Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

Figure 2:

Mandach Deckenschotter

outcrop sampled by

Joachim Kuhlemann at

the beginning of the

pilot project.

profile» below a buried soil layer, so an ancient

buried exposed surface [12]. The main advantage

of isochron-burial dating is that it is independent

of erosional modification of the top surface of the

deposit. This method is extremely promising but

has been applied in only a few settings [15].

Work carried out and

results obtained

Our project was approved in Mai 2012, and officially

began in October 1 2012. We chose a PhD

candidate: Anne Claude. Anne Claude completed

her MSc study in ETH Zurich under the supervision

of PD Dr. Susan Ivy-Ochs (title of MSc thesis: Geomorphology

and landscape evolution at the Chironico

landslide, Leventina). To establish the direct

chronology of the landslide, she applied surface

exposure dating with cosmogenic 10 Be and 36 Cl.

Therefore, she has the basic knowledge on and

training for the sample preparation. Since October

1 st , her focus has been i) literature survey, ii) learning

in detail about the Swiss Deckenschotter, and

iii) refining nuclide extraction laboratory skills. She

recently submitted her first abstract, introducing

the Deckenschotter, to the 8 th International Conference

on Geomorphology of the International

Association of Geomorphologists, which will be

held in August 2013 in Paris.

This project involves challenges on several fronts

both analytical and field related. We propose the

application of several cosmogenic nuclide dating

techniques: burial, isochron-burial and depth-profile

dating. Although formally the methodology is

well established (has been applied at many sites

worldwide) [e.g. 20], for our specific case it involves

the challenge of finding the best outcrop

situation. For this reason close interaction of all

parties is sustained and encouraged.

Apart from the start of the PhD candidate, further

progress was made towards selecting suitable

sites: at a field meeting on March 30 2012

(participants Naki Akçar, Susan Ivy-Ochs, Andreas

Dehnert and Hansruedi Graf). At this meeting we

discussed the results from the pilot project at the

Mandach outcrop (Figure 2). This critical view can

now be incorporated in our ongoing site evaluation.

Results from the pilot project results and lessons

learned at Mandach and Irchel add to information

about choosing an optimal field situation

but also gives us hints about the range of nuclide

atoms per gram to be expected from foreland

Deckenschotter deposits. Based on this latter information,

we can optimize the sample preparation

and AMS measurement parameters.

Burial dating results have been published from

a variety of sites, yet our field situation does increase

the technical challenge. For both burial and

isochron burial dating, concentrations of both 10 Be

and 26 Al are required. Only with analytical errors

that are as low as possible can we estimate ages.

Although AMS can easily attain uncertainties as

low as 3% for these nuclides, the Deckenschotter

sites have rather low concentrations, thus we

are working on several fronts to reduce uncertainties.

Low nuclide concentrations stem from a likely

short period of time in which nuclides built-up

prior to burial, as well as to decay during the long

burial time. Here we do note that recent developments

have led to a notable decrease in 10 Be

measurement uncertainties, especially for samples

with very low 10 Be content [21]. This was achieved

through a combination of optimization of extraction

techniques as well as changes in accelerator

mass spectrometry measurement procedures.

Within the scope of this project, we are working

on similar optimization for 26 Al.

National Cooperation

The scientific collaboration on cosmogenic nuclide

methodology and applications between the Institute

of Geological Sciences at the University of

Bern and the Laboratory of Ion Beam Physics (LIP)

at ETH Zürich, established in the early 90’s, yielded

several research projects, international publications,

PhD and MSc. theses. This consortium has

a long tradition and a wealth of experience in applying

cosmogenic nuclides ( 10 Be, 26 Al and 36 Cl)

to determining the timing of events and rates of

landscape change in four different settings: Quaternary

glaciations, local and large-scale surface

erosion, landslides, and neotectonics. In addition,

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ENSI Erfahrungs- und Forschungsbericht 2012