Ice-marginal processes and sediment transport pathways at Hardangerjøkulen, Norway

Clare M. Bostona, Benedict T.I. Reinardyb, Danni Pearcec

aUniversity of Portsmouth, bUniversitetet i Bergen, cUniversity of Aberdeen Contact: clare.boston@port.ac.uk

1. IntroductionSmall ice masses are important for assessing the impacts of climate change at a local to regional scaledue to their sensitivity to changes in temperature and precipitation. Plateau icefields, such asHardangerjøkulen, are particularly interesting to study since the low gradient of the plateau ice makesthem highly sensitive to changes in the equilibrium line altitude (ELA) (Oerlemans, 2012) (Fig. 1).

Hardangerjøkulen (Fig. 2), in central Norway, has previously been identified as one of the mostvulnerable Norwegian ice masses (Nesje et al., 2008), and is predicted to have completely disappearedby 2100 (Giesen and Oerlemans, 2010). Previous research on Midtdalsbreen, one of the outletglaciers, indicates freeze-on and subsequent elevation of basal sediments to produce annualmoraines, due to winter cold being able to penetrate a thin ice margin (Reinardy et al., 2013). Here,we assess spatial and temporal differences in ice-marginal processes across three neighbouring outletglaciers, Blåisen, Midtdalsbreen and Bukkeskinnsbreen, located on the northeastern side ofHardangerjøkulen, with the aim of better understanding how local factors affect these processes.

3.1 Blåisen

The ice front at Blåsen has a gradient of between 18-22°, whilst the terrainimmediately in front of the ice margin slopes at 12-15° (Fig. 3).

Many of the moraines in the foreland are boulder and clast-rich, andtherefore too coarse to easily excavate. However, in the last few years, themoraines produced have been smaller (0.4 m high), and contain a higherpercentage of finer material, allowing exposures to be created.

3.2 Bukkeskinnsbreen

The northern to central side ofBukkeskinnsbreen is currently flowing up areverse-bed slope, caused by a bedrock knoll.Whilst there is some limited evidence for basalfreeze-on of sediments, the majority of the iceat the contact with the bed is relatively debris-free (Fig. 7). Moraines have been deposited onthe top of the bedrock knoll only, where itwould have acted as a pinning point.

3.3 Midtdalsbreen

2. MethodsFieldwork was undertaken in August 2013. Exposures, perpendicular to the crestline, were created within moraines close to the margins of each outlet glacier,and the sediments logged using a standard approach (e.g. Reinardy et al., 2013). Measurements of clast shape and roundness were collected on 50 phylliteclasts from each exposure, following procedures outlined by Lukas et al. (2013).

3.4 Debris transport pathwaysPhyllite clasts within the moraines at Midtdalsbreen and Blåisen plotwithin a similar zone on the RA-C40 co-variance plot, with the two samplesfrom Blåsen distinguished by higher C40 values. The RA index for allindicates more angular material than the subglacial control, but thatsignificant edge rounding has occurred compared to the supraglacialcontrol. C40 values lie between the subglacial, englacial and fluvialcontrols, showing that the transition from slab-like supraglacial material toa more blocky shape, typical of subglacial material, is incomplete (Lukas etal., 2013), potentially signifying a limited subglacial influence. However,the C40 values are significantly closer to the subglacial control than thoserecorded within the Midtdalsbreen annual moraines, and thus appear todocument less of a fluvial influence here (Reinardy et al., 2013).

4. Conclusions• The moraines that have formed in most recent years at the outlet glacier margins are small ridges,often composed of a structureless diamicton. This is inspite of differences in the slope of the ice frontat Midtdalsbreen and Blåisen. Preservation potential of these moraines is likely to be very low,particularly given the high level of fluvial activity at each glacier margin.• Formation of these ridges is localised and in other parts of the Midtdalsbreen and Bukkeskinsbreenmargin, delivery of sediment to the foreland is via the slow melt-out of debris-covered ice. Thisdelivers sorted sediment to the foreland, which has the potential to be re-worked during subsequentadvances.• Co-variance plots indicate similar sediment transport pathways, irrespective of the mode of sedimentdelivery to the ice-margin. The plots also potentially suggest a more direct glacial route than for clastspreviously examined within the Midtdalsbreen annual moraines, which appear to have been moreinfluenced by fluvial activity (Reinardy et al., 2013).

5. ReferencesGiesen, R.H., Oerlemans, J., 2010. Response of the ice cap Hardangerjøkulen in southern Norway to the 20th and 21st century climates. The Cryosphere 4, 191-213.Lukas, S., Benn, D.I., Boston, C.M., Brook, M.S., Coray, S., Evans, D.J.A., Graf, A., Kellerer-Pirklbauer-Eulenstein, A., Kirkbride, M.P., Krabbendam, M., Lovell, H., Machiedo, M., Mills, S.C., Nye, K., Reinardy, B.T.I., Ross, F.H., Signer, M., 2013. Clastshape analysis and clast transport paths in glacial environments: A critical review of methods and the role of lithology. Earth-Science Reviews 121, 96-116.Nesje, A., Bakke, J., Dahl, S.O., Lie, Ø., Matthews, J.A., 2008. Norwegian mountain glaciers in the past, present and future. Global and Planetary Change 60, 10-27.Oerlemans, J., 2012. Linear modeling of glacier fluctuations. Geografiska Annaler. Series A, physical geography 94 , 183-194.Reinardy, B.T.I., Leighton, I., Marx, P.J., 2013. Glacier thermal regime linked to processes of annual moraine formation at Midtdalsbreen, southern Norway. Boreas 42, 896-911.Winkler, S., Matthews, J.A., 2010. Observations on terminal moraine-ridge formation during recent advances of southern Norwegian glaciers. Geomorphology 116, 87-106.

Three of these moraines were examined(B1 to B3), all within 20 m from the icefront. All contained a structurelessdiamicton with clay matrix (Fig. 4). Thesemoraines typically exhibited anasymmetric cross-profile, with a steeperproximal (42°) than distal slope (35°).

This asymmetry is opposite to thatexpected to occur through basal freeze-on of sediment slabs (Reinardy et al.,2013). Combined with the lack ofstructures within the moraine, thissuggests formation through seasonalbulldozing of sediment at the ice-margin(Winkler and Matthews, 2010).

In contrast, the southern part of the ice margin iscovered by glaciofluvial sediment (Fig. 8),deposited by a meltwater stream emerging froma portal on the southern lateral margin, whichpotentially flowed over the ice margin previously.

The foreland in this area consists of a series oflinear ridges composed of a very loose gravelwith sandy matrix (Fig. 9), similar to that beingdeposited at/on the southern part of the icemargin. These ridges may therefore markdeposition by former meltwater streams thatflowed around this part of the ice margin, thusmarking the recessional pattern of the ice.

Northwestern lateral marginThe foreland in this area contains a set of small (0.5 m high) moraines (MBL1,MBL2), formed of a structureless diamicton, similar to those at Blåisen,although the matrix here is siltier (Fig. 10). Both moraines examined vary interms of their asymmetry; MBL1 has a proximal slope of 20° and distal slopeof 12°, whilst MBL2 has proximal and distal slopes of 22° and 25°,respectively, which alongside the lack of structures within them suggestsformation through seasonal bulldozing (Winkler and Matthews, 2010).

Central marginThe steepness of the ice front in the central zone is c. 12° and similar small moraines are found here (MB3).They are within 10 m of the ice margin, and therefore formed in the last couple of years after the annualmoraines documented by Reinardy et al. (2013). In terms of morphology they are similar to those found onthe northwestern lateral margin and at Blåsen, possessing a similar asymmetric profile and height,indicative of seasonal bulldozing (Winkler and Matthews, 2010). However, structurally, the moraines arecomposed of dipping units of clay, surrounded by gravel (Fig. 11), indicative of basal freeze-on andemplacement of sediment slabs during moraine formation. Yet, there is a significant difference in sizebetween these moraines and the annual moraines previously documented, suggesting a reduction in thisprocess, potentially due to changing dynamics at the ice margin during retreat.

Southeastern margin

The southeastern margin of Midtdalsbreen is debris-covered due tomeltout of an englacial debris septa further upglacier, followed bytransport and sorting into fines, sands and gravels by supraglacialmeltwater (MBI1, MBI2). The immediate foreland is therefore ice-cored, evidenced by tension cracks, and the debris cover is re-workedby gravity (debris flows, slumping and faulting) as the ice melts.

At the lateral margins of thecontrolled moraine, however, thereis more linearity to the ridges (MB1,MB2). Their composition variesbetween sorted and non-sortedmaterial, there is evidence ofshearing and folding, indicative ofproglacial to subglacial deformation,suggesting re-working of sedimentsin the foreland during a readvance.

Fig. 1. Illustration of how thesame rise in ELA on a lowgradient glacier (b) will causea larger proportion of theglacier to move into theablation zone, compared tofor a steep glacier (a).

Fig. 2. Map of Hardangerjøkulen and study area.

Fig. 3. Ice front of Blåisen.

Fig. 4. Section through moraine B2.

Fig. 5. Bulldozing of sediment at the ice-margin.

Fig. 6. a) Southern part of and b) Central to southern part of the ice front of Bukkeskinnsbreen.

Fig. 10. a) Northwestern (*) and central ($) parts of the Midtdalsbreen foreland; b) section log of MBL2.

Fig. 11. Section log of MB3.

Fig. 13. a) The debris-covered southeastern margin; b) faulting of debris cover as the ice melts.

Fig. 12. Section logs of a) MB1 and b) MB2, from different places within the same moraine.

Fig. 14. RA-C40 Co-variance plot for phyllitesfrom Blåisen and Midtdalsbreen, including samples MBI1 and MBI2 from the debris cover on Midtdalsbreen showing the fluvial influence on clast shape and roundness down-glacier due to transport by supraglacialmeltwater streams.

Fig. 7. Central zone of glacier margin.

Fig. 8. Debris-covered southern ice front.

Fig. 9. Glaciofluvial debris ridges.

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