Search

The Anthropocene has been framed around humanity's impact on atmospheric, biologic, and near-surface processes, such as land use and vegetation change, greenhouse gas emissions, and the above-ground hydrologic cycle. Groundwater extraction has lowered water tables in many key aquifers but comparatively little attention has been given to the impacts in the deeper subsurface. Here, we show that fluid fluxes from the extraction and injection of fluids associated with oil and gas production and inflow of water into mines likely exceed background flow rates in deep (>500 m) groundwater systems at a global scale. Projected carbon capture and sequestration (CCS), geothermal energy production, and lithium extraction to facilitate the energy transition will require fluid production rates exceeding current oil and co-produced water extraction. Natural analogs and geochemical modeling indicate that subsurface fluid manipulation in the Anthropocene will likely appear in the rock record. The magnitude and importance of these changes are unclear, due to a lack of understanding of how deep subsurface hydrologic and geochemical cycles and associated microbial life interact with the rest of the Earth system.

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.

Groundwater accounts for the majority of accessible freshwater on Earth, provides drinking water for over two billion people, and supplies over 40% of agricultural demands for water. Numerous aquifer systems around the world, in semi-arid regions, are undergoing substantial depletion due to water demands that exceed modern recharge. Many of these aquifers contain ‘fossil’ groundwater (> 12,000 years), generally considered to be non-renewable because it recharged in the geologic past under different climatic conditions. However, others have suggested that groundwater age should not be the sole indicator of sustainability, as age is also a function of aquifer size and travel distance from the recharge area. To better understand what information groundwater age provides about sustainability, susceptibility to contamination, and whether these systems are still responding to past climates, this study uses multiple age tracers of varying timescales and paleoclimate indicators at semi-arid sites in Tucson, Arizona and Saskatchewan, Canada. Additionally, groundwater age data from nine other regional aquifer systems around the world were compiled to further assess modern recharge rates, aquifer response times, and groundwater velocities.In the Tucson Basin, 3H, 85Kr, 39Ar, 14C, and 4He were applied to constrain groundwater age distributions from modern to tens of thousands of years. Effects of groundwater mixing were deconvolved using lumped parameter modeling, and mean groundwater ages are then used in conjunction with noble gas temperatures (NGTs), the first-ever measurements of Kr and Xe isotopes for the region that reconstruct water table depths (WTDs), and stable water isotopes to identify distinct climatic periods. We find a period of enhanced aridity and reduced recharge from 2.9–1.6 ka, after which NGTs cooled and WTDs quickly rebounded from 1.6–0.34 ka, possibly capturing effects from the Little Ice Age. Recharge was somewhat continuous after this point through toda

Author Posting. © American Geophysical Union, 2022. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 49(11), (2022): e2021GL097618, https://doi.org/10.1029/2021GL097618., Krypton-81 dating provides new insights into the timing, mechanisms, and extent of meteoric flushing versus retention of saline fluids in the subsurface in response to changes in geologic and/or climatic forcings over 50 ka to 1.2 Ma year timescales. Remnant Paleozoic seawater-derived brines associated with evaporites in the Paradox Basin, Colorado Plateau, are beyond the 81Kr dating range (>1.2 Ma) and have likely been preserved due to negative fluid buoyancy and low permeability. 81Kr dating of formation waters above the evaporites indicates topographically-driven meteoric recharge and salt dissolution since the Late Pleistocene (0.03–0.8 Ma). Formation waters below the evaporites (up to 3 km depth), in basal aquifers, contain relatively young meteoric water components (0.4–1.1 Ma based on 81Kr) that partially flushed remnant brines and dissolved evaporites. We demonstrate that recent, rapid denudation of the Colorado Plateau (<4–10 Ma) activated deep, basinal-scale flow systems as recorded in 81Kr groundwater age distributions., Funding for this research was provided by the W.M. Keck Foundation, CIFAR Earth 4D Subsurface Science and Exploration program, NSF EAR (#2120733), National Key Research and Development Program of China (Grant No. 2016YFA0302200), and National Natural Science Foundation of China (Grants No. 41727901). McIntosh and Ballentine are fellows of the CIFAR Earth4D Subsurface Science and Exploration program.

Understanding evolution of paleofluid flow through the Earth’s shallow crust is important for water, mineral, and energy resource management, including extraction of subsurface resources and storage of alternative energy and waste products. The migration of water, gas, and life between surface and deep subsurface systems and its consequence by water-rock-gas-microbe interactions can be evaluated by characterizing hydrogeochemical features of fluids (e.g., formation water and natural gas) in sedimentary basins. This study focuses on the Paradox Basin in the Colorado Plateau, which has iconic manifestations of multiple episodes of paleofluid flow, including widespread sandstone bleaching, ore mineralization (e.g., Cu; U; Fe; Mn), and hydrocarbons, CO2, and He accumulation. Based on molecular and isotopic signatures in formation water and natural gas samples, the origin, types, composition, distribution, and residence time of remnant fluids in the Paradox Basin have been evaluated to constrain the hydrochemical and geological histories responsible for paleofluid flow and solute transport. Highly evaporated paleo-seawater derived brines (i.e., connate brines), associated with the Pennsylvanian Paradox Formation evaporites, migrated into adjacent under- and/or over-lying formations through faults by compaction. These H2S- and hydrocarbon-bearing reduced, saline fluids were responsible for much of the sandstone bleaching in overlying Triassic-Cretaceous shallow sediments, forming reduced traps for later Cu and U deposition. Natural gas throughout the basin is primarily thermogenic in origin recording different thermal maturities of gas generation. Microbial methanogenesis may have been inhibited by the deep burial history of the Paradox Basin and abundance of sulfate. Salt dissolution above and below the evaporites, by topographically-driven meteoric recharge, provided a source of more oxic, sulfate-rich shallow brines. Deep meteoric circulation (up to 3 km depth in the l

Understanding the origin, diagenetic history, chemical composition, and migration pathways of saline fluids in sedimentary basins is important for extraction of subsurface resources (e.g., critical elements, such as Li), storage of alternative energy (e.g., H2) and saline produced waters, and long-term sequestration of anthropogenic CO2 and spent nuclear fuel. The presence of confining units, such as evaporites and shale, may impede basinal-scale fluid flow and can be a source of highly saline fluids to adjacent formations. Upward migration of saline fluids may contaminate surface waters or shallow aquifers. Identifying the source of saline fluids (e.g., fluids sourced from specific oil/gas reservoirs) through unique fluid isotopic signatures may help to remediate contamination. This issue is complicated by the fact that many subsurface reservoirs have been altered by extensive fluid injection activities (e.g., water flooding or disposal of produced, saline waters), which may modify original formation water chemistry. This dissertation aims to evaluate Sr isotopes as an adequate tracer for fingerprinting distinct sources of produced waters from overlapping oil/gas fields, influence of fluid surface storage and subsurface injection activities, and cross-formational fluid migration through evaporite confining units, combined with major ion, Br, and water stable isotope chemistry. Furthermore, this dissertation aims to explore the potential of Li production from sedimentary basin brines.To test the utility of Sr isotopes as a tracer of subsurface, saline fluid sources, Sr isotopes (87Sr/86Sr) of formation waters were evaluated from hydrocarbon reservoirs within three major oil/gas producing regions: the Williston, Appalachian, and Permian basins in North America. Based on a non-parametric statistical test, Sr isotope ratios of formation waters from multiple stacked oil and gas reservoirs within each basin have overlapping (i.e., non-unique) values. In regions where Sr