Collaborative Research: High temporal resolution paleomagnetism of speleothems

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.