Collaborative Research: Pedogenic fractionation of Fe and Mo isotopes: The role of soil redox status, organic matter and Fe-(oxyhydr)oxide composition

本项目研究了土壤中铁（Fe）和钼（Mo）的天然同位素的行为。铁和钼各同位素的质量差异使它们的行为发生了微小的变化。实验室实验表明，土壤中同位素行为变化的方向和幅度与土壤有机质的存在、土壤中铁矿物的组成和土壤氧状态相关的特定生物地球化学过程有关。然而，这些同位素在土壤中的现场测量非常有限，这种知识差距是将其发展为土壤生物地球化学过程诊断指标的关键障碍，类似于碳、氧和氮同位素的常用方法。例如，铁和钼同位素在确定过去土壤对气候、植被类型和含水量变化的反应方面具有很大的前景。我们的实地测量将在夏威夷群岛自然存在的土壤年龄和气候的良好约束梯度中进行（夏威夷气候年龄矩阵）。夏威夷群岛是土壤研究的理想天然实验室，因为它们极端孤立，所有岛屿都有类似的玄武岩土母物质。在本研究项目中，我们将：(1)建立夏威夷气候-年龄矩阵中Fe和Mo同位素变化的范围；(2)利用先进的分子尺度技术表征铁矿物的组成；(3)建立有机质、Fe矿物和土壤氧化还原状态（水和氧含量）对Fe和Mo同位素变化的相对影响。除了开发关键土壤过程的新同位素探针外，我们的工作还将提供夏威夷气候时代矩阵遗址的铁固相组成的详细知识，这将有利于这些遗址正在进行的生态和地球科学研究。它还将为陆地风化向河流和海洋沉积物流出的铁和钼的基线同位素组成提供约束条件，这对于使用这些同位素解释古代海洋沉积物过去的氧化还原状态至关重要。最后，这项工作将在三年的资助期内培养两名研究生和两名本科生，并每年为三名来自弱势高中的学生提供暑期研究经验。

This project investigates the behavior of the natural isotopes of iron (Fe) and molybdenum (Mo) in soils. Differences in the mass of individual isotopes of Fe and Mo confer small variations in their behavior. Laboratory experiments suggest the direction and magnitude of these variations in isotope behavior in soils are tied to specific biogeochemical processes associated with the presence of soil organic matter, the composition of Fe minerals in the soil, and the soil oxygen status. However, field measurements of these isotopes are extremely limited in soils and this knowledge gap is a key barrier for developing them as diagnostic indicators of soil biogeochemical processes in ways similar to how carbon, oxygen, and nitrogen isotopes are commonly used. For instance, Fe and Mo isotopes hold great promise for determining past responses of soils to changes in climate, vegetation type, and water content. Our field measurements will be made across well-constrained gradients in soil age and climate that naturally exist across the Hawaiian Islands (Hawaiian Climate-Age Matrix). The Hawaiian Islands are an ideal natural laboratory for soil research because of their extreme isolation and a similar basaltic soil parent material across all the islands. In this research project we will: (1) establish the range of Fe and Mo isotopic variation across the Hawaiian Climate-Age Matrix; (2) characterize the composition of Fe minerals using advanced molecular-scale techniques; and (3) establish the relative influence of organic matter, Fe minerals and soil redox status (water and oxygen content) on Fe and Mo isotopic variation. In addition to developing a new isotopic probe of key soil processes, our work will provide detailed knowledge of the Fe solid-phase composition of the Hawaiian Climate-Age Matrix sites, which will benefit ongoing ecological and earth science research at those sites. It will also provide constraints on the baseline isotopic composition of Fe and Mo efflux from terrestrial weathering to rivers and oceanic sediments, which is critical for the use of these isotopes to interpret the past redox status of ancient marine sediments. Lastly, this work will train two graduate students and two undergraduate students over the three-year grant period as well as provide summer research experiences for three students each year from disadvantaged high schools.