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

Abstract

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 ∼350°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.

Interglacial Paleoclimate in the Arctic

“…Additional age data come from amino acid racemization (Kaufman et al, ), strontium dating (Dipre et al, ) and Optically Stimulated Luminescence (Jakobsson et al, ). Magnetostratigraphy has also been attempted in Arctic sediments (reviewed in Xuan & Channell, ), but complications from self‐reversals and anomalous excursions preclude its use as a chronological tool (Channell & Xuan, ).…”

Magnetic mineral diagenesis

“…It is unclear whether short-period paleomagnetic signals are compromised by magnetofossil maghemitization, but the slowly deposited pelagic carbonates studied by Florindo and do not provide a useful test of this possibility. A better test is provided by Arctic Ocean sediments in which partial oxidation gives rise to particles with a titanomagnetite core and titanomaghemite rim (Channell and Xuan, 2009;Xuan and Channell, 2010). This oxidation has been argued to cause self-reversal where the titanomaghemite carries a chemical remanent magnetization (CRM) and the host titanomagnetite carries a detrital remanent magnetization (DRM).…”

“…This oxidation has been argued to cause self-reversal where the titanomaghemite carries a chemical remanent magnetization (CRM) and the host titanomagnetite carries a detrital remanent magnetization (DRM). Paleomagnetic recording complexities associated with partial oxidation of titanomagnetite have been proposed as the cause of spurious geomagnetic excursions (Channell and Xuan, 2009;Xuan and Channell, 2010) reported from the Arctic Ocean (e.g., Nowaczyk and Antonow, 1997). Magnetic mineral oxidation, therefore, has apparently variable effects on the paleomagnetic record of sediments and care is required to assess its possible influence.…”

“…Nevertheless, prolonged oxygen diffusion into the sediment can lead to partial or complete oxidation of detrital magnetite or titanomagnetite to form maghemite (e.g., Cui et al, 1994) or titanomaghemite (e.g., Xuan and Channell, 2010) rims on the surface of (titano)magnetite grains. Complexities resulting from such oxidation can make oxic pelagic sediments unsuitable for paleomagnetic recording (e.g., Xuan and Channell, 2010).…”

“…Nevertheless, prolonged oxygen diffusion into the sediment can lead to partial or complete oxidation of detrital magnetite or titanomagnetite to form maghemite (e.g., Cui et al, 1994) or titanomaghemite (e.g., Xuan and Channell, 2010) rims on the surface of (titano)magnetite grains. Complexities resulting from such oxidation can make oxic pelagic sediments unsuitable for paleomagnetic recording (e.g., Xuan and Channell, 2010). Hematite or goethite formation in oxic diagenetic environments can also be important in some pelagic and hemi-pelagic settings (e.g., Henshaw and Merrill, 1980;Channell et al, 1982;Bouilloux et al, 2013), while diagenetic magnetite dissolution and destruction of the paleomagnetic record can be important below productive surface waters.…”