INTEGRATED PETROGRAPHIC AND GEOCHEMICAL ANALYSIS OF A MESOPROTEROZOIC PALEOKARST: DYNAMICS OF SPELEOTHEM FORMATION IN A SEMI-ARID 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.