19 juin 2015, par Irene Schimmelpfennig

The past changes of glaciers are usually determined from glacier landform dating (moraines, roches moutonnées, peatbogs …), providing results about maximum glacier extents.
Moraines were deposited along the margins of an advancing or stagnating glacier just before it started retreating due to warmer/dryer conditions. We envision complementing the existing moraine record from the Mont Blanc Massif with additional data in different catchments. Some sites feature moraine ridges slightly outboard of late LIA limits, presumably of Holocene age. The ridges will be mapped and dated according to up-to-date Be-10 dating procedures.

Due to the recent methodological development of the cosmogenic nuclide dating tool “in situ C-14”, a new geochronological approach now allows studying periods of glacier retreat based on the combined measurements of cosmogenic in situ Be-10 and C-14 in recently deglaciated bedrock. The exposure duration of the bedrock (glacier retracted), burial duration (glacier advance) and a mean subglacial abrasion rate may be deduced for the period of the Holocene (past 11,000 yr).

Combining the dating and mapping results from moraine and glacial bedrock, we will reconstruct a well-defined chronology of Holocene glacier behavior at the Mont Blanc Massif. Because the preserved geomorphic features do not represent a continuous high-resolution archive, but a combination of key extents of advanced glaciers and the total duration of glacier retraction over the Holocene, we will model a scenario of continuous glacier fluctuations, which best fits our geochronological and geomorphic data. For this, we envision following the approach by Goehring et al. (2012) who modeled the Holocene dynamics of the Rhone Glacier using independent paleoclimate records as templates for paleo-equilibrium line altitude variations.

Our first mapping and sampling campaign is planned for October 2015.

Glaciers targeted to investigate Holocene glacier dynamics. Left panel : locations in the MBM with glacier names. Middle panel : example for a moraine dating site ; moraines indicated as white lines. Right panel : example for a bedrock dating site ; potential sampling profile indicated as red line.

Reference :
Goehring et al., 2012, Holocene dynamics of the Rhone Glacier, Switzerland, deduced from ice flow models and cosmogenic nuclides, Earth and Planetary Science Letters, v. 351–352, p. 27–35.

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