The First Billion Years of the Geodynamo

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The onset and nature of the geomagnetic field is important for understanding the evolution of the core, atmosphere and life on Earth. The geomagnetic field is generated in the liquid outer core, and hence is a probe of core conditions. The field also protects Earth from energetic particles streaming from the Sun (the "solar wind"); without this protective shield Earth might have developed into a dry and barren planet. A record of the early core geodynamo that generated the field is preserved in ancient silicate crystals from igneous rocks that contain minute magnetic grain inclusions. Our data indicate the presence of a geodynamo between 3.4 and 3.45 billion years ago, near the limit for the start of growth of the solid inner-core. While the magnetic field sheltered Earth's atmosphere from erosion at this time, the standoff of the solar wind was greatly reduced, and similar to that seen during modern extreme solar storms. These conditions suggest that intense radiation from the young Sun may have modified the atmosphere of the young Earth by promoting loss of light elements and water. Such effects would have been more pronounced if the field were absent prior to 3.45 billion years ago, as suggested in some hypotheses, or if an older geodynamo prior to inner core growth produced a weak field. In general, these considerations suggest the young Earth was more water-rich than today. The new frontier to learn more about these issues is obtaining geomagnetic field records that are more than 3.45 billion-years-old.We are investigating the first billion years of geodynamo history and its implications for Earth evolution through the study of well-dated rock units from 6 ancient cores of continents (cratons). We are using a combination of existing methods of single silicate crystal magnetic measurements from in situ igneous host rocks and new approaches involving magnetic analyses of grains from sedimentary units. These measurements are achievable using highly sensitive magnetometers at the University of Rochester. Determination of the presence and strength of the geomagnetic field during the first billion years of Earth history is of broad interest to a range of scientists who study early Earth environments (atmosphere and biosphere) and the core. The investigation involves international collaboration with geologists from several countries, and multidisciplinary collaborations spanning astrophysics and space physics. Our program also integrates research and educational efforts. The study is contributing to graduate theses and undergraduates are receiving training in the field and the laboratory.