Investigating the influence of local insolation on near-surface snow grain properties to constrain the mechanisms of pore close-off and associated elemental fractionation in polar firn

Abstract

<p>Processes controlling pore closure in deep polar firn are broadly understood, yet defining the physical mechanisms remains ambiguous. Firn densification models predict pore close-off depths which are subsequently used in firn air models to predict the gas-ages of entrapped air bubbles. However, current firn models require observational tuning which causes variable model performance for sites with different characteristics. Furthermore, layering in the deep firn, which is not simulated in many firn densification models, is expected to cause a large distribution of pore lock-in depths. Observations from numerous firn cores have identified neighbouring layers with different physical properties, such as density, grain size and impurities, which experience pore-closure at different depths. These properties are strongly influenced by 1) snow metamorphism due to temperature gradients within the snowpack, and 2) accumulation rate. The relative influence of each of these properties on pore closure remains in question.</p><p>Based on current understanding, we propose to quantify the changes in density and snow microstructural properties near the surface as a result of the interplay between accumulation rate and insolation using the Crocus snowpack model. To support the modelling effort, we have compiled δO<sub>2</sub>/N<sub>2</sub> records - a proxy for local summer solstice insolation - from several polar ice cores. The relationship between insolation and δO<sub>2</sub>/N<sub>2</sub> is understood to be linked to near-surface snow metamorphism, which largely determines the properties of deep-firn layers, and thus, the pore-closure process. By first identifying how insolation and accumulation rate influence the near-surface snow properties, we aim to implement this effect into firn models to develop our understanding of the physical mechanisms controlling pore-closure and the associated elemental fractionation.</p>