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

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.