Firn Core Evidence of Two-Way Feedback Mechanisms Between Meltwater Infiltration and Firn Microstructure From the Western Percolation Zone of the Greenland Ice Sheet

The relationship between firn microstructure and water movement is complex: firn microstructure controls the routing of meltwater through the firn while continuously being altered by liquid water flow processes. Importantly, microstructural transitions within the firn column can stall vertical meltwater percolation, which creates heterogeneities in liquid water content resulting in different rates of firn metamorphism. Physics-based firn models aim to describe these processes to accurately predict ice layer or firn aquifer formation, but require detailed observations of firn structure to validate and inform percolation schemes. Here, we present grain size measurements and ice layer stratigraphy from seven firn cores collected in western Greenland's percolation zone during the 2016 Greenland Traverse for Accumulation and Climate Studies (GreenTrACS). Grain size transitions within the cores are negatively correlated with all temperature proxies for meltwater supply. Additionally, the number of grain size transitions are strongly anticorrelated with the number of ice layers within each core, despite these transitions, particularly fine-over-coarse transitions, promoting meltwater ponding and potential ice layer formation. To investigate if these negative correlations can be understood with firn model physics, we simulate water movement along stratigraphic transitions using the SNOWPACK model. We find that grain size transitions diminish from rapid grain growth in wet firn where ice layers can form, suggesting these microstructural transitions are unlikely to survive repeated meltwater infiltration. Incorporating these microstructure—meltwater feedbacks in firn models could improve their ability to model processes such as ice slab formation or firn aquifer recharge that require accurate predictions of meltwater infiltration depth.