EAGER: Collaborative Research: Development of an isotope-enabled reactive transport tool to simulate carbon transformations in karst environments

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Understanding the processes that control the storage and transformation of carbon near Earth's surface -- the zone encompassing the vegetation canopy, through the soil, to ground water -- is essential for predicting the long-term impacts of climate change on Earth's carbon cycle. Cave minerals are capable of recording these processes because as they grow they record the chemistry of groundwater seeping into the cave through time. Carbon isotope ratios in cave minerals have the potential to provide essential clues as to how Earth's carbon cycle responded to past climate changes. However, though carbon isotope ratios in cave minerals are easily measured, they can be difficult to interpret given the number of factors that influence cave systems. The goal of this research is to use observations of water chemistry variability from the surface through the soil and into a Tennessee cave system to construct and verify a computer model that simulates the transport and reaction of carbon in the Earth's surface. This software can be used to question the environmental factors that are most important for controlling carbon isotopes in cave systems. Through this research, a new and versatile computational capability will emerge to help determine how climate is recorded in cave minerals. This will provide a deeper understanding of carbon cycle response to past climate changes and thus help to make critical predictions for the future. This project is led by two early career female scientists and will involve the training of two new Ph.D. students and at least two undergraduate students.Carbon isotope records of speleothem hold great potential for reconstructing past changes in the Earth's surface response to climate change, such as transient vegetation, soil respiration, carbon stabilization in deep soils, and/or chemical weathering in the epikarst. Yet, because of their inherent complexity, these data often go unpublished and un-interpreted, despite being regularly collected during more common oxygen isotope investigations. By sampling waters from soils and drip sites within a cave on a multi-year timescale, it is possible to observe how seasonal environmental signals are translated and modified along heterogeneous flow paths in the soil and epikarst. However, interpreting time-averaged speleothem records representing climate changes occurring over years to centuries requires integration of observational data and simulation studies that provide a temporal bridge between long and short-term processes. The current research tackles this problem through the development of a carbon isotope enabled reactive transport model that is calibrated and tested in a karst system, providing a novel tool for the investigation and interpretation of speleothem carbon isotope records. This will be accomplished through modification of the reactive transport model CrunchTope to accommodate three-isotope systems to simulate stable and radioactive carbon isotope fractionation and decay within a multicomponent reaction network. Validation of the model for karst settings will be accomplished using data from an ongoing environmental monitoring program at Blue Spring Cave in Tennessee. Once functionality of the CrunchTope model has been demonstrated for the Blue Spring karst environment, this approach can be expanded to investigate carbon isotope systematics and speleothem carbon isotope records from a variety of cave settings and climate regimes. Looking beyond carbon, this model will provide a versatile tool for evaluating the partitioning and resultant distribution of numerous isotope systems in karst settings (e.g. Calcium, Magnesium, Strontium, Uranium), and thus the model will facilitate development and interpretation of a variety of cutting-edge paleoclimate proxy records from cave environments.