Baltic Shield Research Articles

A shear-wave velocity model for the Scandinavian lithosphere from Rayleigh waves and ambient noise - Implications for the origin of the topography of the Scandes mountain range

We present a new 3D shear-wave velocity model and Moho map of Scandinavia, which is based on the inversion of the merged phase dispersion curves from ambient noise and earthquake-generated Rayleigh waves. A classic two step inversion scheme is used where first maps of phase velocities at different periods are derived, and then a 1D transdimensional Bayesian method is applied to determine the VSV-depth structure. We assess the question of what compensates for the unusual high Scandes mountains and aim to identify the different tectonic domains of the adjacent continental lithosphere (Baltic Shield).While the southern Scandes lacks a pronounced crustal root, we observe a crustal root below the northern Scandes that is decreasing towards the central Scandes. A ∼10 km thick high-density lower crustal layer is present below the northern Scandes and generally thickening to the east below the Baltic Shield. The lithosphere-asthenosphere boundary (LAB) below the Scandes is deepening as well from west to east with a sharp step and a strong VSV decrease with depth of 9% in the north and of 5.5% in the south. The LAB of the thinner lithosphere is at 150 km depth in the north and varies from 90 to 120 km depth in the south. Both LAB steps coincide with the mountain front. The central area shows rather smoothly varying structures (170 km LAB depth, −4% VSV with depth) towards the east and no clear spatial match with the front. We infer therefore distinct uplift mechanisms along the Scandes. The southern Scandes might sustain their topography due to dynamic support from the mantle, while the northern Scandes experience both crustal and mantle lithosphere isostasy. In both cases, we suspect a dynamic support from small-scale edge-driven convection that developed at the sharp lithospheric steps.Beneath the Archean Karelia craton in northern Finland, we find low-velocity areas below 150 km depth while a 250 km deep lithospheric keel is imaged below the Paleoproterozic southern Finland. The Norrbotten craton in northern Sweden can be identified at mantle depths as a unit different from the Karelia craton, Scandes and Paleoproterozic central Sweden.

U-Pb (SHRIMP-RG) age of zircon from rare-metal (Li, Cs) pegmatites of the Okhmylk deposit of the Kolmozero-Voron’ya greenstone belt (northeast of the Fennoscandian shield)

The results of isotopic and geochronological study of zircon from rare-metal pegmatites of the Okhmylk deposit are presented. There were no reliable data on the age of lepidolite-spodumene-pollucite pegmatites of this and the other deposits spatially located within the Archean Kolmozero-Voron’ya greenstone belt. The earlier estimates of the pegmatite age indicate a broad time range from 2.7 to 1.8 Ga. Zircon in the studied pegmatites is characterized by inner heterogeneity, where core and rim zones are distinguished. Minor changes are observed in the core zones, they have a spotted structure and contain numerous uranium oxide inclusions. According to X-ray diffraction analysis, zircon crystallinity is preserved completely in these areas. Complete recrystallization with modification of the original U-Pb isotopic system occurred in the zircon rims. New U-Pb (zircon) isotopic and geochronological data of 2607±9 Ma reflect the time of crystallization of pegmatite veins in the Okhmylk deposit. Isotopic data with ages of ~1.7-1.6 Ga indicate later hydrothermal alteration. The obtained results testify to the Neo-Archean age of the formation of the Okhmylk deposit 2.65-2.60 Ga, reflecting the global age of pegmatite formation and associated the world's largest rare-metal pegmatite deposits.

Litsa uranium ore occurrence (Arctic zone of the Fennoscandian Shield): new results of petrophysical and geochemical studies

Mineralogical, petrophysical and geochemical studies have been carried out to determine the sequence and formation conditions of uranium mineralization within the Litsa ore occurrence (Kola Region). Mineralogical studies show the following formation sequence of ore minerals: uraninite – sulfides – uranophane, coffinite, pitchblende. Two stages of uranium mineralization are distinguished: Th-U (1.85-1.75 Ga) and U (400-300 Ma). The distribution of physical properties of rocks in the area is consistent with the presence of two temporal stages in the formation of mineralization with different distribution and form of uranium occurrence in rocks. The factors that reduce rock anisotropy are the processes of migmatization and hydrothermal ore mineralization, which heal pores and cracks. Fluid inclusions in quartz studied by microthermometry and Raman spectroscopy contain gas, gas-liquid and aqueous inclusions of different salinity (1.7-18.4 wt.% NaCl-eq.). According to homogenization temperatures of inclusions in liquid phase, the temperature of the Paleoproterozoic and Paleozoic stages of uranium mineralization at the Litsa ore occurrence is ~ 300 and 200 °С, respectively. Correlations of the spatial distribution of elastic anisotropy index with an elevated radioactive background allow using this petrophysical feature as one of the prognostic criteria for uranium and complex uranium mineralization when carrying out uranium predictive work.

Seismic reflectivity, fracturing and stress field data from the FFC-1 exploratory geothermal project in SW Skåne, Sweden

Enhanced geothermal systems (EGS) are a potential heat source in many parts of the world, even in locations where the temperature gradient is relatively low. We present here an integrated study of reflection seismic data, borehole logs and seismicity analysis performed in conjunction with a geothermal exploratory project operated by E.ON in Malmö, Sweden. In 2020, the pre-existing 2.1 km deep FFC-1 borehole through the sedimentary cover was deepened into the crystalline basement to about 3.1 km vertical depth. Combined interpretation of the reflection seismic data and geophysical wireline logs show that most of the reflectivity in the Precambrian basement is likely generated by lenses of mafic amphibolite embedded in a felsic gneissic matrix. The general structural bedding and foliation is gently dipping to sub-horizontal, similar to other locations in southwest Sweden. Fracture frequency is relatively high in the crystalline rock mass, with heavy fracturing in the uppermost part of the crystalline basement, obscuring a clear reflection from the top of the Precambrian. Highly fractured and hydraulically conductive intervals are also found between 2,562 and 2,695 m based on a temperature drop and the interpretation of the geophysical data. Open fractures, both natural and induced, have a clear N–S orientation, contrasting with the expected NW–SE direction based on the orientation of the Sorgenfrei-Tornquist Zone and earthquake fault plane solutions to the north. This difference may be partly explained by local variations in the stress field near the FFC-1 borehole and vairations in the stress field with depth. Despite this, the data from the FFC-1 well provide novel and unique information on the complex physical state of the crystalline basement on the margin of the Fennoscandian Shield, which further addresses the need for obtaining in-situ stress data to fully understand the local stress field prior to any stimulation. A temperature of 84°C measured at 3 km depth indicates that a desired EGS temperature of 120–140°C may be reached at 5–6 km depth, assuming a temperature gradient of about 20°C. If the relatively high fracture frequency and occurrence of fracture zones down to 3.1 km are also present at these target depths, then the FFC-1 location may be suitable for heat extraction if the rock mass is properly characterized before stimulation.

Target-scale prospectivity modeling for gold mineralization within the Rajapalot Au-Co project area in northern Fennoscandian Shield, Finland. Part 1: Application of knowledge-driven- and machine learning-based-hybrid- expert systems for exploration targeting and addressing model-based uncertainties

This two-part paper reports results of data integration using different machine learning methods for target-scale prospectivity modeling of gold mineralization. The study area is the Rajapalot project area in the Northern Fennoscandian Shield in Finland. In this Part 1 paper, prospectivity modeling was implemented using knowledge-driven- and hybrid- expert systems. Prospectivity analysis-related systemic and model uncertainties were addressed using statistical and machine-learning based methods.The knowledge-driven Fuzzy Inference Systems (FISs) were optimized using Monte Carlo Simulations (MCS) to address and quantify mathematical model-related uncertainties and consequently present the prospectivity values at varying confidence levels. In the FIS results, as the confidence level increases, the high-prospectivity zones mapped by the models are spatially reduced and the mineralization trend becomes progressively well-defined. The resulting prospectivity map has 80% capture efficiency. The supplementary prospectivity map reporting prospectivity values at 90% confidence level has enhanced area-reduction capability. Machine-learning based Adaptive Neuro Fuzzy Inference System (ANFIS) was optimized under the neuro-fuzzy environment. This hybrid ANFIS-based result has 88% capture efficiency.Three exploration targets identified by the prospectivity maps together comprise about 1.5 km2 of the ∼20 km2 study area (i.e., 7.5% of the study area). This indicates considerable search-space reduction by the modeling results for follow-up ground exploration. Hence demonstrating that the prospectivity mapping methods, which are primarily ‘predictive tools’ at the regional scale, can be harnessed to serve as an aid to detection technologies at the target scale. Implementation of knowledge-driven and hybrid approaches deliver results that can (i) incorporate human cognition into prospectivity modeling, (ii) reduce bias of conceptual model, and (iii) are not overfitted to the training dataset. Statistical methods such as hypothesis testing and MCS, and machine learning methods such as ANFIS are therefore collectively beneficial for addressing both systemic and fuzzy systems-related modeling uncertainties.

Target-scale prospectivity modeling for gold mineralization within the Rajapalot Au-Co project area in northern Fennoscandian Shield, Finland. Part 2: Application of self-organizing maps and artificial neural networks for exploration targeting

This paper is part of a two-publication series reporting target-scale prospectivity modeling results for gold enrichment in the Rajapalot project area. The study area is located in the Northern Fennoscandian Shield in Finland and consists of high-grade epigenetic-hydrothermal Au-Co prospects. The first publication of this series described the implementation of knowledge-driven- and hybrid- expert systems. In this second paper we demonstrate the application of machine learning methods such as unsupervised self-organizing maps (SOM) and K-means clustering and supervised artificial neural networks (ANNs). The results from SOM allowed the mapping of deposit-related geological patterns in the evidential layers. K-means clustering of the SOM results identified data clusters favorable for gold enrichment. Quantitative prospectivity values were computed using an ANN model trained on the SOM codebook vectors. This improved the resolution of the prospective-exploration areas mapped within the geospatial domains of the clusters, and reduced the exploration search areas. The ANN result has a significant area under curve value of 0.869 in the receiver operating characteristic plots, confirming the predictive ability of the model. Results identify new areas for exploration, as well as help prioritise drilling targets around the currently less explored prospects. In this second paper we therefore conclude that (1) SOM is an effective method for mapping and visualization of patterns (in the evidential layers) related to mineral deposits at the target scale, and (2) supervised classification of the SOM results, such as using an ANN, can help define new target areas for drilling around the existing prospects. Finally, this two-publication series collectively demonstrates that advanced machine-learning based pattern recognition and prospectivity modeling routines can be used for exploration targeting at camp- and mine- scales. The knowledge-driven methods are efficient at modeling mineralization processes and the data-driven methods are effective at identifying geological attributes related to mineral deposits and occurrences.

Genesis of Convolutions in Lacustrine Complexes in Regions with Comparatively Low (Baltic Shield) and High (Tien Shan) Paleoseismic Activity

Abstract —Structural and lithological comparison of intraformational plicative liquefaction deformations (convolutions) in Late Quaternary lacustrine complexes of the Baltic Shield (sediments of small glacial lakes) and the Tien Shan (sediments of a large stationary basin in the Issyk-Kul depression) is carried out in order to clarify their genesis. These regions have sharply different levels of seismotectonic activity, which makes it possible to most fully determine the role of a seismic factor in the formation of bedding disturbances. Convolutions in the Baltic Shield are represented by load casts, flame structures, pseudonodules, and regular folds with more pronounced anticlinal bends. It is shown that convolutions occur here only in the most liquefied soils (siltstones and fine-grained sands) with low cohesion and fluid permeability in the case of unstable (increased density and coarsening of the sediment from bottom to top) or neutral (homogeneous composition) stratification of weakly consolidated sediments, and the ratio of the widths of the synclinal and anticlinal parts of the folds (Ksyn) for these structures is 1.0–7.5. These signs point to the formation of disturbances because of a spontaneous instability or an instability initiated by weak mechanical influences in the sedimentary stratum under conditions of a normal vertical gradient of the sediment viscosity. Deformations in the second region are structures typical of the Baltic Shield, columnar structures of deep interpenetration of layers, diapirs, and clastic dikes. There are also flexural folds with Ksyn ≈ 1, which formed at a higher degree of sediment consolidation than the actual liquefaction structures. The convolutions of the Issyk-Kul depression are developed not only in sand–silty sediments but also in relatively lowly liquefied soils (clay, gravel, and pebbles). These structures formed under both unstable and stable density stratification of the sediment (for example, clay-on-sand), and Ksyn = 0.3–2.5. Thus, the formation of convolute structures in the Issyk-Kul depression is impossible without strong dynamic impacts on stratified sediments during their accumulation. The results obtained make it possible to substantiate the predominantly diagenetic (convective and landslide) genesis of convolutions in the lacustrine complexes of the Baltic Shield and the seismogenic genesis in the limnogenic deposits of the Tien Shan. A number of new criteria for identifying seismites are proposed, which determines the practical significance of the study.

Multiple Sulfur Isotope Compositions in Mesoarchean Sulfide Deposits of the Karelian Craton: Implications for Determining the Sulfur Source, Biogeochemical Processes, and Deposit Genesis

Abstract —In the present paper we demonstrate that most sulfides of the studied deposits of the Archean Sumozero–Kenozero greenstone belt within the Karelian Craton on the Fennoscandian Shield have nonzero Δ33S values. This indicates that proportions of seawater sulfate and elemental sulfur in Mesoarchean, included into the ores and resulting from UV photolysis, are different. Our results show that systematics of sulfur isotopes of sulfides generally reflects the mixing of mass-independently fractionated sulfur reservoirs with positive and negative Δ33S values. Pyrite is depleted in 34S isotope, which was interpreted as evidence for microbial sulfate reduction. Variations in the positive Δ33S anomalies of the Leksa deposit and the general tendency for Δ33S sulfide content to increase with stratigraphic levels in certain boreholes most likely reflect the change in temperature and the fluid mixing throughout the life of the hydrothermal system. The presence of sulfides with strongly negative Δ33S anomalies suggests that atmospheric sulfur and seawater sulfate, rather than volcanic sulfur, were the prevailing source for mineral systems of the studied deposits. The presented data require the Mesoarchean seawater to contain sulfates at least locally.

Stages of Gold Deposits Formation in the Precambrian of the North-Eastern Fennoscandia

Two main stages of formation of gold deposits are identified in the north-eastern part of the Fennoscandian Shield—Neoarchean (2.7–2.6 Ga) and Paleoproterozoic (1.92–1.74 Ga). These were the stages of rapid growth of the continental crust of the Earth and the consolidation of the ancient supercontinents, Kenorland in the Neoarchean, and Fennoscandia (or Columbia) in the Paleoproterozoic. Gold deposits and occurrences, which formed in the Archean (Oleninskoe and Nyalm in the Kolmozero–Voron’ya belt, and the Olenegorsk group of BIF deposits) were later metamorphosed in the Paleoproterozoic, and isotope data for the rocks and minerals of these deposits detect both Archean and Paleoproterozoic events. The Paleoproterozoic stage was the most important for the formation of gold deposits in the region, as at this stage the deposits formed not only in the Proterozoic greenstone belts, but in those Archean belts as well, which were involved in the Svecofennian tectonic processes in the Paleoproterozoic. As it is shown in the example of the deposits in the Central Lapland Paleoproterozoic belt, gold mineralization formed with a series of impulses of hydrothermal activity, and these impulses correlate with the formation of different generations of minor granite intrusions and dykes.

Metamorphism of the Korvatundra Structure of the Lapland–Kola Orogen (Arctic Zone of the Fennoscandian Shield)

AbstractWe study the P–T conditions and age of metamorphic evolution of the rocks that make up the Korvatundra structure in the northeast of the Fennoscandian Shield. The rocks underwent progressive metamorphism of the amphibolite facies at 625–660 ºC and 8.7–8.8 kbar 1945 ± 34 Ma (Sm–Nd data). The pegmatite cutting the metamorphic paragenesis that formed at this stage has an age of 1917 ± 6 Ma (zircon U–Pb data). Metamorphic transformations after 1917 Ma are manifested locally as discrete zones of blastomylonites in the rocks of the northern part and some inner sites of the Korvatundra structure. Both local increases and decreases in temperature and pressure are possible in these zones. The formation of light titanite with an age of 1863 ± 44 Ma marks the next stage of shear strain. Low-temperature alterations (chloritization and silicification) took place in the zones of final deformations 1722 ± 5 Ma (Rb–Sr data). Beginning from 1.94 Ga, the general deformational and metamorphic history of the Korvatundra structure, Lapland Granulite Belt, and Tana Belt confirms the assumption of the formation of a single inverted metamorphic zoning within the Korvatundra structure and the overlying Lapland–Kolvitsa Collision Belt in the Paleoproterozoic. The obtained data supplement the idea of the Paleoproterozoic geodynamic evolution of the Lapland–Kola orogen.

Graphite-bearing ore areas of the Fennoscandian Shield

The article highlights the demand for natural graphite in the global mineral economy and the relevance of super pure graphite deposits. Due to its high electrical conductivity, polarizability and reduced thermal conductivity, graphite is a versatile material for use in high technologies in the 21st century (a key component of highly efficient refractory lining for steel production, anodes with high charge capacity for lithium-ion batteries, source of graphene, etc.). An overview of the graphite-bearing ore areas of the Fennoscandian Shield is given. It is shown that in Norway, Sweden and Finland the complex of prospecting, evaluation and exploration activities is carried out using geophysical techniques, mainly electric exploration methods of natural and induced polarization electric fields of different modifications, and electric logging, with limited use of other types of geophysical research. It is remarked that SEM electron microscopy, XRD and Raman spectroscopy have been used in studies of flake graphite. Graphite deposits in Norway are concentrated in four provinces: Senja, Lofoten-Vesterålen, Holandsfjord, and Bample. Two are depleted (Rendalsvik mine and Jennestad mine) and one is in operation (Skaland mine). In Sweden, a promising graphite ore field, Woxna, stands out. It contains four graphite deposits – Kringel, Gropabo, Mattsmyra, and Mansberg. The Kringel deposit is one of the best explored sites. Woxna Graphite AB, a subsidiary of the international Leading Edge Material Corp, is reviving the development of the Woxna project on the principles of circular economy, with the Kringel graphite deposit as a priority. Graphite is a common mineral in the shale formations of the Svekofennian province in Finland. The Piippumäki ore field is an important graphite deposit. The effect of Raman spectroscopy on the graphite-bearing Rautalampi and Käypsuo shales in the Savo belt (central Finland) is demonstrated. Information is given on graphite occurrences in the Karelian-Kola region. Characteristics of the Ihala graphite-bearing field of the Ladoga mobile region and the Salnetundra graphitebearing zone of the Kola Peninsula are reported.

Expansion of the seismological monitoring network on the Kola Peninsula

In July 2021, the seismological monitoring network of the Kola Branch of the Geophysical Survey of the Russia Academy of Sciences was expanded by installing a new seismic station. It is installed in the southern part of the Kola Peninsula in the village of Umba (station code UMBA). The expansion of the seismological network to the south has improved the monitoring detail of the Kandalaksha earthquake source zone. According to modern concepts, the Kolvitsky and Kandalaksha grabens are potentially the most seismically hazardous object in the eastern part of the Baltic Shield. To select the location of the new seismic station, reconnaissance work was carried out. This work included an assessment of the level of background seismic noise. The article presents an analysis of noise power spectral density for seismic station UMBA. A low level of background noise is shown in comparison with the NHNM and NLNM models. The following equipment is used at the seismic station: Guralp 6T analog seismometer (registration range 30 s - 50 Hz) and seismic signal recorder “Ermak-5”. Seismic data is transmitted in near real time to the Kola Regional Processing Center via mobile Internet channels. The ability to register weak seismic events with a local magnitude of less than 2 in the area of the Kandalaksha seismic zone and accurately determine the coordinates of their epicenters is shown by the example of a weak earthquake.

1.80–1.75 Ga granite suites in the west Troms Basement Complex, northern Norway: Palaeoproterozoic magma emplacement during advancing accretionary orogeny, from field observations

The Ersfjord Granite is part of a suite of c.1.80–1.75 Ga syeno-granites in the West Troms Basement Complex, northern Norway, presumed to belong to the Transscandinavian Igneous Belt (TIB-1) in the Fennoscandian Shield. Previous data suggest the granite formed post-collisional and ascended as a batholith pluton from a source generated by delamination of mafic-intermediate lower crust. We argue that the Ersfjord Granite was emplaced initially (c. 1.80 Ga) as multiple tabular sills in an extensional setting, then as successive melt injections (c. 1.78–1.75 Ga) in an evolving Andean/Cordilleran type accretionary orogen at the waning stages of the Svecofennian orogen. Field observations indicate melt ascent initiated as successive sills (EG-I) into well-foliated Meso/Neoarchaean TTG gneisses. Some sills preserved a magmatic layering, others injected and assimilated the host rock gneisses leaving pendants of mafic gneiss/migmatite residuum in between the granite sills. The first tectonic patches of melts (EG-II) ascended into the middle/upper crust along regional shear zones and injected into ductile imbricate thrust stacks (D1 event) during NE-SW directed crustal shortening and medium grade P-T conditions, using the sills and ancestor migmatite pendants as melt pathways. Then the tabular EG-I and II granite sheets and adjacent gneisses were coaxially folded by upright macro-folds (D2 event) and steep, granitic pegmatite dyke swarms (EG-III) intruded parallel to the fold axial surface and in related D2 thrusts, at low grade metamorphic conditions. The final melt emplacement (D3 event) included granite pegmatite dykes and sills (EG-IV) along subvertical D3 fold limbs and steep strike-slip shear zones. Our provisional extension, and successive advancing accretionary orogenic emplacement model for the Ersfjord Granite may explain ascent of many other TIB-1 magmas in the Fennoscandian Shield.

Heterogeneous lithospheric structure of the Fennoscandian shield: seismic data

A generalization has been obtained based on the data for deep seismic studies on longitudinal DSS profiles (explosion seismology) and seismological evidence found for local and distant earthquakes (earthquake seismology) in the Fennoscandian Shield over the past 40 years. This provided an opportunity to obtaine quantitative data on the horizontal and vertical heterogeneities of the lithosphere. 3D seismological models were first constructed for the individual parts of the Fennoscandian Shield which show that the crystalline crust structure acquires a block hierarchy pattern, with no persistent seismic boundaries found within the entire shield. The upper crust is characterized by the local occurrence of both waveguides and high-velocity zones correlated with geological bodies. The structural plans of the velocity isolines of various deep sections were found to be inconsistent. It has been proved that major geotectonic provinces have a thick crust and that the Early Proterozoic volcanogenic belts are characterized by elevated velocities and a large crustal thickness. In the upper mantle of the Fennoscandian Shield there were identified the layers with anomalously high velocities. No robust evidence was found for a waveguide, which is sometimes related to the concept of the lithosphere.

Large-scale, flat-lying mafic intrusions in the Baltican crust and their influence on basement deformation during the Caledonian orogeny

Abstract The Fennoscandian Shield in central Sweden displays a complex structural and compositional architecture that is mainly related to the Proterozoic history of the Baltica paleocontinent. In its western parts, the Precambrian basement is covered by the allochthonous rocks of the Caledonide orogen, and direct information about the underlying crust is restricted to a few unevenly distributed basement windows in western Sweden and Norway. In this study, we use preliminary results from the second borehole of the Collisional Orogeny in the Scandinavian Caledonides project (COSC-2), new gravity data, forward gravity, and magnetic modeling and interpretation of seismic reflection profiles to assess the 3-D architecture of the basement. Our results reveal a wide (~100 km) and dense network of mainly flat-lying and saucer-shaped dolerites intruding the volcanic and granitic upper crustal rocks of the Transscandinavian Igneous Belt. Similar intrusion geometries related to 1.2 Ga dolerites can be recognized in the Fennoscandian Shield. We discuss that the formation of these sill complexes occurred in a lithologically and structurally heterogeneous crust during transtension, which is in disagreement with the current understanding of sill emplacement that involves crustal shortening, layering, or anisotropy of the host rock. Our seismic interpretation and the structural observations from the COSC-2 drilling show that part of the Caledonian-related basement deformation was localized along the margins of the dolerite sheets. We propose that the dolerite intrusion geometry, akin to a flat-ramp geometry, guided the basement deformation during the Caledonian orogeny.

Two High-Pressure Metamorphic Events in Early Precambrian Eclogites of the Gridino Area, Belomorian Province of the Fennoscandian Shield: Petrology and Geochronology

Two High-Pressure Metamorphic Events in Early Precambrian Eclogites of the Gridino Area, Belomorian Province of the Fennoscandian Shield: Petrology and Geochronology

Fluid Regime of Gneiss Formation in the Meyeri Thrust Zone of the Northern Ladoga Area (South–Eastern Fennoscandian Shield)

Fluid Regime of Gneiss Formation in the Meyeri Thrust Zone of the Northern Ladoga Area (South–Eastern Fennoscandian Shield)

Active anaerobic methane oxidation and sulfur disproportionation in the deep terrestrial subsurface

Microbial life is widespread in the terrestrial subsurface and present down to several kilometers depth, but the energy sources that fuel metabolism in deep oligotrophic and anoxic environments remain unclear. In the deep crystalline bedrock of the Fennoscandian Shield at Olkiluoto, Finland, opposing gradients of abiotic methane and ancient seawater-derived sulfate create a terrestrial sulfate-methane transition zone (SMTZ). We used chemical and isotopic data coupled to genome-resolved metaproteogenomics to demonstrate active life and, for the first time, provide direct evidence of active anaerobic oxidation of methane (AOM) in a deep terrestrial bedrock. Proteins from Methanoperedens (formerly ANME-2d) are readily identifiable despite the low abundance (≤1%) of this genus and confirm the occurrence of AOM. This finding is supported by 13C-depleted dissolved inorganic carbon. Proteins from Desulfocapsaceae and Desulfurivibrionaceae, in addition to 34S-enriched sulfate, suggest that these organisms use inorganic sulfur compounds as both electron donor and acceptor. Zerovalent sulfur in the groundwater may derive from abiotic rock interactions, or from a non-obligate syntrophy with Methanoperedens, potentially linking methane and sulfur cycles in Olkiluoto groundwater. Finally, putative episymbionts from the candidate phyla radiation (CPR) and DPANN archaea represented a significant diversity in the groundwater (26/84 genomes) with roles in sulfur and carbon cycling. Our results highlight AOM and sulfur disproportionation as active metabolisms and show that methane and sulfur fuel microbial activity in the deep terrestrial subsurface.

Tectonic evolution, volcanic features and geochemistry of the Paleoproterozoic Salla belt, northern Fennoscandia: From 2.52 to 2.40 Ga LIP stages to ca. 1.92–1.90 Ga collision

This study adapts the LIP and mantle plume model for magmatic events in the northern Fennoscandian Shield at 2.51–2.49 Ga and 2.45–2.44 Ga, corresponding globally to the Mistassini LIP and Matachewan LIP systems in the Superior craton. We propose a new model for the Salla belt, where the 2.45–2.44 Ga represents the plume arrival, and both radiating swarm and the circumferential stages. Associated 2.45–2.43 Ga layered intrusions and granites form intrusions ringing the plume centre close to the present NNW part of Fennoscandia.In the Salla belt, the subaerial magmatism of the Sumi system (Salla group) initiated at ca. 2.45–2.44 Ga and followed by volcanism of the subaqueous to subaerial Sariola system (Kuusamo group) at ca. 2.43 to 2.40 Ga. A more primitive magmatic stage at 2.43–2.40 indicates a final gravitational collapse of the LIP mantle plume. Calc-alkaline to tholeiitic affinities indicate that volcanism resulted from the content of the core-related fluids in the plume heads. Enriched in LILE elements with negative Nb and Ta anomalies compared to primitive mantle, suggest continental crustal contamination in a within-plate plume-driven rift setting. Volcanic structures indicate lava cooling, gas expansion and escape processes, and vapor differentiation processes and devolatilization of lava, which are suggestive of continental flood basalts and/or pahoehoe-type lava flows.Based on our structural-tectonomagmatic model, shield-wide systems suggest a correlation of the Seidorechka formation in the Imandra–Varzuga Belt and the upper part of the Sumi sequence within the Lekhta and Shombozero Belts with the 2.45–2.44 Ga Salla group Sumi volcanics in the Salla belt. In addition, the volcanics of the Sariola system (Kuusamo group) are correlated with the Vetreny Belt, the Kairala–Noukajärvi–Aapajärvi formations in Kuolajärvi, and the Polisarka formation in the Imandra–Varzuga Belt. The most peripheral part of the Lapland foreland fold and thrust belt (1.92–1.90 Ga) includes the Salla belt, where E–W contraction first led to E-vergent thrusting followed by a thick-skinned tectonic style with reverse faults and long-wavelength folds.

Geochemistry and mineralogy of Paleoproterozoic metasediments in the Imandra-Varzuga Greenstone Belt: Implications for sediment provenance, tectonic settings and weathering intensity at the transition to oxygenated surface environments

The oxygenation of the atmosphere during the Great Oxygenation Event (GOE) at around 2400 Ma triggered an intensification of chemical weathering on Earth’s surface and caused an acidic attack related to the oxidation and dissolution of sulphidic minerals in surface rocks. The oxidation and selective mobilization of specific elements or, vice versa, their retention in weathering crusts during the GOE is well exemplified in the behaviour of redox sensitive elements such as chromium (Cr), uranium (U) and molybdenum (Mo). Intensification in overall silicate weathering rates during the GOE, however, has remained poorly constrained. The Paleoproterozoic volcano-sedimentary succession of the Imandra-Varzuga Greenstone Belt (deposited ca. 2450–2000 million years ago; Ma) on the northern Fennoscandian Shield records the transformation of Earth’s surface conditions from anoxic to oxic. However, the silicate weathering indices of the sediments deposited in the Imandra-Varzuga Basin across the anoxic–oxic transition do not signal a measureable effect of this transition on the weathering regime. Weathering intensity of the Seidorechka Sedimentary Formation and the Umba Sedimentary Formation – deposited prior and after the GOE, respectively – are similarly moderate. A multitude of physical and chemical factors including the sediment protolith, climate, topography and atmospheric pCO2 control overall weathering intensity. Our results suggest that the oxygenation of the atmosphere and the punctuated (sulphuric) acidic weathering did not significantly overwhelm the former controls over the long term. Silicate weathering following the GOE was, either, tempered by removal of weathered materials by the Huronian global glaciations and dropping atmospheric pCO2 levels, or the felsic protolith and higher latitudional position led to locally muted weathering intensities to be recorded in the post-GOE Umba Sedimentary Formation despite increased background weathering rates.