Determining magnetic field intensity from anisotropy of magnetic remanence in sediments

Anisotropy of magnetic remanence (AMR) holds promise to quantify relative paleointensity records from sedimentary rocks. A proof of concept was established in applied fields that exceeded the intensity of the geomagnetic field. Under Earth-like fields, large uncertainties necessitate the development of a method to improve the estimation of the anisotropy tensor. To this aim, we designed a numeral method that demonstrates the applicability and resolution needed to optimize the experimental protocol. We then implemented a 30-position procedure for AMR measurements and compared the numerical and experimental data with a typically-used, 12-position procedure. Redeposition experiments with sediments rich in single domain magnetite were carried out in fields with intensities of 0, 10, 50 and 100 µT; 25 individual samples were redeposited and measured at each field condition. The 30-position protocol facilitates the isolation and resolution of the field-aligned prolate fabric (1-2% of the total) from the oblate sedimentary fabric, and more so when applying tensor subtraction of the fabric obtained in a null field. Scatter of the anisotropy parameters is inversely proportional to field strength, which together with the high-resolution protocol and tensor subtraction method, should lead to robust relative paleointensity corrections and/or estimates.