Earth Planets Space, Vol. 59 (No. 7), pp. 815-824, 2007
Andrew P. Roberts1, Anisch Bakrania1, Fabio Florindo1,2, Christopher J. Rowan1, Christopher R. Fielding3, and Ross D. Powell4
1National Oceanography Centre, University of Southampton, European Way, Southampton, SO14 3ZH, UK
2Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143, Rome, Italy
3Department of Geosciences, 214 Bessey Hall, University of Nebraska, P.O. Box 880340, Lincoln, NE 68588-0340, USA
4Department of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115, USA
(Received October 7, 2006; Revised May 14, 2007; Accepted May 15, 2007; Online published July 20, 2007)
We report a high-resolution record of a Miocene polarity transition (probably the Chron C6r-C6n transition) from glacimarine sediments in McMurdo Sound, Ross Sea, Antarctica, which is the first transition record reported from high southern latitudes. The transition is recorded in two parallel cores through a 10.7 m stratigraphic thickness. The sediments are interpreted as having been deposited in a marine environment under the influence of floating ice or seaward of a glacier terminus from which a large sediment load was delivered to the drill site. The core was recovered using rotary drilling, which precludes azimuthal orientation of the core and determination of a vector record of the field during the transition. However, constraints on transitional field behaviour are provided by the exceptional resolution of this record. Large-scale paleomagnetic inclination fluctuations in the two cores can be independently correlated with each other using magnetic susceptibility data, which suggests that the sediments are reliable recorders of geomagnetic field variations. Agreement between the two parallel transition records provides evidence for highly dynamic field behaviour, as suggested by numerous large-scale inclination changes (∼90°) throughout the transition. These large-scale changes occur across stratigraphically narrow intervals, which is consistent with the suggestion of rapid field changes during transitions. In one intact portion of the core, where there is no apparent relative core rotation between samples, declinations and inclinations are consistent with the presence of a stable cluster of virtual geomagnetic poles within the transition (although the possibility that this cluster represents a rapid depositional event cannot be precluded). These observations are consistent with those from other high-resolution records and provide a rare detailed view of transitional field behaviour compared to most sedimentary records, which are not as thick and which appear to have been smoothed by sedimentary remanence acquisition processes.
Key words: Paleomagnetism, geomagnetic field behaviour, polarity reversal, glacial processes, Antarctica.