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Abstract While it has been known for some time that reducing fluids have bleached red beds adjacent to fault zones and regionally across the Colorado Plateau, the volumes of fluids expelled along faults have never been quantified. We have developed and applied a suite of one-dimensional hydrologic models to test the hypothesis that internally generated, reducing fluids migrated up sub-basin bounding faults across the Paradox Basin and bleached overlying red beds. The internal fluid driving mechanisms included are mechanical compaction, petroleum and natural gas generation, aquathermal expansion of water, and clay dewatering. The model was calibrated using pressure, temperature, porosity, permeability, and vitrinite reflectance data. Model results indicate that sediment compaction was the most important pressure generation mechanism, producing the majority of internal fluids sourced during basin evolution. Peak fluid migration occurred during the Pennsylvanian–Permian (325–300 Ma) and Cretaceous (95–65 Ma) periods, the latter being concurrent with simulated peak oil/gas generation (87–74 Ma), which likely played a role in the bleaching of red beds. Batch geochemical advection models and mass balance calculations were utilized to estimate the volume of bleaching in an idealized reservoir having a thickness (~100 m) and porosity (0.2) corresponding to bleached reservoirs observed in the Paradox Basin. Bleaching volume calculations show that internal fluid driving mechanisms were likely responsible for fault-related alteration observed within the Wingate, Morrison, and Navajo Formations in four localities across the Paradox Basin in the Colorado Plateau, Utah and Colorado, USA. The volume calculation required that 33%–55% of the total basinal fluids, composed of hydrogen-sulfide and paleo-seawater, migrated into an overlying red bed reservoir (0.5 wt% Fe2O3). 

Abstract Deeply fractured rocks of meteorite impact craters are suggested as prime niches for subsurface microbial colonization. Methane can be a product of such microbial communities and seeps of methane from impact craters on Earth are of strong interest as they act as analogs for Mars. Previous studies report signs of ancient microbial methanogenesis in the Devonian Siljan meteorite impact structure in Sweden, but the proportion of microbial methane, metabolic pathways, and potential modern activity remain elusive. In this study, gas composition, hydrochemistry, oil organic geochemistry, and microbial community analyses are reported in 400 m deep fractures of the Siljan impact structure. The results showed a dominantly microbial origin for methane, which was supported by highly negative δ¹³C_CH4and positive δ¹³C_CO2values along with multiply substituted isotopologues (Δ¹³CH₃D) that indicated disequilibrium fractionation due to microbial kinetic isotope effects. The presence of C₂to C₅hydrocarbons suggested a minor thermogenic input in the gas mix. Characterization of the microbial community via 16S rRNA gene amplicon sequencing and real-time PCR indicated a low abundance of several methanogenic archaeal populations, which is common for settings with active methanogenesis. Evidence of oil biodegradation suggested that secondary microbial hydrocarbon utilization was involved in the methanogenesis. Low sulfate and high alkalinity in the groundwaters also suggested a dominantly microbial methane formation driven by infiltration of freshwater that was coupled to sulfate reduction and secondary utilization of early mature thermogenic hydrocarbons. 

Abstract Deep fracture‐hosted fluids of Precambrian bedrock cratons are relatively stagnant over long time spans compared to near‐surface systems. However, episodic events, such as fracture reactivations, transgressions, and deglaciations, may introduce dilute water, replacing, and mixing with the deep continental brines, thereby sparking microbial activity. Secondary minerals that line bedrock fractures serve as important geochemical archives for such episodic events. Here we explore the fracture mineral record of Archean rocks of Western Greenland by analyzing samples from deep boreholes with the aim to trace and characterize episodic paleofluid flow and paleomicrobial activity. A sequence of hydrothermal to low temperature fluid flow events is demonstrated. For the youngest generation, microscale S‐isotope analysis of pyrite reveals substantial³⁴S‐depletion (minimum δ³⁴S:−58‰V‐CDT) compared to fracture‐hosted barite (δ³⁴S:13‰ ± 2‰) and gypsum (δ³⁴S:2.6‰–10.6‰). This suggests the formation of pyrite following S isotope fractionation during microbial sulfate reduction. This metabolism is further indicated by several methyl‐branched fatty acids preserved in calcite. A general discrepancy between calcite and groundwater δ¹⁸O‐values suggests that calcite formed from water different from the presently residing glacial meltwater‐influenced groundwater mix. High spatial resolution U‐Pb carbonate geochronology of the youngest generation of calcite yielded ages for two samples: 64 ± 3, 75 ± 7 Ma (2σ). These ages overlap with tectonic events related to early stages, or prestages, of the opening of the Atlantic and Labrador Seas. This suggests that deep fracture networks in Western Greenland were colonized by microorganisms, such as sulfate reducers, in the course of this extensional event. 

Abstract Groundwater is one of the largest reservoirs of water on Earth but has relatively small fluxes compared to its volume. This behavior is exaggerated at depths below 500 m, where the majority of groundwater exists and where residence times of millions to even a billion years have been documented. However, the extent of interactions between deep groundwater (>500 m) and the rest of the terrestrial water cycle at a global scale are unclear because of challenges in detecting their contributions to streamflow. Here, we use a chloride mass balance approach to quantify the contribution of deep groundwater to global streamflow. Deep groundwater likely contributes <0.1% to global streamflow and is only weakly and sporadically connected to the rest of the water cycle on geological timescales. Despite this weak connection to streamflow, we found that deep groundwaters are important to the global chloride cycle, providing ~7% of the flux of chloride to the ocean. 

Abstract How subsurface microbial life changed at the bottom of the kilometers‐deep (hypo) Critical Zone in response to evolving surface conditions over geologic time is an open question. This study investigates the burial and exhumation, biodegradation, and fluid circulation history of hydrocarbon reservoirs across the Colorado Plateau as a window into the hypo‐Critical Zone. Hydrocarbon reservoirs, in the Paradox and Uinta basins, were deeply buried starting ca. 100 to 60 Ma, reaching temperatures >80–140°C, likely sterilizing microbial communities present since the deposition of sediments. High salinities associated with evaporites may have further limited microbial activity. Upward migration of hydrocarbons from shale source rocks into shallower reservoirs during maximum burial set the stage for microbial re‐introduction by creating organic‐rich “hot spots.” Denudation related to the incision of the Colorado River over the past few million years brought reservoirs closer to the surface under cooler temperatures, enhanced deep meteoric water circulation and flushing of saline fluids, and likely re‐inoculated more permeable sediments up to several km depth. Modern‐ to paleo‐hydrocarbon reservoirs show molecular and isotopic evidence of anaerobic oxidation of hydrocarbons coupled to bacterial sulfate reduction in areas with relatively high SO₄‐fluxes. Anaerobic oil biodegradation rates are high enough to explain the removal of at least some portion of postulated “supergiant oil fields” across the Colorado Plateau by microbial activity over the past several million years. Results from this study help constrain the lower limits of the hypo‐Critical Zone and how it evolved over geologic time, in response to changing geologic, hydrologic, and biologic forcings. 

Abstract Extensive regions of yellow and white (“bleached”) sandstones within the terrestrial Jurassic red bed deposits of the Colorado Plateau reflect widespread interaction with subsurface reduced fluids which resulted in the dissolution of iron‐oxide grain coatings. Reduced fluids such as hydrocarbons, CO₂, and organic acids have been proposed as bleaching agents. In this study, we characterize an altered section of the Slick Rock member of the Jurassic Entrada Sandstone that exposes bleached sandstone with bitumen‐saturated pore spaces. We observe differences in texture, porosity, mineralogy, and geochemistry between red, pink, yellow, and gray facies. In the bleached yellow facies we observe quartz overgrowths, partially dissolved K‐feldspar, calcite cement, fine‐grained illite, TiO₂‐minerals, and pyrite concretions. Clay mineral content is highest at the margins of the bleached section. Fe₂O₃concentrations are reduced up to 3× from the red to gray facies but enriched up to 50× in iron‐oxide concretions. Metals such as Zn, Pb, and rare‐earth elements are significantly enriched in the concretions. Supported by a batch geochemical model, we conclude the interaction of red sandstones with reduced hydrocarbon‐bearing fluids caused iron‐oxide and K‐feldspar dissolution, and precipitation of quartz, calcite, clay, and pyrite. Localized redistribution of iron into concretions can account for most of the iron removed during bleaching. Pyrite and carbonate stable isotopic data suggest the hydrocarbons were sourced from the Pennsylvanian Paradox Formation. Bitumen in pore spaces and pyrite precipitation formed a reductant trap required to produce Cu, U, and V enrichment in all altered facies by younger, oxidized saline brines. 

Fossil groundwaters make up a substantial fraction of the Earth's fresh water and are being targeted for water supply wells at increasing rates. These groundwaters were recharged more than 12,000 years ago, often in climate conditions that were much different from those of today. Because of the long renewal times involved, fossil groundwaters have often been classified as nonrenewable. However, groundwater ages provide little insight into how water levels and fluxes will change as the result of pumping. The relationship between groundwater ages and these outcome-based metrics of renewability is not straightforward. Therefore, whether a groundwater is fossil or not may have little to do with its renewability. The hydraulic response of an aquifer system to pumping is not strongly related to groundwater age. The use of both modern and fossil groundwater can be unsustainable.