Understanding marine environmental change and associated biological turnover across the Palaeocene–Eocene Thermal Maximum (PETM; ~56 Ma)—the most pronounced Cenozoic short-term global warming event—is important because of the potential role of the ocean in atmospheric CO2 drawdown, yet proxies for tracing marine productivity and oxygenation across the PETM are limited and results remain controversial. Here we show that a high-resolution record of South Atlantic Ocean bottom water oxygenation can be extracted from exceptionally preserved magnetofossils—the bioinorganic magnetite nanocrystals produced by magnetotactic bacteria (MTB) using a new multiscale environmental magnetic approach. Our results suggest that a transient MTB bloom occurred due to increased nutrient supply. Bottom water oxygenation decreased gradually from the onset to the peak PETM. These observations provide a record of microbial response to the PETM and establish the value of magnetofossils as palaeoenvironmental indicators.

As samples of ever decreasing sizes are being studied paleomagnetically, care has to be taken that the underlying assumptions of statistical thermodynamics (Maxwell‐Boltzmann statistics) are being met. Here we determine how many grains and how large a magnetic moment a sample needs to have to be able to accurately record an ambient field. It is found that for samples with a thermoremanent magnetic moment larger than 10−11Am2 the assumption of a sufficiently large number of grains is usually given. Standard 25 mm diameter paleomagnetic samples usually contain enough magnetic grains such that statistical errors are negligible, but “single silicate crystal” works on, for example, zircon, plagioclase, and olivine crystals are approaching the limits of what is physically possible, leading to statistic errors in both the angular deviation and paleointensity that are comparable to other sources of error. The reliability of nanopaleomagnetic imaging techniques capable of resolving individual grains (used, for example, to study the cloudy zone in meteorites), however, is questionable due to the limited area of the material covered.

We build micromagnetic models to investigate the magnetic properties of biogenic magnetitea common type of magnetic minerals that is responsible for recording a wide range of biological, geophysical and geological processes on earth. The geometry of modelled particles is based on realistic size and shape distributions from nanoscale observations. Systematic changes in microstructures of biogenic magnetite ensembles are built and their magnetic properties are calculated, which enables a quantitative and separate assessment of the effect of crystal morphology and chain structures. Although the same particle size and shape distributions are used in all calculations, simulation results document large variations in magnetic properties, i.e., wide distributions of coercivity (B c = ~10-60 mT), coercivity of remanence (B cr = ~14-81 mT), dispersion parameter (DP = ~0.1-0.5), and skewness values (S = ~0.7-1.1) due to variable degree of anisotropy and magnetostatic interactions. Previously, the commonly observed "biogenic soft" and "biogenic hard" components on biogenic magnetite-bearing samples were often interpreted to reflect crystal Revision Notes -Manuscript with Track ChangesMagnetofossil identification and robust interpretation of magnetofossil records require a thorough understanding of the natural variability of their magnetic properties. Several studies demonstrate the potential importance of magnetofossil morphologies in controlling magnetic properties. By analyzing a large set of lacustrine and marine sediments, Egli (2004) identified two distinct groups of biogenic magnetite: biogenic soft (BS) and biogenic hard (BH) components, which were suggested to correspond to different magnetofossil morphologies, i.e. BH and BS components are related to more elongated and more equant magnetite crystals, respectively. Such magnetic fingerprints are suggested for tracing environmental conditions . The presence of BS and BH signatures was widely reported and was used for various paleoenvironmental reconstructions (e.g., . The origin of BS and BH components in sedimentary records was tested by transmission electron microscopic (TEM) imaging, which indicated a casual link between magnetofossil morphologies and magnetic properties (e.g.,

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A new protocol using the viscous remanent magnetization (VRM) of boulders to date cataclysmic geological events such as tsunamis, glacial floods, and landslides is presented and its performance is assessed against two jökulhlaups (glacial floods) of known age in Iceland. High-intensity jökulhlaups have the ability to break off large boulders from bedrock and emplace and rotate them. These rocks originally carried a remanent magnetization parallel to the geomagnetic field during their formation. After being rotated by the flood, they acquire a VRM parallel with Earth's magnetic field. In continuous thermal demagnetization experiments the unblocking temperature of the VRM can be determined, and subsequent rock magnetic VRM acquisition experiments can be used to establish a relationship between the unblocking temperature and the acquisition time, from which the time since the flood can be determined. The protocol was tested on 44 boulders from 2 historical jökulhlaups in Iceland and found to yield good order-of-magnitude estimates: 72 yr (confidence limits 11–360 yr) versus known 155 yr at the Sólheimajökull jökulhlaup and 290 yr (confidence limits 80–2300 yr) versus known 288 yr for the Kotarjökull jökulhlaup. The method can therefore be a valuable tool for future dating of cataclysmic events