Modelling the firn thickness evolution during the last deglaciation: constrains on sensitivity to temperature and impurities
Camille Bréant1,2, Patricia Martinerie2, Anaïs Orsi1, Laurent Arnaud2, and Amaëlle Landais11Laboratoire des Sciences du Climat et de l’Environnement, UMR8212, CEA-CNRS-UPS/IPSL, Gif-sur-Yvette, France 2Univ. Grenoble Alpes/CNRS, Laboratoire de Glaciologie et de Géophysique de l’Environnement (LGGE), UMR 5183, Grenoble, F-38041, France
Received: 14 Sep 2016 – Accepted for review: 11 Oct 2016 – Discussion started: 12 Oct 2016
Abstract. The transformation of snow into ice is a complex phenomenon difficult to model. Depending on surface temperature and accumulation rate, it may take several decades to millennia for air to be entrapped in ice. The air is thus always younger that the surrounding ice. The resulting gas-ice age difference is essential to document the phasing between CO2 and temperature changes especially during deglaciations. The air trapping depth can be inferred in the past using a firn densification model, or using δ15N of air measured in ice cores.
All firn densification models applied to deglaciations show a large disagreement with δ15N measurements in several sites of East Antarctica, predicting larger firn thickness during the Last Glacial Maximum, whereas δ15N suggests a reduced firn thickness compared to the Holocene. We present here modifications of the LGGE firn densification model, which significantly reduce the model-data mismatch for the gas trapping depth evolution over the last deglaciation, while preserving the good agreement between measured and modelled modern firn density profiles. In particular, we introduce a dependency of the activation energy to temperature and impurities in the firn densification rate calculation. The temperature influence reflects the existence of different mechanisms for firn compaction at different temperatures. We show that both the new temperature parameterization and the influence of impurities contribute to the increased agreement between modelled and measured δ15N evolution during the last deglaciation at sites with low temperature and low accumulation rate, such as Dome C or Vostok. However, the inclusion of impurities effects deteriorates the agreement between modelled and measured δ15N evolution in Greenland and Antarctic sites with high accumulation.
Bréant, C., Martinerie, P., Orsi, A., Arnaud, L., and Landais, A.: Modelling the firn thickness evolution during the last deglaciation: constrains on sensitivity to temperature and impurities, Clim. Past Discuss., doi:10.5194/cp-2016-91, in review, 2016.