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

合作研究：利用多系统深时热年代学破译新元古代的时空折返模式

• 批准号: 2044800
• 负责人: Christopher Keller
• 金额: $ 34.93万
• 依托单位: Dartmouth College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-07-15 至 2024-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2044800&HistoricalAwards=false
• 关键词: Collaborative; Research; Using; Multisystem; Deep

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.

岩石中记录的地球历史在任何一个地方通常都是不完整的，存在被称为不整合面的缺失时间间隙。虽然这种差距很常见，但通常会在不同时间、不同地点发生。一个主要的例外是，在当前地质纪元开始前不久，以及在著名的寒武纪大爆发中贝类化石多样化之前不久，多个大陆上存在异常丰富的此类间隙。一段时间以来，岩石记录中这种全球性缺口（称为大不整合面）的起源一直是争论的话题。最近提出的假设之一将不整合面与大约 715-660 和 740-7.35 亿年前发生的全球“雪球地球”冰河时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），因此对这一假设的检验之一涉及称为温度计的矿物。这些矿物记录了它们随着时间的推移所经历的过去温度，这是由于（例如）这些矿物中通过放射性衰变以已知速率产生的同位素的扩散速率。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受到限制。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，我们建议使用从稳定的大陆内部和不太稳定的大陆边缘收集的温度计，不仅研究这一时期侵蚀的时间，而且研究侵蚀的空间模式。由于冰川侵蚀假说预测大陆内部稳定地壳会遭受严重侵蚀，而构造假说则预测仅在构造活动区域附近发生侵蚀，因此侵蚀的空间分布将使我们能够确定与大不整合面相关的侵蚀是否是冰川过程、构造过程或两者兼而有之的结果。这些结果将使我们能够提出和测试有关大不整合面的环境后果以及大不整合面与寒武纪大爆发之间关系的新问题。除了制作同行评审的出版物和开源软件之外，我们的研究结果还将被纳入专业视频内容中，旨在供更广泛的公众访问并在本科课程中使用。 新元古代发生了地球系统的许多重大变化，包括生物圈的重大多样化和复杂化、极端冰川作用以及罗迪尼亚超大陆的分裂。初步数据表明，这个时间间隔还经历了一段长达数公里的令人惊讶的强烈侵蚀折返时期。如果这种挖掘的范围足够广泛、规模足够大并且时机正确，那么它可能成为连接地球系统过程的关键环节。最近，凯勒等人。 （2019）提出了广泛的冰川作用和克拉通折返之间的联系，特别将新元古代“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，这一提议随后遭到弗劳尔斯等人的质疑。 (2020)，他将新元古代晚期的折返和大不整合归因于与罗迪尼亚分裂相关的正常构造过程，并提出新元古代冰川作用几乎没有侵蚀影响。不幸的是，许多先前的热年代学研究，包括Flowers等人的研究，都集中在被新元古代断层切割的区域，这些区域在构造上并不真正稳定——需要仅通过时间来区分这两种假设，考虑到热年代学时间-温度（t-T）反演的时间不确定性，这是一个棘手的命题。在这里，我们将提出一种新的热年代学测试，该测试基于稳定的克拉通内部和不太稳定的构造活跃区域之间的构造和冰川机制预测的折返空间模式的对比。 该项目将为第一代博士后研究员提供综合研究经验和专业发展培训，他将在三个机构（里哈伊大学、达特茅斯大学和伊利诺伊州）进行合作，并将支持两名早期职业生涯的 PI。所有三个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，包括实验规划和结果交流。鉴于公众对大不整合现象的兴趣程度，我们将与 Kindea Labs 合作制作可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价值和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

Cryogenian glacial erosion of the central Canadian Shield: The “late” Great Unconformity on thin ice

加拿大地盾中部的低温冰川侵蚀：薄冰上的“晚期”大不整合面

• DOI: 10.1130/g50315.1
• 发表时间: 2022
• 期刊: Geology
• 影响因子: 5.8
• 作者: McDannell, Kalin T.;Keller, C. Brenhin
• 通讯作者: Keller, C. Brenhin