Central Iceland Research Articles

Seasonal overturning variability in the eastern North Atlantic subpolar gyre: a Lagrangian perspective

Abstract. Both observations and ocean reanalyses show a pronounced seasonality in the strength of the Atlantic meridional overturning circulation (MOC) within the eastern North Atlantic subpolar gyre (eSPG). However, attributing this overturning seasonality to seasonal dense water formation remains challenging owing to the wide distribution of recirculation timescales within the Iceland and Irminger basins. Here, we investigate the nature of seasonal overturning variability using Lagrangian water parcel trajectories initialised across the Overturning in the Subpolar North Atlantic Program (OSNAP) East section within an eddy-permitting ocean sea ice hindcast. By adopting a Lagrangian perspective, we show that the seasonal minimum of the Eulerian overturning at OSNAP East in autumn results from a combination of enhanced stratification and increased southward transport within the upper East Greenland Current. This convergence of southward transport within the MOC upper limb is explained by decreasing water parcel recirculation times in the upper Irminger Sea, consistent with a gyre-scale response to seasonal wind forcing. To account for the diversity of recirculation times within the eSPG, we also quantify the Lagrangian overturning (LMOC) as the total dense water formation along water parcel trajectories. The majority of water parcels, sourced from the central and southern branches of the North Atlantic Current, fail to return to OSNAP East prior to experiencing wintertime diapycnal transformation into the lower limb, and thus they determine the mean strength of the LMOC within the eSPG (8.9 ± 2.2 Sv). The strong seasonality of the LMOC is explained by a small collection of upper-limb water parcels, circulating rapidly (≤ 8.5 months) in the upper Irminger and central Iceland basins, whose along-stream transformation is determined by their month of arrival at OSNAP East.

Deep magma degassing and volatile fluxes through volcanic hydrothermal systems: Insights from the Askja and Kverkfjöll volcanoes, Iceland

Mantle volatiles are transported to Earth's crust and surface by basaltic volcanism. During subaerial eruptions, vast amounts of carbon, sulfur and halogens can be released to the atmosphere during a short time-interval, with impacts ranging in scale from the local environment to the global climate. By contrast, passive volatile release at the surface originating from magmatic intrusions is characterized by much lower flux, yet may outsize eruptive volatile quantities over long timescales. Volcanic hydrothermal systems (VHSs) act as conduits for such volatile release from degassing intrusions and can be used to gauge the contribution of intrusive magmatism to global volatile cycles. Here, we present new compositional and isotopic (δD and δ18O-H2O, 3He/4He, δ13C-CO2, Δ33S-δ34S-H2S and SO4) data for thermal waters and fumarole gases from the Askja and Kverkfjöll volcanoes in central Iceland. We use the data together with magma degassing modelling and mass balance calculations to constrain the sources of volatiles in VHSs and to assess the role of intrusive magmatism to the volcanic volatile emission budgets in Iceland.The CO2/ΣS (10−30), 3He/4He (8.3–10.5 RA; 3He/4He relative to air), δ13C-CO2 (−4.1 to −0.2 ‰) and Δ33S-δ34S-H2S (−0.031 to 0.003 ‰ and −1.5 to +3.6‰) values in high-gas flux fumaroles (CO2 > 10 mmol/mol) are consistent with an intrusive magmatic origin for CO2 and S at Askja and Kverkfjöll. We demonstrate that deep (0.5–5 kbar, equivalent to ∼2–18 km crustal depth) decompression degassing of basaltic intrusions in Iceland results in CO2 and S fluxes of 330–5060 and 6–210 kt/yr, respectively, which is sufficient to account for the estimated CO2 flux of Icelandic VHSs (3365–6730 kt/yr), but not the VHS S flux (220–440 kt/yr). Secondary, crystallization-driven degassing from maturing intrusions and leaching of crustal rocks are suggested as additional sources of S. Only a minor proportion of the mantle flux of Cl is channeled via VHSs whereas the H2O flux remains poorly constrained, because magmatic signals in Icelandic VHSs are masked by a dominant shallow groundwater component of meteoric water origin. These results suggest that the bulk of the mantle CO2 and S flux to the atmosphere in Iceland is supplied by intrusive, not eruptive magmatism, and is largely vented via hydrothermal fields.

Recycled Crustal Components of the Iceland Plume Centre: Ultra-Dehydrated Crust and Oxidised Water-Bearing Fertile Mantle

AbstractPrimitive basalts erupted mainly above the proposed centre of the Iceland plume have been analysed geochemically. The data shed light on the distribution of plume heterogeneities and constrain their nature and origin. Low-Ca (down to 1420 ppm) olivine macrocrysts and chromite inclusions in Snæfell rocks of the eastern flank zone indicate that the magmas were hydrous (c. 2 wt% H2O) and relatively oxidised (fO2 = c. ΔQMF+1). Al-in-olivine crystallisation temperatures indicate that magmas from central Iceland magmas are not hotter than from other parts of Iceland. Some Gæsafjöll rocks with high Nb/U = 110–120 from the northern rift zone (NRZ) represent magmas widespread in Iceland that are also characterised by low Ba/La and high La/U, La/Th, Nb/Th and Ce/Pb. They reflect a mantle component with Nb/U ≈ 200 present in many basalts of all rift zones. This mantle apparently consists of peridotite refertilised by ultra-dehydrated recycled oceanic crust (ROC). This component is in clear contrast to the main source of the Snæfell magmas, which is modelled to be peridotite strongly refertilised by enriched ROC more hydrous than Mid-Ocean Ridge Basalt. In contrast to more-depleted rift zone melts, which are extracted from depths just below the thin rifted lithosphere, some rift zone basalts are extracted deeper and produced by intermediate degrees of melting of the two types of refertilised mantle. Snæfell-type melts are common in the NRZ and eastern rift zones (ERZ) and most strongly present at Upptyppingar. Mixing of magmas is indicated at crustal levels. Mantle melting may be modelled for homogeneous sources of peridotite or olivine websterite. Double-spike Pb isotope analyses suggest that the depleted mantle sources of the ERZ basalts together with Snæfell and high-Nb/U sources, both with 206Pb/204Pb = 18.3–18.6, Δ207Pb ≈ 0 and Δ208Pb = 20–30, explain both ERZ and NRZ magmas, except for Theistareykir. They are distinct from magmas of the western rift zone, which is characterised by melt contributions from a differently enriched mantle with more radiogenic lead, negative Δ207Pb, Δ208Pb ≈ 0. This difference in type of enriched mantle suggests that the Iceland mantle plume is laterally zoned.

Diverse mantle components with invariant oxygen isotopes in the 2021 Fagradalsfjall eruption, Iceland

The basalts of the 2021 Fagradalsfjall eruption were the first erupted on the Reykjanes Peninsula in 781 years and offer a unique opportunity to determine the composition of the mantle underlying Iceland, in particular its oxygen isotope composition (δ18O values). The basalts show compositional variations in Zr/Y, Nb/Zr and Nb/Y values that span roughly half of the previously described range for Icelandic basaltic magmas and signal involvement of Icelandic plume (OIB) and Enriched Mid-Ocean Ridge Basalt (EMORB) in magma genesis. Here we show that Fagradalsfjall δ18O values are invariable (mean δ18O = 5.4 ± 0.3‰ 2 SD, N = 47) and indistinguishable from “normal” upper mantle, in contrast to significantly lower δ18O values reported for erupted materials elsewhere in Iceland (e.g., the 2014–2015 eruption at Holuhraun, Central Iceland). Thus, despite differing trace element characteristics, the melts that supplied the Fagradalsfjall eruption show no evidence for 18O-depleted mantle or interaction with low-δ18O crust and may therefore represent a useful mantle reference value in this part of the Icelandic plume system.

Water mass transformation in the Iceland Sea: Contrasting two winters separated by four decades

Dense water masses formed in the Nordic Seas flow across the Greenland–Scotland Ridge and contribute substantially to the lower limb of the Atlantic Meridional Overturning Circulation. Originally considered an important source of dense water, the Iceland Sea gained renewed interest when the North Icelandic Jet — a current transporting dense water from the Iceland Sea into Denmark Strait — was discovered in the early 2000s. Here we use recent hydrographic data to quantify water mass transformation in the Iceland Sea and contrast the present conditions with measurements from hydrographic surveys conducted four decades earlier. We demonstrate that the large-scale hydrographic structure of the central Iceland Sea has changed significantly over this period and that the locally transformed water has become less dense, in concert with a retreating sea-ice edge and diminished ocean-to-atmosphere heat fluxes. This has reduced the available supply of dense water to the North Icelandic Jet, but also permitted densification of the East Greenland Current during its transit through the presently ice-free western Iceland Sea in winter. Together, these changes have significantly altered the contribution from the Iceland Sea to the overturning in the Nordic Seas over the four decade period.

Linking Coherent Anticyclonic Eddies in the Iceland Basin to Decadal Oceanic Variability in the Subpolar North Atlantic

AbstractThe Iceland Basin in the eastern Subpolar North Atlantic is an eddy‐rich region characterized by intense anticyclonic eddy activity. Our study present the variability of coherent Anticyclonic Eddies (AEs) generated in this region, using satellite altimetry and two ocean eddy tracking algorithms. The yearly count of AEs in the Iceland Basin reveals a decadal variability similar to that of ocean heat content change in the eastern subpolar gyre. Periods with higher number of AEs coincide with periods of increased ocean heat content, and vice versa. However, both algorithms agree that more than 50% of the detected AEs are confined to the central Iceland Basin. The annual number of AEs also tracks zonal shifts of the subpolar front, a variable that can explain about 53 (77)% of the interannual (decadal) variability of AEs in the Iceland Basin. Finally, a Lagrangian approach is used to demonstrate that the amount of subtropical versus subpolar water masses reaching the Iceland Basin appears to influence, via baroclinic instability, the generation of AEs.

Boron isotope evidence for devolatilized and rehydrated recycled materials in the Icelandic mantle source

Enriched mantle heterogeneities are widely considered to be generated through subduction, but the connections between specific subducted materials and the chemical signatures of mantle heterogeneities are not clearly defined. Boron is strongly isotopically fractionated at the surface and traces slab devolatilization, making it a potent tracer of previously subducted and recycled materials. Here, we present high-precision SIMS boron concentrations and isotope ratios on a comprehensive suite of quenched basaltic glasses from all neovolcanic zones in Iceland, two rhyolite glasses, and a set of primitive melt inclusions from central Iceland. Boron isotope ratios (δ11B) in Icelandic basalts and melt inclusions range from −11.6‰ to −1.0‰, averaging −4.9‰, which is higher than mid-ocean ridge basalt (MORB; δ11B=−7.1‰). Because the δ11B value of the Icelandic crust is low, the high δ11B compositions of the Icelandic lavas are not easily explained through crustal assimilation processes.Icelandic basalt glass and melt inclusion B/Ce and δ11B values correlate with trace element ratio indicators of the degree of mantle partial melting and mantle heterogeneity (e.g. Nb/Zr, La/Yb, Sm/Yb), which indicate that the boron systematics of basalts are controlled by mantle heterogeneity. Additionally, basalts with low B/Ce have high 206Pb/204Pb, further indicating mantle source control. These correlations can be used to deduce the boron systematics of the individual Icelandic mantle components. The enriched endmember within the Iceland mantle source has a high δ11B value and low B/Ce, consistent with the composition of “rehydrated” recycled oceanic crust. The depleted endmember comprises multiple distinct components with variable B/Ce, likely consisting of depleted MORB mantle and/or high 3He/4He mantle and two more minor depleted components that are consistent with recycled metasomatized mantle wedge and recycled slab gabbro.The compositions of these components place constraints on the devolatilization history of recycled oceanic crust. The high δ11B value and low B/Ce composition of the enriched component within the Iceland mantle source is inconsistent with a simple devolatilization process and suggests that the recycled oceanic crust component may have been isotopically overprinted by B-rich fluids derived from the underlying hydrated slab lithospheric mantle (i.e. “rehydration”). Further, the B/Ce and δ11B systematics of other OIBs can be used to constrain the devolatilization histories of recycled components on a global scale. Globally, most OIB B/Ce compositions suggest that recycled components have lost >99% of their boron, and their δ11B values suggest that rehydration may be a sporadic process, and not ubiquitous.

Changes in groundwater trace element concentrations before seismic and volcanic activities in Iceland during 2010–2018

We analysed temporal variations of trace element concentrations in groundwater from a 101 m-deep borehole (HA01) in northern Iceland during 2010–2018 and compared them with seismic and volcanic events that occurred in the same period to identify potential hydrogeochemical precursors. An increase of B, Al, V, Li and Mo concentrations started from eight months to one month before the 2014 Bárðarbunga eruption (~115 km from HA01), a major rifting event in central Iceland, while Ga and V concentrations began to increase one day and one month after the onset of the event, respectively. We also found that concentrations of some trace elements (Li, B, Ga, Mo, Sr, Rb and Fe) significantly increased before an Mw 5.0 earthquake that occurred ~80 km from the borehole in 2018. However, other notable hydrogeochemical changes were detected during the monitoring period without apparent correlation with the seismic and volcanic events in the region. This study shows that the systematic long-term hydrogeochemical monitoring in seismic and volcanic areas is critical to advance the science of seismic and eruptive precursors. Furthermore, the use of statistical tools, such as Principal Component Analysis (PCA) and Change Point (CP) detection can help identify the most useful chemical elements and validate the trend variability of those elements in the time series, reducing arbitrary choices of pre-seismic and pre-volcanic hydrogeochemical anomalies as potential precursors.

Tectonic position, structure, and Holocene activity of the Hofsjökull volcanic system, central Iceland

The Hofsjökull volcanic system is located at the northern border of the Hreppar microplate on the Mid-Atlantic plate boundary in Iceland, between the more active Western and Eastern volcanic zones. In this study, fractures, and faults within the fissure swarms of the volcanic system were mapped and the throw and orientation of faults measured. This was done using both aerial photographs and ArcticDEM digital elevation models, as well as during fieldwork. The Hofsjökull volcanic system contains 3–4 fissure swarms extending northwards or southwards from the glacially covered Hofsjökull central volcano. Although these fissure swarms have been active during the Holocene, no clear sink holes were found along the faults, suggesting that they have been filled by sediments. This indicates that the fissure swarms have been less active than fissure swarms in other branches of the plate boundary in Iceland where GPS geodetic measurements show current spreading. Unbroken hyaloclastite covering a fault in the northern Hofsjökull fissure swarm suggests that this part of the northern Hofsjökull fissure swarm has not been active since the earliest part of Holocene, or during the latest stage of glaciation in the area. Still, the fault scarps in the northern Hofsjökull fissure swarm are rather sharp, indicating little erosion by glaciers. This may suggest increased activity in the Hofsjökull fissure swarm during the end of the last glaciation or at the beginning of the Holocene, which is in line with other studies showing increased magmatic activity in Iceland during that period. Fractured Holocene lava flows in the southern and western Hofsjökull fissure swarms indicate that they have been active during the Holocene. The Kerlingarfjöll rhyolitic massif is located south of the Hofsjökull central volcano. The southern fissure swarms are located both east and west of Kerlingarfjöll, but no clear indications are found of the fissure swarm in the rhyolitic massif itself. This may occur as dike intrusions (which likely form the faults) are prevented from penetrating the rhyolite due to density differences, and/or due to the topographic high of Kerlingarfjöll.

Internal structure and palsa development at Orravatnsrústir Palsa Site (Central Iceland), investigated by means of integrated resistivity and ground‐penetrating radar methods

AbstractThe natural cyclical development of palsas makes it difficult to use visible signs of decay as reference points for environmental change. Thus, to determine the actual development stage of a palsa, investigations of the internal structure are crucial. Our study presents 2‐D and 3‐D electrical resistivity imaging (ERI) and 2‐D ground‐penetrating radar (GPR) results, measurements of surface and subsurface temperatures, and of the soil matric potential from Orravatnsrústir Palsa Site in Central Iceland. By a joint interpretation of the results, we deduce the internal structure (i.e., thickness of thaw zone and permafrost, ice/water content) of five palsas of different size and shape. The results differentiate between initial and mature development stages and show that palsas of different development stages can exist in close proximity. While internal characteristics indicate undisturbed development of four palsas, one palsa shows indications of environmental change. Our study shows the value of the multimethod geophysical approach and introduces measurements of the soil matric potential as a promising method to assess the current state of the subsurface.

Bacterial and archaeal community structure in benthic sediments from glacial lakes at the Múlajökull Glacier, central Iceland

Glacial lakes and their sediments are highly sensitive temporal markers of environmental variability. The combination of particular geographical conditions with climate changes makes the Mulajokull lakes an appropriate model for revealing some site-specific and common relationships in psychrophilic microbial ecology. The aim of this study was to evaluate the taxonomic and functional diversity of microbial community structures in five glacial lake sediments situated at different distances from the glacier, i.e., 50 m to 1750 m from the ice margin. The Shannon diversity indices varied in the range from 1.99 to 2.94 (with the lowest in C sample) tested by EcoPlates™ and from 1.69 to 1.89 at the phylum level (with the lowest in A sample) tested by the shotgun metagenomic sequencing, respectively. An inter-sample comparison was also conducted. Overall, six bacteria phyla (Proteobacteria, Cyanobacteria, Bacteroidetes, Actinobacteria, Verrucomicrobia and Planctomycetes) and one microalgae phyla (Bacillariophyta) were detected in five lake sediments tested at abundancy that exceeded 1%. Obtained data brings new knowledge related to the cold biosphere on the local and global Earth systems.

Changes in shear-wave splitting at the 2014 Bárðarbunga seismic crisis and dyke intrusion in Iceland compared with earthquakes and other eruptions

We use shear-wave splitting (SWS) above small earthquakes to monitor stress-variations before the 2014 Bárðarbunga seismic crisis and dyke intrusion in central Iceland. SWS is sensitive to stress-induced variations in the internal geometry of the distributions of stress-aligned fluid-saturated microcracks pervading almost all crustal rocks. This allows stress-accumulation before both impending earthquakes and impending volcanic eruptions to be monitored and potentially the events stress-forecast. We compare the behaviour of SWS before Bárðarbunga with the behaviour before large earthquakes and other volcanic episodes where SWS has been monitored. On the basis of the duration of stress-changes, we classify volcanic episodes into three broad classes. During analysis of SWS before Bárðarbunga, temporal variation in polarizations and another retrospective stress-forecast earthquake were recognized.

The Photochemistry of Gas Molecules in Earth’s Atmosphere Determines the Structure of the Atmosphere and the Average Temperature at Earth’s Surface

A molecule of oxygen absorbing solar ultraviolet-C radiation is photo-dissociated into two atoms of oxygen that fly apart at high velocity, converting kinetic energy of oscillation of the molecular bond directly and completely into kinetic energy of linear motion of the oxygen atoms. This increases air temperature. Two oxygen atoms can then collide forming a new oxygen molecule that can then be dissociated again as long as sufficient ultraviolet-C radiation exists. This continual dissociation of oxygen molecules is the primary reason for the stratopause being 30-40 degrees warmer than the tropopause and for all ultraviolet-C radiation being absorbed before reaching the lower stratosphere. Furthermore, an oxygen molecule and an oxygen atom can collide to form a molecule of ozone, which is photo-dissociated by solar ultraviolet-B radiation. Normally, 97-99 percent of ultraviolet-B radiation is absorbed in the ozone layer, warming the lower stratosphere. By 1970, however, humans manufacturing chlorofluorocarbon gases caused up to 70% depletion of ozone, cooling the ozone layer and allowing more ultraviolet-B to reach Earth where it photo-dissociates ground-level ozone pollution, raising air temperatures, especially in the most polluted areas. Ultraviolet-B also penetrates oceans tens of meters, efficiently raising ocean heat content. Earth’s surface warmed 0.6°C from 1970 to 1998 with warming twice as great in the northern hemisphere containing 90% of global population. In 2014, Bárðarbunga volcano in central Iceland extruded 85 km² of basaltic lavas in six months, depleting the ozone layer and warming Earth another 0.3°C by 2016. Throughout Earth history, basaltic lava flows covering areas of up to millions of square kilometers are contemporaneous with sudden global warming—the larger the lava flow, the greater the warming. Large explosive volcanic eruptions, on the other hand, typically form aerosols in the lower stratosphere that spread throughout the world, reflecting and scattering sunlight, cooling Earth approximately 0.5°C for two to four years. Computer modelling shows the effects of this global cooling can still be observed in ocean temperatures a century later. Several large explosive volcanic eruptions per century, continuing for millennia, cool oceans incrementally down into ice-age conditions. Detailed measurements of air temperatures in ice cores at Summit Greenland over the past 122,000 years show that the footprints of climate change are sudden warming within years, followed by slow, incremental cooling over millennia, in highly erratic sequences averaging only a few thousand years in length. Ozone depletion and aerosols are particularly effective because they occur worldwide.

Variation in b-value of caldera earthquakes during recent activity of the Bárðarbunga Volcano in Iceland

The magnitude distribution of caldera earthquakes in the subglacial Bárðarbunga volcano in Central Iceland, characterized by the b-value, shows a systematic variation that is consistent with stress changes anticipated in the roof of an inflating magma chamber beneath the caldera. The b-value was 0.83 prior to the rupture of the chamber in August 2014 when a dike propagated laterally from the volcano to feed the eruption in Holuhraun. The b-value was relatively high following the collapse of the caldera, reflecting low stress in the magma chamber roof. Half a year later a decrease was observed in the b-value, concurrent with an increase in the seismicity, consistent with indications of recharging of the volcano magma chamber. The magnitude distribution was anomalous during the slow collapse of the caldera in association with the eruption. During this period the earthquake sequence appeared to consist of two populations, only one of which followed the conventional Gutenberg–Richter distribution. For a subglacial volcano, where geodetic methods are difficult to implement, the b-value of caldera earthquakes provides an important additional parameter for the monitoring of magma pressure variations.

Crustal seismic velocity responds to a magmatic intrusion and seasonal loading in Iceland's Northern Volcanic Zone.

Seismic noise interferometry is an exciting technique for studying volcanoes, providing a continuous measurement of seismic velocity changes (dv/v), which are sensitive to magmatic processes that affect the surrounding crust. However, understanding the exact mechanisms causing changes in dv/v is often difficult. We present dv/v measurements over 10 years in central Iceland, measured using single-station cross-component correlation functions from 51 instruments across a range of frequency bands. We observe a linear correlation between changes in dv/v and volumetric strain at stations in regions of both compression and dilatation associated with the 2014 Bárðarbunga-Holuhraun dike intrusion. Furthermore, a clear seasonal cycle in dv/v is modeled as resulting from elastic and poroelastic responses to changing snow thickness, atmospheric pressure, and groundwater level. This study comprehensively explains variations in dv/v arising from diverse crustal stresses and highlights the importance of deformation modeling when interpreting dv/v, with implications for volcano and environmental monitoring worldwide.

Temporal evolution of magma and crystal mush storage conditions in the Bárðarbunga-Veiðivötn volcanic system, Iceland

The depth(s) of magma storage reservoirs beneath active volcanic regions may change with time. Determining the rates and causes of millennial-scale changes in magmatic system architecture is critical for the development of realistic time-integrated models of crustal evolution. Here we examine a suite of samples from the exceptionally well-exposed Bárðarbunga-Veiðivötn volcanic system in central Iceland in order to resolve the temporal evolution of magma storage conditions within one of Iceland’s most productive volcanic systems. We have measured the major and minor elemental composition of glass, mineral and melt inclusion from five erupted units that span a full glacial cycle, from a <100 ka subglacial eruption to a historical eruption in 1477 AD. All samples contain macrocrysts (>500 μm), polymineralic clots and high-crystallinity nodules, consistent with derivation from crystal mush bodies. Macrocryst rims are in chemical equilibrium with their respective carrier melts, while macrocrysts cores are too primitive to have crystallized from these melts. Each sample records a distinct range of macrocryst compositions, indicating that the composition and/or eruptibility of stored crystal mush has changed with time. Macrocrysts from the oldest units are the most primitive, and the macrocryst compositional range becomes wider and, on average, more evolved, with time. Clinopyroxene-melt and melt-based (OPAM) geobarometers reveal temporally invariant crystallization conditions of 1.9–2.2 ± 0.7 (1σ) kbar pressure, corresponding to depths around 6.8–7.8 ± 2.5 km. All the samples also contain melt inclusions trapped at mid-crustal pressures of ∼2.6 kbar (9.6 km). In addition, melt inclusions hosted in most primitive olivines and plagioclases from subglacial and early Holocene eruptions preserve evidence of crystallization in a lower-crustal storage level(s) located at 17.5 km (4.9 kbar). This petrological record of deep crystallization may be linked to a surge in eruption rates, tapping of lower-crustal magma reservoirs, consistent with a crustal response associated with postglacial isostatic rebound. In contrast, the absence of a deep crystallization signature in the younger eruptive units may reflect lower magma production rates under steady-state conditions of the crust, and new magma pathways favouring melt storage in the mid-crust.

Ice-confined construction of a large basaltic volcano—Austurfjöll massif, Askja, Iceland

Austurfjoll is the largest basaltic glaciovolcanic massif at Askja volcano (Central Iceland), and through detailed studies of its volcanological and geochemical characteristics, we provide a detailed account of the sequence and structure of the ice-confined construction of a large Icelandic basaltic volcano. In particular, Austurfjoll represents a geometry of vents, and resulting glaciovolcanic morphology, not previously documented in ice-confined basaltic volcanoes. Austurfjoll was constructed during two major phases of basaltic volcanism, via seven eruptive episodes through disperse fissure-dominated eruptions. The earliest episode involved a rare and poorly exposed example of subaerial activity, and this was succeeded by six episodes involving the eruption of ice-confined pillow lavas and numerous overlapping fissure eruptions of phreatomagmatic tephra. Evidence of local subaerial lavas and tephras indicates the local growth of eruptive centers above englacial lake levels, and subsequent flooding, but no prolonged subaerial activity. Localized ice-contact facies, paleowater levels, and diamictons indicate the position and thickness of the ice was variable during the construction of Austurfjoll, and eruptive activity likely occurred in multiple and variable level meltwater lakes during the last glacial period. Lithofacies evidence including gradational transitions from effusive to explosive deposits, superposition of fragmental facies above coherent facies, and drainage channels suggest that changes in eruptive style were driven largely by external factors such as drainage and the increasing elevation of the massif. This study emphasizes the unique character of Austurfjoll, being composed of large pillow lava sheets, numerous (> 40) overlapping glaciovolcanic tindars, and only localized emergent deposits, as a product of its prolonged ice-confined eruptive history, contrasts with previous descriptions of tuyas and tindars.

Seafloor Structure in the North Atlantic Region between the Kolbeinsey Ridge and the Jan Mayen Microcontinent

Deep seismic sounding, continuous seismic profiling, and bathymetric data are used to discuss the structure of the Iceland Plateau and adjacent areas of the Norway–Greenland region. We tentatively suggest the existence of regional nearly NS-trending tectonic discontinuities; these are normal faults that make up the stepwise character of the Iceland Plateau. The steps are successively descending westward. The fault zones contain magmatic features in the form of dikes that penetrate the sediment in several cases, thus providing evidence of their young age. A rift feature confined to magnetic anomaly C5b is tentatively identified in the central Iceland Plateau based on some crustal peculiarities. The central Iceland Plateau and the southern Jan Mayen Basin show an increased heat flow (92–217 mW/m2) whose lateral distribution suggests a boundary between the regions of increased and normal values. The boundary passes through the center of the Jan Mayen Basin and trends nearly NS. It seems to bound the zone of influence due to the Iceland mantle plume from the northeast at present.

Subglacial drumlins and englacial fractures at the surge‐type glacier, Múlajökull, Iceland

AbstractThe interaction between drumlins and overriding glacier ice is not well studied, largely due to the difficulty of identifying and accessing suitable active subglacial environments. The surge‐type glacier Múlajökull, in central Iceland, overlies a known field of actively forming drumlins and therefore provides a rare opportunity to investigate the englacial structures that have developed in association with ice flow over the subglacial drumlins. In this study detailed ground‐penetrating radar surveys are combined with field observations to identify clear sets of up‐glacier and down‐glacier dipping fractures at Múlajökull's margin. These are interpreted as conjugate shear planes or P‐ and R‐type Reidel shears that developed and filled with saturated sediment derived from the glacier bed, during a previous surge. The fracture sets exhibit focused spatial distributions that are influenced by the subglacial topography. In particular, down‐glacier dipping fractures are strongly focused over drumlin stoss slopes. These fractures, although well developed at depth, were mostly unable to transmit basal water and sediment up to the glacier surface during the surge cycle. In contrast, up‐glacier dipping fractures formed over drumlin lee sides and in more gently sloping swales, and more frequently connected to the glacier surface, providing a pathway for the evacuation of basal water and water‐saturated sediment. The study suggests that the subglacial drumlins under Múlajökull's margin have influenced the nature and distribution of englacial fractures, which could potentially contribute to spatial variations in basal water pressure during a surge. BGS © UKRI 2018

Seismicity of the Askja and Bárðarbunga volcanic systems of Iceland, 2009–2015

A large seismic network deployed in the Icelandic highlands recorded >100,000 earthquakes from 2009 to 2015. We develop a local magnitude scale, appropriate for use in central Iceland, which is similar to the scale used by the Iceland Meteorological Office. Using this large catalogue of earthquakes, we analyze the spatial and temporal changes in seismicity rates and b-values. In microearthquakes recorded from the usually ductile lower crust we find that b-values are high, reflecting the presence of high thermal gradients and low stresses driving seismicity associated with the movement of melt. In contrast, b-values in the upper crust are variable. Low b-values, indicative of a high stress environment, are observed during seismic swarms such as those around Mt. Herðubreið and around Bárðarbunga caldera. A persistently seismically active area around a geothermal area within Askja caldera has a b-value around 1 but has a strong annual cycle of seismicity. We attribute the annual cycle to varying load from the snow cover modulating the seismicity. Seismicity driven by the intrusion of a large dyke has a b-value well above 1, driven by the high pore fluid pressures and thermal gradients around the dyke.