Paleomagnetic results
Results on “first-sampled” specimens
Initial NRM intensities in “first-sampled” specimens (labeled here as “GSDS1-xxcm-AF”) were mostly in the order of 0.01 A/m with a minor variation but three occasional exceptions of larger than 0.1 A/m in specimens from 9, 21 and 39 cm depths from the top (Additional file 1: Table S2).
NRM-SAFD experiments revealed that remanent magnetization for each of most analyzed samples was identified by one or two directional components after soft demagnetizations up to 2–10 mT in peak AF which is probably associated with recently induced remanence during or after sampling (for example, 0–4 mT AFs for specimen GSDS1–27 cm-AF in Fig. 3; named here as LFC0). Then demagnetization patterns could be classified into six types as shown in Fig. 3. Type IA (Fig. 3a), which was found in 9 samples, could be characterized by a single magnetic component decaying toward the origin after removal of the soft remanence (LFC0). This single component indicated a normal-polarity direction. One specimen, GSDS1–19 cm-AF (Fig. 3b), had a similar pattern as type IA but the defined single component did not seem to direct to the origin (type IB). Other 11 samples had two directional components, of which the high-coercivity one could not be recognized unless AF demagnetization persists up to > 10–30 mT and had a normal-polarity direction. Note that in several specimens of this case the high-coercivity component had unstable in some degree and/or did not appear to direct to the origin. This kind of the patterns corresponded to type II (Fig. 3c). Both types IIIA (Fig. 3d) and IIIB (Fig. 3e) could be characterized by a high-coercivity component with reverse-polarity directions, separate from normal-magnetized overprint not to anti-parallel to each other, was visible or seemed to be. Two samples of type IIIA (29 and 65 cm depth) displayed apparently a stable high-coercivity component, whereas other two with type IIIB (7 and 17 cm depth) did indicate incomplete isolation of the reverse-polarity magnetization from the overprint. There were five results showing two different components, in which the high-coercivity one could be defined by AF demagnetizations above 25 mT at least and appeared to be intermediate-polarity magnetized (type IV; Fig. 3f). Two remaining results (23 and 63) were unclassified because the demagnetization behavior above the 10–18 mT AF steps was chaotic.
For all analyzed samples, MDF values during the SAFD of NRM were in the limited range of 10–29 mT, but for 29, 39 and 65 cm depth levels that have much higher MDFs.
Low-coercivity remanence component defined in AF steps between 2–10 and up to 35 mT (LFC1), where visible, had a direction being scattered around the GAD-expected direction (D = 0°, I = 53°), as seen in Fig. 3c–f. On the other hand, high-coercivity remanence component (HFC) was generally defined in AF intervals including peak fields above ~ 35 mT by applying the PCA without anchoring the origin, having a maximum angular deviation (MAD) value smaller than 15°, for 27 out of 33 specimens analyzed. This non-anchoring-determined HFC generally had no significant difference from the anchoring-derived direction at the same AF interval (hereafter called “DANG”) by less than 5°, except for 8 specimens (for example, DANG = 8.7° in Fig. 3d) (see also Additional file 1: Table S3). More importantly, there were unusual HFC directions (for the 5,9, 29, 43 and 65 cm depth levels) apart largely from the GAD direction (in other words, low latitudes in corresponding VGP, more than ~ 45° away from the North Pole), besides normal-polarity HFC directions within the apparent secular variation range of the Brunhes Chron.
Results on “second-sampled” specimens
Twenty-one pairs of adhering specimens for the 7.1-cm-to-50-cm-depth “second-sampled” profile (labeled here as “GSDS2-xx.xcm-AF/TH”) yielded paired STHD and SAFD results of NRMs for the same depth levels. It is found that TH demagnetization behavior was more complex, compared to AF one, in many of them. Figure 4 shows three pairs of TH and AF results. These TH results recognized generally three but four, in maximum, different directional components as follows:
-
(1)
the lowest temperature component determined in the room temperature (20 °C)–120 °C range (LTC0),
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(2)
the low-temperature component determined in TH steps between 120 and 240–320 °C (LTC1),
-
(3)
the high-temperature component determined in TH steps between 240–320 and 520–580 °C (HTC1), and
-
(4)
the highest temperature component determined in TH steps between 520–580 and 620–660 °C (HTC2).
The LTC0, having dominantly a direction with D ~ 10°–50° and I ~ 120°–170°, was considered as viscous remanences induced during the sampling and/or the storage. This corresponded to the lowest component with AFs up to 4–8 mT (LFC0) in the AF result. The LTC1 directions all were around the GAD direction (apparently within the secular variation range of a stable normal-polarity period). This appeared to correspond to the low-coercivity component with the AF steps between 6–10 and 30 mT or higher (LFC1). On the other hand, The HTC1 direction had highly varied directions, in which some were reversed and intermediate-magnetized. This HTC1 direction was not often apparent in the AF demagnetization result, as shown in Fig. 4. The HTC2 component (for example, 7.1 cm in Fig. 4) was found only in some depth levels, of which seven could define the HTC2 which had directions with northerly declinations and shallow positive inclinations. Our demagnetization results indicate that AF demagnetizations often resulted in incomplete isolation of remanence components corresponding to the TH-derived high-temperature ones, such as HTC1 and HTC2, being screened by a large portion of the low-coercivity components (LTC0 and/or LTC1). These LTC1, HTC1 and HTC2 direction data are also listed in Additional file 1: Table S4.
Additionally, there were two particular specimens (13.7 and 15.9 cm depth) where NRMs did not decay toward the origin during heating up to 520–560 °C and the NRMs above the 580 °C step were highly unstable, but their directions were apparently anti-parallel to the NRM directions for the 320–560 °C range (HTC1) (Fig. 5).
Rock magnetic analyses
Magnetic properties for k, ARM and IRM
Magnetic susceptibility (k), kARM, and SIRM are often used as a primary indicator of concentration of magnetic minerals. kLF and kARM showed a similar variation characterized by relatively large amplitudes in fluctuation and slightly high mean values above 30 cm depth, whereas the lower 40 cm interval represented low values than their averages with a suppressed fluctuation but a slight increasing trend with increasing depth (Fig. 6a, b). SIRM variation broadly resembled those of kLF and kARM, but it had impressively high peaks in the value at 7–9, 17, 29, 39, and 65 cm depth levels (Fig. 6c). Of these, the 39 and 65 cm peaks did have common kLF and kARM variations, suggesting that these peaks might have been affected dominantly by an additional magnetic mineral rather than the concentration. Variations in kARM/kLF, kARM/SIRM, and SIRM/kLF did not appear to show a similarity between them (Fig. 6d–f), in accordance with the possible effect of an additional magnetic mineral. Rather, the SIRM/kLF variation was analogous to the MDF of ARM during AF demagnetization (MDFARM), showing remarkably high peaks in their values at 39 and 65 cm depth, and apparently 7 cm depth (Fig. 6f–g). While most of the SIRM/kLF values were between 9 and 13 kA/m, for the peak intervals the values were indicated by up to 35 kA/m. Such high peaks in SIRM/kLF (and MDFARM) may indicate relative abundance of, for example, greigite or other kind of minerals with slightly higher coercivities. The S−0.3T values were as high as 0.98 and most of the S−0.1T values are around 0.90, showing the dominance of low-coercivity minerals throughout the samples (Fig. 6h). However, at 7, 17, 23, 39–41, and 65 cm depth levels, there were relatively low S−0.1T values varying between 0.71 and 0.85, inferring relative abundance of higher coercivity magnetic particles.
Magnetic mineralogy
Figure 7 shows the results of k–T curve, Ms–T curve, and Lowrie experiment for four samples at different depth levels (7, 29, 33, 39 cm) in the “first-sampled” (GSDS1) profile. In all k–t heating curves (Fig. 7a) k increases up to about 300 °C and subsequent decrease was visible and followed by a significant drop at temperatures between 500–570 and 590 °C. The decrease at above ~ 300 °C was occasionally followed by the abrupt increasing between 470 and 550 °C (hump-forming) that were seen for the 33 and 39 cm depth levels. During cooling, there was strong enhancement in k for all cases. Ms–T curves (Fig. 7b) during heating displayed, in addition to a minor inflection at ~ 100 °C, a gradual decrease below ~ 400 °C and subsequent hump-forming approximately between 400 and 500 °C, with a final inflection temperature of ~ 550 °C. The cooling curves did not appear strong enhancement in Ms. In the k and Ms heating curves, the drop approximately between 300 and 400 °C and the hump-like form seen in some samples can infer either exsolution of titanomagnetite to magnetite and ilmenite (e.g. Özdemir 1987), or inversion of (titano)maghemite to magnetite (e.g. Özdemir 1987), or inversion of (ferrimagnetic) iron sulfides to magnetite (e.g. Roberts et al. 2011). The difference between the k–T and Ms–T cooling curves might be associated with the degree of the initial oxidation of samples and/or laboratory conditions during heating, which may cause the inversion partly to hematite upon heating of the Ms–T analysis leading to no remarkable increase during cooling. Lowrie experiments (Fig. 7c) gave us more direct and evident information regarding the remanence carrier. In general, for all analyzed samples IRMs were dominated by low-coercivity (magnetically soft; 0–0.1 T) contribution and they did not have Tubs above 600 °C for all spectra of the coercivities (0–1.2 T). For the 7 and 39 cm samples, more or less medium (0.1–0.4 T) and hard (0.4–1.2 T) components were visible. Both the soft and medium components represented two tubs of 300 and 580 °C, respectively, indicating the co-existence of another “low-to-middle coercivities” magnetic phase with Tub of 300 °C with magnetite (the Curie temperature, Tc ~ 580 °C) as the remanence carrier. The demagnetization behavior of the small hard (0.4–1.2 T) component differed from sample to sample: The 7 cm sample had two Tubs of 300 and 580 °C, and only 580 °C for the 29 cm, only 300 °C for the 39 cm, and none for the 33 cm depth level. The presence of the hard Tub ~ 300 °C magnetic phase (7, 39 cm depth) appeared to be correlated to the S−0.1T lows (Fig. 6h).
In addition, Fig. 8 shows two biplots of SIRM/kLF versus ARMdem40mT/ARM, and IRM-0.1T/SIRM versus ARMdem40mT/ARM, as introduced by Peters and Thompson (1998), which allowed us to discriminate ferrimagnetic iron oxides from ferrimagnetic iron sulfides among “low-coercivity” magnetic minerals. These indicate no presence of greigite and pyrrhotite. Also, no presence of greigite could be supported by the following data: (1) extremely low values (nearly zero or 0.06 at most) in gyroremanent magnetization (GRM) index (GI), which is a measure to quantify the potential GRM effect by greigite and is calculated as the ratio of the difference between the intensity value at the highest demagnetization level (80 mT hereto, J80 mT) and the minimum intensity reached during demagnetization (Jmin) and the difference between the initial value (J0) and the same minimum in intensity, GI = [J80 mT − Jmin]/[J0 − Jmin] (Fu et al. 2008), with respect to the NRMs, and (2) absence of FORC maxima above about 40 mT in Bc with considerable magnetostatic interaction (e.g. Roberts et al. 2000; Rowan and Roberts 2006; see Fig. 10b), and by much lower than about 40 mT in Bcr (Roberts et al. 2011 and references cited therein; see Fig. 9). Furthermore, there were some cases for the TH demagnetization where NRM intensity persisted by larger than 10% of the total even after demagnetization at 580 °C and the remaining NRM appeared to be fully demagnetized by heating to ~ 620 °C or higher (7.1, 20.3, 23.1, 25.3, 29.7, 34.1, and 36.3 cm depth in the GSDS2 profile). Given the observation of the S−0.3T ratios near saturation (Fig. 6h), this may imply a possibility of some presence of maghemite with thermal stability, apparently consistent with the previous studies showing the possible high-temperature Curie temperature and its stability by up to approximately 645 °C of maghemite (e.g. Özdemir and Banerjee 1984; Gehring et al. 2009).
Therefore, it is possible to interpret that in the entire sediment, besides magnetite (or Ti-poor titanomagnetite), a magnetic phase of ~ 300 °C in Tc with a different range of coercivities, possibly suggestive of (titano)maghemite or relatively Ti–rich titanomagnetite or both (i.e. partially maghemitized titanomagnetite), is pervasive as the remanence carrier and that occasionally maghemite with high-temperature stability also would be present. Also, it is obvious that these two Tcs (or tubs) identified above are in excellent agreement with the lowest and/or the highest temperatures that discriminate the partial remanence components in the TH demagnetization results for all analyzed specimens.
Magnetic granulometry
Magnetic hysteresis loop and stepwise DC back-field demagnetization of IRM and FORCs were obtained on each of subsamples from selected 8 depth levels (7, 17, 29, 33, 39, 57, 65, and 67 cm depths in the GSDS1 profile). The hysteresis loops after correcting for the paramagnetic contribution using a high-field slope are shown in Fig. 9, and Day plot of the hysteresis parameters is illustrated in Fig. 10a. FORC diagrams for the 7 and 65 cm depth are presented in Fig. 10b.
Analyzed samples all displayed consistently an identical hysteresis loop in the shape which was thinly opened but not “wasp-waisted”, characterized as a pseudo-single-domain (PSD) loop (Tauxe 2010), with narrow ranges of 10–12 mT in Bc and 25–28 mT in Bcr (Fig. 9). This is in agreement with the result on the Day plot (Fig. 10a) that define cluster of data points of limited Mrs/Ms (0.21–0.25) and Bcr/Bc (2.3–2.6), falling in the medial leftward side of the PSD region, and close to the theoretical curve 3 for a mixture of single-domain (SD) and multi-domain (MD) particles derived by Dunlop (2002a, b). As shown in Fig. 10b, the FORC diagrams have very similar characteristics that were expressed by remarkable spreading of closed contours along the Bc axis, centered at 10–12 and < 5 mT (SD contributions), and modest-to-little degrees of divergent contours toward the Bu axis (MD contribution) and restricted vertical dispersion (supressed magnetostatic interaction), but no vertical spreading of the contours at Bc = 0 mT along the Bu axis (contribution of SP particles). From these results, granulometry of the magnetic particles could be interpreted as a mixture of SD particles with two slightly different mean coercivities and coarser (PSD to possibly less MD) particles, with supressed magnetostatic interaction.
Combining paleomagnetic data
Prior to discussing the origin of unusual NRM directions, it was necessary to check the stratigraphic relationship of two paleomagnetic data sets from between the GSDS1 and the GSDS2 profiles in the GSDS site through comparison between their rock magnetic properties. First, we found that variation pattern in ratio of NRM intensity after TH demagnetization at 580 °C to initial NRM intensity (NRM580°C/NRM20°C) of the GSDS2 profile closely mimic the S−0.1T variation pattern of the GSDS1 profile. This similarity might be explained by the degree of contribution of possible maghemite with the high-temperature stability and relatively high coercivity of remanence. Note that in the GSDS2 profile IRMs were not measured because of the inapplicable shapes and sizes of the specimens to the related instruments. Second, we found a close similarity in variation of between ratios of MDFNRM to MDFARM (MDFNRM/MDFARM) from the two profiles. This can support within-site consistency in AF spectra of the acquired NRMs between the same depth levels. Figure 11 shows a possible stratigraphic correlation between the two profiles, which is made by matching major peak values both in between the S−0.1T ratio and the NRM580°C/NRM20°C ratio, and between the two MDFNRM/MDFARM ratios, with variations of both NRM0mT/ARM0mT and NRM40mT/ARM40mT ratios at depth (possible proxies of paleointensity variation) as auxiliary data. This depth correlation makes downward shifts by 1.1–5.0 cm for the GSDS2 depth profile, confirming no significant difference in the upper and middle parts between the two profiles but more or less difference between the lower parts probably arisen from the sampling in the field. A line of estimated corrected depth (cmcd) data of the GSDS2 profile for the GSDS1 depth profile are given in Additional file 1: Table S4.
Figure 12 shows AF-derived NRM declinations and inclinations of the GSDS1 profile, and TH-derived NRM declinations and inclinations of the GSDS2 profile on the cmcd scale. In the figure, AF-derived LFC1 and HFC directions in the GSDS1 profile are illustrated as blue and red stars, respectively, and TH-derived LTC1, HTC1 and HTC2 directions in the GSDS2 profile are displayed as blue, red and yellow diamonds, respectively. Accordingly, note that the 8.2-to-55-cmcd (equivalent to the depth scale of the GSDS1 profile) interval has paleomagnetic data derived both from AF and TH demagnetization techniques. However, a first-order character is that there does not appear to be a similar pattern of variation between the high-demagnetization-level AF-derived (HFC) and TH-derived (HTC1 and, if present, HTC2) directions. For the 15-cmcd level (see Fig. 12), the HFC direction is identical to the LTC1 one, thus suggesting failure of the isolation of a higher-level remanence component in AF demagnetization. For the 55-cmcd level the LFC1 or HFC direction is analogous to that of the vector sum of the LTC1 and HTC1, indicating strong overlapping AF spectra of two possibly existing remanence components. Hence, we believe that some discrepancies of the high-level demagnetization-derived directions at the same levels are likely due to the incomplete separation of remanences by AF demagnetization resulting from the two remanence carriers (different Tc phases) with little difference in coercivity spectra, rather than suspicion of less within-site consistency of the directions.
The unusual NRM directions: rock magnetic anomalies or geomagnetic excursional behavior?
As described above, unusual NRM directions are seen in the high-demagnetization-level components (i.e. HTC1s or HFCs). We believe that the unusual directions were not influenced by the NRM of the underlying plateau-forming basalts (Kwanghaeak Basalts). Unfortunately, there is no NRM data of the nearby basalts so far, but the low-latitude plateau-forming basalts (which also belong to the Kwanghaeak Basalts) exposed along a southeast coast of Jeju Island, ~ 16 km away from the GSDS site, are broadly of normal polarity with D = ~ 330°–355° and I = ~ 50°–60° in the direction of characteristic remanence (ChRM) (Ahn, unpublished data). So, a key question is which one is the primary remanence (i.e. the geomagnetic field during or just after deposition) between LTC1 and HTC1.
One could say that the LTC1 directions carried by possibly (occasionally maghemitized) Ti–rich titanomagnetite with Tc ~ 300 °C are primary. If so, the LTC1s should be DRMs. Also, even if partially low-temperature oxidation (maghemitization) of the detrital titanomagnetite grains occurs, the timing of the oxidation should be just after deposition, in which CRM can partially contribute to the NRM acquisition but its direction has to have little difference from the original DRM one. In this case, the magnetite (or Ti-poor titanomagnetite) grains must be chemical origin after deposition, because if the magnetite (or Ti-poor titanomagnetite) grains were also detrital in the origin, the HTC1 carried by the magnetite (or Ti-poor titanomagnetite) would have a similar direction as the LTC1 one.
Tarduno et al. (1996, 1998) suggested that, in pelagic sediments, chemical remanences carried by bacterial magnetite that is produced between the Mn- and Fe-redox boundaries can be acquired significantly after deposition with non-steady-state conditions. The CRM acquisition by bacterial magnetite could be a reason why the HTC1 directions are variable highly. However, there is no rock magnetic signature indicating the presence of bacterial magnetite [for example, dominance of SD magnetite expressed as e.g. kARM/SIRM > ~ 1 × 10−3 m/A and kARM/kLF > ~ 10 and a sharp central ridge in the FORC diagram (e.g. Liu et al. 2015)].
Another possibility is that the HTC1 directions, carried possibly by magnetite (or Ti-poor titanomagnetite), are primary. If so, the HTC1s should be the original DRMs. Then the LTC1s should be post-depositional CRMs after deposition, compatible with the observation that the highest TH step that define the LTC1s is associated with the Tub (Tc) of the low-temperature mineral phase. This scenario is possible when the low-temperature oxidation of the original detrital (titano)magnetite to (titano)maghemite occurs (e.g. Bina and Prévot 1989; Zhou et al. 2001; Fischer et al. 2008; Channell and Xuan 2009) in oxidizing condition. It is also consistent with the LTC1 directions that are apparently within the secular variation range of the normal-polarity Brunhes Chron. Furthermore, this possibility may explain the occasional presence of the HTC2 having Tubs above 580 °C: some maghemite with high-temperature stability formed during the low-temperature oxidation might also acquire post-depositional CRM with Tubs with above 580 °C, having a normal-polarity direction, but this acquisition might not significantly affect the temperature spectra below 580 °C of its NRM previously carried by the detrital magnetite (or Ti-poor titanomagnetite), presumably because of a relatively little amount compared to the pre-existing magnetite (or Ti-poor titanomagnetite). This possible scenario may also explain the HTC1 decaying not toward the origin during TH demagnetization, for example, as shown in the case of specimen GSDS2–7.1 cm-TH of Fig. 4. On the other hand, some studies (e.g. Channell and Xuan 2009; Xuan and Channell 2010; Xuan et al. 2012) reported apparently negative inclinations unblocked below ~ 300–350 °C in late Brunhes-aged Arctic deep-sea sediments, which were often anti-parallel in direction to the remanence with higher Tubs. The low-temperature negative inclinations were interpreted by the authors as partially self-reversed CRMs carried by titanomaghemite. Even considering this, the current possibility that the HTC1 s were originated from the geomagnetic field during or just after deposition is still valid.
It would be also noted that in five depth levels (14.8, 21.4, 26.5, 31.5, 43.0 cmcd; 13.7, 20.3, 25.3, 29.7, 38.0 cm in the raw depth scale of the GSDS2 profile) the TH demagnetizations could isolate two different directions in the 280–580 °C range (Fig. 5; Fig. 12; Additional file 1: Table S4). It is difficult to find an exact explanation of the mechanism of these two remanence directions, but it is worth noting that at least for these distinct intervals there are some directional similarities to artifacts created by rock magnetic complexity discussed earlier. For solving this issue, more detailed identification of magnetic grains, for example, electron microscopy and X-ray analyses of magnetic separates may be helpful.
Figure 13a illustrates directional change of the HTC1s with time as the reversal angle (Valet et al. 2012) which denotes the angle between the local magnetic vector and the GAD-expected direction at the GSDS site (D = 0°, I = 53°). An estimated age-depth model based on linear interpolations between the AMS 14C dates could assign a sequence of our paleomagnetic data to be in the age range from 21.3 to 17.1 cal. kyr BP (see also Additional file 1: Table S5), which is generally thought to be within the time of a stable normal magnetic field polarity (i.e. within the Brunhes Chron). The TH-derived paleomagnetic data possibly indicate variation of the geomagnetic field spanning approximately from 20.5 to 17.4 cal. kyr BP (or ka). This directional variation is characterized by several directional swings, where some reaches nearly the opposite polarity (~ 17.8 and ~ 18.6 ka), concentrated in the interval spanning approximately from 18.8 to 17.4 ka and by a possible high-amplitude departure at before ~ 20 ka. Figure 13b shows time-dependent variation in NRM40mT/ARM40mT ratio as a possible proxy of relative paleointensity (RPI) variation. This RPI proxy variation shows that there is extremely high at ~ 19.2 ka and relatively low values appear to persist over the periods before and after ~ 19.2 ka. A number of steady paleomagnetic studies with high-resolution and quasi-continuous deep-sea sediments have revealed that most excursions are associated with field intensity minima (e.g. Guyodo and Valet 1999; Stoner et al. 2002; Stott et al. 2002; Valet et al. 2005; Yamazaki and Oda 2005; Laj and Channell 2007; Channell et al. 2009; Roberts et al. 2013). However, we should note that in the studied sediment such RPI proxies based on AF-derived properties do not seem to ensure their validity probably because NRMs in the studied sediment are considered to be influenced entirely by the post-depositional CRMs having strongly overlapping AF spectra with those of the possible original DRMs, as discussed above.
In Table 1, previous paleomagnetic records of anomalous features during the 15–30 ka period are summarized. None of our unusual directional features, according to the estimated age model, could be correlated with the Rockall excursion (~ 26 ka; Channell et al. 2016). In the ~ 17–22 ka period that our paleomagnetic record covers, there are some documents reporting excursional features from different sites on the globe. Singer et al. (2014b) presented new 40Ar/39Ar dates of 17.1 ± 0.9 (2σ uncertainty; Fig. 13c) ka from comenditic lavas atop Tianchi Volcano in NE China, in which Zhu et al. (2000) reported excursional directions (corresponding to the intermediate-to-southerly VGP latitudes, labeled as “lava 3”and “lava 4” in Fig. 13c) and paleointensity lows, then they called it “Tianchi excursion”. The authors suggested that the ~ 17 ka Tianchi excursion could be correlative with the Hilina Pali excursion that was first documented in Hawaii by Coe et al. (1978) and later confirmed by Laj et al. (2002b) and Teanby et al. (2002) (see also Fig. 13d, e), and even with some excursional records approximately between 17 and 20 ka from other sites (e.g. Peck et al. 1996; Nowaczyk and Knies 2000; Nowaczyk et al. 2003; Turrin et al. 2013). However, because of uncertainties on their chronologies the correlation still remains uncertain, as also pointed out in Singer et al. (2014b). On the other hand, it should be noted that in deep-sea sediment cores in eastern Arctic Ocean an apparent excursional feature (negative/shallow NRM inclinations) across the ~ 17–22 ka period was also observed (e.g. Nowaczyk and Knies 2000; Nowaczyk et al. 2003; Channell and Xuan 2009; Xuan and Channell 2010; Xuan et al. 2012), but some studies suggest that the Arctic excursional features are rock magnetic artifacts (e.g. Channell and Xuan 2009; Xuan and Channell 2010; Xuan et al. 2012). Moreover, in the given time interval, no excursional paleomagnetic directions but apparently high-amplitude secular variations in the direction were found in sedimentary cores from e.g. Laguna Potrok Aike maar lake in southern Argentina (Lisé-Pronovost et al. 2013) and western North Atlantic Ocean (Figures S1 to S3 in Lund et al. 2017).
To summarize, our recording of the unusual directions in the ~ 17–19 ka period may be correlative with the Tianchi excursion or elsewhere and, in conjunction with the possible large directional departure before ~ 20 ka, our paleomagnetic data may infer a possibility of more complex picture of the ~ 17–22 ka geomagnetic field than expected.
On the other hand, it is also fact that the previous quasi-continuous paleomagnetic records from lake or marine sediments in Northeast China (e.g. Frank 2007) and Japan (e.g. Yamazaki et al. 2003; Hayashida et al. 2007) did not preserve any excursional features (Additional file 1: Figure S1) despite relatively high sediment rates (~ 15 cm/kyr in Yamazaki et al. 2003; ~ 27 cm/kyr in Frank 2007; ~ 35 cm/kyr in Hayashida et al. 2007). However, the Tianchi lava sequence, although it is near Erlongwan maar lake, recorded the ~ 17 ka excursion (Singer et al. 2014b). One might say that the GSDS excursional features may be potentially correlated with any of inclination lows in the previous East Asia records (see Additional file 1: Figure S1), given that post-depositional remanence lock-in can smoothen geomagnetic signals with durations on time scales of hundreds of years (e.g. Channell and Guyodo 2004; Roberts and Winklhofer 2004).
Finally, we would like to emphasize the unusual directions we have observed need further investigation to confidently determine whether they record true geomagnetic instability or rock magnetic recording complexity.