A REACTIVE TRANSPORT DESCRIPTION OF CARBON TRANSPORT AND TRANSFORMATION IN KARST ENVIRONMENTS

Speleothem carbon isotope records (δ¹³C) hold great potential for reconstructing past changes in Critical Zone response to climate change, such as changes in vegetation, soil respiration, carbon stabilization in deep soils, and/or chemical weathering in the epikarst. However, interpreting time-averaged speleothem records requires integration of observational data and simulation studies that provide a temporal bridge between long and short-term processes. To address this requirement, we have developed a novel reactive transport modeling approach capable of simulating surface – to – cave carbon transformations to determine the resultant isotope signatures in speleothems.

Model development involves modification of the existing isotopic solid solution model implemented in CrunchTope, the newly developed isotope-enabled module of the reactive transport model CrunchFlow, to accommodate the three-isotope carbon system (¹²C, ¹³C, ¹⁴C) with concurrent radioactive decay of ¹⁴C in both fluid and solid phases. To parameterize the model, we use the chemical, isotopic, and mineralogical properties of the soils and host rocks from an ongoing environmental monitoring project in Blue Spring Cave, TN. Model validation is based on data from coordinated measurements of the carbon isotope (δ¹³C and ¹⁴C) signatures of waters and gases moving along different flow paths within and between the soil, epikarst, and cave.

Model results demonstrate a narrow range of solution space in which the carbon isotope signature of speleothems record information from the surface environment, rather than acquiring the compositional signature of the epikarst. This signature is highly depended upon (1) the rate at which fluid moves between the surface and cave, and (2) the extent to which this water is subject to an ‘open’ environment capable of exchange with gaseous CO₂. In this sense, the current modeling approach extends the seminal work of Hendy (1971) from the original box model to a discretized, reactivity + transport framework. Through this process, we aim to produce a tool that can be widely used to question how carbon cycling in karst systems responds to climate change in differing climate regimes and on a variety of temporal scales.