Evidence for a Hadean to Paleoarchean geodynamo: Implications for the survivability of planetary hydrospheres

Abstract:

Knowledge of when the geodynamo started is important for understanding the evolution of the atmosphere and life on Earth. Full vector paleointensity measurements of Archean to Hadean zircons bearing magnetic inclusions from the Jack Hills conglomerate (Western Australia) allow a for reconstruction of the early history of the geodynamo (Tarduno et al., 2015). Data from zircons between 3.3 and 4.2 billion-years-old record magnetic fields varying between 1.0 and 0.12 times recent equatorial field strengths. These magnetizations can only be read using highly sensitive cryogenic magnetometers such as the ultra-high resolution 3-component DC SQUID small bore magnetometer at the University of Rochester. The direct measurement of each component of magnetization separates this class of magnetometer from one-axis instruments that have an inherent limitation due to non-uniqueness. Only a few laboratories have instrumentation/procedures with the resolution to accurately record directions and paleointensities (see presentations by Dare et al., 2015, and Cottrell et al., 2015). For example, data from Weiss et al. (2015) are too noisy to examine Paleoarchean to Hadean magnetic fields. In contrast, our high-resolution paleointensity and age data suggest the presence of a terrestrial core dynamo more than 750 million years earlier than prior estimates. This early start for the geodynamo is similar to that suggested for the early Martian magnetic field. However, the subsequent collapse of the Martian dynamo probably facilitated atmospheric stripping. In contrast, the early start and persistence of atmospheric shielding associated with the long-lived geodynamo was likely a key factor in the development of Earth as a habitable planet. Paleomagnetic evidence for these ancient magnetic fields will be discussed as will the role of planetary magnetic shielding.