Acquisition of remanent magnetisation in sediments

The natural remanent magnetization (NRM) of sediments can be used to reconstruct variations of Earth’s magnetic field in the past. The variability of the geomagnetic field provides information on Earth’s internal dynamics and its sedimentary record can also serve as a stratigraphic tool. In addition to its relationship with the Earth’s magnetic field, sediment magnetization is constrained by the sedimentary environment. To succeed in faithfully tracking the paleomagnetic variations, it is necessary to determine which processes are involved during the acquisition of the magnetization. The aim of this thesis is to better understand processes responsible for the acquisition of magnetization by sediments by studying sedimentary and magnetic parameters involved in the blocking of magnetic grains within the sediment. For this purpose, varved sediments of the proglacial Lake Ojibway, as well as turbidites, have been studied.

The first chapter is about the limits of the use of U-channels for the study of rapid variations of the Earth’s magnetic field, especially for excursions and reversals. For this, a comparison between discrete measurements and U-channels, from different inversions and excursions, was made. We created different excursion sizes and found out that excursions recorded over less than 7.5 cm are barely detected in U-channel measurements. Regarding reversals, U-channel measurements smooth the signal of low-resolution records and generate artifacts. We tested the convolution of discrete samples measurements by different response functions. The results show that even small response function changes can generate significant differences in results.

In the second chapter, we focused on sediments from the proglacial Lake Ojibway (~ 8.5 ka cal BP) and more particularly on several centimeter-thick summer and winter beds that were sampled individually. Paleomagnetic, granulometric and geochemical analyses were conducted on each bed. Magnetic declinations do not show strong systematic deviations from the expected direction, contrary to inclinations, which are much shallower than expected. Inclination shallowing is systematically more pronounced in winter (25.5 ° ± 4.3 °) than in summer (12.5 ° ± 3.3 °). Summer beds are thicker than winter beds and characterized by stronger magnetic susceptibility, higher Ca/Fe ratio due to Fe dilution by the large increase in carbonate content, and coarser sedimentary and magnetic grains. These observations reflect the input of coarser detrital particles during summer, while the finer fraction remained in suspension until deposition in winter. Differential compaction of the winter and summer beds and variations of magnetic grain composition might be responsible for the observed differences. These results indicate that lithological changes can play a dominant role on magnetic sediment records.

In the third chapter, we have compiled magnetic data of 17 rapid deposit layers (RDL) with varying thicknesses ranging from 7.1 cm to 1510 cm. This study was conducted from a statistical point of view to highlight the mechanisms of acquisition of magnetization, which play a major role during this type of depositions. We found a logarithmic relationship between the amplitude of inclination changes, as well as the amplitude of magnetic grain sizes and the RDL thickness. Inclination and grain sizes are themselves correlated to each other by a logarithmic law. As there is no relationship between the deviation and depth, compaction cannot account for such significant deviations. Flocculation likely varies with grain size, but again, the amplitude of the inclination deviations is difficult to explain. Turbulence inherent to the depositional process of such events is most likely the dominant factor. This interpretation is supported by calculations aimed at describing the impact of bottom currents.

In the end, the main aim of this thesis was to improve our knowledge of the mechanisms of magnetization’s acquisition by sediments. This study highlighted two new mechanisms, which have an influence on the recording of magnetization in sediments: turbulence and a little variation in magnetic composition.