Reversal of Earth’s magnetic field polarity every 10 5 to 10 6 years is among the most far-reaching, yet enigmatic, geophysical phenomena. The short duration of reversals make precise temporal records of past magnetic field behavior paramount to understanding the processes that produce them. We correlate new 40 Ar/ 39 Ar dates from transitionally magnetized lava flows to astronomically dated sediment and ice records to map the evolution of Earth’s last reversal. The final 180° polarity reversal at ~773 ka culminates a complex process beginning at ~795 ka with weakening of the field, succeeded by increased field intensity manifested in sediments and ice, and then by an excursion and weakening of intensity at ~784 ka that heralds a >10 ka period wherein sediments record highly variable directions. The 22 ka evolution of this reversal suggested by our findings is mirrored by a numerical geodynamo simulation that may capture much of the naturally observed reversal process.

INTRODUCTION

Reversals of Earth’s magnetic field are global manifestations of instability that develops within the outer core geodynamo. To understand processes in the core that propel reversals, and to predict how the Earth system would respond to a future reversal, it is critical to map out a detailed description of the global geomagnetic field that occurs during the transition between long-lived, relatively stable, polarity states (1–3). Yet, despite decades of study, the geometric structure, timing, and duration of reversals remain enigmatic, principally because they are short-lived phenomena and thus high-resolution recordings are sparse and difficult to date. The Matuyama-Brunhes (M-B) reversal is the most studied because it has been identified in dozens of marine sediment cores [e.g., (2, 4–11)], exposed sedimentary successions [e.g., (12, 13)], and a few lava flow sequences (14–17). Moreover, the M-B reversal is an important global temporal marker (18) key to defining the Lower-Middle Pleistocene boundary and, thus, to correlating among records associated with the profound shift of global climate dynamics into a state dominated by 100 ka (ka = thousand years) oscillations (19, 20).

Although the vast majority of M-B transition records come from marine sediment cores, sedimentary archives can be degraded with respect to magnetic fidelity and temporal resolution; thus, inferences about the reversing field are controversial (1, 2, 21). Key issues that may complicate sedimentary records include depositional processes, low deposition rates (22, 23), weak magnetization, and, in some cases, remagnetization [e.g., (24)]. When compared to volcanic records, the impact of deposition rates lower than 10 cm/ka and increased magnetic lock-in depth results in smoothing or “smearing” of geomagnetic field directions (1, 2) and thus loss of geometric and temporal resolution. Moreover, even at sediment deposition rates of 10 cm/ka, bioturbation and magnetic lock-in processes can convolute magnetic remanence signals and decrease the temporal resolution to 1 to 2 ka or less. These effects may be amplified owing to the response of magnetometers used to measure U-channels from long sediment cores that integrate signals and lead to smoothing of records over several centimeters of core (1, 25).

On the other hand, the thermoremanent magnetization of lava flows can provide well-understood “spot” recordings of the magnetic field geometry as virtual geomagnetic poles (VGPs), as well as intensity, as they cool (3, 14, 15, 26). Sequences of flows are among the best materials also because successive lavas can provide temporal records of paleointensity together with paleodirection, both tied to 40Ar/39Ar dating [e.g., (16, 27)]. An examination of lava flow records led to the bold proposal by Valet et al. (26) that all reversals share a common three-phase evolution including a precursor, a 180° reversal of the dipole field, and a rebound in which weak nondipole fields emerge during the first and third phases. We examine this proposal in light of our new, more accurate timeline for lava flows associated with the M-B reversal.

We present new 40Ar/39Ar dates within six volcanic sections that record transitional geomagnetic field directions and, in some cases, intensity variations, associated with the M-B reversal. The VGPs of these lava flow sequences are correlated to several high–deposition rate sediment recordings of paleomagnetic field direction and intensity for which astrochronologic age models have been derived. Cosmogenic 10Be proxy records of paleomagnetic field intensity in Antarctic ice and Indian Ocean sediment are also correlated to the volcanic record. The integration of 40Ar/39Ar-dated lava, sediment, and ice recordings reveals extraordinary complexity in the evolution of the geomagnetic field during a ≥22 ka period leading up to and including the final M-B reversal. Moreover, during this period, lava flow VGPs define paths and clusters over Australasia, North America, and Southern South America. The observed behavior is consistent with the hypothesis that demise of the axial dipole field allows a weak non-axial dipole (NAD; the residual field left after subtracting out the axial dipole) field to emerge from the shallow core (28) repeatedly during the waning stages of the Matuyama Chron.

Lava flow sequences thought to record fragmentary pieces of the M-B process are known on the south (29) and north (16) walls of Punaruu Valley, Tahiti; in two adjacent superposed sequences on the west wall of Quebrada Turbia, Chile (30); in Guadeloupe (17); in Los Tilos canyon, La Palma (31); and in the Haleakala “caldera,” Maui (Fig. 1) (14). We note that the two lava sequences in Chile are superposed on the same wall of the Quebrada Turbia canyon such that the entire section records reverse-transitional-normal polarities with a reversely magnetized lava dated at 816 ka at the base of the QTW (Quebrada Turbia West) 10 section, and 1 km to the north, the top of the entire section is recorded in the QTW 11 sequence that terminates with three normally magnetized lavas (30). More than two decades ago, the 40Ar/39Ar ages determined from 11 lava flows in the sections at Punaruu South, La Palma, Chile, and Haleakala led to the conclusion that the M-B reversal may have lasted >12 ka (32). Subsequent 40Ar/39Ar experiments yielded additional ages from these sections (14, 30, 31) plus the sequence of three R-T-R (reverse-transitional-reverse) lava flows on Guadeloupe (17). Singer et al. (15) integrated the geochronology and paleomagnetic directions of 23 lavas at Punaruu South, Chile, La Palma, and Haleakala to conclude that most of these sequences record geodynamo instability associated with an M-B precursor that occurred ~18 ka before the reversal, which is well recorded only at Haleakala. These 40Ar/39Ar ages were determined from large samples (~200 to 450 mg) using single-collector mass spectrometers and gas extraction systems that used resistance furnaces, which had background levels of argon that were relatively high, non-atmospheric in composition, and infrequently measured compared to modern procedures (see the Supplementary Materials) (18). Low and frequently measured system blanks and careful monitoring of mass discrimination are paramount to obtaining accurate and precise dates from Pleistocene lava samples in which the majority of 40Ar is not radiogenic (see Methods and the Supplementary Materials). Moreover, a novel calibration of the 40Ar/39Ar method against astronomically dated marine sediments yields an age for the widely used Fish Canyon sanidine (FCs) standard of 28.201 ± 0.046 Ma that improved both the accuracy and precision of the method by an order of magnitude (33). Yet, recalculating the 23 ages determined by Singer et al. (15) using this new age of FCs results in a mismatch by 8 ka, or about 1%, between the lavas that record the reversal at Haleakala and the astronomical ages of 773 ± 1 ka determined for the M-B reversal in five North Atlantic sediment cores (34) and 773 ka in the Chiba section, Japan (20), that is also independently calibrated by a 206Pb/238U zircon age of 772.7 ± 7.2 ka for the ByK-E tuff (13). We emphasize that, because of the issues noted above regarding how system blank measurements associated with the 23 40Ar/39Ar dates summarized by Singer et al. (15) limit their accuracy to ~2% relative, it is inappropriate to simply recalculate these using the astronomically calibrated age of the FCs standard as has been done by several researchers (34–36) (see the Supplementary Materials). To capitalize on the astronomical calibration of FCs (33), as well as the exceptionally low blanks and counting statistic advantages of an ion counting multicollector mass spectrometer (37, 38), we obtained new 40Ar/39Ar dates from 40 lavas in the six sequences outlined above using the IH MCMS (incremental-heating, multicollector mass spectrometry) approach (see Methods) to improve both the accuracy and precision of the volcanic record of the M-B reversal process.