The ~1.3 Ga Dismal Lakes Group, NWT, Canada contains a laterally extensive paleokarst system that features grikes extending up to 12 m into underlying bedrock and spectacular cave-collapse breccias. Speleothem deposits within the grike system represent carbonate precipitation under both subaerial and subaqueous conditions. Largely subaerial deposits consist of a variety of meniscus and drip cements and inversely graded vadose pisoids. Subaqueous deposits are marked by precipitation of isopachous cement crusts and microsparitic carbonate. Complex interlamination of these fabric types suggests speleothem deposition from a hydrodynamically active system, and the occurrence of detrital quartz throughout deposits requires that the grike system remained in active communication with the terrestrial environment.

C- and O-isotope composition of individual phases uniquely partitions inferred subaerial and subaqueous depositional fabrics. Subaqueous components (δ¹³C -2.0 to -0.0‰; δ¹⁸O -8.0 to -6.0‰) define a field that is isotopically lighter than, yet broadly similar to, host-rock compositions (δ¹³C=0.5‰; δ¹⁸O=-5.5‰). By contrast, subaerial components (δ¹³C -2.5 to -0.5‰; δ¹⁸O -3.5 to -1.5‰) show a strongly covariant isotopic trend (R>0.995) strongly enriched in ¹⁸O with respect to host-rock and subaqueous phases.

Isotopic data is critical to constraining environments of speleothem formation in this open karst system, where continuous re-equilibration with atmospheric CO₂ should favor dissolution rather than precipitation. We envision strongly variable inflow in a semi-arid environment. During periods of low flow, high rates of evaporation resulted in concentration of carbonate alkalinity, evaporation-driven CO₂ degassing, and carbonate precipitation. Episodes of high flow resulted in active transport of fluids to a deep groundwater aquifer where closed-system fractional dissolution of largely dolomitic host rocks brought the solution to carbonate saturation. Vertical expansion of this aquifer and its intersection with a hydrodynamically open grike system resulted in CO₂ degassing and carbonate deposition under subaqueous conditions. Limited C-isotope range recorded in these speleothems indicates <20% contribution from terrestrial organics to the dissolved CO₂ reservoir.