Grand challenges in geomagnetism and paleomagnetism

Geomagnetism and paleomagnetism is a broad sub-discipline of the Earth Sciences. I present the Grand Challenges from the perspective of a paleomagnetist and rock magnetist, hence I feel more comfortable with the paleomagnetic side of this short article than the geomagnetic side. Nevertheless, exciting things are happening in geomagnetism and the future is bright for both paleomagnetists and geomagnetists. In paleomagnetism, one of the important challenges is the extension of accurate apparent polar wander paths back into the Precambrian to facilitate the construction of continental paleogeography deep into Earth’s history. In particular, the existence and paleogeography of supercontinents will tell us more about the nature of plate tectonics, and the Wilson cycle, in the Precambrian. Paleomagnetic data have been critical in delineating the supercontinent Pangea from 320–180Ma. The fragmentation of Pangea has lead to the current configuration of the continents. Critical to investigations of Snowball Earth episodes in the Neoproterozoic (Sturtian from 730–705Ma and Marinoan from 663–635Ma) is the evolution in our understanding of the existence, paleogeography, and fragmentation of the supercontinent Rodinia (900–700Ma). Rodinia has been built from paleomagnetic data collected from nearly all the continental cratons. The enhanced silicate weathering, and consequent drawdown of CO2 in the atmosphere, caused by the increase of continental margins resulting from the fragmentation of Rodinia, has been suggested to be the cause of Snowball Earth episodes (Hoffman, 1999), showing how important the delineation of supercontinent assembly and fragmentation is to understanding the Earth system through geologic time. More recently evidence is building for a mid-Proterozoic supercontinent, called either Nuna or Columbia (Zhang et al., 2012) from 1.74– 1.59Ga. Nuna is built mainly on data collected from Laurentia, Baltica, and the North China block for 1.78–1.40Ga. Australia adds good data coverage for 1.80–1.60Ga. Data from the remaining cratons (Amazonia, India, Siberia, Slave) is spotty (Zhang et al., 2012), illustrating the need for more robust apparent polar wander paths in the Precambian. As these ancient apparent polar wander paths are constructed, it will be important to identify and correct the effects of compaction-caused inclination shallowing in sedimentary rocks and the effects of grain-scale strain on the paleomagnetism of deformed rocks that will be almost unavoidable paleomagnetic targets for ancient Precambrian rocks (Kodama, 2012). The challenges for building accurate and well-constrained Precambrian pole paths are great, but ultimately critical to the Earth sciences. A second grand challenge for paleomagnetists is more definitive understanding of the directional and intensity variations of paleosecular variation of the geomagnetic field through Earth history. A critical, fundamental assumption for using paleomagnetism to reconstruct continental paleogeography is that the Earth’s magnetic field has been nearly an axial, geocentric dipole throughout Earth history. Some workers have suggested that significant non-dipole field components, particularly the octupole (e.g., Torsvik and Van der Voo, 2002), have been important components of the geomagnetic field in the Paleozoic and Precambrian. Kent and Smethurst (1998) have shown that a shallow bias in paleomagnetic directions in the Paleozoic and Precambrian is consistent with a 25% contribution from an octupolar non-dipole field. However, shallow paleomagnetic inclinations could also be caused by compaction-induced inclination shallowing in sedimentary rocks, so part of the challenge is to tease out accurate paleomagnetic directions from sedimentary rocks as the nature of geomagnetic field behavior is studied into deep time. If observations of geomagnetic field behavior are limited to the past 5 million years, paleomagnetic data from igneous rocks collected over a wide range of latitudes (78◦S–53◦N) do not show similar secular variation behavior of the geomagnetic field either during reversed or normal polarity periods or from the southern and northern hemispheres (Johnson et al., 2008). The 5 million year time period is used because plate motions will not have caused movements greater than several hundred kilometers (∼250 km assuming 50 km/myr as a typical plate velocity), which is within typical paleomagnetic resolution (∼2–3◦ and knowing that on the Earth’s surface that there is 111 km/degree of great circle distance). The time-averaged field would deviate more from dipolar behavior in the southern hemisphere during the reversed polarity Matuyama epoch, with a relatively stronger octupolar non-dipole field contribution, than if it were observed from the northern hemisphere during the normal polarity Brunhes epoch. The challenge, then, is to fully document the global behavior of the geomagnetic field over the past 5 million years, and then to push that understanding back in time, to answer the question how far and when the field has strayed from dipolar geometry.