The origin of the geomagnetic field is important for understanding the evolution of Earth's deep interior, surface environment and atmosphere. But defining the nature of the early geomagnetic field is challenging; even the best preserved rocks from the Archean eon (more than 2.5 billion-years-old) have seen low grade metamorphism (at 200 degrees C to 320 degrees C) related to geologic events after their formation. The acquisition of later magnetizations by these rocks is expected, precluding use of conventional paleointensity techniques. A newly developed approach to the problem, utilizing CO2 laser heating and DC SQUID magnetometer measurements, provides a means to obtain paleodirections and intensities from single silicate crystals which host magnetite inclusions. Using these techniques, 3.2 billion-year-old field strengths have been reported from Archean rocks of South Africa that are within 50% of the present-day value. This indicates that a viable magnetosphere sheltered the early Earth's atmosphere from solar wind erosion.
Rocks older than 3.2 billion years old are found in South Africa and Swaziland, potentially holding a 300 to 400 million-year-long record of geomagnetic field behavior preserved in single silicate crystals. The new paleointensity approach can also be used to examine these rocks. If the geomagnetic field was present during this interval, these data should provide bounds on its strength, and they may be used to make inferences on field morphology. Such constraints on the nature of the earliest geomagnetic field are of interest to a broad range of sciences interested in core and mantle processes.
Among the broader impacts of these investigations are that they form the basis for the dissertation studies of graduate and undergraduate students. Students take part in field and laboratory work, and assist in K-12 activities, integrating graduate and undergraduate teaching efforts at the University of Rochester with area school programs.
Scientific Merit: The onset and nature of the geomagnetic field is important for understanding the evolution of the core, atmosphere and life on Earth. A record of the early geodynamo is preserved in ancient silicate crystals containing minute magnetic inclusions. We have obtained the oldest field strength measurement reported to date, from 0.25 mm to 1.00 mm sized silicate crystals. Our data indicate the presence of a geodynamo between 3.4 and 3.45 billion years ago, near the limit of model estimates for the onset of growth of the solid inner core. While the magnetic field sheltered Earth's atmosphere from erosion by powerful solar winds at this time, the standoff of the solar wind by the ancient magnetic field was greatly reduced, and similar to that during modern extreme solar storms. These conditions suggest that intense radiation from the young Sun may have modified the atmosphere of the young Earth. Broader impacts: The work has supported undergraduate and graduate students, and fostered international collaboration (between students and scientists at the University of Rochester and South African universities). Our work has generated considerable interest in the media; we have been interviewed by several news organizations, and this has served to better communicate our results to the public. News items about our study have appeared in US News and World Report, MSNBC, Science Daily, Discovery News, BBC and others. An electronic slide based on our work has been developed by the American Museum of Natural History and distributed to other museums.
