The impact of CO2-charged fluids on the aqueous geochemistry of terrestrial aquifers

Understanding the formation and evolution of subsurface CO₂-rich terrestrial fluids and reservoirs is key for modelling the storage of CO₂ in the crust, fracture-controlled fluid flow, and/or diffusive transport of fluids from deep crustal settings. However, migration and exchange mechanisms associated with CO₂-rich environments remain poorly constrained. In at least one natural CO₂-rich setting, a Precambrian crystalline brine component has been postulated based on δ¹⁸O and δ²H signatures which plot above the Global Meteoric Water Line. In this study we evaluate the intriguing potential role of migration and mixing of deep fluids in the shallow subsurface using a novel geochemical and microbiological framework approach which incorporates noble gas, stable isotope, and microbial diversity-based approaches. Through targeting a series of 10 springs which sample depths of up to 250 m at Saratoga Springs, NY, USA, this integrated approach finds no evidence of a deep shield brine component and reveals only migration of a principally deep crustally-derived CO₂ (87.2–99.3 %) with a minor mantle component. Instead, this study reveals how putative CO₂ dissolution-enhanced water–rock interaction coupled with ≤17 % CO₂-H₂O ¹⁸O isotopic exchange can produce the observed aqueous geochemical composition, including the apparent shield brine signal. Microbial community data presented here also suggest distinct assemblages between shallow, freshwater and deep, saline spring fluids in line with geochemical interpretations. Crucially, our novel integrated approach highlights how migrating CO₂-rich phases in subsurface environments can overprint and drive geochemical reactions in the subsurface to produce aqueous geochemistries which mimic characteristics of unrelated deep fluid systems.