The geography of basal ice and its relationship to glaciohydraulic supercooling: Svínafellsjökull,southeast Iceland

Glaciohydraulic supercooling is potentially an important mechanism of basal ice formation with significant implications for Quaternary Science, but remains controversial. Some studies cite similarities between basal ice and the ice forming at vents of upwelling supercooled subglacial water as evidence that where supercooling occurs it dominates basal ice formation. Other studies find no evidence linking supercooling to basal ice even at glaciers where supercooling occurs, questioning the supposed genetic link between basal ice and vent ice. At Svínafellsjökull, southeast Iceland, we compare the physical characteristics and geographical distribution of stratified basal ice with the characteristics and distribution of phenomena such as upwellings and anchor ice terraces associated with supercooling. Two physically and geographically distinct basal ice populations emerge: one occurs only where there is evidence for glaciohydraulic supercooling and has physical characteristics consistent with an origin by supercooling; the other is ubiquitous, even at locations where there is no evidence for supercooling, and has characteristics consistent with non-supercooling modes of origin. We suggest that glaciohydraulic supercooling does not account for all of the basal ice at Svínafellsjökull, and that other mechanisms of basal ice formation are also significant.     

Origin of stratified basal ice in outlet glaciers of Vatnajökull and Öræfajökull,Iceland

Larson, G.J., Lawson, D.E., Evenson, E.B., Knudsen, Ó., Alley, R.B. & Phanikumar, M.S. 2010: Origin of stratified basal ice in outlet glaciers of Vatnajökull and Öræfajökull, Iceland. Boreas, Vol. 39, pp. 457–470. 10.1111/j.1502‐3885.2009.00134.x. ISSN 0300‐9483. During the period 2000–2005, we collected samples of englacial ice, vent water, frazil/anchor ice and stratified basal ice from warm‐based outlet glaciers of Vatnajökull and Öræfajökull, Iceland, and analysed them for ³H, ¹⁸O and D. Results of ³H analyses show that the stratified basal ice contains ³H from atmospheric thermonuclear testing and is younger than the englacial ice. Results of the ¹⁸O and D analyses show that frazil/anchor and stratified basal ice are both enriched by an average of 2.4‰ in ¹⁸O and 11‰ in D relative to vent water. These values are consistent with fractionation during partial freezing of supercooled subglacial water in an open system, one in which the remaining water is continuously removed and replenished by water of similar composition. The isotopic data and field observations do not support either a regelation or a thermal ad‐freeze‐on origin for the stratified basal ice.     

Glacier margin fluctuations, Skaftafellsjökull, Iceland: implications for sandur evolution

This paper relates recent proglacial fluvial channel change at Skaftafellsjökull, southeast Iceland, to glacier margin fluctuations. Observations of the western portion of the proglacial braided sandur were made annually between 1996 and 2000. Between 1996 and 1998, during a period of glacier advance, the proximal proglacial outwash surface at the western end of the glacier margin was characterized by a complex braided channel pattern active over the entire sandur surface. Retreat of the glacier margin since 1998 led to rapid incision, so that by 1999 abandonment of the proximal terrace surface and reorganization of the proglacial fluvial system into a single, entrenched channel had occurred. Further retreat and incision occurred during 1999–2000. These observations demonstrate that glacier retreat at Skaftafellsjökull is accompanied by short-lived rapid incision events and terrace formation, separated by long intervals of relatively minor change rather than progressive incision over long time periods. The margin of Skaftafellsjökull is thought to be particularly sensitive to retreat, as the glacier occupies an overdeepening behind the snout and results in lowering of the river's point of exit from the glacier, necessitating adjustment of the river's long profile.     

The Little Ice Age glacier maximum in Iceland and the North Atlantic Oscillation: evidence from Lambatungnajökull, southeast Iceland

This article examines the link between late Holocene fluctuations of Lambatungnajökull, an outlet glacier of the Vatnajökull ice cap in Iceland, and variations in climate. Geomorphological evidence is used to reconstruct the pattern of glacier fluctuations, while lichenometry and tephrostratigraphy are used to date glacial landforms deposited over the past ˜400 years. Moraines dated using two different lichenometric techniques indicate that the most extensive period of glacier expansion occurred shortly before c . AD 1795, probably during the 1780s. Recession over the last 200 years was punctuated by re-advances in the 1810s, 1850s, 1870s, 1890s and c . 1920, 1930 and 1965. Lambatungnajökull receded more rapidly in the 1930s and 1940s than at any other time during the last 200 years. The rate and style of glacier retreat since 1930 compare well with other similar-sized, non-surging, glaciers in southeast Iceland, suggesting that the terminus fluctuations are climatically driven. Furthermore, the pattern of glacier fluctuations over the 20th century broadly reflects the temperature oscillations recorded at nearby meteorological stations. Much of the climatic variation experienced in southern Iceland, and the glacier fluctuations that result, can be explained by secular changes in the North Atlantic Oscillation (NAO) Advances of Lambatungnajökull generally occur during prolonged periods of negative NAO index. The main implication of this work relates to the exact timing of the Little Ice Age in the Northeast Atlantic. Mounting evidence now suggests that the period between AD 1750 and 1800, rather than the late 19th century, represented the culmination of the Little Ice Age in Iceland.     

Landsystems of Morsárjökull,Skaftafellsjökull and Svínafellsjökull,outlet glaciers of the Vatnajökull Ice Cap,Iceland

This study utilizes a landsystem approach to analyse the landforms and sediments exposed on the forefields of three closely spaced outlet glaciers of the Vatnajökull Ice Cap, southeast Iceland; Morsárjökull, Skaftafellsjökull and Svínafellsjökull, in order to determine how individual glacier and environmental characteristics influence landscape development. Analysis of satellite imagery and fieldwork were used in conjunction to examine the geomorphology and sedimentology of the forefields, and to define the characteristic landsystems of each of the glaciers. Morsárjökull and Skaftafellsjökull have similar proglacial fields, with similarities in the distribution and scale of the landforms, and their characteristics conform to the established active temperate landsystem. Svínafellsjökull differs significantly from the other glaciers having a proglacial field that more closely resembles an early stage debris‐charged landsystem. Variations between the glaciers in terms of their ice distribution (hypsometry, equilibrium line altitude), bedrock type, topography and debris content are important factors that contribute to the landsystem variability evident in their proglacial fields. The forefields of these three glaciers may be used as analogues to enhance understanding of palaeoenvironmental conditions that existed along the southern margin of Pleistocene glaciers that covered much of northern North America and Europe in the past.     

Progressive formation of modern drumlins at Múlajökull,Iceland: stratigraphical and morphological evidence

The drumlin field at Múlajökull, Iceland, is considered to be an active field in that partly and fully ice‐covered drumlins are being shaped by the current glacier regime. We test the hypothesis that the drumlins form by a combination of erosion and deposition during successive surge cycles. We mapped and measured 143 drumlins and studied their stratigraphy in four exposures. All exposures reveal several till units where the youngest till commonly truncates older tills on the drumlin flanks and proximal slope. Drumlins inside a 1992 moraine are relatively long and narrow whereas drumlins outside the moraine are wider and shorter. A conceptual model suggests that radial crevasses create spatial heterogeneity in normal stress on the bed so that deposition is favoured beneath crevasses and erosion in adjacent areas. Consequently, the crevasse pattern of the glacier controls the location of proto‐drumlins. A feedback mechanism leads to continued crevassing and increased sedimentation at the location of the proto‐drumlins. The drumlin relief and elongation ratio increases as the glacier erodes the sides and drapes a new till over the landform through successive surges. Our observations of this only known active drumlin field may have implications for the formation and morphological evolution of Pleistocene drumlin fields with similar composition, and our model may be tested on modern drumlins that may become exposed upon future ice retreat.     

Role of particle size in till-fabric characteristics: systematic variation in till fabric from Vestari-Hagafellsjökull, Iceland

Till-fabric analysis has often been used to interpret glacier flow directions and subglacial dynamics using vector-based statistics, but recent data suggest that such analysis may also effectively indicate former glacier dynamics. The results of a fabric investigation of subglacially strained till deposited during a surge of Vestari-Hagafellsjökull, Iceland, are presented. Till fabric was collected at four sites within a limited area where ice-flow direction during deposition was known from subglacial bedforms at the site. Analysis was carried out on elongate grains (axial ratio <1.5:1) at seven size fractions, with the a-axis length ranging from 0.25 to 32 mm. The largest grains tend to be parallel to ice flow, whereas smaller grains reflect a mix of parallel and transverse orientations. The implications of these data for the role and validity of till-fabric analysis are discussed, with reference to vector analyses and the compilation of fabric shape envelopes. It is noted that, in contrast to laboratory experimental data, neither March nor Jeffery mechanisms explain the fabric configurations reported. It is concluded that a standardized approach is necessary for collecting fabric data, and in many situations analysis of data populations at a range of particle sizes is desirable.     

Fluvial adjustments in response to glacier retreat: Skaftafellsjökull,Iceland

This study describes changes to the proglacial drainage network of Skaftafellsjökull, Iceland from 1998 to 2011. Proglacial landscapes are highly sensitive to glacier retreat, and the retreat of glaciers around the world has accelerated since the mid‐1990s. Skaftafellsjökull has retreated at an average rate of 53 m per year since 1999. From 1999 to 2003, the river incised and formed a sequence of now abandoned channels and fluvial terraces extending ～1 km downstream from the glacier. Retreat of the glacier from an over‐deepened ice‐contact slope meant that there was a positive correlation between the distance of glacier retreat and the amount of fluvial incision. Incision was episodic, occurring annually in response to drainage reactivation and reorganization. On an annual basis, the rate of retreat is moderately negatively correlated with the rate of incision. This is partly because the ice‐contact slope decreases away from the position of maximum glacier extent, and also because more sediment is released with faster retreat, counteracting the effect of retreat down an ice‐contact slope. From 2003 onwards, proximal terrace formation ceased, as a proglacial lake became established. Downstream of the lake outlet further incision deepened the channel, with most change occurring during a flood in 2006, where incision in the upstream confined reach was accompanied by downstream aggradation and terrace formation. These observations indicate that proglacial changes in response to glacier retreat are a result of the interactions of river channel incision and terrace formation, aggradation, lake development, and flooding, which together control river channel changes, sediment redistribution and sandur stratigraphy.     

Climate change and 'anomalous' glacier fluctuations: the southwest outlets of Mrdalsjökull, Iceland

Evidence of past glacier fluctuations is valuable palaeoenvironmental data, but determining their relationship to climatic change is sometimes complex because of differing glacier sensitivities and patterns of response. In Iceland, a diverse range of glaciation creates changing geographical patterns of response to climatic changes. The outlet glaciers of the Márdalsjökull ice cap in southern Iceland have produced detailed, but differing, records of change. For a key southwestern sector of the ice cap, we specifically searched for evidence equivalent to the c . 4500 BP, c . 3100 BP and c . 1200 BP advances of Sólheimajökull reported earlier. A combination of geomorphological mapping and dating by tephrochronology and lichenometry was used to constrain the glacier advances and determine the relative magnitude of Neoglacial glacier episodes. This is a key step towards creating a record of the changes for the entire ice cap. Major glacier advances c . 4500–1000 BP previously identified on the southern margin of Márdalsjökull are shown not to have occurred in this sector, where Neoglacial maxima occur post-1755 AD.     

The final phase of dead-ice moraine development: processes and sediment architecture, Kötlujökull, Iceland

Consecutive phases of de-icing of ice-cored moraines and the formation of dead-ice moraine were monitored over a 4-year period at the terminus of the Kötlujökull glacier, Iceland. Particularly, the transition from partially ice-cored moraine with isolated dead-ice blocks to the ice-free landscape receives attention in this paper in order to link the final melting processes to the architecture of the sedimentary end product. In the current humid sub-polar climate of south Iceland de-icing of partially ice-cored moraines results chiefly from melting along the bottom surface of ice-cores with an annual average rate of 25 cm. The final de-icing is associated with an interrelated group of re-sedimentation processes and surface features. Series of sinkholes evolve at the toe of dead-ice blocks, which initiate retrogressive rotational sliding or backslumping of the ice-cored slopes and the formation of distinct edges and fractures in the adjacent basins. Although backslumping is the dominant process in this phase of re-sedimentation, structures resulting from this process are rarely recognized in the ice-free landscape. As ice-cores gradually diminish the effect of the latest re-sedimentation events will overprint or destroy most existing sedimentary characteristics. Thus, in the ice-free landscape, structures mainly related to the formation of sinkholes and fractures remain imprinted on the sediment succession. Generally, no inversion of the topography occurs during the final phase of de-icing. The overall topography recognized in the late phase of the fully ice-cored terrain is merely lowered and the amplitude of the relief reduced as de-icing progresses. The sediment architecture of the ice-free landscape is characterized by heterogeneous and often slumped diamict sediments with variable thickness and lateral distribution; clast orientation is related to the direction of slopes, and boulders are found in isolated groups or in linear arrangements.     

Subglacial bedforms and conditions associated with the 1991 surge of Skeiðarárjökull, Iceland

Much previous research at surge-type glaciers has sought to identify features diagnostic of surge-type behaviour. However, in comparatively little work have subglacial landform–sediment characteristics been used to reconstruct changing basal processes and conditions during surge events. Subglacial bedforms described in this article are associated with the 1991 surge of Skeiðarárjökull, Iceland, and include a series of drumlins with superimposed flutes and basal crevasse-fill ridges. The drumlins were formed by the subglacial erosion of ice-contact fans. Sedimentary evidence indicates a shift from rigid-bed to soft-bed conditions during the surge. The presence of eroded but undeformed fan sediments suggests that they acted as a rigid bed when initially overridden. Subsequent deposition of a layer of deformation till resulted in a change to soft-bed conditions and the generation of flutes and subglacial crevasse-fill ridges. The lack of mixing between this till and the underlying stratified sediments indicates that subglacial sediment deformation was restricted to a thin layer and that its deposition resulted in a cessation of subglacial erosion. The drumlin is therefore a composite of both rigid-bed and soft-bed processes that illustrates changes in basal conditions and processes during the course of the event. The limited time frame in which the drumlin formed and the presence of kettleholes across its surface are distinctive features that may warrant further investigation in the search for features diagnostic of past surge events.     

Petrogenetic Evolution of the Torfaj?kull Volcanic Complex, Iceland II. The Role of Magma Mixing

In southern Iceland, tholeiitic basalt magmas propagating laterallyfrom the active Eastern Rift Zone into the older cmstal segmentof the South Eastern Zone have been injected into Torfaj?kull,a mature volcanic centre dominated by rhyolites. Eruptions ofcomplex suites of mixed and hybrid rocks have been triggered,involving tholeiites of the rift zone and transitional basaltsand rhyolites of the Torfaj?kull centre. Three-component hybridsare an unusual feature of the activity. The distribution ofvarious magma mixing and hybrid types is related to the periodicinjection of tholeiite into a magma chamber, or chambers, whererhyolite overlies parental transitional basalts. Pre-postglacial rhyolites (>10000 y) at Torfajokull are predominantlyperalkaline, whereas later rhyolites are, with few exceptions,subalkaline. Furthermore, the injection of rift zone magmas,and the consequent abundance of rhyolite-basalt mixing, havebeen important features of magmatism at the centre only in postglacialtimes. Reduced repose times in the magma reservoirs have preventedthe production of peralkaline rhyolites. These trends are interpretedin terms of the southerly migration of the Eastern Rift Zone.     

Origin and de-icing of multiple generations of ice-cored moraines at Brúarjökull, Iceland

Mature dead-ice has been overridden repeatedly by the Brúarjökull glacier, and multiple generations of ice-cored landforms occur, with ice cores originating at least from glacier surges in 1963-1964, 1890 and 1810. Ice-cores are located on the proximal slopes of end moraines and in the valleys, as ice-cored outwash and eskers, ice-cored drumlins and ice-cored moraine patches. This dictates that the sediments and internal architecture might not always match their end-products as de-icing progresses. Analysis of multi-temporal aerial photographs integrated with annual field measurements showed that the time required for a total de-icing in the forefield exceeds the duration of the quiescent phases between the surges, even in the current climate at the limit of permafrost. Quantifying melting progression suggests that complete de-icing of ice-cored landforms is not likely to occur. The mean de-icing rate is c. 9.8 cm/yr in 1890 ice-cored moraines, and c. 17.7 cm/yr in 1963-1964 ice-cored moraines. Backwasting of ice-cored slopes (c. 30 cm/yr) is the fastest melt process. Long-term downwasting rates derived from multi-temporal digital elevation models provide a superior insight into the impact of multiple glacier surges on the formation of dead-ice moraines in front of Brúarjökull.     

Water-eroded crescentic scours and furrows associated with subglacial flutes at Breidamerkurjökull, Iceland

Several subglacially formed short flutes show crescentic scours around the boulders at their proximal end and furrows along their sides. The crescentic scours and furrows are partly filled by sorted sand and gravel. and it is suggested here that they were formed by fast flowing, turbulent meltwater in the subglacial cavity where the flute was formed.     

Time-scales for magmatic differentiation at the Snaefellsjökull central volcano, western Iceland: Constraints from U-Th-Pa-Ra disequilibria in post-glacial lavas

We present major and trace element and Sr-Nd-Pb and U-Th-Pa-Ra isotope data for a small sample suite of primarily post-glacial, mildly alkalic volcanic rocks from the Snaefellsjökull central volcano situated off the main rift systems in western Iceland. The volcanic rocks are crystal-poor and range from olivine alkali basalt to trachyte and show tight correlations of major and trace elements that are explained by fractional crystallization involving removal of olivine, plagioclase, clinopyroxene, Fe-Ti oxide and apatite. Sr-Nd-Pb isotopes are practically invariant, consistent with derivation from the same source region. During fractionation from primitive basalt to evolved trachyte, (²³⁰Th/²³²Th), (²³⁰Th/²³⁸U) and (²³¹Pa/²³⁵U) decrease progressively at broadly constant (²³⁸U/²³²Th). A continuous closed-system fractionation model that assumes constant initial (²³⁰Th/²³²Th) in the basaltic precursor melt indicates that hawaiite was derived from olivine basalt by ∼50% fractional crystallization within and trachyte by ∼80% fractionation within . An overrepresentation of evolved basalts and hawaiites with young inferred magma ages in the dataset is consistent with the parental precursor to these magmas intruded into the sub-volcanic magma plumbing system as a consequence of lithospheric rebound caused by deglaciation. Lavas affected by apatite removal have higher (²³¹Pa/²³⁵U) than predicted for simple radioactive decay, suggesting apatite significantly fractionates U from Pa. The proposed fractionation model consistently explains our U-series data assuming and and . If applicable, these D values would indicate that the effect of apatite fractionation must be adequately considered when assessing differentiation time scales using (²³¹Pa/²³⁵U) disequilibria data.     

Rates and causes of proglacial river terrace formation in southeast Iceland: an application of lichenometric dating techniques

The immediate proglacial areas of most of the Oraefajökull outlet glaciers in southeast Iceland are characterized by well-developed river terraces, formed by the recent downcutting of the major meltwater streams. This paper examines the rates and causes of dissection in two contrasting cases, using lichenometric dating to establish the ages of individual terraces. An age–size curve for the aggregated Rhizocarpon sub-genus is developed from lichen measurements on dated recessional moraines, and is compared with similar curves obtained by previous workers. Levelling profiles of the terraces are then used in conjunction with the lichenometric dates to determine mean rates of net erosion between each dated surface, and to study the associated variations in channel slope. The results obtained for the Svinafellsá show that the timing and rates of downcutting have been closely related to frontal movements of the Svinafellsjökull glacier. The Kotá terraces, however, may have been formed independently of glacier fluctuations, and are thought to represent stages in the gradual recovery of the stream from the aggradational effects of the 1727 jökulhlaup.     

Sedimentary facies and environmental history of the Late-glacial glaciomarine Fossvogur sediments in Reykjavík, Iceland

A detailed account of facies relationships in the Late-glacial Fossvogur beds is presented for the first time. A new interpretation in terms of sedimentary processes has been synthesized in a palaeoenvironmental reconstruction that incorporates the results of a recently completed, systematic ¹⁴C dating project for the Fossvogur beds. The present sedimentological analysis has revealed three marine fossiliferous facies and several diamictite facies. The two uppermost marine facies are separated by a horizon of local deformation and erosion which is ascribed to increased activity at a tidewater glacier margin in Fossvogur. The fact that marine fossiliferous sediments below and above this horizon have been confined to the Allerød chron indicates a temporary expansion of glaciers in the Reykjavík region towards the end of, but within, the Allersd. The relative sea level must have been at least 20 m higher than at present before the expansion, and it was probably even higher during and after the expansion. The total absence of facies indicating either lodgement or melt-out processes, and the abundance of diamictites interpreted as debris-flow deposits as well as frequent erratics in the marine mudrocks favour an interpretation based on a glaciomarine model for the Allerød deposition in Fossvogur. It is suggested that the relatively quiet, submarine conditions indicated by facies towards the top of the Fossvogur beds display continued transgression and an increased distance to the source of sediment supply during the Younger Dryas.     

Lichenometry in dating recent glacial landforms and deposits, southeast Iceland

Lichenometry curves were derived for the glacier forelands of Breidamerkurjokull and Skalafellsjökull using measurements of the collective species, Rhizocarpon geographicum , from independently dated surfaces deglaciated during the period since the late-19th century ice maximum episode in the area. The form of the curves appears to be linear. Several methods of predicting the ages of undated surfaces were compared for specific test examples, including single lichenometry curves, families of curves and multiple curves based on randomly selected largest lichens. The last method provided a useful means of assessing the reproducibility of predicted ages. Where substrate conditions were highly variable, a lichenometry curve based on single largest lichens provided a preferred result.