Magnetotactic bacteria, cosmogenic beryllium and the Earth's magnetic field: obtaining high-quality records of magnetofossil preservation and relative paleointensity over the last 1.1 million years

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Sedimentary records are one of the few means providing continuous information about the past geomagnetic field changes and thus the evolution of geodynamo and the Earth’s interior. Field variations are recorded by the sediment through three independent mecha-nisms: (a) the partial alignment of magnetic minerals of detrital origin, (b) the partial align-ment of magnetofossils produced by magnetotactic bacteria, and (c) the cosmogenic isotope 10Be, whose production rate is modulated by the field-dependent screening of cosmic rays. Only few studies have documented the recording mechanisms (b) and (c), showing a correla-tion with relative paleointensity records, but also significant differences caused by different environmental contaminations. Magnetofossils potentially provide a fourth field recording mechanism of interest for rocks formed during the early history of our planet, where strong metamorphism might have erased their original magnetization. This requires checking whether more recent magnetofossil records are affected by low field intensities during field excursions and reversals. The proposed project aims at tackling the following fundamental questions for the abovementioned recording mechanisms: (1) How can we eliminate environmental overprints from sedimentary records of the Earth magnetic field? (2) Which factors govern the efficiency and timing of magnetic particle alignment in sediment? (3) How do magnetofossils affect relative paleointensity determinations, and how can magnetofossils be used to improve paleomagnetic records. (4) Do reversals and excursions affect magnetotactic bacteria popu-lations and the associated magnetofossil record? The main goal of this project is to answer the above questions through the study of a continuous marine sedimentary sequence from Western Pacific Ocean covering the last 1.1 Ma, using the unique infrastructure of the paleomagnetic laboratory at Ludwig-Maximilians University and advanced modelling. The availability of a high-resolution 10Be record for this core enables unique comparisons between magnetic and cosmogenic records. Such comparisons allow a better identification and elimination of environmental overprints, paving the way to more reliable relative paleointensity records. Identification of climatic overprints is also essential for disentangling the responses of magnetotactic bacteria populations to environ¬mental variations and low-field conditions. The marine sedimentary sequence contains 16 excursions and the Matuyama-Brunhes reversal. Unbiased records of these events, along with realistic confidence intervals obtained from the comparison of the three-abovementioned recording mechanisms, are essential for the improvement of global field models and a better integration of sedimentary and cosmogenic records with other datasets.