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

了解控制地球表面附近碳的储存和转化的过程——包括植被冠层，通过土壤，到地下水的区域——对于预测气候变化对地球碳循环的长期影响至关重要。洞穴矿物能够记录这些过程，因为随着它们的生长，它们记录了随着时间推移渗入洞穴的地下水的化学成分。洞穴矿物中的碳同位素比率有可能为了解地球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因素很多，它们很难解释。这项研究的目的是利用对水化学变化的观察，从地表到土壤，再到田纳西州的一个洞穴系统，建立并验证一个计算机模型，模拟碳在地球表面的运输和反应。这个软件可以用来质疑对控制洞穴系统中的碳同位素最重要的环境因素。通过这项研究，一种新的、通用的计算能力将出现，以帮助确定如何在洞穴矿物中记录气候。这将使我们更深入地了解碳循环对过去气候变化的响应，从而有助于对未来做出关键的预测。该项目由两名早期职业女性科学家领导，将培养两名新的博士生和至少两名本科生。洞穴层的碳同位素记录在重建过去地球表面对气候变化响应的变化方面具有很大的潜力，例如瞬态植被、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，尽管在更常见的氧同位素研究中定期收集这些数据，但这些数据往往未被发表和解释。通过在多年时间尺度上从洞穴内的土壤和滴落点取样，可以观察到季节性环境信号是如何沿着土壤和表层喀斯特的非均匀流动路径被转换和修改的。然而，解释代表数年至数世纪气候变化的时间平均洞穴记录需要整合观测数据和模拟研究，这些研究在长期和短期过程之间提供了一个时间桥梁。目前的研究通过开发碳同位素反应输运模型解决了这一问题，该模型在喀斯特系统中进行了校准和测试，为洞穴碳同位素记录的调查和解释提供了一种新的工具。这将通过修改反应输运模型CrunchTope来实现，以适应三同位素系统，以模拟多组分反应网络中的稳定和放射性碳同位素分馏和衰变。对喀斯特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。一旦证明了CrunchTope模型在蓝泉喀斯特环境中的功能，该方法就可以扩展到研究各种洞穴环境和气候条件下的碳同位素系统和洞穴碳同位素记录。除了碳之外，该模型将为评估喀斯特环境中众多同位素系统（如钙、镁、锶、铀）的分配和最终分布提供一个通用的工具，因此该模型将有助于开发和解释来自洞穴环境的各种前沿古气候代理记录。

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.