4D high resolution monitoring of glacier evolution, Virkisjökull, Iceland
Virkisjökull is an outlet glacier draining the western flanks of Oraefajökull, the southern extremity of the Vatnajökull ice cap in south-east Iceland (Figure 1).
The glacier is sensitive to climatic forcing due to a relatively low accumulation–ablation area ratio and significant altitudinal difference between summit and margin.
The long-term BGS monitoring project at Virkisjökull aims to quantify glacial and geomorphological changes, and link these to variations in local climate.
Evidence of glacier retreat
The glacier has undergone significant lateral retreat and vertical thinning since its 20th century maximum position, between 1900–1920 (Figure 2a and 2b). This retreat appears to have accelerated since 2005.
Monitoring programme
There are three strands of the monitoring programme:
- Two automated weather stations operate on site, one at the glacier margin (156 m a.s.l., Figure 3), and a second at the icefall (444 m a.s.l., Figure 4). These work in conjunction with a third low altitude station run by Vegagerðin (National Roads Authority).
- Terrestrial LiDAR scans are conducted each year, generating centimetre-accurate digital elevation models (DEMs) of the glacier margin and foreland. These can be directly compared year-on-year to quantify absolute changes in ice and foreland surface morphology, and model changes in ice volume at the highest resolution.
- Ground penetrating radar and passive seismology have been conducted across the site to determine internal sedimentary architecture of the glacier foreland, establishing the presence of buried ice masses, and mapping out subsurface drainage. The latter is most likely controlled by the distribution of buried ice.
Weather stations
Two automated weather stations have been installed at Virkisjökull. Both stations are recording data continuously and are linked to BGS Murchison House via satellite phone, allowing us to monitor conditions on a daily basis.
All meteorological data is now a free download from the BGS website.
The first station is sited 100 m from the current ice margin at 156 m a.s.l.. This is continuously recording temperature, relative humidity, precipitation, wind speed and direction, and solar intensity. This station is also fitted with a webcam, enabling us to create timelapse videos of glacier motion and foreland evolution, and also it lets us know if the station is still upright.
The second station is positioned 300 m higher, adjacent to the glacier icefall, and records temperature, humidity and wind speed and direction.
Records of local climate right next to the glacier margin are vital to demonstrate the link between climate change and glacier response, and multiple stations at different altitudes allow us to determine environmental lapse rates and the influence that the glacier has on local climate.
Terrestrial LiDAR
A Riegl LPMi800 LiDAR scanner was used in September 2009, and again in September 2010 to generate centimetre-accurate DEMs covering 2 km × 1 km of the glacier margin and foreland. The point clouds of over 80 million scanned points are precisely positioned using dGPS and are given photorealistic colour values via a digital SLR camera incorporated into the scanner (Figures 5a & b).
The scans are repeated annually, allowing sequential DEM surfaces to be superimposed.
Subtraction of later scans from the 2009 scan will provide quantitative data on the evolution of the glacier foreland, in terms of the melting out of buried ice masses and ongoing geomorphological change; it will also will reveal the precise extent of ice thinning and retreat at the margin.
Subsurface architecture
Ground penetrating radar was undertaken using a MALA 50 MHz RTA system system; 30 transects were completed across the glacier foreland, and two on the glacier margin (Figures 6a & b). The towed array allowed a high resolution and precisely positioned (dGPS) grid of GPR measurements to be completed relatively quickly.
The grid of GPR traces will be tied to the LiDAR-derived DEM, enabling a fence diagram of subsurface geophysics to be generated. First interpretations suggest the presence of extensive bodies of buried ice, effectively detached from the active glacier margin. These bodies maintain a network of linked-cavity drainage, often indicated by ‘swallowed’ ground (Figure 6a), possibly inherited from the pre-existing englacial drainage network.
Passive seismic profiling has been carried out using a TROMINO Zero ® single station system, allowing the debris/buried ice and buried ice/bedrock boundaries to be located across the study area (Figures 6c, d, e & f).
Using these techniques in combination the 3D architecture of the glacier bed and basin has been revealed; repeat surveys will show how this system evolves over time.
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Local climate and the 'glacier cooling effect'
Air temperatures in winter at the ice margin are very similar and in phase with temperatures on the sandur 3 km away (Figure 7). However, in spring, mean air temperatures on the sandur are consistently higher than those at the glacier margin.
This trend continues into summer, with the offset increasing notably in July and August. The lower panel highlights this offset: on warm summer days the temperature on the sandur is typically 3°C higher than at the ice margin.
This temperature difference is the 'glacier cooling' effect, sometimes known as the 'temperature jump' — the direct affect of the glacier on the thermal properties of the surrounding air. The data shows that ‘glacier cooling’ effectively reduces air temperatures during the melt season by 1–3°C in maritime south-east Iceland.
In proxy mass-balance studies this cooling effect should be taken into account, particularly when estimating ablation using models such as the positive degree-day (PDD) method. Future research aims to explore this relationship in more detail.
Beneath the glacier
In numerous places along the ice margin, ice caves and abandoned glacial stream channels allow glimpses of the glacier bed and the processes acting beneath the ice today and in the recent past.
The sediments under the margin of Virkisjökull are locally sourced, poorly sorted and unlithified, but have an unusually low water content.
Much of this subglacial debris is volcanic in origin and may have been transported during previous eruptions of the Oraefajökull volcano, when the ice cap was probably smaller than today.
Thicker glacier evidence
Closer examination reveals large sediment-filled cracks, 1–5 m long, strongly aligned within the glacier bed.
Such features are known as hydrofractures that form when glacial meltwater bursts through sediment under high pressure (Figure 8).
Their presence in these sediments at Virkisjökull is telling, as they must relate to a time when the glacier was substantially thicker and readvanced over pre-existing volcanic debris.
Ongoing work is trying to understand the formation of these hydrofractures, and reconstruct the former size of the glacier.
We are also trying to date the surrounding volcanic rocks and sediments using a range of techniques; in collaboration with the University of Liverpool.
Contact
Contact Jez Everest or Tom Bradwell for further information.
