Digital elevation models (DEMs) of several j & ouml;kulhlaup source areas were measured during the mapping of Icelandic glaciers with aerial lidar in 2008-2013. This included i) the j & ouml;kulhlaup in M & uacute;lakvisl from M & yacute;rdalsj & ouml;kull on 9 July 2011, which destroyed the bridge on the ring road, and ii) the j & ouml;kulhlaup in Vestari-J & ouml;kuls & aacute; from Hofsj & ouml;kull in late August 2013. Comparison of these DEMs with lidar measurements from before the j & ouml;kulhlaups reveals valuable information about the volume of the floods and the nature of subglacial water flow from j & ouml;kulhlaup source areas created by subglacial geothermal activity. DEM differencing shows lowering of the glacier surface of M & yacute;rdalsj & ouml;kull along several kilometres of the subglacial flood path from the main cauldrons that released the initial flash flood, which is most likely caused by melting of the glacier sole by warm flood water. It is not possible to derive an explicit estimate of the temperature of the flood water near the main cauldrons, but the DEM differencing indicates temperature on the order of 10 degrees C to tens of degrees Celsius. In recent years, three j & ouml;kulhlaups have been observed from Hofsj & ouml;kull and cauldrons have formed in the ice cap surface at three different locations. These measurements and observations of several other j & ouml;kulhlaups in Iceland in recent years show that the initial temperature of j & ouml;kulhlaup flood water may be quite variable from near the freezing point of water to tens of degrees Celsius. This variable temperature is likely to have important consequences for j & ouml;kulhlaups dynamics and may be part of the explanation for the highly variable development of j & ouml;kulhlaups. The j & ouml;kulhlaup in M & uacute;lakvisl in 2011 and from Hofsj & ouml;kull in 2013, as well as several other j & ouml;kulhlaups from M & yacute;rdalsj & ouml;kull and some other glaciers and ice caps in Iceland, highlight the importance of improving the understanding of j & ouml;kulhlaups and related natural hazards.
The caldera Lake & Ouml;skjuvatn, is at 1050 m elevation in Iceland's interior. It is a deep (217 m) dimictic lake formed after an eruption in 1875. Geothermal activity with gas and liquid inflows, down to 84 m depth, maintains a similar to 0.13 km2 permanent ice opening in winter. Remote sensing data revealed a progressively disappearing ice cover in the winter 2012. Physical and chemical conditions were explored in April 2012 (ice-free) and April 2013 (ice-cover). Measurements included CTD profiles and continuous temperature records. Meteorological observations from an automatic station show frequent southwesterly winds in the first quarter of 2012. Near-linear temperature increase with depth in April 2012 indicated effective whole lake vertical mixing. In contrast, the lake was weakly stratified in April 2013 with heat stored below 60 m depth. Moored temperature records in the winters of 2013 and 2014, revealed sustained under ice temperature, and hence a density rise in the upper 60 m, which is half the lake volume. The April 2012 concentrations of geochemical temperature indicators gave no indications of enhanced thermal activity. However, the concentrations of dissolved mineral constituents had decreased since 1975. Chloride and lithium decreased by 30 degrees lo but the geothermal indicators, silicate and sulphate, had decreased less, at similar to 15 degrees lo. The inflowing water, from local precipitation, had not changed. The estimated annual lake-air flux of carbon dioxide, 19000 tons in 2012, had substantially decreased. The unexpected winter ice loss in Lake & Ouml;skjuvatn in February-March 2012 was driven by a complex interplay of wind stress, the lake's seasonal deep water heat storage and geothermal activity. Inflow of geothermal gas at 84 m depth off the western shore enhances the vertical transport of heat to the lake's upper layer which, together with frequent southwesterly wind stress 2012, eventually induced vertical instability, whole lake turnover and a complete ice melt.
- The Skaft & aacute; cauldrons, a pair of surface depressions NW of Grimsv & ouml;tn on the Vatnaj & ouml;kull glacier, signify subglacial lakes created by geothermal heat sources within the underlying bedrock. These subglacial lakes continuously grow in volume and each produce j & ouml;kulhlaups every 1-5 years at the glacier outlet 35-40 km away. Seismic activity associated with the eight 2015-2021 j & ouml;kulhlaups from the Skaft & aacute; cauldrons was analyzed, reflecting striking similarities. Seismic activity associated with subglacial flood propagation and cauldron deepening was dominated by small transient signals, interpreted as icequakes from ice deformation and hydrofracturing as the water starts to migrate subglacially. Low-amplitude, low-frequency highly repetitive events were discovered during this period for the larger magnitude j & ouml;kulhlaups, possibly relating to stick-slip motion of the glacier at the bedrock. Seismic tremor events coincide with these large floods, but the origin of the tremor has been disputed. Sustained tremor (1-3 Hz) persists for 1.5-3 days, while high-amplitude tremor bursts (0.5-4 Hz) with durations up to tens of minutes and a strong relationship with increased electrical conductivity in the flood water are observed once most of the water has drained from the cauldrons, exhibiting higher amplitudes during larger j & ouml;kulhlaups. A probabilistic location method reveals that this 0.5-4 Hz tremor is co-located with the cauldrons and temporally coincides with the end of cauldron subsidence, indicating that the tremor is related to the rapid depressurization of the bedrock and is likely generated by either confined shallow level magmatic activity or enhanced geothermal activity such as vigorous boiling or hydrothermal explosions.
Numerous jokulhlaups have followed the river Jokulsa a Fjollum in northeast Iceland during the Holocene. Some of these fall under the category of catastrophic floods that carved out the present-day Jokulsargljufur canyon, over 100 km north of the present-day Vatnajokull. Sedimentary beds from 16 jokulhlaups between 6.3 and 4.1 ka ago are preserved in Vesturdalur, near the north end of the canyon. Chemical composition of volcanic glass in the jokulhlaup deposits correlates to three volcanic systems beneath Vatnajokull. The characteristics of the Bardarbunga volcanic system dominate in 12 beds, and those of Grimsvotn and Kverkfjoll in one bed each, two remain unsolved. The characteristics of the Bardarbunga glass in the jokulhlaup sediments are mostly low TiO2 and high MgO (TiO2 <1.6, MgO >7.3 w%). Seventeen basaltic "Low-Ti" tephra layers from Bardarbunga have been identified in soil sections in north Iceland from this period. Grain characteristics of the tephra indicate phreatomagmatic origin. Dispersal maps confirm source area below northwest Vatnajokull and tephra volume (bulk) of the order of 1 km3 for the largest layers. As the preserved soil sections are distal (>50 km) from source, it is likely that only the largest tephra layers have been preserved. The mid-Holocene floods confirm the existence of glaciers on Bardarbunga, Kverkfjoll, and Grimsvotn at that time. The magnitude of these jokulhlaups is not well constrained, but apparent cross sections out of Vesturdalur fit a peak discharge of order 50,000 m(3)/s and likely total volume of a few km3. These repeated jokulhlaups 6.3 to 4.1 ka ago did not cause large erosion at Vesturdalur. Their source areas were most likely the calderas of the central volcanoes, which may have changed in size and form since the mid-Holocene. Eruptions within the Bardarbunga caldera are a possible source for 12 of the floods. Bardarbunga may have hosted a geothermal area and a subglacial caldera lake similar to present day Grimsvotn, which may explain the repeated, apparently similar-magnitude jokulhlaups over this long period.
Glaciologists and meteorologists described shrinkage of glaciers in the northern hemisphere in the early 20th century. Warm southerly currents in the atmosphere and in the ocean caused the decrease. Scientists did not advance theories about the causes of the changes but discussed fluctuations of solar radiation and changes in trade winds. They encouraged colleagues to expand monitoring of glaciers which might lead to improved understanding and thanks to the interest of physicists, knowledge advanced of the natural causes of the glacier variations. ...
The Preboreal ice-sheet in Northeast Iceland received major tephra falls during its southwards retreat. Such air fall events can affect melting of the ice sheet and cause temporary standstills or even advances if tephra thickness on ablation areas exceeds a critical limit. Tephra deposits on ablation areas may then be removed by subsequent seasonal meltwater run-off and accumulate as recognizable deposits at the former glacier margins, defining the position of the margin at a particular time. A major tephra fall of greyish-white silicic tephra with the chemical characteristics of the 10.9 ka Askja-S tephra was deposited over the ice sheet as it retreated from the northeastern lowlands (& THORN;istilfjordur-Vopnafjordur) towards the Fjallgardar highlands. Substantial quantities of tephra were washed off the glacier snout and ablation area onto lateral terraces at Svartfell and Langadalsa where the tephra accumulated as thick, ash-rich deposits. A fines depleted tephra containing rounded grains also forms a layer within proglacial sediments deposited in front of the presumed glacier margin. These deposits lie inside a previously recognized standstill/advance known as Bruni or Melur stage. Another major tephra fall of black tephra with chemical characteristics corresponding to those of the 10.3 ka Saksunarvatn (Grimsvotn volcano) tephra may have affected the ice sheet at the time of the eruption, causing a standstill/advance near Arnardalsa shortly after those recognized as Anavatn and & THORN;vera/Fiskidalur stages. A distance of about 40 km separates the two dated locations. Given the uncertainties concerning the age of the two tephra layers, an average retreat rate of the inland ice of some 60 +/- 10 m/year seems realistic. Using this rate of retreat the Anavatn stage can be dated to 10.5 ka, the & THORN;vera/Fiskidalur to 10.4 ka and the inland ice would have retreated behind the present edge of northern Vatnajokull at 9.5-9.6 ka. The dating of these paleoice-margins calls upon some revision of the areal extent of the Preboreal ice sheet in Northeast Iceland and implies that a corridor may have formed at Langidalur around 10.9 ka splitting the ice sheet into an isolated northern dome, separated from a southern ice-margin retreating with several advances/standstills towards south.
- Nine nunataks have been exposed during the post-Little Ice Age recession of Brei & eth;amerkurj & ouml;kull, an outlet glacier of the Vatnaj & ouml;kull ice cap in Southeast Iceland. The outlet is fed by three branches that originate from the central ice cap. We analyse from digital elevation models (spanning 131 years, with the Equilibrium-Line Altitude (ELA) rising from 1000 m to 1200 m) the changes in the surface elevation and the development of the medial moraines originating from the nunataks. Elevation changes surrounding the nunataks in the central ablation zone show that ice flow compensated for about half of the lowering due to negative mass balance (up to 40 m from 2000 to 2022). During the 20th century, the internal driving stresses of the large ice branches (flow units) caused lateral displacement of medial moraines (up to 500 m), but as the glacier receded in the early 21st century, the subglacial topography started to dictate the migration patterns of the medial moraines. The Brei & eth;amerkurj & ouml;kull medial moraines are of the Eyles and Rogerson (1978) ISI and AD1 and AD3 types, whose historical lateral migration in some areas will disrupt their eventual accumulation as unbroken linear features on the deglaciated foreland, giving rise instead to gently inclined or folded englacial debris banding and ultimately the production of a supraglacial veneer.
The annual J & Ouml;RF & Iacute; expedition to Vatnaj & ouml;kull took place from May 28th to June 3rd, 2023. A total of 29 people participated. The weather was favorable, calm, sunny and mostly cloudless skies. This aided in successfully carrying out all the planned survey and maintenance work. Measurement of winter mass balance at Kverkfj & ouml;ll, B & aacute;rBarbunga and Grimsv & ouml;tn sites and a few hundred kilometers of radio -echo soundings within and around the Grimsv & ouml;tn, Skaft & aacute; cauldrons and B & aacute;rBarbunga area were collected. Installation and maintenance of GPS and seismic stations at various locations on Vatnaj & ouml;kull were carried out, as well as gas measurements in Kverkfj & ouml;ll, B & aacute;rBarbunga and at Saltarinn. Seismic sensors recorded tremor in Grimsv & ouml;tn for the duration of the trip, and seismometers were dug into the snow pack at B & aacute;rBarbunga and were supposed to be recording during the summer. Some maintenance and preparatory work on the huts of the society were carried out by the cabin committee (sk & aacute;lanefnd). Camera brackets were installed at K & aacute;rasker in BreiBamerkurj & ouml;kull and scouting of other possible locations for additional brackets as part of the project EISI (Extreme Ice Survey Iceland). Below average wintersnow and rain in the weeks prior to the trip led to a weak snow pack in some areas.
For the glaciological year 2022-23 mass balance was measured at similar to 120 locations in Iceland. At Vatnaj & ouml;kull 72 observation points were used, 20 points at Hofsj & ouml;kull, 25 at Langj & ouml;kull and 2 at M & yacute;rdalsj & ouml;kull. Five automatic weather stations (AWSs) were in fullyear operation at Vatnaj & ouml;kull and Hofsj & ouml;kull but additional 7 AWSs were operated during the melt season. Winter mass balance was below average, due to less winter snowfall than in an average year. Thus glacier ice beneath the snow cover was exposed relatively early in the summer, during which warm and relatively cloud -free conditions prevailed, leading to high levels of summer ablation. Figure 1 shows the main results for the three largest ice caps in Iceland. At Vatnaj & ouml;kull winter mass balance was similar to 10% below average and summer ablation was significantly (20%) above average, resulting in a negative net balance of - 1.01 m w. eq. ( - 1.21 m w. eq. if other mass loss (geothermal melt, calving etc.) is included). At Hofsj & ouml;kull winter snow thickness, as measured during the spring expedition, was about 25% below the long-term average. Summer ablation was about 10% above average, resulting in a negative net mass balance of - 1.50 m w. eq. Only 6 years have seen more negative mass balance since the start of mass balance measurements on Hofsj & ouml;kull, 36 years ago. Net surface mass balance at Langj & ouml;kull was 20% below the long term average, - 1.43 m w. eq. ( - 1.48 if estimates of other mass loss are included). Winter mass balance was 14% under the long-term average (1.53 m w. eq.) and summer ablation was close to the long-term average, - 2.96 m w. eq.
- The Gr & iacute;msv & ouml;tn volcano, one of the most active volcanoes in Iceland, is covered to a large extent by the Vatnaj & ouml;kull glacier. High geothermal activity within its caldera maintains an ice-covered caldera lake with variable water level. Large floods from the lake (j & ouml;kulhlaups) are initiated when the water breaks through an ice dam and flows out of the caldera. In several cases the falling lake level is known to have triggered eruptions of the volcano, e.g, in 1922, 1934, and 2004. The eruptions of 1983, 1998, and 2011, however, were not triggered by j & ouml;kulhlaups, and most j & ouml;kulhlaups have not triggered eruptions, including those of 2008 and 2010. All these processes, i.e. volcanic activity, water floods, and geothermal activity, are accompanied by seismic tremor that is detectable by the surrounding network of seismic stations. By comparing tremorplots of the j & ouml;kulhlaups of 2008 and 2010, and the eruptions of 2004 and 2011, we can identify three types of tremor: Water flood tremor. J & ouml;kulhlaups from the caldera are always accompanied by high-frequency tremor (2-9 Hz), recorded on the seismic stations near the caldera. It starts when the lake level begins to drop and increases gradually with increasing water discharge from the lake. This tremor is usually detected a few days before the subglacial flood reaches the glacier edge. Geothermal tremor. The second type of tremor appears to be switched on when the drop in water level reaches 10-30 m. It remains after all water has been drained from the lake. The tremor is characterized by relatively high frequency (2-6 Hz) and sudden changes in amplitude. The distance range of this tremor is short, it is seldom recorded beyond the edge of the glacier. We suggest that it is generated by flash-boiling of the geothermal system within the caldera, triggered by the pressure drop of the lake level. Eruption tremor. Eruptions of Gr & iacute;msv & ouml;tn are accompanied by tremor that begins simultaneously with the eruption and is distinctly different from the other two types of tremor. It contains lower frequencies (0.5-4 Hz) and has a wider distance range. It is recorded beyond the edge of the glacier, possibly because of its frequency content, but other effects such as crustal structure and depth of the tremor sources may also play a part.
