Holocene sediment fluxes in a fragile loess landscape (Saxony, Germany)

Catena 103 (2013) 87–102

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Catena j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t e n a

Holocene sediment fluxes in a fragile loess landscape (Saxony, Germany) Daniel Wolf ⁎, Dominik Faust Department of Physical Geography, Dresden University of Technology, Helmholtzstr. 10, D-01069 Dresden, Germany

a r t i c l e Keywords: Sediment fluxes Soil erosion Sediment budget Holocene Saxony Loess dells

i n f o

a b s t r a c t The loess region of the Central Saxon Hill Country is characterized by a relief of gently rolling hills, whose most distinctive landscape elements are well-defined shallow valleys, called dells. The aim of this research is to gather morphodynamic information on the dells in a qualitative and quantitative way by means of high-resolution field mapping and laboratory-based analyses. The intensity of sediment shifting has been modelled via sediment budgeting on a small catchment scale. Research objectives are to determine the character and magnitude of geomorphic processes for a Holocene time frame. Concerning the absence of currently obtainable dating, this study represents a reflection, which summarises all sediment shifting related to anthropogenic land use during the Holocene. As a specific parameter in terms of landscape development, the relief is considered to be strongly associated with varying process intensities. Agrarian land use of the loess region since the late Neolithic has resulted in severe soil erosion. Lower middle slope positions are the apparent spots were soil erosion processes take place in terms of rill erosion. A detailed survey of three dells provided a very heterogeneous pattern of the relocation of soil material. Dell 1 (Zehren) shows medial Holocene soil profile truncation of 1090 mm. In contrast, dell 3 (Löbschütz) shows soil loss of around 660 mm. All systems have similarly high slope gradients up to 12°. Actually, the deepening of the dell´s depth contours and therewith the extent of erosive slope lengths is varying. Thus, a higher longitudinal decline of the valley floors and a more intensive incision, lead to enlarged erosive slope lengths, which causes increasing sediment transport. Hence, land-use patterns trigger the process of material displacement, whereas relief modifies the intensity of geomorphic processes. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Loess landscapes of the temperate zone are highly fertile areas and characterized by long-lasting agrarian land use. The worldwide spatial distribution amounts to 10% of the earth´s surface. That illustrates the importance of loess areas in a way of being living space and settlement area. Furthermore, it demonstrates the geographic spread of a highly fragile landscape unit. Slight interventions may force extraordinary impacts concerning processes of soil erosion and material displacement (Pecsi and Richter, 1996). In this context, most hazardous to loess landscapes is the high erodibility of loess and loess-like sediments caused by soil-physical properties of the specific substrate (Gerlinger, 1997; Pecsi and Richter, 1996). Unprotected soils without vegetation cover are subjected to strong degradation of soil structure. This may initiate efficient causal chains leading to destabilization and transportation of soil aggregates due to soil erosion phenomena. Loess landscapes in central Europe attracted new settlers by providing fertile soils right from the beginning of the colonization.

⁎ Corresponding author. Tel.: + 49 351 463 35140; fax: + 49 351 463 37064. E-mail address: [email protected] (D. Wolf). 0341-8162/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2011.05.011

Consequently, early deforestation and intensive forest use have taken place since the end of the Mesolithic (Bentley et al., 2002; Lang, 2003; Litt, 1992). In such old settlement areas conditions have emerged, which forced soil erosion processes starting as early as 7500 years ago, depending on the regional onset of cultivation (Davison et al., 2006; Dotterweich, 2008; Houben et al., 2006; Turney and Brown, 2007). Extensive studies exist for Central European loess areas situated in Belgium, Southwestern Germany or Northeastern Germany (cf. Dotterweich, 2008). A number of studies focus on the process of soil erosion, dealing with specific properties of loessic substrates and their erodibility, dependent on changing internal and external parameters (Evrard et al., 2010; Kehl, 1997; Knapen et al., 2007; Le Bissonnais et al., 1995; Rejman et al., 1998; Vanwalleghem et al., 2005). Further studies analyze colluvial or alluvial archives to elucidate the progress of soil erosion processes in time (Bork et al., 1998; Lang, 2003). And yet others pursue the objective to gather all processes of soil erosion and sedimentation within enclosed catchments, to build sediment budgets including a spatiotemporal linkage of sediment shifting. The observed time span ranges between the status quo (Walling and Collins, 2008), a sequence of recorded statuses of the recent past (Trimble, 2009) and a synopsis of all the sediment shifting that has taken place over historical periods and the Holocene as whole (Houben, 2008; Seidel and Mäckel, 2007; Verstraeten et al., 2009).

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Sediment budgeting provides an opportunity to establish a relation between soil erosion processes and sedimentation, within catchments across different scales. Furthermore, the regional and trans-regional comparison of sediment flux quantities can reveal various factors influencing the sediment shifting. In addition to the variable appearance of natural environments, also alternating land use systems and varying durations of land use determine absolute values for soil erosion, sedimentation and sediment delivery. Hence, of particular importance is the survey of past sediment shifting within loess landscapes in a widespread spatial distribution. This study fills a gap for an extensive loess area in Eastern Germany. Although this area is mainly used as farm land and despite the fact of being the most productive region of cultivation within Saxony, there is no knowledge about sediment shifting, except for a few general descriptions from the seventies and eighties (e.g. Kramer, 1971; Lieberoth, 1982). To unveil Holocene sediment fluxes, a facile but appropriate approach was chosen. Due to the fact that this loess region exhibits a highly undulating relief, the surface is structured by various dell systems stringing together. Generally, these dells are not drained by watercourses and usually they are not exceeding a longitudinal extension of

more than two kilometers. The importance of dells as main pathways for soil erosion processes, linear erosion processes, as well as sedimentation processes was sufficiently emphasized by Semmel (1961), Thiemeyer (1988) or Bork (1983) and Bork et al. (1998) for other loess areas in Germany. The aim of this study was to unravel the complex patterns and processes of soil erosion and sedimentation in dell systems developed in loess. For that, several dells were examined by a high resolution field mapping and finally, the results of more than 450 drillings found entrance into the characterization of sediment fluxes. Special attention was given to determining the erosion processes that took place and attempting to quantify soil erosion and sedimentation processes. A further focus was on the parameters which modify the intensity of sediment shifting. 2. Geographical setting The loess region of the Central Saxon Hill Country is situated at the northern border of the Central European loess belt and occupies an area of 675 km². Its interior, an arable area with an extension of barely half of the loess region, is called Lommatzscher Pflege (Fig. 1). This so-

Area of investigation

Riesa

Hamburg

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dell 2

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town / village

studied dells

northern loess plateau

stream network

Elevation m a.s.l.

90 - 120 120 - 150 150 - 180 180 - 210 210 - 240

Fig. 1. Topographic map of the study area within the Saxon loess area and studied dell systems: dell 1 (Zehren), dell 2 (Schwochau), dell 3 (Löbschütz).

D. Wolf, D. Faust / Catena 103 (2013) 87–102

called breadbasket of Saxony is distinguished by a plateau-like character in its northern part. The area is characterized by a continuous loess deposit originating from the Weichselian glacial (MIS 2–4) with a depth of at least 3 m (Eissmann, 2002). In fact, the loess achieves a thickness between 10 m and 20 m in sheltered positions (Haase et al., 1970) and especially, the research area presented in this paper shows the highest depths of loess within the whole loess region. The regional climate reveals precipitation of around 600 to 650 mm per year and is predominantly characterized by oceanic influence. Despite the prevalent cyclonic weather situation, the loess landscape is not affected by high rain falls due to the precipitation shadow of the westward located ranges of the German Uplands. The northern part of the Lommatzscher Pflege with rainfalls around 600 mm per year represents one of the driest areas in Saxony. A view to the natural scenery of the loess region shows a highly undulating relief, characterized by gently rolling hills and shallow valleys (dells) in between. Additionally, the hill country is eminently dissected by a stream network, creating a highly energetic relief in the proximity of the incised valleys. The relevance of this dissection related to the sediment fluxes inside the dells will be discussed later.

se Tran

A

longitudinal section

2.1. Local conditions of the investigated dell systems Within the loess region, the relief is a markedly varying parameter. In case of a strong and intensive dissection by the river network, the relief energy is high. On the other hand, if a given part of the loess area is just slightly dissected and the rivers show weak incision, the relief energy is reduced. In order to verify the influence of the relief on Holocene sediment shifting, each mapped dell is situated in another part of the plateau with differing values for the associated relief energy. Dell 1 (Fig. 2) is situated close to the margin of the loess plateau, next to the village Zehren (Fig. 1). The dell is incised up to 20 or 30 m and shows a longitudinal extension of 800 m, before it runs into the main recipient Elbe after another 800 m. Therefore, the relief has to be characterized as highly energetic. Dell 2 (Fig. 3) is situated in a peripheral plateau area nearby the village Schwochau. The dell runs into the river Ketzerbach, which is characterized by intensive valley formation and incision. Approximately 8 km farther, the river discharges into the Elbe. Therefore, dell 2 is located distant to the Elbe, but due to the strong incision of the Ketzerbach, the relief energy reaches notably high values.

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³ 19

volume of the whole sediment body: 146.000 m³ /26,5 ha

190 cm

C

.20

0m

³ 37

.60

0m

³

Fig. 2. Dell 1 (Zehren). A) Map of the catchment and positions of drilling cores (size of the catchment area amounts to 26,5 ha). B) Map of the catchment with subdivision in several soil loss-classes. C) Extension of colluvial sediments and calculation of the sediment volume. D) Standard soil profile.

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A

Tra nse c

S17

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0 m³

volume of the whole sediment body: 93.600 m³ /17 ha

m

50.35

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24.05

0 m³

0

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200

300

400 m

Fig. 3. Dell 2 (Schwochau). A) Map of the catchment and positions of drilling cores (size of the catchment area amounts to 17 ha). B) Map of the catchment with subdivision in several soil loss-classes. C) Extension of colluvial sediments and calculation of the sediment volume.

Dell 3 (Fig. 4) is situated in the center of the loess plateau, nearby the village Löbschütz (Fig. 1). It is sixfold larger than dell 1 but consists of six dells of about the same size, which confluence in a finger-shaped way. Dell 3 is connected to the small stream Keppritzbach, which runs another 17 km until the confluence to the main recipient Elbe is arrived. Due to the far distance to the Elbe, the river Keppritzbach is less incised and the relief energy is on a low level. 3. Methods 3.1. Fieldwork For the examination of the initial raised questions, the chosen dells were intensively studied to identify sediment shifting taken place inside these systems. The pursued approach intended to use soils or rather truncation and preservation states of soils, to identify positions as being affected by soil erosion processes. In return, the covering by colluvial sediments documents sedimentation processes. Due to the pronounced undulation of the hill scenery, the distribution of soil erosion and sedimentation processes is heavily dependent on the relief. Hence, for mapping the dells, the borings were drilled along transects perpendicular to the dell's depth contours, following the catena concept. Average boring distance was 10 to 15 m, if required less. The fieldwork is based on some 450 borings with 1 m-Pürckhauer and at least 30 borings with hand auger and percussion drill up to a depth of 5 m (Figs. 2A, 3A and 4A). All results were used to compile graphic charts of the cross sections, to describe spatial interrelations of soil erosion and sedimentation. For a better perception, the profiles are super-elevated fivefold to sevenfold. In addition to soil truncation, also sedimentation is represented by the extension of colluvial layers. This basic information is needed to prepare the sediment budgets.

3.2. Sediment budgeting Soil erosion has been incorporated into the budgets as given below. (1) First of all, for each drilling profile, the particular soil profile truncation was estimated. In this context, the occurrence of a few isolated remnants of well-preserved soil profiles (Fig. 6A boring N14, Fig. 7 boring 18, X4, 27 and 26 and coring 8) helped to reconstruct a standard soil profile with a thickness of approximately 130 cm (Fig. 2D). (2) Next, soil profile truncation was divided into five classes, ranging from 10 cm to 300 cm of soil loss. Thus, it was possible to subdivide the cross sections into different segments of averaged soil profile truncation. (3) With the aid of aerial photographs, we succeeded to interconnect these segments over the whole surface of the dell catchments (Figs. 2B, 3B and 4B). On the aerial photographs, several circumstances concerning the substratum, like exposing subsoil horizons or outcropping calcareous loess, are obvious to an extremely high degree. (4) Finally, the surface area of each unit was defined. By multiplication with the ordinal number of the medial soil profile truncation, the absolute volume of soil loss due to soil erosion was calculated. Sedimentation was rated by calculating the volume of all Holocene sediment bodies within the dells. (1) As a first step, the cross sections of all colluvial layers together were taken from the plotted transects by integral calculus. Per

D. Wolf, D. Faust / Catena 103 (2013) 87–102

A

91

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RKS4 Q6

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ct

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longitudinal section along the depth contour

soil profil truncation:

C

300 cm

north-western catchment 184.000 m³ /29,5 ha

71.000 m³ /28,5 ha

200 cm

north-eastern catchment

120 cm 60 cm

210 m

eastern

15.700 m³

54.500 m³ /22 ha south-western catchment

69.500 m³ /28 ha 10

100 m

0

extension of colluvial sediments

volume of the whole sediment body: 487.000 m³ /153 ha

southern catchment

m 00

3

22.500 m³

0m

m³/ha volume of colluvial

77.000 m³ /31 ha

23

sediments in each sub-catchment in relation to its size

m

13.050 m³

area with estimated sediment quantity

10 cm

south-eastern catchment

19.700 m³

200 m

catchment area of the dell system

catchment 30.600 m³ /14 ha

6.250 m³

transect

Fig. 4. Dell 3 (Löbschütz). A) Map of the catchment and positions of drilling cores (size of the catchment area amounts to 153 ha). B) Map of the southern sub-catchment with subdivision in several soil loss-classes. C) Extension of colluvial sediments and calculation of sediment volumes. For each sub-catchment, sediment volumes are listed beside the sub-catchment sizes in the dark boxes. Extensions of colluvial sediments are only marked on the map for the field-mapped southern, eastern and north-western sub-catchments. Also the sediment volumes were directly calculated only for the mentioned sub-catchments. The sediment volumes of the remaining sub-catchments (south-western, north-eastern and south-eastern sub-catchments, marked with criss-cross lines) were extrapolated from the results of the field-mapped sub-catchments.

(2) For the calculation of a three-dimensional sediment body, the size of the cross section of the dell's sediment body was multiplied with the respective distance to the next transect along the course of the depth contour (Fig. 5).

definition, plough horizons are considered as colluvial sediments, if they reach a depth of ≥40 cm (according to AG Boden, 1996) or if they, a priori, are developed within a colluvial sediment body.

calculation of sediment storage: volume of colluvial sediment body

=

V1+V2+V3+V4

( e.g.: V1 = h(Q1+ Q1Q2+Q2)/3 ) 400 m 00 m

3 ons ecti ss s our (h) o r c t f m con es o 100 anc e depth dist h t in with

2

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Q3

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0m

model of colluvial sediment body

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00 m

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model for calculation of the abstracted volume

Fig. 5. Schematic calculation of the colluvial sediment body. The abstracted section of the headwaters (V4) was multiplicated with the number of converging initial dells (e.g. dell 1 in Fig. 2: multiplier 3).

4

158

V8 V6 V4 42 V7 V5 V3 38 36 34 V2 V1 37 35 33 32

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slope hollow, cut at an angle of 45° 0

1m

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FU

3m

260

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Fig. 6. Cross sections A) Q1 and B) Q2 of dell 1 (Zehren) with analytics of coring 1 and coring 4 in the depth contour.

solifluction / aqueous loess

D. Wolf, D. Faust / Catena 103 (2013) 87–102

0

4m

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parallel running tributary dell

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organic matter (%)

pH-value

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25 24 23 22 21 20 17

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coring 2

146

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soil thickness (m)

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92

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D. Wolf, D. Faust / Catena 103 (2013) 87–102

elevation (m)

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soil thickness (m)

Transect Zehren Q3 + Q5

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solifluction / aqueous loess

Fig. 7. Cross sections Q3 and Q5 of dell 1 (Zehren) with analytics of coring 6 and coring 8 in the depth contour.

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D. Wolf, D. Faust / Catena 103 (2013) 87–102

(3) In case of several converging tributary dells, usually one of them was field-mapped and the results were transferred to the other ones. After all, the sediment discharge on a catchment scale was calculated by subtracting the total stored sediment volume from the total soil loss. 4. Results The field mapping resulted in a chief presence of Luvisols or rather truncated Luvisols within the study area. A tendency toward the formation of hydromorphic features turned out along depth contours and intra slope depressions. As expected, a characteristic distribution pattern of soil truncation and preservation arises from the close relation between the relief and the geomorphic processes that must have taken place there. Especially lower middle-slope positions are strongest affected by soil erosion processes. In contrast, higher upper-slopes and watershed positions are most sheltered to soil erosion processes. It is very surprising that recently the depressions show maximum values of sedimentation, whereas they were strongest affected by erosion in former periods. It only becomes allegeable, if a transition of process is assumed, which left linear erosion mechanisms behind and led to more or less planar denudations (in terms of Mortensen, 1954/55). The time of such a transition keeps unknown, as solid dating are not conducted yet. 4.1. Geomorphological features of dell 1 (Zehren) The main observations related to soil erosion and sedimentation are given below, using the example of dell 1. For the other dells, only the distinctions are listed.

4.1.1. Soil erosion The best-preserved soils are to be found in the uppermost part of dell 1 (Fig. 7, transect Q5 – right section), where even a complete buried Luvic soil, including A-horizon, was mapped in the depth contour (Fig. 7, profile records and analytics of coring 8). After converging with two other tributary dells between Q3 und Q5, the soils have been eroded due to concentrated runoff (Fig. 7, transect Q3 – left section). Also the lower part of dell 1, ranging as far as the outlet, has been totally eroded within the depth contour. It is assumed that up to three meter of soil material and underlying substrate have been removed due to a kind of linear erosion process. Below the erosion surface, the depth contour is lined with a weathered clayey substrate (profile records and analytics of Figs. 6A, B and 7 – left side). It represents aqueous loess and solifluction loess, which were deposited during the late glacial. The erosion surface is covered by a pitch-black colluvial layer, which contains a high amount of organic carbon (profile records and analytics of Figs. 6A, B and 7). This material originates from former Holocene top soils and indicates the onset of planar soil erosion processes. Apart from transect Q5, some other reaches of dell 1 show a spatially isolated occurrence of well-preserved soil profiles as well. These positions are altogether limited to the hilltops of the watersheds (e.g. Fig. 6A, boring N14; Fig. 7, boring 18), where they are buried by plough horizons. Due to the extensive effect of soil erosion processes, most Luvisols of dell 1 have been truncated. Along the upper slopes, commonly Bt-material merges into the plough horizons. The lower slopes are often characterized by calcareous or decalcified loess, appearing at the surface (Fig. 8A). Consequently, Bt-horizons have been cut in middle slope position (Fig. 6A, between borings 7 and 8; Fig. 6B, between borings 8 and 9). These positions are marked by a bend in the slope, with an increasing slope angle toward the depth contour.

Fig. 8. Examples of erosion and sedimentation processes on hill slopes. A) Photograph of the south-facing slope of cross section Q1 in dell 1 (2008). The truncation of a complete soil profile within a hill slope becomes visible by the help of plough horizons. Dark colours on upper slope positions mark the appearance of Bt-horizons at the surface. Light colours on lower middle slope positions point out calcareous loess below the Holocene soil. Blackish colours of the foot slope positions demonstrate the accumulation of colluvial sediments. B) Sediment fans along the foot slope of dell 1 (same position as shown in A, 2006). C) Initial rill on a middle slope position. D) Fine lamination of the sediment fan, shown in B. The deposition of 40 mm of sediments occurred in spring 2006. D) Erosive sediment pathway in an agricultural vehicle lane merges into a sediment fan in foot slope position (dell 3, 2008).

D. Wolf, D. Faust / Catena 103 (2013) 87–102

A

SW

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Transect Schwochau Q1

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Fig. 9. Cross sections A) Q1 and B) Q2 of dell 2 (Schwochau) with analytics of coring 13 in the depth contour.

260 (m)

soil thickness (m)

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96

A SSO

Transect Löbschütz Q3

NNW

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Cv (decalcified) Cv (calcareous)

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transition

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9

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148

Bt

soil thickness (m)

O

120

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Fig. 10. Cross sections A) Q3 and B) Q5 of dell 3 (Löbschütz) with analytics of coring 22 in the depth contour.

220 (m)

D. Wolf, D. Faust / Catena 103 (2013) 87–102

0

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D. Wolf, D. Faust / Catena 103 (2013) 87–102

transition

154

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1.5 0m

0m

M1 M2

8

M3

148

1m

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M4

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Fig. 11. Cross sections A) Q4 and B) Q6 of dell 3 (Löbschütz) with analytics of coring 25 in the depth contour. The sedimentary layer M8 indicates either a short period of stabilisation or a reworking of underlying M(Ah) material.

97

98

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4.2. Sedimentation patterns Similar to soil erosion processes, also sedimentation is strongly linked to the shape of the relief. In this respect, we see four particular positions of sedimentation within dell 1 as follows: (1) Initial dell sections with limited transport capacity due to high sediment load and/or low runoff. Almost the whole initial part of dell 1 (Fig. 7, transect Q5 – right side) is covered with up to 1 m of colluvial sediments. Mobilized material is primarily left in place after a short-distance translocation. (2) Foot slope positions with decreasing transport capacity in transition to the depth contour (Fig. 8B and E). (3) Slope hollows or tributary dells with low inclination angles and sediment overload. Depth contours of tributary dells have generally been eroded and filled up with colluvial sediments again (Fig. 6A, borings 32 to 37; Fig. 6B, borings 22 to 31). Sediment fluxes are concentrated by converging runoff and cause a refilling due to transport capacity overload. (4) Hilltop positions fed by overlain areas. Watersheds must be considered as ridges, dipping parallel to the dell´s courses. Therefore, all along the upper surface of the ridges, translocation processes take place. Sediments are accumulated at flat sections or depressions within the ridges (Fig. 6A, borings 37 to 42). However, the most important sediment storage takes place along the main depth contour, where a voluminous sediment body was accumulated. These colluvial sediments can reach a depth of up to 3.5 m. Several layers can be differentiated, but there are no indications of intermediate soil formations or humus enrichment (see profile records and analytics of Figs. 6A, B and 7). The upper part of dell 1 shows a more or less horizontally persistent sediment sequence, because sediment input originates from the immediate vicinity only. In contrast, the depositions of the lower part of dell 1 appear much more chaotic. Due to the fact that the whole catchment is drained through the center of this lowermost depth contour, converged runoff could have caused considerable linear erosion processes. This phenomenon is recently testified by an annual formation of ephemeral gullies. The subsequent refill of these gullies creates an interlocking of mixed sediments from distant positions and sediments from the flanking hill slopes. 4.3. Geomorphological features of dell 2 (Schwochau) Some noticeable variations are observable compared to the geomorphological features off dell 1. An important difference is the stronger erosion of the depth contour that reaches even the upper part of the dell (dell 2; Fig. 9A). Furthermore, the whole catchment shows no occurrences of preserved soil profiles. Even the watershed positions have been strongly affected by soil erosion processes. It seems that soil erosion has taken place more extensive than in dell 1.

However, apart from the depth contour, selective erosion has been much more moderate, since nowhere else calcareous or decalcified loess appears at the surface. In contrast to the sedimentation patterns of dell 1, large parts of the south facing hill slope are covered by colluvial sediments (Fig. 9A, borings 16 to 21 and 25 to 28; Fig. 9B, borings 20 to 24). 4.4. Geomorphological features of dell 3 (Löbschütz) Generally, soil erosion and sedimentation processes are similar to those taking place in dell 1. However, some major differences appear regarding the intensity of processes. The upper part of the southern sub catchment of dell 3, as well as the watershed positions, show well-preserved soil profiles just like dell 1 (Fig. 10A). Likewise, lower middle slope positions are strongest affected by soil erosion processes as calcareous loess appears at the surface (Fig. 11A, borings 14 and 15; Fig. 10B, borings 9 to 11; Fig. 11B, borings 6 and 13 to 15). Compared to dell 1 and dell 2, a significant difference concerns the depth contour of the southern part of the dell. All along this depth contour, Bt-horizons of Luvisols have been preserved (profile records and analytics of Fig. 10B). That means that linear erosion processes have not been as effective as in other areas of the loess plateau. Only when several dells run together into the main depth contour, the soils have totally been eroded as obvious in Fig. 11B. Sedimentation patterns turned out to be similar to dell 1 as well. Hence, a mentionable occurrence of colluvial sediments is limited to the depth contours. These sediments are horizontally layered and a pitch-black and carbon rich material was accumulated at the base of the sediment body (profile records and analytics of Figs. 10B and 11B). 4.5. Sediment budgets The calculated values for soil erosion and sedimentation are shown in Table 1 for every three dell systems. For example, dell 1 features a total erosion of 288,000 m³ of soil material. That implies a soil loss of 10,900 m³/ha or 16,300 t/ha, assuming a bulk density of 1.5 g/cm³ (following Roth, 1997 and Verstraeten et al., 2009) and an average profile truncation of around 1090 mm. Considering the colluvial sediments of dell 1, a volume of 145,000 m³ arises. The remaining difference illustrates the sediment delivery out of the system. Thus, about 49.7% of the eroded material yielded to the proximate river systems, meanwhile, the second half still remains inside the dell. As expected, the values for mean soil erosion quantity are varying, but it is quite astonishing that the values differ as much. Comparing dell 1 and dell 3, mean soil profile truncation ranges between 1090 mm and 660 mm. In contrast, the sediment delivery out of the dells is surprisingly similar for each catchment. The arising fact is that the specific dells show very different intensities of soil erosion and sedimentation processes, but ultimately a process-related equilibrium is reached, which creates a balanced interrelation between soil erosion and sediment storage. In order to define annual soil erosion ratios (Table 1) a human land use duration of 7000 years is assumed. Due to the lacking time control

Table 1 Sediment budgets of the studied dell systems. Location

Dell Dell Dell Dell a

1 (Zehren) 2 (Schwochau) 3 (Löbschütz) 3 (southern catchment)

b

Catchment area (ha)

Erosion

Sedimentation

SDR

10³ m³

t/ha

mm

t/ha/yra

10³ m³

%

26.5 17.0 153.0 31.1

288 190 1,012 200

16,300 16,700 9900 9600

1090 1120 660 640

2.3 2.4 1.4 1.4

145 94 487 77

49.7 51.5 51.9 61.6

Mean annual soil erosion rates are based on land use duration of 7000 years. Refers to the southern catchment of dell 3 that is mapped in Fig. 4B. Sedimentation is less intensive than within the total catchment, which causes the higher sediment delivery out of the sub-catchment. The main depth contour of dell 3 is a further sediment storage for the southern sub-catchment. This compensates the high SDR. b

D. Wolf, D. Faust / Catena 103 (2013) 87–102

5. Interpretation and discussion

Table 2 General view of different relief components inside the dells.

Average depth contour incision Average slope length b Maximum slope inclination Depth contour gradient

a

99

Dell 1

Dell 2

Dell 3

10 m 130 m 13° 2.9%

9m 120 m 12° 3.2%

7m 90 m 12° 2.2%

a Floor level difference from the points of culmination to the ground of the dell´s depth contour. Values were taken from the mapped cross sections. b Values were taken from the cross sections. A stronger incision of the depth contour coincides with an enlargement of the average slope length. Additionally, the amount of soil loss is in line with the gradient of the depth contour. That means that a high gradient, which moreover indicates a strong incision of the connected stream network, is accompanied by high soil loss.

of sedimentation processes, the earliest known date of a permanent formation of settlements within the loess region is taken from literature (Czok, 1989; Hartsch et al., 2005; Kalis et al., 2003; Litt, 1992; Strobel et al., 2009). The resulting soil erosion rates range between 1.4 and 2.4 t/ha/yr.

4.6. Relief configuration One of the main objectives was to prove parameters controlling the intensity of sediment shifting. Therefore, a simple characteristic of each dell´s relief components is shown in Table 2. It is obvious that parameters like medial slope length, deepening of the depth contour and mean gradient of the depth contour are varying. As a basic relation, a stronger deepening of the depth contour comes along with a more extensive slope length, as long as slope angles remain more or less constant. The high soil erosion of dell 1 coincides with very long slopes due to the intensive deepening of this depth contour. Additionally, the gradient of the depth contour is relatively steep. In contrast, dell 3 shows the lowest soil erosion, accompanied by the lowest depth contour gradient, the lowest incision of the depth contour and hence, the shortest slope lengths. A further linkage to the relief arises from the drainage pattern in Fig. 12. It shows the difference in altitude between the dells and the river Elbe, depending on the respective distance to the junction with this river. The Elbe represents the preliminary base level of erosion within the loess area. Again, the high soil erosion of dell 1 is coincident with the least distance to the main recipient and the highest gradients of the pathways. The low soil erosion of dell 3 is associated with the longest distance to the Elbe and the slightest pathway gradients.

5.1. Implications Considering the results in chapter 4 some basic relations regarding soil erosion processes can be mentioned. In the past, erosion of Bt-horizons started mainly at lower middle slope positions. Because of regressive erosion phenomena, there was an upward movement right to the actual knick point. This finding is in contrast to the often described observation that especially the upper slopes are mostly affected by soil erosion processes (e.g. Pecsi and Richter, 1996). After heavy rainfall the topsoil surface becomes sealed and crusted due to splash effects. This compaction causes runoff and induces the incision of rills. For a widening of initial micro-rills a specific magnitude of runoff is necessary (cf. Faust and Schmidt, 2009), and thus the formation of rills depends on a certain distance to the watersheds, and is therefore most intensive along the lower middle slopes. If calcareous loess reaches the surface, the unfavorable properties in relation to soil erosion processes reveal (cf. Le Bissonnais et al., 1995). The absence of soil structure and the strong dehydration cause a weak pre-moistening before rainfall events (Faust, 2003; Gerlinger, 1997). Therefore, in such positions soil erosion processes operate much stronger, compared to positions containing well-aggregated and clayey soil material (Fig. 7, borings 3 to 6; Fig. 6A, borings 12 to 14). In the case of dell 1, soil erosion processes on calcareous loess become additionally intensified by the southern slope exposure due to stronger dehydration (cf. Cammeraat and Imeson, 1998). The lower slopes of the north exposed hillside, especially in transect Q1 (Fig. 6A, borings 24 to 28) are apparently better protected against soil erosion processes. Due to the stronger dissection of this hillside by slope hollows and tributary dells, the erosive slope lengths, and thus the erosive pathways are shortened. Therefore, erosive runoff and soil erosion on the lower slopes are less intensive.

5.2. Relief as main factor controlling sediment flux intensity in the study area If we recapitulate the main points resulting from chapter 4, following facts should be accentuated. (1) Generally, the initial parts of the upper dells show wellpreserved soils and a persistent colluvial cover. (2) Lower middle slopes are most affected by soil erosion processes, while watersheds are the most protected positions.

Floor level difference to main recipient [m]

100

dell 3 (Löbschütz)

80

dell 2 (Schwochau)

60

dell 1 (Zehren)

A

40

B 20 0 0

C 5

10

15

20

Distance to main recipient Elbe [10≥ m] Fig. 12. Relief energy expressed as the drainage pathways of the three studied dells. The graphs indicate the floor level difference between the dells and the river Elbe (base level of erosion), depending on the respective horizontal distance between the headwater of the dell and the confluence into the Elbe, for each catchment. The signatures represent A) zero-order catchments with the beginning at the highest elevation of the depth contour and the lowest point at the outlet of the dell, B) intermediary courses of first-order streams, starting at the dell´s outlet and ending at the river mouth into higher-order streams, C) stream sections, which discharge directly into the main recipient Elbe. Average gradients are (%): dell 1: A) 2.9, C) 5; dell 2: A) 3.2, B) 1.5, C) 0.4; dell 3: A) 2.2, B) 0.3, C) 0.1.

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(3) The lower parts of the dells show a strong pre-sedimentary erosion of the depth contours. Subsequent sedimentation is first and foremost limited to these depth contours. As a consequence, we draw the conclusion that soil erosion on slopes depends on a critical distance to the watershed positions. With increasing distance, erosive runoff cumulates. Soil erosion principally takes place in terms of rill erosion; therefore, enlarged runoff raises the number of rills and increases the rill´s volume (Morgan, 1999; Pecsi and Richter, 1996). A further implication is the fact that a longer slope provides an even larger contact surface for rill erosion processes. Thus, the wide-stretched slopes of dell 1 show higher soil erosion than the short slopes of dell 3. For a further specification the development of the current relief must be considered, as Holocene sediment shifting inside the dells seems to be directly linked to the slope length, which in turn is a product of the relief development. The pre-Holocene landscape evolution of the loess area was dominated by a fragmentation due to river incision. Based on the subsidence of the Elbe basin (Eissmann, 2002; Wagenbreth and Steiner, 1990) as main base level of erosion, the loess region was dissected by tributary streams, starting about 400,000 years ago. That resulted in a hierarchical system of a river network according to the following pattern. (1) The direct tributaries, e.g. the river Ketzerbach, show the highest incision rates nearby the confluence to the main recipient. The intensity of incision decreases with increasing distance to the main river Elbe. (2) With increasing distance to the main recipient, the tributaries branch finer and finer. (3) Incision intensity is dependent on the flow rate. Therefore, lowest order streams (after Strahler, 1964) show lowest discharge, and thus lowest incision rates. The courses of the streams were fed by numerous dells. Due to the strong connectivity of the dells to the receiving streams, which are characterized by continual deepening of its valley floors, expansive

erosion and denudation processes took place within the dells already during the Pleistocene (Meszner and Faust, in press; cf. Semmel, 1985; Semmel and Stäblein, 1971). If it now appears, that a dell is discharged into a deeply incised river valley, the dell´s depth contour will be deeply incised as well (Fig. 13). That causes consequences like extended erosive slope lengths and raised soil loss. This case is exemplified by dell 1, which runs directly into the Elbe valley (Fig. 1and Table 2). On the other hand, if a dell is situated in a central plateau position and the runoff pathway to the main recipient is notably extended and shows interposed lower order tributaries, the incision of these tributaries is reduced. Accordingly, the dell's depth contour is less incised, the slope lengths are shortened and the soil loss due to soil erosion processes is reduced. This relationship is reflected by dell 3 (Fig. 1and Table 2). A critical reflection is demanded by dell 2. Following the above mentioned model, dell 2 should feature soil loss amounts ranging between dell 1 and dell 3 as it is located at the margin of the loess plateau, but at some distance to the Elbe valley. Actually, dell 2 shows the highest values for soil loss (Table 1). First of all, the derived model in Fig. 13 is based on quite simple assumptions and considerations. However, actual circumstances are partially much more complex and do not correspond to the pattern without exception. For instance, dell 2 is running into the river Ketzerbach, which shows a higher incision than average for a distance of about 10 km to the Elbe valley (Fig. 1). Reasons for this configuration may be manifold. Perhaps it is due to more discharge caused by higher precipitation values within its southern catchment or maybe it is due to its course along an old fault line. Anyway, a further explanation emerges out of the studies of Hartsch et al. (2005), who discovered about 14 findings on the south-facing ridge, parallel to cross section Q2 (Fig. 9B). They provide evidence of an almost continuous settlement from the Neolithic to the Roman Iron Age, lasting for about 5000 years. It might be that dell 2 was subject to intensified utilization due to the close proximity to the settlement sites. This would be expressed in higher soil erosion amounts, even in comparison with dell 1. This circumstance could also explain the widespread colluvial deposition along the south-facing hill slope of dell 2 (Fig. 9A, borings 17 to 21 and 25 to 28; Fig. 9B, borings 20 to 24).

parameters concerning dell systems

D3 increasing order within stream network

parameters concerning stream network

rise in soil loss / intensity of sediment shift

D2

decreasing erosive slope lengths

D1

decreasing incision of stream network

5.3. Sediment fluxes in the Saxon loess belt in relation to other mid-European loess regions

dells

increasing distance to main recipient

stream network hierarchy

Table 3 gives a general idea which values one must expect concerning soil erosion processes within Central European loess areas. The scale of soil erosion as well as the sediment delivery, detected by the study in hand, is quite comparable to the other results. Nevertheless, it is obvious that some catchments of the Saxon loess belt show notably higher soil loss. In this context, possible sources of errors should be mentioned. First, the adopted approach tends to an overestimation of soil profile truncation and an underestimation of sedimentation. The cross sectional field mapping represents an illustration of the floor conditions in particular positions within a catchment and neither an area-wide measure. Sediment bodies, especially of subordinated depressions and slope hollows, might be undetected.

D1-3: dells, dependent on the relativ position to main recipient

Fig. 13. Causal relations between the hierarchical stream network that drains the loess landscape and particular features of the dells, which are directly linked to different sections of the stream network. Regarding the stream network, the distance to the main recipient, as well as the order of the stream determine the level of incision. Concerning the dells, the incision of the receiving water course determines the incision of the dell's depth contour. Thus, the dell´s slope lengths are varying. As slope lengths control the intensity of soil erosion processes, different values for soil loss are produced.

Table 3 Holocene erosion ratios for selected studies of Central European loess regions. Area

Soil loss (t/ha)

SDR

Saxon loess area (E-Germany) Kraichgau (SW-Germany) Central Belgium Wetterau (SW-Germany) Central Belgium

9900–16,700

0.5–0.52

7800–12,900 10,800 9780 9200

– 0.6 0.38 0.5

Reference

Clemens and Stahr, 1994 Notebaert et al., 2009 Houben, 2008 Rommens et al., 2006

D. Wolf, D. Faust / Catena 103 (2013) 87–102

Overestimation of soil profile truncation is due to the unknown depth of profile development in slope positions. The derived standard soil profile (Fig. 2D) with a depth of 130 cm is applicable for flat positions on hilltops, initial dell sections and depth contour positions. Presumably, steep slopes featured shorter soil profiles; however, as no undisturbed soil profile was found in slope position, the calculation was oriented toward the available values. Frequently, depth contours were characterized by a soil profile truncation of about 300 cm (Fig. 2C), which is derived by projecting the Early- to Mid-Holocene ground surface onto recent depth contour positions. It remains questionable, if the pre-colluvial incision (“mega-gullies”) should find entrance into the sediment budgets, as involved processes are considered to have taken place prior to the process of extensive soil erosion (cf. Thiemeyer, 1988). The bottoms of the pre-colluvial gullies (“mega-gullies”) show discordances with a partial removal of Holocene soils (Fig. 7, coring 8 and coring 6; borings 9 and 10). As long as a Holocene age of the Luvisol formation is not being disproved, the post-pedogenic erosion of soil material has to be included in a Holocene sediment budget. Nevertheless, in case of dell 1, the material that was removed during the pre-colluvial incision, equals 20% of the total removed material. Therefore, the pre-colluvial incision contributes significantly to the raised soil loss inside the dells. Focusing on Table 3, the shown case studies should be discussed on a supra-regional scale. Clemens and Stahr (1994) analyzed four different catchments within the Kraichgau Basin, a loess area in SW-Germany. The catchment size ranges between 20 and 180 ha and total soil loss amounts to 7800, 9000, 12,600 and 12,900 t/ha for a time span of about 5000 years since the first deforestation. Apparently, the different catchments feature no interrelation between soil loss and a specific shape of the relief. Therefore, a longer duration of land use was considered to be the main driving factor causing the higher soil loss of about 13,000 t/ha. Apart from that, the altogether lower values in comparison to the Saxon area cannot be explained by the relief, as relief energy is at least as high or even higher. It can rather be assumed that land use systems and especially the field sizes are the major parameters to influence varying soil loss amounts. The average present field sizes vary from 1.6 ha within the Kraichgau catchments (Dabbert et al., 1999) to 15.3 ha within Central Saxony (Ernst and Förster, 2009). Especially the last centuries were most relevant regarding soil loss, as agriculture continuously intensified (Bork et al., 1998; Dotterweich, 2008). The Kraichgau Basin traditionally represents an area with a strong fragmentation of arable land, due to a kind of agricultural inheritance law, which caused the subdivision of lots at least since the early 19th century (Huber, 2005). In contrast, in Central Saxony, field sizes equaled 7 to 10 ha (1 hide) already during the inland colonization in the High Middle Ages. In the middle of the 19th century, fields in Saxony extended to a size of 10 to 50 ha due to land consolidation and grew to an extent of 18 to 51 ha in average during the forced collectivization in the 1960th (Lieberoth, 1982). Based on the importance of hill slope fragmentation on soil erosion intensity (Houben, 2008), slopes of the study area show no interruptions, and enable extensive sediment shifting due to undisturbed sediment pathways. Moreover, a distinct relief dependency of soil erosion and sedimentation processes becomes prevalent for Central Saxony. The results of Rommens et al. (2006) and Notebaert et al. (2009) show soil loss between 9200 and 10,800 t/ha for loess areas in Central Belgium for the Holocene period (Table 3). Relief configuration seems quite comparable to the Saxon loess belt, but precipitation increases to values of 700 to 800 mm per year due to a more oceanic climate. It is arguable, whether beside different land use systems and land use durations, an unsteady climate with a higher variability of precipitation can cause slightly increased soil loss amounts within the Saxon loess belt. A further numerical value for soil loss is provided by Houben (2008) from the Wetterau Basin, a loess covered area in SW-Germany

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(Table 3). A catchment with a size of 1000 ha points to an average soil loss of 9780 t/ha for the arable cultivation period of 7500 years. If soil profile truncation would be estimated on equal criteria with the work in hand, the soil loss within the Wetterau Basin would be reduced by approximately 25%. Besides the fact of the already mentioned hill slope fragmentation of the Wetterau area, a much more moderate relief presumably prevents from increased soil erosion. Finally, the soil erosion phenomenon is and will remain a very complex matter with a vast number of influencing, mutually dependent factors. Beside parameters like geological substrate (Macaire et al., 2002), relief, precipitation or land use systems and land use duration, also the scale of consideration plays a major role regarding soil erosion phenomena (De Vente et al., 2007; Notebaert et al., 2009; Slaymaker, 2006; Smith et al., 2001; Verstraeten and Poesen, 2001). Definitive relations between different parameters and specific process structures and process intensities remain uncertain to some degree. Nevertheless, this study discovered a strong correlation between the relief and the intensity of soil erosion processes inside the Saxon loess area. Because of the homogeneity of land utilization and the limited spatial extent of the study area, several parameters, like fragmentation of hill slopes, duration of land use, as well as climate could be excluded with regards to the intensity of sediment shifting. In fact, the slope length was identified as main controlling factor for soil loss quantities. Even in a supra-regional comparison with other Central European loess areas, large field sizes associated with high relief energy seem to be responsible for more intensive soil erosion within the Saxon loess area. 6. Conclusions The loess landscape of the Saxon loess belt shows a strongly varying pattern of soil erosion intensity within small-scale catchments (dells). Different values of soil loss arise from the close relation between soil erosion intensity and relief. Soil erosion principally takes place in form of rill erosion. Thus, the extent of soil erosion depends on a certain runoff as main control factor concerning formation and enlargement of rills. A specific distance to the watershed is necessary to intensify rill erosion as indicated by the lower middle slopes, which turned out to be most affected by soil erosion processes. Consequently, a long slope implies a higher efficiency of soil erosion processes, and therefore a higher soil loss. The causal relation for the varying slope lengths is to be found in the differing incision of the dell´s depth contours. That was caused by the differing incision of the linked river network that took place during the Pleistocene. Therefore, if valley incision is strong and relief energy is high, connected dells show an increased volume of sediment shifting, whereas low relief energy leads to a moderate mobilization of soil material. Furthermore it shows the effect and importance of inherited relief forming processes from the Pleistocene. The strong dissection of the Saxon loess belt causes high relief energy, and accordingly an extensive sediment shifting that points to an average of 9900 to 16,700 t/ha of soil loss. In a supra-regional comparison, these values surmount soil erosion quantities of other loess regions in Central Europe. Responsible for that, is the strongly accentuated relief, as well as uninterrupted sediment pathways caused by large field sizes, that exist already since historical periods. References Bentley, R., Price, D., Lüning, J., Gronenborn, D., Wahl, J., Fullagar, P., 2002. Prehistoric migration in Europe: strontium isotope analyses of early Neolithic skeletons. Current Anthropology 43, 799–804. Boden, A.G., 1996. Bodenkundliche Kartieranleitung, 4th ed. Hannover. 392 p. Bork, H.R., 1983. Die holozäne Relief- und Bodenentwicklung in Lößgebieten -Beispiele aus dem südöstlichen Niedersachsen. In: Bork, H.R., Ricken, W. (Eds.), Bodenerosion, Holozaene und Pleistozaene Bodenentwicklung. Catena Supplement, 3. Catena, Braunschweig, pp. 1–93. Bork, H.-R., Bork, H., Dalchow, C., Faust, B., Piorr, H., Schatz, T., 1998. Landschaftsentwicklung in Mitteleuropa, 1st ed. Klett-Perthes, Gotha. 328 p.

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