Firn Rheology Revealed by Contrasting Shear Regimes

<jats:p>Abstract. Firn densification is commonly modelled using 1D column approaches that neglect the deformation history associated with ice flow. Here, we present a flowline firn model that explicitly accounts for the accumulation and horizontal strain experienced by firn along its trajectory and use it to infer density-dependent rheology directly from observations. Applying the method to two ice-core sites in northeast Greenland with contrasting strain regimes, we constrain both the total densification response and its partitioning into shear and volumetric components. While steady-state and flowline inversions agree at large scales, the combined function governing vertical strain is tightly constrained, whereas individual rheological parameters remain poorly identifiable. The inferred rheology departs markedly from standard formulations, with enhanced near-surface densification, reduced viscous Poisson ratios, and pronounced non-monotonic structure. These behaviours are incompatible with a single power-law rheology and instead require the combined action of multiple deformation mechanisms, including grain-size-dependent creep and evolving anisotropy. The resulting parameters should therefore be interpreted as effective representations of these processes. These results further imply that firn-derived gas proxies such as Δage and δ15N may contain a dynamical component linked to horizontal strain, with potential implications for the interpretation of ice-core chronologies.</jats:p>