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经常是不完整的，遗失时间的空白被称为不整合。虽然这种差距很常见，但在不同的地方，这种差距通常出现在不同的时间。一个主要的例外是，在当前地质年代开始前不久，以及在著名的寒武纪大爆发中雪莱化石多样化之前不久，多个大陆上出现了异常丰富的此类缺口。摇滚唱片中这种被称为大不整合的全球鸿沟的起源，一段时间以来一直是一个争论的话题。最近提出的一个假说将不整合与大约715-660年和7.4亿-7.35亿年前发生的全球雪球地球冰期期间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)，这一假设的一项检验涉及到被称为温度计时器的矿物。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，这是由于(例如)这些矿物中产生的同位素通过放射性衰变以已知的速度扩散的速度。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，温度计时器的时间分辨率有限。因此，为了区分冰川侵蚀和与板块构造正常运行相关的侵蚀，我们建议不仅研究这一时期的侵蚀时间，还研究这段时间内侵蚀的空间模式，使用从稳定的大陆内部和不稳定的大陆边缘收集的温度计时器。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动区附近的侵蚀，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阐述和测试关于大不整合的环境后果，以及大不整合和寒武纪爆发之间的关系的新问题。除了制作同行评议的出版物和开源软件外，我们的成果还将被纳入专业视频内容，以供更广泛的公众使用，并用于本科课程。新元古代包括了地球系统的一些重大变化，包括生物圈的重大多样化和复杂化，极端冰川时期，以及罗迪尼亚超大陆的解体。初步数据表明，这段时间间隔也出现了一段令人惊讶的强劲的侵蚀挖出时期，大约在几公里左右。如果这种挖掘的范围足够广、规模足够大、时间安排得当，那么它可能是连接地球系统过程的关键环节。最近，Keller等人提出了一个新的观点。(2019)提出了广泛的冰川作用和克拉通折返之间的联系，特别是将新元古代的“雪球地球”冰川与横跨新元古代晚期的广泛不整合现象联系起来。然而，这一提议随后遭到了Flowers等人的质疑。(2020)，他将新元古代晚期的折返作用和大不整合归因于与Rodinia裂解有关的正常构造过程，并提出新元古代冰川作用即使有侵蚀影响也很小。不幸的是，许多以前的热年代学研究，包括Flowers等人的研究，都集中在新元古代断裂切割的区域，而不是真正的构造稳定--需要仅通过时间来区分这两个假说，考虑到热年代学时间-温度(t-T)反演的巨大时间不确定性，这是一个棘手的命题。在这里，我们将提出一个新的热年代学测试，取而代之的是基于构造和冰川机制预测的稳定克拉通内部和不太稳定的构造活动区之间的反差空间折返模式。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，该博士后将在三个机构(利哈伊、达特茅斯和伊利诺伊州)合作，并将支持两个职业生涯早期的个人评价。所有三个合作机构的本科生也将以暑期实习生的身份参与该项目，并接触到研究过程，包括实验规划和成果交流。鉴于公众对《大不一致》的关注程度，我们将与Kindea实验室合作，制作可以整合到本科课程和科普媒体中的内容。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.