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

沉积记录是为数不多的提供有关过去地磁场变化以及地球发电机和地球内部演化的连续信息的手段之一。沉积物通过三种独立的机制记录了磁场的变化：(A)碎屑成因的磁性矿物的部分排列，(B)趋磁细菌产生的磁性化石的部分排列，以及(C)宇宙成因同位素10Be，其产生速率受场相关宇宙线屏蔽的调节。只有很少的研究记录了记录机制(B)和(C)，显示出与相对古强度记录的相关性，但也显示了不同环境污染造成的显著差异。磁化石可能为地球早期形成的岩石提供了第四种感兴趣的磁场记录机制，在那里强烈的变质作用可能抹去了它们最初的磁化作用。这需要检查在磁场漂移和反转期间，较新的磁铁记录是否受到低场强的影响。该项目旨在解决上述记录机制的以下基本问题：(1)如何从地球磁场的沉积记录中消除环境叠加；(2)哪些因素决定了沉积物中磁粒排列的效率和时间；(3)磁化石如何影响相对古强度的测定，以及如何利用磁化石来改进古地磁记录。(4)反转和漂移是否影响趋磁细菌的繁殖和相关的磁化记录？该项目的主要目标是利用路德维希-马克西米利安大学古地磁实验室独特的基础设施和先进的模拟技术，通过研究过去1.1 Ma以来西太平洋的连续海洋沉积序列来回答上述问题。这个核心的高分辨率10Be记录的可用性使磁记录和宇宙起源记录之间能够进行独特的比较。这样的比较可以更好地识别和消除环境叠加，为获得更可靠的相对古强度记录铺平道路。气候叠加的识别对于弄清趋磁细菌种群对环境变化和低场条件的反应也是必不可少的。海相沉积层序包括16个漂移和松山-布鲁内斯反转。对这些事件的无偏见记录，以及通过比较上述三种记录机制获得的实际可信区间，对于改进全球场模型和更好地将沉积和宇宙成因记录与其他数据集结合起来至关重要。

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.