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Avery, MS, Gee JS, Bowles JA, Jackson MJ.  2018.  Paleointensity estimates from ignimbrites: The Bishop Tuff Revisited. Geochemistry Geophysics Geosystems. 19:3811-3831.   10.1029/2018gc007665   AbstractWebsite

Volcanic ash flow tuffs (ignimbrites) may contain single domain-sized (titano) magnetite that should be good for recording geomagnetic field intensity, but due to their complex thermal histories also contain other magnetic grains, which can complicate and obscure paleointensity determination. An initial study of the suitability of the similar to 767ka Bishop Tuff for measuring paleointensity found an internally consistent estimate of 43.03.2T. This initial study also showed a spatial heterogeneity in reliable paleointensity estimates that is possibly associated with vapor-phase alteration and fumarolic activity, which motivated resampling of the Bishop Tuff to examine spatial changes in magnetic properties. Three new stratigraphic sections of the Bishop Tuff within the Owens River gorge were sampled, and the paleointensity results from the initial study in the same locality were reinterpreted. The mean of all sites is 41.911.8T; this agrees with the initial study's finding but with substantially greater scatter. Two sections show evidence of vapor-phase alteration where the presence of titanohematite, likely carrying a thermochemical remanence, produces nonideal behavior. This thermochemical remanence in the upper portion of the section also produces some paleointensity estimates of technically high quality that have significantly higher intensity than the rest of the tuff. Our best estimate for paleointensity, 39.69.9T, comes from the densely welded ignimbrite that was emplaced above the Curie temperature of magnetite. The low permeability of this unit likely shielded it from vapor-phase alteration. Our results suggest that care must be taken in interpreting paleointensity data from large tuffs as nonthermal remanence may be present. Understanding past variations of Earth's magnetic field help us understand processes in Earth's core and help us to better understand current field behavior, which is important to life on Earth. Earth's field is recorded by magnetic-minerals in rocks as they form. Variations in the strength of the magnetic field (paleointensity) are less well known than large variations in direction. This is partially due to the difficulty in identifying rocks that are suitable for paleointensity experiments. Rocks made of volcanic ash (ignimbrites) have been shown to successfully record the field strength during recent volcanic eruptions. However, we show evidence that ignimbrites may not all be suitable for paleointensity studies. The Bishop Tuff, located in eastern California, erupted about 767 thousand years ago, emplacing a large volume (similar to 200km(3), i.e., about 80 million Olympic swimming pools or slightly bigger than Lake Tahoe) of ash and lava over a few days. With samples from the Bishop Tuff we test variations in magnetic-mineralogy that may be related to venting volcanic gas, interaction with water, eruption temperatures, or the degree to which the ash compacted and solidified into rock. These factors affect the magnetic-minerals' ability to record paleointensity and the success rate of our experiments.

Avery, MS, Gee JS, Constable CG.  2017.  Asymmetry in growth and decay of the geomagnetic dipole revealed in seafloor magnetization. Earth and Planetary Science Letters. 467:79-88.   10.1016/j.epsl.2017.03.020   AbstractWebsite

Geomagnetic intensity fluctuations provide important constraints on time-scales associated with dynamical processes in the outer core. PADM2M is a reconstructed time series of the 0-2 Ma axial dipole moment (ADM). After smoothing to reject high frequency variations PADM2M's average growth rate is larger than its decay rate. The observed asymmetry in rates of change is compatible with longer term diffusive decay of the ADM balanced by advective growth on shorter time scales, and provides a potentially useful diagnostic for evaluating numerical geodynamo simulations. We re-analyze the PADM2M record using improved low-pass filtering to identify asymmetry and quantify its uncertainty via bootstrap methods before applying the new methodology to other kinds of records. Asymmetry in distribution of axial dipole moment derivatives is quantified using the geomagnetic skewness coefficient, sg. A positive value indicates the distribution has a longer positive tail and the average growth rate is greater than the average decay rate. The original asymmetry noted by Ziegler and Constable (2011) is significant and does not depend on the specifics of the analysis. A long-term record of geomagnetic intensity should also be preserved in the thermoremanent magnetization of oceanic crust recovered by inversion of stacked profiles of marine magnetic anomalies. These provide an independent means of verifying the asymmetry seen in PADM2M. We examine three near bottom surveys: a 0 to 780 ka record from the East Pacific Rise at 19 degrees S, a 0 to 5.2 Ma record from the Pacific Antarctic Ridge at 51 degrees S, and a chron C4Ar-C5r (9.3-11.2 Ma) record from the NE Pacific. All three records show an asymmetry similar in sense to PADM2M with geomagnetic skewness coefficients, s(g) > 0. Results from PADM2M and C4Ar-C5r are most robust, reflecting the higher quality of these geomagnetic records. Our results confirm that marine magnetic anomalies can carry a record of the asymmetric geomagnetic field behavior first found for 0-2 Ma in PADM2M, and show that it was also present during the earlier time interval from 9.3-11.2 Ma. (C) 2017 The Authors. Published by Elsevier B.V.