CAREER: Linking the stable isotope record of Earth's surface and interior across the Great Oxidation Event

在大约45亿年的历史中，地球大气中的氧含量在两次主要转变中上升。第一次被称为大氧化事件（GOE），发生在大约23 - 24亿年前，当时大气中的氧气含量增加到现在的1-10%。氧含量的上升通过氧化风化的开始和稳定同位素组成的变化在沉积岩记录上留下了不可磨灭的印记。在同一时期，火成岩（或岩浆岩）记录经历了重大的化学和同位素变化。该研究将通过研究一组被称为强过铝花岗岩（SPGs）的独特火成岩的稳定同位素，来调查沉积岩和火成岩记录的变化是否存在联系。当沉积岩变质和部分熔融形成花岗质熔体时，SPGs就形成了。因此，SPGs代表了火成岩记录中纯沉积（通过部分熔融和同化）贡献的末段火成岩档案。通过分析GOE两侧SPGs的同位素，本研究将确定沉积成分的变化是否以及如何影响地球历史上这一关键转变时期的火成岩记录。PI将领导一项综合研究和教育计划，旨在通过多层次的指导制度鼓励代表性不足的少数民族参与和保留以海外外劳为基础的研究。该项目支持加州理工学院的一名女性或少数族裔研究生，3名本科生暑期研究员，3名来自帕萨迪纳联合学区（PUSD）的高中生和教师实习生，以及PUSD中学生每年到当地圣盖博山脉的课堂课程和实地考察。本CAREER项目拟开展的研究将探讨GOE如何通过沉积物的部分熔融或同化作用影响火成岩的O、S和Fe同位素记录。研究考察了太古宙和元古代强过铝花岗岩，它们代表了变质沉积岩的纯部分熔体。这项研究有三个主要目标。首先，它将揭示在整个GOE的火成岩稳定同位素记录中观察到的变化的潜在原因。例如，岩浆锆石中O同位素比值的增加被不同地归因于板块构造的开始和沉积岩的再循环，或归因于氧化的上升，陆上风化作用产生沉积物中更重的O同位素比值。如果后者是正确的，那么应该观察太古宙和元古宙的锆石氧同位素比值的变化。其次，本研究将通过对两个保存有代表性源区变质沉积岩的“焦点”地点以及衍生SPG的详细野外研究，详细探讨铁和S同位素是如何从沉积物向火成岩转移的。第三，SPGs为沉积记录提供了另一种存档方法，因为它们将大量沉积岩均质化，从而可以获得平均沉积成分的信息。重要的是，这项工作将建立一个spg的“参考集”，这些参考集将由加州理工学院管理，并提供给外部研究人员来研究其化学的其他方面。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Oxygen levels in Earth’s atmosphere have risen throughout its ~4.5 billion-year history in two major transitions. The first, known as the Great Oxidation Event (GOE), occurred circa 2.3-2.4 billion years ago when atmospheric oxygen levels increased to 1-10% of present day levels. This rise in oxygen levels left an indelible imprint on the sedimentary rock record through the onset of oxidative weathering and shifts in stable isotope compositions. During this same time interval, the igneous (or magmatic) rock record underwent significant chemical and isotopic changes. The proposed research will investigate whether the changes in the sedimentary and igneous rock records are linked through studying the stable isotopes of a unique suite of igneous rocks known as strongly peraluminous granites (SPGs). SPGs form when sedimentary rocks are metamorphosed and partially melted to form granitic melts. SPGs therefore represent an endmember igneous archive of a pure sedimentary contribution (through partial melting and assimilation) to the igneous rock record. Through analyzing the isotopes of SPGs on either side of the GOE, this study will determine if and how changing sedimentary compositions affected the igneous rock record across this critical transition in Earth’s history. The PI will lead an integrated research and education plan designed to encourage participation and retention of under-represented minorities in OFW-based research through a multi-tiered mentoring system. This project supports one female or minority graduate student at Caltech, 3 undergraduate summer fellows, 3 high-school student and teacher interns from the Pasadena Unified School District (PUSD), and annual classroom lessons and field trips for PUSD middle schoolers to the local San Gabriel Mountains. The proposed research in this CAREER project will explore how the GOE influenced the O, S, and Fe isotope record of igneous rocks through partial melting or assimilation of sediments. The research examines Archean and Proterozoic strongly peraluminous granites, which represent pure partial melts of metasedimentary rocks. This research has three main objectives. First, it will shed light on the underlying causes for shifts observed in the stable isotope record of igneous rocks across the GOE. For example, the increase in O isotope ratios in magmatic zircon has been variably ascribed to the onset of plate tectonics and recycling of sedimentary rocks or to the rise of oxidative, subaerial weathering generating heavier O isotope ratios in sediments. If the latter is true, a shift in the oxygen isotope ratios of zircon from Archean and Proterozoic SPGs should be observed. Second, this research will explore in detail how Fe and S isotopes are transferred from sediments to igneous rocks through detailed field studies of two “focus” localities which preserved representative source metasedimentary rocks, as well as, the derivative SPG. Third, the SPGs provide an alternative archive to the sedimentary record in that they homogenize large volumes of sedimentary rocks and thus can yield information on average sedimentary compositions. Importantly, this work will establish a “reference collection” of SPGs, which will be curated at Caltech and made available to external researchers to investigate other aspects of their chemistry.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.