Geomagnetic Polarity Transition Records from Five Hydraulic Piston Core Sites in the North Atlantic

Eleven geomagnetic polarity transition records from five hydraulic piston core sites in the North Atlantic are presented. The Matuyama-Brunhes reversal was sampled at five sites and the upper and lower Jaramillo and upper Olduvai transitions were each sampled at two sites. The wet sediment was sampled by taking U-channel samples across the reversals, as defined by conventional shipboard magnetostratigraphic studies. The U-channel samples were subsampled on shore by taking successive 0.5-cm-thick slices of the sediment. The amount of detail contained in the records of these transitions is not always directly related to sedimentation rate. In seven out of eleven transitions, detailed sampling of cores deposited at moderate to high sedimentation rates (30-50 m/m.y.) failed to produce detailed transition records. The lack of intermediate directions demonstrated to be carried by single-component magnetizations indicates that these sections do not contain accurate records of transitional field behavior. The four transitions sampled from Hole 609B (82.5 m/m.y.), however, exhibit intermediate directions that are carried by univectorial magnetizations. A progression of directions through these reversals is also observed. These records therefore satisfy the basic criteria required to allow this interpretation in terms of geomagnetic field behavior. Data from these mid-northern latitude transitions place additional constraints on existing transitional field models of the Matuyama-Brunhes reversal. The lower Jaramillo and upper Olduvai transitions obtained in this study are compared with records of these same reversals at a midsouthern latitude site. Comparison of these transitions reveals symmetries that are dependent upon the sense of the reversal with respect to the hemisphere on which the site is located. These symmetries are predicted by two different transitional field models in which an axisymmetric quadrupole term is dominant

A detailed record of the Lower Jaramillo polarity transition from a southern hemisphere deep sea sediment core

A detailed record of the lower Jaramillo (reversed to normal) polarity transition was obtained from a southern hemisphere, deep-sea sediment core (latitude = 35.91°E, longitude = 59.97°E) The record consists of over 850 samples taken across 140 cm of section. The transition itself is recorded across approximately 70 cm and is represented by more than 475 specimens from about 160 levels giving intermediate directions. The transition is identified by a nearly 180° shift from directions in good agreement with a reversed, axial dipole field to those closely aligned with a normal, axial dipole field for the core site latitude. The inclinations shallow gradually early in the reversal and pass through very steep negative values (-80°) late in the transition. The declinations show little appreciable variation until the inclinations have moved through the near vertical, and then slowly approach values in agreement with a normal polarity field. An intensity low accompanies the directional change during which the intensity drops to less than 15% of the maximum values observed in this sample interval. The intensity fluctuation spans a wider interval than the directional change, decreasing prior to any systematic change in the directions and then increasing to pre-transition levels by the same depth at which the directions have stabilized. The VGP path constructed for this reversal is longitudinally constrained to a certain extent, between 140° and 230° for intermediate VGP latitudes and is roughly centered 120° from the site longitude. This path is therefore a far-sided VGP path in Hoffman's [1977] terminology. Assuming a constant sedimentation rate (67m/Ma) through the Jaramillo Subchron, the duration of the transition is estimated to be 11,200 years to 4,500 years (depending on the criteria) for the directional change, whereas the associated intensity variation occurred over 15,000 to 20,000 years. Considered together with records of the most recent reversal (Matuyama/Brunhes) in light of current transitional field models, this record strongly suggests that the lower Jaramillo transitional field was dominated by different harmonics than the Matuyam/Brunhes transitional field

Short polarity intervals within the Matuyama: transitional field records from hydraulic piston cored sediments from the North Atlantic

Detailed sampling of two short magnetozones within the Matuyama Chronozone recorded at DSDP Site 609 (49.86°N, 335.77°E) confirms that one, the Cobb Mountain Subchronozone (1.12 Ma), is a very short, full normal polarity interval and that the other, the older interval, is a record of a geomagnetic excursion which occurred at approximately 1.55 Ma. The Cobb Mountain Subchron lasted approximately 25,000 years, one third the duration of the Jaramillo Subchron. The normal polarity interval is bounded by two transition zones which document an antisymmetry in the sequence of directions in the reverse to normal and normal to reverse polarity transitions. We interpret the antisymmetry as reflecting a dependence upon the sense of the reversal, without significant changes in the relative contributions of non-dipole terms. The polarity interval recorded at 1.55 Ma lasted only 8,800 years with what may be regarded as full polarity directions observed across only 3 cm of stratigraphic section. This feature is interpreted as an excursion of the geomagnetic field and appears to be correlative with the Gilsa Subchron. Similarities between the transition bounding these two magnetozones suggest that these features occur as the result of the same process or triggering mechanisms in the earth's outer core

Latitudinal dependency of geomagnetic polarity transition durations

Palaeomagnetic records of the Matuyama-Brunhes polarity transition were obtained from seven, low sedimentation rate, deepsea 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

A synthesis of magnetostratigraphic results from Pliocene-Pleistocene sediments cored using the hydraulic piston corer

We present a summary evaluation of the distribution and qualitative ranking of the Plio-Pleistocene magnetostratigraphic results obtained to date from Deep Sea Drilling Project and Ocean Drilling Program piston-cored sites. A review of the published magnetostratigraphic records provides insights into the important extrinsic and intrinsic factors which affect the quality of the paleomagnetic records. The extrinsic factors originate with drilling processes, such as core barrel remagnetization, and steps can be taken to reduce these effects and improve the data quality. The distribution of the high-quality records correlates well both with areas of terrigenous sediment input as well as regions of moderate biological productivity. This suggests that important intrinsic factors include the origin of the original magnetic carrier in the sediment (lithogenic or biogenic) and the degree to which the magnetic carrier has been affected by reduction diagenesis

A comparison of two sequential geomagnetic polarity transitions (upper Olduvai and lower Jaramillo) from the Southern Hemisphere

Two normal to reverse (N to R) geomagnetic polarity transitions were obtained from a Southern Hemisphere deep-sea sediment core (35.91°S, 59.97°E). The upper Jaramillo reversal was continuously sampled by taking 0.5 cm thick samples across 55 cm of section, and the upper Olduvai reversal was sampled in a similar manner across 60 cm. Both records are characterized by full normal and reverse polarity directions which are in very good agreement with those predicted by axial dipole fields for the core site latitude. Each record also exhibits a zone yielding intermediate directions. Because of a large increase in both the within-level and the between-level scatter in the upper Jaramillo transition zone, it is not considered to represent an accurate record of the geomagnetic field. The upper Olduvai transition, however, appears to be a more coherent record. It is characterized by directions that shallow early in the reversal but then rapidly steepen to nearly vertical, upward directions. The declinations do not change until the inclinations have passed through the vertical. The total directional change occurs within a broad NRM intensity minimum. The virtual geomagnetic pole (VGP) path calculated for this record is not longitudinally constrained but instead exhibits a westward progression through the reversal. When considered together with the lower Jaramillo transition previously reported from this core, these records constitute a set of sequential Southern Hemisphere transitions. A number of striking similarities exist in the two records such as steep, upward directions and a shallowing which occurs between the near vertical and the full polarity directions. The results of zonal harmonic modeling of these records are consistent with the idea of a standing field which persisted across both the upper Olduvai and lower Jaramillo reversals

A negative test of orbital control of geomagnetic reversals and excursions

A ~41 Kyr periodic component has been reported in some sedimentary paleointensity records, allowing speculation that there may be some component of orbital control of geomagnetic field generation such as by obliquity modulation. However, no discernable tendency is found for astronomically-dated geomagnetic reversals in the Plio-Pleistocene (0 to 5.3 Ma) or excursions in the Brunhes (0 to 0.78 Ma) to occur at a consistent amplitude or phase of obliquity cyclicity, nor of orbital eccentricity. An implication is that paleointensity lows which are characteristically associated with these features are not distributed in a systematic way relative to obliquity and eccentricity, supporting the idea that orbital forcing does not power the geodynamo

The time-averaged paleomagnetic field

We review indications of persistent deviations from the geocentric axial dipole model of the time-averaged geomagnetic field and present a zonal harmonic model derived from 185 deep-sea sediment piston cores taken from low to middle latitudes (to approximately ±45°). Analysis of the paleomagnetic inclination recorded in these cores for the Brunhes (normal polarity; 0-73 Ma) and Matuyama (reverse polarity; 0.73-2.47 Ma) chrons, after plate motion correction, gives well-constrained estimates of the dominant long-term nondipole contributions (the axial quadrupole and axial octupole) and shows no significant deviation from axial symmetry. The amplitude of the axial quadrupole is found to vary with polarity (2.6% of the geocentric axial dipole for normal; 4.6% for reverse), while the axial octupole does not show appreciable change (-2.9% for normal; -2.1% for reverse). These estimates of the quadrupole contribution agree well with prior determinations for the Plio-Pleistocene (0-5 Ma); however, the octupole contribution we find is opposite in sign to previous estimates. We suggest that a negative octupole is representative of the actual time-averaged paleomagnetic field, while prior positive octupole estimates probably reflect spurious inclination shallowing. The lack of polarity asymmetry in the octupole suggests that this nondipole component may be more closely linked to the main dipole field than is the quadrupole and so supports models of the geodynamo in which dipole and quadrupole families do not interact