Bimodal volcanism at the Katla subglacial caldera,Iceland: insight into the geochemistry and petrogenesis of rhyolitic magmas

The Katla subglacial caldera is one of the most active and hazardous volcanic centres in Iceland as revealed by its historical volcanic activity and recent seismic unrest and magma accumulation. A petrologic and geochemical study was carried out on a suite of mid-Pleistocene to Recent lavas and pyroclastic rocks originated from the caldera. The whole series is characterised by a bimodal composition, including Fe-Ti transitional alkali basalts and mildly alkalic rhyolites. Variations in trace-element composition amongst the basalts and rhyolites show that their chemical differentiation was mainly controlled by fractional crystallisation and possible assimilation. The petrology and chemistry of the few intermediate extrusive rocks show that they were derived from magma mingling or hybridisation. The absence of extrusive rocks of true intermediate magmatic composition and the occurrence of amphibole-bearing felsic xenoliths support the hypothesis of partial melting of the hydrated basalt crust as the main process leading to the generation of rhyolites. The ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr values of Katla volcanic rocks fit the general isotopic array defined by late Quaternary to Recent lavas from Iceland. A few rock specimens are distinguished by low ¹⁴³Nd/¹⁴⁴Nd values suggesting assimilation and mixing of much older crustal material. Despite their similar whole-rock chemical compositions, the postglacial rhyolitic extrusives differ from the felsic xenoliths by their glass composition and the absence of amphibole. This, together with the general chemical trend of volcanic glasses, indicates that the postglacial rhyolitic extrusives were probably derived by a process involving late reheating and partial melting of crustal material by intrusion of basaltic magmas.     

The emplacement of an obsidian dyke through thin ice: Hrafntinnuhryggur, Krafla Iceland

An eruption along a 2.5 km-long rhyolitic dyke at Krafla volcano, northern Iceland during the last glacial period formed a ridge of obsidian (Hrafntinnuhryggur). The ridge rises up to 80 m above the surrounding land and is composed of a number of small-volume lava bodies with minor fragmental material. The total volume is < 0.05 km³. The lava bodies are flow- or dome-like in morphology and many display columnar-jointed sides typical of magma–ice interaction, quench-fragmented lower margins indicative of interaction with meltwater and pumiceous upper surfaces typical of subaerial obsidian flows. The fragmental material compromises poorly-sorted perlitic quench hyaloclastites and poorly-exposed pumiceous tuffs. Lava bodies on the western ridge flanks are columnar jointed and extensively hydrothermally altered. At the southern end of the ridge the feeder dyke is exposed at an elevation  95 m beneath the ridge crest and flares upwards into a lava body.Using the distribution of lithofacies, we interpret that the eruption melted through ice only 35–55 m thick, which is likely to have been dominated by firn. Hrafntinnuhryggur is therefore the first documented example of a rhyolitic fissure eruption beneath thin ice/firn. The eruption breached the ice, leading to subaerial but ice/firn-contact lava effusion, and only minor explosive activity occurred. The ridge appears to have been well-drained during the eruption, aided by the high permeability of the thin ice/firn, which appears not to have greatly affected the eruption mechanisms. We estimate that the eruption lasted between 2 and 20 months and would not have generated a significant jökulhlaup (< 70 m³ s^− 1).     

Seismic imaging of the geothermal field at Krafla,Iceland using shear-wave splitting

Shear-wave splitting is emerging as a useful exploration method for geothermal reservoirs as it can detect the geometry of the fracture system, the intensity of cracking and possibly, changes in fluid pressure within the reservoir. The method is based on the analyses of polarizations and time delays of shear-waves that have been distorted by the anisotropy of the medium through which the seismic waves have propagated. Observations of shear-wave splitting within the Krafla–Leirhnúkur geothermal field, Iceland, using a 20-station 3-component portable seismic array have provided evidence for at least two major crack systems of microfractures, oriented approximately N–S and E–W. Located microearthquakes align roughly along the E–W direction of the geothermal field, with shallow focal depths mostly around the injection well, probably related to the ongoing injection. This unexpected direction is however consistent with results from a simultaneous MT (magnetotelluric) survey.     

Surface deformation at the Krafla volcano,North Iceland, 1982–1992

Extensive measurements of ground deformation at the Krafla volcano, Iceland, have been made since the beginning in 1975 of a series of eruptions and intrusions into the fissure system that extends north and south of the volcano. I concentrate on measurements before and after the eruption of September 1984, the last event of this series when the largest volume of lava was erupted. The patterns of ground deformation associated with the 1984 eruption, determined by precision levelling, electronic distance measurements and lake level observations, were similar to earlier intrusions and eruptions, in that the surface of the volcano subsided and the fissure system widened as magma moved laterally from a shallow central reservoir into the fissure system. The shallow magma reservoir of Krafla continued to expand for about five years after the eruption, but a slow subsidence of the central area began in 1989. Besides the presence of an inflating and deflating shallow magma reservoir at a depth of 2.5 km beneath the Krafla caldera, another inflating magma reservoir may exist at much greater depth below Krafla. The accumulation of compressive strain by numerous rift intrusions and eruptions since 1975 along the flanks of the north-south Krafla fissure swarm is being released slowly and will probably be reflected in the results of deformation measurements near Krafla for the next several decades. The total horizontal extension of the Krafla rift system in 1975–1984 was about 9 m, equal to about 500 years of constant plate divergence. The extension is twice the accumulated divergence since previous rifting events and eruptions in 1724–1729     

Geology of the Heimaey volcanic centre, south Iceland: early evolution of a central volcano in a propagating rift?

Heimaey is the southernmost and also the youngest of nine volcanic centres in the southward-propagating Eastern Volcanic Zone, Iceland. The island of Heimaey belongs to the Vestmannaeyjar volcanic system (850 km²) and is situated 10 km off the south coast of Iceland. Although Heimaey probably started to form during the Upper Pleistocene all the exposed subaerial volcanics (10 monogenetic vents covering an area of 13.4 km²) are of Holocene age. Heimaey is composed of roughly equal amounts of tuff/tuff-breccias and lavas as most eruptions involve both a phreatomagmatic and an effusive phase. The compositions of the extrusives are predominantly alkali basalts belonging to the sodic series. Repeated eruptions on Heimaey, and the occurrence of slightly more evolved rocks (i.e. hawaiite approaching mugearite), might indicate that the island is in an early stage of forming a central volcano in the Vestmannaeyjar system. This is further substantiated by the development of a magma chamber at 10–20 km depth during the most recent eruption in 1973 and by the fact that the average volume of material produced in a single eruption on Heimaey is 0.32 km³ (dense rock equivalent), which is twice the value reported for the Vestmannaeyjar system as a whole. We find no support for the previously postulated episodic behaviour of the volcanism in the Vestmannaeyjar system. However, the oldest units exposed above sea level, i.e. the Norðurklettar ridge, probably formed over a 500-year interval during the deglaciation of southern Iceland. The absence of equilibrium phenocryst assemblages in the Heimaey lavas suggests that magma rose quickly from depth, without long-time ponding in shallow-seated crustal magma chambers. Eruptions on Heimaey have occurred along two main lineaments (N45°E and N65°E), which indicate that it is seismic events associated with the southward propagation of the Eastern Volcanic Zone that open pathways for the magma to reach the surface. Continuing southward propagation of the Eastern Volcanic Zone suggests that the frequency of volcanic eruptions in the Vestmannaeyjar system might increase with time, and that Heimaey may develop into a central volcano like the mature volcanic centres situated on the Icelandic mainland.     

Three magmatic components in the 1973 eruption of Eldfell volcano,Iceland: Evidence from plagioclase crystal size distribution (CSD) and geochemistry

The 1973 eruption of Eldfell volcano, Iceland, appears to have been a short, simple event, but textural and geochemical evidence suggest that it may have had three different magmatic components. The first-erupted fissure magmas were chemically evolved, rich in plagioclase (∼ 18%) and had shallow, straight crystal size distribution (CSD) curves. The early lavas were less evolved chemically, had lower plagioclase contents (∼ 13%) and steeper, slightly concave up CSDs. The late lavas were chemically similar to the early lavas, but even richer in plagioclase than the initial magmas (∼ 24%) and had the steepest CSDs. There was no chemical evidence for plagioclase fractionation, but compositional diversity could be produced by clinopyroxene fractionation which must have occurred at depth. We propose that the eruption started with old, coarsened (Ostwald ripened) magma left over from a previous eruption, possibly that which produced Surtsey Island ten years earlier. The early flows may be mixtures of small amounts of this old magma with a new, low crystallinity, uncoarsened magma or a completely new magma. The late flows are another new magma from depth, chemically similar to the early flows, but which has grown plagioclase under increasing saturation (undercooling) perhaps during its ascent. All three magmatic components may have originated from the same parent, but had varying degrees of clinopyroxene fractionation, plagioclase nucleation and growth, and coarsening.     

Plagioclase ultraphyric basalts in Iceland: the mush of the rift

Glassy, plagioclase ultraphyric basalts from six locations in Iceland have bimodal phenocryst size distributions where microphenocrysts (ol+plg±cpx±mt) are in equilibrium with the matrix glass, but macrophenocrysts (ol+plg±cpx) are too primitive to be so. Matrix glass compositions are similar to those of other rift zone glasses from Iceland, and oxygen isotope variations suggest they interacted with the Icelandic crust. A lack of negative Eu-anomalies in matrix glasses precludes large amounts of plagioclase crystallisation from their parental liquids. Compositions of glass inclusions in plagioclase and olivine macrophenocrysts indicate that parental magma compositions of the macrophenocryst assemblage are similar to those of primitive, Icelandic rift zone glasses. Temperatures for plagioclase macrophenocryst crystallisation obtained from Linkam^® heating stage experiments, and from glass inclusion compositions corrected for post-entrapment crystallisation, give temperatures up to 1260°C, corresponding to crystallisation at middle to deep crustal levels. Temperature differences of less than 100°C between plagioclase-hosted glass inclusions before and after post-entrapment plagioclase crystallisation show that the macrophenocrysts must have been kept at elevated temperatures prior to incorporation in their present host magmas. We suggest that the macrophenocrysts of the plagioclase ultraphyric basalts accumulated in crystal mush bodies underneath the rift zone and were picked up by their present hosts during a rifting event with increased magma supply from the mantle.     

Temporal development of the 1999 intrusive episode in the Eyjafjallajökull volcano, Iceland, derived from InSAR images

Eyjafjallajökull volcano, located in southern Iceland, is characterized by its quiet nature. Only about a handful of earthquakes associated with the volcanic system had been detected prior to the 1990s. Earthquake swarms did, however, occur in 1994 and 1999. Here we investigate the spatio-temporal evolution of a magmatic intrusion associated with the 1999 earthquake swarm via analysis of produced surface deformation. A series of interferometric synthetic aperture radar (InSAR) images, spanning various periods of the intrusion, show that in 1999 surface deformation occurred mainly on the southern flanks of the volcano. The deformation amounts to more than 20 cm of range change. Inverse modeling resolves the deformation source to be a sill intrusion at 6.3 km depth. Sill opening was up to 1 m and the total intruded volume amounts to ～0.03 km³. The InSAR data display a migration of the center of deformation through time, enabling us to create time-dependant sill-opening models. Furthermore, we investigate the spatio-temporal distribution of earthquakes and find that the distribution supports the InSAR derived model and additionally provides indications for a possible site of a feeder channel. Magmatic flow-rate estimates indicate an initial intrusion rate of 4–6 m³/s, declining over a few weeks.     

Katla volcano,Iceland: magma composition,dynamics and eruption frequency as recorded by Holocene tephra layers

The Katla volcano in Iceland is characterized by subglacial explosive eruptions of Fe–Ti basalt composition. Although the nature and products of historical Katla eruptions (i.e. over the last 1,100 years) at the volcano is well-documented, the long term evolution of Katla’s volcanic activity and magma production is less well known. A study of the tephra stratigraphy from a composite soil section to the east of the volcano has been undertaken with emphasis on the prehistoric deposits. The section records ∼8,400 years of explosive activity at Katla volcano and includes 208 tephra layers of which 126 samples were analysed for major-element composition. The age of individual Katla layers was calculated using soil accumulation rates (SAR) derived from soil thicknesses between ¹⁴C-dated marker tephra layers. Temporal variations in major-element compositions of the basaltic tephra divide the ∼8,400-year record into eight intervals with durations of 510–1,750 years. Concentrations of incompatible elements (e.g. K₂O) in individual intervals reveal changes that are characterized as constant, irregular, and increasing. These variations in incompatible elements correlate with changes in other major-element concentrations and suggest that the magmatic evolution of the basalts beneath Katla is primarily controlled by fractional crystallisation. In addition, binary mixing between a basaltic component and a silicic melt is inferred for several tephra layers of intermediate composition. Small to moderate eruptions of silicic tephra (SILK) occur throughout the Holocene. However, these events do not appear to exhibit strong influence on the magmatic evolution of the basalts. Nevertheless, peaks in the frequency of basaltic and silicic eruptions are contemporaneous. The observed pattern of change in tephra composition within individual time intervals suggests different conditions in the plumbing system beneath Katla volcano. At present, the cause of change of the magma plumbing system is not clear, but might be related to eruptions of eight known Holocene lavas around the volcano. Two cycles are observed throughout the Holocene, each involving three stages of plumbing system evolution. A cycle begins with an interval characterized by simple plumbing system, as indicated by uniform major element compositions. This is followed by an interval of sill and dyke system, as depicted by irregular temporal variations in major element compositions. This stage eventually leads to a formation of a magma chamber, represented by an interval with increasing concentrations of incompatible elements with time. The eruption frequency within the cycle increases from the stage of a simple plumbing system to the sill and dyke complex stage and then drops again during magma chamber stage. In accordance with this model, Katla volcano is at present in the first interval (i.e. simple plumbing system) of the third cycle because the activity in historical time has been characterized by uniform magma composition and relatively low eruption frequency.     

Explosive silicic volcanism in Iceland and the Jan Mayen area during the last 6 Ma: sources and timing of major eruptions

Fifty-three major explosive eruptions on Iceland and Jan Mayen island were identified in 0–6-Ma-old sediments of the North Atlantic and Arctic oceans by the age and the chemical composition of silicic tephra. The depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 Ma and paleoclimatic proxies (δ¹⁸O, ice-rafted debris) for the last 1 Ma. Major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off-rift volcanic zones in Iceland, and to the Jan Mayen volcanic system. The tholeiitic central volcanoes along the Iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4.9 Ma. They were the source of at least 26 eruptions of dominant rhyolitic magma composition, including the late Pleistocene explosive eruption of Krafla volcano of the Eastern Rift Zone at about 201 ka. The central volcanoes along the off-rift volcanic zones in Iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition, with three distinct episodes recorded at 4.6–5.3, 3.5–3.6, and 0–1.8 Ma. The longest and last episode recorded 11 Pleistocene major events including the two explosive eruptions of Tindfjallajökull volcano (Thórsmörk, ca. 54.5 ka) and Katla volcano (Sólheimar, ca. 11.9 ka) of the Southeastern Transgressive Zone. Eight major explosive eruptions from the Jan Mayen volcanic system are recorded in terms of the distinctive grain-size, mineralogy and chemistry of the tephra. The tephra contain K-rich glasses (K₂O/SiO₂>0.06) ranging from trachytic to alkali rhyolitic composition. Their normative trends (Ab–Q–Or) and their depleted concentrations of Ba, Eu and heavy-REE reflect fractional crystallisation of K-feldspar, biotite and hornblende. In contrast, their enrichment in highly incompatible and water-mobile trace elements such as Rb, Th, Nb and Ta most likely reflect crustal contamination. One late Pleistocene tephra from Jan Mayen was recorded in the marine sequence. Its age, estimated between 617 and 620 ka, and its composition support a common source with the Borga pumice formation at Sør Jan in the south of the island.     

ICEARRAY: the first small-aperture, strong-motion array in Iceland

A small-aperture, strong-motion array, the ICEARRAY, has been deployed in South Iceland, a region with a history of destructive earthquakes, some exceeding magnitude 7. The array’s purpose is: (1) monitoring future significant events in the region, (2) quantifying spatial variability of strong-motion over short distances and (3) shedding light on earthquake source processes. The number of array stations and their arrangement were based on an optimisation of the shape of the corresponding array transfer function (ATF). The optimal ICEARRAY configuration comprises 14 stations, has an aperture of ~1.9 km and a minimum interelement distance of ~50 m and possesses a near-azimuthally independent ATF with a sharp main lobe, negligible sidelobes and a wavenumber range of 1.5–24 rad/km. Accordingly, the ICEARRAY has the intended capabilities of capturing seismic waves in the frequency range of 1–20 Hz, which is of main interest to earthquake engineering and engineering seismology applications.     

Miocene silicic volcanism in southwestern Idaho: geochronology,geochemistry, and evolution of the central Snake River Plain

New ⁴⁰Ar-³⁹Ar geochronology, bulk rock geochemical data, and physical characteristics for representative stratigraphic sections of rhyolite ignimbrites and lavas from the west-central Snake River Plain (SRP) are combined to develop a coherent stratigraphic framework for Miocene silicic magmatism in this part of the Yellowstone ‘hotspot track’. The magmatic record differs from that in areas to the west and east with regard to its unusually large extrusive volume, broad lateral scale, and extended duration. We infer that the magmatic systems developed in response to large-scale and repeated injections of basaltic magma into the crust, resulting in significant reconstitution of large volumes of the crust, wide distribution of crustal melt zones, and complex feeder systems for individual eruptive events. Some eruptive episodes or ‘events’ appear to be contemporaneous with major normal faulting, and perhaps catastrophic crustal foundering, that may have triggered concurrent evacuations of separate silicic magma reservoirs. This behavior and cumulative time-composition relations are difficult to relate to simple caldera-style single-source feeder systems and imply complex temporal-spatial development of the silicic magma systems. Inferred volumes and timing of mafic magma inputs, as the driving energy source, require a significant component of lithospheric extension on NNW-trending Basin and Range style faults (i.e., roughly parallel to the SW–NE orientation of the eastern SRP). This is needed to accommodate basaltic inputs at crustal levels, and is likely to play a role in generation of those magmas. Anomalously high magma production in the SRP compared to that in adjacent areas (e.g., northern Basin and Range Province) may require additional sub-lithospheric processes. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

Effect of glacier loading/deloading on volcanism: postglacial volcanic production rate of the Dyngjufjöll area,central Iceland

Tephrochronological dating of postglacial volcanism in the Dyngjufjöll volcanic complex, a major spreading center in the Icelandic Rift Zone, indicates a high production rate in the millennia following deglaciation as compared to the present low productivity. The visible and implied evidence indicates that lava production in the period 10 000–4500 bp was at least 20 to 30 times higher than that in the period after 2900 bp but the results are biased towards lower values for lava volumes during the earlier age periods since multiple lava layers are buried beneath younger flows. The higher production rate during the earlier period coincides with the disappearance of glaciers of the last glaciation. Decreasing lithostatic pressure as the glacier melts and vigorous crustal movements caused by rapid isostatic rebound may trigger intense volcanism until a new pressure equilibrium has been established.     

Petrochemistry of the Gjálp-1996 subglacial eruption, Vatnajökull, SE Iceland

In October 1996 a subglacial fissure to the north of the Grimsvötn caldera in W-Vatnajökull produced about 0.4 km³ of Fe-rich basaltic andesite–icelandite—in an area characterized mostly by tholeiitic basalt. In this paper the chemical composition of volcanic systems in the region is discussed with the help of six new analyses and others from the literature, and a tentative model for their evolution is proposed, in which magma produced by the partial melting of a two-component mantle mixes with hydrous, silicic melt in the crust. The Vatnajökull 1996 magma belongs to a separate volcanic system, intermediate between Bardarbunga and Grimsvötn.     

Geothermal control on flow patterns in the Last Glacial Maximum ice sheet of Iceland

Because it is located both on the Mid‐Atlantic Ridge and on a mantle plume, Iceland is a region of intense tectonics and volcanism. During the last glaciation, the island was covered by an ice sheet approximately 1000 m thick. A reconstruction of the ice flow lines, based on glacial directional features, shows that the ice sheet was partly drained through fast‐flowing streams. Fast flow of the ice streams has been recorded in megascale lineations and flutes visible on the currently deglaciated bedrock, and is confirmed by simple mass balance considerations. Locations of the major drainage routes correlate with locations of geothermal anomalies, suggesting that ice stream activity was favoured by lubrication of the bed by meltwater produced in regions of high geothermal heat flux. Similar control of ice flow by geothermal activity is expected in ice sheets currently covering tectonically and volcanically active area such as the West Antarctic ice sheet. Copyright © 2000 John Wiley & Sons, Ltd.     

Volcano-stratigraphy and geochemistry of collision-related volcanism on the Erzurum–Kars Plateau, northeastern Turkey

The Eastern Anatolia Region exhibits one of the world's best exposed and most complete transects across a volcanic province related to a continental collision zone. Within this region, the Erzurum–Kars Plateau is of special importance since it contains the full record of collision-related volcanism from Middle Miocene to Pliocene. This paper presents a detailed study of the volcanic stratigraphy of the plateau, together with new K–Ar ages and several hundred new major- and trace-element analyses in order to evaluate the magmatic evolution of the plateau and its links to collision-related tectonic processes. The data show that the volcanic units of the Erzurum–Kars Plateau cover a broad compositional range from basalts to rhyolites. Correlations between six logged, volcano-stratigraphic sections suggest that the volcanic activity may be divided into three consecutive Stages, and that activity begins slightly earlier in the west of the plateau than in the east. The Early Stage (mostly from 11 to 6 Ma) is characterised by bimodal volcanism, made up of mafic-intermediate lavas and acid pyroclastic rocks. Their petrography and high-Y fractionation trend suggest that they result from crystallization of anhydrous assemblages at relatively shallow crustal levels. Their stratigraphy and geochemistry suggest that the basic rocks erupted from small transient chambers while the acid rocks erupted from large, zoned magma chambers. The Middle Stage (mostly from 6–5 Ma) is characterised by unimodal volcanism made up predominantly of andesitic–dacitic lavas. Their petrography and low-Y fractionation trend indicate that they resulted from crystallization of hydrous (amphibole-bearing) assemblages in deeper magma chambers. The Late Stage (mostly 5–2.7 Ma) is again characterised by bimodal volcanism, made up mainly of plateau basalts and basaltic andesite lavas and felsic domes. Their petrography and high-Y fractionation trend indicate that they resulted from crystallization of anhydrous assemblages at relatively shallow crustal levels. AFC modelling shows that crustal assimilation was most important in the deeper magma chambers of the Middle Stage. The geochemical data indicate that the parental magma changed little throughout the evolution of the plateau. This parental magma exhibits a distinctive subduction signature represented by selective enrichment in LILE and LREE thought to have been inherited from a lithosphere modified by pre-collision subduction events. The relationships between magmatism and tectonics support models in which delamination of thickened subcontinental lithosphere cause uplift accompanied by melting of this enriched lithosphere. Magma ascent, and possibly magma generation, is then strongly controlled by strike-slip faulting and associated pull-apart extensional tectonics.     

Evidences of the Blake and Iceland Basin magnetic excursions in southeastern Iberia and chronological implications for the Padul sedimentary record

The Padul-15-05 sediment core provides an exceptional perspective of the paleoenvironmental and climate change in the Western Mediterranean region for the last ca. 200 kyr. However, even though a robust chronology mainly relying on radiometric dating is available for the last 50 ka, the chronology for the older sediments is not yet fully resolved. Ages for the bottom part of the core (>21 m) were previously inferred from amino-acid racemization dating and sediment accumulation rates. In this work, we provide a more accurate chronology for the older part (>100 kyr) of the Padul-15-05 sediment core record based on the recognition of past Earth's magnetic excursions. We identify an interval prone of reversed polarity samples close to MIS-5e/5 d transition that we correlate to the Blake geomagnetic excursion (116.5 kyr–112 kyr). In addition, we identify an interval of low inclinations and two reversed samples that we interpret as the Iceland Basin geomagnetic excursion (192.7 kyr–187.7 kyr: wide scenario of VGP <40°). Our new results, which include IRM acquisition curves that contribute to understand the magnetic mineralogy, enhances the robustness of the age model for the Padul-15-05 sedimentary sequence by adding an independent age dataset with new accurate tie-points. Our refined age control together with the available paleoenvironmental and paleoclimate multiproxy data provide insightful information to unveil the response of the western Mediterranean environments to regional environmental and climate change.     

Long‐term variability of proglacial groundwater‐fed hydrological systems in an area of glacier retreat,Skeiðarársandur,Iceland

Proglacial groundwater‐fed features, such as seeps, substantially impact proglacial geomorphology, hydrology, and ecology. However, there is a paucity of research on the impacts of climate change and glacier retreat on the extent of these important features. This paper aims to investigate the impact of glacier retreat on proglacial groundwater levels and on the extent of groundwater‐fed seeps. Research has taken place in western Skeiðarársandur, the large proglacial outwash plain of Skeiðarárjökull, a retreating temperate glacier in southeast Iceland. Changes in the extent of proglacial groundwater seeps were mapped using historical aerial photographs from 1986, 1997, and 2012. Proglacial groundwater levels were monitored in shallow boreholes between 2000 and 2012. The western margin of Skeiðarárjökull has retreated approximately 1 km beyond its position in 1986. However, this retreat was punctuated by short periods of readvance. The geomorphology and groundwater systems at the site were substantially impacted by the November 1996 jökulhlaup, whose deposits altered approximately 18% of the area of groundwater seeps. The surface areas of groundwater seeps and lakes in the study area have declined by ~97% between 1986 and 2012. Most of the decline took place after 1997, when the mean annual rate of retreat increased three‐fold. Groundwater levels also declined substantially between 2000 and 2012, although this trend varies spatially. The paper provides a conceptual model of the controls on proglacial shallow groundwater systems. Direct impacts of glacier retreat are suggested as the main cause for the declines in proglacial groundwater levels and in the extent of groundwater seeps. These declines are expected to adversely impact sandur ecology. Copyright © 2014 John Wiley & Sons, Ltd.