Long-Term Geomagnetic Variations: Linking Paleomagnetic Observations, Statistical Analyses, and Numerical Geodynamo Simulation

An interdisciplinary approach that combines the gathering of paleomagnetic observations with the use of sophisticated analysis and numerical geodynamo simulations can help progress our knowledge of long-term variations in paleomagnetic intensity. Paleointensity is difficult to determine accurately, and made more difficult by the limited geologic materials suitable for paleointensity experiments. In this dissertation we add a new paleointensity estimate from the Bishop tuff ignimbrite, which erupted in central California ~767 thousand years ago. We study the impact of post-emplacement alteration on the recording of a thermal remanent magnetization (TRM). Ignimbrites are potentially valuable for estimating paleointensity, but we find care must be taken to avoid regions of alteration. The seafloor also records a TRM and therefore paleointensity, and because seafloor is continuously spreading from mid-ocean ridges it has the advantage of preserving a time series of geomagnetic field behavior. We analyze near-bottom marine magnetic anomaly data. We find confirmation of an asymmetry between rates of dipole growth and decay previously observed in sedimentary data, by low-pass filtering stacked magnetization solutions and assessing the distribution of its derivatives. We also observe this field behavior during another time period, 9.3--11.2 Ma in addition to 0--2 Ma. The combined effects of magnetic induction and diffusion control changes in magnetic energy, but with different characteristic time and length scales. We use geodynamo simulations to help evaluate if different processes control dipole growth and decay. We introduce power spectral tools for assessing the energy balance as a function of frequency. Within our collection of geodynamo simulations we find several examples of Earth-like asymmetry in dipole rates of change. In these simulations changes in magnetic energy are more coherent with ohmic dissipation than with induction within the frequency range where the dynamos display an Earth-like distribution of dipole derivatives. We find this asymmetry between growth and decay is associated with a transition in the dominant length scale of the flow. Geomagnetic dipole variability provides an important constraint on the temporal dynamics of the geodynamo; these studies of the paleomagnetic record and computational dynamo simulations complement each other.