Multidecadally resolved polarity oscillations during a geomagnetic excursion

U-Th Dating and Paleomagnetic Analyses

Stalagmite SX11 was collected in a chamber 800 m from the entrance of Sanxing Cave (107°11′ E, 27°22′ N, 2 km in length, 720-m altitude), Guizhou Province, southwestern China (Fig. 1 A and B), in 2011. The stalagmite is 1 m in length and 8 cm in diameter, and it has a candle-like shape and yellow to dark brown color (Fig. 1C). At the micrometer scale, irregularly shaped detrital particles are observed within laminae (SI Appendix, Fig. S2). Subsamples with 5-mm thickness were used for U-Th dating to provide 2σ precision of ±10s of years (Methods Summary). The stalagmite, which was deposited from 106.66 to 91.23 ka, contains a hiatus from 100.81 to 98.77 ka (Fig. 1C and SI Appendix, Figs. S3 and S4).

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Fig. 1.

Sampling location and photograph of Sanxing stalagmite SX11. (A) Location map of Sanxing Cave (red circle). (B) The sampling site of SX11 in the cave chamber (red star). (C) Stalagmite photograph. The white line is a tape measure, and the dashed brown curved line denotes a hiatus at a depth of 45.5 cm.

One hundred ninety-four subsamples were cut along horizontal growth laminae to avoid possible distorted paleomagnetic directions at stalagmite slope edges (Methods Summary). The natural remanent magnetization (NRM) ranges from 10⁻³ to 10⁻⁵ A/m and averages 10⁻⁴ A/m. Alternating field (AF) demagnetization results for most (180 of 194) subsamples indicate the presence of a weak viscous remanent magnetization (VRM) that is removed after AF treatment at 10–25 mT. At high demagnetization fields, the NRM is dominated by one stable paleomagnetic component that generally decays linearly to the origin of demagnetization diagrams and that defines the characteristic remanent magnetization (ChRM) with AF < 50 mT (Methods Summary, Fig. 2 A and C, SI Appendix, Fig. S5, and Dataset S2). Thermal demagnetization of a three-axis isothermal remanent magnetization (IRM) (27) indicates that the soft component (B_cr < 50 mT) is fully thermally demagnetized at 575 °C, which indicates that the ChRM carrier is dominantly magnetite (Fig. 2 B and D and SI Appendix, Fig. S6). The ratio of anhysteretic remanent magnetization (ARM) to saturation isothermal remanent magnetization applied in a 1-T pulse field (SIRM_1T) suggests that the magnetite particles are in the single domain to vortex state size range (SI Appendix, Fig. S7). These properties are typical of the pedogenic and detrital magnetite particles that are likely to be delivered to the stalagmite by drip waters. The stability of the recorded paleomagnetic directions throughout the stalagmite (SI Appendix, Fig. S5) confirms that the magnetite particles must be in this magnetically stable particle size range. A one-component ChRM with a signal dominated by magnetite indicates that the magnetic particles acquired a depositional remanent magnetization without chemical alteration/precipitation of magnetic carriers during stalagmite growth (28).

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Fig. 2.

Orthogonal projections of AF demagnetization results for the NRM and thermal demagnetization results for a three-axis IRM. Typical orthogonal vector component plots for depth intervals of (A) 565–570 and (C) 970–975 mm with vector endpoints projected onto the vertical (white squares) and horizontal (black squares) planes. Letters of E, W, S, N, U, and D denote east, west, south, north, up, and down, respectively. ChRM line fits (red lines) indicate the presence of one stable component with NRM values of 6.10 × 10⁻³ A/m at 970–975 mm and as low as 5.30 × 10⁻⁵ A/m at 565–570 mm. Thermal demagnetization curves for a three-axis IRM at depth intervals of (B) 560–580 and (D) 960–980 mm imparted with fields of 2.5 (red circles), 0.5 (yellow circles), and 0.05 T (blue triangles). Unblocking up to 575 °C indicates that magnetite is consistently the dominant magnetic mineral.

Geomagnetic excursions are often defined as a deviation of the virtual geomagnetic pole (VGP) from the normal paleosecular variation range (NPSVR) (29). Stalagmite SX11 was not oriented azimuthally, and VGPs could not be calculated from paleomagnetic directions. NPSVR for inclination at the site was calculated with a paleosecular variation model for VGP scatter (30). The gradient color bands in Fig. 3B indicate inclination distributions from modeled paleosecular variation for normal (yellow in Fig. 3B) and reversed (blue in Fig. 3B) polarities at 27°22′ N, where the most intense color expresses the highest probability; geocentric axial dipole field inclinations for both polarities ±45.8° are also plotted with dashed lines in Fig. 3B (Methods Summary and SI Appendix, Fig. S8). Two excursions, post-Blake 1 (PB1) and PB2, which are defined based on inclination, are recorded in the stalagmite inclination record at 105.8–103.5 and 98.5–92.0 ka, respectively (Fig. 3). These excursions coincide with the PB excursion (PBE) events at 105 and 95 ka, respectively, which were discovered in Lac du Bouchet, France (31). ⁴⁰Ar/³⁹Ar dating of lava flows from Lipari Island and New Mexico indicates that the beginning of the PBE was at 104–105 ka (Lipari: 105 ± 1 ka; New Mexico: 104 ± 12 ka) (17). The PBE is also recorded in Icelandic lava flows at 94 ± 8 ka (32).

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Fig. 3.

Paleomagnetic record for Sanxing stalagmite SX11 at 107–91 ka. (A) Two excursions, PB1 and PB2, are indicated in the magnetostratigraphic column. (B) Inclination record with 95% confidence limit of ChRM (calculated following ref. 51 and shaded in gray). ²³⁰Th ages and 2σ errors are plotted. Numbers with uncertainties are ²³⁰Th ages per data point (unit: years before AD 1950). Ages in italics were calculated using the linear age model to interpolate between two adjacent determined ²³⁰Th ages with propagated errors. Horizontal dashed lines are inclinations for a geocentric axial dipole field at the site. Yellow and blue color gradient bands denote the probability distributions of modeled paleosecular variation at 27°22′ N for normal and reversed polarity, respectively. The letters a–f and numbers 1–4 represent different intervals of geomagnetic field behavior described in the text. (C) Relative declination record with 95% confidence limit of ChRM (shaded in gray). (D) Virtual axial dipole moment (VADM) records of the SINT-800 (blue line), PISO-1500 (red line), and PADM2M (black line) RPI stacks with 95% confidence limits (39⇓–41). The vertical light gray bar indicates the location of the hiatus.

For the PB1 excursion, at least three submillennial to millennial directional drift cycles (Fig. 3, a and b, 650 y; c and d, 1,100 y; e and f, 500 y) with inclinations that change from the NPSVR to shallow excursional angles are identified (Fig. 3). The first cycle (PB1-a and PB1-b) involves a rapid inclination deviation from 27.3° to 0.8° and a westward declination drift of 60° in 100 y (a in Fig. 3B; inclination enlarged in SI Appendix, Fig. S9) followed by a 150-y northward inclination shift back to NPSVR values and an eastward declination drift of 60°. Directions then remain in the NPSVR for 400 y (b in Fig. 3B; inclination enlarged in SI Appendix, Fig. S9). The second cycle (PB1-c and PB1-d in Fig. 3B; inclination enlarged in SI Appendix, Fig. S9) has a similar pattern, where the inclination deviated from 49.7° to −2.9° in 70 y (PB1-c), rebounded back to NPSVR values in 70 y, and then remained in the NPSVR for 900 y. The counterpart declination change involved a rapid 120° eastward movement followed by westward drift back to NPSVR values (PB1-d). The structure of the third cycle (PB1-e and PB1-f) differs from that of the first two cycles. The inclination deviated slowly from 27.3° to 8.8° in 350 y (PB1-e) and then returned in 150 y (PB1-f). The declination changed within a 60° range. The PB1-e and PB1-f record has large uncertainties with low stalagmite growth rates, which prevent clear visualization of field behavior. After 103 ka, inclinations remained within the NPSVR, and relative declinations were roughly constant for at least two millennia.

The PB2 excursion has two large inclination drift cycles (Fig. 3, 1 and 2, 2,300 y; 3 and 4, 3,900 y) from 98 to 92 ka (Fig. 3). The cycle at 98–96 ka involved an abrupt drift to full opposite polarity with inclination changing from 30.0° to −64.6° (one data point at 98.22 ka) and a westward declination shift of 120° from 98,360 ± 39 to 98,216 ± 43 y (1 in Fig. 3B; inclination enlarged in SI Appendix, Fig. S9) followed by a 1,400-y period with excursional inclinations from −19.5° to −0.1° and an inclination return to NPSVR over 800 y, with declination drifts of >60° (PB2-2). The second cycle at 96–92 ka records a reversal process with a gradual 40° inclination shift with four 250-y fluctuations, each recorded by multiple data points, from 62.3° to −20.9°. During this 1,050-y interval of PB2-3, the declination rotated eastward by >60° over the first 950 y and rotated westward by >60° during the final 100 y. Through the 2,950-y PB2-4 interval, excursional inclinations between −9.0° and 3.5° occurred over a 2,350-y period, with declination drifts of >30°. Directions then returned to NPSVR values slowly, with declination rotating >60° eastward in 300 y. The geomagnetic structure of these multimillennial cycles is similar to that of the first two submillennial cycles in the PB1 excursion (a–b and c–d), although with different timescales.

Asymmetrical Interhemispheric Polarity Oscillations

The multicycle variations during the PBE illustrated in Fig. 3 are also observed for the BE over the 119- to 112-ka interval in a stalagmite from northern Spain (22) that lacks precise age control. Such a detailed multidecadally resolved drift structure is only precisely dated in the Sanxing stalagmite. The multicentennial to millennial inclination drift cycles (Fig. 3B) are characterized by an abrupt southward shift and slow relaxation back to the NPSVR (except PB1-e and PB1-f), which are expressed as an asymmetrical saw-tooth oscillation. For the PB1 excursion, the three observed inclination events only drift to ∼0°, whereas one fully reversed polarity direction is observed in the PB2 excursion. The longest saw-tooth inclination oscillation is 4,000 y for the cycle from PB2-3 to PB2-4 (Fig. 3B), and the shortest is only 650 y for the period from PB1-a to PB1-b. Even with large differences in oscillation durations, a similar geomagnetic structure is depicted for the multicentennial to millennial cycles with an abrupt southward shift and slower rebound (Fig. 3B). The periodicity and amplitude of directional deviations of saw-tooth oscillations are amplified in PB2 with respect to PB1. The stalagmite BE record from northern Spain has a low inclination departure a few thousand years before the full reversed polarity event (22). The PBE record from SX11 also has a low inclination PB1 period before PB2. The low inclination event could be a precursor before the large amplitude geomagnetic excursion.

The most noticeable features of our record are the occurrence of two dynamical changes toward negative inclination at PB2. The one-millennium PB2-3 drift period from 96.0 to 94.9 ka is characterized by four 250-y fluctuations (Fig. 3B). Another abrupt movement occurred in 144 ± 58 y during PB2-1 (Fig. 3B and SI Appendix, Fig. S9). These polarity transition periods are much shorter than previously estimated millennial-scale reversal durations from volcanic records (33). The most surprising observation is the abrupt centennial polarity shift at PB2-1, which possibly occurred within one century. Such an extremely rapid polarity drift has not been shown before. Previous hotly debated rates from lava (34) and lacustrine sedimentary (35) records suggested extreme directional changes of 1° per week (34) or less than a century for the transition between polarity states (35). However, the rapid reversal indicated by lava flows was rescinded in later studies (36), and the lacustrine records (35) have been suggested (37) and then shown to be affected by chemical remagnetization (38).

Mechanisms of Asymmetrical Polarity Drifts

Transitional field morphologies and underlying physical mechanisms have never been established fully due to the lack of detailed geomagnetic excursion records with precise dating. Our Sanxing stalagmite record offers a radioisotopically dated dynamic view of geomagnetic field behavior on multidecadal to millennial timescales associated with reversals or excursions (Fig. 3 and SI Appendix, Fig. S9). The saw-tooth inclination oscillations represent asymmetrical interhemispheric polarity drifts at 105.8–103.5 and 98.5–92.0 ka, respectively (Fig. 3). A rapid reversal event within a century at 98 ka (PB2-1) (SI Appendix, Fig. S9) coincides with a global relative paleointensity (RPI) minimum (39⇓–41) (Fig. 3D), which indicates a strong link between the amplitude of the directional deviation and RPI.

A hypothesis for geomagnetic excursions has suggested that rapid field changes (Fig. 3) are associated with an intrinsically oscillating dynamo during periods with low dipole intensity (42), although exact timescales are unknown. Our stalagmite record provides a precisely radioisotopically dated excursion time series, which reveals that directional geomagnetic stability correlated inversely with field intensity (Fig. 3 B and D) and that the geodynamo was intrinsically less stable when dipole field intensities were low. Model simulations (43) indicate that, under a low-friction scenario with small Ekman number, disturbances can propagate in Earth’s liquid outer core. The enhanced outer core convective flow can generate a geomagnetic field with a particular polarity. This gradually increasing field may become unstable and then eventually collapse as depicted by inclination dynamics during PB2-3 and PB2-1. Ongoing convection continues to produce dynamo action and generates magnetic fields that could have the opposite polarity. Inclination variability shown in Fig. 3 suggests that such a nonlinear process probably repeats several times on centennial to millennial timescales in response to chaotic geomagnetic variations. Our results support the argument that the geodynamo was unstable and that it undergoes continuous change to reestablish a full polarity state between occasional collapses (43).

Full geomagnetic polarity reversals are relatively rare (3). The intensity of Earth’s magnetic field varies continuously, and it has decreased by ∼10% over the past century, which has led to suggestions of an impending reversal (11, 12), although it is controversial (44). Our Sanxing stalagmite record indicates that short-lived but rapid field changes occur on centennial timescales when the geomagnetic field intensity decreases into a reversal or excursion. Such rapid geomagnetic changes can disturb animal navigation (45), damage satellites, and disturb radio communications (46). A much-improved view of past geomagnetic fluctuations without the smoothing or temporally aliased spot readings provided by sedimentary or volcanic archives is, thus, important for obtaining additional insights into geomagnetic transition processes.