Information about the geometry, stability, and intensity of Earth's magnetic field during the Proterozoic is of great importance for understanding the early geodynamo and in the interpretation of paleomagnetic data with regards to paleogeographic reconstructions, true polar wander, global glaciations and other problems. What are the space-time characteristics of the Proterozoic field, such as the relative significance of the dipole and non-dipole components, and are these characteristics significantly different from their Phanerozoic counterparts? In the absence of strict theoretical constraints, paleomagnetic data are the principal source of information about the Precambrian field. Field geometry can be estimated by combining paleomagnetic data with independent latitudinal indicators such as evaporites and glacial deposits. A hotter Precambrian Earth coupled with absence of the inner core could have resulted in higher field variation, including more frequent reversals. The current magnetostratigraphic database is insufficient to test whether the early geodynamo reversed its polarity with the same range of frequencies as in the Mesozoic-Cenozoic interval. However, Precambrian rocks may preserve directional information useful for constraining paleosecular variation (PSV). The estimates of PSV based on the Precambrian paleomagnetic database will be discussed. Paleointensity data provide information on the energy state of geodynamo and may prove crucial for constraining models of the thermal evolution of the Earth (e.g., the timing of nucleation and growth of the solid inner core). However, many Precambrian rock sequences have been affected by alteration, which hinders the measurement of paleointensity using bulk rock samples. In addition, many paleointensity determinations for that time period come from intrusive rocks in which the paleointensity signal may be complicated by thermochemical remanent magnetization and other factors. Therefore, a caution should be exercised when concluding on the long-term behavior of paleointensity based on a very limited database for the Precambrian. Alternatively, single silicate crystals are less susceptible to alteration in nature and during experiments may be used as paleointensity recorders. Data from plagioclase crystals separated from mafic dikes, together with directional data from whole rocks, indicate a dipole-dominated field at 2.5--2.7 Ga. The bulk of available data indicate that on a long-term scale the Proterozoic field was not grossly different from the present-day field.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1521 Paleointensity;
- 1522 Paleomagnetic secular variation;
- 1527 Paleomagnetism applied to geologic processes;
- 1560 Time variations: secular and longer