Collaborative Research: High temporal resolution paleomagnetism of speleothems

洞穴沉积物(石笋、钟乳石、花石)是由上覆基岩中含有溶解元素的滴水、流水和渗水形成的洞穴矿藏。洞穴群的生长可能跨越数万年，并提供了这段时期气候和环境条件的自然档案。通常，它们还包括来自上覆土壤的微量磁性矿物，这最终可能提供该时间间隔内地球磁场的连续记录。洞穴化石的精确年代测定为从这些磁记录中获取有关过去短期地磁不稳定事件(例如，地球磁场强度或方向的快速变化)的宝贵信息提供了可能性。反过来，这些不稳定性可能揭示有关地球磁场产生机制(称为地球发电机)的关键信息，提前调查地磁事件和气候事件之间可能存在的联系，并提供有关地球磁场强度波动如何影响卫星通信性能的重要数据。到目前为止，从洞穴中恢复高分辨率磁记录一直具有挑战性，因为它们携带的磁化强度非常弱。然而，最先进的磁测量技术和测年技术的最新发展使得对最近地质历史上过去的磁场进行高分辨率分析成为可能。这项研究项目将在史密森国家自然历史博物馆开展外展活动，支持一名博士后研究员和一名研究生的兼职参与，为STEM中未被充分代表的少数族裔的本科生实习提供支持，促进本科生的研究项目，并为来自全球的20名初学研究生提供洞穴磁学实践经验，作为岩石磁学暑期课程的一部分。该项目专注于使用洞穴，它锁定了次年时间尺度上的磁化，以研究LasChamp地磁偏移(~41ka)，这是地质记录中最重要的地磁不稳定之一。来自北半球中纬度(美国密苏里州)和南半球低纬度(巴西巴伊亚州和米纳斯吉拉斯州)的时间同步观测将提供日期准确、高分辨率的古地磁记录，填补最近球谐模型中的关键空白，并允许检查被认为显示了过去10万年来一些最快地磁变化的时间段。利用保存在洞穴中的连续古地磁记录，将通过超导量子干涉装置(SQUID)显微镜、量子钻石显微镜(QDM)、扫描电子显微镜(SEM)断层扫描和透射电子显微镜(TEM)等技术的组合，实现高空间和时间分辨率(1-10年)。具体目标包括：(1)利用从巴西和美国收集的拉尚山脉中已知的定向洞穴沉积物研究短期地磁行为；(2)调查控制洞穴沉积物剩磁获取的精细过程，并评估从这种洞穴形成获得的磁记录的稳健性；(3)进一步发展扫描磁学工作流程，以自信地从最大限度利用年度分层的洞穴沉积物中恢复全矢量记录。这些古地磁数据将代表拉尚漂移中分辨率最高、最可靠的年代记录。它们将提高未来球谐模型的地理复盖面和年代学精确度，并有助于加深对地球发电机不稳定性的理解。此外，先进的扫描电子显微镜断层扫描和基于电子显微镜的矿物学分析将提供在这些材料中保留古地磁记录的磁性矿物子集的原位视图，帮助解决与获取剩磁有关的悬而未决的问题。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。

Speleothems (stalagmites, stalactites, flowstone) are cave mineral deposits that form by dripping, flowing, and seeping water containing dissolved elements from the overlying bedrock. Speleothem growth may span tens of thousands of years and provide a natural archive of climatic and environmental conditions during that period. Usually, they also include minute concentrations of magnetic minerals from overlying soils, which could ultimately provide a continuous record of Earth’s magnetic field during that time interval. Precise dating of speleothems has opened up the possibility of accessing invaluable information from those magnetic records about past episodes of short-duration geomagnetic instability (for example, rapid changes in the strength or direction of Earth’s magnetic field). In turn, these instabilities may reveal critical information about the mechanism by which Earth’s magnetic field is generated (known as the geodynamo), advance investigations of possible links between geomagnetic and climatic events, and provide important data about how fluctuations in the Earth’s magnetic field strength may affect the performance of satellite communications. Retrieval of high-resolution magnetic records from speleothems has been challenging until now owing to the very weak magnetization they carry. However, latest developments in state-of-the-art magnetometry technologies and dating techniques have made it possible to conduct high-resolution analysis of past magnetic fields in recent geological history. This research project will enable outreach activities at the Smithsonian National Museum of Natural History, support a postdoctoral researcher and part-time participation of a graduate student, provide support for an undergraduate internship targeting underrepresented minorities in STEM, facilitate undergraduate research projects, and provide hands-on experience in speleothem magnetism for twenty beginning graduate students from around the globe as part of a summer school on rock magnetism.This project focuses on using speleothems, which lock in magnetizations on sub-annual timescales, to study the Laschamp geomagnetic excursion (~41 ka), one of the most important geomagnetic instabilities in the geologic record. Time-synchronous observations from the mid-latitudes of the northern hemisphere (Missouri, US) and the low-latitudes of the southern hemisphere (Bahia and Minas Gerais, Brazil) will provide well-dated, high-resolution paleomagnetic records that fill critical gaps in the most recent spherical harmonic models and allow examination of a time period thought to display some of the fastest geomagnetic changes of the last 100,000 years. High spatial and temporal resolutions (1-10 years) will be achieved by leveraging the continuous paleomagnetic recordings preserved in speleothems through a combination of techniques, including superconducting quantum interference device (SQUID) microscopy, quantum diamond microscopy (QDM), scanning electron microscopy (SEM) tomography, and transmission electron microscopy (TEM). Specific goals include (1) studying short-term geomagnetic behavior using oriented speleothems known to have grown across the Laschamp excursion collected from Brazil and the United States; (2) investigating the fine-scale processes controlling acquisition of remanence in speleothems and assessing the robustness of the magnetic record obtained from such cave formations; and (3) further developing the workflow for scanning magnetometry to confidently recover full-vector records from speleothems that take maximum advantage of their annual layering. These paleomagnetic data will represent some of the highest resolution, most robustly dated records across the Laschamp excursion. They will improve the geographic coverage and geochronological rigor of future spherical harmonic models and contribute to a deeper understanding of geodynamo instability. Additionally, advanced SEM tomography and TEM-based mineralogic analyses will provide an in situ view of a subset of the magnetic minerals that retain the paleomagnetic recording in these materials, helping address unresolved questions related to the acquisition of remanence.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.