AGU journal highlights - July 2, 2009

Authors: A.-M. Berggren and A. Aldahan: Department of Earth Sciences, Uppsala University, Uppsala, Sweden;

J. Beer and J. Abreu: Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland;

G. Possnert: Tandem Laboratory, Uppsala University, Uppsala, Sweden;

P. Kubik and M. Christl: Laboratory for Ion Beam Physics, ETH, Zurich, Switzerland;

S. J. Johnsen and B. M. Vinther: Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038004, 2009; http://dx.doi.org/10.1029/2009GL038004

10. Isotope data suggest new history of the Arctic

Recent studies have suggested that beginning about 44 million years ago the Arctic underwent a transition from lake to ocean conditions. On the basis of new evidence from rhenium (Re) and osmium (Os) isotopes in the sedimentary record, Poirier and Hillaire-Marcel propose a new chronology of that change. To trace the Arctic's shift from a freshwater to a marine environment, the authors analyze Re and Os concentrations and Os isotope ratios in sedimentary material collected from the Lomonosov Ridge in the Arctic. Their analysis suggests that the Arctic was an enclosed freshwater lake until about 38 million years ago, when tectonic activity caused the Fram Strait to widen, connecting the Arctic to global ocean circulation and bringing about a fairly rapid transition from lake to ocean conditions. Previous studies had suggested that the transition from lake to ocean conditions was completed only about 17.5 million years ago, after a 26-million-year hiatus. The authors conclude that if the results are confirmed by further studies, they will give scientists a new perspective on the tectonic evolution of the Arctic.

Title: Os-isotope insights into major environmental changes of the Arctic Ocean during the Cenozoic

Authors: Andre Poirier and Claude Hillaire-Marcel: GEOTOP, Université du Québec à Montréal, Montreal, Quebec, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL037422, 2009; http://dx.doi.org/10.1029/2009GL037422

11. Mixing in Earth's outer core causes geomagnetic dipole to collapse

For the past 160 years, the Earth's magnetic dipole has been weakening at a rate of nearly six percent per century. To gain an improved understanding of the mechanisms contributing to the dipole moment collapse, Liu and Olson perform numerical calculations that show how convective mixing flows in the Earth's liquid outer core can lead to a decreasing dipole moment. Their simulations show that as fluid mixes in the outer core, magnetic energy is transferred from the dipole to smaller scales, producing patches of reversed magnetic field at the core-mantle boundary and weakening the dipole. They demonstrate that the rate of dipole moment decay is weakly sensitive to the particular mixing flow pattern but varies with the magnetic Reynolds number, a measure of the velocity of the flow. In particular, the authors find that a mixing flow in the outer core with magnetic Reynolds number in the range of 200�, which they suggest is a physically reasonable range, could account for the historically measured rate of decrease of the geomagnetic dipole moment.