Testing the directional recording ability of natural chemical remanent magnetisations using historical sediments

Abstract

Rocks containing magnetic minerals capture the Earth's magnetic field during their formation and growth, and acquire a chemical remanent magnetisation (CRM). However, the ability of magnetic minerals in sediments to accurately record the direction of the Earth's magnetic field during CRM acquisition has yet to be field tested. In this study, the directional recording ability of CRMs in nature was tested using historical salt marsh sediments from Norfolk, UK. Our results find greigite is the dominant remanence carrier in the salt marsh sediments. Evidence for this includes a gyroremanent magnetisation acquired during alternating field demagnetisation and abundant authigenic iron sulphides identified in SEM-EDX analysis. These iron sulphides appeared as clusters and framboids of equidimensional grains. This morphology is typical of natural iron sulphides. SEM analysis shows these grains on the surface of existing grains and within cracks, indicating that the iron sulphides grew authigenically. During the authigenic growth of greigite, it will acquire a grain-growth CRM of the geomagnetic field. A consistent direction (declination 356°, inclination 68°) with an α 95 of 8° was found throughout the sediments. This direction is indistinguishable from the average geomagnetic field direction during greigite formation for the last 100 years. Therefore, the CRM carried by greigite has accurately recorded the Earth's field direction. This is the first study to directly demonstrate that natural grain-growth CRMs reliably record magnetic field direction.