High-resolution records of non-dipole variations in the intensity of the Earth's magnetic field

Abstract

Our understanding of the short-term behavior of the Earth’s magnetic field is currently mainly hampered by a lack of high-resolution records of geomagnetic intensity variations that are well distributed over the globe and cover the same timespan. Over the past decades many efforts have been made to improve the methodology to obtain reliable estimates of the paleointensity from lavas – the only recorder of the intensity of the Earth’s magnetic field that is available for all parts of the globe on geological time scales, and is capable of registering short-term fluctuations. In this thesis we demonstrate that some paleointensity methods will work on certain lavas with specific thermomagnetic behavior, but fail for others. We therefore propose a ‘multi-method paleointensity approach’ that consists of Thellier-style, MSP-style, and pseudo-Thellier-style experiments to construct high-resolution records of non-dipole variations in the intensity of the Earth’s magnetic field. By applying our new approach to suites of lavas from Hawai’i (USA), and Tenerife (Canary Islands, Spain) we obtain important constraints for short-term local geomagnetic intensity highs, and their possible driving mechanisms. Our new record for Hawai’i indicates that, approximately 1000 years ago, the local field intensity increased on the order of 50% for 200 years – a qualitatively similar phenomenon to that observed 200 years earlier in western Europe and 500 years later in southwestern USA. When these records are combined with a record for Japan, a coherent picture emerges that includes the dipole component decaying steadily since at least 1000 years ago. Superimposed onto this at a regional level are strong but shorter-term intensity variations whose asynchronicity necessitates a highly non-dipolar nature. Our paleointensity data obtained for Tenerife reveals high paleointensities coinciding with an intensity highs occurring in the Middle East around 1000 BC. If our findings are related to this jerk, it must have extended more than 50 degrees westwards in longitude. With the multi-method paleointensity approach outlined in this thesis it is feasible to obtain records of regional variations in the paleofield’s intensity from lavas. Hence, we can set out to produce similar records for other volcanic edifices – especially on the Southern hemisphere – and thereby increase the global description of the short-term behavior of the geomagnetic field, along with regional, non-dipole, features. A better spatial and temporal global coverage of these records will lead to an improved understanding of the occurrence of the short-lived regional intensity highs observed in some records, and may elucidate their driving mechanism. Furthermore, they will provide essential observational constraints for (numerical) models of the geomagnetic field and the geodynamo, constraints that cannot be obtained otherwise.