Deep Subsurface Research Articles

In situ oxidation of sulfide minerals supports widespread sulfate reducing bacteria in the deep subsurface of the Witwatersrand Basin (South Africa): Insights from multiple sulfur and oxygen isotopes

Dissolved sulfate is a crucial electron acceptor for the subsurface biosphere, particularly for the living microbial ecosystems in the long-isolated (on the order of millions to billions of years) deep subsurface fracture waters in Precambrian cratons, e.g., in the Witwatersrand Basin of the Kaapvaal Craton, South Africa. Aiming to understand the role of sulfate in the sustainability of the subsurface habitat and the spatial extent of the terrestrial subsurface biosphere, we carried out a basin-scale examination on the sources and producing mechanisms of dissolved sulfur (sulfate and sulfide) in the subsurface fracture waters in the Witwatersrand Basin using multiple sulfur isotopes (32,33,34,36S) and oxygen isotopes (16,18O). Dissolved sulfates in 14 fracture water samples collected from 900 to 3,413 meters below the surface at 6 sites show isotopic ranges from −5.3‰ to +19.4‰ for δ34S, −0.40‰ to +0.50‰ for Δ33S, and −1.1‰ to +10.9‰ for δ18O. These isotopic signatures are distinct to those of the sulfate minerals (e.g., the carbonate-associated sulfate in local Transvaal Supergroup dolomite sequences: δ34S = +31.4‰ to +39.2‰; Δ33S = −0.01‰ to +0.16‰), but identical to those of the sulfide minerals in the host rocks. This indicates that the dissolved sulfate in the fracture waters were dominantly generated by in situ oxidation of sulfide minerals at basin scale, although mixing of a small amount of surface sulfate at some sites cannot be completely ruled out. The dissolved sulfates in the less deep and less saline fracture waters are in oxygen isotope disequilibrium with their host waters, a surprising result given that the water residence times are orders of magnitude longer than the time required for oxygen isotope exchange to reach equilibrium. This implies that vigorous in situ sulfate production has occurred after the fracture waters were isolated. In contrast, the dissolved sulfate in the deeper, more saline waters are in apparent oxygen isotope equilibration or close to equilibration with their host waters. This might be attributed to a combined effect of faster oxygen isotope exchange between sulfate and water at higher temperatures, larger extent of sulfate reduction, and/or less efficient sulfate production. The dissolved sulfide in the fracture waters has similar Δ33S values but is 3.0‰ to 26.4‰ lower in δ34S than coexisting sulfate, suggesting that the dissolved sulfide is mostly generated from bacterial sulfate reduction, which is consistent with the widespread existence of sulfate-reducing bacteria down to >3.4 km below surface in the Witwatersrand Basin. Overall, these novel isotopic results demonstrate that geological processes can provide a steady long-term sulfate source in deep fracture fluids by in situ oxidation of sulfide minerals in their host rocks, and thereby a mechanism to sustain the terrestrial subsurface biosphere, even in deep, high-temperature, long-isolated water systems. Thus, sulfate as a terminal electron acceptor is not the limiting factor for the spatial expansion of terrestrial subsurface biosphere.

Ions in the Deep Subsurface of Earth, Mars, and Icy Moons: Their Effects in Combination with Temperature and Pressure on tRNA-Ligand Binding.

The interactions of ligands with nucleic acids are central to numerous reactions in the biological cell. How such reactions are affected by harsh environmental conditions such as low temperatures, high pressures, and high concentrations of destructive ions is still largely unknown. To elucidate the ions’ role in shaping habitability in extraterrestrial environments and the deep subsurface of Earth with respect to fundamental biochemical processes, we investigated the effect of selected salts (MgCl2, MgSO4, and Mg(ClO4)2) and high hydrostatic pressure (relevant for the subsurface of that planet) on the complex formation between tRNA and the ligand ThT. The results show that Mg2+ salts reduce the binding tendency of ThT to tRNA. This effect is largely due to the interaction of ThT with the salt anions, which leads to a strong decrease in the activity of the ligand. However, at mM concentrations, binding is still favored. The ions alter the thermodynamics of binding, rendering complex formation that is more entropy driven. Remarkably, the pressure favors ligand binding regardless of the type of salt. Although the binding constant is reduced, the harsh conditions in the subsurface of Earth, Mars, and icy moons do not necessarily preclude nucleic acid–ligand interactions of the type studied here.

Redefining the Subsurface Biosphere: Characterization of Fungi Isolated From Energy-Limited Marine Deep Subsurface Sediment.

The characterization of metabolically active fungal isolates within the deep marine subsurface will alter current ecosystem models and living biomass estimates that are limited to bacterial and archaeal populations. Although marine fungi have been studied for over fifty years, a detailed description of fungal populations within the deep subsurface is lacking. Fungi possess metabolic pathways capable of utilizing previously considered non-bioavailable energy reserves. Therefore, metabolically active fungi would occupy a unique niche within subsurface ecosystems, with the potential to provide an organic carbon source for heterotrophic prokaryotic populations from the transformation of non-bioavailable energy into substrates, as well as from the fungal necromass itself. These organic carbon sources are not currently being considered in subsurface energy budgets. Sediments from South Pacific Gyre subsurface, one of the most energy-limited environments on Earth, were collected during the Integrated Ocean Drilling Program Expedition 329. Anoxic and oxic sediment slurry enrichments using fresh sediment were used to isolate multiple fungal strains in media types that varied in organic carbon substrates and concentration. Metabolically active and dormant fungal populations were also determined from nucleic acids extracted from in situ cryopreserved South Pacific Gyre sediments. For further characterization of physical growth parameters, two isolates were chosen based on their representation of the whole South Pacific Gyre fungal community. Results from this study show that fungi have adapted to be metabolically active and key community members in South Pacific Gyre sediments and potentially within global biogeochemical cycles.

Potential Activities and Long Lifetimes of Organic Carbon-Degrading Extracellular Enzymes in Deep Subsurface Sediments of the Baltic Sea.

Heterotrophic microorganisms in marine sediments produce extracellular enzymes to hydrolyze organic macromolecules, so their products can be transported inside the cell and used for energy and growth. Therefore, extracellular enzymes may mediate the fate of organic carbon in sediments. The Baltic Sea Basin is a primarily depositional environment with high potential for organic matter preservation. The potential activities of multiple organic carbon-degrading enzymes were measured in samples obtained by the International Ocean Discovery Program Expedition 347 from the Little Belt Strait, Denmark, core M0059C. Potential maximum hydrolysis rates (Vmax) were measured at depths down to 77.9mbsf for the following enzymes: alkaline phosphatase, β-d-xylosidase, β-d-cellobiohydrolase, N-acetyl-β-d-glucosaminidase, β-glucosidase, α-glucosidase, leucyl aminopeptidase, arginyl aminopeptidase, prolyl aminopeptidase, gingipain, and clostripain. Extracellular peptidase activities were detectable at depths shallower than 54.95mbsf, and alkaline phosphatase activity was detectable throughout the core, albeit against a relatively high activity in autoclaved sediments. β-glucosidase activities were detected above 30mbsf; however, activities of other glycosyl hydrolases (β-xylosidase, β-cellobiohydrolase, N-acetyl-β-glucosaminidase, and α-glucosidase) were generally indistinguishable from zero at all depths. These extracellular enzymes appear to be extremely stable: Among all enzymes, a median of 51.3% of enzyme activity was retained after autoclaving for an hour. We show that enzyme turnover times scale with the inverse of community metabolic rates, such that enzyme lifetimes in subsurface sediments, in which metabolic rates are very slow, are likely to be extraordinarily long. A back-of-the-envelope calculation suggests enzyme lifetimes are, at minimum, on the order of 230days, and may be substantially longer. These results lend empirical support to the hypothesis that a population of subsurface microbes persist by using extracellular enzymes to slowly metabolize old, highly degraded organic carbon.

Phylogenetic Diversity and Potential Activity of Bacteria and Fungi in the Deep Subsurface Horizons of an Uranium Deposit

Phylogenetic Diversity and Potential Activity of Bacteria and Fungi in the Deep Subsurface Horizons of an Uranium Deposit

Tracking Microbial Evolution in the Subseafloor Biosphere.

ABSTRACTThe deep marine subsurface constitutes a massive biosphere that hosts a multitude of archaea, bacteria, and viruses across a diversity of habitats. These microbes play key roles in mediating global biogeochemical cycles, and the marine subsurface is thought to have been among the earliest habitats for life on Earth. Yet we have a poor understanding of what forces govern the evolution of subsurface microbes over time. Here, I outline why evolutionary trajectories in the subsurface may be different than those of microbes living on the surface of the planet and describe how we can take advantage of technological advancements to study the evolutionary dynamics of subsurface microbes and their viruses. The sequencing revolution, in tandem with marine infrastructure advancements, promises that we will soon gain a much deeper understanding of how the vast majority of the microbial biosphere changes, adapts, and evolves over time.

Gothic-Arch Calcite from Speleothems of the Bohemian Karst (Czech Republic): Its Occurrence, Microscopic Ultrastructure and Possible Mechanism of Growth

Gothic arch calcite, a specific crystallographic variety of calcite known from some hot springs and tufa streams, has been newly recognized in the Koněprusy Caves. The gothic-arch calcite occurs on the exteriors of exotic coralloid speleothems where it coexists with scalenohedral (dogtooth) spar crystals. The crystals exhibit microscopic ultrastructural features including deeply eroded topography, etch pits, and spiky and ribbon calcite crystallites, pointing to its extensive natural etching. Many gothic-arch calcites originated as late-stage, secondary overgrowths on older, etched dogtooth calcite crystals. Its characteristic outward curvature resulted from the recrystallization of etching-liberated fine carbonate grains and newly formed needle-fiber calcite laths, which were accumulated and bound on the faces and at the bases of corroded crystals. These intimately coexisting destructive and constructive processes of carbonate crystal corrosion and growth were probably mediated by bacteria, fungi, or other microorganisms. Fluid inclusions embedded in calcite crystals point to a vadose setting and temperatures below ~50 °C. This, combined with the wider geological context, indicates that the gothic arch calcite crystals originated only during the late Pleistocene to Holocene epochs, when the cave, initially eroded by hypogene fluids in the deeper subsurface, was uplifted to the subaerial setting and exposed to the meteoric waters seeping from the topographic surface. The radiocarbon analysis shows that gothic-arch calcite crystals are generally older than ~55,000 years, but the surface layers of some crystals still reveal a weak 14C activity, suggesting that microbiologically mediated alterations of the speleothems may have been occurring locally until now.

Lytic archaeal viruses infect abundant primary producers in Earth\u2019s crust

The continental subsurface houses a major portion of life’s abundance and diversity, yet little is known about viruses infecting microbes that reside there. Here, we use a combination of metagenomics and virus-targeted direct-geneFISH (virusFISH) to show that highly abundant carbon-fixing organisms of the uncultivated genus Candidatus Altiarchaeum are frequent targets of previously unrecognized viruses in the deep subsurface. Analysis of CRISPR spacer matches display resistances of Ca. Altiarchaea against eight predicted viral clades, which show genomic relatedness across continents but little similarity to previously identified viruses. Based on metagenomic information, we tag and image a putatively viral genome rich in protospacers using fluorescence microscopy. VirusFISH reveals a lytic lifestyle of the respective virus and challenges previous predictions that lysogeny prevails as the dominant viral lifestyle in the subsurface. CRISPR development over time and imaging of 18 samples from one subsurface ecosystem suggest a sophisticated interplay of viral diversification and adapting CRISPR-mediated resistances of Ca. Altiarchaeum. We conclude that infections of primary producers with lytic viruses followed by cell lysis potentially jump-start heterotrophic carbon cycling in these subsurface ecosystems.

Stemflow Infiltration Hotspots Create Soil Microsites Near Tree Stems in an Unmanaged Mixed Beech Forest

In stemflow, rainfall is collected and channeled to a concentrated soil water input. It can constitute up to 30% of incident precipitation in some ecosystems. However, the size of the zone influenced by stemflow is unclear, and statistically representative measurement of stemflow (on and in between sites) is scarce. Therefore, whether stemflow creates hotspots of infiltration and potential impacts on forest soils remain subject to controversy. In this study, we investigated the areal dimension of infiltrating stemflow fluxes as well as effects on near-stem soils. We measured throughfall, stemflow and soil properties in high-resolution statistical designs on a mixed forest plot in Germany receiving moderate stemflow. From this data, we modeled the spatial distribution of net precipitation infiltration depth on the plot. Furthermore, we examined soil chemical and physical properties around tree stems to test for and assess a stemflow impact. Results show that stemflow infiltration areas are much smaller than typically assumed and constitute strong infiltration hotspots compared to throughfall. This is also mirrored in soil properties, which are significantly altered near stems. Here, accelerated soil formation and enhanced translocation processes indicate increased soil water fluxes due to high inputs. Additionally, altered soil hydraulic properties enable quicker soil water fluxes near stems. Our findings attest that even comparatively low stemflow fractions (of gross precipitation) can generate strong hotspots of water and matter inputs, which are impactful to subsequent hydrological and biogeochemical processes and properties. Trees shape their direct soil environment, thereby establishing pathways of preferential water flow connecting the canopy and the deeper subsurface.

Recoding of stop codons expands the metabolic potential of two novel Asgardarchaeota lineages

Asgardarchaeota have been proposed as the closest living relatives to eukaryotes, and a total of 72 metagenome-assembled genomes (MAGs) representing six primary lineages in this archaeal phylum have thus far been described. These organisms are predicted to be fermentative heterotrophs contributing to carbon cycling in sediment ecosystems. Here, we double the genomic catalogue of Asgardarchaeota by obtaining 71 MAGs from a range of habitats around the globe, including the deep subsurface, brackish shallow lakes, and geothermal spring sediments. Phylogenomic inferences followed by taxonomic rank normalisation confirmed previously established Asgardarchaeota classes and revealed four additional lineages, two of which were consistently recovered as monophyletic classes. We therefore propose the names Candidatus Sifarchaeia class nov. and Ca. Jordarchaeia class nov., derived from the gods Sif and Jord in Norse mythology. Metabolic inference suggests that both classes represent hetero-organotrophic acetogens, which also have the ability to utilise methyl groups such as methylated amines, with acetate as the probable end product in remnants of a methanogen-derived core metabolism. This inferred mode of energy conservation is predicted to be enhanced by genetic code expansions, i.e., stop codon recoding, allowing the incorporation of the rare 21st and 22nd amino acids selenocysteine (Sec) and pyrrolysine (Pyl). We found Sec recoding in Jordarchaeia and all other Asgardarchaeota classes, which likely benefit from increased catalytic activities of Sec-containing enzymes. Pyl recoding, on the other hand, is restricted to Sifarchaeia in the Asgardarchaeota, making it the first reported non-methanogenic archaeal lineage with an inferred complete Pyl machinery, likely providing members of this class with an efficient mechanism for methylamine utilisation. Furthermore, we identified enzymes for the biosynthesis of ester-type lipids, characteristic of bacteria and eukaryotes, in both newly described classes, supporting the hypothesis that mixed ether-ester lipids are a shared feature among Asgardarchaeota.

Biological production of H2 , CH4 and CO2 in the deep subsurface of the Iberian Pyrite Belt.

Most of the terrestrial deep subsurfaces are oligotrophic environments in which some gases, mainly H2 , CH4 and CO2 , play an important role as energy and/or carbon sources. In this work, we assessed their biotic and abiotic origin in samples from subsurface hard-rock cores of the Iberian Pyrite Belt (IPB) at three different depths (414, 497 and 520 m). One set of samples was sterilized (abiotic control) and all samples were incubated under anaerobic conditions. Our results showed that H2 , CH4 and CO2 remained low and constant in the sterilized controls while their levels were 4, 4.1 and 2.5 times higher respectively, in the unsterilized samples compared to the abiotic controls. The δ13 CCH4 -values measured in the samples (range -31.2 to -43.0 ‰) reveals carbon isotopic signatures that are within the range for biological methane production. Possible microorganisms responsible for the biotic production of the gases were assessed by CARD-FISH. The analysis of sequenced genomes of detected microorganisms within the subsurface of the IPB allowed to identify possible metabolic activities involved in H2 (Rhodoplanes, Shewanella and Desulfosporosinus), CH4 (Methanobacteriales) and CO2 production. The obtained results suggest that part of the H2 , CH4 and CO2 detected in the deep subsurface has a biological origin.

Shallow vs. Deep Subsurface Structures of Central Luconia Province, Offshore Malaysia Reveal by Aeromagnetic, Airborne Gravity and Seismic Data

Across the Luconia continental shelf, the nature and structures of the crust are lacking geological understanding and precise characterization. Newly acquired, aeromagnetic, and airborne gravity data were used to assess deep and shallow sub-surface signals within the Central Luconia Province, off the coast of Sarawak, offshore Malaysia. Regional aeromagnetic anomalies appear to primarily reflect deep crustal features while depth (Z) tensors of airborne gravity anomalies evidence shallow subsurface structures. Strike directions of the interpreted structural trend on aeromagnetic and airborne gravity anomalies maps are measured and plotted into rose diagrams to distinguish the structural orientations for all datasets. Signature patterns extracted from the depth profiles were correlated with parallel seismic lines and nearest exploration wells and coincide well with the top of carbonate for Cycle IV/V and structures seen within the Cycle I and II sediments. The orientation of faults/lineaments at shallower depth is dominated by a NW-SE orientation, similar with the faults extracted from two recently published structural maps. Deeper subsurface sections yielded E-W to NWW-SEE dominant directions which were never presented in the published literature. The E-W oriented anomalies are postulated to represent the remnants of the accretion between the Luconia crustal block and southern boundary of the Palawan block. The NW-SE trend follows the same direction as prominent faults in the region. The insight into shallow and deep subsurface structures in Central Luconia Province imaged through airborne gravity and aeromagnetic data should provide guidelines and complementary information for regional structural studies for this area, particularly in combination with detailed seismic interpretation. Further evaluation on the response of Air-FTG® gravity and aeromagnetic could lead to the zonation of potential basement highs and hydrocarbon prospects in this area.

Human kidney stones: a natural record of universal biomineralization.

GeoBioMed - a new transdisciplinary approach that integrates the fields of geology, biology and medicine - reveals that kidney stones composed of calcium-rich minerals precipitate from a continuum of repeated events of crystallization, dissolution and recrystallization that result from the same fundamental natural processes that have governed billions of years of biomineralization on Earth. This contextual change in our understanding of renal stone formation opens fundamentally new avenues of human kidney stone investigation that include analyses of crystalline structure and stratigraphy, diagenetic phase transitions, and paragenetic sequences across broad length scales from hundreds of nanometres to centimetres (five Powers of 10). This paradigm shift has also enabled the development of a new kidney stone classification scheme according to thermodynamic energetics and crystalline architecture. Evidence suggests that ≥50% of the total volume of individual stones have undergone repeated in vivo dissolution and recrystallization. Amorphous calcium phosphate and hydroxyapatite spherules coalesce to form planar concentric zoning and sector zones that indicate disequilibrium precipitation. In addition, calcium oxalate dihydrate and calcium oxalate monohydrate crystal aggregates exhibit high-frequency organic-matter-rich and mineral-rich nanolayering that is orders of magnitude higher than layering observed in analogous coral reef, Roman aqueduct, cave, deep subsurface and hot-spring deposits. This higher frequency nanolayering represents the unique microenvironment of the kidney in which potent crystallization promoters and inhibitors are working in opposition. These GeoBioMed insights identify previously unexplored strategies for development and testing of new clinical therapies for the prevention and treatment of kidney stones.

Carbon isotopic composition of Frutexites in subseafloor ultramafic rocks

Micrometer sized stromatolitic structures called Frutexites are features observed in samples from the deep subsurface, and hot-spring environments. These structures are comprised of fine laminations, columnar morphology, and commonly consist of iron oxides, manganese oxides, and/or carbonates. Although a biological origin is commonly invoked, few reports have shown direct evidence of their association with microbial activity. Here, we report for the first time the occurrence of subsurface manganese-dominated Frutexites preserved within carbonate veins in ultramafic rocks. To determine the biogenicity of these putative biosignatures, we analyzed their chemical and isotopic composition using Raman spectroscopy and secondary ion mass spectroscopy (SIMS). These structures were found to contain macromolecular carbon signal and have a depleted 13C/12C carbon isotopic composition of – 35.4 ± 0.50‰ relative to the entombing carbonate matrix. These observations are consistent with a biological origin for the observed Frutexites structures.

Water column baseline assessment for offshore Carbon Dioxide Capture and Storage (CCS) sites: Analysis of field data from the Goldeneye storage complex area

Monitoring operations at the seafloor above a storage complex area are required to demonstrate that offshore storage of CO2 in the deep subsurface is safe and effective. Within the framework of the STEMM-CCS project, the Goldeneye area was identified as an offshore experimental site for CO2 storage. In this work, the physico-chemical characteristics of the water column at the Goldeneye site were determined by an in-depth analysis of measurements collated from CTD casts, discrete water samples and seafloor landers. The results showed a clear tidal, seasonal and inter-annual variability in the measured parameters within the monitoring period (Oct 2017- May 2019). Variations in pH and pCO2 over a single tidal cycle were in the order of ±0.008 and ±1.5 μatm, respectively. The temporal variability of water column carbonate chemistry parameters was further defined with the aim to provide a solid background for the discrimination from natural changes of potential impacts of CO2 emissions at the potential storage site. Here, we demonstrated how the combination of new and existing methods and technologies could be used for an effective assessment of water column conditions at a potential offshore carbon storage site.

Influence of Agricultural Managed Aquifer Recharge (AgMAR) and Stratigraphic Heterogeneities on Nitrate Reduction in the Deep Subsurface

AbstractAgricultural managed aquifer recharge (AgMAR) is a strategy whereby surface water is used to intentionally flood croplands to recharge underlying aquifers. However, nitrate (NO3−) contamination in agriculturally intensive regions poses a threat to groundwater resources under AgMAR. We use a reactive transport model to understand the effects of AgMAR management strategies (i.e., by varying the frequency, duration between flooding events, and amount of water) on NO3− leaching to groundwater under different stratigraphic configurations and antecedent moisture conditions. We examine the potential of denitrification and nitrogen retention in deep vadose zone sediments (∼15 m) using variable AgMAR application rates on two‐dimensional representations of differently textured soils, soils with discontinuous bands/channels, and with preferential flow paths characteristic of agricultural fields. Simulations indicate finer textured sediments, alone or embedded within/adjacent to high flow regions, are important reducing zones providing conditions needed for denitrification. Simulation results suggest that applying water all‐at‐once rather than in increments transports higher concentrations of NO3− deeper into the profile, which may exacerbate groundwater quality. This transport into deeper depths can be aggravated by wetter antecedent soil moisture conditions. However, applying water all‐at‐once also increases denitrification within the vadose zone by promoting anoxic conditions. We conclude that AgMAR can be designed to enhance denitrification in the subsurface and reduce NO3− leaching to groundwater, while specifically accounting for lithologic heterogeneity, antecedent soil moisture conditions, and depth to the water table. Our findings are potentially relevant to other systems that experience flooding inundation such as floodplains and dedicated recharge basins.

Earth-like Habitable Environments in the Subsurface of Mars.

In Earth's deep continental subsurface, where groundwaters are often isolated for >106 to 109 years, energy released by radionuclides within rock produces oxidants and reductants that drive metabolisms of non-photosynthetic microorganisms. Similar processes could support past and present life in the martian subsurface. Sulfate-reducing microorganisms are common in Earth's deep subsurface, often using hydrogen derived directly from radiolysis of pore water and sulfate derived from oxidation of rock-matrix-hosted sulfides by radiolytically derived oxidants. Radiolysis thus produces redox energy to support a deep biosphere in groundwaters isolated from surface substrate input for millions to billions of years on Earth. Here, we demonstrate that radiolysis by itself could produce sufficient redox energy to sustain a habitable environment in the subsurface of present-day Mars, one in which Earth-like microorganisms could survive wherever groundwater exists. We show that the source localities for many martian meteorites are capable of producing sufficient redox nutrients to sustain up to millions of sulfate-reducing microbial cells per kilogram rock via radiolysis alone, comparable to cell densities observed in many regions of Earth's deep subsurface. Additionally, we calculate variability in supportable sulfate-reducing cell densities between the martian meteorite source regions. Our results demonstrate that martian subsurface groundwaters, where present, would largely be habitable for sulfate-reducing bacteria from a redox energy perspective via radiolysis alone. We present evidence for crustal regions that could support especially high cell densities, including zones with high sulfide concentrations, which could be targeted by future subsurface exploration missions.

Maximum Credible Earthquake Ground Motions with Focus on Site Amplification due to Deep Subsurface

Since an Earthquake Ground Motion (EGM) is amplified from the propagation through the ground, different models are required for each ground type in the seismic design of structures. While the shallow subsurface indicators are used for the classification of ground types, a deep subsurface has a significant impact on the amplification of the EGMs. This study discusses the maximum credible EGMs for seismic design reflecting seismic amplification due to deep subsurface. The design spectra, reflecting the site amplification factor of the target location, are presented by the calculation of the EGMs with the same source and path characteristics and different site amplification factors as recent major Japanese earthquake records have shown, from the perspective of establishing the maximum credible EGMs that may occur in the future at a target site. The present design spectra, which are based on the natural period of a shallow subsurface, are compared with those based on the site amplification factors, considering the effect of deep subsurfaces. Although there are almost no differences in the design spectra with the present design methods according to the surface ground type, the proposed method provides significantly different design spectra for each site amplification factor.

Rock-Hosted Subsurface Biofilms: Mineral Selectivity Drives Hotspots for Intraterrestrial Life.

The continental deep subsurface is likely the largest reservoir of biofilm-based microbial biomass on Earth, but the role of mineral selectivity in regulating its distribution and diversity is unclear. Minerals can produce hotspots for intraterrestrial life by locally enhancing biofilm biomass. Metabolic transformations of minerals by subsurface biofilms may occur widely with the potential to significantly impact subsurface biogeochemical cycles. However, the degree of impact depends upon the amount of biofilm biomass and its relationship to host rock mineralogy, estimates that are currently loosely constrained to non-existent. Here, we use in situ cultivation of biofilms on native rocks and coupled microscopy/spectroscopy to constrain mineral selectivity by biofilms in a deep continental subsurface setting: the Deep Mine Microbial Observatory (DeMMO). Through hotspot analysis and spatial modeling approaches we find that mineral distributions, particularly those putatively metabolized by microbes, indeed drive biofilm distribution at DeMMO, and that bioleaching of pyrite may be a volumetrically important process influencing fluid geochemistry at this site when considered at the kilometer scale. Given the ubiquity of iron-bearing minerals at this site and globally, and the amount of biomass they can support, we posit that rock-hosted biofilms likely contribute significantly to subsurface biogeochemical cycles. As more data becomes available, future efforts to estimate biomass in the continental subsurface should incorporate host rock mineralogy.

Utilizing the towed Transient ElectroMagnetic method (tTEM) for achieving unprecedented near-surface detail in geological mapping

The subsurface in areas affected by the Pleistocene glaciations often reveal very complex architectures and because of this, the near-surface geology is generally difficult to map and model in high detail. A number of geophysical methods focus on the uppermost part of the subsurface and are capable of mapping details, but no single method has hitherto been able to provide the detail, the data density and the resolution required to map the near-surface Quaternary geology in 3D. Driven by the demands for high detail in the uppermost parts of the subsurface related to for instance surface water and groundwater vulnerability assessments and climate-change related projects, a new high-resolution electromagnetic survey method, tTEM, has been developed. We present examples and discuss the method and its applicability in four study areas where data from tTEM surveys has been combined with geological data and knowledge to map near-surface geological features that could not be resolved in 3D using other geophysical methods focusing on the deeper subsurface or methods with a wider data spacing. • A new geophysical method (tTEM) for high-detailed mapping of the uppermost 50–70 m. • The method provides a high data coverage combined with a considerable penetration depth. • New and improved possibilities for modelling complex near-surface geology in high detail. • Especially useful in aquifer mapping, surface water and groundwater vulnerability assessments, climate change adaptation projects, mapping of aggregates, and risk assessments at contaminated sites.