Krypton‐81 Dating Constrains Timing of Deep Groundwater Flow Activation

Abstract

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.

Groundwater Circulation Within the Mountain Block: Combining Flow and Transport Models With Magnetotelluric Observations to Untangle Its Nested Nature

“…(2024) argued that in mountainous terrains of the Paradox Plateau, USA enhanced mountain from recharge occurs where the crystalline basement cropped out. These findings are consistent with a permeable (10 −14 –10 −16 m 2 ) crystalline basement and supported groundwater age dates ( 14 C, 81 Kr) and salinity inversions within lowland basal sedimentary aquifer systems (Kim et al., 2022). Hydrothermal investigations within the desert southwest of the United States suggest MBR fluid circulation depths of 3–6 km (Barroll & Reiter, 1990; Mailloux et al., 1999; Pepin et al., 2014).…”

Burial and Denudation Alter Microbial Life at the Bottom of the Hypo‐Critical Zone

“…Minor to moderate amounts of oil biodegradation are found up to ca. 2 km depth, in contact with circulating meteoric waters (Kim, Ferguson, et al., 2022), while non‐biodegraded oils are found from near the surface to ca. 5 km depth.…”

“…40 ka (Gardner et al., 2020; Noyes et al., 2021), possibly approaching the tens to hundreds of pore volumes required to biodegrade HC reservoirs. Slightly longer residence times of deeper aquifers (e.g., up to 1.2 Ma in Permian Cutler and Pennsylvanian Honaker Trail Fm (Kim, Ferguson, et al., 2022; Tyne et al., 2022)) and fewer pore volume turnovers may explain the general decrease in HC biodegradation extents with depth (Figure 7). However, these estimates of water fluxes required for oil biodegradation (32.5–325 pore volumes) are likely low, as exposed oil reservoirs in the Colorado Plateau are more degraded than the North Sea, and HCs often contain less oxygen than shown in the simple formula for organic matter above.…”

“…We hypothesize that shale source rocks and hydrocarbon (HC) reservoirs in the Colorado Plateau were sterilized during maximum burial and re-inoculated recently during rapid denudation (<4 Ma; Karlstrom et al, 2014;Murray et al, 2019Murray et al, , 2016 that brought reservoirs closer to the surface at cooler temperatures and in contact with circulating meteoric waters (Kim, Bailey, et al, 2022;Kim, Ferguson, et al, 2022) containing microorganisms. The oil in the Colorado Plateau reservoirs could have been partially removed by anaerobic oxidation, coupled to bacterial sulfate reduction (BSR) or other electron acceptors in the subsurface, and aerobic oxidation as reservoirs were breached and exposed by incision.…”

Acceleration of Deep Subsurface Fluid Fluxes in the Anthropocene

“…Solute transport responses typically take place over longer time periods due differences between rates of advection and hydraulic diffusion (Ferguson et al., 2023). Evidence for increases in subsurface paleofluid fluxes, solute transport and microbial activity have been tied to geological events such as continental scale glaciations (McIntosh et al., 2012) or extensive denudation and incision by large rivers (Kim et al., 2022; Li et al., 2023).…”