Anomalous high coercivity natural remanent magnetization acquired by maghemite in Deep-sea Sediments off Wilkes Land, East Antarctica

The magnetic properties and minerals of a deep-sea sediment core obtained from off Wilkes Land have been investigated in order to obtain paleomagnetic, rock-magnetic and paleoenvironmental data for Antarctic region. The core used in this study was collected from a continental rise site 3060m deep at the western part of the Antarctic Wilkes Land margin during the TH94 cruise (1994-1995) of R/V HAKUREI-MARU, carried out by the Technology Research Center, Japan National Oil Corporation. The core consists of siliceous silt and is characterized by its brownish gray color, an absence of paleoclimatically induced lithological variations and abundant foraminiferal contents in good preservation. Paleomagnetic measurements reveal that the sediment core has an anomalous natural remanent magnetization (NRM). The original NRM intensities of sediments are 10-100 times greater than those commonly observed at different localities. The high median destructive field value of about 60mT in overall average suggests a remarkable high stability of NRM. The high coercive components of NRM have never been demagnetized completely and the more than 20% of the primary NRM can survive even after the AF demagnetization with peak fields of 100mT. The NRM intensity decay curves of most samples at AF demagnetization depict unusual straight line, indicating that each sample has an anomalous flat distribution in the coercivity of NRM carrier grains. Magnetostratigraphy proves that the core covers the last 1 Ma. The employment of 1 cc small-volume samples for paleomagnetic measurements proves its advantage of deriving the higher time resolution records from sediments by comparison with the paleomagnetic data obtained by using common U-channel and 7cc samples. The employed 1 cc samples provide a long-term secular variation in declination, which correlates with those of well-dated cores in previously published study. That correlation enables an age control for the core, and the sedimentation rate are estimated to be 2-9 mm kyr-1 with the significant change about 0.4 Ma. The combination of optical microscopic observations, transmission electron microscopy observations, energy dispersive analyses of X-ray emission analyses, X-ray diffraction analyses and inductive coupled plasma/mass spectrometry analyses identify the magnetic minerals in the fractions extracted magnetically from sediments as pure iron oxide minerals namely magnetite and maghemite. The simultaneous measurements of temperature dependences of five magnetic properties and thermostructural analyses successfully reveals the hysteretic changes in heating-cooling cycles, which contributes to rule the possible presence of other magnetic minerals in sediments out. This useful approach to identify magnetic minerals from a mixture in sediments might be applicable to other studies dealing with sediments. The presence of maghemite suggested that the anomalous NRM is mainly a chemical remanent magnetization (CRM), which is proved by the drastic decay of NRM intensity at the inversion temperature of maghemite to hematite in thermal demagnetization experiments, while a small portion of detrital remanent magnetization (DRM) carried by magnetite also exists in the NRM. It is assumed that the CRM was acquired by maghemite in its generation from magnetite under the oxic system at sea bottom by the high-oxygen water namely Antarctic Bottom Water (AABW). The magnetization zone for CRM is presumed to have been near the surface of sediments (most oxidized zone) and the zone for DRM is also assumed to have been there, since the directions of CRM by maghemite and DRM by magnetite are almost consistent. Secondary magnetizations in many samples, are also inferred to be CRM acquired at the sub-surface of sediments, because the polarity of the secondary magnetizations in the sediments for Jaramillo Subchron are the reversed (Matuyama) polarity and opposite to that of the present geomagnetic field. Since a CRM intensity correlates the external field intensity as well as thermal remanent magnetization, relative geomagnetic paleointensity estimation is bravely carried out with using the NRM (mainly a CRM) of sediments, though it is not usual DRM but mainly a CRM and it is scarcely considered as preferable condition for paleointensity estimation in the present paleomagnetic studies. Downcore changes of magnetic concentration represented by various magnetic parameters are a factor of five or less. It demonstrates that the core is rock-magnetically homogeneous, which is essential for paleointensity estimation. The saturation isothermal remanent magnetization (SIRM) is selected for normalizing the NRM, since its stability in AF field is almost consistent with that of the anomalous NRM of the sediments. The normalized intensity (NRM/SIRM) record shows the similarities to other worldwide paleointensity records and the feature trends corresponding to a global reference paleointensity curve.The characteristics of magnetic properties in thermo-magnetic experiments are almost consistent through out the core. It indicates that maghemite exists in overall sedimentary sequence, namely that it has been yielded constantly over at least the past 1 Ma. Thus it is assumed that the AABW has continuously existed supplying the high-oxygen water to sea bottom and the sedimentary environment has been maintained at oxidized condition for such a long period. This fact will greatly contribute to future studies of paleoenvironments, paleoclimates and paleoceanography for Antarctic region