumulations down- BEDROCK WIND EROSION IN THE MU US DESERT Wind streaks and dune geometries in the Mu Us Desert indicate westerly to northwesterly geomorphically effective wind directions (black arrows in Fig. 1); these are approximately parallel surface wind vectors that were recorded during modern wind-storm events (Liu et al., 2005; Mason et al., 2008), which are most frequent during spring (Roe, 2009). The northwestern Mu Us Desert locally exposes linear ridges of *Current address: Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. GEOLOGY, September 2015; v. 43; no. 9; p. 835–838 weakly cemented Cretaceous strata with meters to several tens of meters of relief, and lengths ranging from hundreds of meters to several kilometers (Fig. 3A). The distribution and orientations of the ridges are indicated by the red shading and red arrows in Figure 1; the red arrows show mean orientations of multiple individual ridges, the number of which scale inversely with their size. Many of the ridges exhibit steep windward faces and/or show evidence of being streamlined in map view, and thus can be classified as yardangs, whereas other linear bedrock ridges are not obviously streamlined. Where present, elongate troughs between the ridges are variably floored by bedrock (Fig. 3A), vegetation, or dunes (both active and vegetation stabilized). Linear bedrock ridges and locally welldeveloped fields of yardangs are also present along the windward and leeward margins of an approximately north-south–aligned, ~60-kmlong, ~50-m-high, and to 8-km-wide mesa of Cretaceous bedrock west of Otog city (Fig. 1; Figs. DR1A and DR1B in the GSA Data Repository1). East of Otog Mesa is an ~8-km-wide mesa-parallel trough, presumably wind excavated, and then dunes of sand at higher elevation (Fig. 2B; Fig. DR1A). These observations indicate the importance of eolian processes in sculpting the landscape of the Mu Us Desert. | Data Repository item 2015283 | doi:10.1130/G36724.1 © 2015 Geological Society America. permission to copy, contact editing@geosociety.org. GEOLOGY 43 | ofNumber 9 For | Volume | www.gsapubs.org Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/43/9/835/3548979/835.pdf by Utrecht University Library user 1 GSA Data Repository item 2015283, figures showing satellite images, rose diagrams, and maps of near-surface wind vectors, is available online at www.geosociety.org/pubs/ft2015.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. | Published online 28 July 2015 835 within the Mu Us Desert and the incised Loess Plateau (Figs. 1 and 2), and in many places forms a drainage divide along which wind gaps are present (Fig. 3B) as a result of stream capture. The Miocene Red Clay Formation, which underlies Loess Plateau strata in many places, is locally exposed in the Mu Us Desert adjacent to extensions of the escarpment (Fig. 3B; Li, 2006; our observations). Figure 1. A: Location map of Ordos Basin, China. Stippled pattern indicates sand deserts. B: Shaded relief map (www.geomapapp.org) of Ordos Basin. Yellow contours of mean annual precipitation (in mm) are from Porter et al. (2001). Red shading and arrows show the distribution and orientation of linear bedrock ridges in the Mu Us Desert and linear Loess Plateau topography. Black arrows show geomorphically effective wind directions, based on our interpretations of satellite images. The A-A’ and B-B’ dashed lines correspond to topographic profiles in Figure 2. Approximate distribution of Mesozoic and Cretaceous strata is shown. C: Rose diagram of linear loess topography orientations, plotted as unidirectional (wind parallel). Dark gray population is representative of windward margin of Loess Plateau. Light gray population is representative of linear topography to the south and east. Mean azimuth values and one standard deviations are indicated. wind (Mason et al., 1999). In many other places, however, the boundary is an escarpment within loess, hundreds of meters high along the northern margin of the central Loess Plateau (Fig. 2A) and less pronounced but still identifiable in the east (Fig. 2B). There is no indication that the escarpment is a barrier to sand transport or related to Quaternary faulting (Zhang et al., 1998; our observations). The escarpment roughly follows the boundary between internal drainage 836 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/43/9/835/3548979/835.pdf by Utrecht University Library user LINEAR LOESS PLATEAU TOPOGRAPHY Superimposed on the dendritic incision pattern of the Loess Plateau (Fig. 1) is a smallwavelength (<1 km) linear topographic fabric defined by kilometer-scale-long aligned ridges and parallel valleys (Fig. 3B) in the red shaded regions of Figure 1. The ridges are rilled and the valleys show evidence of fluvial incision. In places the linear topography is prominent and ubiquitous (Figs. DR2A–DR2D), whereas in others it is spatially patchy and/or more cryptic at the 10 km scale, but the overall linear orientation is still resolvable (Figs. DR2E–DR2G). Quaternary loess in the United States locally exhibits similar linear topography; although it is debated whether it is primarily of depositional or erosional origin (Flemal et al., 1972), it is accepted to be wind parallel and the role of wind erosion has been demonstrated (Sweeney and Mason, 2013). The linear topography is developed within the Malan Loess of the last glacial period, and therefore must have (or continued to have) formed since that time. To document spatial variations in linear loess topography orientation, we mapped orientations in Google Earth (n = 2869 localities), spaced across the area of the Loess Plateau where linear topography is evident. A rose diagram of all orientation measurements is shown in Figure 1C, and Figure DR3 shows rose diagrams for ~1° × 1° geographic areas. The red arrows in Figure 1 show the dominant orientation of the linear loess topography, which generally varies <5° at the scale of ~10 km (Figs. DR2A–DR2E) and deviates 5°–10° from a mean value at the scale of ~100 km (Fig. DR3). Exceptions are where there are spatially abrupt variations in linear topography orientation between the two distinct azimuth populations (Fig. 1C). The linear loess topography is oriented 118° ± 14° (mean ± one standard deviation) along the windward margins of the Loess Plateau, parallel to the geomorphically effective wind directions in the adjacent Mu Us Desert (Fig. 1). Over a distance of <10 km, the linear topography orientation rotates clockwise to a north-south azimuth (179° ± 11°; mean ± standard deviation) over the central Loess Plateau (Figs. DR2F–DR2H), and the eastern Loess Plateau where it abuts with the Luliang Mountains (Fig. 1). To evaluate whether the wind directions resolved from the geomorphology are consistent www.gsapubs.org | Volume 43 | Number 9 | GEOLOGY Elevation (m) Elevation (m) 1800 A (N) former extent of Loess Plateau? 1400 Yellow River rock ous bed 1000 Cretace 600 200 1800 0 50 B (NW) 1400 600 escarpment dune fields Mu Us Desert internally drained A 1000 (S) A’ Otog trough Otog Mesa 50 B 0 150 250 escarpment dune fields Yellow River loes s loess incised 250 300 350 Mu Us Desert internally drained 100 150 200 (SE) B’ distance along profile (km) wind-parallel ridge Cretaceous sandstone 60 m 450 eou 650 750 Figure 2. Topographic profiles across Ordos Basin, China, along A-A’ and B-B’ (see Fig. 1). A: Approximately north-south profile showing the topography of the Loess Plateau where it is least incised, its escarpment margin, and its possible former extent across the Mu Us Desert. B: Approximately east-west profile across Otog Mesa and trough, and the escarpment margin of the eastern Loess Plateau. N drainage divide 37°24’N wind gaps n pressio d de s-floore 550 distance along profile (km) d se n clo ssio e pr de dunes on leeward face Cretac Loess Plateau fluvially incised B Mu Us Desert 30 m A 350 Wei graben c es t en m p ar Loess Plateau linear loess topography wind gaps107°E with the modern climatology, we compared them to observed near-surface (10 m height) wind vectors averaged over different seasons from A.D. 1979 to 2010 and over a 6 h time period of maximum wind speed during spring windstorm events (Fig. 4; Figs. DR4A–DR4G). The seasonal wind pattern most similar to the orientations of the linear topography is that of winter (Fig. DR4D). It shows the first-order clockwise rotation in wind vectors over the Ordos Basin, but with a stronger westerly component over the Mu Us and northern half of the central Loess Plateau. The four spring wind-storm events analyzed were all characterized by northwesterly winds over the Mu Us Desert, consistent with the geomorphology, but variable winds over the Loess Plateau, from northwesterly, to northerly, and even southwesterly to southerly (Fig. 4; Figs. DR4E–DR4G). The wind-storm event of 14 March 2010, however, shows a wind pattern strikingly similar to that indicated by the geomorphology (Fig. 4). Additional comparative studies are needed, but our preliminary analysis implicates modern wind storms in sculpting the eolian geomorphology. HYPOTHESES AND IMPLICATIONS We propose that at the onset of the Quaternary, Loess Plateau strata extended farther across the Ordos Basin (Fig. 2A), consistent with the subsequent ~200 km of Mu Us Desert expansion suggested from loess grain-size studies (Ding et al., 2005). The increase in Northern GEOLOGY | Volume 43 | Number 9 | 10 km Hemisphere ice volume during the Quaternary may have brought more frequent and farther southeastward-reaching cold air surges that generated dust storms in Central Asia (Roe, 2009) and concomitantly limited the penetration of the East Asian Monsoon into Central Asia, which in turn would decrease the spatial extent of dusttrapping vegetation cover (Ding et al., 1999). Consequently, regions of former loess accumulation transitioned into regions dominated by wind erosion. We attribute the development of the linear bedrock ridges and closed depressions in the Mu Us Desert and the Loess Plateau windward escarpment and linear topography to wind erosion. The paleo–Yellow River provided a continuous supply of sand that could be reworked by wind (Stevens et al., 2013). The Loess Plateau escarpment retreated as it was sandblasted while loess continued to accumulate downwind. At a spatial scale greater than that of the scale of regions of closed drainage (kilometers to tens of kilometers), the Mu Us Desert exhibits a subtle decrease in elevation as it approaches the Loess Plateau escarpment (Fig. 2). Here, wind erosion is enhanced because of wind speed acceleration associated with streamline compression over the escarpment (e.g., Jackson and Hunt, 1975) in combination with the higher erodibility of the loess compared to the underlying Red Clay Formation and older bedrock. We propose that through localized scour, wind erosion helped generate internal drainage in the Mu Us Desert www.gsapubs.org Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/43/9/835/3548979/835.pdf by Utrecht University Library user 37°12’N 107°20’E Figure 3. A: Wind-parallel linear bedrock ridges and adjacent bedrock-floored depression, Mu Us Desert (location indicated in Fig. 1B). B: Google Earth™, image of linear loess topography and Loess Plateau escarpment (location indicated in Fig. 1). The top of the escarpment is a drainage divide (blue line). Blue circles indicate locations of wind gaps. Coordinates are 37.294°N, 107.394°E (image date 17 March 2013). at a range of spatial scales (Fig. 2). Where the escarpment forms a drainage divide and exhibits wind gaps (e.g., Fig. 3B), the late Quaternary rate of escarpment retreat exceeded that of headward river incision, such that the upper reaches 106°E 112°E 41°N 35°N 12 m/s Figure 4. Near-surface (10 m height) wind vectors averaged over a 6 h time period of maximum wind speed during a dust storm event over the Loess Plateau on 14 March 2010 (7 a.m. Greenwich Mean Time). Data are from Saha et al. (2010). Black box indicates location of Figure 1. 837 of formerly southward- and eastward-flowing rivers were beheaded and reversed flow direction. The envisioned process at the million-year time scale is analogous to using a leaf blower to generate an expanding region of wind erosion, a windward-retreating escarpment made of leaves, and a growing pile of wind-transported material downwind. At the shorter time scale, the history was more complex, as regions transitioned between net eolian erosion and sediment accumulation in response to glacial-interglacial and millennial time scale climate variability (e.g., Sun and Ding, 1998; Ding et al., 1999; Xiao et al., 2002; Lu et al., 2005; Zhou et al., 2009). This explains local preservation of Quaternary paleosol and loess in the Mu Us Desert, presumably where they have been shielded from wind erosion by topography or vegetation, and eolian sand interbedded with loess along the windward margins of the Loess Plateau (e.g., Sun and Ding, 1998). Given that the linear loess topography developed since the last glacial period, and that the surface wind pattern in the Ordos Basin resolved from the geomorphology is similar to surface wind patterns observed during at least some modern spring wind storms (Fig. 4), the Loess Plateau should be considered today as not only a sink, but also a source of dust. We anticipate that wind erosion of the Loess Plateau was more severe during glacial periods when it was less vegetated and subjected to a larger flux of windblown sand (Ding et al., 1999). This is consistent with the presence of deflation-attributed unconformities within some Loess Plateau sections (Lu et al., 2005; Stevens et al., 2006; Buylaert et al., 2008). A final implication pertains to provenance studies, a large number of which have been conducted on Loess Plateau strata with the aim of reconstructing potential glacial versus interglacial atmospheric circulation pattern variations, yet complications due to sediment reworking are only rarely considered (e.g., Stevens et al., 2013). Our proposal of extensive sediment reworking by wind in the Ordos Basin would serve to homogenize the composition of glacialage loess and dust in interglacial-age paleosol. Therefore, even if the loess and paleosols show an indistinguishable provenance (e.g., Che and Li, 2013), this does not negate the possibility of temporal-spatial variability in wind patterns and dust source regions. ACKNOWLEDGMENTS This research was supported by U.S. National Science Foundation grants AGS-1203427, AGS-1203973, and EAR-1323148. Comments by J. Mason, editor J. Spotila, T. Stevens, M. Sweeney, and three anonymous reviewers helped improve this manuscript. 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