Landsvirkjun (Iceland)

• Krafla fluids originate from a deep two-phase aquifer (272–320°C) with vapor fractions of 0.26–0.93. • These temperature estimates match the main feed zones, whereas solute geothermometers indicate cooler, shallower liquid aquifers. • The H 2 O-H 2 -CO 2 -CO-CH 4 gas system also tracks downflows from shallow liquid aquifers and/or reinjection effects. • Water isotopes reveal geothermal fluids originate from meteoric and reinjection water. • N (δ 15 N: –2.78 to –6.04‰) and Ar ( 40 Ar/ 36 Ar down to 292.8) isotopes suggest atmospheric gas fractionation from meteoric-like liquids. Equilibrium in the H 2 O-H 2 -CO 2 -CO-CH 4 gas system has been extensively applied to fumarole data for geothermal exploration and volcano monitoring. However, little is known about its application to two-phase (vapor and liquid) geothermal well fluids, which can show an excess of enthalpy. Here, we applied the H 2 O-H 2 -CO 2 -CO-CH 4 gas indicators to two-phase geothermal well discharges from the Krafla geothermal system, Iceland, to estimate aquifer temperatures and identify secondary processes during resource exploitation. Results suggest that the Krafla resource is drawn from a deep (approximately between –500 and –1,600 m a.s.l.), two-phase aquifer with temperatures ranging from 272 to 320 °C and vapor fractions between 0.26 and 0.93, explaining the excess enthalpy observed in well fluids. These estimates align with the temperatures of the main production zones of geothermal wells, whereas solute geothermometers (SiO 2 and Na/K) appear to record lower temperatures of minor, shallower, liquid aquifers. Wells with liquid-like enthalpy are sourced from the two-phase aquifer but are also influenced by water reinjection or downflows from a colder, shallower aquifer, consistent with the isothermal zone extending approximately between 400 and –900 m a.s.l. in Leirbotnar and Vesturhlíðar subfields. Water isotopes indicate the main aquifer is recharged by meteoric and reinjection fluids. Excess-enthalpy discharges show an influx of Ar- and N 2 -rich vapor, with depleted 40 Ar/ 36 Ar and δ 15 N values, suggesting fractionations of atmospheric gases dissolved into the reservoir liquid. On the other hand, δ 13 C CO2 and 3 He/ 4 He values point to a mantle origin, despite the lower δ 13 C CO2 and P CO2 levels that reflect a degassed magma (i.e., a noneruptive phase). These findings underscore the usefulness of the H 2 O-H 2 -CO 2 -CO-CH 4 gas system and isotopic methods in tracking geothermal reservoir temperatures, their sources, and secondary processes, such as water reinjection or downflows from shallower aquifers.

The dataset includes Pan-European maps of timber volume (Vol), above-ground biomass (AGB), and deciduous-coniferous proportion (DCP) with a pixel size of 10×10 m for the reference year 2020. In addition, a measure of prediction uncertainty is provided for each pixel. The maps have been created using a combination of a Sentinel-2 mosaic, Copernicus layers, and National Forest Inventory (NFI) data. The mapping was done with the k-Nearest Neighbour (kNN, k=7) approach with harmonized data of species-specific Vol and AGB from 14 NFIs consisting of approximately 151 000 field plots across Europe. The maps cover 40 European countries, forming a continuous coverage of the western part of the European continent. A sample of 1/3 of NFI plots was left out for validation, whereas 2/3 of the plots were used for mapping. Maps were created independently for 13 multi-country processing areas. Root-mean-squared-errors (RMSEs) for AGB ranged from 53 % in the Nordic processing area to 73 % in the South-Eastern area. The maps are on average nearly unbiased on European level (1.0 % of the mean AGB), but show significant overestimation for small biomass values (53 % bias for forests with AGB less than 150 t/ha) and underestimation for high biomass values (-55 % bias for forests with AGB higher than 500 t/ha). The created maps are the first of their kind as they are utilizing a large number of harmonized NFI plot observations and consistent remote sensing data for high-resolution forest attribute mapping. While the published maps can be useful for visualization and other purposes, they are primarily meant as auxiliary information in model-assisted estimation where model-related biases can be mitigated, and field-based estimates improved. Therefore, additional calibration procedures were not applied, and especially high Vol and AGB values tend to be underestimated. We therefore discourage from summarizing map values (pixel counting) over areas in interest, as this may inadvertently result in biased estimates.

Icelandic native ecosystems and soils have been severely degraded since settlement in the 9th century. Today, barren landscapes occupy about 45% of the land surface and only 1.5% is covered by native birch woodlands versus 20-40% in pre-settlement times. Iceland's soils are mainly Andisols, among the most carbon-rich soil orders owing to their unique colloidal characteristics. Hence, there is tremendous potential to sequester soil carbon in degraded soils through revegetation activities. The restoration of birch woodlands is considered a national priority, which may significantly impact the nation's carbon budget. The objective of this study was to determine soil carbon concentrations and stocks across chronosequences (0 to 60+ years) of birch woodlands under diverse geographical conditions comprised of ten study areas across Iceland. The highest carbon stocks were found in old birch woodlands with a mean of 7.4kg C m-2 in the top 30cm soils, which is unusually high compared to other Nordic deciduous woodlands. We attribute this to andic soil properties that effectively stabilize and sequester soil organic matter. Calculated soil carbon accumulation rates were 0.01kgm-2yr-1 for the first 30years of birch woodland establishment and 0.04-0.07kgm-2yr-1 in mature woodlands (30-60years old). These accumulation rates, if applied to large-scale birch woodland restoration plans, would amount to 20% of the current total CO2 emissions of Iceland (not counting LULUCF). Importantly, we found a significant impact of dust deposition (up to 1mmyr-1) on soil carbon stocks, contributing to carbon burial (∼26gm-2yr-1) in areas close to dust hotspots. Birch restoration further stabilizes soils from erosion and the above-ground biomass serves as an efficient dust collector. This study documents the potential of birch restoration as a highly effective strategy to address soil degradation and promote soil carbon sequestration across Iceland.

Abstract. Spaceborne digital elevation models (DEMs) of glaciers are essential to describe their health and their contribution to river runoff and sea level rise. Publicly available DEMs derived from sub-meter satellite stereo imagery were, up to now, mainly available in the polar regions and High Mountain Asia. Here, we present the Pléiades Glacier Observatory (PGO), a scientific program acquiring Pléiades 0.7 m satellite stereo pairs for 140 sites from Earth's glacierized areas. The PGO product consists of freely available DEMs at 2 and 20 m ground sampling distance together with 0.5 m (panchromatic) and 2 m (multispectral) ortho-images. PGO stereo acquisitions began in July 2016 in the Northern Hemisphere and February 2017 in the Southern Hemisphere. Each site is revisited every 5 years (cloud permitting), close to the end of the melt season, to measure glacier elevation change with an average uncertainty of 0.49 m (95 % confidence level, for a glacierized area of 1 km2), i.e., 0.1 m yr−1. PGO samples over 20 000 km2 of glacierized terrain, which represents about 3 % of the Earth's glacier area. This small sample, however, provides a first-order estimate (within 0.07 mw.e.yr-1) of the global glacier mass change and its decadal evolution.

Fault’s frictional strength and particularly its ability to heal during the interseismic period (fault frictional healing Δμ) is critical to understand the seismic cycle, yet the understanding of temperature and phase-dependent healing characteristics of natural geothermal conditions remains limited. Here we examined the frictional healing of both simulated fresh and chlorite-altered basaltic gouge from Krafla geothermal field (Iceland) under realistic geothermal conditions of water temperature Tf = 100-400 ˚C and pressure Pf = 10-30 MPa (water in liquid, vapor and supercritical state) by performing Slide-Hold-Slide (SHS) experiments. All experiments were performed under a constant effective normal stress of 10 MPa and initiated with a 5-mm run-in slip at a loading point slip rate V of 10 mm/s before the SHS sequence. For each SHS sequence, shearing was held from 3 s to 10,000 s, separated by a slip interval of 1mm. Our mechanical results indicate that frictional healing, the difference between peak friction reached upon re-shear and the steady-state friction before the hold, increases with increasing logarithm of hold time in all experiments, as suggested by previous studies. Meanwhile, frictional healing rate (β = Δμ/log(1+ thold/tcutoff)), commonly regarded as the quantification of the rate of healing, increases with increasing temperature for both fresh and altered basalt. For fresh basalt, β increases from 0.007 at Tf = 100 ˚C to 0.060 at Tf = 300 ˚C (liquid) before dropping to 0.036 at Tf = 400 ˚C (vapor) and eventually increases to 0.096 at Tf = 400 ˚C (supercritical). For altered basalt, β  increases continuously from 0.003-0.007 at Tf = 100 ˚C to 0.013-0.022 at Tf = 300 ˚C and reaches its maximum value of β = 0.024-0.035 at Tf = 400 ˚C (vapor) and β = 0.030 at Tf = 400 ˚C (supercritical). Besides this temperature-dependent relationship, the dramatic decrease of β in fresh basalt to values similar to those of altered basalt when water changed from liquid to vapor state also suggests that the physical state of water can control the healing rate. Subsequent microanalytical analyses (XRPD, XRF, SEM-EDS) performed on the deformed gouges from altered basalts suggest an increase in hydrothermal alteration with increasing temperature, as shown by a depletion in K2O at Tf ≥ 300 ˚C. SEM-BSE images of fine platy matrices in shear bands formed at Tf = 400 ˚C point towards a dissolution of quartz, pyroxene and plagioclase. Therefore, we suggest that the healing rate of both fresh and altered basalt not only scales with the ambient temperature but is also affected by the physical state of water, particularly in the case of fresh basalt, potentially related to more intense fluid-rock interactions with increasing temperature.Keywords: frictional healing, frictional healing rate, hydrothermal fluids, basaltic gouge, Krafla geothermal field

Magmatic-hydrothermal systems are key environments to study the transfer of elements from the deep Earth to the surface and the mobilization and re-distribution of elements within the crust. These systems have been recognized as potential active analogue sites for ore deposition. The source of fluids and their metal load is split between the relative contributions from degassing of underlying magma and interactions between the hydrothermal fluid and the host rock. Here, we combine analyses of noble gases and volatile metals in fluids and rocks from the þeistareykir geothermal field (NE Iceland) to provide constraints on the relative contribution of these two sources. Helium isotope data suggest 80–85% originated from magma degassing. The 3He/4He ratio, corrected for atmospheric contamination (Rc/Ra) correlates with volatile metal abundances in surface fluids and indicates that Bi and Hg are predominantly derived from magma degassing. It is also shown that the deep geothermal reservoir fluid is dominated by magmatic input, except for Mn, Fe, Co, Cu, Ti and V, using the elemental signature of magmatic degassing and water-rock interaction. The spatial variations in Rc/Ra and surface fluid volatile metal contents among the wells suggest an impact of the local and regional structures on the fluid's pathway from depth to surface.

Abstract Pressure and stress perturbations associated with volcanic activity and geothermal production can modify the porosity and permeability of volcanic rock, influencing hydrothermal convection, the distribution of pore fluids and pressures, and the ease of magma outgassing. However, porosity and permeability data for volcanic rock as a function of pressure and stress are rare. We focus here on three porous tuffs from Krafla volcano (Iceland). Triaxial deformation experiments showed that, despite their very similar porosities, the mechanical behavior of the three tuffs differs. Tuffs with a greater abundance of phyllosilicates and zeolites require lower stresses for inelastic behavior. Under hydrostatic conditions, porosity and permeability decrease as a function of increasing effective pressure, with larger decreases measured at pressures above that required for cataclastic pore collapse. During differential loading in the ductile regime, permeability evolution depends on initial microstructure, particularly the initial void space tortuosity. Cataclastic pore collapse can disrupt the low‐tortuosity porosity structure of high‐permeability tuffs, reducing permeability, but does not particularly influence the already tortuous porosity structure of low‐permeability tuffs, for which permeability can even increase. Increases in permeability during compaction, not observed for other porous rocks, are interpreted as a result of a decrease in void space tortuosity as microcracks surrounding collapsed pores connect adjacent pores. Our data underscore the importance of initial microstructure on permeability evolution in volcanic rock. Our data can be used to better understand and model fluid flow at geothermal reservoirs and volcanoes, important to optimize geothermal exploitation and understand and mitigate volcanic hazards.

Abstract. Access to mountainous regions for monitoring streamflow, snow and glaciers is often difficult, and many rivers are thus not gauged and hydrological measurements are limited. Consequently, cold-region watersheds, particularly heavily glacierized ones, are poorly represented in large-sample hydrology (LSH) datasets. We present a new LSH dataset for Iceland, termed LamaH-Ice (LArge-SaMple DAta for Hydrology and Environmental Sciences for Iceland). Glaciers and ice caps cover about 10 % of Iceland and, while streamflow has been measured for several decades, these measurements have not previously been published in a consistent manner. The dataset provides daily and hourly hydrometeorological time series and catchment characteristics for 107 river basins in Iceland, covering an area of almost 46 000 km2 (45 % of Iceland's area), with catchment sizes ranging from 4 to 7500 km2. LamaH-Ice conforms to the structure of existing LSH datasets and includes most variables contained in these datasets as well as additional information relevant to cold-region hydrology, e.g., time series of snow cover, glacier mass balance and albedo. LamaH-Ice also includes dynamic catchment characteristics to account for changes in land cover, vegetation and glacier extent. A large majority of the watersheds in LamaH-Ice are not subject to human activities, such as diversions and flow regulations. Streamflow measurements under natural flow conditions are highly valuable to hydrologists seeking to model and comprehend the natural hydrological cycle or estimate climate change trends. The LamaH-Ice dataset (Helgason and Nijssen, 2024) is intended for the research community to improve the understanding of hydrology in cold-region environments. LamaH-Ice is publicly available on HydroShare at https://doi.org/10.4211/hs.86117a5f36cc4b7c90a5d54e18161c91 (Helgason and Nijssen, 2024).

Spaceborne digital elevation models (DEMs) of glaciers are essential to describe their health, and their contribution to river runoff and to sea level rise. Publicly available DEMs derived from submeter satellite stereo-imagery were, up to now, mainly available in the polar regions and High Mountain Asia. Here, we present the Pléiades Glacier Observatory (PGO), a scientific programme acquiring Pléiades stereo pairs for 140 sites from Earth's glacierized areas. The PGO product consists of DEMs at 2 m and 20 m ground sampling distance together with 0.5 m (panchromatic) and 2 m (multispectral) ortho-images. The DEMs are freely available to all registered users whereas ortho-images are available after signing a licence. PGO commenced in July 2016 in the North Hemisphere and February 2017 in the South Hemisphere. Each site is revisited every five years (cloud permitting), close to the end of the melt season, to measure glacier elevation change with an average uncertainty of 0.49 m (95% confidence level, for a glacierized area of 1 km²), i.e. 0.1 m a -1 . PGO samples over 20,000 km² of glacierized terrain which represents about 3% of the Earth's glaciers area. This small sample, however, provides a first order estimate of the global glacier mass change and its decadal evolution.

Abstract The Krafla geothermal system is located within a volcanic center that periodically emit basaltic lavas, and has recently attracted an economic interest due to supercritical fluids forming nearby a shallow magma intrusion (~ 2 km depth). Here, we discuss new soil CO2 flux and stable isotope data of the CO2 efflux (δ13C) and hydrothermal calcites (δ13C, δ18O) of drill cuttings, to estimate both the current magmatic outgassing from soils and the thermal flows in the geothermal system. Soil CO2 emission is controlled by the tectonic, following the NNE-SSW fissure swarm direction and a WSW-ENE trend, and accounts for ~ 62.5 t d–1. While the δ18O of the H2O in equilibrium with deep calcites is predominantly meteoric, both the δ13C of the soil CO2 efflux and of the fluids from which calcite precipitated have a clear magmatic origin, overlapping the δ13C estimated for the Icelandic mantle (–2.5 +/- 1.1 ‰). Estimates based on the soil CO2 emission from the southern part of the system show that these fluxes might be sustained by the ascent and depressurization of supercritical fluids with a thermal energy of ~800 MW. Such significant amount of energy might reach 1.5 GW, if supercritical conditions extended below the whole investigated area. Finally, we report an increase in the soil CO2 emission of about 3 times with respect to 14 years ago, likely due to recent changes in the fluid extracted for power production or magmatic activity. Pairing the soil CO2 emission with stable isotopes of the efflux and calcite samples has important implications for both volcano monitoring and geothermal exploration, since it can help us to track magmatic fluid upflows and the associated thermal energy.