Collaborative Research: High temporal resolution paleomagnetism of speleothems

洞穴矿物（石笋、钟乳石、流石）是由滴落、流动和渗透的水形成的洞穴矿床，这些水含有来自上覆基岩的溶解元素。洞穴的生长可能跨越数万年，并提供了该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质，这些矿物质最终可以提供那段时间内地球磁场的连续记录。对洞穴的精确测年使我们有可能从这些磁记录中获取有关过去短时间地磁不稳定事件的宝贵信息（例如，地球磁场强度或方向的快速变化）。反过来，这些不稳定性可能揭示关于地球磁场产生机制（称为地球发电机）的关键信息，推进对地磁与气候事件之间可能联系的调查，并提供关于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，由于洞穴石的磁化强度非常弱，从洞穴石中提取高分辨率磁记录一直是一个挑战。然而，最先进的磁力计技术和测年技术的最新发展使得对最近地质历史中过去的磁场进行高分辨率分析成为可能。该研究项目将支持史密森国家自然历史博物馆的外展活动，支持一名博士后研究员和一名研究生的兼职参与，为针对STEM中代表性不足的少数民族的本科生实习提供支持，促进本科生研究项目，并为来自世界各地的20名刚开始的研究生提供洞穴磁学的实践经验，作为岩石磁学暑期学校的一部分。该项目重点利用洞穴主题，锁定亚年时间尺度上的磁化，研究地质记录中最重要的地磁不稳定性之一Laschamp地磁偏移（~41 ka）。来自北半球中纬度地区（美国密苏里州）和南半球低纬度地区（巴西巴伊亚州和米纳斯吉拉斯州）的时间同步观测将提供年代准确、高分辨率的古地磁记录，这些记录将填补最近球面调和模型中的关键空白，并允许对一段被认为显示过去10万年中一些最快地磁变化的时期进行检查。通过超导量子干涉装置（SQUID）显微镜、量子金刚石显微镜（QDM）、扫描电子显微镜（SEM）断层扫描和透射电子显微镜（TEM）等技术组合，利用保存在洞穴中的连续古磁记录，将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Laschamp地区生长的定向洞穴研究短期地磁行为；(2)研究控制岩洞中剩余物捕获的精细过程，并评估从这些岩洞中获得的磁记录的鲁棒性；(3)进一步开发扫描磁强计工作流程，以自信地从洞穴中恢复全矢量记录，最大限度地利用其年度分层。这些古地磁数据将代表拉尚考察中一些最高分辨率、最可靠的年代记录。它们将提高未来球谐模型的地理覆盖范围和地质年代学的严严性，并有助于更深入地了解地球动力学的不稳定性。此外，先进的SEM层析成像和基于tem的矿物学分析将提供一个保留这些材料中古磁记录的磁性矿物子集的现场视图，有助于解决与剩余物获取相关的未解决的问题。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。

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.