A Thick Negative Polarity Anomaly in a Sediment Core From the Central Arctic Ocean: Geomagnetic Excursion Versus Reversal

Abstract

There are two distinctly different views on Quaternary sedimentation rates in the central Arctic Ocean, namely, that they were on cm/kyr or on mm/kyr scales, largely as a result of divergent interpretations of magnetic reversal stratigraphy. This study provides new evidence to help resolve this controversy in the form of an almost 1-m-thick negative polarity interval located at~1.15 m below the seafloor in a 4.15-m-long sediment core from the Lomonosov Ridge, central Arctic Ocean. This thick polarity anomaly was first revealed through alternating field demagnetization of natural remanent magnetization on u-channel samples. It was subsequently confirmed to be an authentic signal of a syndepositional geomagnetic field by integrating detailed rock magnetic studies, transmission electron microscopy with energy-dispersive X-ray spectroscopy analyses of magnetic extracts, and both alternating field and thermal demagnetization of discrete paleomagnetic samples. Here we conclude that it is more reasonable to interpret the reversed interval as a record of the Matuyama Chron rather than a Brunhes-aged excursion, which yields mean sedimentation rates of~1.5 mm/kyr and~0.5 mm/kyr in the Brunhes and Matuyama Chrons, respectively. These estimates are remarkably lower than assumed in most recent studies, but in good agreement with both previous magnetostratigraphic and recent geochemical studies relative to the decay of Be and U-series isotopes, which might indicate "sediment-starved" environments at least in some parts of this region. Overall, our results not only imply that chronostratigraphy in the central Arctic Ocean remains open to debate but also warrant further investigations in this microfossil-barren region.

Sedimentary Record of Glacial Impacts and Melt Water Discharge off the East Siberian Continental Margin, Arctic Ocean

“…We recall that a prominent magnetostratigraphic unit was identified in the bottom sediment, while a similarly significant inclination drop can also be traced in the records of previous cores from the MR to the central Arctic Ocean, as described in Section 3.2 (Adler et al., 2009; Cronin et al., 2013; Jakobsson et al., 2000; Liu et al., 2019). As an independent comparison tool, the unique paleomagnetic inclination drop has been used in the interpretation of Arctic Ocean glacial‐interglacial cycles and was attributed to MIS7 (Jakobsson et al., 2003; Xiao et al., 2020).…”

“…As an independent comparison tool, the unique paleomagnetic inclination drop has been used in the interpretation of Arctic Ocean glacial‐interglacial cycles and was attributed to MIS7 (Jakobsson et al., 2003; Xiao et al., 2020). However, because such a signal may also be triggered by diagenesis processes at the sediment–water interface, we maintain that the assignment of the bottom age is still tentative (Channell & Xuan, 2009; Liu et al., 2019; Xuan & Channell, 2010).…”

“…Typical layers of condensed deposition suggest extremely low sedimentation rates (∼0.2 cm/kyr), for example, under the cover of thick sea ice or glaciation (Figure 4; Poirier et al, 2012;Polyak et al, 2009;Railsback et al, 2015). The boundaries between B2, overlying G1 and underlying G2 were blurred; however, B2 could be constrained by four AMS We recall that a prominent magnetostratigraphic unit was identified in the bottom sediment, while a similarly significant inclination drop can also be traced in the records of previous cores from the MR to the central Arctic Ocean, as described in Section 3.2 (Adler et al, 2009;Cronin et al, 2013;Jakobsson et al, 2000;Liu et al, 2019).…”

“…Paleomagnetic inclination was measured at 2‐cm intervals on u‐channel samples using a 2G Enterprises Model 760R high‐resolution long‐core cryogenic magnetometer (2G760). The magnetometer was installed in a magnetically shielded (<300 nT) space at the Third Institute of Oceanography, China (Liu et al., 2019).…”

The Arctic Ocean Manganese Cycle, an Overlooked Mechanism in the Anomalous Palaeomagnetic Sedimentary Record

“…These difficulties in interpreting palaeomagnetic data in the Arctic led to the exclusion of magnetostratigraphy in the construction of the Arctic Coring Expedition (ACEX, IODP Expedition 302) age model for the Neogene . Some recent studies challenge the ACEX-based age model through U-Th ages (Hillaire-Marcel et al, 2017) and palaeomagnetic interpretations (Liu et al, 2019) that support the low-sedimentation rate scenario of Clark (1970). While some studies (e.g., Spielhagen et al, 2004) of Arctic Ocean sediments assumed inclination changes to be coeval and used them for stratigraphic correlation, more recent studies that placed inclination changes into the lithostratigraphic context of the Arctic Ocean (e.g., O'Regan et al, 2008;Wiers et al, 2019) have confirmed the abnormal nature of the palaeomagnetic record by showing that inclination changes are not stratigraphically coeval.…”