The geomagnetic field acts both as an umbrella, shielding us from cosmic radiation and as a window, offering one of the few glimpses of the inner workings of the Earth. Ancient records of the geomagnetic field can inform us about dynamics of the early Earth and changes in boundary conditions through time. Thanks to its essentially dipolar nature, the field has acted as a guide, pointing to the axis of rotation thereby providing latitudinal information for both explorers and geologists. Human measurements of the geomagnetic field date to about a millenium and are quite sparse prior to about 400 years ago. Knowledge of what the field has done in the past relies on accidental records carried by the magnetization of geological and archaeological materials. Obtaining meaningful information from such materials requires an understanding of complex recording processes, which are poorly understood for most natural materials as these deviate from the ideal tractable to theory. Some of the most important archives for the history of the geomagnetic field are found in sediments and sedimentary rocks. Such materials can preserve quasi-continuous records of the geomagnetic field vector, which can be turned into time series with rather tight age control. Because of their many advantages, sediments and sedimentary rocks have been the subject of intense study by paleomagnetists for over sixty years. There are over 100 publications reporting results of ancient field intensity based on sedimentary materials. Yet how sediments get magnetized and how best to retrieve the geomagnetic field vector are still not well understood. This is the topic of the present proposal. In this study, we are constructing numerical models designed to explain experimental data that we are obtaining through laboratory redeposition experiments under carefully controlled conditions. These models with then be used to make specific predictions testable with published data sets.
Our work has significant impacts throughout the geosciences, contributing to the understanding of one of the most important physical properties of planet Earth, its magnetic field. It will contribute to the understanding of many fundamental processes, including plate motion constraints from paleomagnetic data, the role of the inner core in controlling the geomagnetic field, and establishing the average strength of the dipole moment to place recent observations in perspective. Constraints on the geomagnetic field are important to those in the deep Earth community who study the dynamical processes in Earth's deep interior. Through this grant we will augment the publicly accessible database for magnetic information (MagIC), greatly expanding its utility to the broader geoscience community. This is particularly important as the role of the Earth's magnetic field in the climatic system undergoes increased scrutiny. Finally, it will serve as the doctoral dissertation project of a graduate student at the Scripps Institute of Oceanography.
Direct observation of the geomagnetic field has been carried out for the last four centuries; prior to that time we must be cull data from archeological and geological proxies. One of the first motivations for investigating the magnetism of rocks, therefore, was to study the behavior of Earth's magnetic field in the past. Paleo-geomagnetism is almost unique among geophysical endeavors in that a historical perspective is possible, and the potential of the rock record was realized from very early on. The magnetic field is a vector field, having both direction and intensity, and a complete understanding of it requires study of the full vector properties. However, paleointensity determinations are much more difficult than directional ones and by far the majority of paleo-geomagnetic studies are concerned solely with directional variability of the field. The advantage of sedimentary sequences over igneous records is that they can provide more or less continuous records that can be dated with relative precision. Good global coverage is also a possibility. Great strides were made under this grant in gathering, compiling and stacking relative paleointensity records spanning the few million years. Although there is impressive agreement overall among various recent "global" relative paledointensity models, there are periods during which the agreement is poor. For us to make further progress in understanding paleintensity variations, we must first understand the sources of "noise" in the sedimentary paleointensity database. Despite decades of effort there is little consensus on how sediments get magnetized. Our laboratory experiments and numerical modeling efforts demonstrate a link between inclination shallowing, the linearity of the magnetic response (or lack thereof) and the degree of flocculation in the sediments. These studies hint at potentially serious problems in current practices of normalization. Under this grant we employed a comprehensive approach of 1) augmentation and examination of the database of published sedimentary records, 2) laboratory redeposition experiments on natural sediments, particularly those that will contribute to resolving the cause of discrepancies between two recent global stacks, and 3) numerical modeling of the redeposition results.
