Although biomarker, trace element, and isotopic evidence have been used to claim that oxygenic photosynthesis evolved by 2.8 giga-annum before present (Ga) and perhaps as early as 3.7 Ga, a skeptical examination raises considerable doubt about the presence of oxygen producers at these times. Geological features suggestive of oxygen, such as red beds, lateritic paleosols, and the return of sedimentary sulfate deposits after a Ϸ900-million year hiatus, occur shortly before the Ϸ2.3-2.2 Ga Makganyene ''snowball Earth'' (global glaciation). The massive deposition of Mn, which has a high redox potential, practically requires the presence of environmental oxygen after the snowball. New age constraints from the Transvaal Supergroup of South Africa suggest that all three glaciations in the Huronian Supergroup of Canada predate the Snowball event. A simple cyanobacterial growth model incorporating the range of C, Fe, and P fluxes expected during a partial glaciation in an anoxic world with high-Fe oceans indicates that oxygenic photosynthesis could have destroyed a methane greenhouse and triggered a snowball event on timescales as short as 1 million years. As the geological evidence requiring oxygen does not appear during the Pongola glaciation at 2.9 Ga or during the Huronian glaciations, we argue that oxygenic cyanobacteria evolved and radiated shortly before the Makganyene snowball.oxygen ͉ Makganyene glaciation ͉ Huronian glaciations ͉ cyanobacteria

Magnetotactic bacteria produce intracellular crystals of magnetite or greigite, the properties of which have been shaped by evolution to maximize the magnetic moment per atom of iron. Intracellular bacterial magnetite therefore possesses traits amenable to detection by physical techniques: typically, narrow size and shape distributions, single-domain size and arrangement in linear chains, and often crystal elongation. Past strategies for searching for bacterial magnetofossils using physical techniques have focused on identifying samples containing significant amounts of single domain magnetite or with narrow coercivity distributions. Searching for additional of traits would, however, increase the likelihood that candidate magnetofossils are truly of biological origin. Ferromagnetic resonance spectroscopy (FMR) is in theory capable of detecting the distinctive magnetic anisotropy produced by chain arrangement and crystal elongation. Here we present analyses of intact and lysed magnetotactic bacteria, dilutions of synthetic magnetite, and sedimentary samples of modern carbonates from the Great Bahama Bank, Oligocene-Miocene deep-sea muds from the South Atlantic, and Pleistocene lacustrine deposits from Mono Basin, California. We demonstrate that FMR can distinguish between intact bacterial magnetite chains, collapsed chains, and linear strings of magnetite formed by physical processes. We also show that sediments in which the magnetization is likely carried by bacterial magnetite have FMR spectra resembling those of intact or altered bacterial magnetite chains.

Ferromagnetic resonance spectroscopy (FMR) can be used to measure the effective magnetic field within a sample, including the contributions of both magnetic anisotropy and magnetostatic interactions. One particular use is in the detection of magnetite produced by magnetotactic bacteria. These bacteria produce single-domain particles with narrow size and shape distributions that are often elongated and generally arranged in chains. All of these features are detectable through FMR. Here, we examine their effects on the FMR spectra of magnetotactic bacteria strains MV-1 (which produces chains of elongate magnetite crystals), AMB-1 (which produces chains of nearly equidimensional magnetite crystals), and two novel mutants of AMB-1: mnm13 (which produces isolated, elongate crystals), and mnm18 (which produces nearly equidimensional crystals that are usually isolated). Comparison of their FMR spectra indicates that the positive magnetic anisotropy indicated by the spectra of almost all magnetotactic bacteria is a product of chain alignment and particle elongation. We also find correlations between FMR properties and magnetic measurements of coercivity and magnetostatic interactions. FMR thus provides a rapid method for assessing the magnetic properties of assemblages of particles, with applications including screening for samples likely to contain bacterial magnetofossils. Index terms: 1505 biogenic magnetic minerals, 1518 magnetic fabrics and anisotropy, 1540 rock and mineral magnetism, 0419 biomineralization, 0465 microbiology.Key words: ferromagnetic resonance spectroscopy, magnetotactic bacteria, magnetofossils, magnetic anisotropy, magnetostatic interactions, rock magnetism An edited version of this paper was published by AGU. Copyright (C) 2006 American Geophysical Union. 2 IntroductionFerromagnetic resonance spectroscopy (FMR), a form of electron spin resonance spectroscopy, can serve as a rapid technique for assessing the magnetic anisotropy of and magnetostatic interactions between individual particles in a polycrystalline sample. It is based upon the Zeeman effect, which is the splitting between electron spin energy levels that occurs in the presence of a magnetic field. The Zeeman effect allows a ground-state electron to absorb a photon with energy equal to the splitting between the energy states. In a magnetic material, magnetic anisotropy (whether magnetocrystalline, shape, or stress-induced) and interparticle interactions contribute to the energy of the particles within a sample and thereby alter the resonance energy. As a result, FMR can be used to probe these parameters [Griscom, 1974;Griscom, 1981;Kittel, 1948;Kopp, et al., 2006;Schlömann, 1958;Weiss, et al., 2004].Techniques for measuring anisotropy and magnetostatic interactions have a number of applications in the field of rock magnetism. The example on which we will focus here is the identification of magnetite produced by magnetotactic bacteria, a topic of great interest for understanding the magnetization of sediments. Fossil magnetotactic bacte...

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