Ancient Microbial Activity in Deep Hydraulically Conductive Fracture Zones within the Forsmark Target Area for Geological Nuclear Waste Disposal, Sweden

Sorption behavior and mechanism of U(VI) on Tamusu clay in the presence of U(VI)-CO3 complexes

The sorption behavior of U(VI) on Tamusu clay sampled from a pre-selected high-level radioactive waste (HLW) disposal site in Inner Mongolia (China) was studied systematically in the U(VI)-CO3 solution at pH 7.8 by batch experiments. The results demonstrated that the distribution coefficients (Kd) decreased with the increasing values of pHinitial, [U(VI)]initial, and ionic strength, but increased with the extended time and the rising temperature. The sorption was a pH-dependent, heterogeneous, spontaneous, and endothermic chemical process, which could be better described by Freundlich isothermal model and pseudo-second-order kinetic model. The presence of humic acid (HA) or fulvic acid (FA) significantly inhibited the U(VI) sorption, due to the enhanced electrostatic repulsion between the negatively charged HA/FA adsorbed on the clay surface and the negative U(VI) species, as well as the well dispersed HA/FA aggregates in solution wrapping the U(VI) species. The FTIR and XPS spectra indicated that the HCO3- groups on the surface of Tamusu clay after hydroxylation and the ‒OH groups in HA/FA were involved in the U(VI) sorption. The results reported here provide valuable insights into the further understanding of U(VI) migration in geological media.

Rock Surface Fungi in Deep Continental Biosphere-Exploration of Microbial Community Formation with Subsurface In Situ Biofilm Trap

Fungi have an important role in nutrient cycling in most ecosystems on Earth, yet their ecology and functionality in deep continental subsurface remain unknown. Here, we report the first observations of active fungal colonization of mica schist in the deep continental biosphere and the ability of deep subsurface fungi to attach to rock surfaces under in situ conditions in groundwater at 500 and 967 m depth in Precambrian bedrock. We present an in situ subsurface biofilm trap, designed to reveal sessile microbial communities on rock surface in deep continental groundwater, using Outokumpu Deep Drill Hole, in eastern Finland, as a test site. The observed fungal phyla in Outokumpu subsurface were Basidiomycota, Ascomycota, and Mortierellomycota. In addition, significant proportion of the community represented unclassified Fungi. Sessile fungal communities on mica schist surfaces differed from the planktic fungal communities. The main bacterial phyla were Firmicutes, Proteobacteria, and Actinobacteriota. Biofilm formation on rock surfaces is a slow process and our results indicate that fungal and bacterial communities dominate the early surface attachment process, when pristine mineral surfaces are exposed to deep subsurface ecosystems. Various fungi showed statistically significant cross-kingdom correlation with both thiosulfate and sulfate reducing bacteria, e.g., SRB2 with fungi Debaryomyces hansenii.

In situ Rb-Sr dating of slickenfibres in deep crystalline basement faults

Establishing temporal constraints of faulting is of importance for tectonic and seismicity reconstructions and predictions. Conventional fault dating techniques commonly use bulk samples of syn-kinematic illite and other K-bearing minerals in fault gouges, which results in mixed ages of repeatedly reactivated faults as well as grain-size dependent age variations. Here we present a new approach to resolve fault reactivation histories by applying high-spatial resolution Rb-Sr dating to fine-grained mineral slickenfibres in faults occurring in Paleoproterozoic crystalline rocks. Slickenfibre illite and/or K-feldspar together with co-genetic calcite and/or albite were targeted with 50 µm laser ablation triple quadrupole inductively coupled plasma mass spectrometry analyses (LA-ICP-MS/MS). The ages obtained disclose slickenfibre growth at several occasions spanning over 1 billion years, from at least 1527 Ma to 349 ± 9 Ma. The timing of these growth phases and the associated structural orientation information of the kinematic indicators on the fracture surfaces are linked to far-field tectonic events, including the Caledonian orogeny. Our approach links faulting to individual regional deformation events by minimizing age mixing through micro-scale analysis of individual grains and narrow crystal zones in common fault mineral assemblages.

Instant Attraction: Clay Authigenesis in Fossil Fungal Biofilms

Clay authigenesis associated with the activity of microorganisms is an important process for biofilm preservation and may provide clues to the formation of biominerals on the ancient Earth. Fossilization of fungal biofilms attached to vesicles or cracks in igneous rock, is characterized by fungal-induced clay mineralization and can be tracked in deep rock and deep time, from late Paleoproterozoic (2.4 Ga), to the present. Here we briefly review the current data on clay mineralization by fossil fungal biofilms from oceanic and continental subsurface igneous rock. The aim of this study was to compare the nature of subsurface fungal clays from different igneous settings to evaluate the importance of host rock and ambient redox conditions for clay speciation related to fossil microorganisms. Our study suggests that the most common type of authigenic clay associated with pristine fossil fungal biofilms in both oxic (basaltic) and anoxic (granitic) settings are montmorillonite-like smectites and confirms a significant role of fungal biofilms in the cycling of elements between host rock, ocean and secondary precipitates. The presence of life in the deep subsurface may thus prove more significant than host rock geochemistry in directing the precipitation of authigenic clays in the igneous crust, the extent of which remains to be fully understood.

A New Frontier for Palaeobiology: Earth's Vast Deep Biosphere

Diverse micro-organisms populate a global deep biosphere hosted by rocks and sediments beneath land and sea, containing more biomass than any other biome except forests. This paper reviews an emerging palaeobiological archive of these dark habitats: microfossils preserved in ancient pores and fractures in the crust. This archive, seemingly dominated by mineralized filaments (although rods and coccoids are also reported), is presently far too sparsely sampled and poorly understood to reveal trends in the abundance, distribution, or diversity of deep life through time. New research is called for to establish the nature and extent of the fossil record of Earth's deep biosphere by combining systematic exploration, rigorous microanalysis, and experimental studies of both microbial preservation and the formation of abiotic pseudofossils within the crust. It is concluded that the fossil record of Earth's largest microbial habitat may still have much to tell us about the history of life, the evolution of biogeochemical cycles, and the search for life on Mars.

Re-Evaluating the Age of Deep Biosphere Fossils in the Lockne Impact Structure

Impact-generated hydrothermal systems have been suggested as favourable environments for deep microbial ecosystems on Earth, and possibly beyond. Fossil evidence from a handful of impact craters worldwide have been used to support this notion. However, as always with mineralized remains of microorganisms in crystalline rock, certain time constraints with respect to the ecosystems and their subsequent fossilization are difficult to obtain. Here we re-evaluate previously described fungal fossils from the Lockne crater (458 Ma), Sweden. Based on in-situ Rb/Sr dating of secondary calcite-albite-feldspar (356.6 ± 6.7 Ma) we conclude that the fungal colonization took place at least 100 Myr after the impact event, thus long after the impact-induced hydrothermal activity ceased. We also present microscale stable isotope data of 13C-enriched calcite suggesting the presence of methanogens contemporary with the fungi. Thus, the Lockne fungi fossils are not, as previously thought, related to the impact event, but nevertheless have colonized fractures that may have been formed or were reactivated by the impact. Instead, the Lockne fossils show similar features as recent findings of ancient microbial remains elsewhere in the fractured Swedish Precambrian basement and may thus represent a more general feature in this scarcely explored habitat than previously known.

Transport of oxygen into granitic rocks: Role of physical and mineralogical heterogeneity

The rock matrix of granites is expected to be an important buffer against the dispersion of contaminants, e.g. radionuclides, and against the ingress of oxygenated glacial meltwater. The influence of matrix heterogeneity on O₂ diffusive transport is assessed here by means of numerical experiments based on a micro-Discrete Fracture Network (micro-DFN) representation of the diffusion-available pore space along with random realisations of idealized biotite grains, to simulate the heterogeneous nature of granitic rocks. A homogeneous-based analytical solution is also presented and used to assess possible deviations of the numerical experiments from the assumption of homogeneity. The analytical solution is also used to test upscaled values of mineral surface area. The numerical experiments show that the matrix behaves as a composite system, with the coexistence of fast and slow diffusive pathways. This behavior is more evident at low Damköhler numbers. Our interpretation of the numerical experiments points out the importance to properly characterise the heterogeneity of the rock matrix.