Mesozoic Variations in Polarity Reversal Rate – In my postdoctoral work as an NSF Earth Sciences Postdoctoral Fellow at the University of California, Berkeley I am continuing this interdisciplinary approach combining paleomagnetic observations and geodynamo numerical simulations to look even farther back in time to the Mesozoic Era. The Jurassic (201.3-145.5 Myr ago) and Cretaceous (145.5-65.5 Myr ago) have very different geomagnetic behavior. The Cretaceous normal-polarity superchron (CNS) occurred during the Cretaceous; a period when the field strength was high while the dipole polarity reversal rate was zero. During the Jurassic the magnetic field had low strength with a high rate of reversals, a phase known as the Mesozoic Dipole Low (MDL). I am building a compilation of Mesozoic paleomagnetic datasets and running numerical geodynamo simulations exploring outer core processes that could generate this transition between different geomagnetic behaviors.
Core Mantle Boundary Heat Flux – My study of the effects core-mantle interactions on geomagnetic behavior has also benefited from the Cooperative Institute for Dynamic Earth Research (CIDER) summer 2018 workshop where I began a new collaboration with a small group to make multidisciplinary assessment of heat flux at the core mantle boundary. We examined the potential thermal boundary layers which may exist on both sides of the core mantle boundary and explored the possibility of constraining their thicknesses and structures with seismic and geomagnetic observations.
My PhD work addressed how to improve understanding of long-term geomagnetic field variations from 3 distinct perspectives: (1) extending the range of geological materials suitable for paleointensity studies, (2) seeking verification for distinctive properties of dipole variations found in the paleomagnetic field record for 0-2 Ma in the marine magnetic anomaly record, and (3) reproducing statistical properties of the paleofield in geodynamo simulations.
Ignimbrite Paleointensity – [This project was featured in EOS!] There are a limited number of paleomagnetic data, especially paleointensity data before the Holocene period. The relative shortage of paleointensity information is caused in part by a lack of geological materials that are suitable for paleointensity studies. Volcanic ash flows (ignimbrites) could potentially be useful as they occur throughout the global geologic record and are usually well suited for dating. Ignimbrite samples from the 0.76 Ma Bishop Tuff were collected from the Owens’ River Gorge. The theoretic underpinning of the IZZI variant of the Thellier-Thellier method used to estimate paleointensity requires a thermoremanent magnetization (TRM). The most likely place in the Bishop Tuff to find a pure TRM is in the densely welded ignimbrite; these sections estimate the field was 36.7±5.7 μT. The mean of all sites means is 42.0 ± 12.7 μT this estimate is higher and more scattered due to the contribution of a chemical thermoremanent magnetization (CTRM). The effect of post-emplacement alteration on the ignimbrite’s magnetization was evaluated by measuring various rock-magnetic properties. Future studies of paleointensity determined from ignimbrites should be very specific in their documentation of the geologic material (density and alteration) and where it came from within the ash flow, and demonstrate the magnetization is a TRM.
Asymmetry Between Dipole Growth and Decay in Marine Magnetic Anomaly Records – An asymmetry between the growth and decay rates of the axial dipole moment has previously been observed in PADM2M (a reconstruction of the 0 to 2 Ma axial dipole moment from sedimentary relative intensity records calibrated by igneous and archeological absolute intensity data). At periods longer than 15 kyr PADM2M spends more time decaying than growing: thus its average growth rate is greater than its decay rate. However, there was also discussion that this signal could be an artifact of the sedimentary recording process. A long-term record of geomagnetic intensity should also be preserved by the TRM of oceanic crust and stacks of marine magnetic anomalies are inverted to provide an independent means of assessing the asymmetry between field growth and decay seen in PADM2M. I examined three near-bottom marine magnetic: a 0 to 780 kyr record from the East Pacific Rise, a 0 to 5.2 Ma record from the Pacific Antarctic Ridge, and a chron C4Ar-C5r (9.3-11.2 Ma) record from the NE Pacific. All three records show an asymmetry between growth and decay similar in sense to PADM2M; the distribution has a positive tail and the average growth rate is greater than the average decay rate. This result indicates that other recording media carry a record of the asymmetric behavior first found in PADM2M and that it was present during other times.
Asymmetry Between Dipole Growth and Decay in Geodynamo Simulations – I used this asymmetric behavior of the ADM as a criterion for evaluating geodynamo simulations as Earth-like, and assessed what insight this observation can offer about the energy balance of the geodynamo as a function of frequency. I examined the coherence spectra for the various terms of the magnetic induction equation to assess changes in the force balance as a function of frequency. Some simulations exhibited asymmetry between growth and decay similar to that observed in the paleomagnetic record and it was associated with changes in magnetic energy that are more coherent with ohmic heating than with the work done by the Lorentz force in that frequency band. Visualization of their magnetic fields at the core mantle boundary and their internal fields revealed a link between the number of convective upwellings and ADM variations. I also mapped the contributions to changes in axial dipole moment from advection and diffusion at the core-mantle boundary (CMB). For the Earth-like case dipole decay is mostly caused by diffusion although both advection and diffusion are in play, during dipole growth advection at the CMB is stronger and acts to increase the axial dipole moment.