Mud, microbes and magnetism
Paleomagnetism has made fundamental contributions to Earth science through helping to establish the global plate tectonic paradigm and by providing the geomagnetic polarity timescale for calibrating geological time. Paleomagnetic analysis also provides an understanding of the Earth’s magnetic field and the deep-Earth dynamo processes that generate the geomagnetic field. This is all possible because nanoparticulate magnetic rock-forming minerals (e.g., magnetite, hematite, maghemite, pyrrhotite, greigite, goethite) commonly occur in nature within the ideal single domain (SD) grain size range (for magnetite, the SD range is ~30 to 100 nm). The Nobel laureate, Louis Néel, demonstrated that SD materials (in which magnetic particles have homogeneous magnetization) can retain stable magnetizations for periods exceeding the age of the Earth. The long-term stability of these magnetizations provides the basis for the widespread usefulness of paleomagnetism in Earth science. Within sediments and sedimentary rocks, there are many processes that give rise to interactions between microbes and minerals that can produce magnetizations with varying ages. Understanding these processes is therefore fundamental to interpretation of paleomagnetic data. I will discuss two such processes. The first involves the microbial degradation of organic matter, which gives rise to a series of reactions that can dissolve primary magnetic iron oxide minerals and produce secondary magnetic iron sulphide minerals. This is often bad news for paleomagnetism; but rigorous data interpretation requires recognition of the processes involved. The second involves direct assimilation of available dissolved iron by bacteria. These so-called magnetotactic bacteria intracellularly biomineralize ideal SD magnetic minerals that they use to orient along geomagnetic field lines to enable them to reduce the dimensions of their search for ideal habitats in highly stratified chemical environments. When they die, their mineral remains can provide ideal paleomagnetic signals. However, because of the eventual burial of many such sediments into anoxic environments, these particles are prone to dissolution. Few pre-Quaternary records of such magnetofossils have therefore been recorded in the literature. New developments enable us to better identify magnetofossils within sediments and we now have evidence that they dominate the paleomagnetic signature in many ancient sedimentary environments, particularly pelagic marine carbonates. These examples demonstrate that microbes can be a major determinant of the magnetism of mud. Recognition of the widespread presence of magnetofossils in the geological record is now providing a new understanding of the mechanisms by which sediments acquire paleomagnetic signals.
Professor Andrew Roberts is Dean of the College of Physical and Mathematical Sciences at the Australian National University (ANU). Prior to taking on this role, he was Director of the Research School of Earth Sciences at ANU (2010-2012) and was previously Head of the School of Ocean and Earth Science at the University of Southampton and Associate Director of the National Oceanography Centre (2005-2009) in the U.K. He has served on scientific advisory committees in the U.K., U.S.A., China, France, Germany, Norway, Australia and New Zealand. He has published 180 papers in peer-reviewed international scientific journals and is a Fellow of the American Geophysical Union and the Royal Society for the Arts (London), and is an Honorary Fellow of the Royal Society of New Zealand.
Snacks will be provided at 11:30am prior to the Colloquium in the RSPE tearoom Oliphant Building 60
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