Freeze-on limits bed strength beneath sliding glaciers

• Published in: Nature communications 2018-08, Vol.9 (1), p.3242-6, Article 3242
• Main Authors: Meyer, Colin R., Downey, Anthony S., Rempel, Alan W.
• Format: Article
• Language: English
• Subjects: 704/106/125; 704/2151/2809; Deformation mechanisms; Glacial drift; Glaciers; Gravity; Humanities and Social Sciences; Ice; Ice sheets; Liquid surfaces; Mechanical stimuli; multidisciplinary; Pore size; Porosity; Rheological properties; Rheology; Science; Science (multidisciplinary); Sea level; Sediments; Shear stress; Sliding; Stresses; Surface energy; Surface properties
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-018-05716-1

Discharge from sliding outlet glaciers controls uncertainty in projections for future sea level. Remarkably, over 90% of glacial area is subject to gravitational driving stresses below 150 kPa (median ∼70 kPa). Longstanding explanations that appeal to the shear-thinning rheology of ice tend to overpredict driving stresses and are restricted to areas where ice sheets only deform (roughly 50%). Over the more dynamic portions that slide, driving stresses must be balanced by thermo-mechanical interactions that control basal strength. Here we show that median bed strength is comparable to a threshold effective stress set by ice–liquid surface energy and till pore size. Above this threshold, ice infiltrates sediment to produce basal layers of debris-rich ice, even where net melting takes place. We demonstrate that the narrow range of inferred bed strengths can be explained by the mechanical resistance to sliding where roughness is enhanced by heterogeneous freeze-on. Across all glaciers, ice caps, and ice sheets, the gravitational driving stress, and therefore the average basal shear stress falls in a narrow range that tops out around 1 bar. Here, the authors show that the mechanical resistance posed by heterogeneous infiltration of ice into sediments governs the peak bed strength.