The paleolatitude of Midway Atoll

Midway Atoll of the Hawaiian chain was drilled in 1965, with the principal result obtained at the Reef Hole (Fig. 1, Supplementary Fig. 1), where 120 m of lavas were penetrated24 (see Drilling at Midway Atoll and prior analyses, in Methods). The recovered tholeiitic basalt flows show weathered flow boundaries; of special note is a thick soil horizon (minimum thickness of 5.8 m) between lava flows25. A shorter basalt section (~16 m) was recovered from the Sand Island Hole (Fig. 1, Supplementary Fig. 1). The highest quality K–Ar radiometric age data from mugearite and hawaiite from conglomerate pebbles above the Reef Hole basement yield an age of 27.6 ± 0.6 Ma26,27. This compares well with 40Ar/39Ar ages of 27.5 ± 1.2 Ma and 27.6 ± 0.9 Ma on shield phase samples recovered from subsequent dredging28. Grommé and Vine29 sampled thirteen of the Midway lavas, applying only partial alternating field demagnetization on about one half of the samples. The nominal paleolatitude \(\left( {14.7_{ - 4.2^\circ }^{ + 7.6^\circ }} \right)\) is generally not considered in recent analyses of Pacific plate motions because the paleomagnetic analysis techniques are incomplete (Methods).

Fig. 1 Location maps and plate-circuit predictions. a Hawaiian-Emperor chain, with episode of rapid hotspot motion highlighted. Predicted position of the Hawaiian hotspot using plate-circuit analyses (from ref. 6, 7, 13); (i) East-West Antarctica4; (ii) Australia-Lord Howe Rise5, using the updated Antarctica-Australia spreading history and rotations of Whittaker et al.44 and Müller et al.45. Red and purple arrows mark differences in plate-circuit prediction and seamount trace. b Midway Atoll with Reef and Sand Island drill sites highlighted Full size image

We sampled throughout the sections of basalts, including weathered core tops and the thick soil (see Paleomagnetic sampling and analyses, in Methods). The magnetization directions show linear decay to the origin of orthogonal vector plots after the removal of spurious magnetizations at low unblocking temperatures (Fig. 2). Rapid intensity decay in the basalts commences over a range of temperatures (~275–400 °C) and extends to ~585 °C, indicating the dominance of titanomagnetite carriers. A small, and variable amount of remanence is carried by higher unblocking temperature minerals (indicative of hematite). Weathered flow tops, as well as soils, show a dominance of these higher blocking temperatures (>585 °C) highlighting hematite in these samples. The characteristic remanent magnetizations, determined by principal component analysis of the demagnetization data, record both normal and reversed magnetization zones (Supplementary Tables 1 and 2). Two positive contact tests (Fig. 2) and the different polarities strongly support to the preservation of a primary magnetization. Basalts from the Sand Island hole are of normal polarity and show demagnetization characteristics similar to those of the Reef Hole (Supplementary Tables 1 and 2, Methods).

Fig. 2 Paleomagnetic data and contact tests. Orthogonal vector plots of stepwise thermal demagnetization of basalt (a) and soil (b) samples. Red is inclination, blue is declination (cores are azimuthally unoriented). c Stratigraphy adjacent to thick soil horizon in the Reef Hole (Supplementary Fig. 1). Here the unit feet is retained as it was the unit of record. Red, soil or weather flow top. Dark grey, basalt. Light grey, unrecovered. d Characteristic remanent magnetization inclination. Blue, basalt samples. Red, soil samples. The uppermost soil sample from the thick soil horizon (circled) records the polarity of the overlying lava flow (reversed polarity) whereas, the lower samples record a different (normal polarity). This constitutes a positive contact test indicating that a primary magnetization has been preserved. Similarly, the uppermost sample of the weathered lava flow top of the reversed flow (circled) records the direction of the overlying lava flow (normal polarity), constituting a second positive contact test Full size image

We identify 12 lava flow means from the Reef Hole, and three from the Sand Island Hole (Supplementary Table 1). The 15 lava flow averages yield a mean paleoinclination of 30.1 ± 7.8° (Supplementary Table 3), corresponding to a paleolatitude of \(16.2_{ - 4.6^\circ }^{ + 5.1^\circ }\,{\mathrm{N}}\), with an estimated angular dispersion of \(S = 14.0_{ - 3.4^\circ }^{ + 10.6^\circ }\) (see Secular variation, in Methods). The mean S value is close to, but slightly lower than predicted from data of the last 5–10 million years30,31, much lower than predicted from Miocene-Oligocene data32, and slightly lower than predicted from Oligocene-Eocene data32 (Supplementary Figs. 2 and 3). Although there are numerous indicators that time has elapsed in the Midway basement stratigraphy, the S estimates at best barely reach predicted time-averaged values. This reflects the limited number of temporally independent lava units (Supplementary Fig. 4) available rather than the age span of the basement penetrated.

Additional paleolatitude estimates, however, are available from the thick Reef Hole soil horizon. Magnetostratigraphy and geochronology suggest that the soil horizon represents weathering over at least several hundred thousand years during chron 9 (27.027–27.972 Ma)33, a duration consistent with soil development on Hawaii34 (see Duration of Reef Hole soil formation, in Methods). This age assignment suggests that the soil horizon represents time averaging that is equivalent to or greater than the cumulative sampling by the lava flows. In this case, it is most appropriate to give the soil horizon samples and lava means equal weight. This approach yields a paleolatitude of \(19.2_{ - 3.4^\circ }^{ + 3.7^\circ }\,{\mathrm{N}}\) (Supplementary Table 3), which is our preferred estimate. This value is indistinguishable from the present-day latitude of the Hawaiian hotspot (~19°). It also reveals as spurious the claim that the recording location of Hawaii is anomalous to the level it would affect paleomagnetic tests of hotspot drift17. It should also be noted that Cromwell et al.31, in an analysis of volcanic data for the last 10 m.y. corrected for serial correlation effects (model LN3-SC), find a negligible anomaly at the location of Hawaii.

The Midway paleolatitude is very different from paleolatitudes recorded by drilling Emperor Seamount lavas (Fig. 3). Before moving on to an additional test, we first perform a formal analysis of the Emperor trend paleomagnetic data to determine a mean rate of latitudinal change. We use a Bayesian approach (specifically, Markov Chain Monte Carlo, see Methods)35,36 to linear regression, incorporating uncertainties in age and paleolatitude. This yields a rate estimate of 47.8 (±15.3) mm yr−1 (95% confidence). This analysis indicates that low rates of motion (i.e. 10 mm yr−1) sometimes interpreted for hotspot drift during creation of the Emperor seamounts14, have a negligible probability (less than 0.01%, Supplementary Fig. 5).

Fig. 3 Paleomagnetic and inter-hotspot distance tests of hotspot fixity. a Paleolatitude versus time plot1 with time-averaged results from paleomagnetic analyses of samples recovered from scientific drilling of seamounts augmented with paleolatitude value from Midway Atoll (blue triangle) presented here. Age uncertainty shown is 2 sigma, paleolatitude uncertainty is 95% confidence. b Distance between Hawaiian-Emperor and Louisville seamounts with formation ages within 3 Myr with 95% confidence. (Supplementary Table 4) Full size image

Changes in inter-hotspot distance

Paleomagnetic and radiometric age analyses of samples recovered during IODP Expedition 330 suggest only limited (3–5°) latitudinal drift of the Louisville hotspot between 50 and 70 Ma20. This result highlighting the independent motion of the Hawaiian plume affords the possibility of measuring not just the relative motion documented by Konrad et al.21, but also absolute motion. If the Louisville plume was only slowly moving while the Hawaiian plume was moving rapidly southward, this difference should be preserved as a change in distance between the volcanic edificies comprising the two hotspots tracks. Continued efforts to improve the age assignments of the Hawaiian-Emperor and Louisville Seamounts through 40Ar/39Ar radiometric analysis20,21,28 allows this analysis (see Hawaii-Louisville seamount distances, in Methods). We select eight seamount pairs where the ages are within 3 m.y. (Supplementary Table 4). These data show a dramatic decrease in distance between 63 and 52 Ma (see Fig. 3, Supplementary Fig. 6, Methods) at a rate of 32.2 ± 6.7 mm yr−1 (95% confidence interval). Distance differences between 80 and 70 Ma are indistinguishable and may point to a similar rate of southward hotspot motion of the two hotspots. There is a hint of this southward motion in the paleolatitude (42.9° S) from the oldest Louisville seamount drilled (Canopus)20, which is more northerly than those of the younger seamounts, but the data do not fully average secular variation. Both tracks were probably influenced by ridges in the Late Cretaceous. After 25 Ma, distances between the hotspots increase, but at this time the Louisville hotspot magmatic output had waned to the point that lithospheric structure probably has a large effect on the plume location as recorded by its volcanic constructs.

True polar wander revisited

The change in hotspot distance19,21 between 63 and 52 Ma provides spectacular confirmation of the rapid Hawaiian hotspot southward motion. Because this distance change is on a single plate it cannot reflect TPW, and therefore inferring that this process is responsible for the observed Emperor seamount paleomagnetic data trend14 is incorrect. A principal set of analyses that have been used for continued calls for TPW have been marine magnetic anomaly skewness. Anomaly skewness values are model-dependent, as the data also reflect a complex mixture of oceanic crustal accretion processes. Nevertheless, the paleolatitudes predicted from the skewness-based models tend to scatter about the more robust paleolatitude trend defined by paleomagnetic analyses of seamount lava (Supplementary Fig. 7). However, these skewness models have also been used to call for a much younger TPW event (after 11 Ma) that would be responsible for North Hemisphere glaciation37. The new data from Midway also afford the opportunity to examine this hypothesized rotation. The closest skewness-based modeled paleolatitude to the age of Midway is a 32 Ma value (marine magnetic anomaly chron 12r)38, which yields 22.3° N. Applying a Bayesian formalism, there is only an 8.9% chance of observing the skewness inclination given the distribution of paleomagnetic inclinations observed at Midway (Supplementary Fig. 8). Thus, we conclude that the apparent latitudinal discrepancy from the skewness models is inconsequential, and the related TPW rotation14,37 unnecessary (see True polar wander based on marine magnetic anomaly skewness, in Methods).

Creating the Hawaiian-Emperor Bend morphology

The confirmation of rapid hotspot motion indicates that this process must be central to the formation of the HEB. This conclusion is in concert with the lack of clear geological evidence for a profound event, on the Pacific margins39 or in the seafloor spreading record40, corresponding to the age of the HEB and commensurate with its prior designation as the largest and most rapid change in plate motion for a large oceanic plate. Some of the confusion about the HEB arises from a narrow focus on relative plate motions41 or ages42 immediately before and after the bend. As a result, the HEB is considered out of context with respect to the overall morphology of the Hawaiian-Emperor chain.

Plate-circuit predictions provide the means of understanding the origin of the overall morphology. They can be used to illustrate a hypothetical Hawaiian-Emperor chain produced by a hotspot fixed in the deep mantle. The standard plate circuit through East Antarctica shows that the HEB (Supplementary Fig. 9A) is replaced by a small wiggle in the chain6 (Supplementary Fig. 9B) similar to those before and after 47 Ma. Thus, even if there is a change in absolute plate motion direction at ~47 Ma, the change appears to have been imprinted in the volcanic track as only a wiggle similar to those that can be related to early and late Paleogene plate motion changes43. Similar conclusions can be reached based on the alternative plate circuit, through Lord Howe Rise5, updated by the revised Antarctic-Australian spreading history44,45 (Supplementary Fig. 9C).

While plate motion changes played only a minor role in the HEB morphology, the analyses here do not exclude the presence of plate motion changes in the Pacific basin. Detailed analyses of Indo-Atlantic hotspot tracks fail to find a signal, which might indicate an important global change in absolute plate motion corresponding to chron 21 (47.9 Ma) and the HEB46. But the still nascent Pacific apparent polar wander path does hint at an acceleration of the Pacific plate in the Cenozoic. Cottrell and Tarduno47 noted that very slow Pacific apparent polar wander in the Late Cretaceous was followed by faster early Cenozoic motion. This is largely consistent with the development of subduction zones in the western Pacific48.

Causes of the rapid hotspot motion

There are are least two processes that can explain the rapid hotspot motion, and its sudden slowdown, at the HEB. They are not mutually exclusive. In the first the ancestral Hawaiian plume is drawn at mid-mantle depths toward rapid spreading at the Late Cretaceous Kula/Pacific ridge6, a process supported by numerical simulations49. As spreading wanes, and eventually ceases altogether, the ridge influence on the plume diminishes. During this time of ever diminishing influence, the plume moves toward a more vertical position in the mantle. In the second, the plume position is affected by motions in the deeper mantle6. Numerical modeling of Hassan et al.50 shows that the interaction of the plume with the Pacific LLSVP can give rise to the observed motions. Geochemistry lends support to this interpretation. The Hawaiian Islands are characterized by two distinctive lava trends, enriched Loa lavas and less primitive Kea lavas, best characterized by 208Pb*/206Pb*, which gauges radiogenic ingrowth in the formation of Earth51. The Emperor trend seamounts lack the Loa trend signature. Although there are several ways to explain double geochemical traces on young volcanic chains52, Harrison et al.53 have focused on a mechanism that can give rise to the observed longterm temporal signature without a change in plate motion. Southward motion and anchoring of the plume on the LLSVP (Fig. 4a) results in the gradual entrainment of LLSVP material and the appearance of Loa lavas on the Hawaiian chain53. This explanation is viable if the Loa geochemical anomaly is a passive marker54 of subducted material stored in the Pacific LLSVP. The potential for LLVSP deformation implicit in the Hassan et al.50 model raises the question of whether they are fixed in the deep mantle, a question that we address below.