Midtvejskollokvium: Sarah M. P. Berben – Niels Bohr Institute - University of Copenhagen

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Midtvejskollokvium: Sarah M. P. Berben

Midtvejskollokvium: Sarah M. P. Berben

Academic supervisors: Sigfus J. Johnsen and Hans Christian Steen-Larsen

Abstract
The stable water isotope signal throughout an ice core is a well known and often used proxy for past temperature reconstructions, and is important in our understanding of the climate system. The knowledge about the post-depositional processes influencing the isotope signal within the snowpack is therefore important. As wind blow across the snow surface, micro high and low pressure areas arise because of sastrugi. These pressure differences create forced ventilation through the snow pack, which then affects the interstitial mass exchange between water vapor and snow crystals, and therefore perhaps the climatic signal stored in the snow.

In order of understanding the physics behind this ongoing exchange, a combination of modeling and a controlled experiment has been set up. The process of forced ventilation –as it is believed to occur on Greenland and Antarctica- has been simulated. The snow within this experiment is collected in Greenland during the 2009 field season of the new deep drilling project in North-West Greenland (NEEM).

Within this experiment, dry air is pushed through a snow sample with fixed thickness. This sample has a known isotopic content and is kept at different sub-zero temperatures. The flow rate of the air has been controlled between 0.15 and 1.50 l/min. After the interaction between the water vapor and the ice crystals, the isotope signal is been studied. New in this research are the continuous measurements of the isotope content with a Picarro WS-CRDS analyzer of the water vapor.

Eventually -to understand the effect of ventilation forced through snow on the stable isotope content of the water vapor- the results of the experiment are compared with the output from a Rayleigh type model.

This research investigated the interstitial mass exchange between vapor and snow crystals and delivered thereby some consistently experimental data. Although the remaining non understood issues, it still can be seen as a contribution to the understanding of the effect of forced ventilation on the mean isotope signal in the snow, and its possible implications for the derived temperature signal from the water isotope ratio of an ice core.