Glaciers in a warming world: Lessons from Holocene records based on the novel cosmogenic nuclide dating tool “in situ carbon-14” – WarHol

In this project we develop a novel surface exposure/burial dating method in France that allows exploring the past glacier retreat: pairing measurements of the cosmogenic nuclides in situ 14C and 10Be in pro- and subglacial bedrock that is exposed by the current glacier retreat and therefore accessible for analyzing the cosmogenic nuclide inventory, which was produced during periods of smaller-than-today glaciers in the course of the Holocene. Combining this innovative approach with the groundbreaking ‘high-resolution 10Be dating’ of Holocene moraines, we produce chronologies of the timing and magnitude of past glacier extents, with particular focus on the periods of retracted glaciers, on a transect along the Western and Central European Alps (France and Switzerland). Our emphasis is on the chronological reconstructions of the glacier fluctuations to well establish the new chronometer.

After several sampling campains in France and Switzerland, we obtained preliminary results, which allow us to determine the exact moment when three glaciers started to retreat from their last large extent during the glacial/Holocene transition (about 12 000 years ago). The determined chronologies of the glacier fluctuations at the beginning of the Holocene were very similar in the Western and Cetral Alps suggesting that the glacier dynamics in this region were driven by the same climate variations, and that the local climate conditions had little influence.
Further results from a small glacier in the Swiss Central Alps suggest that the glacier was in retracted position for at least 5000 years.

Once the chronological data acquisition will be finished, we plan to couple them with precise digital mapping of the preserved glacio-geomorphic features to model equilibrium-line-altitude changes and propose associated climate (temperature/precipitation) scenarios.

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This project deals with the implementation of the novel cosmogenic dating tool “in situ carbon-14” in France and the establishment of a new research team of motivated experts at CEREGE dedicated to the timely topic of shrinking glaciers as a response to climate change.
The current near-global retreat of mountain glaciers is among the most visible and worrisome evidence of the warming earth. What is the significance of this rapid ice retreat today and within a longer time perspective? Glaciers sensitively react to small climate variations, in particular temperature and to a minor degree precipitation changes. Understanding the response of glaciers to climate changes during the Holocene, the current interglacial which started ~11,500 years ago, offers the opportunity to assess the effects of the ongoing climate change on land ice masses, because the Holocene is characterized by moderate-amplitude climate variability with climate conditions similar to those existing at present. It has been shown that in the Northern Hemisphere extended warm periods prevailed during the Mid-Holocene (~8-5 kyr ago), which were presumably warmer than today. How did glaciers react to these paleo-warming pulses? Did they recede to extents that were smaller than today? Did all glaciers recede for the same duration? If yes, for how long and what does this mean for the ongoing glacier retreat? Preserved glacial deposits, in particular moraines, bear witness to the sensitive response of glaciers to climate cooling in the past and are subject to extensive investigations. The response of glaciers to warm pulses in the past, such as those during the mid-Holocene, is much harder to reconstruct, because the geological evidence has since been overrun by late-Holocene glacier re-advances.
In this project we will develop a novel surface exposure/burial dating method in France that allows exploring the past glacier retreat: pairing concentrations of the in situ produced cosmogenic nuclides 14C and 10Be measured in pro- and subglacial bedrock that is exposed by the current glacier retreat and therefore accessible for analyzing the cosmogenic nuclide inventory, which was produced during periods of smaller-than-today glaciers in the course of the Holocene. Combining this innovative approach with the groundbreaking ‘high-resolution 10Be dating’ of Holocene moraines, both developed at the Lamont-Doherty Earth Observatory (LDEO, USA), we will produce chronologies of the timing and magnitude of past glacier extents, with particular focus on the periods of retracted glaciers, on a transect along the Western and Central European Alps (France and Switzerland). While our emphasis will be on the chronological reconstructions of the glacier fluctuations to well establish the new chronometer, we also envision coupling the chronological data with precise digital mapping of the preserved glacio-geomorphic features to model equilibrium-line-altitude changes and propose associated climate (temperature/precipitation) scenarios.
During her postdoctoral studies at LDEO, the project coordinator I. Schimmelpfennig acquired the scientific and analytical state-of-the-art knowledge underlying these innovative dating approaches required for the realization of this project. Supported by her national and international collaborators, she is currently establishing the first French in situ 14C laboratory and will build a new research team at CEREGE dedicated to studying glacier retreat periods during the Holocene. This project matches particularly well at CEREGE due to its state-of-the-art facilities: the national accelerator mass spectrometry facility ASTER (coordinated by Prof. D. Bourlès) for routine measurements of several cosmogenic nuclides such as 10Be, and the brand-new accelerator mass spectrometer AixMICADAS dedicated to radiocarbon measurements of microsamples, installed this year (coordinated by Prof. E. Bard).