The Earth's magnetic field is generated by convection of molten iron in the planet's outer core. This process, or the geodynamo, is powered by density settling and heat of fusion of the crystallizing inner core. The Earth's magnetic field can be well approximated by that of a dipole (i.e. bar magnet) positioned at Earth's center and aligned with the rotational axis. This allows us to apply the dipole equations to reconstruct past positions of continents using the direction of ancient magnetic field recorded in rocks (fossil magnetism). It is well established that the field has had this dipolar geometry for at least 500 milion years, and that the field has been strong enough to provide magnetic shielding of the biosphere and atmosphere from solar radiation. However, our knowledge of the field characteristics for earlier times remains limited. Current thinking suggests that the solid inner core formed sometime during the Proterozoic eon (2500 to 542 million years ago). Before the inner core formation, the geodynamo could have produced a weaker and less stable magnetic field with large episodic deviations from the dipolar geometry. An attendant weaker magnetic shielding would allow solar radiation to affect the life evolution and atmospheric chemistry. Our research seeks to obtain high-quality data on the Proterozoic field by investigating the fossil magnetism of several suites of quickly-cooled intrusive Proterozoic rocks in Peninsular India. This study will provide important insight in our understanding the mechanism that generates Earth's magnetic field and would have important implications in how we use the magnetic field records to decipher the geological history of our planet, including the age of the inner core. Broader implications of the study include better understanding of a potential link between the evolution of Earth's magnetic field and the evolution of biosphere and atmosphere. The project will involve undergraduate students at Michigan Tech in the laboratory analyses of the samples thus training the next generation of scientists. This research will also become a part of a Ph.D. thesis. In order to increase the general public awareness of Earth science, the results will be disseminated through a series of science exploration sessions.

Data on the long-term behavior and configuration of the geomagnetic field during the Precambrian are crucial in understanding the nature of Earth's early geodynamo. These data are also important for investigating potential causative links between the evolution of geomagnetic field and other components of the Earth system. For example, a weak or unstable field of an early geodynamo could result in weaker magnetosphere shielding and, hence, a stronger effect on the atmosphere and biosphere from solar and cosmic radiation. In addition, long-term trends in the strength and stability of geomagnetic field may provide insight into the timing of some important transitions in the Earth's interior such as the formation and growth of the solid inner core. In the absence of strict theoretical constraints, paleomagnetic data become a principal source of information about the Precambrian field. However, our knowledge of the field characteristics during the first four billion years of Earth history remains very limited. In particular, the database on the field strength contains only a handful of reliable data points. In our project, we will investigate the strength, morphology, and stability of the Proterozoic geodynamo by detailed paleodirectional and paleointensity analyses of five mafic dike swarms in Peninsular India, which have been reliably dated and shown to contain pristine paleomagnetic records. Our proposed targets are the~2.37 Ga and 2.18 Ga swarms in the Dharwar craton, the ~1.99 Ga swarm in the Bundelkhand craton, the ~1.89 Ga swarm in the southern Bastar craton, and ~0.75 Ga swarm of the Malani Igneous Suite in the Marwar Terrain. In this study, we will use both traditional and novel approaches such as single crystal technique for paleointensity determinations. This study will provide a synoptic view of the Proterozoic geodynamo, including time-averaged paleointensity values, estimates of the relative contributions from dipole and non-dipole field components, and paleolatitude-dependent estimates of paleosecular variation.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Raffaella Montelli
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Michigan Technological University
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