Firn Microstructure Influences the Frequency of Ice Layer Formation in the Shallow Firn Column in Western Greenland

Ice layers in polar firn serve as proxies for previous ice sheet summer surface temperatures and melt, while also reducing the ability for additional meltwater to vertically percolate deep into the firn column in the future. Therefore, determining the factors controlling ice layer formation will refine paleoclimate reconstructions relying on trends in melt feature percentages and will improve estimates of firns capacity to accept meltwater and buffer sea level rise in a warmer climate. Here, we show ice layer frequency in the top 10 m of seven firn cores collected in the western Greenland Ice Sheet percolation zone does not correspond to the climate gradient, which suggests a more complicated relationship between ice layer formation and climate. These cores were collected as part of the 2016-2017 Greenland Traverse for Accumulation and Climate Studies (GreenTrACS). We hypothesize microstructural changes within the firn column, such as grain size transitions resulting in variations in firn permeability, generate the discrepancy between ice layer frequency and the position of the cores along the climate gradient. We document ice core stratigraphy and grain size transitions as well as develop an ice layer classification scheme by digitally analyzing photographs detailing the top 10 m of the cores. These results can serve as validation for microstructural modeling of the firn column in western Greenland, which can improve estimates of past surface melt and inform future simulations of meltwater percolation and refreezing or firn aquifer formation.