Influence of periglacial cover beds on in situ-produced cosmogenic 10Be in soil sections

Influence of periglacial cover beds on in situ-produced

cosmogenic 10Be in soil sections

M. Schaller a,*,1, F. von Blanckenburg a,2, H. Veit b, P.W. Kubik c

aInstitut fur Geologie, Universitat Bern, Erlachstrasse 9a, CH-3012 Bern, SwitzerlandbGeographisches Institut, Universitat Bern, Hallerstrasse 12, CH-3012 Bern, Switzerland

cPaul Scherrer Institut, c/o Institute of Particle Physics, ETH Honggerberg, CH-8093 Zurich, Switzerland

Received 21 September 2001; received in revised form 18 June 2002; accepted 21 June 2002

Abstract

Cover beds, widespread on hillslopes of temperate climate zones, represent layers of allochthonous material laterally

transported by periglacial processes during the Late Pleistocene. Two soil sections comprised of cover beds from the Bavarian

Forest, SE Germany, have been analysed for in situ-produced cosmogenic 10Be. Major changes in the nuclide concentration

agree well with soil section boundaries defined by field observations and grain size analyses. Numeric modeling of these

cosmogenic nuclide sections demonstrates that simple continuous erosion and regolith mixing models fail to explain the

measured nuclide concentrations. Instead, the measured data can be best described by modeling an admixture of material such

as loess or reworked allochthonous material, which have different nuclide concentrations.

A comparison of cosmogenic nuclide concentrations from the two cover bed sections with concentrations from river bed-

load sediments of the Regen catchment reveals that cover bed formation might affect the result of basin-wide erosion rate

determinations based on cosmogenic nuclides. Nuclide concentration of river bedload is potentially a binary mixture produced

by (1) spatial erosion of the soil surfaces; and (2) spatially nonuniform incision into deep cover bed layers that contributes

sediment low in nuclide concentration.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Soil; Cover bed; Cosmogenic nuclide; Erosion; Europe

1. Introduction

European soils frequently consist of several

allochthonous layers and are not an in situ weath-

ering product of a homogeneous bedrock substratum

materials (Semmel, 1993). These layers are called

‘‘cover beds’’ and differ in the lithology of their rock

fragments, texture, grain size distribution, mineral-

ogy, and heavy mineral association. Cover beds in

Central Europe are predominantly Late Pleistocene

in age and consist of material transported laterally

down-slope during climate cold stages, mostly by

periglacial processes such as solifluction or cryotur-

bation. Additionally, loess may have been incorpo-

0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0169 -555X(02 )00189 -7

* Corresponding author.

E-mail address: [email protected] (M. Schaller).1 Present address: Department of Earth Sciences, University of

Cambridge, Downing Street, Cambridge CB2 3EQ, UK.2 Present address: Institut fur Mineralogie, Universitat Hann-

over, Callinstrasse 9, D-30167 Hannover, Germany.

www.elsevier.com/locate/geomorph

Geomorphology 49 (2002) 255–267

rated into some of the layers due to periglacial

processes (Semmel, 1993). Mixing by floral and

bioturbation processes during the Holocene some-

times locally overprints these periglacial features.

The spatial and chronological correlation between

these layered deposits have been systematically

investigated in Germany (overviews in e.g. AK

Bodensystematik, 1998; Kleber, 1992; Mailander

and Veit, 2001; Semmel, 1993; Volkel, 1995). The

existence of cover beds as soil parent materials has

been proven in many mid-latitude regions (Kleber,

1997; Mailander and Veit, 2001). The presence of

layered deposits has important ecological consequen-

ces. Slope hydrology and the distribution of soil

types are governed by the physical and mineralogical

characteristics of cover beds. The identification of

cover beds is crucial for any type of geochemical

balance in soil profiles, whether of nutrients or

pollutants.

Cover beds in Germany may be classified into

three main layers (e.g. AK Bodensystematik, 1998;

Semmel, 1968): the Basal layer, the Intermediate

layer, and the Upper layer (Table 1). The age of

formation of the Upper layer is believed to be

Younger Dryas (e.g. Semmel, 1968; AG Boden der

Geologischen Landesamter, 1994) or Oldest Dryas

(Mahr, 1998; Volkel et al., 2001). The lower two

layers lack chronostratigraphic definition, but are

believed to be products of an evolutionary succession

of processes independent of the age of any particular

event. Formation ages of the Basal and Intermediate

layers vary from Early Wurmian to Late Glacial time

(Kleber, 1992). It has recently been shown that floral

and bioturbation processes occurred in the Upper

layer during the Holocene period (Mahr, 1998; Rus-

sow and Heinrich, 2001).

To date, the different layers have been identified by

field observations, grain size, bulk density and heavy

mineral analysis. In this study, in situ-produced cos-

mogenic nuclides are used to examine cover beds.

Cosmogenic nuclides represent a novel means of

quantifying the history of complex soil sections (e.g.

Braucher et al., 1998, 2000; Brown et al., 1994;

Phillips et al., 1998). These rare nuclides are produced

by nuclear reactions in minerals bombarded with

cosmic rays. The abundance of cosmogenic nuclides

in minerals is a function of depth and time and the

activity decreases exponentially with depth (Lal,

1991). The time a mineral resides at a certain depth

is given by the soil’s erosion or deposition history.

The highest activity of cosmogenic nuclides occurs at

a stable surface.

Two cover bed sections from the Bavarian Forest

(SE Germany) have been analysed for in situ-pro-

duced cosmogenic 10Be in quartz. The cosmogenic

nuclide data are compared with a classification of

layers based on field observations and grain size

analyses. Model calculations are performed to fit the

complex cosmogenic nuclide pattern. Finally, the

observed cosmogenic nuclide concentrations with

depth are compared with the nuclide concentrations

derived from the river sediment load of the Regen

catchment (Schaller et al., 2001).

2. Theory

The depth and time dependence of cosmogenic

nuclides in soil sections may be used to detect a

variety of processes operating in complex hillslope

settings. Some of the potential complex cosmogenic

nuclide patterns are discussed in the following

section.

Table 1

Characteristics of Late Pleistocene periglacial cover beds in

Germany (AK Bodensystematik, 1998)

Layer Occurrence Thickness

(cm)

Frequently

observed

characteristics

Upper layer widespread 30–70 basal enrichment

of rock fragments

cryoturbation

admixture of loess

low bulk density

Intermediate

layer

Restricted to

non-erosive

positions

0–50 basal enrichment

of rock fragments

orientated clasts

cryoturbation/

solifluction

admixture of loess

Basal layer widespread 20–100 cryoturbation/

solifluction

orientated clasts

mainly loess free

high-bulk density

M. Schaller et al. / Geomorphology 49 (2002) 255–267256

2.1. Simple continuous erosion

The simplest case is a rock section that is eroding

steadily by constant removal of material from the

surface. The nuclide concentration with depth can be

expressed by Eq. (8) of Lal (1991),

CðxÞ ¼P0

k þ e � qK

� � � e �x�qKð Þ ð1Þ

where C(x) is the nuclide concentration (atoms/g(qtz))

at depth x (cm), P0 is the cosmogenic nuclide pro-

duction rate at the surface (atoms/(g(qtz)� year)), q is

the rock density (g/cm3), K is the absorption mean

free path (g/cm2), k is the radioactive decay constant

(/year) and e is the erosion rate (cm/year). Without

radioactive decay (k= 0) or for high erosion rates

(kb((e� q)/(K))), this equation would take the form

CðxÞ ¼P0 � Ke � q

� e �x�qKð Þ ð2Þ

The cosmogenic nuclide concentration is related to

the erosion rate (Fig. 1a). In practice, the depth

Fig. 1. Cosmogenic nuclide depth sections modeled from different scenarios are compared to a reference scenario of continuous erosion with a

rate of 20 mm/kyear (dashed line). The production rate parameters used are those determined for sea-level high-latitude (see also text). (a) Simple

continuous erosion with different erosion rates e. (b) Simple continuous erosion taking into account a density change towards the surface. (c)

Simple continuous erosion and steady state regolith mixing as well as one recent instantaneous regolith mixing event. (d) Addition of material

with low- and high-nuclide concentration to the mixed surface. Note that for comparison, a different depth scale has been added to the right y-axis

for the case where no material has been added. (e) Material, e.g. a loess cover, acting as shield for the underlying cosmogenic nuclide section.

M. Schaller et al. / Geomorphology 49 (2002) 255–267 257

dependence of the nuclide concentration is the sum of

all nucleonic and muonic production mechanisms

(Schaller et al., 2001). Eqs. (1) and (2) have to be

expanded accordingly. High erosion rates lead to a

low-nuclide concentration at the surface with a mod-

erate decrease in nuclide concentration with depth.

Low erosion rates result in high-nuclide concentration

at the surface and a strong decrease in nuclide con-

centration with depth (Fig. 1a).

2.2. Simple continuous erosion and density change

In eroding sections with regolith (Fig. 1b), a

density change occurs at a given depth as hard rock

(q = 2.7 g/cm3) is converted into regolith (q = 1.5 g/

cm3). The effect of such a decrease in density on

cosmogenic nuclide activity is that the mean

absorption depth, given by q/K, increases. The

nuclide concentration is higher at a given depth

of regolith than at the equivalent depth of rock. If

the mass loss rate for regolith is the same as for

rock, the surface nuclide concentrations are the

same for sections with regolith or sections consist-

ing of pure bedrock.

2.3. Simple continuous erosion and regolith mixing

One of the most likely complexities in a regolith

section is that the surface layer has been mixed by

processes such as bioturbation, wind-throw, or con-

tinuous soil creep. Complete mixing would result in

uniform 10Be concentrations (Phillips et al., 1998;

Braucher et al., 2000). In this case, the concentration

of all samples over the mixed depth interval is equal to

the concentration of the surface samples (Granger et

al., 1996). Surface regolith mixing for both an instan-

taneous mixing scenario and a continuous steady state

scenario is shown in Fig. 1c.

2.4. Addition of material

During mixing processes such as solifluction,

external material that has a different nuclide concen-

tration can be incorporated into the regolith. Depend-

ing on the nuclide concentration of the added material,

the resulting 10Be concentrations will be either lower

or higher than in the case of simple continuous

erosion and regolith mixing (Fig. 1d). Assuming

complete mixing, the nuclide concentrations are uni-

form within the mixed layer. Incorporation of non-

quartz material has the long-term effect of decreasing

the production of cosmogenic nuclides in quartz by

providing shielding from cosmic rays. The same effect

results from incorporation of fine-grained material,

which will not be used for cosmogenic nuclide anal-

ysis. For example, the nuclide concentration measured

in a coarse-grained sample at a depth of a cover bed

admixed with loess is lower than in an undisturbed

cover bed (Fig. 1e).

2.5. Variable erosion rates

Eq. (1) assumes constant long-term erosion rates

and is therefore not valid if erosion rates change

through time. In the case of variable erosion rates,

Eq. (1) must be replaced by forward modeling. For

variable erosion rates, the nuclide concentration in

quartz at a given depth is the sum of all nuclides

produced as a certain regolith increment is trans-

ported towards the surface. To calculate the nuclide

concentration at a given depth, Eq. (3) of Lal

(1991) is used:

dCðxÞdt

¼ �CiðxÞ � k þ PðxÞ ð3Þ

Fig. 2. Overview map showing the localities of the Rossberg and

Brennersried sections situated in the Bavarian Forest, SE Germany.


where

PðxÞ ¼ Pð0Þ � e�x�q

Kð Þ ð4Þ

and

CiðxÞ ¼Xx�1

x¼max depth

PðxÞ � Dti ð5Þ

Here, each discrete time step is assigned an index i.

Dti is the period a particle is irradiated at a certain

depth x. Dti is given by the time it takes to erode

one soil depth increment Dx by the erosion rate ei(Dti =Dx/ei).

2.6. Cover beds

Cover beds consisting of different layers may be a

product of all of the above mentioned processes. Non-

steadily eroding surfaces may be mixed several times

and external material may be incorporated. A density

change occurs in the surface layer. Furthermore, as

regolith moves towards the surface by continuous

erosion, quartz may be enriched relative to feldspar

Table 2

Characterstics of section Brennersried and section Rossberg

Section Sample Layer Soil Depth Grain size analysisb Total 10Be

classificationa classification (cm)Sand

(wt.%)

Silt

(wt.%)

Clay

(wt.%)

concentrationc

(105 atoms/g(qtz))

Brennersriedd rep-1.16 Upper layer Ah 5F 5 61.2 29.8 8.9 2.11F 0.23

(48j06VN, 12j59VE rep-1.15 Upper layer Bv 30F 10 60.7 32.6 6.7 1.99F 0.18

640 m a.s.l.) rep-1.14 Upper layer Bv 45F 10 62.7 30.6 6.7

rep-1.13 Upper layer Bv 50F 10 60.6 27.2 12.2 1.86F 0.17

rep-1.12 Upper layer Bv 70F 10 62.0 23.9 14.1 1.92F 0.26

rep-1.11 Gully fill IIC 100F 10 77.9 20.1 1.9 1.04F 0.14





rep-1.6 Basal layer IIIC 195F 10 77.5 17.2 5.3 0.428F 0.086



rep-1.3 Saprolite IVCv 230F 10 73.2 19.7 7.1 0.279F 0.075

rep-1.2 Saprolite IVCv 260F 10 72.4 14.2 13.3

rep-1.1 Saprolite IVCv 290F 10 0.224F 0.089

Rossberge rep-2.9 Upper layer Ae 5F 5 57.9 34.4 7.7 3.05F 0.61

(49j11VN, 12j49VE rep-2.8 Upper layer Bv 20F 10 59.8 26.2 13.9 3.38F 0.22

420 m a.s.l.) rep-2.7 Upper layer Bv 35F 10 58.2 27.5 14.3 3.66F 0.26

rep-2.6 Basal layer IIC 55F 10 66.1 21.4 12.5 2.45F 0.26

rep-2.5 Basal layer IIC 85F 10 65.1 21.1 13.8 2.79F 0.21

rep-2.4 Saprolite IIICv 100F 10 74.9 15.0 10.1 2.12F 0.17



rep-2.1 Saprolite IIIC 205F 20

a Classification after AK Bodensystematik (1998).b Grain size analysis as described in text.c 10Be concentration in quartz corrected for blank, reported error includes analytical uncertainties (1r).d Nucleonic production rate Pn = 9.06 atoms/(g(qtz)� year), stopped muonic production rate PA= 0.144 atoms/(g(qtz) year), and fast

muonic production rate PAfast = 0.096 atoms/(g(qtz) year).e Nucleonic production rate Pn = 7.45 atoms/(g(qtz)� year), stopped muonic production rate PA= 0.130 atoms/(g(qtz) year), and fast

muonic production rate PAfast = 0.095 atoms/(g(qtz) year).


by continued weathering. The general result of non-

quartz mineral dissolution is that quartz material irra-

diated at a certain depth is located, after dissolution, at a

higher level in the section relative to the surface

(Braucher et al., 1998; Small et al., 1999). The process

of non-quartz mineral dissolution adds a level of

complexity that has not been modeled in cover beds.

3. Study area and characteristics of soil sections

3.1. Study area

The cover beds studied are located in the Bavarian

Forest, which forms part of the middle European

uplands of the Bohemian Massif (Fig. 2). Precambrian

metamorphic rocks and Variscan granites exposed in

the Bavarian Forest were eroded and severely weath-

ered during the Tertiary (Mahr, 1998). The Bavarian

Forest is located within the boundary between a

maritime dominated climate and an increasingly con-

tinental climate. This combination leads to heavy

winter snowfall (173 days/year with >1 cm snowfall

and complete snow cover). Despite today’s climatic

conditions, only the highlands of the Bavarian Forest

were glaciated during the Late Pleistocene (Raab,

1999). The lower parts of the Bavarian Forest have,

however, been influenced by periglacial processes that

formed cover beds during the Pleistocene cold phases.

The two soil sections occur outside of the glacial

margins of the Last Glacial Maximum. The periglacial

cover beds of the Bavarian Forest show typical char-

acteristics of the cover beds defined in the classifica-

tion known from Germany (Table 1). The formation of

the Upper layer in the Bavarian Forest occurred during

the Late Glacial probably during Oldest Dryas (Volkel

and Mahr, 1997). Pollen analyses indicate that the area

was not forested during the Oldest Dryas (Stalling,

1987). During the Bølling and Allerød periods

(12,600–12,000 and 11,800–10,750 14C year BP,

respectively) the vegetation changed from shrubs to

forest. The vegetation during the Older Dryas period

(12,000–11,800 14C year BP) is not well defined due

to its short duration. During the Younger Dryas time, a

change in non-tree pollen content, as well as a retreat

of birches and forest clearance, have been determined.

At the end of the Younger Dryas, a sharp decrease in

non-tree pollen and the first appearance of thermo-

phile tree pollen are reported and reflect the transition

to the warmer Holocene epoch (Stalling, 1987).

3.2. Characteristics of soil sections

Two soil sections from the Bavarian Forest have

been analysed for grain size distribution and classified

according to the nomenclature known from Germany

(AK Bodensystematik, 1998). The sand-sized fractions

of sediment samples were separated by wet sieving

(dispersion with Na2CO3). The silt and clay grain size

fractions were analysed using a Micromeritics Sedi-

Graph 5100 (dispersion with (NaPO3)12–13�Na2O and

NaHCO3).

Fig. 3. Sketch of Rossberg and Brennersried sections. (a) The Rossberg section consists of an Upper layer, a Basal layer, and a saprolite. (b) In

the Brennersried section, an Upper layer, a gully fill, a Basal layer, and a saprolite are present. The different units contain granitic (white) and

gneissic (light grey) rock fragments. Horizontal and vertical distances have the same scale.


The cover bed of the Rossberg section (Table 2;

Figs. 3a and 4a) shows the commonly observed

succession of Upper and Basal layers and is situated

on granitic saprolite. The high silt content (26–34%)

in the Upper layer is attributed to the admixture of

loess. The Basal layer is characterised by a higher

sand content and a lower silt content than the Upper

layer. A continuous increase in the amount of sand

and a decrease in the amount of silt and clay are

observed in the saprolite with increasing depth. The

layers of the Rossberg section do not contain any rock

fragments. The soil type of section Rossberg is a

Cambisol. The lower boundary of the Bv horizon

coincides with that of the Upper layer.

In the Brennersried section (Table 2; Figs. 3b

and 5a), the Upper layer contains granitic and

Fig. 4. (a) Layers of cover beds (bold lines) from the Rossberg section defined by grain size analyses and field observations. Samples (triangles)

have been separated into sand- (dark grey), silt- (white) and clay-fractions (light grey). The reported values are weight percentages. (b)

Significant changes in the 10Be concentration of samples from the Rossberg section coincide with layers defined by grain size analyses and field

observations. Uncertainties in nuclide concentration are analytical errors (1r). Error bars with depth indicate the depth range in which the

samples have been collected.

Fig. 5. (a) Grain size analyses from samples of the Brennersried section. See Fig. 4 for further explanations. (b) 10Be concentrations of quartz

material from different depths. Some major changes coincide with layer boundaries defined by field observations and grain size analyses.


gneissic rock fragments and is deposited uncon-

formably over all discontinuities observed in the

lower part of the soil section. The lower part

consists of a Basal layer and a gully incised into

the Basal layer. The gully fill, containing few small

granitic and gneissic rock fragments, consists of

several small sublayers. The Basal layer overlies a

granitic saprolite and shows no internal sedimento-

logical layering. The soil type of the Brennersried

section is a slightly podzolic Cambisol. Again, the

lower boundary of the Bv horizon coincides with

that of the Upper layer.

4. Cosmogenic nuclide analysis

Samples were taken in at least 40 cm depth

intervals within a depth range of at least 10 cm in

order to obtain a suitable weight for cosmogenic

nuclide analysis The sample depth indicated in Table

2 is the mean sample depth. Quartz grains (0.5–1.0

mm) were separated from the bulk samples. This grain

size range was selected to avoid changes in the

cosmogenic nuclide concentration due to different

grain size distributions. Furthermore, it ensures that

no wind-blown material is incorporated into the cos-

mogenic nuclide measurement. To eliminate possible

contamination by 10Be produced in the atmosphere,

the quartz was leached with hydrofluoric acid. After

dissolution of the quartz (approximately 50 g per

sample) and addition of the 9Be spike, Be was

separated by extraction and precipitation using the

modified method of von Blanckenburg et al. (1996).

Accelerator mass spectrometry to measure the 10Be/

Be ratios was carried out at the PSI/ETH facility in

Zurich. The measured 10Be/Be ratios were corrected

as described in Kubik et al. (1998). The blank-

corrected 10Be concentrations and the analytical errors

(reported throughout as 1r errors), including blank

error, are listed in Table 2.

For model calculations, use was made of sea-level

high-latitude production rates of 5.33F .34 atoms/

(g(qtz) � year) for nucleonic, 0.106 atoms/

(g(qtz)� year) for stopped muonic, and 0.093

atoms/(g(qtz)� year) for fast muonic production

mechanisms. The sea-level high-latitude values were

scaled to the sample altitude and latitude using com-

bined scaling factors of Dunai (2000) and Heisinger et

al. (2002a,b). Total production rates are 7.73 and 9.30

atoms/(g(qtz)� year) for profile Rossberg and Bren-

nersried, respectively (see also Table 2). In order to

take into account the exponential decrease of produc-

tion rate with depth, we made use of the revised

absorption laws published in Heisinger et al.

(2002a,b) that were converted into multiexponential

fits similar to Schaller et al. (2001). Absorption of

nucleons was calculated by a double-exponential fit

with coefficients a1 = 1.0747, b1 = 157 g/cm2 and

a2 =� 0.0747, b2 = 5.887 g/cm2. Stopped muonic

absorption was approximated with a three-exponential

fit: a1 =� 0.050, b1 = 160 g/cm2; a2 = 0.845, b2 = 1030

g/cm2; a3 = 0.205, b3 = 3000 g/cm2. Fast muonic

absorption was approximated with a three-exponential

fit: a1 = 0.010, b1 = 100 g/cm2; a2 = 0.615, b2 = 1520 g/

cm2; a3 = 0.375, b3 = 7600 g/cm2.

5. Results

The Rossberg section reveals 10Be concentrations

in quartz ranging from 0.7� 105 to 3.7� 105 atoms/

g(qtz) (Fig. 4b). The highest 10Be concentrations are

measured in the Upper layer. Using Eq. (1), an

apparent mean erosion rate of about 20 mm/kyear is

determined from the nuclide concentration of the

uppermost sample. The samples of the Upper layer

are identical within error and no trend with depth can

be determined. A 1.4-fold decrease in the average10Be concentration of the samples from the Upper

layer to the uppermost sample of the Basal layer is

observed. A sample taken from the saprolite has a10Be activity 25% less than the lower sample of the

Basal layer. A 3-fold decrease of the 10Be concen-

tration is observed in the samples collected from

within the saprolite (depth difference 80 cm).

The 10Be concentrations of quartz from the Bren-

nersried section range from 0.2� 105 to 2.1�105

atoms/g(qtz) (Fig. 5b). Four samples from the Upper

layer have identical nuclide concentrations within

error. An apparent mean erosion rate of about 35

mm/kyear is calculated from the nuclide concentration

of the uppermost sample. A 2-fold decrease in the

nuclide concentrations from the Upper layer to the

gully fill defines a clear boundary between the Upper

layer and the gully fill. The 10Be concentration

measured in quartz decreases steadily from the top


of the gully fill to the bottom. Samples analysed from

the Basal layer have a 10Be activity that is identical

within error. The two samples collected from the

saprolite contain lower nuclide concentrations than

the overlying Basal layer.

In comparison, the two sections show similar

relative changes in 10Be concentration. However,

the 10Be concentration at the surface of the Ross-

berg section is higher than that at the surface of the

Brennersried section. Layer boundaries determined

by field observations and grain size analyses coin-

cide well with changes in the 10Be concentrations.

6. Discussion

6.1. Cover beds

The physics of cover bed formation are complex.

In central Europe, long-term monitoring of modern

solifluction and process studies have been under-

taken in the alpine/sunivale ecotone of the Alps

Fig. 6. (a) Comparison of measured with modeled 10Be concen-

trations of the Rossberg section, taking into account simple

continuous erosion (solid lines) and regolith mixing (stippled and

dotted lines). Vertically well-mixed regolith eroding with 20 mm/

kyear models the data of the surface layer (dotted line). To model

the surface data with one instantaneous regolith mixing event, an

erosion rate of 15 mm/kyear is required (stippled line). (b) Fit of

measured 10Be concentrations by modeling cover beds.

Fig. 7. (a) A continuous erosion rate of 37 mm/kyear and vertical

mixing of the regolith model the measured 10Be concentrations of

the surface layer from the Brennersried section (dotted line).

Instantaneous regolith mixing requires a continuous erosion rate of

27 mm/kyear to calculate the cosmogenic nuclide concentration of

the surface layer (stippled line). (b) Fit of measured 10Be

concentrations is achieved by modeling cover beds.


(Gamper, 1986; Jaesche, 1999; Veit et al., 1995). It

is difficult to determine the exact timing of the Late

Pleistocene cover bed formation as datable material

is scarce. The characteristic patterns of cosmogenic

nuclides may reveal some information about cover

bed genesis.

The 10Be of both the Rossberg and Brennersried

sections have been modeled by simple continuous

erosion taking into account a density change from

rock (q = 2.7 g/cm3) to soil (q = 1.5 g/cm3) (Figs. 6a

and 7a and Section 2). Simple continuous erosion

with a density change cannot explain the complex

cosmogenic nuclide pattern. Also, simple continuous

erosion combined with regolith mixing of the surface

layer by floral and bioturbation or solifluction pro-

cesses fails to explain the measured nuclide concen-

trations (Figs. 6a and 7a and Section 2). The

complex features in the measured nuclide concen-

trations can best be explained by several mixing

processes and the addition of allochthonous material

as described for cover bed formation (Figs. 6b and

7b; Table 3 and Section 2). The modeled formation

processes described below are not unique but possi-

ble solutions, most of which agree with the published

concepts of cover bed genesis.

We modeled the Basal layers of the two sections

by adding external material at 15 and 39 kyear BP,

respectively. No loess has been added to the mod-

eled Basal layer, which is in agreement with the

low silt content of the Basal layer. Basal layer

formation ages are within the range of literature

ages, which differ significantly from each other due

to the lack of datable material. Formation ages of

the Basal and Intermediate layers postulated in the

literature vary from Early Wurmian to Late Glacial.

The incorporation of a large quantity of material

with low-nuclide concentration into the gully fill at

section Brennersried can be attributed to the fact

that the stratified gully fill is not the product of

periglacial solifluction but is produced by other

mechanisms such as erosion by water and transport

by water during or since periglacial conditions. The

modeled admixture of fine-grained material (e.g.

loess) into the Upper layer is in agreement with

the characteristics of cover beds in Germany (AK

Bodensystematik, 1998). The modeled loess depo-

sition took place at the Pleistocene to Holocene

transition. This age agrees well with the reported

Younger or Oldest Dryas age of the Upper layer.

Furthermore, the modeling of the cover beds indi-

cates continuous mixing of the surface layers. This

observation supports postulated mixing by floral

and bioturbation processes during the Holocene

period. (Mahr, 1998; Russow and Heinrich, 2001).

Table 3

Parameters used for modeling cover beds

Section Time interval

(kyear BP)

Erosion rate

(mm/kyear)

Mixing interval

(cm)

Admixture

of quartz-material

(cm)

Nuclide

concentration

of admixture

(atoms/g(qtz))

Admixture

of nonquartz

material (cm)

Rossberg >15 20

15 0–65 65 170,000 0

15–10 80

10 0–30 10 600,000 10

10–0 10 0–45 0 0

Brennersried >39 60

strong erosion event

39 0–20 10 50,000 0

39–37 60

37 0–20 100 60,000 0

37–10 30

10 0–80 10 600,000 10

10–0 10


6.2. Catchment-wide erosion rates

In situ-produced cosmogenic nuclides offer an

approach to quantify catchment-wide erosion esti-

mates from alluvial sediments. The concept addresses

the mixing of a multitude of individual bedrock and

soil surface samples by hillslope and fluvial processes

(Brown et al., 1995; Bierman and Steig, 1996;

Granger et al., 1996). This approach has been applied

to large catchments (1�103–4� 104 km2) in Middle

Europe, among them the Regen catchment from

where the samples of the present study have been

taken (Schaller et al., 2001). Schaller et al. (2001)

made the observation that cosmogenic nuclide-

derived erosion rates are 1.5–4 times higher than

river load gauging-derived erosion rates. A similar

trend has been observed in the temperate USA by

Kirchner et al. (2001). Schaller et al. (2001) offer

three potential explanations: (1) systematic underesti-

mation of modern river loads, for example by failing

to include high-magnitude, low-frequency transport

events in the gauging period; (2) spatially nonuniform

erosion (e.g. linear dissection) and selective tapping of

deeper segments of the irradiation section; and (3)

inheritance of a Late Pleistocene erosion signal.

The two soil sections from the Bavarian Forest

give a first insight into the depth dependency of cos-

mogenic nuclide concentrations and address explan-

ation (2). The range encompassed by nuclide con-

centrations of surface samples from Rossberg and

Brennersried sections are at the lower and upper

end, respectively, of the nuclide concentrations mea-

sured in bedload of the Regen catchment (Fig. 8).

The spread of nuclide concentrations in river bed-

load can be explained (1) by eroding only the sur-

face of the soil sections. However, river concen-

trations are also compatible with (2) delivery of

deep sediment by linear incision. For example,

material from the Basal layer of section Rossberg

(100 cm) would deliver sediment with the cosmo-

genic nuclide concentration observed in river bed-

load. Therefore, remobilisation of cover bed sedi-

ments from depth cannot be excluded as one of two

sources of sediment to the river Regen, or to any

river in areas affected by these periglacial hillslope

processes.

7. Conclusion

Major changes in cosmogenic nuclide concentra-

tions in two soil profiles agree well with slope deposit

boundaries defined by grain size analyses and field

observations. Measured nuclide concentrations are

incompatible with cover bed genesis involving simple

continuous erosion and regolith mixing only. Nuclide

concentrations of cover beds can only be fitted with

complex modeling, taking into account several stages

of soil mixing and material addition.

A comparison of cosmogenic nuclides measured in

the bedload of rivers with those in slope deposits

indicate that the nuclide concentrations in river sedi-

ments used for the determination of catchment-wide

erosion rates can be produced by surface material of

cover beds. However, the dissection of cover beds by

modern streams might potentially contribute bedload

low in nuclide concentrations.

Acknowledgements

We gratefully acknowledge laboratory assistance by

I. Hebeisen and A. Brockman. Comments by P.

Bierman and W. Blake helped to improve an earlier

version of the manuscript. This study was supported by

Fig. 8. Comparison of 10Be concentrations measured in the

Rossberg section (open circles) and the Brennersried section (open

squares), as well as of sediments from rivers (triangles), draining

parts of the Bavarian Forest. 10Be concentrations of the surface

samples from the two sections are at the lower and upper ends of the

range observed in river sediments.


the Schweizerische Nationalfonds (Grant 2100-053971

to FVB).

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Influence of periglacial cover beds on in situ-produced cosmogenic 10Be in soil sections

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