Fate of lithium and strategic metals under Pleistocene ice streams

We analysed 35,944 samples of last glaciation till across 149,000 km 2 of the beds of paleo ice streams and adjacent interlobate areas in Central and Southern Finland. Ice streams were short-lived and were triggered during a 2–300-year phase between c. 13,000–10,500 ybp within the rapidly melting Scandinavian Ice Sheet. Analysis of high-resolution LiDAR topographic imagery of ice stream beds shows flow sets of highly elongated till ridges (mega-scale glacial lineations) resulting from erosion of the bed below fast flowing wet-based ice. In contrast, adjacent interlobate areas unaffected by ice streaming show irregular hummocky till surfaces formed by melt-out of debris from slow moving ice partly frozen to its substrate. Geochemical analysis shows that melt-out tills have elevated Li concentrations reflecting local bedrock sources and short distances of subglacial of transport of Li-bearing bedrock. These tills also retain a secondary component of glacially incorporated Li-bearing marine clays of last (Eemian) interglacial age. In contrast, tills below ice stream beds are characterized by statistically validated reduced concentrations of lithium which is attributed to extended long distance comminution of mineralized debris under fast flowing ice. Other strategic metals (Ni, Cu, Co) show the same regional pattern independent of changing bedrock substrates, indicating a pervasive glaciological control on till geochemistry related to changing ice flow velocity. These findings are the foundation for a new glacio-geochemical model that relates regional variations in till geochemistry to ice flow dynamics and subglacial processes below fast and slow flowing ice.