Axford Medalist

Andrew P. ROBERTS
Australian National University

Fri- 02 Aug, 1:30-2pm
Nicoll Room, Level 3

"Sedimentary Paleomagnetism: What is New and Exciting?"

Abstract
Paleomagnetic analysis provides an understanding of Earth’s magnetic field and the deep-Earth dynamo processes that generate the geomagnetic field. 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. This is all possible because nanoparticulate magnetic rock-forming minerals (e.g., magnetite, hematite, maghemite, pyrrhotite, greigite, goethite) occur commonly 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. Despite the fact that sedimentary paleomagnetism has a history of more than 65 years of active investigation, we have recently made significant progress in understanding the recording of paleomagnetic signals by sediments. In particular, the widespread role of so-called magnetotactic bacteria in contributing to the magnetization of sediments has come to be recognized over the last decade. These bacteria biomineralize intracellular magnetic minerals in chains with ideal SD properties 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 if these magnetic particles are preserved over geological timescales. Recent developments enable us to identify such magnetofossils within sediments and we have shown that they dominate the paleomagnetic signature in many ancient sedimentary environments, particularly pelagic marine carbonates. Recognition of the widespread presence of magnetofossils in the geological record is providing a new understanding of the mechanisms by which sediments acquire paleomagnetic signals.

Biography
Andrew Roberts is a Professor in the Research School of Earth Sciences at the Australian National University (ANU). From 2012 to 2017 he served as Dean of the College of Physical and Mathematical Sciences at ANU. He was previously Director of the Research School of Earth Sciences at ANU (2010-2012) and 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, Taiwan, France, Germany, Norway, Australia, and New Zealand. He has published more than 235 papers in peer-reviewed international scientific journals, and he is a Fellow of the American Geophysical Union and an Honorary Fellow of the Royal Society of New Zealand.