South Iceland Research Articles

Historical lava flow fields at Hekla volcano, South Iceland

Hekla volcano is known to have erupted at least 23 times in historical time (last 1100 years); often producing mixed eruptions of tephra and lava. The lava flow volumes from the 20th century have amounted 80\% to almost 100\% of the entire erupted volume. Therefore, evaluating the extent and volume of individual lava flows is very important when assessing the historical productivity of Hekla volcano. Here we present new maps of the historical lava flow fields at Hekla in a digital format. The maps were produced at a scale of 1:2000–10000 using a catalogue of orthophotos since 1945, acquired before and after each of the last five eruptions, combined with field observation of stratigraphy, soil profiles, tephra layers and vegetation cover. The new lava flow maps significantly improve the historical eruptive history of Hekla, prior to the 1947 eruption. The historical lava flow fields from Hekla cover ∼233 km² and the lavas reach up to 16 km from Hekla volcano. Flow lengths up to 20 km are known, though lava flows only travelled up to 8–9 km from Hekla in the last 250 years. Identified historical vents are distributed between 0 and 16 km from Hekla volcano and vents are known to have migrated up to 5 km away from Hekla during eruptions. We have remapped the lava flow fields around Hekla and assigned the identified flow fields to 16 eruptions. In addition, ca. 60 unidentified lava units, which may be of historical age, have been mapped. It is expected that some of these units are from known historical Hekla eruptions such as the 1222, 1341, 1510, 1597, 1636 and potentially even from the previously excluded eruptions such as 1436/1439.

A Preseismic Strain Anomaly Detected Before M6 Earthquakes in the South Iceland Seismic Zone From GPS Station Velocities

AbstractThe South Iceland Seismic Zone (SISZ) accommodates E‐W translation due to oblique spreading between the North American and the Eurasian plates in South Iceland with ruptures on N‐S faults. Strain is released in earthquake sequences that last days to years, at average intervals of 80–100 years. Two M6.5 earthquakes struck in the SISZ in June 2000, and two M6 earthquakes in May 2008. These events released only half of the strain accumulated since the last earthquake sequence in 1896–1912. GPS station velocities are estimated from annual campaigns and continuous measurements in the SISZ during 2001–2015. The GPS station velocities are used to calculate strain rates from a new method using the vertical derivatives of horizontal stress. Our new strain rates are obtained by integrating vertical derivatives of horizontal stress rates, rather than differentiating interpolated GPS velocities, allowing finer resolution of strain. Estimating the strain rates for eight time intervals, we find high strain rates in the SISZ (0.5–1 microstrain per year) with significant temporal and spatial variation. The strain rates in the SISZ reveal contraction in the Hengill area and dilatation over a large area in eastern SISZ, indicating inflation of the magma system beneath Hekla volcano. A prominent strain anomaly is evident in the epicentral area prior to the May 2008 earthquakes. We suggest that this signal is primarily caused by plate motion and an increase in the rate of contraction in the Hengill area, located ∼10 km west of the epicentral area.

Extending the known distribution of the Vedde Ash into Siberia: occurrence in lake sediments from the Timan Ridge and the Ural Mountains, northern Russia

Tephra shards from the Vedde Ash eruption have been identified in two lakes from northwestern Russia and the Polar Ural Mountains. This is the most distal and easternmost occurrence of this regional tephra marker horizon found so far and it extends the area of the Vedde Ash tephra more than 1700 km further east than previously documented. This means that particles the size of fine sand have travelled more than 4000 km from the Katla volcano source, south Iceland. These findings offer a new possibility to correlate archives over a very long distance in the time period around the Younger Dryas.

Of mosses and men: Plant succession, soil development and soil carbon accretion in the sub-Arctic volcanic landscape of Hekla, Iceland

Lava flows pose a hazard in volcanic environments and reset ecosystem development. A succession of dated lava flows provides the possibility to estimate the direction and rates of ecosystem development and can be used to predict future development. We examine plant succession, soil development and soil carbon (C) accretion on the historical (post 874 AD) lava flows formed by the Hekla volcano in south Iceland. Vegetation and soil measurements were conducted all around the volcano reflecting the diverse vegetation communities on the lavas, climatic conditions around Hekla mountain and various intensities in deposition of loose material. Multivariate analysis was used to identify groups with similar vegetation composition and patterns in the vegetation. The association of vegetation and soil parameters with lava age, mean annual temperature, mean annual precipitation and soil accumulation rate (SAR) was analysed. Soil carbon concentration increased with increasing lava age becoming comparable to concentrations found on the prehistoric lavas. The combination of a sub-Arctic climate, gradual soil thickening due to input of loose material and the specific properties of volcanic soils allow for continuing accumulation of soil carbon in the soil profile. Four successional stages were identified: initial colonization and cover coalescence (ICC) of Racomitrium lanuginosum and Stereocaulon spp. (lavas <70 years of age); secondary colonization (SC) – R. lanuginosum dominance (170−700 years); vascular plant dominance (VPD) (>600 years); and highland conditions/retrogression (H/R) by tephra deposition (70−860 years). The long time span of the SC stage indicates arrested development by the thick R. lanuginosum moss mat. The progression from SC into VPD was linked to age of the lava flows and soil depth, which was significantly deeper within the VPD stage. Birch was growing on lavas over 600 years old indicating the development towards birch woodland, the climax ecosystem in Iceland.

Fault kinematics at the Hengill Triple Junction, SW-Iceland, derived from surface fracture pattern

The Hengill Triple Junction in Iceland is formed by the intersection of the Western Volcanic Zone (WVZ), Reykjanes Peninsula Oblique Rift (RPOR) and a transform zone, the South Iceland Seismic Zone (SISZ). The 19 × 13 km2 study area thus contains extensional structures related to volcanic systems of the two rift zones, Hengill and Hrómundartindur, as well as strike-slip structures characteristic of the transform zones. High definition aerial photographs, satellite images, and digital elevation models are used, supported by field studies, to accurately map and classify tectonic features in an effort to separate N-S trending strike-slip faults from the fissure swarms of the volcanic systems. A portion of the southern Hengill fissure swarm lies within the area. It trends NE-SW and exhibits extensional features characteristic of spreading ridges, i.e. normal faults, open fractures, and eruptive fissures. It is clearly delineated by surface fractures, and is traced a length of 28 km southwest from the central volcano, terminating before reaching the coastline. The fissure swarm is 2–3.5 km wide in the study area, widening southwards. Elevation profiles across the fissure swarm suggest a spreading rate comparable to the southern WVZ, which spreads at 4–5 mm/year. A portion of the Nesjahraun eruptive fissure is anomalously smooth and may represent secondary rifting. East-west shearing in the RPOR and SISZ is accomplished by many smaller and less conspicuous, northerly oriented strike-slip faults in a ‘bookshelf faulting’ manner. These parallel faults are overprinted on the volcano-tectonic structures of the RPOR. Evidence of 11 northerly striking strike-slip faults is found within the study area. The zone of bookshelf faulting is thus shown to be continuous across the triple junction area, from the SISZ to the RPOR. Finally, several structures (eruptive fissures, exposed dikes, and faults) strike near-perpendicular to the fissure swarm and may be the result of changes in the direction of maximum stress caused by rifting events, however, their tectonic significance remains uncertain.

Subpolar North Atlantic sea surface temperature since 6 ka BP: Indications of anomalous ocean-atmosphere interactions at 4-2 ka BP

Atmospheric circulation may change with future climate change in response to modification of meridional temperature gradients, but the potential influence on ocean circulation is as yet unclear. Over the mid-late Holocene, atmospheric circulation in the North Atlantic region has fluctuated on millennial timescales; therefore, the ocean response to these changes can be investigated using the paleoceanographic records that have been developed in the north-eastern subpolar North Atlantic. Here, we present a diatom-based sea surface temperature reconstruction from the Iceland Basin, south of Iceland; the reconstruction shows the warmest temperatures of the record at 6.1–4 ka BP, cooler temperatures at 4-2 ka BP and warmer temperatures thereafter. Inter-record comparisons indicate that the cold period at c. 4-2 ka BP may have resulted from a strengthened East Greenland Current and/or melting of the Greenland ice sheet, in response to a negative North Atlantic Oscillation. The findings highlight that atmospheric circulation changes are likely to cause pronounced variations in the latitudinal exchange of heat, which may have consequences for deep-water formation and global ocean circulation.

Þórarinn í þágufalli

The ancient inflection of the Icelandic male name Þórarinn (nom.), Þórarin (acc.), Þórarni (dat.), Þórarins (gen.), is identical in the modern language. But in addition to the usual dative form, Þórarni, four other forms have been used, Þórarin, Þórarini, Þórarinum, Þórarininum. As examples of the use of these innovative forms are not com-mon, it is difficult to date their emergence with certainty. The oldest known example of Þórarinum is from around 1700 in South Iceland, from where it may have spread. Examples are found in several places in the South, in the West and in the North. Examples of Þórarin in the dative are found in the latter half of the 18th century. Þórarini occurs in the language of a man born in 1850. The rarest and presumably the youngest form, Þórarininum, occurs in the 20th century, in Rangárvallasýsla and Barðastrandarsýsla, and is probably extinct. The other three innovative forms are still occasionally found, especially Þórarin and Þórarini.

The structure of seismogenic strike-slip faults in the eastern part of the Reykjanes Peninsula Oblique Rift, SW Iceland

A considerable part of the seismic risk to the capital area of Iceland is provided by a series of strike-slip faults at the mid-Atlantic plate boundary within a distance range of 15–35 km. About two thirds of the Icelandic population lives within this distance range from the source areas of M 6–6.5 earthquakes. The structure of the plate boundaries in SW-Iceland is relatively complex. Three branches meet in the Hengill Triple Junction, the oblique rift of the Reykjanes Peninsula, the South Iceland Seismic Zone (a transform zone) and the Western Volcanic Zone (a rift zone). The Reykjanes Peninsula oblique rift has an over-all trend of 70°, highly oblique with respect to the spreading direction, 101° in this region. It contains both volcanic systems and seismogenic strike-slip faults. The fissure swarms of individual volcanic systems contain normal faults and fissures, with a NE-trend, also quite oblique to the plate boundary. The fissure swarms fade out towards the NE and SW as they extend into the plates on either side. Overprinting this pattern of volcano-tectonic structures are sets of parallel, northerly striking transcurrent faults that generate the largest earthquakes in this zone. Their surface expressions are en echelon fracture arrays and push-up structures. The sense of displacement is right-lateral. In the 15 km long section of the rift studied here we find evidence for at least six faults of this type. The length of individual faults may exceed 12 km. The distance between them varies from 1 to 5 km, and together they form a bookshelf-type fault system taking up the left-lateral component of plate movements across the oblique rift zone.

Development and predictors of psychological outcomes following the 2008 earthquake in Iceland: a longitudinal cohort study.

On 29 May 2008, an earthquake struck in South Iceland. The aim of this study was to explore the trajectories of post-traumatic stress, depressive and anxiety symptoms among exposed inhabitants during the first year following the earthquake, as well as predictors for symptomology. This was a longitudinal cohort study based on a sample that was randomly selected from the earthquake-stricken area ( n = 1301). Participants answered a questionnaire assessing demographic and disaster-related factors 2 months after the earthquake. In addition, symptoms of post-traumatic stress disorder (PTSD), depression and anxiety were assessed 2, 4, 8 and 12 months post-disaster. Two months after the earthquake, 5.2% of the participants reported PTSD symptoms, 6.7% depression and 6.4% anxiety symptoms. When comparing first and last time points only, we found a significant decrease in anxiety ( p = 0.05), particulary among females ( p = 0.05), those with a primary education ( p = 0.01), prior history of accidents/disasters ( p = 0.02) and those experiencing damage to their home ( p = 0.02). No significant trends were found when the development of other symptoms between the four time points was assessed. Findings indicate a reduction in anxiety symptoms between 2 and 12 months post-disaster, with PTSD and depression symptoms remaining fairly constant across time. No trends in symptomology were observed over time. The results highlight the need for continued monitoring of those affected by disasters and the identification of subgroups at risk in the aftermath of natural disasters.

Tool for analysis of multichannel analysis of surface waves (MASW) field data and evaluation of shear wave velocity profiles of soils

Multichannel analysis of surface waves (MASW) is a fast, low-cost, and environmentally friendly technique to estimate shear wave velocity profiles of soil sites. This paper introduces a new open-source software, MASWaves, for processing and analysing multichannel surface wave records using the MASW method. The software consists of two main parts: a dispersion analysis tool (MASWaves Dispersion) and an inversion analysis tool (MASWaves Inversion). The performance of the dispersion analysis tool is validated by comparison with results obtained by the Geopsy software package. Verification of the inversion analysis tool is carried out by comparison with results obtained by the software WinSASW and theoretical dispersion curves presented in the literature. Results of MASW field tests conducted at three sites in south Iceland are presented to demonstrate the performance and robustness of the new software. The soils at the three test sites ranged from loose sand to cemented silty sand. In addition, at one site, the results of existing spectral analysis of surface waves (SASW) measurements were compared with the results obtained by MASWaves.

Seismic attractor can assist in finding of geothermal area?

The seismic active zone is a distributed system consisting of various subareas. To understand a state of the system during a given period of time so called ‘the linear gradient frame’ technique was developed to search the stable characteristics for subareas. The search is based on analysis of geographic components, least-squares inversion and statistical estimators of seismic data. The stable subarea includes hypocenters of earthquakes, the space-time trajectories of which are restricted by a certain domain (seismic attractor). Fractal structures obtained in south of Iceland are in an agreement with geological history. The results are promising for the geothermal exploration.

First tomographic observations of gravity waves by the infrared limb imager GLORIA

Abstract. Atmospheric gravity waves are a major cause of uncertainty in atmosphere general circulation models. This uncertainty affects regional climate projections and seasonal weather predictions. Improving the representation of gravity waves in general circulation models is therefore of primary interest. In this regard, measurements providing an accurate 3-D characterization of gravity waves are needed. Using the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA), the first airborne implementation of a novel infrared limb imaging technique, a gravity wave event over Iceland was observed. An air volume disturbed by this gravity wave was investigated from different angles by encircling the volume with a closed flight pattern. Using a tomographic retrieval approach, the measurements of this air mass at different angles allowed for a 3-D reconstruction of the temperature and trace gas structure. The temperature measurements were used to derive gravity wave amplitudes, 3-D wave vectors, and direction-resolved momentum fluxes. These parameters facilitated the backtracing of the waves to their sources on the southern coast of Iceland. Two wave packets are distinguished, one stemming from the main mountain ridge in the south of Iceland and the other from the smaller mountains in the north. The total area-integrated fluxes of these two wave packets are determined. Forward ray tracing reveals that the waves propagate laterally more than 2000 km away from their source region. A comparison of a 3-D ray-tracing version to solely column-based propagation showed that lateral propagation can help the waves to avoid critical layers and propagate to higher altitudes. Thus, the implementation of oblique gravity wave propagation into general circulation models may improve their predictive skills.

Colonization of woodland species during restoration: seed or safe site limitation?

A strategy of assisted natural regeneration uses selective restoration interventions to remove potential barriers to passive natural regeneration and succession toward target ecosystems. In woodland restoration, this may involve the planting of woodland clusters that serve as seed sources for further colonization and/or site amelioration to enhance colonization and growth of the target species. We tested the importance of seed and safe site limitations for the colonization of birch and willows in experimental treatments representing a matrix of revegetation (+/−) and planted birch and willow clusters (+/−) on an eroded area in South Iceland. The revegetation was initiated in the fall of 1999 and clusters of birch and willows planted in 2002. Density of birch and willow seedlings and their distribution among microsites was surveyed in 2005, 2006, 2010, and 2015. Willow seedlings were first noted in 2006 but birch seedlings in 2010. In 2015, seedling density in revegetated plots with planted clusters was over 20 times greater than that of revegetated plots without clusters, indicating a strong seed limitation. No seedlings were found in plots that had not been revegetated, showing very strong microsite limitation. However, revegetation increased the cover of biological soil crust and other microsites favorable for seedling establishment. The results demonstrate that both seed and safe site limit the colonization of birch and willows, key species in woodland restoration, and highlight the importance of identifying barriers to spontaneous succession before selecting restoration strategies.

Ambient-noise tomography of Katla volcano, south Iceland

A shear-wave velocity model of subglacial Katla volcano, southern Iceland, has been developed using ambient seismic noise tomography based on data from a temporary network operating between May 2011 and August 2013 and permanent stations around the volcano. Phase-velocity dispersion curves were obtained using cross-correlations of vertical components of 136 station pairs and non-linearly inverted for phase-velocity maps between 1.7 and 7.5s. Local dispersion curves were inverted for shear-velocity variation with depth using a grid search imposing a fixed ice layer at the top. The resulting one-dimensional (1-D) velocity models were combined to obtain a pseudo three-dimensional (3-D) model with estimated lateral resolution of 8km and depth resolution varying from close to 1km near the surface to about 8km at 10km depth. Shear wave velocities are generally higher within the Katla central volcano than in its surroundings. The most striking feature of the model is a high-velocity anomaly beneath the caldera at >6km depth interpreted to be due to cumulates resulting from differentiation of shallower magma intrusions and remelting of subsiding upper crust. No shallow low-velocity anomaly is resolved beneath the central caldera, but a low-velocity region is found at 2–4km depth beneath the western half of the caldera. VP/VS ratios, estimated from average velocity-depth profiles from surface-wave data and higher frequency P-wave data, are anomalously high (>1.9) compared to average Icelandic crust, particularly in the top 2–3km. This is argued not to be an artifact due to lateral refraction or topography. Instead, this anomaly could be explained as an artifact caused by velocity dispersion due to attenuation and a difference in frequency content, and possibly to a degree by the compositional difference between the transalkalic Fe-Ti basalts of Katla and average tholeiitic Icelandic crust.

Spatial distribution of helium isotopes in Icelandic geothermal fluids and volcanic materials with implications for location, upwelling and evolution of the Icelandic mantle plume

Abstract The distribution of helium isotope ratios (3He/4He) in Icelandic geothermal fluids, volcanic glasses and phyric lavas is investigated. Along with presenting a new helium isotope dataset using phyric lavas largely from off-rift regions, we compiled published data and constructed a database of all available helium isotope data from Iceland. The new dataset reveals an exceptionally high 3He/4He ratio from a phyric lava in NW-Iceland (47.5 RA, where RA is the 3He/4He ratio of air), which is among the highest values measured in any mantle-derived magma to date. Modifications of primary (i.e., mantle-derived) helium isotope ratios, due to additions of air-derived helium and He from radiogenic ingrowth, were evaluated and the database was filtered accordingly. The geographical information system ArcGIS (ESRI) was used to perform spatial analysis on the filtered database and the interpolation method, Natural Neighbor, was used to calculate representative helium isotope ratios for all parts of Iceland, including off-rift regions. The results show that helium isotope ratios for the whole of Iceland vary from 5.1 to 47.5 RA. However, this study allows for a fine-scale distinction to be made between individual rift segments and off-rift regions. The results clearly reveal that each rift zone has its own distinctive mean isotope signature: 12–17 RA in the Western Rift Zone, 8–11 RA in the Northern Rift Zone and 18–21 RA in the Eastern Rift Zone. Our isoscape map places new constraints on a previously inferred high-helium plateau region in central Iceland (Breddam et al., 2000). The plateau continues southward along the propagating Eastern Rift Zone and through to the South Iceland Seismic Zone and the Mid-Iceland belt. Its location coincides with many geological features, e.g., eruption rates, location of abandoned rift segments, seismic velocity and gravity anomalies. Such high helium isotope ratios have been associated with undegassed and primordial mantle sources that have been isolated in the lower mantle over Earth's history. Thus, high-helium domains throughout Iceland are interpreted to mark the loci of present and past plume conduits which help explain the considerable spatial variation in the sampling of a primordial mantle He component beneath the Iceland hotspot.

THE KERAUGA CAVE AND LÆKJARBOTNAVEITA IN SOUTH ICELAND ‒ GROUNDWATER SAFETY AND HYDROGEOLOGY

This research looks at water protection of Lækjarbotnar and Kerauga springs in South Iceland. Discharge measurements show that Minnivalla- and Tjarnalækur streams lose 350 and 1350 l/s respectively into the ground. The equivalent amount springs to surface along the edge of the Þjórsá Lava and Búði Glacial Moraine in a few springs. Tracer tests indicate that lost surface water of Minnivallalækur transits partly along a fracture to Kerauga in 65 days and to Lækjarbotnar in 125 days through the Þjórsá Lava over a similar distance of about 5 km. Hence the lost surface water of Tjarnalækur in the east is unlikely to travel past this drainage divide west of the fracture to Lækjarbotnar but more likely to drain into Ytri-Rangá. Thus a proposed poultry farm in that area proposes minimum direct risk to water safety of Lækjarbotnar. As a result of this connection Kerauga is not a safe source of drinking water. The aquifer is considered unconfined, heterogeneous and anisotropic. It is recommended to define inner and outer protection zones for Lækjarbotnar further north and to springs of Minnivallalækur respectively. It is hypothesized that Kerauga cave is manmade.

Directional effects of tectonic fractures on ground motion site amplification from earthquake and ambient noise data: A case study in South Iceland

The geology of SW-Iceland is characterized by alternating basaltic lava units, hyaloclastite formations, postglacial sedimentary filled valleys and alluvial plains, as well as highly fractured bedrock within the Reykjanes Peninsula volcanic rift zone and the South-Iceland transform fault system. Historic earthquakes within this region reach magnitudes 6.5–7. Using earthquake and ambient noise recordings from 15 seismic stations within the rift and transform zones we compared wavefield polarization and seismic site response in order to assess characteristics of local amplification of ground motion. Ambient noise and earthquake ground motion spectral ratios are comparable in frequency and can qualitatively be subdivided into three groups: one with a spectral ratio characterized by a single predominant frequency of horizontal amplification, one with a bi- or multimodal and one characterized by a relatively constant amplitude across the frequency range. Seismic wavefield polarization within the transform zone has a prevailing direction of amplification towards 110°−150°N in the frequency range 1.0–3.0Hz, having a quasi-perpendicular relationship with mapped faults and fractures. Shear wave splitting results show that the wavefield polarization and fast S wave directions tend to be orthogonal, i.e. highly dependent on the anisotropy of the medium.

Pathways of high-latitude dust in the North Atlantic

The contribution of mineral dust from high-latitude sources has remained an under-examined feature of the global dust cycle. Dust events originating at high latitudes can provide inputs of aeolian sediment to regions lying well outside the subtropical dust belt. Constraining the seasonal variability and preferential pathways of dust from high-latitude sources is important for understanding the potential impacts that the dust may have on wider environmental systems, such as nearby marine or cryospheric domains. This study quantifies dust pathways from two areas exhibiting different emission dynamics in the north and south of Iceland, which is a prominent Northern Hemisphere dust source. The analysis uses air parcel trajectory modelling, and for the first time for high-latitude sources, explicitly links all trajectory simulations to time-specific (meteorological) observations of suspended dust. This approach maximises the potential for trajectories to represent dust, and illustrates that trajectory climatologies not limited to dust can grossly overestimate the potential for dust transport.Preferential pathways emerge that demonstrate the role of Iceland in supplying dust to the Northern Atlantic and sub-Arctic oceans. For dust emitted from northern sources, a dominant route exists to the northeast, into the Norwegian, Greenland and Barents Seas, although there is also potential for delivery to the North Atlantic in summer months. From the southern sources, the primary pathway extends into the North Atlantic, with a high density of trajectories extending as far south as 50°N, particularly in spring and summer. Common to both southern and northern sources is a pathway to the west-southwest of Iceland into the Denmark Strait and towards Greenland. For trajectories simulated at ≤500 m, the vertical development of dust plumes from Iceland is limited, likely due to the stable air masses of the region suppressing the potential for vertical motion. Trajectories rarely ascend high enough to reach the central portions of the Greenland Ice Sheet. The overall distribution of trajectories suggests that contributions of Icelandic dust are relatively more important for neighbouring marine environments than the cryosphere.

Geochemistry of CO2-rich waters in Iceland

CO2-rich waters in Iceland from volcanic systems of the Mid-Atlantic Ridge and off-rift volcanic areas of Iceland provide insights into the interactions between CO2-rich fluids and basaltic crust. These waters, from the Snæfellsnes Peninsula, within the South Iceland Seismic Zone and on the margin of the Torfajökull central volcano, are chemically and isotopically distinct from surface waters, groundwaters and low and high-temperature geothermal fluids in Iceland. Our conceptual model for the evolution of thermal and non-thermal CO2-rich waters (~4–80mmol/kg ΣCO2 (aq); 6.6–192°C) suggests that lower subsurface temperatures during retrograde alteration and cooling of a geothermal system increase the aqueous solubility of CO2, which allows anomalously high aqueous CO2 concentrations. The elevated CO2 concentrations result in slightly acidic pH (average in situ pH of 6.40), which leads to increased dissolution of primary and hydrothermal silicate minerals. Equilibrium with respect to carbonate minerals, calculated using measured solute concentrations, suggests that carbonate mineral reactions control the water compositions in these systems more than in high-temperature systems, and that hydrothermally altered basalts are sufficiently reactive to promote carbonate mineralization during geologic CO2 sequestration (GCS). Water-rock interaction with previously hydrothermally altered basalts releases trace elements to the aqueous phase to levels above the World Health Organization drinking water limit. The elements of environmental concern include arsenic (<0.01 to 1.08μmol/kg) and manganese (<0.1 to 184μmol/kg), highlighting the need for careful site selection during GCS to prevent groundwater contamination.

An inflated subpolar gyre blows life toward the northeastern Atlantic

Deep convection in the Labrador and Irminger Seas inflates the cold and low-saline subpolar gyre, which is a rich nutrient and zooplankton source for the surrounding warmer waters of subtropical origin. The zooplankton abundances on the south Iceland shelf show characteristic sub-decadal variability, which closely reflect the oceanic abundances of the ecologically most important zooplankton species – Calanus finmarchicus. Much higher abundances of this species are observed during years when the winter mixed layer depths (MLD) in the Labrador-Irminger Sea, and over the Reykjanes Ridge are deep. Furthermore, a tight relationship is identified between on-shelf zooplankton abundances and lateral shifts of the biologically productive subarctic front southwest of Iceland. Thus, we suggest that northeastward expansion of the subpolar gyre results in biologically productive periods in the waters southwest of Iceland – both oceanic and on the shelf. In addition to local atmospheric forcing, we find that the MLD and frontal position are also impacted by remote heat losses and convection in the Labrador Sea, through northward advection of unstable mode waters. The sub-decadal oceanic and on-shelf biological production peaks are possibly predictable by half a year (local winter convection to subsequent summer production), and the advective time-lag from the Labrador Sea might induce an even longer predictability horizon (up to 1.5years).