Paleomagnetic inclination variations during the last 200 kyr in the Okhotsk Sea and their relation to persistent non-axial-dipole field

Continuous records of past geomagnetic intensity variations during the last few million years recovered from marine sediments revealed that paleointensity fluctuations between polarity reversals contain variations with timescales of 10⁴ years and longer (e.g., Guyodo and Valet 1999; Yamazaki and Oda 2005; Valet et al. 2005; Channell et al. 2009; Tauxe and Yamazaki 2015). It is expected that paleomagnetic direction also has secular variations of such timescales. However, discussion on directional secular variations has mostly been for centennial to millennial timescales utilizing datasets during Holocene (e.g., Korte and Constable 2005; Lund et al. 2006; Yang et al. 2009; Constable and Korte 2015). Directional secular variations of 10⁴ or longer timescales, if exist, would have an amplitude of ~5° or smaller, similar to differences between the time-averaged field during the last few million years and the geocentric axial dipole (GAD) field. Such variations are close to sampling and measurement errors for studies using sediment cores, and not easy to be detected. To enhance signal-to-noise ratios, precise inter-core correlations among many cores are required, which is also not easy to be performed.

Occurrence of inclination variations with timescales of 10⁴ years and longer was previously reported using sediment cores from the western equatorial Pacific (Yamazaki and Ioka 1994; Yamazaki and Oda 2002; Yamazaki et al. 2008). Using a continuous inclination record for the last 2 m.y., a possibility of orbital influence on inclination variations was argued (Yamazaki and Oda 2002; Roberts et al. 2003). Correlation between paleointensity and inclination was also investigated using some records from this region, and a model explaining the correlation was proposed (Yamazaki and Oda 2002, 2004; Yamazaki 2002). For better understanding of the geomagnetic field behavior of 10⁴ year and longer timescales, further accumulation of datasets with global site distribution is required. It is also necessary to understand long-term secular variations for tectonic application of paleomagnetism assuming the GAD field; we need to know a period of time required for averaging out secular variations in order to detect differences of several degrees in paleolatitudes.

In this paper, we present inclination variations during the last 200 kyr recorded in sediment cores from the Okhotsk Sea. Three piston cores and nine gravity cores adjacent to each other were used for stacking. We show that synchronous inclination shifts occur in the Okhotsk Sea and western equatorial Pacific and that the inclination variations may correlate with paleointensity. We then present a model for the coherent variations.

Three piston cores of about 20 m long were obtained from the central part of the Okhotsk Sea during the R/V Mirai MR06-04 cruise in 2006, and nine gravity cores of about 6 m or less in length were obtained from the same area during the R/V Yokosuka YK07-12 cruise in 2007 (Fig. 1; Table 1). Three gravity cores out of nine were re-occupation of the three piston-core sites to make up for disturbed surface sediments. Composite core sections, GC1 + PC7, GC8 + PC6, and GC9 + PC5, were established at the three sites (Yamazaki et al. 2013). Discrete samples for paleo- and rock magnetic measurements were taken sequentially from the half-split core surfaces using plastic cubes of 7 cm³ each.

Site	Latitude (N)	Longitude (E)	Depth (m)	GAD I	Mean I	∆I
GC1 + PC7	51°16.5′	149°12.5′	1253	68.2	71.2	3.0
GC3	52°36.1′	150°08.3′	1048	69.1	69.9	0.8
GC5	52°39.6′	149°08.8′	1084	69.1	75.0	5.9
GC6	53°13.5′	148°56.5′	1456	69.5	64.6	−4.9
GC8 + PC6	53°16.9′	150°04.7′	1145	69.5	69.2	−0.3
GC9 + PC5	54°19.0′	149°16.1′	828	70.2	70.9	0.7
GC10	54°43.0′	149°17.9′	513	70.5	70.8	0.3
GC11	53°25.7′	148°58.3′	1381	69.6	68.8	−0.8
GC12	53°10.4′	148°56.5′	1299	69.5	74.2	4.7

I inclination, GAD geocentric axial dipole, ∆I inclination anomaly (observed inclination minus GAD inclination)

Relative paleointensity and magnetic properties of the three major sites, GC1 + PC7, GC8 + PC6, and GC9 + PC5, were already reported by Inoue and Yamazaki (2010) and Yamazaki et al. (2013). The procedure of paleo- and rock magnetic measurements of other gravity-core sites, GC3, GC5, GC6, GC10, GC11, and GC12, was the same as that of Inoue and Yamazaki (2010) and Yamazaki et al. (2013). Stepwise alternating-field (AF) demagnetization showed univectorial behavior in general except for a soft secondary component that was removed at AF of 10 mT or less. Most samples have maximum standard deviation (MAD) of <10° at principal component analysis (Kirschvink 1980); a small number of samples with MAD >10° were discarded. For relative paleointensity estimation, anhysteretic remanent magnetization (ARM) was chosen as a normalizer of natural remanent magnetization (NRM) for compensating differences in NRM acquisition efficiency, and NRM and ARM intensities after AF demagnetization at 30 mT were used for calculating relative paleointensity.

The scheme of inter-core correlations is shown in Fig. 2. Correlations and age estimations of the three major sites, GC1 + PC7, GC8 + PC6, and GC9 + PC5, were based on relative paleointensity, which was tied to the PISO-1500 curve of Channell et al. (2009), as presented in Yamazaki et al. (2013). The cores of the three sites cover the last 360–520 kyr with the average sedimentation rates of 37–59 m/m.y. The age model based on the oxygen-isotope (δ¹⁸O) stratigraphy at Site GC1 + PC7 well agrees with that of relative paleointensity (Yamazaki et al. 2013). Coincidence of the relative paleointensity records of the three major sites is generally good (Fig. 3a), although relatively large temporal and spatial lithological changes in the Okhotsk Sea sediments (Nürnberg and Tiedemann 2004; Yamazaki et al. 2013) are not ideal for relative paleointensity estimations, and thus, the records may partly be influenced by lithological changes (Tauxe and Yamazaki 2015).

Other short gravity-core sites were, on the other hand, tied to the Site GC1 + PC7 or GC9 + PC5 based on inter-core correlation using magnetic susceptibility (Fig. 2). We chose magnetic susceptibility for the correlation rather than relative paleointensity because the number of conspicuous features that can be used for correlation is larger in magnetic susceptibility for the cores that cover a relatively short period of time. The Site GC10, the northernmost site, was tied to Site GC9 + PC5, whereas other southern sites are correlated to Site GC1 + PC7 (Fig. 2). This is because environmental changes and thus magnetic property changes including magnetic susceptibility were asynchronous between the northern and southern parts of the Okhotsk Sea (Yamazaki et al. 2013); this is why relative paleointensity was used for the correlation of the three major sites. The southern part (Sites GC1 + PC7 and GC8 + PC6) was in mobile sea-ice conditions even in full glacials, and accumulation of ice-rafted debris (IRD) increased in glacial and deglacial periods. This was succeeded by extremely enhanced ocean productivity induced by nearly ice-free conditions in early interglacials. The northern part (Site GC9 + PC5) was, on the other hand, covered with perennial sea ice in glacials, and IRD accumulation was low in glacials and increased in early interglacials. Succeeding ocean-productivity enhancement was delayed compared to the southern part (Yamazaki et al. 2013).

The correlation between Site GC3 and Site GC1 + PC7 is shown in Fig. 4; the correlations of other sites are presented in Additional file 1, Additional file 2, Additional file 3, Additional file 4, and Additional file 5. A constant sedimentation rate was assumed between tie points. The inter-core correlations using magnetic susceptibility yielded relative paleointensity variations consistent with each other. Estimated ages of the bottom of the gravity cores range from about 57 (Site GC11) to 197 ka (Site GC3). Age–depth curves of individual cores are shown in Additional file 6. The average sedimentation rate is from 21 (Site GC5) to 92 m/m.y. (Site GC6), which corresponds to time intervals of 1100 and 250 years for each discrete sample, respectively.

Inclination records of the three major sites are shown in Fig. 3, which suggests that variations of a timescale of a few tens of 1000 years occur in common. Inclination records of each gravity-core site are presented in Fig. 4 and Additional file 1, Additional file 2, Additional file 3, Additional file 4, and Additional file 5 together with the records of the target sites of the inter-core correlations with magnetic susceptibility. The agreement of inclination records between the coupled sites is generally good. Inclination data of all nine sites are superimposed in Fig. 5a. Inclination variations with a timescale of a few tens of thousand years are visible. Sudden inclination decreases were observed at about 28, 35–40, 64 85, 113, and 188–195 ka in two or more cores. Part of them are close in age to known geomagnetic excursions, the Mono Lake excursion at ~33 ka, the Laschamp at ~41 ka, the Blake at ~120 ka, and the Iceland Basin at ~188 ka (Roberts 2008), and they may have recorded the excursions. On the other hand, low inclination spikes at about 128 and 180 ka, which were recorded in only one core (Site GC9 + PC5), may not be of geomagnetic origin.

We constructed a stacked inclination record of the Okhotsk Sea. The age interval of the stack was limited for the last 200 kyr because the number of available cores for older ages is small, three or less. First, the mean inclination of each site was calculated between 0 and 100 ka (Table 1). Inclination data from the uppermost 20 cm of each core were removed because of possible physical disturbance of surface sediments during coring. For calculating the mean inclination, incfish.py of the pmag.py programs for inclination-only data (Tauxe 2010) was used. Inclination anomaly (∆I) of each site, which is defined as observed mean inclination minus GAD inclination, ranges from −4.9° to 5.9° (Table 1), and the mean ∆I of the nine sites is 1.0° ± 3.2° (the mean ∆I is 2.1° ± 1.7° when calculated between 0 and 200 ka for four sites that cover this time interval). Next, we chose Site GC1 + PC7 as a representative location, and differences in inclinations expected from the differences in site latitudes were corrected using GAD inclinations. In addition, inclinations of each site were shifted slightly so that the ∆I of each site becomes zero. Then, the mean and standard deviation were calculated at 1-kyr intervals after resampling of each record (Fig. 5b). Long-term inclination variations are visible on the stacked record; intervals of shallower inclinations occur at about 25–45, 75–90, 110–135, and 185–200 ka in the Okhotsk Sea.

We then compare the inclination stack from the Okhotsk Sea with the stacks of relative paleointensity and inclination from the West Caroline Basin in the western equatorial Pacific (Yamazaki et al. 2008; Fig. 5). The inclination records of the two regions show parallel variations, although the two regions are about 5000 km apart; the periods of shallower inclinations in the Okhotsk Sea at about 25–45, 75–90, 110–135, and 185–200 ka coincide with those of negative steeper inclinations in the West Caroline Basin. These inclination shifts are synchronous with relative paleointensity lows in general (Fig. 5), as pointed out in the West Caroline Basin by Yamazaki and Oda (2002) and Yamazaki et al. (2008), although a period of paleointensity low around 100 ka contradictorily has steeper, but not shallower, inclinations in the Okhotsk Sea.

Yamazaki and Oda (2002, 2004) and Yamazaki et al. (2008) explained the inclination and paleointensity variations observed in the western equatorial Pacific that relative contribution of a persistent quadrupole component increased when the strength of the GAD field decreased. The western equatorial Pacific is known to have a large ∆I associated with a quadrupole component, and the sign of ∆I flips with polarity reversals (Johnson and Constable 1997). From sediment cores in the West Caroline Basin, ∆I of −6.5° ± 2.8° (N = 13) in the Brunhes Chron was reported (Yamazaki et al. 2008). The phase of the paleointensity–inclination correlation also flipped with polarity reversals, in-phase in the Brunhes Chron and anti-phase in the Matuyama Chron (Yamazaki and Oda 2002). The coeval inclination variations observed in the Okhotsk Sea, however, cannot be explained by this model because ∆I in this area is near zero, as observed in our cores.

Hoffman and Singer (2008) proposed that a magnetic field at Earth’s surface during polarity transitions and excursions is dominated by a field generated only in the shallower part of the core, designated the SCOR field. They also proposed that the SCOR field is represented by persisting higher-degree terms other than the GAD field and lasts for a timescale of ~10⁶ years. Both the Okhotsk Sea and western equatorial Pacific are within a region of outward directed flux in the 1590–1990 time-averaged non-axial-dipole (NAD) field, which extends from Europe and Asia to the south of Australia (Hoffman and Singer 2008; Constable and Korte 2015). Thus, the observed inclination shifts around excursions, shallower in the Okhotsk Sea and deeper toward negative in the western equatorial Pacific, may be explained by a larger contribution of the SCOR field when the GAD was weak. In the western North Atlantic, the 1590–1990 time-averaged NAD field has opposite inward flux. In this region, periods of steeper inclinations than the site averages, in which rapid excursional directional swings are intercalated, were observed near the Laschamp excursion (Lund et al. 2001, 2005) and “excursions 13a, 15a, and 17a” (~510, ~573, and ~666 ka, respectively; Lund et al. 2001). This may support the SCOR field model. The coherent shifts in inclinations among the Okhotsk Sea, western equatorial Pacific, and North Atlantic may also be explained by hypothetical dipole wobbles. However, the correspondence of the inclination shifts to paleointensity lows prefers the SCOR field model.

In this study, we obtained a stacked inclination record for the last 200 kyr in the Okhotsk Sea. The mean inclination anomaly of nine sites is close to zero. Inclinations shallower than the average occurred at 25–45, 75–90, 110–135, and 185–200 ka. These are synchronous to inclination shifts toward negative reported in the western equatorial Pacific and coincide in general with paleointensity lows. The synchronous inclination shifts associated with decreased paleointensity may be explained by a larger contribution of the SCOR field proposed by Hoffman and Singer (2008) when the GAD was weaker; both the Okhotsk Sea and western equatorial Pacific are within a region of outward directed flux in the persistent NAD field.