Krypton‐81 Dating Constrains Timing of Deep Groundwater Flow Activation. Issue 11 (10th June 2022) - Kim, Ji‐Hyun
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 81 Kr dating range (>1.2 Ma) and have likely been preserved due to negative fluid buoyancy and low permeability. 81 Kr 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 81 Kr) 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 81 Kr groundwater age distributions. Plain Language Summary: Landscape changes over geological time alter hydraulic gradients and the presence or absence of near‐surface confining units, which drive the evolution of subsurface flow systems. However, our understanding of the time required for groundwater flow systems to respond to geological processes, such as shifts in topography, stratigraphy, and permeability structures, is still limited. This study uses krypton‐81 dating to constrain the age of meteoric waters in the Paradox Basin in the Colorado Plateau and constrain the timing of groundwater recharge into basinal aquifers. We discovered that rapid, widespread erosion and incision in the Colorado Plateau in the last 10 Ma activated deep meteoric circulation, partially flushing residual ancient seawater‐derived brines from aquifers above and below thick, evaporite confining units and dissolving salt. Krypton‐81 dating may provide insights into timescales and drivers of subsurface fluid flow and connectivity with the near‐surface in other environments. Key Points: Meteoric waters up to 3 km in basinal aquifers are <1.1 Ma Recent, rapid denudation of the Colorado Plateau enabled deep circulation of meteoric water and flushing of connate brines Krypton‐81 dating can illuminate the timescales and extent of meteoric circulation in response to geologic and/or climatic forcings