Geomagnetic Polarity Transition Research Articles

New 40Ar/39Ar age of the full vector Upper Mammoth geomagnetic polarity transition recorded in the Pu’u Kualakauila volcanic sequence, Hawaii

Paleomagnetic measurements, coupled with 40Ar/39Ar dating, are improving our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the Earth's magnetic field. As part of an investigation of the Wai'anae Volcano, Oahu, and the short-term behavior of the geomagnetic field, we sampled a long volcanic section located on the volcano's collapsed flank at a locality known as Pu’u Kaulakauila. Prior paleomagnetic investigations of the Kamaile'unu Volcanic Series (i.e. Herrero-Bervera and Coe, 1999, Herrero-Bervera and Valet, 1999, 2005) revealed transitional directions. The fresh lava flows, easy access, and close geographical proximity to KAr dated flows made this ~215-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 47 successive flow sites, were stepwise demagnetized by both alternating field and thermal methods. Magnetostratigraphic results indicate the existence of four excursions or aborted reversals occurring at approximately ~36, ~75, ~130, and ~ 151 m above flow#1. The mean directions of magnetization of the entire section sampled indicate a reversed polarity, with ∼10 m of the section characterized by truly excursional/Cryptochron directions (~7 lava flows). Paleointensity (P.I.) determinations indicate a steady decreasing of the PI values from 75 μ-T to ~12.2 μ-T reaching a minimum of 7.1 μ-T. These very low PI values are within the transitional/excursional part of the record. The corresponding VGPs are located on the western part of Australia. 40Ar/39Ar incremental heating experiments on groundmass from transitional flow sites at different stratigraphic levels yield a weighted mean plateau age of 3.201 ± 0.041 Ma, which, combined with the overall reversed polarity and two older polarity reversals, strongly suggests that the transitional lavas correspond to the Upper Mammoth polarity transition.

Mechanism of rift flank uplift and escarpment formation evidenced by Western Ghats, India

The Western Ghats is one of the largest escarpments on earth, containing Reunion plume derived Deccan Traps, it is an excellent example to probe epeirogenic uplift, extension and subsidence in volcanic continental margins. The most continuous unbiased stratigraphic section of basalt down to the basement within a 1250 m drill hole of the Continental Scientific Deep Drilling Project is a valuable resource to investigate the above aspects. The flows across the entire drill core are geologically subaerial in character with basement exposed ~300 m below the mean sea level; they clearly display more evolved compositions from primary melts of mantle in terms of petrology, and only a single geomagnetic polarity transition in palaeomagnetic data. These results, combined with existing geological and geophysical data, constitute a multi-method approach that demonstrates (a) igneous underplating caused uplift prior to frequently suggested flexural isostasy (b) plume impact and eruption are near-simultaneous and extension/rifting essentially followed soon after volcanism and (c) lithosphere beneath the continental margin, while returning to normal temperatures following the Seychelles-India breakup, experienced thermal collapse and subsidence causing slumping of basalt basement below sea level.

Extension of the Middle Miocene Kleifakot geomagnetic instability event in Ísafjörður, Northwest Iceland

Detailed records of past geomagnetic polarity transitions and excursions are seldom found in extrusive volcanics. One type of such events seems to involve very irregular variations of the magnetic field direction. Several examples of that type have been discovered in paleomagnetic surveys on the Neogene lava pile of Iceland. Lava sequences spanning the most notable event are accessible in the two tributary fjords Mjóifjörður and Ísafjörður south of Ísafjarðardjúp, Northwest Iceland. Paleofield direction results from about 80 sampling sites in these sequences have been described in previous publications. The present paper adds 20 sampled sites in Ísafjörður, extending the area where parts of this „Kleifakot instability event“ of the geomagnetic field are recorded, to 5–6 kilometers along the fjord. Intermediate paleofield directions in the collection often agree closely with each other in correlated strata, even more than 4 kilometers apart. Events like this can provide valuable correlation tools for future stratigraphic mapping and various studies on volcanological features in the relatively uncharted region around Ísafjarðardjúp and beyond. Globally, recognition of the existence of such events will aid in the interpretation of results in other kinds of paleomagnetic studies. The role of geomagnetic paleo-intensity determinations is discussed briefly in this context.

A record of the upper Olduvai geomagnetic polarity transition from a sediment core in southern Yokohama City, Pacific side of central Japan

A detailed paleomagnetic record of the upper Olduvai polarity transition was obtained from a 106.72 m-long sediment core drilled in southern Yokohama City, located on the northern Miura Peninsula, on the Pacific side of central Japan. The core spans the upper part of the Nojima Formation and the lowermost part of the Ofuna Formation, both of which correspond to the middle Kazusa Group (Lower Pleistocene forearc basin fill). The record was reconstructed using discrete specimens taken throughout mudstone and/or sandy mudstone sequences in the Nojima Formation. In this record, the virtual geomagnetic pole (VGP) fluctuation accompanying the polarity transition was determined to occur between depths of 66.99 and 63.60 m. These depths have been dated at 1784.4 and 1779.9 ka, respectively, and the duration of the polarity transition is estimated to be 4.5 kyr using an age model based on a δ18O record from that core. The VGP paths during the transition do not appear to show any preferred longitudinal bands. However, the VGP positions cluster in five areas: (A) eastern Asia near Japan, (B) the Middle East, (C) eastern North America (North Atlantic), (D) off southern Australasia, and (E) the southern South Atlantic off South Africa. The primary locations of the observed VGP clusters coincide with the areas on the Earth’s surface that possess a strong downward flux of the vertical component of the present geomagnetic non-axial dipole field. The relative paleointensity rapidly decreased approximately 1 kyr before the beginning of the polarity transition and gradually recovered to its initial level in 12 kyr.

Dipole fluctuations and the duration of geomagnetic polarity transitions

AbstractGeomagnetic polarity transitions are often accompanied by a decrease in magnetic intensity. The time required to reestablish the magnetic intensity after a transition is usually longer than the duration based on magnetic direction. Analysis of the paleomagnetic axial dipole moment from the PADM2M model shows a return to the time‐averaged intensity after 42 kyr. A shorter time is required to recover a fraction of the time‐averaged moment, but the correspondence between recovery time and fraction of recovery is nonlinear. Predictions of a stochastic model reproduce the general trends in recovery time and suggest that fluctuations in dipole generation increase substantially during polarity transitions relative to times of stable polarity. These large fluctuations could reflect larger convective velocities in the core or represent a change in the efficiency of dipole generation. In either case, large fluctuations during polarity transitions can flip the sign of the axial dipole field several times before a polarity transition is completed.

Paleomagnetic constraints on Miocene rotation in the central Japan Arc

AbstractWe present new Middle Miocene paleomagnetic data for the central Japan Arc, and discuss their implications for Miocene rotation. To obtain a refined paleodirection, we made magnetic measurements on basaltic to andesitic lavas and intrusive rocks from 12 sites in the Tsugu volcanic rocks (ca 15 Ma) in the northern part of the Shitara area, Japan. Significant secondary magnetizations in samples with strong magnetic intensities are interpreted as lightning‐induced components. Mean directions carried by magnetite and/or titanomagnetite were determined for all sites. An overall mean direction with a northerly declination was obtained from dual‐polarity site means for nine sites. This direction is indistinguishable from the mean direction for coeval parallel dikes in the northern part of the Shitara area, and also indistinguishable from the Miocene reference direction derived from the paleopole for the North China Block in the Asian continent. These comparisons suggest little or no rotation or latitudinal motion in the study area with respect to the North China Block since 15 Ma. We obtained a refined early Middle Miocene paleodirection (D = 9.7°, I = 52.5°, α95 = 4.8°; 30 sites) and paleopole (82.0°N, 230.8°E, A95 = 5.6°) for Shitara by combining data from the Tsugu volcanic rocks and a coeval dike swarm. An anomalous direction found at three sites could be a record of an extraordinary field during a geomagnetic polarity transition or excursion. Paleomagnetic data from Shitara suggest that: (i) the western wing of the Kanto Syntaxis, a prominent cuspate geologic structure in central Honshu, underwent a counterclockwise rotation with respect to the main part of the southwestern Japan Arc between ca 17.5 Ma and 15 Ma; (ii) collision between the Japan and Izu–Bonin (Ogasawara) Arcs began prior to 15 Ma; and (iii) clockwise rotation of the entire southwestern part of the Japan Arc had ceased by 15 Ma.

Geomagnetic polarity transitions recorded in the Miocene lavas of the Wuyu basin, Tibet

Geomagnetic polarity transitions were the most important phenomena in the study of the geomagnetic field evolution and the earth’s deep interior dynamics. The Miocene postcollisional ultrapotassic-potassic rocks are widely distributed in the Lhasa Block of southern Tibet. Paleomagnetic studies on the Wuyu Formation indicated that these volcanic rocks recorded the C5Bn.2n-C5Bn.1r and C5ACr-C5ACn geomagnetic polarity transitions. The virtual geomagnetic poles (VGPs) of these two polarity transitions were located in the Atlantic and the central Pacific, respectively. Due to data scarcity, it remains unclear whether VGP paths of Miocene geomagnetic polarity transitions were longitudinal preference and/or 90° away from the sampling sites.

Magnetic Polarity Transitions and Biospheric Effects

This review addresses possible biospheric effects of geomagnetic polarity transitions. During a transition the magnetic field at the surface of the Earth decreases to about 10% of its current value. If the geomagnetic field is a shield against energetic particles of solar or cosmic origin then biospheric effects can be expected. We review the early speculations on the problem and discuss in more detail its current status. We conclude that no clear picture of a geomagnetic link, a causal relation between secular magnetic field variations and the evolution of life on our planet can be drawn.

Comment to “Distribution and geochronology of the Oregon Plateau (U.S.A.) flood basalt volcanism: The Steens Basalt Revisited” by M.E. Brueseke et al. [J. Volcanol. Geotherm. Res. 161 (2007), 187–214

The 40Ar/ 39Ar stepheating results of Brueseke et al. (2007) are critically evaluated, firstly for possible alteration of the samples dated. Steps passing the alteration index test (Baksi, 2007a) can then be used to derive plateau ages, which must satisfy rigorous statistical tests. Of the eleven samples studied by Brueseke et al. (2007), only four yield plateau values that reflect eruption ages. These suggest Steens Basalt was erupted within a relatively short period of time, ∼ 0.2 m.y., considerably less than the ∼ 1 m.y. suggested by the authors. The seeming difference between the plateau ages for the Steens Mountain Basalts obtained by these authors and two earlier attempts, is resolved by (a) proper adjustment of ages at various laboratories for interlaboratory calibration differences, and by rejecting ages from altered samples. The age of the important Steens Mountain geomagnetic polarity transition is 16.59 ± 0.03 Ma.

Cosmic ray and solar energetic particle flux in paleomagnetospheres

The geomagnetic field is part of the shield prohibiting energetic particles of solar and cosmic origin directly hitting the Earth surface. During geomagnetic polarity transitions the geomagnetic field strength significantly decreases with energetic particles having a much better access to the atmosphere and surface. To study in more detail the flux of energetic particles into the paleomagnetosphere we use a potential field approach to model the paleomagnetosphere which generalizes the parametric model of Voigt (1981) by taking into account a non-zero quadrupole moment of the core field. We study in particular the quadrupolar situation as a geomagnetic polarity transition is also characterized by a significant increase of non-dipolar contributions. Our model is used as a tool for tracing particle trajectories in such paleomagnetospheres and to assess variations of high-energy particle fluxes into the atmosphere. As a first application of the particle tracing scheme we determine cutoff latitudes and impact areas for different paleomagnetospheric configurations. For configurations with equivalent magnetic field strength or magnetic energy the impact area is very similar.

The Sun, geomagnetic polarity transitions, and possible biospheric effects: review and illustrating model

AbstractThe Earth is embedded in the solar wind, this ever-streaming extremely tenuous ionized gas emanating from the Sun. It is the geomagnetic field which inhibits the solar wind plasma to directly impinge upon the terrestrial atmosphere. It is also the geomagnetic field which moderates and controls the entry of energetic particles of cosmic and solar origin into the atmosphere. During geomagnetic polarity transitions the terrestrial magnetic field decays down to about 10% of its current value. Also, the magnetic field topology changes from a dipole dominated structure to a multipole dominated topology. What happens to the Earth system during such a polarity transition, that is, during episodes of a weak transition field? Which modifications of the configuration of the terrestrial magnetosphere can be expected? Is there any influence on the atmosphere from the intensified particle bombardment? What are the possible effects on the biosphere? Is a polarity transition another example of a cosmic cataclysm? A review is provided on the current understanding of the problem. A first, illustrating model is also discussed to outline the complexity of any biospheric reaction on polarity transitions.

Palynology of OGS-6a and OGS-7, two new 2.6 Ma archaeological sites from Gona, Afar, Ethiopia: Insights on aspects of Late Pliocene habitats and the beginnings of stone-tool use

Fossil pollen grains extracted from sediments sampled from OGS-6a and OGS-7, two newly excavated Late Pliocene sites of Gona, in the Afar Depression of Ethiopia, indicate a high percentage of afromontane forest and highland taxa (36.8–43.3%), among which Podocarpus cf. gracilor is dominant (19.6–24.2%). Forest taxa typical of the afromontane phytogeographic region have also been identified in the samples. The overall pollen spectrum suggests a mosaic of open and closed habitats, with a considerable wooded environment present during the time associated with the beginnings of early hominid use of flaked stones at Gona. Here we provide details of the results from the pollen analyses. The two sites lie 4–8 m directly below a volcanic tuff dated by 40Ar/ 39Ar to 2.53 ± 0.15 million years (Ma). The materials were recovered within fine-grained sediments located just above a geomagnetic polarity transition identified as the Gauss-Matuyama boundary dated close to 2.6 Ma, also corroborating the earlier age reported for the East Gona artifacts from EG-10 and EG-12. Thus, the stone artifacts, fossilized broken fauna and pollen grains from OGS-6a and OGS-7 represent archaeological materials from the world's oldest securely dated sites, and offer insights on aspects of the palaeoenvironments around the time of the beginnings of ancestral hominid use of flaked stones, ca. 2.6 Ma.

Behaviour of the geomagnetic field during the Matuyama–Brunhes polarity transition

The ability to derive Gauss coefficients, up to and including degree 3, and their variation through a geomagnetic polarity transition is studied using simulated palaeomagnetic data. It is concluded that for a specified distribution of palaeomagnetic sites reasonable estimates of the behaviour of the coefficients can be derived even when uncertainties in the data, and in the compilation of contemporaneous records, are considered. Published palaeomagnetic records of the Matuyama–Brunhes transition are then used as basis for deriving the variation of the Gauss coefficients over a 32 kyear period encompassing the reversal. Individual records are interpolated to uniform time intervals of 0.5 kyear and put on to a common time scale by correlating between sites the variation in the latitude of VGP's through the reversal. Relative palaeointensity data are scaled by the geocentric axial dipole field intensity for 2000 at each site, and the Gauss coefficients derived by a matrix inversion employing singular value decomposition. The derived variation with time of the Gauss coefficients suggests that, over the time span of the data, the dipole and non-dipole fields have approximately equal intensities. Plots of the variation of the surface vertical magnetic field through the reversal suggest that immediately prior to the reversal a large patch of reverse flux appears in the southern hemisphere. This may subsequently have been responsible for the weakening of the vertical field leading into the reversal. A similar patch of reverse flux is observed some 20–15 kyear prior to the actual reversal and may be associated with an observed excursion in VGPs at several sites.

Geochemical evolution of Ngorongoro Caldera, Northern Tanzania: Implications for crust–magma interaction

Ngorongoro Caldera is the largest and best-preserved of nine Plio-Pleistocene volcanoes that make-up the Ngorongoro Volcanic Highlands (NVH) complex situated at the southern bifurcation of Gregory Rift, part of the East African Rift system of northern Tanzania. We report here, major and trace element abundances, Sr–Nd–Pb isotope analyses and 40Ar/ 39Ar laser incremental-heating and total fusion ages on lava and tephra sampled from stratigraphic sections exposed within the Ngorongoro Caldera. Major and trace elements measured on samples collected from the Ngorongoro Caldera wall indicate a stratified magma chamber whose silicic top and basaltic bottom was inverted by sequential eruptions. Samples from the lower part of the exposed Ngorongoro Caldera wall are high in silica, alkalis and HFSE (High Field Strength Elements). The Zr, Nb and Hf concentrations are highly correlated with each other and decrease up-section, indicative of the extent of magma evolution. Modeling of major, trace as well as Sr, Nd and Pb isotope data suggests that assimilation fractional crystallization processes were essential in producing the observed geochemical variations. The Sr and Nd isotope ratios from the Ngorongoro samples are widely dispersed ( 87Sr/ 86Sr = 0.70405 to 0.70801, 143Nd/ 144Nd = 0.512205 to 0.512623) and Pb isotope ratios are consistent with previous studies: 206Pb/ 204Pb = 18.73 to 19.37, 207Pb/ 204Pb = 15.64 to 15.69, 208Pb/ 204Pb = 39.52 to 39.55. Although Sr isotopic ratios are similar to Oceanic Island Basalt (OIB), the more radiogenic samples ( 87Sr/ 86Sr > 0.705) from the lower part of the section suggest crust–magma interaction; this is supported by Ce/Pb ratios (< 25 ± 5) which are lower than those expected from primary mantle derived fluids. Moderate normalized Tb/Yb ratios (2.7–2.1) of these samples suggest lithospheric mantle melting, a finding consistent with other studies in the region. Laser incremental-heating of basalt samples and single grain total fusion of anorthoclase from tephra samples collected from the Ngorongoro Caldera wall section yield 40Ar/ 39Ar ages of 2.25 ± 0.02 Ma to 2.01 ± 0.02 Ma, constraining a duration of volcanism of the order of ~ 240 kyr. These ages suggest correlation of a normal to reverse geomagnetic polarity transition measured in the Ngorongoro Caldera wall section with the ~ 2.14 Ma C2r.1n–1r (normal to reverse) Réunion–Matuyama boundary.

Jurassic–Early Cretaceous intermediate virtual geomagnetic poles and Pangaean subduction zones

The objective of this paper is to show that the distribution of Jurassic–Early Cretaceous intermediate virtual geomagnetic poles (VGPs) seems to be conditioned by Pangaean subducted slabs. Palaeomagnetic data from between ∼ 200Ma and 125Ma were compiled from reliable studies and their VGPs repositioned in their Jurassic–Early Cretaceous geographic location considering a “zero-longitude” motion of Africa over the last 200m.y. and the corresponding palaeomagnetic poles from each sequence. Those repositioned VGPs lying between latitudes of ± 60° were considered to be intermediate. To avoid bias as a function of simple sampling numbers for those sequences with more data, each VGP was weighted by Love's methodology. A colour-scale map (shadow-scale map in printed issue) of density of the weighted intermediate VGPs was obtained and compared with the Pangaean subduction zones. There is a good visual correlation between the distribution of these VGPs and the location of the subduction zones during the Jurassic, suggesting that there is a relationship between the Jurassic–Early Cretaceous geomagnetic reversals and the plate tectonic setting at that time. Minima of intermediate VGPs correlate well with the absence of VGPs predicted with a tomographic model and the intermediate VGP distribution is also well correlated with zones of faster seismic wave propagation in the lower mantle (just above of the core–mantle boundary), which suggests that the Jurassic geomagnetic polarity transitions could have been controlled by a structure of the core–mantle boundary similar to that at the Present time. We suspect that the subducted lithospheric slabs refrigerated the deepest mantle causing more heat than average flowing out from the core and controlling the geometry of the Jurassic–Early Cretaceous polarity transitions. The Earth's lithospheric plate motion history could have played a controlling role in the geometry of the geomagnetic reversals.

Energetic particles in the paleomagnetosphere: Reduced dipole configurations and quadrupolar contributions

The Earth’s magnetosphere acts as a shield against highly energetic particles of cosmic and solar origin. On geological timescales, variations of the internal geomagnetic field can drastically alter the magnetospheric structure and dynamics. This paper deals with energetic particles in the paleomagnetosphere. Scaling relations for cutoff energies and differential particle fluxes during periods of reduced dipole moment are derived. Particular attention is paid to high rigidity galactic cosmic ray particles in the GV range and to lower rigidity solar energetic particles. We find that in the paleomagnetosphere of a strongly reduced dipole moment, solar protons of several tens of MeV may access the Earth’s atmosphere even at midlatitudes. Higher‐order core‐field components can widen the polar caps and open new particle‐entry regions around the equator. Potential field modeling of the paleomagnetosphere and magnetohydrodynamic (MHD) simulations are utilized to study the impact of solar particles on the Earth’s atmosphere for more complex field configurations that are likely to occur during geomagnetic polarity transitions.

Millennial‐ to submillennial‐scale features of the Matuyama‐Brunhes geomagnetic polarity transition from Osaka Bay, southwestern Japan

A high‐resolution magnetic record of the Matuyama‐Brunhes (MB) reversal transition was obtained from homogeneous marine clay in a 1700‐m core from Osaka Bay. The transition stretches over a core length of 6.4 m and yields four short reversal episodes. Two predate the main MB boundary (MBB), and two postdate it. We made diatom analyses of sediments to estimate relative sea level changes and constructed an age model by correlation with the astronomically calibrated marine oxygen isotope curve. The age model shows the short reversal episodes, with durations of &lt;1 kyr, occurred at 4–5 kyr intervals within stage 19, and the MBB occurred about 5 kyr after the sea level highstand corresponding to isotopic event 19.3. It also shows the paleointensity minimum of 15–20% of the postreversal mean occurred at about 6 kyr before the MBB and another minimum at about 1 kyr after. Four virtual reverse‐to‐normal polarity boundaries found in this study suggest different ages for the MBB could be observed in lava flow studies. Some millennial‐ to submillennial‐scale transition features found in this study are compatible with the previous numerical simulations of geomagnetic field reversal.

Scaling relations in the paleomagnetosphere derived from MHD simulations

[1] On geological timescales the Earth's dipole moment can vary between 0.1 and 2 times the present value. The weakest internal magnetic fields occur during geomagnetic polarity transitions when the Earth's internal dipole field reverses. Theoretically, the size of the paleomagnetosphere is expected to change in the function of the dipole moment according to a power law scaling relation. We carried out a series of numerical magnetohydrodynamic (MHD) simulations of axial dipolar paleomagnetospheres, gradually decreasing the relative dipole moment from 1 to 0.1, in order to test the validity of the theoretical scaling relations for different values of the north-south interplanetary magnetic field component (IMF B z ). We study the dipole moment dependence of the standoff distance, the flank distances, and the polar cap size, and derive power law scaling relations with significantly differing scaling exponents as compared to the theoretically expected ones. The extent of deviation from the theoretical scaling exponents is controlled by the magnitude of the southward B z . We quantify the B z -dependence of the size of the magnetosphere and validate our results with the Roelof-Sibeck bivariate function of magnetopause shape obtained from in-situ satellite measurements for the present-day magnetosphere. We conclude that the Roelof-Sibeck function cannot be applied for southward B z values stronger than 5 nT. A new B z -dependent dipole scaling relation is suggested for the magnetospheric scale size. Inserting our corrected scaling relation in the Hill model of magnetosphere-ionosphere coupling, a better fit can be obtained between simulated and theoretically calculated paleomagnetospheric transpolar potentials.

Assessing the timing of greigite formation and the reliability of the Upper Olduvai polarity transition record from the Crostolo River, Italy

The Crostolo River section in Italy yielded a detailed record of the Upper Olduvai geomagnetic polarity transition that has been used to support the hypothesis of deep mantle control on the transitional geomagnetic field. The paleomagnetic record is carried by the authigenic iron sulphide, greigite, which was interpreted to have formed shortly after deposition. Our detailed scanning electron microscope investigations indicate the presence of at least 3 generations of pyrite, which usually forms with greigite as a precursor. This suggests that the total magnetization is a complex composite that produced a smoothed record of transitional field behaviour.

A coherent middle Pliocene magnetostratigraphy, Wanganui Basin, New Zealand

We document magnetostratigraphies for three river sections (Turakina, Ran‐gitikei, Wanganui) in Wanganui Basin and interpret them as corresponding to the Upper Gilbert, the Gauss and lower Matuyama Chrons of the Geomagnetic Polarity Timescale, in agreement with foraminiferal biostratigraphic datums. The Gauss‐Gilbert transition (3.58 Ma) is located in both the Turakina and Wanganui River sections, while the Gauss‐Matuyama transition (2.58 Ma) is located in all three sections, as are the lower and upper boundaries of the Mammoth (3.33–3.22 Ma) and Kaena (3.11–3.04 Ma) Subchrons. Our interpretations are based in part on the re‐analysis of existing datasets and in part on the acquisition and analysis of new data, particularly for the Wanganui River section. The palaeomagnetic dates of these six horizons provide the only numerical age control for a thick (up to 2000 m) mudstone succession (Tangahoe Mudstone) that accumulated chiefly in upper bathyal and outer neritic palaeoenvironments. In the Wanganui River section the mean sediment accumulation rate is estimated to have been about 1.8 m/k.y., in the Turakina section it was about 1.5 m/k.y., and in the Rangitikei section, the mean rate from the beginning of the Mammoth Subchron to the Hautawa Shellbed was about 1.1 m/k.y. The high rates may be associated with the progradation of slope clinoforms northward through the basin. This new palaeomagnetic timescale allows revised correlations to be made between cyclothems in the Rangitikei River section and the global Oxygen Isotope Stages (OIS) as represented in Ocean Drilling Program (ODP) Site 846. The 16 depositional sequences between the end of the Mammoth Subchron and the Gauss‐Matuyama Boundary are correlated with OIS MG2 to 100. The cyclothems average 39 k.y. in duration in our age model, which is close to the 41 k.y. duration of the orbital obliquity cycles. We support the arguments advanced recently in defence of the need for local New Zealand stages as a means of classifying New Zealand sedimentary successions, and strongly oppose the proposal to move stage boundaries to selected geomagnetic polarity transitions. The primary magnetisation of New Zealand mudstone is frequently overprinted with secondary components of diagenetic origin, and hence it is often difficult to obtain reliable magnetostratigraphic records. We suggest specific approaches, analytical methods, and criteria to help ensure robustness and coherency in the palaeomagnetic identification of chron boundaries in typical New Zealand Cenozoic mudstone successions.