Colluvial sediment in Central Europe

Colluvial
sediment in
Central Europe
Dr Thomas Hoffmann discusses his research which investigates colluvial sediment with the
aim to further knowledge on the long-term impacts of human-induced soil erosion in Europe
dr thomas hoffmann
What are the chief goals of your project?
To understand the impact of the first farmers in
Central Europe on soil erosion and degradation.
We hope to achieve this by: i) quantifying spatial
variation in the onset of colluvial process along
the trajectories of human migration pathways,
and iii) developing a human impact proxy of
colluvial hillslope systems.
Could you define ‘colluvial sediment’ and it’s
geomorphological significance?
Colluvial sediments are hillslope deposits, many
of which are caused by anthropogenic soil
erosion. A significant proportion of soil eroded
on gently inclined hillslopes is not exported into
river channels and the ocean, but is transported
over a short distance and remains on hillslopes
for several thousand years. Therefore, colluvial
deposits can be directly linked to phases of
increased soil erosion and provide an effective
means to reconstruct periods of increased soil
erosion in Central Europe since the Neolithic
(starting 7,500 years ago).
How did you initially develop an interest in
this research area?
As a physical geographer, I have always been
interested in how humans impact on their
natural environment. I was curious to know if
humans have only very recently modified their
environment or if substantial transitions from
natural to human dominated environments
already took place several thousand years ago.
Living in Central Europe, it was rather obvious
that human-induced soil erosion did not start
during the Industrial Revolution, but is eminent
since the first farmers arrived in the fertile Loess
countries of Central Europe.
Could you provide an overview of the
methods you are using to achieve your
project’s aims?
We use a sediment budget approach as an
organising framework to synthesise case studies
on colluvial sediments. A sediment budget
compares the sources, transport pathways
and sinks of eroded soil. It thus considers the
intermediate sediment storages (eg. storage
in colluvial footslopes, debris fans, floodplains
and lakes) as sediments travel from hillslopes
to the sea, and takes into account complex
responses of soil erosion and transport
resulting from external environmental changes
and internal feedback, which are intrinsic to
sediment fluxes.
The temporal evolution of sediment budget
components in Central Europe is studied by
compiling a database of age estimates (eg.
through radiocarbon and optically stimulated
luminescence) in different depositional
environments. These dates are analysed in
terms of frequency distributions and changing
nature of sediment accumulation rates.
Have you made any interesting findings so
far that you would like to highlight?
Despite massive soil erosion in Central
Europe since the Neolithic, only a fraction
of eroded soils were transported beyond the
hillslopes. The majority of the eroded soils
are transported only a short distance and are
deposited within the eroded field or at the
foot of the eroded hillslope. This is especially
true for Neolithic land use, which was limited
to small patches, within a predominantly
forest covered landscape. The results suggest
that soil erosion during the Neolithic did
not result in a significant soil degradation
and decline of soil nutrients. Thus, it is very
unlikely that soil erosion provides a critical
boundary condition for regional human
migration during that period.
How might your research contribute to a
deeper understanding of anthropological
effects and global climate?
Today, soil degradation affects 38 per cent
of global cropland and thus has undesirable
impacts on sustainable food production.
Soil erosion, however, is not limited to the
last decades. Much (pre)historical evidence
indicates that water erosion caused a
substantial decline of food resources in ancient
societies that may have been a critical driver
of their collapse. Already, Plato commented
on the deforestation of Attica: “What now
remains compared with what then existed
is like the skeleton of a sick man, all fat and
soft earth having wasted away, and only
the bare framework of the land being left”.
Therefore, we intend to evaluate when and
where anthropogenic soil erosion became a
controlling factor of human migration as a
consequence of declining crop yields.
It has been argued that early humaninduced
deforestation is a major cause
of increasing atmospheric CO 2 starting
7,000 years ago. However, increasing soil
erosion caused by early deforestation
removes organic rich topsoils, which
are stored in hillslope and floodplain
deposits. Carbon burial, in conjunction
with replacement of carbon on eroded
sites, results in a net removal of CO 2 from
the atmosphere and might potentially
offset the effects of carbon emission of
human-induced deforestation. Gaining a
detailed knowledge of the mechanisms of
soil erosion on the global carbon cycle is
therefore imperative.
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dr thomas hoffmann
Understanding the impacts of soil erosion
Soil erosion by water remains the greatest global cause of soil degradation, yet the impact of (pre)historical
land management on soil erosion is still poorly understood. Analysing colluvial deposits may help us understand
past and present human-environmental interactions with respect to changing soil fertility and productivity
Environmental change is an important
topic, and one which is increasingly impossible
to ignore. Whether this change is precipitated
by human impacts or natural climatic factors
is a highly contentious issue, particularly in the
context of climate change politics. Increasingly, the
interrelation of different human factors and natural
forces is reassessed and re-evaluated. Needless
to say, it is not simply a dichotomy between CO 2
emissions and carbon mitigation initiatives.
Changing land use, for example, is also an important
factor to be considered in any conversation around
environmental change, and a project led by Dr
Thomas Hoffmann suggests that the long term
environmental impact of soil erosion is much
greater than one might expect. Whether natural or
human induced, soil erosion mobilises organic-rich
topsoil and can lead to the burial of organic carbon
in terrestrial and aquatic ecosystems. Because of
this, soil erosion can result in the degradation of
land and reduction of soil fertility, and can therefore
threaten the livelihoods of many who live off the
land.
In addition, however, the erosion and re-deposition
of organic carbon attached to sediment is an
imporant part of the global carbon cycle, yet a
part which is poorly understood – particularly in
the context of historic, or prehistoric, interactions
between humans and the environment.
LUCIFS & CRC 806
This is where the Land Use and Climate Impacts on
Fluvial Systems (LUCIFS) network comes in. As part
of the Past Global changES (PAGES) programme,
LUCIFS is chaired by Dr Thomas Hoffmann and
supported by an international cohort of top
geoscientists, with the aim of seeking to understand
past interactions between climate, human activity
and fluvial systems.
By utilising a geomorphological and
sedimentological approach, LUCIFS aims to
gain a greater understanding of the mid- to
long-term impact of humans on the landscape.
Hoffmann employs expertise from numerous
disciplines, including archaeology, paleo-botany,
soil science and geomorphology, and believes
that a multidisciplinary approach will lead to a
better understanding of the impact of human
history on soil erosion and degradation. Through
Hoffmann, LUCIFS has teamed up with CRC
806 (Collaborative Research Centre 806 ’Our
Way to Europe’) – a multidisciplinary research
initiative, bringing scientists together to research
cooperatively on the complex interrelation of
climatic, environmental, and socio-cultural factors
which drove (or enabled) the dispersal of Homo
sapiens, or modern man, from Africa to Central
Europe and resulted in human induced changes of
the natural environment.
Examining colluvial deposits
As part of CRC 806’s aim to shed light on historic,
or prehistoric, human-environmental interactions,
Hoffmann has led a LUCIFS project to analyse
colluvial deposits across Central Europe. When soil
erodes, either naturally or through agriculture, it
rarely travels as far as the sea, but often builds up
as deposits on hillslopes. Since colluvial sediment
can lie here for thousands of years, it can be dated
to provide a picture of historic climatic conditions
and land use practices. Colluvial sediment therefore
has the potential to open the door to humanenvironmental
interactions from the Neolithic to
the present day.
In addition, quantifying sediment storage has the
potential to gauge the effects of related carbon
storage on atmospheric CO 2 consumption, and
could therefore inform current land use practices
and carbon mitigation policy.
Soil erosion can result in
the degradation of land and
reduction of soil fertility, and can
therefore threaten the livelihoods
of many who live off the land
Driving the project forwards, Hoffmann is
coordinating various research methods to arrive
at as accurate a quantification of past humanenvironment
interactions as possible. These
methods include sediment budgeting on different
spacial and temporal scales, spatial modelling with
GIS and geostatistics, mathematical and numerical
modelling, and geophysics. Key to the project’s
approach is providing data on a dynamic large
scale. “Research on colluvial deposits is traditionally
focused on local archaeological sites or neighbouring
hillslopes,” explains Hoffmann. “Soil erosion in
prehistoric times, however, is very scattered due to
disordered land use activity, causing a high spatial
variability of colluvial deposits.”
As a result, Hoffmann’s team is bringing together a
large dataset from across Central Europe to try and
gain a more complete picture of colluvial sediment
and therefore also historic, and prehistoric, humanenvironmental
interactions.
Rising to the challenge
Their research is not without obstacles: “On
steep terrain, deposition of eroded soil is typically
controlled by slope steepness and curvature, and
can therefore be predicted from digital elevation
models,” Hoffmann examines. On gentle terrain
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however, eroded sediment is generally deposited
at local features such as field borders, roads and
shallow depressions, which block the down-slope
surface water flow. “These features are not static,
but change over time, due to changing land use
practises, causing a changing pattern of colluvial
deposition in different land use periods. These
features are difficult to reconstruct and thus add a
contingent dimension to the spatial and temporal
pattern of colluvial accumulation.”
One further challenge has been encountered in
the form of colluvial deposits having a limited
preservation potential. Whereas lakes provide
a long-term sedimentary sink with very limited
post-depositional modification, colluvial deposits
are subject to erosion in subsequent storm events.
Consequently, prehistoric colluvial deposits are
less frequent than younger deposits and they
are less likely to be detected. On account of
this bias, Hoffmann believes that we may have
underestimated human-induced soil erosion during
early prehistoric times.
As well as benefiting from multidisciplinary
cooperation and the involvement of the LUCIFS
network and CRC 806, this project has been
helped by a number of advances in geoscience.
Hoffmann comments: “Our understanding of
temporal changes has greatly advanced through
the development and advancement of several
dating techniques, of which radiocarbon dating and
optically stimulated luminescence (OSL) has been
most promising”.
Yet the research also benefits from recent
developments in data handling and the (re-)
construction of large scale datasets. Large
scale maps of historic and prehistoric land use
have recently become available. Coupled with
extensive high resolution digital elevation models,
Hoffmann and his team have been able to study
soil degradation of early cultural epochs on a much
wider scale.
Future focus
The results of the project so far suggest that limited
sediment delivery from hillslopes to river channels
and the long-term effects of carbon burial in human
induced sediment is of significant importance.
Through collaboration with the LUCIFS network,
Outcrop with 3 m thick colluvial sediments in the loess
lower Rhine (GER). The dark brown layer on top shows
a young colluvium deposited since 1950AD, the light
brown layers were depsoited since the Bronze age.
Hoffmann has reassessed the relevance of carbon
burial in hillslopes and floodplains and called for
their inclusion in carbon budgets.
Looking forwards, not only is the project likely to
inform us about our past and the impacts of humanenvironmental
interactions on Holocene human
migration, but it also has a present relevance. By
investigating the factors that played a role in the
human-environmental interactions of our early
ancestors, the projects could contribute towards
a greater understanding of environmental change,
and feed into current and future land and river
management policy.
The project’s next landmark is the Open PAGES
Focus 4 Workshop ’Towards a more accurate
quantification of human-environment interactions
in the past Human-Climate-Ecosystem
Interactions’, which will be held in Leuven,
Belgium, 3-7 February, 2014. More information
about the workshop can be found at http://ees.
kuleuven.be/pages/.
Intelligence
Human Impact on Colluvial
sediment storages in Europe since
the Neolithic
OBJECTIVES
To improve the understanding of human
impact on soil degradation and soil erosion
and it’s feedback to human migration during
the past 7,500 years in Central Europe.
KEY COLLABORATORS
Professor Richard Dikau (University of
Bonn) • Professor Andreas Lang (University
of Liverpool) • Professor Bob Wasson
(University of Signapore) • Dr Gert Vertraeten
(University of Leuven) • Professor Oliver
Korup (University of Potsdam) • Professor
Andreas Zimmermann (University Cologne)
• Professor Renate Gerlach (LVR, Bonn) •
Professor Wulf Amelung (University of Bonn)
• members of the CRC 806 (Universities of
Cologne, Bonn and Aachen)
FUNDING
A 36-month project was founded by the
German Research Foundation as part of the
Collaborative Research Centre 806 ‘Our way
to Europe: Culture-Environment Interaction
and Human Mobility in the Late Quaternary’
(SFB 806/1)
CONTACT
Dr Thomas Hoffman
Project Coordinator
University of Bonn
Department of Geography
Meckenheimer Allee 166
53115 Bonn, Germany
T +49 228 73 7507
E thomas.hoffmann@uni-bonn.de
www.geomorphology.uni-bonn.de/
persons/hoffmann
Thomas Hoffmann is a geomorphologist
at the Department of Geography, University
of Bonn. He is the leader of the PAGES Focus
4 working group on soils and sediments. The
main field of his research is on soil erosion and
sediment fluxes through river systems.
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