Preliminary paleomagnetic and rock magnetic results from 17 to 22 ka sediment of Jeju Island, Korea: Geomagnetic excursional behavior or rock magnetic anomalies?

Geomagnetic field excursions are short-lived but globally recorded periods of the Earth’s magnetic field fluctuation, shorter than a few 10³ years, during which virtual geomagnetic poles (VGPs) deviate beyond the normal range of secular variation associated with the geocentric axial dipole (GAD) (see Laj and Channell 2007). Paleomagnetic investigations from both lava flow sequences and various types of sediments, especially quasi-continuous deep-sea sediments, in the last two decades have made significant advances in establishing the number, duration and field geometry of geomagnetic excursions, especially during the Quaternary. However, many of them still remain an enigma. Exploring potentially correlative excursions at different sites of the world and increasing their global areal coverage is essential not only to understand how physical processes of the geodynamo modulate frequent occurrence of excursions and what processes discriminate between the geomagnetic reversal and excursion (e.g. Gubbins 1999; Zhang and Gubbins 2000; Hoffman and Singer 2008; Singer et al. 2008), but also to avoid correlation mistakes between temporally different paleomagnetic records from distant sites on the globe (e.g. Laj and Channell 2007; Singer et al. 2008, 2014a; Singer 2014).

In addition, some researchers have suggested that such an unusual magnetic field is not always manifested equally at all locations on the globe (e.g. Tarduno et al. 2015). This also emphasizes that paleomagnetic investigation in previously un-investigated regions is always helpful for resolving this issue.

In paleomagnetic studies to identify and characterize such geomagnetic excursions from sediments, a well-established age model, high sedimentation rate (due to the limited duration of geomagnetic excursions), and identification of the magnetic mineralogy in particular are essential. Ferrimagnetic iron sulfides (i.e. greigite, pyrrhotite) and oxidized titanomagnetite (or titanomaghemite) have been often reported as problematic magnetic carriers. Greigite (Fe₃O₄) is widely distributed over the world, found in anoxic and sulfidic sedimentary environments like estuaries, fluvial fans, hemipelagic marines and gas hydrate systems, forming authigenically in reductive diagenetic processes (e.g. Fu et al. 2008; Roberts et al. 2011). It can generate a “false” magnetic record of the geomagnetic fluctuation when the age of greigite formation significantly postdates the age of deposition of the surrounding sediments (e.g. Horng et al. 1998; Jiang et al. 2001; Roberts and Weaver 2005; Sagnotti et al. 2005). Pyrrhotite (Fe₇S₈) often generates such false geomagnetic fluctuations by such late diagenetic formation (e.g. Weaver et al. 2002) or by the self-reversed magnetizations (e.g. Bhimasankaram 1964; Bina and Daly 1994). In addition, some studies on marine sediments suggest that non-steady-state magnetic mineral reductions can result in delayed remanence acquisition leading to anomalous features in natural remanent magnetization (NRM) data (e.g. Tarduno et al. 1996, 1998). Tarduno et al. (1998), for example, suggest that the production of biogenetic magnetite near the Fe-redox boundary at depth can lead to the acquisition of a biochemical remanence. Changes in the redox boundary depth with time in pelagic sediments can result in complexities in magnetic records.

Another complexity can be caused by the presence of titanomaghemite. Titanomaghemite, which is usually formed by low-temperature oxidation (maghemitization) of titanomagnetite in nature, can acquire a partial or complete self-reversed chemical remanent magnetization (CRM) that replaces the original detrital remanent magnetization (DRM) or thermoremanent magnetization (TRM) (e.g. Doubrovine and Tarduno 2004; Krása et al. 2005; Channell and Xuan 2009; Xuan and Channell 2010; Xuan et al. 2012). As a possible mechanism of the self-reversal, ionic reordering (Doubrovine and Tarduno 2004, 2006a) and magnetic coupling between titanomagnetite and oxidized titanomaghemite in a close side-by-side assemblage (Krása et al. 2005) have been suggested. Moreover, observations of self-reversal for titanomaghemites by Doubrovine and Tarduno (2005, 2006a, b) suggest that a finite range of high oxidation states of z (O’reilly 2012) of no less than 0.9 and relatively high Ti contents (x) of no less than 0.6 are required to generate self-reversed remanences.

Here we firstly document a sequence of unusual NRM data, of which the directions deviate visually from the apparent secular variation range during a stable normal-polarity period, from one site of ~ 17–22 ka unconsolidated nonmarine sediment (named Gosan Formation) in Jeju Island, Korea. This study shows (1) that, from thermal (TH) demagnetizations, the unusual paleomagnetic directions all are from remanence components with an unblocking temperature (Tub) interval ranging from 240–320 to 520–580 °C (probably carried by magnetite or Ti-poor titanomagnetite) which are separate from remanence overprints with Tubs up to 240–320 °C (probably carried by (titano)maghemite and/or Ti–rich titanomagnetite) having directions apparently within the secular variation range of the Brunhes Chron, and (2) that alternating field (AF) demagnetization technique often appears to fail to isolate these two different remanences presumably because of their strongly overlapping coercivity spectra. Hence, we address interpretation of these results about whether the unusual directional features are associated with geomagnetic excursional behavior or rock magnetic anomalies.

Jeju Island, located to the south off the Korean Peninsula, is a Quaternary volcanic field formed in an intraplate tectonic setting on the eastern continental margin of the Eurasian plate (Fig. 1a). The island was generated by extensive hydrovolcanism between c. 1.8 and 0.5 Ma and later plateau- and shield-forming, basaltic-to-trachytic lava effusions that produced a wide and low-altitude lava plateau and a peak of shield volcano (Mt. Halla) dotted with hundreds of monogenetic volcanic scoria cones and tuff rings/cones (e.g. Sohn and Park 2004; Sohn et al. 2008; Koh et al. 2013). Besides, paleosols, lake or wetland sediments, and other unspecified, unconsolidated sediments are often observed between or on the top of such volcanic deposits in the island (e.g. Park et al. 1998, 2000a, b; Lee et al. 2014a, b; Park et al. 2014; Lim et al. 2015; Ahn and Choi 2016).

The Gosan Formation (GSF) is a muddy sedimentary formation, a few meters thick and massive, intercalated between the Late Pleistocene plateau-forming basaltic lava, named the Kwanghaeak Basalt, and the basaltic tuff of the Suwolbong tuff ring (Sohn and Chough 1989). The GSF was dated to be approximately 18 ka by optical stimulated luminescence (OSL) method (Cheong et al. 2007). The GSF is exposed approximately 4.5 km along the western coast of Jeju Island, and extending ~ 2 km landward and used as a rice field at present (Park et al. 2000a; Fig. 1b). The main source of the GSF sediment was Dangsanbong, which is a 450 kyr-old, horseshoe-shaped tuff cone with a nested scoria cone at its center (Sohn and Park 2005; Brenna et al. 2015). The stratigraphic relationship near Dangsanbong suggests that the tuff cone was overlain by the Kwanghaeak Basalt after the cone was lithified and significantly eroded (e.g. Park et al. 2000a). Dangsanbong continued to supply volcaniclastic materials to nearby areas, forming a scree deposit upon the basalt. The GSF muddy sediment is laterally correlative with the Dangsanbong-derived scree deposit and thins and pinches out toward the south, i.e., away from Dangsanbong. The sediment is massive and has a fairly level upper surface without any features related to fluvial incision, wave reworking or bioturbation. It is therefore interpreted to have been deposited by suspension setting of fines in a wetland near the toe of the scree that surrounds Dangsanbong.

OSL age of the GSF was obtained earlier by Cheong et al. (2007) and was estimated at 23.2 ± 1.0 ka. Afterward, a research group of the Korea Institute of Geoscience and Mineral Resources (KIGAM) carried out more detailed, absolute age dating and geochemical analyses over a wider area of the GSF exposures (Lee et al. 2014b). Part of these results including a suite of radiocarbon (¹⁴C) ages with an accelerator mass spectrometry (AMS), granulometry, contents of total organic carbon (TOC) and its δ¹³C (=¹³C/¹²C) ratios for a site at the coast near Dangsanbong (hereafter referred to as site GSDS; see Fig. 1B) were published in Lim et al. (2015). Median grain sizes for the GSDS site varied between ca. 12 and 18 µm, having an apparently consistent value around 15 µm. The TOC contents ranging between 2.0 and 0.8% (totally low throughout the sediment) monotonously decrease with depth. In combination with the AMS ¹⁴C ages, they suggested that the GSF sediment was deposited in a terrestrial (and probably subaqueous) environment during a period approximately between 22 and 17 calibrated (cal.) kyr BP (Fig. 2 and Additional file 1: Table S1), and that the age of the Suwolbong tuff ring should be younger than the previous estimate (~ 18 ka; Cheong et al. 2007). Lee et al. (2014b)’s work at another GSF site near Suwolbong (hereafter referred to as GSSW; see Fig. 1b) has also yielded a line of AMS ¹⁴C dates, suggesting a wide range of the GSF sedimentation age from 24 to 16 cal. kyr BP (Fig. 2 and Additional file 1: Table S1). Age-depth model for each site was made by linear interpolations between AMS ¹⁴C age estimates, as shown in Fig. 2. The GSSW and GSDS age-depth models were similar for, at least, the upper ca. 70 cm sediments, where the estimated sedimentation rates are much higher than 10 cm/kyr (in the range of 13–25 cm/kyr). The above-mentioned age data with related additional information are also listed in Additional file 1: Table S1.

We made paleomagnetic sampling at the same site analyzed in Lim et al. (2015) (i.e. site GSDS, 33.30917°N, 126.16556°E; Fig. 1b). The site GSDS was found to be lithologically homogeneous, comprising dark-brown, unconsolidated and massive clayey silt deposit with scattered gravel-sized clasts. The estimated sedimentation rate varying between 13 and 25 cm/kyr (the blue thin line in Fig. 2) is high enough to record such “short-lived” geomagnetic events. After removing more than 10 cm of surface soils to avoid possible adverse effects of post-deposition alteration and making a planar face, a total of 33 oriented samples were collected at 2-cm-spacing between 5 and 69 cm depths from the top (i.e. from the contact with the Suwolbong tuff), carefully pushing 7 cm³ non-magnetic plastic cubes into the face and orientating with a magnetic compass with an attachment allowing to mount the plane to be measured (Fig. 1c).

Experimental sequences [1]–[5] and [9] were carried out at the paleomagnetism and rock magnetism laboratory of the Center for Advanced Marine Core Research (CMCR) of Kochi University in Japan, and sequences [6] and [7] to [8] were performed at the paleomagnetic laboratories of Pusan National University and KIGAM in Korea, respectively.

The remanence measurement, AF demagnetization, ARM acquisition were made using a 2G Enterprises 760R pass-through cryogenic magnetometer with in-line 3-axis static AF demagnetization and ARM acquisition. The SAFD experiments of remanent magnetizations (NRMs and ARMs) were made in generally 23 steps of peak AFs from 0 to 80 mT (the increments of 2 mT for the 0–20 mT peak AFs and 5 mT for the 20–80 mT peak AFs). The ARM acquisitions were imposed in a DC field of 0.1 mT and AF field decaying from a peak AF of 80 mT (k_ARM can be calculated following e.g. Banerjee et al. 1981). The IRMs for each bulk cube-shaped sample were acquired using a Magnetic Measurements MMPM10 pulse field magnetizer. The dual k_LF and k_HF were measured at AF external fields with frequencies of 0.47 and 4.7 kHz, respectively, using a Bartington MS2-MS2B magnetic susceptibility system. The thermomagnetic analysis of saturation magnetization was made by a single heating–cooling run between room temperature and 700 °C with a constant field of 0.3 T and a heating rate of ~ 15 °C/min in air, using a Natsuhara Giken NMB-89 magnetic balance. The thermomagnetic analysis of magnetic susceptibility was conducted, heating from room temperature up to 700 °C and then cooling down to 40 °C, at 300 A/m field and a heating rate of ~ 12 °C/min in air, using an AGICO KLY-4 Kappabridge system equipped with a CS-3 furnace. The magnetic hysteresis loop and the stepwise back-field IRM demagnetization were measured on each small-volume subsample, applying fields up to 1 T, by means of a Princeton Measurements Corporation MicroMag 3900 alternating gradient magnetometer (AGM). With the AGM, 170 FORCs were also measured on the same subsamples, at averaging time of 200 ms and a field increment of 1.06 mT up to a maximum applied field of 1 T. During the Lowrie experiments, TH demagnetization experiments were performed, using a Natsuhara Giken TDE-91 thermal demagnetizer (residual field of < 15 nT), by measuring the remanent magnetization before every TH step, using a Natsuhara Giken SMD-88 spinner magnetometer. The TH steps were set to 100, 160, 200, 260, 300, 320, 340, 360, 400, 440, 480, 520, 580, 620, 650, and 680 °C (16 steps).

S ratios, S_−0.1T and S_−0.3T, were calculated following the definition of Bloemendal et al. (1992), to seek the relative fraction of low-coercivity minerals with respect to high-coercivity minerals: S_−0.1T = (1 − IRM_−0.1T/IRM_1T)/2, and S_−0.3T = (1 − IRM_−0.3T/IRM_1T)/2, where IRM_1T is hereto regarded as saturated IRM (SIRM). Median destructive field (MDF) with respect to each laboratory-induced ARM during SAFD was calculated to see a magnetic hardness to the remanence in relation to coercivity and average grain size of the magnetic minerals. With k_LF and k_HF values, k_FD was calculated as k_FD (%) = (k_LF − k_HF)/k_LF * 100, to see the possible contribution of superparamagnetic (SP; < 50 nm in general) minerals (e.g. Evans and Heller 2003). k_ARM/SIRM, k_ARM/k_LF, and SIRM/k_LF ratios for bulk samples were employed as a proxy of grain-size fluctuations in relation to the domain state of magnetic particles (e.g. Banerjee et al. 1981). The SIRM/k_LF ratio can be also used as indicator of the dominance of greigite (e.g. Roberts et al. 2011; Nowaczyk et al. 2012). The hysteresis parameters were processed with a biplot of the Mrs/Ms ratio versus the Bcr/Bc ratio (so-called “Day plot” from Day et al. 1977), to obtain information about the average grain-size of magnetic particles. The measured FORCs were processed with the FORCinel software (Harrison and Feinberg 2008) to display a FORC diagrams, which provide information about distributions on microscale coercivity and magnetostatic interaction. The experiments [5], [6] and [10] give information about the contained magnetic minerals. As introduced by Peters and Thompson (1998), the two biplots of SIRM/k_LF versus ARM_dem40mT/ARM and IRM_-0.1T/SIRM versus ARM_dem40mT/ARM, where ARM_dem40mT denotes the ARM intensity after demagnetization by a peak AF 40 mT, were employed to discriminate magnetic minerals with low coercivities.

Afterward, another set of three oriented block samples with plastic boxes of ca. 13 cm × 18 cm × 4 cm in size were also collected from a 5-to-55-cm-depth profile in close proximity of the “first-sampled” point in the GSDS site. The block samples, in the laboratory, were consolidated with plaster and cut to cube-shaped specimens with 2 or 2.2 cm on each side. Two pairs of Al-foil wrapped cubes for 21 successive depth levels (7.1–50 cm depths in the central position) were then prepared to make STHD and SAFD experiments for NRM, respectively. Central depth levels of the prepared cube-cut specimens from three blocks were 7.1–20.3, 23.1–36.3, and 38.0–50.0 cm, respectively. The STHD experiment of NRM was treated in 40 °C steps in the 120–580 °C range and in 20 °C steps in the 580–640 (660) °C range. The SAFD experiment of NRM was same as in the earlier AF treatments. The specimens with AF treatments and NRM measurements were then subjected to ARM acquisition followed by its SAFD experiment.

Compass readings of the sample orientation were corrected for the locality declination of the present-day Earth’s magnetic field of 7°, calculated using the 12th International Geomagnetic Reference Field (Thébault et al. 2015; http://wdc.kugi.kyoto-u.ac.jp/igrf/point/index-j.html). Demagnetization data were identified with the orthogonal vector diagram of Zijderveld (1967) and equal-area projection. Principal component analysis (PCA) of remanent magnetization for each demagnetization data was performed applying Kirschvink (1980). Mean direction for the isolated remanence components was calculated using a Fisherian statistics (Fisher 1953).

Paleomagnetic and rock magnetic investigations were performed on a 64-cm-thick section of nonmarine unconsolidated muddy sediment (Gosan Formation) in Jeju Island, Korea, having an age interval of deposition between ~ 22 and ~ 17 ka. Interestingly, unusual NRM directions were found at several depths by stepwise AF demagnetizations, manifested by significantly large departures from the direction of today’s axial dipole field. On the other hand, stepwise TH demagnetizations show more complex behavior resulting in the identification of multiple directional components, where two distinct components with TH steps between 120 and 240–320 °C (LTC1), and between 240–320 and 520–580 °C (HTC1), respectively, are predominant in all specimens analyzed. The low-temperature components (LTC1 s) generally indicate normal-polarity directions presumably within the secular variation range in the normal Brunhes Chron, whereas the high-temperature components (HTC1 s) often have unusual directions including negative inclinations and reversed directions, particularly in the ~ 13–34 cmcd (~ 17.6–18.8 ka in inferred age) and possibly below ~ 53 cmcd (before ~ 20.2 ka). A comprehensive interpretation from various rock magnetic analyses infers the co-existence of low-coercivity magnetic minerals having ~ 300 and ~ 580 °C Tcs, suggestive of (titano)maghemite and/or Ti–rich titanomagnetite (not ferrimagnetic iron sulfides) and magnetite (or Ti-poor titanomagnetite). Some differences in directional behavior between AF and TH demagnetizations could be interpreted to result from incomplete separation, by the AF demagnetization, of the two remanence components with strongly overlapping AF spectra each other. Hence, it is possible that the low-temperature components may be associated with CRMs carried by (titano)maghemite probably formed by post-depositional low-temperature oxidation, and the high-temperature components might be primary geomagnetic records that survived from the post-depositional chemical influence. This therefore raises a possibility of geomagnetic instability within the ~ 17–22 ka period, manifested by multiple excursional swings, part of which might be associated with the Tianchi/Hilina Pali excursion. We note, however, that our interpretation of the excursional features is tentative and needs further works including electron microscopy and X-ray analyses of magnetic separates, and more thorough within-site consistency and between-site consistency checks for these excursional features to be considered true records of the geodynamo.