Origin of apparent magnetic excursions in deep-sea sediments from Mendeleev-Alpha Ridge, Arctic Ocean

Arctic deep-sea sediments often record intervals of negative inclination of natural remanence that are tens of centimeters thick, implying magnetic excursions with durations of tens of thousand years that far exceed excursion durations estimated elsewhere, and the lack of tight age control usually provides excessive freedom in the labeling of Arctic excursions. Fortuitous variations in sedimentation rate have been invoked to explain the amplified excursions. Alternating field demagnetization of natural remanent magnetization (NRM) of sediment cores 08JPC, 10JPC, 11JPC, and 13JPC recovered by the Healy Oden Trans-Arctic Expedition in August 2005 (HOTRAX05) to the Mendeleev-Alpha Ridge yields apparent magnetic excursions in sediments deposited in the Brunhes Chron. Thermal demagnetization of the NRM, however, implies multiple magnetization components with negative inclination components usually unblocked below similar to 350 degrees C. Analysis of isothermal remanent magnetization acquisition curves from magnetic extracts indicates two magnetic coercivity components superimposed on one another. Magnetic experiments conducted under high and low temperatures show features that are characteristic for (titano)magnetite and titanomaghemite. Presence of the two magnetic phases is further confirmed by elemental mapping on a scanning electron microscope equipped for X-ray energy dispersive spectroscopy (EDS) and by high-resolution X-ray diffraction (XRD). It is unlikely that anomalously thick intervals of negative inclination in these Brunhes-aged sediments are caused by unusual behavior of the magnetic field in the Arctic area. We therefore attribute low and negative NRM inclinations in these cores to partially self-reversed chemical remanent magnetizations, apparently carried by titanomaghemite and acquired during the oxidation of detrital (titano) magnetite grains. The high Ti contents and high oxidation states indicated by EDS and XRD data provide the conditions required for partial self-reversal by ionic reordering during diagenetic maghemitization, and this process appears to have affected all HOTRAX05 cores collected from the Mendeleev-Alpha Ridge.