Quaternary Sediments in the Arctic Ocean: Towards Solving a Paleomagnetic Conundrum

Abstract:

Understanding of the magnetostratigraphy of Arctic Ocean sediments is muddled by multiple reversals that start a few meters below the sea floor. This level, which was earlier interpreted as the Brunhes-Matuyama boundary, roughly coincides with the base of Marine Isotope Stage 6 (ca. 128 ka). Results of thermal demagnetization and analyses of magnetic extracts led Channell and Xuan (2010) to propose a self-reversal mechanism caused by oxidation of primary titanomagnetite to secondary titanomaghemite. We present a detailed study of the natural remanent magnetization acquired by the 7 m long core LR12-PC06, which was collected from the Lomonosov Ridge during the LOMROG expedition in 2012 and can be directly correlated to the ACEX depth scale by grain size and density profiles. Similar to previous studies, the magnetic directions of many initially normally magnetized samples trended towards reversed during alternating field (AF) demagnetization up to 100 mT, although this change was often associated with a significant increase in intensity. Experiments investigating the possibility of spurious laboratory magnetizations as a cause of the aforementioned intensity increase included increasing the AF level to 120 mT. In contrast to previous studies employing less intense fields, our more intense fields removed the reversed component and directions returned to normal. While our trial study of LR12-PC06 is by no means conclusive, it shows that one component, with a relatively narrow and medium coercivity spectrum, acquired a reversed natural remanence that is superimposed upon a normal magnetization with a wider coercivity spectrum and multiple possible contributors. We suggest that the reversed component, when characterized by a narrow coercivity spectrum, can be considered as a secondary magnetization, probably acquired by a rapidly formed and fine grained diagenetic mineral. Our applied method and results may help discriminate between true and apparent reversal boundaries.