Abstract
Characterization of geomagnetic field behaviour on timescales of centuries to millennia is
necessary to understand the mechanisms that sustain the geodynamo and drive its evolution.
As Holocene paleomagnetic and archeomagnetic data have become more abundant, strategies
for regularized inversion of modern field data have been adapted to produce numerous timevarying
global field models. We evaluate the effectiveness of several approaches to inversion
and data handling, by assessing both global and regional properties of the resulting models.
Global Holocene field models cannot resolve Southern hemisphere regional field variations
without the use of sediments. A standard data set is used to construct multiple models using
two different strategies for relative paleointensity calibration and declination orientation
and a selection of starting models in the inversion procedure. When data uncertainties are
considered, the results are similar overall regardless of whether we use iterative calibration
and reorientation, or co-estimation of the calibration and orientation parameters as part of
the inversion procedure. In each case the quality of the starting model used for initial relative
paleointensity calibration and declination orientation is crucial and must be based on the
best absolute information available. Without adequate initial calibration the morphology of
dipole moment variations can be recovered but its absolute value will be correlated with the
initial intensity calibrations, an effect that might be mitigated by ensuring an appropriate fit
to enough high quality absolute intensity data with low uncertainties. The declination reorientation
mainly impacts regional field structure and in the presence of non-zonal fields will
result in a non-zero local average. The importance of declination orientation is highlighted by
inconsistencies in the West Pacific and Australian sediment records in CALS10k.1b model.
Great care must also be taken to assess uncertainties associated with both paleomagnetic
and age data and to evaluate the effects of poor data distribution. New consistently allocated
uncertainty estimates for sediment paleomagnetic records highlight the importance of adequate
uncertainties in the inversion process, as they determine the relative weighting among
the data and overall normalized misfit levels which in turn influence the complexity of the
inferred field models. Residual distributions suggest that the most appropriate misfit measure
is the L1 norm (minimum absolute deviation) rather than L2 (least squares), but this seems
to have relatively minor impact on the overall results. For future Holocene field modelling
we see a need for comprehensive methods to assess uncertainty in individual archeomagnetic
data so that these data or models derived from them can be used for reliable initial relative
paleointensity calibration and declination orientation in sediments. More work will be needed
to assess whether co-estimation or an iterative approach to inversion is more efficient overall.
This would be facilitated by realistic and globally consistent data and age uncertainties from
the paleomagnetic community.
Original language | English |
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Journal | Geophysical Journal International |
Volume | 202 |
Pages (from-to) | 402-418 |
ISSN | 0956-540X |
DOIs | |
Publication status | Published - 2015 |
Keywords
- Archaeomagnetism
- Magnetic field
- Palaeointensity
- Palaeomagnetic secular variation