The Paradox Basin in the Colorado Plateau (USA) has some of the most iconic records of paleofluid flow, including sandstone bleaching and ore mineralization, and hydrocarbon, CO2, and He reservoirs, yet the sources of fluids responsible for these extensive fluid-rock reactions are highly debated. This study, for the first time, characterizes fluids within the basin to constrain the sources and emergent behavior of paleofluid flow resulting in the iconic rock records. Major ion and isotopic (δ18Owater; δDwater; δ18OSO4; δ34SSO4; δ34SH2S; 87Sr/86Sr) signatures of formation waters were used to evaluate the distribution and sources of fluids and water-rock interactions by comparison with the rock record. There are two sources of salinity in basinal fluids: (1) diagenetically altered highly evaporated paleo-seawater-derived brines associated with the Pennsylvanian Paradox Formation evaporites; and (2) dissolution of evaporites by topographically driven meteoric circulation. Fresh to brackish groundwater in the shallow Cretaceous Burro Canyon Formation contains low Cu and high SO4 concentrations and shows oxidation of sulfides by meteoric water, while U concentrations are higher than within other formation waters. Deeper brines in the Pennsylvanian Honaker Trail Formation were derived from evaporated paleo-seawater mixed with meteoric water that oxidized sulfides and dissolved gypsum and have high 87Sr/86Sr indicating interaction with radiogenic siliciclastic minerals. Upward migration of reduced (hydrocarbon- and H2S-bearing) saline fluids from the Pennsylvanian Paradox Formation along faults likely bleached sandstones in shallower sediments and provided a reduced trap for later Cu and U deposition. The distribution of existing fluids in the Paradox Basin provides important constraints to understand the rock record over geological time.

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 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.

Table S1: (ST1). PHREEQC inverse mixing modeling for the Mississippian Leadville Ls brine (Solution 3) assumed to be evolved from a mixture of the meteoric water endmember (Solution 1) and evaporated paleo-seawater endmember (Solution 2); Table S2: (ST2). PHREEQC inverse mixing modeling for the salt anticline brine (Solution 3) assumed to be evolved from a mixture of the meteoric water endmember (Solution 1) and evaporated paleo-seawater endmember (Solution 2).

Constraining the dynamic interface between circulating meteoric waters and deeper more stagnant saline fluids is important for groundwater supplies (

The Critical Zone (CZ), where water, rock and life interact (Brantley et al., 2006), may extend much deeper and contain much older groundwaters than is typically considered (e.g., a few tens of m and millions of years). Microbial communities are found in fluid-rock systems kilometers beneath the surface associated with millionto billion-year-old fluids (Magnabosco et al., 2018;Onstott et al., 2019). How these populations arrived at their current locations is largely unknown. Circulating meteoric waters extend to several kilometers driving fluidrock reactions and are certainly one potential source. Ancient microbial communities deposited with the rocks, or which migrated in previously, are likely to have experienced sterilization during periods of deep burial and

Table S1: (ST1). PHREEQC inverse mixing modeling for the Mississippian Leadville Ls brine (Solution 3) assumed to be evolved from a mixture of the meteoric water endmember (Solution 1) and evaporated paleo-seawater endmember (Solution 2); Table S2: (ST2). PHREEQC inverse mixing modeling for the salt anticline brine (Solution 3) assumed to be evolved from a mixture of the meteoric water endmember (Solution 1) and evaporated paleo-seawater endmember (Solution 2).