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Panovska, S, Constable CG, Korte M.  2018.  Extending global continuous geomagnetic field reconstructions on timescales beyond human civilization. Geochemistry Geophysics Geosystems. 19:4757-4772.   10.1029/2018gc007966   AbstractWebsite

Study of the late Quaternary geomagnetic field contributes significantly to understanding the origin of millennial-scale paleomagnetic secular variations, the structure of geomagnetic excursions, and the long-term shielding by the geomagnetic field. A compilation of paleomagnetic sediment records and archeomagnetic and lava flow data covering the past 100ka enables reconstruction of the global geomagnetic field on such long-term scales. We use regularized inversion to build the first global, time-dependent, geomagnetic field model spanning the past 100ka, named GGF100k (Global Geomagnetic Field over the past 100 ka). Spatial parametrization of the model is in spherical harmonics and time variations with cubic splines. The model is heavily constrained by more than 100 continuous sediment records covering extended periods of time, which strongly prevail over the limited number of discrete snapshots provided by archeomagnetic and volcanic data. Following an assessment of temporal resolution in each sediment's magnetic record, we have introduced smoothing kernels into the forward modeling when assessing data misfit. This accommodates the smoothing inherent in the remanence acquisition in individual sediment paleomagnetic records, facilitating a closer fit to both high- and low-resolution records in regions where some sediments have variable temporal resolutions. The model has similar spatial resolution but less temporal complexity than current Holocene geomagnetic field models. Using the new reconstruction, we discuss dipole moment variations, the time-averaged field, and paleomagnetic secular variation activity. The new GGF100k model fills the gap in the geomagnetic power spectrum in the frequency range 100-1,000Ma(-1).

Panovska, S, Constable CG, Brown MC.  2018.  Global and regional assessments of paleosecular variation activity over the past 100 ka. Geochemistry Geophysics Geosystems. 19:1559-1580.   10.1029/2017gc007271   AbstractWebsite

We present a global compilation of paleomagnetic data spanning the past 100 ka. Sediment data comprise 61,687 declinations, 70,936 inclinations, and 69,596 relative paleointensities. Many sites are located in the northern Atlantic and western Pacific, with approximately twice as many data from the Northern Hemisphere as from the Southern Hemisphere. The 14,954 volcanic and archeomagnetic data are sparse, especially in the Southern Hemisphere. Directional and intensity information are aggregated under the paleosecular variation (PSV) index to assess occurrence of excursions over the past 100 ka. The Laschamp excursion (approximate to 41 ka) is clearly defined across globally distributed sediment records with an average duration of 1,300 years. Regional stacks obtained using bootstrap resampling show a more pronounced Laschamp excursion in the Northern Hemisphere than in the Southern, and in the Atlantic Hemisphere compared with the Pacific. No anomalous indices occurred around the Mono Lake excursion or other periods in the bootstrap curves. This may result from low sedimentation rates, discrepancies in age scales, large age errors, and/or the lack of global character of any transitional events. These data and associated new uncertainty estimates for the sediment records provide a good foundation for global, time-dependent, spherical harmonic field modeling for the past 100 ka.

Hartl, P, Tauxe L, Constable C.  1993.  Early Oligocene Geomatnetic-Field Behavior From Deep-Sea Drilling Project Site-522. Journal of Geophysical Research-Solid Earth. 98:19649-19665.   10.1029/93jb02019   AbstractWebsite

Hydraulic piston coring operations at Deep Sea Drillng Project site 522 in the South Atlantic retrieved an unusually continuous section of late Eocene to late Oligocene pelagic sediments, which we sampled at 3-4 cm intervals (approximately 3-5 kyr). Natural remanent magnetization demagnetization studies indicate a well-behaved remanence. Various rock magnetic procedures strongly suggest the magnetic carrier is dominated by pseudo-single domain magnetite appropriate for recording relative intensity variations of the paleomagnetic field. Nine zones of unusually low relative paleointensity were identified within the 2 my Chron C12R interval. Seven can be typified by a approximately 20-40 kyr interval of low field intensity accompanied by apparently random, low-amplitude, short-duration directional fluctuations. The other two are of approximately equal duration and intensity but exhibit an orderly progression of directional changes that result in well-defined virtual geomagnetic pole (VGP) paths confined along a preferred meridian of approximately 70-90-degrees-W longitude. We propose that both styles occur when the main dipole term diminishes significantly but that the former result when undimished ''normal'' secular variation is continuous during the period of low axial dipole moment. We propose that the other two lows in relative paleointensity, along with one reversal record, reflect a field structure of low axial dipole moment dominated by a low-degree nonzonal spherical harmonic term. Alternatively, the confined VGP paths could be an artifact of heavy remanence smoothing between nonantipodal, semistable transitional geomagnetic pole positions. Geographical control of VGP paths, particularly along approximately 70-90-degrees-W longitude, has recently been noted for much younger reversals. The site 522 record may indicate that the underlying cause of this phenomenon was present at 32 Ma. We compare our C12R record of paleointensity lows with C12R marine magnetic anomaly ''tiny wiggles''. These data appear to indicate that C12R tiny wiggles resulted from periods of low geomagnetic field intensity that were sometimes accompanied by directional excursions.