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

对于预测气候变化对地球碳循环的长期影响，了解控制地球表面附近碳储存和转化的过程--包围植被树冠的区域，通过土壤，到地下水--至关重要。洞穴矿物能够记录这些过程，因为随着它们的生长，它们记录了地下水随着时间的推移渗入洞穴的化学物质。洞穴矿物中的碳同位素比率有可能为地球的碳循环如何响应过去的气候变化提供重要线索。然而，尽管洞穴矿物中的碳同位素比率很容易测量，但考虑到影响洞穴系统的许多因素，它们可能很难解释。这项研究的目标是利用对水化学可变性的观测，从地表、土壤到田纳西州洞穴系统，以构建和验证一个模拟地球表面碳的运输和反应的计算机模型。该软件可以用来质疑在洞穴系统中控制碳同位素最重要的环境因素。通过这项研究，一种新的、多功能的计算能力将出现，以帮助确定如何在洞穴矿物中记录气候。这将有助于更深入地了解碳循环对过去气候变化的反应，从而有助于对未来做出关键预测。该项目由两名职业生涯早期的女科学家领导，将培训两名新的博士生和至少两名本科生。洞穴群落的碳同位素记录在重建过去地球表面对气候变化的响应方面具有巨大的潜力，例如短暂的植被、土壤呼吸、深层土壤中的碳稳定和/或表层岩溶的化学风化。然而，由于其固有的复杂性，这些数据经常没有公布和解释，尽管这些数据是在更常见的氧同位素调查中定期收集的。通过在多年的时间尺度上对洞穴内土壤和滴水点的水进行采样，可以观察到季节性环境信号是如何沿着土壤和表层岩溶中的不同流动路径进行转换和修改的。然而，要解释代表多年至数百年气候变化的时间平均洞穴喷发记录，需要将观测数据和模拟研究结合起来，在长期和短期过程之间建立时间桥梁。目前的研究通过开发碳同位素激活的反应运移模型来解决这一问题，该模型在岩溶系统中进行了校准和测试，为洞穴相碳同位素记录的调查和解释提供了一种新的工具。这将通过修改反应传输模型CrunchTope来实现，以适应三同位素系统，以模拟多组分反应网络中稳定的和放射性的碳同位素分馏和衰变。对岩溶环境模型的验证将使用田纳西州蓝泉洞正在进行的环境监测项目的数据来完成。一旦CrunchTope模型对蓝泉岩溶环境的功能性被证明，这种方法就可以扩展到研究碳同位素系统学和来自各种洞穴环境和气候制度的洞穴碳同位素记录。展望碳以外，该模型将为评估岩溶环境中众多同位素系统(如钙、镁、锶、铀)的分配和结果分布提供一个通用工具，因此该模型将有助于开发和解释来自洞穴环境的各种尖端古气候替代记录。

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.