CAREER: Reading magnetic fingerprints from deep time: An insight into the geodynamo and early Earth system evolution

职业：从深处读取磁性指纹：洞察地球发电机和早期地球系统演化

• 批准号: 1149434
• 负责人: Aleksey Smirnov
• 金额: $ 47万
• 依托单位: Michigan Technological University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-05-01 至 2018-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1149434&HistoricalAwards=false
• 关键词: CAREER; Reading; magnetic; fingerprints; deep

(a) The first four billion years of Earth history was a time of many critical transitions in the Earth system (e.g., the beginning of plate tectonics, oxygenation of atmosphere, and emergence of life). However, many of these processes and links between them remain poorly understood. This project will advance an understanding of our planet's early evolution by obtaining high-quality data on the strength of ancient Earth's magnetic field. It is well established that for at least 500 million years, the field has been sufficiently strong to provide magnetic shielding of the biosphere and atmosphere from solar radiation. This strong field has been generated by convection of molten iron in the outer core (geodynamo), powered by density settling and heat of fusion of the crystallizing inner core. However, field characteristics for earlier times remain largely unknown. The solid inner core could have formed sometime during the Proterozoic eon (2500 to 542 million years ago). Before the inner core formation, the geodynamo could have produced a much weaker and less stable magnetic field. An attendant weaker magnetic shielding would allow solar radiation to affect the life evolution and atmospheric chemistry. Our research will provide high-quality data on the strength of Proterozoic field by investigating the fossil magnetism of several suites of well dated extrusive and intrusive rocks around the globe. This study will result in nearly doubling the size of current paleointensity database for the Proterozoic, including the time periods for which no determinations of the paleofield strength currently exist. These data will provide important insights into early Earth's geodynamo mechanisms and will have important implications in how we use the magnetic field records to decipher the geological history of our planet, including the age of the inner core. Broader implications of the study include a better understanding of the link between evolution of Earth's magnetic field and evolution of biosphere and atmosphere. The project will involve Michigan Tech undergraduate and graduate students, thus training the next generation of scientists. The project will also establish a sustainable framework for linking the academic research to K-12 science education advancement and to scientific outreach in order to increase the public awareness of Earth Science. In particular, unique summer research experiences will be provided for pre-college science teachers. These partnerships will provide a basis for classroom action research to improve student achievement in science. (b) Data on the long-term behavior and configuration of the geomagnetic field during the Precambrian are crucial in understanding the nature of Earth's early geodynamo. These data are also important for investigating potential causative links between the evolution of geomagnetic field and other components of the Earth system. For example, a weak or unstable field of an early geodynamo could result in weaker magnetosphere shielding and, hence, a stronger effect on the atmosphere and biosphere from solar and cosmic radiation. In addition, long-term trends in the strength and stability of geomagnetic field may provide insight into the timing of some important transitions in the Earth's interior, such as the formation and growth of the solid inner core. In the absence of strict theoretical constraints, paleomagnetic data become a principal source of information about the Precambrian field. However, our knowledge of the field characteristics during the first four billion years of Earth history remains very limited. In particular, the database on the field strength contains only a handful of reliable data points. In our project, we will investigate the strength of the Proterozoic geodynamo by detailed paleomagnetic analyses of several reliably dated, globally distributed basaltic sequences and mafic dike swarms that have been previously shown to contain pristine paleomagnetic records. Our proposed targets are the ~1.1 Ga rocks of the Mid-Continent Rift System (USA and Canada), the 615 Ma Long Range dike swarm (Canada), the 755 Ma Mundine Well Dike, 1070 Ma Warakurna dolerite sills and dikes, and the 1210 Ma Marnda Moorn (Western Australia), the 810 Ma Xiaofeng and 1765 Ma Taihang dike swarms (China), and the ~925 Ma Salvador-Olivença, ~1.5 Ga Curaçá, and ~2.6 Ga Uauá dike swarms (Brasil). In this study, we will use conventional and novel experimental and interpretative approaches to contribute to a synoptic view of the Precambrian geodynamo. This project will significantly increase the number of high-quality paleointensity determinations for the Precambrian, including the time periods for which no determinations of paleofield strength currently exist (for example, for the Neoproterozoic). The outcomes of the proposed research will be integrated into an Earth-system-wide timeline of Neoarchean-Proterozoic geological history, possibly transforming the current view of causative factors between geophysical boundary conditions and evolutionary response of the biosphere and atmosphere.

(a)地球历史的前40亿年是地球系统发生许多关键转变的时期（例如，板块构造的开始，大气的氧化，生命的出现）。然而，对其中许多过程及其之间的联系仍然知之甚少。该项目将通过获得关于古代地球磁场强度的高质量数据，促进对地球早期演变的了解。已经确定的是，至少5亿年来，磁场一直足够强大，可以为生物圈和大气层提供磁屏蔽，使其免受太阳辐射的影响。这种强磁场是由外核（地球发电机）中的熔融铁对流产生的，由密度沉降和结晶内核的熔化热提供动力。然而，早期的磁场特征在很大程度上仍然未知。固体内核可能在元古代（2500至5.42亿年前）的某个时候形成。在内核形成之前，地球发电机可能会产生一个更弱、更不稳定的磁场。随之而来的较弱的磁屏蔽将允许太阳辐射影响生命进化和大气化学。 我们的研究将通过对地球仪周围几套年代较好的喷出岩和侵入岩的化石磁性研究，提供有关元古宙磁场强度的高质量数据。这项研究将使目前元古宙古强度数据库的规模几乎翻一番，包括目前没有古场强测定的时间段。这些数据将为早期地球的地球发电机机制提供重要的见解，并将对我们如何使用磁场记录来破译地球的地质历史，包括内核的年龄产生重要影响。 这项研究的更广泛意义包括更好地理解地球磁场演变与生物圈和大气演变之间的联系。该项目将涉及密歇根理工大学的本科生和研究生，从而培养下一代科学家。该项目还将建立一个可持续的框架，将学术研究与K-12科学教育进步和科学推广联系起来，以提高公众对地球科学的认识。特别是，将为大学预科科学教师提供独特的夏季研究经验。这些伙伴关系将为课堂行动研究提供基础，以提高学生的科学成绩。(b)关于前寒武纪地磁场长期行为和结构的数据对于理解地球早期地球发电机的性质至关重要。这些数据对于研究地磁场演变与地球系统其他组成部分之间的潜在因果关系也很重要。例如，早期地球发电机的弱场或不稳定场可能导致较弱的磁层屏蔽，因此，太阳和宇宙辐射对大气和生物圈的影响更大。此外，地磁场的强度和稳定性的长期趋势可以提供对地球内部一些重要转变的时间的洞察，例如固体内核的形成和增长。在缺乏严格理论约束的情况下，古地磁数据成为有关前寒武纪磁场的主要信息来源。然而，我们对地球历史最初40亿年期间磁场特征的了解仍然非常有限。特别是，关于场强的数据库只包含少数可靠的数据点。在我们的项目中，我们将通过对几个可靠定年的、全球分布的玄武岩序列和基性岩脉群进行详细的古地磁分析来研究元古代地球发电机的强度，这些序列和岩脉群先前已被证明包含原始古地磁记录。我们的目标是中大陆裂谷系的~ 1.1Ga岩石（美国和加拿大）、615 Ma长范围岩墙群（加拿大）、755 Ma Mundine井岩墙、1070 Ma Warakurna粗玄岩岩床和岩墙以及1210 Ma Marnda Moorn 810 Ma Xiaofeng和1765 Ma Taihang岩墙群（中国），~925 Ma Salvador-Olivença、~1.5 Ga Curaçá和~2.6 Ga Uauá岩墙群（巴西）。在这项研究中，我们将使用传统的和新的实验和解释的方法，以有助于前寒武纪地球发电机的天气视图。该项目将显著增加前寒武纪高质量古强度测定的数量，包括目前没有古场强测定的时间段（例如新元古代）。拟议研究的成果将纳入整个地球系统的新太古代-元古代地质历史时间轴，可能改变目前对地球物理边界条件与生物圈和大气的演变反应之间的成因的看法。