title: Characterising the Triassic palaeomagnetic field: Implications for geomagnetic field evolution
Creators: name: Handford, Benjamin
Publisher: name: University of Liverpool Repository
Year: 2023

description: Records of the palaeomagnetic field are unique in their potential to provide insight into the evolution of the Earth’s interior. The geomagnetic field is a product of a dynamo process, and certain variations in the geomagnetic field at the Earth’s surface are reflective of variations in the Earth’s internal dynamics. Across time scales of tens of millions of years, such variations are proposed to reflect changes in core-mantle boundary conditions. Typically, variations in long-term geomagnetic field behaviour are expressed in terms of the frequency at which the geomagnetic field was experiencing polarity reversals, its average strength, and its stability. Of these measurable characteristics, it is changes in average polarity reversal frequency that are most well-defined. In particular, a relatively well-developed, continuous understanding of variations in average polarity reversal frequency has been established for the last 320 million years. Knowledge of average palaeointensity and palaeosecular variation (PSV) of the geomagnetic field, are less well established, with obvious shortcomings for the Triassic (ca. 251.9-201.3 Ma). This study aims to address these gaps by investigating the distribution of virtual geomagnetic pole dispersion, a common measure of PSV, during the Triassic, and conducting palaeointensity experiments on geological units that have previously been assigned Triassic dates. The study on PSV revealed a near-invariant latitudinal dependence of VGP dispersion for a time interval encompassing the Triassic, that was near-identical to that of the preceding time interval, as defined by average polarity reversal frequency. In contrast, the Permo-Carboniferous Reversed Superchron (PCRS), one of two such events during the last 320 million years in which polarity reversals were essentially absent, displayed an extreme example of VGP dispersion pattern that in part suggested that the PCRS was a time interval of enhanced axial dipole dominance. Successful palaeointensity estimates were deemed to be reliable from specimens of pillow lavas of the Fernazza Formation (Italy) and ignimbrites and sills of the Puesto Viejo Group (Argentina). Estimates of virtual (axial) dipole moment ranged between 0.9 and 4.8 x 1022 Am2. These results represent a significant improvement in attempts to define the strength of the geomagnetic field during the middle-late Triassic. Finally, taking advantage of these results, and the resulting improved continuity of palaeomagnetic records across the last 320 million years, statistical models were generated for time intervals of distinct reversal frequency. The resulting models represent a viable description of the palaeomagnetic field across each studied time interval. Exploring a range of different statistical models that aimed to describe the palaeomagnetic field during the PCRS, suggested that the more extreme PSV behaviour observed in the earlier study is likely representative of field behaviour across this time. Comparing preferred models that were representative of the different studied time intervals, suggested that the somewhat unique palaeomagnetic observations for the PCRS might be due to a difference in the relationship between antisymmetric and symmetric terms. Overall, the process deployed throughout this study has demonstrated the effectiveness of analysing palaeomagnetic records alongside statistical models, simultaneously utilising data that describes different characteristics of the palaeomagnetic field. Better constraining the assumptions associated with statistical models, for example through the use of geodynamo simulations, would better facilitate their use as a method to test the reliability of palaeomagnetic datasets. More refined datasets would in turn provide a more accurate description of the palaeomagnetic field.