Tephrochronological dating of Holocene moraines at Icelandic glaciers, and climatic interpretations

Fluctuations of Icelandic glaciers reveal the impact of regional climate change on the cryosphere, filtered by the different response characteristics of individual glaciers. Frequent tephra deposition upon steadily aggrading aeolian soils provides a useful dating environment, in which basal tephras often provide close minimum ages on underlying tills and outwash deposits in areas where the local tephrostratigraphy is well constrained. We have dated moraines at glaciers across Iceland to improve the Holocene glacial chronology in terms of its temporal extent and resolution. Tephrochronology also provides a test of lichenometric dating, an area for further research. At least five groups of regionally-synchronous advances occurred between c. AD 1700 and 1930 during the “Little Ice Age”. The maximum extent of “Little Ice Age” glaciers varies by up to 200 years across Iceland, due more to the response chraracteristics of individual glaciers than to regional climatic variation. At Gígjökull, two glacier advances occurred before the 3rd century AD, with others in the 9th and 12th centuries AD bracketing the Medieval Warm Period. In central and northern Iceland, earlier glacier advances are dated to c. 4.5-5.0, c.3.0-3.5 ka BP, c. 2.0-2.5 ka BP. This classic “Neoglacial” sequence is comparable to other parts of Europe and Scandinavia, but is discernible only at smaller mountain glaciers. In contrast, the 19th-Century advance of large ice caps censored evidence of earlier fluctuations from the moraine record, and preservation potential is preconditioned by glacier type. In general, the forefields of steep, fast-responding glaciers contain more complete archives of Holocene climatic changes than do the margins of the large icefields. Glacier advances appear to be favoured by a weakening of zonal circulation (the negative mode of the North Atlantic Oscillation) associated with cooler, drier winters and cooler, wetter summers

Thermal and dynamic behaviour of supraglacial clasts and the origin of sorting in supraglacial debris covers

The transition zone from a discontinuous to a continuous debris cover is an extensive part of many glacier ablation zones. Although responsible for the highest specific melt rates of debris-covered glaciers, transition zones have received little research and are poorly understood. Here we consider the interactions between emergent clasts and melting ice surfaces at Glacier d'Estelette and Miage Glacier (Italian Alps). Debris-ice interactions are complex because dispersed heterogenous debris both enhances and retards melt rate in the same locality, depending on the distribution of clast sizes. Observations reveal that thermal and dynamic clast interactions with the glacier surface increase the transport rate of coarse clasts, and initiate vertical sorting at the point when a continuous debris layer forms. This happens because, in summer, clasts exceeding the critical thickness for melt slide over the glacier surface. In contrast finer thermally-embedded material is transported at ice surface velocity and become covered by coarser material from upslope. Once established, debris-cover texture allows sorting to develop as the cover thickens downglacier. A two-layer temperature profile results, in which a coarse, drier clast layer of low thermal conductivity overlies a finer-grained, moist layer of higher thermal conductivity. Transition-zone processes establish inverse grading at the initiation of a debris cover, allowing subsequent sorting to operate as the cover thickens downstream. The processes by which this occurs are unknown, but analogy with periglacial active layers suggests convection within a frost-susceptible lower fine layer and eluviation of fines supplied by aeolian deposition and in-situ clast distintegratio

Continuous borehole optical televiewing reveals variable englacial debris concentrations at Khumbu Glacier, Nepal

Surface melting of High Mountain Asian debris-covered glaciers shapes the seasonal water supply to millions of people. This melt is strongly influenced by the spatially variable thickness of the supraglacial debris layer, which is itself partially controlled by englacial debris concentration and melt-out. Here, we present measurements of deep englacial debris concentrations from debris-covered Khumbu Glacier, Nepal, based on four borehole optical televiewer logs, each up to 150 m long. The mean borehole englacial debris content is ≤ 0.7% by volume in the glacier’s mid-to-upper ablation area, and increases to 6.4% by volume near the terminus. These concentrations are higher than those reported for other valley glaciers, although those measurements relate to discrete samples while our approach yields a continuous depth profile. The vertical distribution of englacial debris increases with depth, but is also highly variable, which will complicate predictions of future rates of surface melt and debris exhumation at such glaciers