Matuyama-Brunhes Polarity Transition Research Articles

Eccentric Dipole Evolution during the Last Reversal, Last Excursions, and Holocene Anomalies. Interpretation Using a 360-Dipole Ring Model

The eccentric dipole (ED) is the next approach of the geomagnetic field after the generally used geocentric dipole. Here, we analyzed the evolution of the ED during extreme events, such as the Matuyama-Brunhes polarity transition (~780 ka), the Laschamp (~41 ka) and Mono Lake (~34 ka) excursions, and during the time of two anomalous features of the geomagnetic field observed during the Holocene: the Levantine Iron Age Anomaly (LIAA, ~1000 BC) and the South Atlantic Anomaly (SAA, analyzed from ~700 AD to present day). The analysis was carried out using the paleoreconstructions that cover the time of the mentioned events (IMMAB4, IMOLEe, LSMOD.2, SHAWQ-Iron Age, and SHAWQ2k). We found that the ED moves around the meridian plane of 0–180° during the reversal and the excursions; it moves towards the region of the LIAA; and it moves away from the SAA. To investigate what information can be extracted from its evolution, we designed a simple model based on 360-point dipoles evenly distributed in a ring close to the inner core boundary that can be reversed and their magnitude changed. We tried to reproduce with our simple model the observed evolution of the ED, and the total field energy at the Earth’s surface. We observed that the modeled ED moves away from the region where we set the dipoles to reverse. If we consider that the ring dipoles could be related to convective columns in the outer core of the Earth, our simple model would indicate the potential of the displacement of the ED to give information about the regions in the outer core where changes start for polarity transitions and for the generation of important anomalies of the geomagnetic field. According to our simple model, the regions in which the most important events of the Holocene occur, or in which the last polarity reversal or excursion begin, are related to the regions of the Core Mantle Boundary (CMB), where the heat flux is low.

Complexity in Matuyama–Brunhes polarity transitions from North Atlantic IODP/ODP deep-sea sites

Integrated Ocean Drilling Program (IODP) Expedition 303 to the North Atlantic provided 16 records of the Matuyama–Brunhes polarity transition (MBT), based on u-channel and discrete samples, from holes drilled at three sites (Sites U1304, U1305 and U1306) that have mean Brunhes sedimentation rates of 16–18 cm/kyr. The MBT occurs during the transition from marine isotope stage (MIS) 19c to MIS 18e, with mid-point at ∼773 ka, and a transition duration of ∼8 kyr. Combining the new MBT records, including one new record for the top Jaramillo, with previously published North Atlantic MBT records (ODP Sites 983, 984 and 1063) yields a total of more than 20 high-sedimentation-rate polarity transition records. The MBT yields a repetitive pattern of transitional field states as virtual geomagnetic poles (VGPs) move from high southern latitudes to loop over the Pacific, group in NE Asia, and transit into the mid-latitude South Atlantic before reaching high latitudes in the Northern Hemisphere. The VGPs for the top Jaramillo transition feature a loop over the Pacific, then a NE Asia group before transit over the Indian Ocean to high southerly latitudes. The North Atlantic MBT records described here contrast with longitudinally-constrained VGP paths for the MBT, indicating that relatively low sedimentation rate (∼4 cm/kyr) records of the MBT are heavily smoothed by the remanence acquisition process and do not adequately represent the MBT field. The VGPs at the MBT and top Jaramillo, as measured in the North Atlantic, have similarities with excursion (Iceland Basin) VGP paths, and were apparently guided by maxima in downward vertical flux similar to those seen in the modern non-dipole (ND) field, implying longevity in ND features through time.

Stratigraphy of the Lake Biwa sediment during the past 1 Ma, central Japan

Integrated Ocean Drilling Program (IODP) Expedition 303 to the North Atlantic provided 16 records of the Matuyama–Brunhes polarity transition (MBT), based on u-channel and discrete samples, from holes drilled at three sites (Sites U1304, U1305 and U1306) that have mean Brunhes sedimentation rates of 16–18 cm/kyr. The MBT occurs during the transition from marine isotope stage (MIS) 19c to MIS 18e, with mid-point at ∼773 ka, and a transition duration of ∼8 kyr. Combining the new MBT records, including one new record for the top Jaramillo, with previously published North Atlantic MBT records (ODP Sites 983, 984 and 1063) yields a total of more than 20 high-sedimentation-rate polarity transition records. The MBT yields a repetitive pattern of transitional field states as virtual geomagnetic poles (VGPs) move from high southern latitudes to loop over the Pacific, group in NE Asia, and transit into the mid-latitude South Atlantic before reaching high latitudes in the Northern Hemisphere. The VGPs for the top Jaramillo transition feature a loop over the Pacific, then a NE Asia group before transit over the Indian Ocean to high southerly latitudes. The North Atlantic MBT records described here contrast with longitudinally-constrained VGP paths for the MBT, indicating that relatively low sedimentation rate (∼4 cm/kyr) records of the MBT are heavily smoothed by the remanence acquisition process and do not adequately represent the MBT field. The VGPs at the MBT and top Jaramillo, as measured in the North Atlantic, have similarities with excursion (Iceland Basin) VGP paths, and were apparently guided by maxima in downward vertical flux similar to those seen in the modern non-dipole (ND) field, implying longevity in ND features through time.

Lava flow recordings of the Blake and post-Blake excursions: refining the Quaternary Geomagnetic Instability Time Scale (GITS)

Complex polarity reversals in numerical dynamos driven by thermo-chemical convection are analyzed in terms of magnetic field intensity variations, transitional field structure, and other observable characteristics. Our most Earth-like dynamos are characterized by long stable polarity chrons with dipole-dominant magnetic fields punctuated by occasional polarity reversals, and are found within a transition region of parameter space between non-reversing strongly dipolar dynamos and chaotic multi-polar-type dynamos. Dynamos in the transition region have elevated dipole terms, reduced quadrupole terms, magnetic energy that decreases slowly with spherical harmonic degree, and broadband dipole frequency spectra. Their axial dipole intensity histograms are trimodal, with large modes representing the stable polarity states and an intermediate mode representing the transitional multi-polar state. The dipole family intensity exceeds the quadrupole family intensity on the core–mantle boundary during stable polarity times, but during transitions the two families are similar. A complex dynamo model reversal is compared with paleomagnetic reconstructions around the Matuyama–Brunhes polarity transition. Both start with a gradual decrease of the dipole intensity, followed by a precursor reversal and transient polarity recovery, then a rapid dipole collapse and a final reversal that initiates with reverse flux generation in one hemisphere. Virtual geomagnetic poles (VGPs) from sites near the reverse flux trace complex paths and cross the equator several thousand years before the simpler VGP paths from more distant sites, and magnetic intensity variations during the dynamo model reversal correlate with intensity variations inferred for the Matuyama–Brunhes transition.

Matuyama-Brunhes polarity transition just overlying the latest Homo erectus and meteorite impact evidence in Sangiran, Java

Matuyama-Brunhes polarity transition just overlying the latest Homo erectus and meteorite impact evidence in Sangiran, Java

Paleointensity variation across the Matuyama-Brunhes polarity transition: Observations from lavas at Punaruu Valley, Tahiti

[1] We have conducted a paleointensity study of the Matuyama-Brunhes (M-B) polarity transition recorded in 34 successive lava flows of Punaruu Valley on Tahiti. A reversed polarity is obtained from the lower part of the record, major directional changes are derived from the middle part of the record, and a normal polarity is recorded in the upper part of the record. These paleomagnetic directions and five 40Ar/39Ar ages yielding a weighted mean of 771 ± 8 (1σ) ka indicate that 30 lava flows recorded the geomagnetic field across the M-B transition. The 215 specimens from 32 flows were subjected to the double-heating technique of the Shaw method combined with low-temperature demagnetization (LTD-DHT Shaw method), yielding 73 successful results from 18 flows. For the reversed polarity period just prior to the major directional changes, paleointensity shows an oscillation-like variation between 3 and 38 μT corresponding to virtual dipole moments (VDMs) between 0.9 × 1022 and 9.6 × 1022 Am2. For the major directional changes, a weak paleointensity of 5 μT is obtained, which gives a VDM of 1.0 × 1022 Am2. For the normal polarity period, paleointensities are 14–21 μT, giving VDMs of 3.5–5.2 × 1022 Am2. For the reversed polarity period just prior to the major directional changes, a linear relationship with a correlation coefficient of 0.96 is recognized on the diagram of VDM versus virtual geomagnetic pole latitude. This linear relationship may be a precursory feature of the geodynamo at the onset of the M-B transition.

Complex polarity reversals in a geodynamo model

Complex polarity reversals in numerical dynamos driven by thermo-chemical convection are analyzed in terms of magnetic field intensity variations, transitional field structure, and other observable characteristics. Our most Earth-like dynamos are characterized by long stable polarity chrons with dipole-dominant magnetic fields punctuated by occasional polarity reversals, and are found within a transition region of parameter space between non-reversing strongly dipolar dynamos and chaotic multi-polar-type dynamos. Dynamos in the transition region have elevated dipole terms, reduced quadrupole terms, magnetic energy that decreases slowly with spherical harmonic degree, and broadband dipole frequency spectra. Their axial dipole intensity histograms are trimodal, with large modes representing the stable polarity states and an intermediate mode representing the transitional multi-polar state. The dipole family intensity exceeds the quadrupole family intensity on the core–mantle boundary during stable polarity times, but during transitions the two families are similar. A complex dynamo model reversal is compared with paleomagnetic reconstructions around the Matuyama–Brunhes polarity transition. Both start with a gradual decrease of the dipole intensity, followed by a precursor reversal and transient polarity recovery, then a rapid dipole collapse and a final reversal that initiates with reverse flux generation in one hemisphere. Virtual geomagnetic poles (VGPs) from sites near the reverse flux trace complex paths and cross the equator several thousand years before the simpler VGP paths from more distant sites, and magnetic intensity variations during the dynamo model reversal correlate with intensity variations inferred for the Matuyama–Brunhes transition.

関東平野中央部，埼玉県菖蒲町で掘削されたGS-SB-1 コアの古地磁気・岩石磁気測定

Paleomagnetic and rock-magnetic measurements were performed on the GS-SB-1 core of 350.2 m long drilled at Shobu Town, Saitama Prefecture, central Kanto Plain, Japan. 493 and 16 samples were taken from the core for progressive alternating field and thermal demagnetization experiments. Approximately 95 % of the samples have a stable remanent magnetization component.The core between 0 and 266.755 m in depth yielded a normal polarity, while between 276.9 and 350.2 m in depth was a reversed polarity. The reversal between 266.755 and 350.2 m in depth corresponds to the Matuyama-Brunhes polarity transition.

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.

Pedogenesis affecting the Matuyama-Brunhes polarity transition recorded in Chinese loess?

A detailed record of the Matuyama-Brunhes (M-B) transition has been obtained from the loess unit 8 (L8) at Duanjiapo (34.2°N, 109.2°E), Shannxi Province of China. An investigation of the rock magnetic properties using hysteresis loops, thermomagnetic analyses identifies pseudosingle domain magnetite as the main carrier of the remanence, with a small contribution from maghemite and hematite. The paleo-direction records obtained reveal: (i) The M-B transition was recorded in the middle and lower part of L8, and comprises of five fast reversals. (ii) The duration of the M-B polarity transition related to the directional change is about 4800 a. (iii) The virtual geomagnetic pole (VGP) path during the transition is confined over Africa, peaked 90° away from the sampling site, in contrast with the results obtained from the Weinan loess section. The different VGPs are probably attributed to the pedogenesis.

Geomagnetic palaeointensities and astrochronological ages for the Matuyama–Brunhes boundary and the boundaries of the Jaramillo Subchron: palaeomagnetic and oxygen isotope records from ODP Site 983

We have measured relative geomagnetic palaeointensity proxies, palaeomagnetic directions, and δ 18 O for the 700–1100 ka interval from ODP Site 983 (Gardar Drift, North Atlantic), where mean sedimentation rates are ca .13 cm kyr −1 . The age model was generated by matching the benthic δ 18 O data to the Ice Volume Model and confirmed by tuning the precessional components of both signals. For the Matuyama–Brunhes boundary (MBB) and the boundaries of the Jaramillo Subchronozone, the duration of the polarity reversal process, defined by virtual geomagnetic polar latitudes of less than 45°, is ca .5 kyr. Whereas the generally accepted astrochronological estimates for the boundaries of the Jaramillo Subchronozone lie within the polarity transitions as recorded at Site 983, the astrochronological age for the Matuyama–Brunhes polarity transition (780 ka) is ca .5 kyr older than the onset of this transition at Site 983 (775 ka). The polarity reversals lie within palaeointensity lows, with abrupt recovery of palaeointensity post reversal. There is no progressive (‘sawtooth’) decrease in palaeointensity within the Jaramillo Subchronozone or between the top of the Jaramillo and the MBB, but rather, within polarity chrons, several short intervals of low palaeointensity which sometimes coincide with high–amplitude secular variation. Orbital (100 and 41 kyr) periods are present in the palaeointensity record. As they are not obviously attributable to climate/lithology in these records, they may be a feature of the geomagnetic field itself.

Onset of aridity and dune-building in central Australia: sedimentological and magnetostratigraphic evidence from Lake Amadeus

Sediments from Lake Amadeus, a groundwater discharge playa in central Australia, comprise a surface playa sequence (the Winmatti Beds), varying between 0.6 and 3 m in thickness, overlying a long sequence of fairly uniform fluvial-lacustrine clays (the Uluru Clay). The latter extends down to at least 15 m (the maximum depth cored during this study), and possibly down to 65 m below the present playa surface. In the cores studied the Brunhes-Matuyama boundary (0.73 Ma) was identifiable in all the upper sediments, typically at a depth of between 1 and 2 m, but below this there are two plausible chronological interpretations of the palaeomagnetic data. Deposition rates are very low, typically no more than 1.5 cm/ka in the Uluru Clay Beds, and down to 0.2 cm/ka in the Winmatti Beds. These rates suggest that the fluvial-lacustrine phase lasted for at least 5 Ma. The boundary between the Uluru Clay and the Winmatti Beds bears a close correspondence to the boundary between the Blanchetown Clay (a lacustrine sequence) and the Tyrrell Beds (a playa sequence) in palaeo-lake Bungunnia, in what is now the Murray Basin in southeastern Australia. However the transition occurs at about 0.9 Ma (or possibly 1.6 Ma) at Lake Amadeus, whereas it has been dated palaeomagnetically at less than 0.73 Ma (probably 0.5 Ma) in Lake Bungunnia. Records from other locations in southeastern Australia likewise support a major change from wetter to drier conditions at, or shortly after, the Matuyama-Brunhes polarity transition. Thus the current evidence from Lake Amadeus indicates that in central Australia the onset of aeolian and saline gypseous deposits, characteristic of arid-zone facies, may have pre-dated the corresponding change in the southeastern part of the continent by about 0.4 Ma, or possibly by about 1.1 Ma. In common with most playas in central Australia, Lake Amadeus is surrounded by a ring of gypseous dunes derived from material deflated from the surface of the playa. Magnetostratigraphic evidence from Auger Island, in the middle of Lake Amadeus, suggests that the oldest gypseous dune formation on the island must have pre-dated the start of the Jaramillo subchron at 0.98 Ma (or possibly the start of the Brunhes epoch, 0.73 Ma), thus making these the oldest dated dunes in Australia. The age of the Auger Island dunes is probably characteristic of other gypseous dunes around the margins of playas in central Australia.

Preliminary results of a study of four successive sedimentary geomagnetic reversal records from the Mediterranean (Upper Thvera, Lower and Upper Sidufjall, and Lower Nunivak)

Abstract The results of a study of four successive Early Pliocene geomagnetic reversal records are presented. The polarity transitions have been recorded in the Trubi formation of Calabria (S. Italy) which is composed of alternating carbonate-poor and carbonate-rich marly layers with moderately high (> 5 cm ka−1) accumulation rates. The multicomponent NRM contains a well-defined primary component in which two dominant magnetic phases can be recognized: a magnetite and a low-temperature (LT; i.e., removed at moderate temperatures) phase. Most probably the LT phase is pyrrhotite. The combined presence of pyrrhotite and magnetite in the sediment restricts the validity of a XARM-X test for paleointensity studies. The magnetite magnetizations reveal a smoother record relative to the records of the LT phase magnetizations. Between the two phases, no significant delay in the moment of remanence acquisition is observed. The durations of the major directional changes for the reversals are estimated to be in the range 0.3–3.0 ka. The VGP path of the Upper Thvera record follows a great-circle 90° W of the site meridian, indicating a clearly non-zonal transitional field, whereas the VGP paths of the other reversal records are close to the site meridian, indicating a mainly zonal transitional field. The lower Sidufjall and Lower Nunivak records are nearly identical and have a zonal harmonic content similar to records reported for the Matuyama-Brunhes polarity transition.

Latitudinal dependency of geomagnetic polarity transition durations

Palaeomagnetic records of the Matuyama–Brunhes polarity transition were obtained from seven, low sedimentation rate, deep-sea cores from the Pacific Ocean. The cores were taken near the 180° meridian and provide a latitudinal transect of transition records extending from 45.3° N to 33.4° S. Examination of these records suggests that low sedimentation rate cores may not be capable of recording the fine details of transitional field behaviour, but there are indications that these cores may, in fact, provide accurate records of the more general features of the reversal. Most notable of these features is that the duration of the transition is dependent on the site latitude, with durations at mid-latitudes being more than a factor of 2 longer than at equatorial latitudes.

Quantitative description of the geomagnetic field during the Matuyama-Brunhes polarity transition

A model of the reversing geodynamo based on the assumptions (1) that reversals start in a localized region of the core and (2) that upon its onset this reversed region extends, or “floods”, both north-south and east-west until the entire core is affected, has recently been shown to provide a generally successful simulation of existing paleomagnetic records of the Matuyama-Brunhes transition (Hoffman, 1979). In this paper the modelled solution is analyzed so as to reveal the behavior of the dominant Gauss coefficients during the transition. At the time of total axial dipole decay the controlling components are found to be a zonal octupole ( g 3 0) and a non-axisymmetric quadrupole ( g 2 1, h 2 1). Given the distribution of sites corresponding to the available records of the Matuyama-Brunhes, the existence of a significant zonal quadrupole field component cannot be ruled out; however, the role of any equatorial dipole component can be neglected. Due to the presence of a significant low-order non-axisymmetric term in the analyzed transition field, the predicted minimum intensity experienced during the Matuyama-Brunhes is found to be dependent on both site latitude and longitude. In particular, over a mid-northern circle of latitude, the predicted minimum intensity is found to vary by more than a factor of three, averaging about 10% of the full polarity field strength. Although not a unique solution, the applicability of the findings from this analysis is not tied to the phenomenological model from which they were derived. More specifically, whether the above two-component non-dipole transitional field arises from assumed configurational changes of the reversing geodynamo (as is the case for the flooding model) or, alternatively, is considered to be a stationary (non-reversing) portion of the field during axial dipole decay and regeneration, has little effect on either the calculated path locality of the virtual geomagnetic pole or the minimum intensity experienced at a given site. These two possible situations, in principle, should be distinguishable given the future attainment of detailed paleomagnetic data corresponding to back-to-back (R → N and N → R) polarity transitions.