EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms (2015) Copyright © 2015 John Wiley & Sons, Ltd. Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.3883 Paleotopographic controls on loess deposition in the Loess Plateau of China Li-Yang Xiong,1,2,3,4,5 Guo-An Tang,1,2,3* Josef Strobl4 and A-Xing Zhu5 1 Key laboratory of Virtual Geographic Environment, Ministry of Education, Nanjing Normal University, Nanjing, 210023, China 2 State Key Laboratory Cultivation Base of Geographical Environment Evolution (Jiangsu Province), Nanjing, 210023, China 3 Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing, 210023, China 4 Department of Geoinformatics – Z_GIS, University of Salzburg, Salzburg 5020, Austria 5 Department of Geography, University of Wisconsin-Madison, Madison, USA Received 16 August 2015; Revised 1 December 2015; Accepted 2 December 2015 *Correspondence to: G.-A. Tang, Key laboratory of Virtual Geographic Environment, Ministry of Education, Nanjing Normal University; Nanjing 210023, China. E-mail: [email protected] ABSTRACT: The underlying pre-existing paleotopography directly influences the loess deposition process and shapes the morphol- ogy of current loess landforms. An understanding of the controlling effects of the underlying paleotopography on loess deposition is critical to revealing the mechanism of loess-landform formation. However, these controlling effects exhibit spatial variation as well as uncertainty, depending on a study’s data sources, methodologies and particular research scope. In this study, the geological history of a study area in the Loess Plateau of China that is subject to severe soil erosion is investigated using detailed geological information and digital elevation models (DEMs), and an underlying paleotopographic model of the area is constructed. Based on the models of modern terrain and paleotopography, we introduce a watershed hierarchy method to investigate the spatial variation of the loess- landform inheritance relationship and reveal the loess deposition process over different scales of drainage. The landform inheritance relationships were characterized using a terrain-relief change index (TRCI) and a bedrock terrain controllability index (BTCI). The results show that the TRCI appears to have an inverse relationship with increasing research scope, indicating that, compared with the paleotopography of the region, modern terrain has lower topographic relief over the entire area, while it has higher topographic relief in the smaller, local areas. The BTCI strengthens with increasing drainage area, which demonstrates a strong controlling effect over the entire study area, but a weak effect in the smaller, local areas because of the effect of paleotopography on modern terrain. The results provide for an understanding of the spatial variation of loess deposition in relation to paleotopography and contribute to the development of a process-based loess-landform evolution model. Copyright © 2015 John Wiley & Sons, Ltd. KEYWORDS: loess deposition; paleotopography; spatial variation; watershed hierarchy Introduction on the underlying paleotopography is thus critical to a better understanding of its formation mechanism and landform evolu- In the loess deposition process, dust moves and accumulates tion process. on underlying paleotopography, playing a key role in shaping There has been extensive research on the loess dust transport the surface morphology of loess regions, which exist across and deposition rate, and many methods have been developed the globe (Bradley, 2015). The loess landforms of the Chinese to understand the process of loess dust deposition on underly- Loess Plateau, the vast arid desert areas in Central Asia, and ing paleotopography. The literature includes studies that have the Yellow River system, together form a huge geomorphic unit looked at the origin of loess dust transport using mineralogical that extends over an area of ~500 000 km2, almost completely and geochemical methods (Schaetzl, 2008; Muhs et al., 2013; sitting on underlying paleotopography from the Quaternary pe- Zhang et al., 2013), OSL dating to find the loess deposition rate riod (Liu et al., 2001; Lu et al., 2011). Evolved over 2.6 million (Lai et al., 2007; Chen et al., 2013; Stevens et al., 2013a; Yang years in the East Asian monsoon climate, loess deposits have et al., 2014; Youn et al., 2014) and loess relief reconstruction been differentially deposited and eroded in the windward slope through a pedological analysis (Rodzik et al., 2014). Studies and the leeward slope, and have shaped the well-known spe- of the loess and palaeosol layer have explored the relationship cific loess landscapes (Xiong et al., 2014a, 2014b). This unique between climate change and the loess deposition process, and formation mechanism reflects the loess deposition process, be- are prominent in loess-landform research. These include a cause the specific and diverse loess landforms are the conse- comparison of loess-palaeosol layers in China and Europe quence of its spatial variation. A deeper analysis of the loess (Vasiljević et al., 2014), periodic evidence of loess layers and deposition process and its spatial variation and dependency palaeosol layers in rare loess-deposition areas (Hobbs et al., 2011), L.-Y. XIONG ET AL. vertical variations in the luminescence sensitivity of quartz 2014). However, the effect of the original, underlying pre- grains from the Luochuan loess-palaeosol section (Lü et al., quaternary surface, before the loess deposition evolved, has re- 2014), paleoenvironmental changes that are revealed by the ceived scant attention. The morphology and distribution of the interaction of loess sediment distribution and loess-paleosol paleotopography may have significantly affected spatial variation layers (Pan et al., 2012; Kühn et al., 2013; Lehmkuhl et al., in the loess deposition process. Although there have been studies 2014; Wang et al., 2014), and the influence of climate on the of the spatial variations in loess deposition that are influenced by loess deposition process, using detrital remanent magnetiza- hydrologic and geologic factors (Rokosh et al., 2003), there are tion (Wang and Løvlie, 2010). The severe soil erosion in the still few studies looking at how the underlying paleotopography area has also led to frequent analysis (Stolte et al., 2003; Yang controls the loess deposition and its spatial variation in the Loess et al., 2006; Liu and Liu, 2010; Superson et al., 2014; Geng Plateau. It is still not clear if, during the loess deposition process, et al., 2015) and studies of pedogenic processes (Eger et al., the modern terrain has a lower or higher topographic relief than 2012). the original paleotopography at different scales (Figure 1). Also Previous studies have paid more attention to where the loess unclear is how the original paleotopography controlled the deposits come from, the deposition rate and how relevant the evolution of the modern loess landform. Hence, there is a need loess deposition process is to paleoclimate evolution (Guo to examine the effect of scale on the spatial variation of loess et al., 2002; Pan et al., 2012; Stevens et al., 2013b; Lü et al., deposition in relation to paleotopography. Figure 1. Terrain relationships at different scales of scope. (a) Modern terrain has higher topographic relief compared with original paleotopography at a microscale. (b) Modern terrain has lower topographic relief compared with original paleotopography at a macroscale. This figure is available in colour online at wileyonlinelibrary.com/journal/espl Figure 2. Study area. Area A is a zone of severe soil erosion in the Loess Plateau, Area B is a small area at the intersection of the Wu-Ding River and the Yellow River. This figure is available in colour online at wileyonlinelibrary.com/journal/espl Copyright © 2015 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2015) PALEOTOPOGRAPHIC CONTROLS ON LOESS DEPOSITION With the development of the multiple data source acquisition relationships and to further reveal spatial variation in the loess de- and digital terrain analysis method (Geomorphometry; Evans, position process according to different drainage scales. 2012), it is possible to reconstruct pre-existing underlying paleotopography (Alexander et al., 2008; Campani et al., 2012; Perron and Fagherazzi, 2012; Castillo et al., 2014; Xiong et al., Study Area 2014c; Bergonse and Reis, 2015). A comparative analysis of both dual-layer terrains, including modern terrain and paleotopography, The main study area (Area A) (Figure 2) is a zone of severe soil could help to reveal the landform evolution process. In addition erosion in the upper-middle reach of the Yellow River basin to the pixel-based and rectangular block-based terrain analysis (Upper and Middle Yellow River Bureau, 2012) with a geolog- that uses DEM to describe the spatial variation of landform char- ical history that is relatively tectonically stable, located in the acteristics, a real-landform object or unit-based method could Ordos platform surrounded by mountains and rift valleys (Liu, become a standard method in landform analysis. Just as the 1985; Hu et al., 2012; Pan et al., 2012; Yuan et al., 2012; Xiong watershed has been regarded as the principal hydrologic unit et al., 2014c). The area is between 106.4°–12.7° E and 34.29°– for sediment movement in fluvial geomorphology (DeBarry, 40.11°N, with a total