The δ13C evolution of cave drip water along discreet flow paths in a central Texas cave: Quantifying kinetic isotope fractionation factors

Abstract:

Gaps remain in our understanding of in-cave processes that influence cave water chemistry during speleothem formation. Quantifying environmental controls on the isotopic and chemical evolution of karst groundwater would improve the accuracy of speleothem-based paleoclimate reconstructions. In this study, drip water chemical evolution along flow paths was sampled monthly at two locations in Inner Space Cavern, Texas, over a period of 8 months. In each of the two locations, cave water drips off a stalactite, flows along a flowstone and subsequently drips off a lower stalactite, allowing cave water to be sampled at two points, 1-2 meters apart, along each flow path. The chemical and isotopic evolution of drip water along its flow path shows seasonality, where 1) summer months (high cave-air pCO₂) have small variations in δ¹³C values along the flow paths, high and relatively invariant DIC and Ca values,; and 2) winter months (low cave-air pCO₂) generally have large increases in DIC δ¹³C values along the flow paths, lower DIC and Ca values.

The magnitude of the increase in DIC δ¹³C values along the flow paths, <~1‰ to ~4‰, is controlled by the extent of DIC loss to CO₂ degassing and calcite precipitation which is controlled by the pCO₂ gradient between drip water and cave air. If the DIC loss is less than 15%, then the evolution of the δ¹³C value of the DIC reservoir can be modelled using a Rayleigh distillation model and equilibrium fractionation factors between (CO_2(g)-HCO₃^-_(aq)) and (CaCO₃-HCO₃^-_(aq)). As the loss of the DIC reservoir increases above 15% the DIC δ¹³C values become progressively higher such that the ε (CO_2(g)-HCO₃^-_(aq)) values needed to model the observed results change from equilibrium values of ~8‰ to non-equilibrium values up to ~25‰. The variance in magnitude of carbon isotope fractionation during CO₂ degassing cannot be attributed to changes in temperature, and thus we infer significant kinetic isotope effects at higher rates of DIC loss.