PADM2M: A TIME-VARYING MODEL OF PALEOMAGNETIC AXIAL DIPOLE MOMENT FOR 0-2 MA

Large numbers of Relative Paleomagnetic field Intensity (RPI) and Absolute Paleointensity (API) data are being used to develop models of variations in Earth's magnetic field strength on million year timescales. Such models are used in geomagnetic studies and for large scale regional and global stratigraphic correlations of sedimentary records. Previous efforts to model these variations in terms of Virtual Axial Dipole Moment (VADM) generally involve normalizing, stacking and averaging selected RPI time series and scaling the result using API data. We have developed a maximum likelihood (ML) estimation technique to construct a continuous time-varying model of Paleomagnetic Axial Dipole Moment (PADM) from the joint API and RPI data set. Time variations in PADM are parametrized by cubic B-splines and we find a regularized model with appropriate temporal resolution and reasonable misfit to the data. Scaling factors needed to calibrate RPI data are estimated as part of the ML procedure. The new method has been applied to a global data collection of over 3000 archeomagnetic API data, 1800 igneous API data, and 89 sedimentary RPI time series from the Geomagia50, PINT08, and MagIC paleomagnetic databases. The result is called PADM2M (PADM for 0-2 Ma), and is in reasonable agreement with existing VADM models. It has paleointensity lows at field reversals and excursions, and robustly resolves features at timescales longer than 10 kyr. In the time period 1.3-1.5 Ma there are some differences in timing of various features. Unlike earlier models which lack temporal resolution in the most recent part of the record, PADM2M is in excellent agreement with direct historical measurements of and archeomagnetic variations in dipole moment. PADM2M is used to provide an updated power spectrum for dipole moment variations: there are slight improvements in resolution but no clear signatures at orbital frequencies.