Terminal zone glacial sediment transfer at a temperate overdeepened glacier system

Continuity of sediment transfer through glacial systems is essential to maintain subglacial bedrock erosion, yet transfer at temperate glaciers with overdeepened beds, where subglacial fluvial sediment transport should be greatly limited by adverse slopes, remains poorly understood. Complex multiple transfer processes in temperate overdeepened systems has been indicated by the presence of large frontal moraine systems, supraglacial debris of mixed transport origin, thick basal ice sequences, and englacial thrusts and eskers. At Svinafellsjokull, thrusts comprising decimetre-thick debris-rich bands of stratified facies ice of basal origin, with a coarser size distribution and higher clast content than that observed in basal ice layers, contribute substantially to the transfer of subglacial material in the terminal zone. Entrainment and transfer of material occurs by simple shear along the upper surface of bands and by straininduced deformation of stratified and firnified glacier ice below. Thrust material includes rounded and well-rounded clasts that are also striated, indicating that fluvial bedload is deposited as subglacial channels approach the overdeepening and then entrained along thrusts. Substantial transfer also occurs within basal ice, with facies type and debris content dependent on the hydrological connectedness of the adverse slope. A process model of transfer at glaciers with terminal overdeepenings is proposed, in which the geometry of the overdeepening influences spatial patterns of ice deformation, hydrology, and basal ice formation. We conclude that the significance of thrusting in maintaining sediment transfer continuity has likely been overlooked by glacier sediment budgets and glacial landscape evolution studies

Constraints on ⁸⁷Sr/⁸⁶Sr of Late Ediacaran seawater: insight from Siberian high-Sr limestones

<p>In SE Siberia, carbonate formations with δ¹³C_carb values ranging between −12‰ and −7‰ (V-PDB) and Sr concentrations of up to 2.5% occupy an area of 40 000 km². Several successions exceed 1000 m in thickness and represent the world's largest known exposures of sedimentary carbonates exhibiting extreme depletion in ¹³C. The carbonates were deposited on a carbonate platform evolving from a mixed carbonate–siliciclastic ramp to a carbonate ramp, and then from a peritidal rimmed shelf to a deep-water open shelf. All sequences reveal a facies-independent, upward rise in marine δ¹³C_carb from −12‰ to −7‰. The trend and magnitude of the values mimic those that are characteristic of the 600–550 Ma Shuram–Wonoka isotope event. A coincident stratigraphic rise in ⁸⁷Sr/⁸⁶Sr from 0.70802 to 0.70862 in several sections of limestones, containing 4400 μg g⁻¹ Sr on average, is considered to be by far the best available constraint on a temporal variation of seawater isotopic composition through the Late Ediacaran. If the greatest temporal rate of change in seawater ⁸⁷Sr/⁸⁶Sr observed in the Cenozoic is applied to the Siberian sections, the calculated minimum duration for the Suram–Wonoka event is 10 Ma. </p