Geomagnetic field for 0–3 ka: 1. New data sets for global modeling

Abstract

[1] Paleomagnetic and archeomagnetic records are used in both regional and global studies of Earth's magnetic field. We present a description and assessment of five newly compiled data sets, also used in the companion paper by Korte et al. (2009) to produce a series of time-varying spherical harmonic models of the geomagnetic field for the last 3000 years. Data are drawn from our compilation of lake sediment records and from the online database, GEOMAGIA50v2. The five selections are available from the EarthRef Digital Archive at http://earthref.org/cgi-bin/erda.cgi?n=944. Data are grouped according to the source of material, and we conducted separate assessments of reliability for archeomagnetic artifacts and lava flows (the ARCH3k_dat data set) and for sediments (SED3k_dat). The overall number of data is 55% greater than in previous compilations. Constrained data sets were selected using different criteria for each group. Winnowing of archeological data was based on uncertainties supplied by the original data providers. The lake sediment data assessment relied on preassigned age uncertainties and one or more of the following: comparisons with archeomagnetic data from the same region, regional consistency among several lakes, and consistency with global archeomagnetic models. We discuss relative merits of a larger unconstrained data set or a smaller (possibly) more reliable one. The constrained data sets eliminate a priori up to 35% of the available data in each case and rely on potentially subjective assessments of data quality. Given the limited data available our analyses indicate that iterative rejection of a small number (1-1.5%) of outlying data during global field modeling is a preferable approach. Specific regional comparisons among the models and data support the conclusion that Korte et al.'s outlier-free CALS3k.3 model based on all available measurements from sediments and archeological artifacts currently provides the best global representation of the 0-3 ka field; the ARCH3k.1 model provides a better fit to the denser European archeomagnetic data and may be better in that region.

Limitations in paleomagnetic data and modelling techniques and their impact on Holocene geomagnetic field models

“…The archeomagnetic data set used is the same as that employed in the construction of CALS10k.1b model by and we adopted the same uncertainty estimates for archeomagnetic data. Equations for conversion of α 95 to σ I and σ D are given in Donadini et al (2009). Based on the average deviation of archeomagnetic data and historical gufm1 model, minimum uncertainty estimates of 5 µT and a minimum α 95 of 4.3 • are applied to archeomagnetic data .…”

“…Relative paleointensity (RPI) records used in the CALS3k models Donadini et al 2009) were calibrated using comparisons to a previous model as described by Korte & Constable (2006), and by comparison with archeomagnetic data from nearby locations. Relative declination records were compared to the predictions from the previous models (e.g.…”

Centennial to millennial geomagnetic field variations

“…Finally, we include a comparison to past geomagnetic field estimates obtained with multi-centennial to millennial-scale filtering from 14 C and the world, so that the amount of available data is getting large enough to allow for global inversion models of the Holocene magnetic field. One of the latest, most comprehensive data collections is described by Donadini et al (2009), to which 13 more sediment records have been added for more recent modelling work by . The past field information is gained from a variety of materials through a range of different laboratory procedures (see, e.g., Donadini et al 2010, for a recent review).…”

“…Substantial progress has been made over the past 10 years to compile all published Holocene results (Korte et al 2005;Genevey et al 2008;Korhonen et al 2008;Donadini et al 2009). These global compilations have been used to develop global field models, either describing only the dipole evolution (Genevey et al 2008;Knudsen et al 2008;Valet et al 2008;Nilsson et al 2010) or also including non-dipole contributions (Korte & Constable 2005aKorte et al 2009.…”

Magnetic force microscopy reveals meta-stable magnetic domain states that prevent reliable absolute palaeointensity experiments

“…O ur understanding of both long-and short-term geomagnetic behaviour depends on the availability of reliable well-dated full-vector descriptions of the Earth's magnetic field 1,2 . Such records-preferably evenly distributed over the globe-consist of both palaeodirections and palaeointensities.…”