Drainage system and thermal structure of a High Arctic polythermal glacier: Waldemarbreen, western Svalbard

Understanding glacier drainage system behaviour and its response to increased meltwater production faces several challenges in the High Arctic because many glaciers are transitioning from polythermal to almost entirely cold thermal structures. We, therefore, used ground-penetrating radar data to investigate the thermal structure and drainage system of Waldemarbreen in Svalbard: a small High Arctic glacier believed to be undergoing thermal change. We found that Waldemarbreen retains up to 80 m of temperate ice in its upper reaches, but this thickness most likely is a relict from the Little Ice Age when greater ice volumes were insulated from winter cooling and caused greater driving stresses. Since then, negative mass balance and firn loss have prevented latent heat release and allowed near-surface ice temperatures to cool in winter, thus reducing the thickness of the temperate ice. Numerous reflectors that can be traced up-glacier are interpreted as englacial channels formed by hydrofracturing in the crevassed upper region of the glacier. The alternative cut and closure mechanism of conduit initiation only forms conduits in parts of the lower ablation area. Consequently, Waldemarbreen provides evidence that hydrofracturing at higher elevations can play a major role in englacial water drainage through cold ice.

Glacial processes and landforms

From 1965-2000 glacial geomorphology became increasingly specialised and developed significantly due to technological improvements, particularly in remote sensing, surveying and field-based glaciological process studies. The better understanding of basal thermal regimes in ice sheets and glaciers led to the development of concepts such as spatial and temporal migration of ice divides in dynamic ice sheets that could overprint subglacial landform assemblages, debris entrainment processes related to polythermal glacier systems, and glacier and ice sheet beds composed of cold and warm based mosaics. Process observations at the ice-bed interface led to the discovery of the third glacier flow mechanism, substrate deformation, which provided the impetus to reconstruct the genesis of subglacial bedforms such as drumlins and to evaluate the origins and potential flow law for till. Numerical evaluations of glacial erosion led to a better understanding of abrasion and quarrying as well as the erection of genetic models and erosion rates for larger scale features such as U-shaped valleys and cirques. Linkages were made between debris transport pathways and moraine construction in supraglacial environments, with the role of glacier structure being linked to specific landforms, such as medial, lateral, hummocky and ice-cored moraines as well as rock glaciers. Our appreciation of the erosional and depositional impacts of glacifluvial systems was enhanced significantly with the advent of process observations on the hydrology of modern glaciers as well as the final vindication of J.H. Bretz and his proposed jökulhlaup origins of the Channelled Scablands and the Missoula Floods. In addition to the increasing numbers of studies at modern glacier snouts, the embracing of sedimentology by glacial geomorphologists was to result in significant developments in understanding the process-form regimes of subglacial, marginal and proglacial landforms, particularly the recognition of landform continua and hybrids. Advances resulting from this included the recognition of different modes of moraine and glacitectonic thrust mass development, lithofacies models of the varied glacifluvial depositional environments, and the initial expansion of the sediments and depo-centres of glacimarine settings, the latter being the result of glacial research taking to submersibles and ice-strengthened ships for the first time. A similarly new frontier was the expansion of research on the increasingly higher resolution images returning from Mars, where extraterrestrial glaciations were recognised based on comparisons with Earth analogues. Holistic appreciations of glaciation signatures using landform assemblages were developed, initially as process-form models and later as glacial landsystems, providing an ever expanding set of templates for reconstructing palaeoglaciology in the wide variety of topographic and environmental settings, which also acknowledge spatial and temporal change in glacier and ice sheet systems.

The impact of internal thermal regime of glaciers on climate caused advance and retreat

<p>Most glaciers in Sweden have polythermal temperature regimes, where a temperate core of ice is overlain by a cold surface layer. The cold surface layer prolongs the response time of a glacier, and therefore increases the time it takes for a glacier to start advancing during a cooling climate trend. In the late 1980s and 1990s, some glaciers in Sweden advanced due to prolonged periods of positive mass balance, for example Storglaciären. However, far from all glaciers advanced during this period, coincidentally relating to their cold surface layer thickness. This raises the question: what factors drive how and when a polythermal glacier advances, and what climatic signals can be read from traces of past advances and extents? Here, four polythermal glaciers are described in detail since the early 1900s, when they were close to, or at, their largest Holocene extents. These glaciers lie in relatively similar settings, and thus share many resemblances, but also show many differences. How these glaciers have changed since the early 1900s, how they look today, and what landforms they have left behind, provides an opportunity to explore factors behind their responses. The four studied glaciers are: Mikkaglaciären, Storglaciären, Rabots glaciär, and Mårmaglaciären. The dynamics of glaciers retreating are much better understood than glaciers advancing, as the overwhelming majority of existing data have been collected since the latter 1900s half, during a period of overall negative mass balance. The aim of the study is to describe the current properties of the studied glaciers. Using this knowledge and the landform assemblages in their glacier forefields, we suggest explanations to how they might have responded to climate change in the past and possible causes for differences in their response.</p>

A general theory of glacier surges

We present the first general theory of glacier surging that includes both temperate and polythermal glacier surges, based on coupled mass and enthalpy budgets. Enthalpy (in the form of thermal energy and water) is gained at the glacier bed from geothermal heating plus frictional heating (expenditure of potential energy) as a consequence of ice flow. Enthalpy losses occur by conduction and loss of meltwater from the system. Because enthalpy directly impacts flow speeds, mass and enthalpy budgets must simultaneously balance if a glacier is to maintain a steady flow. If not, glaciers undergo out-of-phase mass and enthalpy cycles, manifest as quiescent and surge phases. We illustrate the theory using a lumped element model, which parameterizes key thermodynamic and hydrological processes, including surface-to-bed drainage and distributed and channelized drainage systems. Model output exhibits many of the observed characteristics of polythermal and temperate glacier surges, including the association of surging behaviour with particular combinations of climate (precipitation, temperature), geometry (length, slope) and bed properties (hydraulic conductivity). Enthalpy balance theory explains a broad spectrum of observed surging behaviour in a single framework, and offers an answer to the wider question of why the majority of glaciers do not surge.

Mass and enthalpy budget evolution during the surge of a polythermal glacier: a test of theory

Analysis of a recent surge of Morsnevbreen, Svalbard, is used to test predictions of the enthalpy balance theory of surging. High-resolution time series of velocities, ice thickness and crevasse distribution allow key elements of the enthalpy (internal energy) budget to be quantified for different stages of the surge cycle. During quiescence (1936–1990), velocities were very low, and geothermal heat slowly built-up enthalpy at the bed. Measurable mass transfer and frictional heating began in 1990–2010, then positive frictional heating-velocity feedbacks caused gradual acceleration from 2010 to 2015. Rapid acceleration occurred in summer 2016, when extensive crevassing and positive air temperatures allowed significant surface to bed drainage. The surge front reached the terminus in October 2016, coincident with a drop in velocities. Ice plumes in the fjord are interpreted as discharge of large volumes of supercooled water from the bed. Surge termination was prolonged, however, indicating persistence of an inefficient drainage system. The observations closely match predictions of the theory, particularly build-up of enthalpy from geothermal and frictional heat, and surface meltwater, and the concomitant changes in ice-surface elevation and velocity. Additional characteristics of the surge reflect spatial processes not represented in the model, but can be explained with respect to enthalpy gradients.

Organochlorine Pollutants within a Polythermal Glacier in the Interior Eastern Alaska Range

To assess the presence of organochlorine pollutants (OCP) in Alaskan sub-Arctic latitudes, we analyzed ice core and meltwater samples from Jarvis Glacier, a polythermal glacier in Interior Alaska. Jarvis Glacier is receding as atmospheric warming continues throughout the region, increasing opportunity for OCP transport both englacially and into the proglacial watershed. Across all meltwater and ice core samples we identify the pesticides DDT, DDE and DDD, α- HCH and ϒ-HCH. OCP concentrations in ice core samples were highest at the 7-14 m depth (0.51 ng/L of DDT) and decreased gradually approaching the bedrock at 79m. Meltwater concentrations from the proglacial creek slightly exceeded concentrations found in the ice core, potentially indicating aggregate OCP glacial loss, with peak OCP concentration (1.12 ng/L of DDD) taken in July and potentially associated to peak melt. Ongoing use of DDT to fight Malaria in Asia, and the extended atmospheric range of HCH may account for concentrations in near-surface ice, correlating with use and atmospheric transport. The opportunity for biota bioaccumulation of OCPs, or human uptake of OCPs from glacial meltwater, may increase as glacial melt continues.

Mechanisms leading to the 2016 giant twin glacier collapses, Aru range, Tibet

In northwestern Tibet (34.0° N, 82.2° E) near lake Aru Co, the entire ablation area of two glaciers (Aru-1 and Aru-2) suddenly collapsed on 17 July 2016 and 21 September 2016, respectively, and transformed into 68 and 83 106 m3 mass flows that ran out up to 7 km, killing nine people. The only similar event currently documented is the 2002 Kolka Glacier mass flow (Caucasus Mountains). Using climatic reanalysis, remote sensing and 3D thermo-mechanical modeling, we reconstructed in detail the glaciers' thermal regimes, thicknesses, velocities, basal shear stresses and ice damage prior to the collapse. We show that frictional change leading to the collapses occurred in the temperate areas of polythermal glacier structures and are not linked to thaw of cold based ice. The two glaciers experienced a similar stress transfer from predominant basal drag towards predominant lateral shearing in the later detachment areas and during the 5–6 years before the collapses, though with a high friction patch on Aru-2 tongue which is inexistent on Aru-1. The latter led to distinctly disparate behaviour making the development of the instability more visible for the Aru-1 glacier compared to Aru-2 through enhanced crevassing over a longer period and terminus advance. Field investigations reveal that those two glaciers are flowing on a soft, highly erodible, and fine-grained sedimentary lithology. We propose that specific bedrock lithology played a key role in the two Tibet, and also in the Caucasus gigantic glacier collapses documented to date by producing low bed roughness and large amount of till rich in clay/silt with low friction angle. The twin Aru collapses would have been driven by a failing substrate linked to increasing water pore pressure in the subglacial drainage system in response to recent increases of surface melting and rain.