Assessing the performance of geophysical survey techniques for characterising the subsurface around glacier margins

<p>Glacier forelands contain valuable information on past glacier dynamics and associated climatic conditions, particularly at small mountain glaciers where responses to climate change are rapid. To maximize the potential of glacial landforms as palaeoclimate indicators, a thorough understanding of the controls on landform genesis and subsequent evolution is required. Traditionally, such landforms have been studied using glacial geological techniques such as sedimentary logging. While these provide valuable in situ information they have numerous limitations, namely poor availability and spatial extent of exposures. Near-surface geophysics provides an efficient and non-invasive means of studying subsurface conditions in numerous sedimentary settings, offering spatially extensive information on substrate material properties and architecture. However, the logistically challenging terrain, remote location and complex structure of proglacial environments has limited the development of geophysical techniques for studying the internal architecture of glacial landforms.</p><p>Here, we explore the application of three geophysical methods to investigate proglacial substrates: ground penetrating radar (GPR), seismic refraction and multi-channel analysis of surface waves (MASW). Three sites with contrasting sediment properties were surveyed at the foreland of Midtdalsbreen glacier in southern Norway; (a) a 100 m<sup>2</sup> area of glaciotectonised sandy sediments, (b) a ~2 m high lateral moraine ridge containing stratified silts, sands, and gravel and (c) a terminal moraine ridge with a peak crest height of ~5 m and an open blockwork of cobbles and boulders at its surface. At all sites, we deployed 25 MHz and 100 MHz GPR antennas and undertook seismic surveys with 50−75 m long geophone spreads and a sledge-hammer source to sample to target depths of around 10−15 m. Through comparing the results from sites (a) to (c), we assess the capabilities and limitations of each of the aforementioned techniques for proglacial substrate imaging and characterisation, we analyse how their performances vary across these settings and outline factors that contribute to a successful geophysical investigation. </p><p>The ease of analysis and achievable investigation depths of the geophysical data and the applicability of seismic interpretation methods varied considerably depending on the surface terrain and structural complexity of the site. Our results show how the combination of GPR and seismic data can assist with the internal characterisation of glacial moraines when a relatively simple subsurface structure is present. However, basic seismic inversions likely lack the sophistication to resolve seismic structure in all but the simplest of layered models. We offer suggestions on how to optimise field time in more complex settings, where more sophisticated seismic inversion algorithms (e.g. tomography) or 3-D GPR surveys could be better-suited.</p><p>Our experience should help advance the use of geophysics in proglacial studies. It should serve as a guide for future survey planning, and help avoid typical pitfalls such that field time can be optimised.  It is hoped that geophysical survey methods will play an increasing role in the understanding of proglacial sedimentary landforms and their associated palaeoenvironments.</p>

How extensive was the Younger Dryas glacier advance in Austria? New insights from the Großsölk Valley

<p>Modern systematic studies on the record of the Alpine Lateglacial (~ 19 – 11.7 ka) are missing for the Eastern Alps east of the Hohe Tauern mountain range. In order to fill this gap, a study has been started in the Niedere Tauern mountain range, which reaches 2862 m in altitude and comprises crystalline rocks. The recently non-glaciated mountain range is famous for a glacially shaped morphology with a series of cirques. During the Last Glacial Maximum (LGM), it was part of the transection glacier complex, which covered the western and central parts of the Eastern Alps. Thus, the conditions for studying the glacial chronology after the LGM are excellent.</p><p>In recent decades, three phases of glacier advances from cirques or higher altitude valleys have been distinguished within the Alpine Lateglacial, i.e. phase of ice-decay (immediately after the breakdown of the large valley glaciers like the Enns glacier), Gschnitz Stadial (correlated with the Heinrich 1 ice rafting event) and Egesen Stadial (marking the beginning of the Younger Dryas). A first step for additional paleogeographic, geochronological and palaeoglaciological studies in the Niedere Tauern is the identification and characterisation of the legacy of these three glacial phases within the Großsölk valley.</p><p>In this paper, we deal with the Egesen Stadial. New fieldwork reveals geomorphological and sedimentological evidence for glacier advances in three cirques in the Großsölk valley. Peaks bounding these east facing cirques are at 2400-2600 m altitude. The cirques contain lateral and end moraine ridges surrounding small tongue-shaped lake basins. These up to 5 m high ridges consist of boulder-bearing sandy to gravelly diamicts, which are interpreted to have formed during discrete phases of glacier stabilisation. The observed features in the three cirques allow us to interpret the following, from south to north:</p><p>1) A glacier at Lake Schimpelsee that extended down to 1930 m and which deposited three sharp crested end-moraines and one marginal moraine ridges during three stabilisation phases. <br>2) A similar glacier at Grünsee that extended down to 1920 m and underwent two stabilisation phases. An end moraine ridge is not observable, because in the suspected position there is a lake today. Evidence for the second stabilisation phase is partly overprinted by a relict rock glacier. <br>3) At Weißensee, large angular boulders along smoothed ridges testify to a debris-covered glacier in this area, which extended to 2000 m a.s.l.</p><p>Considering the altitude of the catchment area, the eastward facing orientation, the altitude of the maximum extent of the ancient glaciers as well as the geomorphologically constrained multiphase glacier retreat, we associate these glacier advances with the Egesen Stadial. Future radionuclide work will provide better age constraints for the Großsölk valley, extending knowledge of the Würmian Lateglacial to less investigated eastern parts of Austria.</p>

A Close Observation to a Typical Continental Valley Glacier Surge in Northeastern Pamir

In May 2015, it is reported that "a glacier slid 20 km long, accompanying with disappearance of ~ 10 km2 grasslands on the north slope of Mt Jiubie, Kongur Mountain, northeastern Pamir, China". Based on expedition and multi-temporal RS image interpretation, this paper confirms that it was a rare continental valley glacier surge occurred in the tributaries of Kalayayilake Glacier, and find: (1) It has significant phenomena difference between quiescent and surging phase, including distorted medial moraine ridge, extruding-bulging ice masses, disappeared superglacial lakes etc; (2) Compared with the oceanic glacier surge, its characteristics are higher ice choking uplift and large chaotic crevasses area extent, but having relatively small integral movement distance with low velocity; (3) Environmental factors of large glacier coefficient, long tongue, low altitude, especially the stagnant downstream tongue and thick superglacial moraine, contribute to continental glacier surge's features, and the long-term temperature rise and rainfall enhancement in study area seem to be consistent with the occurrence of this surge; (4) This surge brings out severe strength reduction of glacier, rapid ablation of ice, congestion in the subglacial passageway, and accumulation of englacial water, which together have being bred the risk of terminus advance suddenly to result in flooding and debris flow.

Paleoflood marks in sandur morphometry as the result of the glacier surge (NW Poland)

In the Pomeranian marginal zone of the last Pleistocene glaciation specific morphometric features are found. They represent indicators of surging events affecting the individual outlet glacier lobes of the Scandinavian Ice Sheet margin. They are assumed to have been active during the climate warming of the Weichselian glaciation decay. Many tunnel valleys deep-rooted in the proximal slopes of the terminal moraine ridge with incised gorges and wide extramarginal sandur fans spread from the mouths to the forefield. The relatively low relief intensity factor, the fluvioglacial sediments covering distal slopes of the terminal moraines and ‘washed out’ older forms may be considered as indirect evidence of outburst floods caused by a re-canalization of the subglacial drainage system. The morphometry of the palaeochannels in the gorge profiles are used to calculate estimates of extreme discharges (c. 5 × 103–105m3s–1).

Little Ice Age fluctuations of Glaciar Río Manso in the North Patagonian Andes of Argentina

Little Ice Age (LIA) fluctuations of Glaciar R"o Manso, north Patagonian Andes, Argentina are studied using information from previous work and dendrogeomorphological analyses of living and subfossil wood. The most extensive LIA expansion occurred between the late 1700s and the 1830"1840s. Except for a massive older frontal moraine system apparently predating ca. 2240 14C yr BP and a small section of a south lateral moraine ridge that is at least 300 yr old, the early nineteenth century advance overrode surficial evidence of any earlier LIA glacier events. Over the past 150 yr the gently sloping, heavily debris-covered lower glacier tongue has thinned significantly, but several short periods of readvance or stasis have been identified and tree-ring dated to the mid-1870s, 1890s, 1900s, 1920s, 1950s, and the mid-1970s. Ice mass loss has increased in recent years due to calving into a rapidly growing proglacial lake. The neighboring debris-free and land-based Glaciar Fr"as has also retreated markedly in recent years but shows substantial differences in the timing of the peak LIA advance (early 1600s). This indicates that site-specific factors can have a significant impact on the resulting glacier records and should thus be considered carefully in the development and assessment of regional glacier chronologies.