Badlands and environmental change

Badlands and environmental change Badlands develop in many climatic regions, on a wide ränge of soüs and in various bedrock types. The physical triggers for development of badlands can be natural, such are tectonic activity and climate change, but more frequently they are human induced, e.g. land clearance to change use of land. The research presented here clearly indicates that clay mineralogy and type and amount of clay in the bedrock play a critical role in the development of surface crust and hillslope morphology and ultimately of badlands. Laboratory experiments on smectite-rich samples subjeeted to simulated rainfall have established a correlation between phased development of surface crust and desiccation cracks and duration of rainfall. A similar correlation could not be found for smectite-poor materials. In addition, evidence was collected on the different responses of smectite-rich and smectite-poor lithologies to wetting and drying periods. Thus, it appears in particular that drying periods play an important role in badland development on smectite-rich materials, an aspect which is directly linked to slope orientation and strongly sensitive to differences that occur with sensu stricto seasonal climatic changes.


Badlands and environmental change
Milica Kasanin-Grubin, Beigrade 1 Introduction Badlands develop in many climatic regjons and on a wide ränge of soils and bedrock (Bryan & Yair 1982;Howard 1994). The one general characteristic of badlands, regardless of location, is the presence of intensely dissected landforms accompanied by steep slopes, rills, gullies, and frequently, extensive pipe or tunnel erosion (Fig. la, b, c, d).
Badlands are often considered to be ideal Seid «laboratories» because their rapid formation allows close investigation of geomorphic processes (Bryan & Yair 1982). However, badlands can also be seen as ideal areas for furthering understanding of the scale and impact of environmental change, a factor which generally lies at the origin of badland initiation. Badland development is either caused by land use change, such as land clearance due to agricultural development or due to overgrazing, or by natural events like tectonic activity or climatic change (Table 1).
Badlands exist under different climatic conditions. Gallart et al. (2002) distinguishes between badlands in arid (precipitation <200 mm), semi-arid (precipitation 200-700 mm) and humid climates (precipitation >700 mm), each with a distinct set of processes, not least due to the differing presence of Vegetation. In arid climates which do not sustain Vegetation, geomorphic processes are controüed by bedrock and regolith properties, in semi-arid climates badland evolution confines plant growth by limiting water avaüabüity in thin regoliths, especially on south-facing slopes, while in badlands in humid climates, freezing rather than dryness, is important for plant growth (Gallart et al. 2002). which evolve rapidly, have steep rüled and gullied slopes, with mass movement as a main process on the side-slopes. Biancana, on the other hand, have gentler slopes with equally active surface and subsurface networks (pipes and subsurface cracks).They are characterised not only by rill erosion, but by mass movement and sheet wash as weü (Alexander 1982). Calanchi, which are usually much larger landforms, generally form in coarser Sediments like clayey silts and sandy clayey silts, while smaüer dorne type biancana have a very high clay content (Battaglia et al. 2002;Torri & Bryan 1997). Biancana Sediments also have a higher Na content (Alexander 1982;Battaglia et al. 2002). Battaglia et al. (2002) found that although sweüing clay content is roughly the same in materials from both landforms, the weathering profiles differ greatly: while they are only a few centimetres thick on biancana, a thick weathered profile is characteristic of calanchi (Vittorini 1977in Battaglia et al. 2002).
As mentioned above, badlands mostly develop in clay-rich lithologies. The relation of specific physical and chemical properties of these materials to erosion processes has been demonstrated in many studies (e.g. Bowyer-Bower & Bryan 1986;Bryan et al. 1978;Bryan et al. 1984;Gerits et al. 1987;Hodges & Bryan 1982;Imeson et al. 1982;Oostwoud Whdenes & Ergenzinger 1998;Yair et al. 1980). Properties such as shrink-swell capacity, slaking potential and dispersivity are controüed by soü texture, clay mineralogy and chemistry, thus strongly influencing the timing and location of runoff generation and the relative significance of surface and subsurface erosional processes (rill erosion and micro-piping). For this reason, Kasanin-Grubin (2006) and Kasanin-Grubin & Bryan (2007) have argued for the Controlling factor role that lithological properties, and in particular clay mineralogy, play in badland hülslope processes and especially in rill development.
Arid and semi-arid climates are most often associated with badlands, however, not all landscapes in this climate develop into badlands, and badlands can form in different climates as well. For this reason, fundamental attention should be gjven to badland materials (Campbell 1997). In badland areas with two or more different lithologies, erosion rates, slope properties and processes differ. This particular phenomenon was highlighted in the description by Schumm (1956) (Iasio et al. 2002;Pardini et al. 1995;Regues et al. 1995).
Temporal changes can produce marked changes in surface characteristics, as described in Schumm & Lusby (1963) on annual rül formation and obliteration, and seasonal change of processes on the smectite-rich Mancos shale, Colorado, USA. During winter, freezing and thawing transforms the less permeable rüled surface into a highly permeable surface without rüls. During spring and summer, compaction of the surface, runoff increase and rüls re-establishment can be observed. Scoging (1982) observed significantly less erosion during winter than summer in the Ugjjar badlands in SE Spain due to short, but high intensity storms in summer months. It was also noted that the last summer rains flush out dry surface material, reducing the amount of material that is ready for transport by winter rainfalls.
Seasonal changes in material response have been observed by Regues et al. (1995) and Regues & Gallart (2004) regarding weathering and erosion in the mountainous Vallcebre Badlands of the SE Pyrenees, Spain. Both studies were based on antecedent moisture and bulk density measurements. Physical weathering was found to be strongest during the winter due to frost action; during summer and fall, the material is easüy removed and erosion is most active. As a consequence, it appears that the Vallcebre Badlands' materials are subject to alternating periods of erosional and weathering activity with conditions being described as stable in autumn and spring and transitional in summer and winter (Regues & Gallart 2004). These authors also indicate a two-season delay between maximal weathering.
Observed erosional response was a consequence of the delay between the season with the strongest weathering and the season with the strongest erosion Slopes with different aspects have various radiation receipts. They also receive different precipitation inputs, which vary with each storm and are influenced by prevailing wind conditions. Aspect-related differences in slope characteristics are likeüer to occur in arid areas because of the more critical nature of moisture conditions here than in areas with abundant moisture (Churchill 1981). In the Brule Formation Badlands, South Dakota, USA, south-facing slopes, which are subjected to more intense wetting and drying, are significantly shorter, steeper and generally straighter in profile than their north-facing counterparts (Churchill 1981). In contrast, the north-facing slopes are densely rilled with deeper regoliths due to deeper infiltration. In the Zin B adlands, Negev, Israel, north-facing slopes have rough, liehen covered surfaces with deep regoliths, while their south-facing counterparts are smoother with greater runoff rates (Yair et al. 1980). Similarly, in the Dinosaur Park Badlands, Alberta, USA, north-facing slopes retain snow longer and have moister regoliths (Harty 1984). Despite the fact that the significance of slope aspect as a control factor of erosion processes has been described in these and other areas, it has not been clearly identified whether the erosional and weathering processes are predominantly climate driven or if the critical variable are the lithological physico-chemical properties.
3 Lithological properties: clay minerals and weathering Most badland lithologies are clay-rich materials and their behaviour is controüed by the type and amount of clay minerals present. Clay minerals are fine-grained with size particles <2|xm. Due to their sheet shape, they have a very large surface area.The clay minerals found predominantly in badland materials are smectite, illite, chlorite and kaolinite.
The properties of active clays change significantly due to weathering at or near the surface as they progressively become exposed (Faulkner et al. 2000;Finlayson et al. 1987). As the crust develops, the physico-chemical properties of the material in the weathered layers change, thereby influencing the activity of the geomorphic processes. Alternation of wetting and drying cycles, presence of joints and fissures and dissolution-crystalü-zation of soluble minerals are the three main influences on mudrock weathering (Canton et al. 2001). Wettingdrying cycles may cause compaction of the internal structure and there appears to be a significant difference in material response to precipitation depending on whether or not it is followed by freezing (during which more deterioration oecurs) (Pardini et al. 1995).
The weathering profiles of mudrock in the Dinosaur Badlands Park, Alberta, Canada (Hodges & Bryan 1982), marls from the Guadix Basin Badlands, Spain , mudrocks from the Chadron Formation, Utah, USA (Howard 1994) and mudrocks from the Zin Valley Badlands, Israel (Yair et al. 1980) have the foüowing typical layers: a) 1-2 cm thick porous crust with desiccation cracks, leached of highly soluble components; b) -10 cm subsurface compact layer rieh in micropores; c) 10-40 cm thick transitional layer with partly weathered shards and d) unweathered material.
The crust characteristics, such as mineralogical and geochemical composition, cracks and thickness of the surface and subsurface layers influence the processes on the hiüslope. The type of crust that develops on the exposed material depends on its physico-chemical characteristics and on the magnitude and frequency of precipitation. Intense shrink-swell activity in smectiterich Sediments can produce desiccation cracks and a loose «popcorn» regoüth that has high macroporosity (Imeson 1986;Schumm 1956). The «popcorn» surface can also form with repeated freezing-thawing cycles and expansion that oecurs due to ice crystal growth. Clay swelüng can also induce stronger alteration of mudrock than caused with wetting and drying (Pardini et al. 1995). The «popcorn» surface has been identified in the Chadron formation, South Dakota, USA (Schumm 1956), Dinosaur Park Badlands, Alberta, Canada (Bryan et al. 1978), Vaücebre Badlands, Spain (Regues et al. 1995) and the Val D'Orcia Badlands, Italy (Torri & Bryan 1997). In the Zin Badlands, Israel, the swelüng of smectite clays was suppressed due to the presence of kaolinite and calcite. Instead of a typical «popcorn» crust with loose aggregates, a dense crust rieh in desiccation cracks with a subsurface coarse shard layer developed (Yair et al. 1980). A similar crust developed on marls in badlands in SE Spain (Canton et al. 2001) and on smectite-poor shales in the Chinguacousy Badlands, Canada (Kasa-nin-Grubin 2006). which was carried out in May, 2001 andMay, 2003 (Fig. la, b; 2a, b). On both occasions, the geometrie characteristics of rüls and rüls Systems were measured. Even though the number of sites in 2001 was substantially smaller than in 2003, differences in geometrie properties were still evident. Rill width and width/depth ratio on mudrock slopes decreased, while rill depth increased during the two years. The limited number of sites investigated and the high Standard deviation of the 2001 data prevented direct comparison of rill network properties between the two years. However, field observations aüowed the conclusion to be drawn that the riü Systems on mudrock appeared to not only be more incised but also denser and characterised by longer first order Channels.
Besides the form of rül Systems, the difference in appearance of the surface crust on the mudrock slopes between 2001 and 2003 was even more striking (Fig. 2b & Campbell 1990). In 2003, the «popcorn» crust was almost completely absent; surfaces were sealed, had less microrelief and were denser, with wider and deeper desiccation cracks. Thus it appears that for Alberta mudrocks, although climatic seasonal Variation may not appear to be particularly significant during an average year, dramatic changes can oeeur under extreme precipitation conditions. This includes the transformation of the surface from its «popcorn» characteristic to a dense, Hat and compacted surface. Similarly, in Mediterranean climates, seasonal distribution of precipitation was found to be more important than the total amount of rain (Yaalon 1997). For example, on simüar material in the Petrer Badlands, Spain, Calvo-Cases & Harvey (1996) observed more changes between seasons than between years.
If the Alberta mudrocks are susceptible to «seasonal» changes, then the same could be assumed for smectiterich shales in other regions due to their similar mineralogical composition. To test this assumption, Kasanin-Grubin (2006) tested smectite-rich and smectite-poor lithologies by means of weathering experiments. Mudrock shards (average 1 x 1 x 0.5 cm) were placed in circular aluminium sample trays (radius 12.5 cm, depth 4 cm). They were subjeeted to 10 cycles of simulated rainfall at 45 mmh4 intensity with duration ranging from 10 to 60 min.
In the smectite-rich lithologies a marked difference in surface crust and desiccation crack development was noticed (Fig. 3a).The shard structure with defined margins was maintained throughout the experiment under rainfall durations of 10 and 20 minutes. This could be due to shard swelüng potential being limited by water availability and the short duration of the wetting period. During drying cycles, minute cracks of < 1 mm in width often appeared on the shard surfaces. In contrast, the effect of water availability could be seen on the samples subjeeted to 50 and 60 minutes of rainfall. Here, maximum swelüng appeared after the first cycle of rainfall (Fig. 3a). After this swift swelling, samples became unstable and dispersive, and after the third rainfall cycle, dispersion became dominant, resulting in flatter surfaces, thinner crust development and narrower desiccation cracks. From the weathering experiments it would seem that even rainfall of very short duration (10-20 min) can lead to swelüng of clay minerals during wetting and formation of «popcorn» surface during drying periods. During subsequent wetting-drying periods, the crust becomes flatter and denser, and desiccation cracks become wider and deeper. Depending on size, continuity and reappearance after wetting, cracks can become flowpaths and may evolve into rüls. The rate at which cracks reappear after sealing is very important for runoff generation, particularly in typical infrequent rainstorms of low intensity and duration (Bryan et al. 1978).
In the smectite-poor materials there was no apparent difference in samples as a result of rainfall duration (Kasanin-Grubin 2006). When exposed to rainfall, smectite-poor shale shards broke apart after each drying cycle (Fig. 3b). During repeated cycles of wetting and drying, large smectite-poor shards broke down into smaller shards due to differential swelüng of üüte and chlorite. Once they were reduced to tiny, flaky shards (0.5 cm x 0.2 cm), the surface became compacted. As indicated above, this process appears to be characteristic of Mancos shale in Utah (Fam & Dusseault 1998).The Mancos shale decomposes after a few tens of hours with only slight sweüing into flaky shards (Howard 1997). These materials are salt-rich and each time shards disintegrate they yield a yellowish liquid rieh in Na and Ca sulphates (Laronne 1981).
If salt leaching does not oeeur, shard disintegration will not oeeur (Howard 1997). The mixed-layer non swelüng clay minerals can cause pressure to increase under repeated wetting and drying conditions, possibly leading to brittle failure of material (Wust & McLane 2000). Cracks that form during slaking promote further permeability and expose more rock surface to water (Sadisun et al. 2005).
Furthermore, smectite-rich and smectite-poor lithologies appear to differ not only in their response to wetting periods, but to drying periods as well (Kasanin-Grubin 2006). Lithologies that have thin regoliths,like Alberta badland sandstone and smectite-poor litholo- gies, do not respond to moisture input/output variations. In contrast, smectite-rich lithologies release up to 4 times more Sediments when moisture inputs occur over long periods (60 min) with drying periods in between than during short rainfalls (10 min) of same intensity. This indicates the importance of drying for smectite-rich materials which can differ with slope orientation. Furthermore it also implies more profound differences that occur with sensu stricto seasonal climatic changes. Even more importantly, this Observation highlights the importance of the response of the material to climatic variations, a response which does not necessarily occur on a regulär cycle.
such are tectonic activity and climate change, but more frequently they are human induced, e.g. land clearance to change use of land. The research presented here clearly indicates that clay mineralogy and type and amount of clay in the bedrock play a critical role in the development of surface crust and hillslope morphology and ultimately of badlands. Laboratory experiments on smectite-rich samples subjeeted to simulated rainfall have established a correlation between phased development of surface crust and desiccation cracks and duration of rainfall. A similar correlation could not be found for smectite-poor materials.
In addition, evidence was collected on the different responses of smectite-rich and smectite-poor lithologies to wetting and drying periods. Thus, it appears in particular that drying periods play an important role in badland development on smectite-rich materials, an aspect which is directly linked to slope orientation and strongly sensitive to differences that occur with sensu stricto seasonal climatic changes.