Collaborative Research: Using Multisystem Deep-Time Thermochronology to Decipher Neoproterozoic Exhumation Patterns in Time and Space

记录在岩石中的地球历史在任何一个地方都是不完整的，缺失的时间间隔被称为不整合面。虽然很常见，但这种间隙通常发生在不同的时间和地点。一个主要的例外是，在当前的地质时代开始前不久，在著名的寒武纪大爆发中，贝壳化石的多样化前不久，在多个大陆上出现了不同寻常的大量这样的裂缝。在岩石记录中，这个被称为“大不整合”的全球裂缝的起源一直是争论的话题。最近提出的一种假设将这种不整合与大约715- 6.6亿年前和7.4 - 7.35亿年前全球“雪球地球”冰期的冰川侵蚀联系起来。由于侵蚀降低了地壳表面下岩石的深度（从而降低了温度），对这一假设的一种测试涉及到被称为温度计的矿物。这些矿物记录了它们随着时间的推移所经历的过去的温度，例如，由于放射性衰变以已知的速率在这些矿物中产生同位素的扩散速率。虽然时间本身对侵蚀的原因提供了一些限制，但在如此古老的岩石中，温度计的时间分辨率是有限的。因此，为了区分冰川侵蚀与与板块构造正常作用相关的侵蚀，我们建议不仅研究时间，而且研究这一时期侵蚀的空间格局，使用从稳定大陆内部和不太稳定大陆边缘收集的热时计。由于冰川侵蚀假说预测了大陆内部稳定地壳的大量侵蚀，而构造假说只预测了构造活跃区域附近的侵蚀，因此侵蚀的空间分布将使我们能够确定与大不整合相关的侵蚀是冰川过程、构造过程还是两者兼而有之的结果。这些结果将使我们能够制定和测试关于大不整合对环境的影响的新问题，以及大不整合与寒武纪大爆发之间的关系。除了制作同行评议的出版物和开源软件外，我们的研究结果还将被纳入专业视频内容中，旨在为更广泛的公众提供服务，并用于本科课程。新元古代包含了地球系统的许多重大变化，包括生物圈的主要多样化和复杂化，极端冰川作用的事件，以及超大陆Rodinia的分裂。初步数据表明，在这段时间间隔内，还出现了一段令人惊讶的、长达数公里的侵蚀挖掘活动。如果这种挖掘活动范围足够广泛，规模足够显著，而且时间正确，那么它可能是连接地球系统过程的关键环节。最近，Keller等人（2019）提出了广泛的冰川作用与克拉通挖掘之间的联系，特别是将新元古代的“雪球地球”冰川作用与跨越新元古代晚期的广泛不整合现象联系起来。然而，这一建议随后受到Flowers等人（2020）的质疑，他们将晚新元古代的发掘和大不整合归因于与Rodinia分裂相关的正常构造过程，并提出新元古代的冰川作用几乎没有任何侵蚀影响。不幸的是，许多以前的热年代学研究，包括Flowers等人的研究，都集中在被新元古代断层切割的区域，而不是真正的构造稳定-要求两种假设仅通过时间来区分，这是一个棘手的命题，因为热年代学时间-温度（t-T）反转的时间不确定性很大。在这里，我们将提出一种新的热年代学测试，基于构造和冰川机制在稳定的克拉通内部和不太稳定的构造活跃区域之间预测的挖掘空间格局的对比。该项目将为第一代博士后提供综合研究经验和专业发展培训，他们将在三个机构（利哈伊、达特茅斯和伊利诺伊）合作，并将支持两个早期职业pi。所有三所合作机构的本科生也将作为暑期实习生参与该项目，并接触研究过程，包括实验计划和结果交流。鉴于公众对“大不一致”的兴趣程度，我们将与kinda实验室合作，制作可以整合到本科课程和大众科学媒体中的内容。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。

Earth’s history as recorded in rocks is frequently incomplete in any one place, with gaps of missing time known as unconformities. While common, such gaps typically occur at different times in different places. A major exception is the unusual abundance of such gaps across multiple continents shortly before the start of the current geological Eon, and shortly before the diversification of shelly fossils in the famous Cambrian Explosion. The origin of this global gap in the rock record, known as the Great Unconformity, has been a subject of debate for some time. One of the recently proposed hypotheses links the unconformity to glacial erosion during the global “snowball Earth” ice ages that occurred about 715-660 and 740-735 million years ago. Because erosion decreases the depth (and thus temperature) of rocks beneath the surface of the crust, one test of this hypothesis involves minerals known as thermochronometers. These minerals record the past temperatures that they have experienced over time, due to (for instance) the rate of diffusion of isotopes produced in these minerals at a known rate by radioactive decay. While time alone provides some constraints on the cause of erosion, the time resolution of thermochronometers is limited in rocks this old. Consequently, in order to distinguish glacial erosion from erosion associated with the normal operation of plate tectonics, we propose to study not only the timing but also the spatial pattern of erosion over this time period, using thermochronometers collected from both stable continental interiors and less stable continental margins. Since the glacial erosion hypothesis predicts substantial erosion of stable crust in the interior of the continents, while the tectonic hypothesis predicts erosion only near tectonically active regions, the spatial distribution of erosion will allow us to determine whether erosion associated with the Great Unconformity was the result of glacial processes, tectonic processes, or both. The results will allow us to formulate and test new questions about the environmental consequences of the Great Unconformity, and the relationship between the Great Unconformity and the Cambrian Explosion. In addition to producing peer-reviewed publications and open-source software, our results will be incorporated into professional video content designed to be accessible to the broader public and for use in undergraduate courses. The Neoproterozoic Era encompassed a number of significant changes in Earth’s systems, including major diversification and complexification of the biosphere, episodes of extreme glaciation, and breakup of the supercontinent Rodinia. Preliminary data suggest that this time interval also saw a period of surprisingly robust erosional exhumation on the order of several km. Such exhumation could be a key link in connecting Earth-system processes, if it were widespread enough in extent, significant enough in magnitude, and had the correct timing. Recently, Keller et al. (2019) proposed a link between widespread glaciation and cratonic exhumation, specifically linking Neoproterozoic “Snowball Earth” glaciations to the phenomenon of widespread unconformity spanning the late Neoproterozoic. However, this proposal has subsequently been contested by Flowers et al. (2020), who instead attribute late Neoproterozoic exhumation and the Great Unconformity to normal tectonic processes associated with the breakup of Rodinia, and propose that Neoproterozoic glaciation had little if any erosive impact. Unfortunately, many previous thermochronologic studies, including that of Flowers et al., have focused on regions that were cut by Neoproterozoic faults, not truly tectonically stable — requiring the two hypotheses to be differentiated by timing alone, a tricky proposition given the large time uncertainty of thermochronologic time-temperature (t-T) inversions. Here we will propose a new thermochronologic test, based instead on the contrasting spatial patterns of exhumation predicted by tectonic and glacial mechanisms between stable cratonic interiors and less stable, tectonically active regions. This project will provide integrated research experience and professional-development training for a first-generation- postdoctoral fellow, who will collaborate across three institutions (Lehigh, Dartmouth, and Illinois), and will support two early-career PIs. Undergraduates at all three collaborating institutions will also be engaged in the project as summer interns and receive exposure to the research process, including experimental planning and communication of results. Given the level of public interest in the Great Unconformity, we will collaborate with Kindea Labs to produce content that can be integrated both in undergraduate class lessons and in popular science media.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.