Climate Reconstructions for the Younger Dryas in Graubünden, Swiss Alps: Using Glacier Geometry and Hypsometry to Estimate Equilibrium Line Altitude

Abstract:

Mountain glaciers serve as important paleoclimate records due to the direct physical link

between glacier extent and climate. The high sensitivity of mountain glaciers to even small

perturbations in the climate has the potential to provide very detailed records of regional

glacier and climate histories. Recent progress in age determination techniques such as

surface exposure dating has greatly improved the temporal precision of glacial records.

The conversion of changes in glacier geometries to a climate signal, however, remains a

significant challenge. A particular need exists for a versatile method easily applicable to

diverse regions and conditions around the globe. Because the equilibrium line altitude

(ELA) provides a more explicit comparison of climate than properties such as glacier length

or area, ELA methods lend themselves well to such a need, and allow for a more direct

investigation of the primary drivers of mountain glaciations during specific events. Here we

present a new, robust ELA model for quantifying changes in climate directly from glacier

geometry. The model derives from a linear flow model based on Glen’s Flow law while fully

accounting for glacier hypsometry. As a preliminary application, we combined our modeled

ELA reconstructions with a new 10Be chronology of late glacial culminations in Graubünden

in the Swiss Alps. These glacier culminations occurred during the Egesen Stadial, which has

been correlated to the Younger Dryas (YD) interval. Results for two related glacier systems

in Graubünden reveal an ELA depression of 365-401 m (depending on the moraines

chosen) during the Egesen stage/YD compared to the modern ELA. This agrees well both

with established estimates for ELA depressions in the region and an additional application

performed using our model and previously determined ages on the nearby Lagrev Glacier

(a 370 m ELA depression). We then reconstruct the temperature and precipitation changes

required to explain the ELA changes for these Swiss glacier systems. We will apply the

model to other regions in order to further elucidate the variability and sensitivity in the

climate system during the YD, and provide insight into the primary drivers of those changes.