VRM Studies in Leg 37 Igneous Rocks

A representative set of igneous rock samples from Hole 332B and Sites 334 and 335 were studied to determine their ability to acquire viscous remanence (VRM). The results for samples from Sites 334 and 335 indicate that VRM cannot be considered to be a serious secondary component in the remanence of these rocks; these samples have stable magnetizations characterized by high median destructive fields (MDF). The ability to develop VRM is quite variable in samples from Hole 332B. In high MDF samples, the developed VRM is of low intensity, but in samples with low MDF and VRM can account for a large portion of the measured NRM intensity. The quantities VRM/NRM and MDF were approximately inversely proportional for samples from the three holes studied here

Viscous Remanent Magnetization in Basalt Samples

Remanent magnetization measurements were made on four specimens of fresh, fine-to coarse-grained basalt from Site 321 and seven specimens of fresh medium-grained diabase at Hole 319A. The natural remanent magnetizations of these rocks were very unstable, characterized by median destructive fields of less than 100 oe in every sample and large directional changes during partial demagnetization. The samples were able to acquire large viscous remanences (VRM) in the laboratory in a 1.0-oe field. Moreover, the intensity of VRM acquired in the presence of the NRM was twice that acquired under similar conditions but after AF demagnetization. Whether acquired from the demagnetized or NRM state, the VRM acquired in 500 hr amounted to a very large fraction of the NRM intensity, particularly at Hole 319A. These results suggest that the large vertical component of magnetization observed in samples from this site may be in part a VRM acquired while drilling the hole in the presence of the drill pipe ambient field

Origin of Magnetic Instability in Sediment Cores From the Central North Pacific

Previous paleomagnetic studies on deep-sea sediment cores from the central North Pacific have shown that the natural remanent magnetization (NRM) of these 'red clay' sediments was unstable below several meters depth in each core. It was also noted that the magnetic instability was related to the presence of a relatively large low-coercivity component of magnetization. The purpose of this investigation was to characterize the rock magnetic properties in three select cores from this region to determine the physical origin of the unstable magnetization. The principal findings of our investigation were as follows. (1) The ability to acquire a viscous remanent magnetization increased with depth in each core, particularly at about the level where unstable magnetization became evident. (2) The magnitude and stability of the observed NRM of the magnetically unstable section of each core can be explained by a viscous remanence acquired in the presence of the earth's magnetic field over a period of time ranging from only several weeks to several thousands of years. (3) The unstable magnetization, believed to be of viscous origin, was attributed to the presence of a magnetic mineral similar in structure and composition to maghemite. This mineral may have resulted from the low-temperature oxidation of very fine grained magnetite at about the time of deposition of these sediments. The extrapolated ages of the levels at which unstable magnetization becomes evident in the cores from this region suggest a close correspondence with the times of established upper Cenozoic climatic changes. Considerations of the alteration of the sedimentary regime resulting from the changes in climate can provide a satisfactory explanation for the observed change in magnetic properties

Geomagnetic Polarity Timescales and Reversal Frequency Regimes

An analysis of geomagnetic reversal history is made for the most reliable polarity timescales covering the last 160 Myr. The timescale of Cande and Kent [1995] (CK95) is the optimum representation of Cenozoic and Late Cretaceous polarity history, and the timescale of Channell et al. [1995] (CENT94) best represents the Early Cretaceous and Late Jurassic. The CK95 timescale can be divided into two nearly linear segments at Chron C12r. The lengths of chrons in the younger segment have no systematic trend, and so this part of the polarity sequence is considered to be stationary for statistical analysis. The mean chron length is 0.248 Myr and the gamma index, k, for the distribution of chron lengths is 1.6 ± 0.4; inserting just 8 additional short subchrons that have been verified from magnetostratigraphic studies as polarity reversals reduces the mean chron length to 0.219 Myr and k to 1.3 ± 0.3. The older segment is stationary if the two long polarity chrons C33n and C33r adjacent to the Cretaceous Normal Polarity Superchron are omitted; in this case the mean chron length is 0.749 Myr and k is 1.2 ± 0.4. The chrons in the CENT94 timescale are stationary with mean length 0.415 Myr and k is 1.3 ± 0.3. The gamma indices of the chron distributions are not significantly different from a Poisson distribution (k = 1), which implies that the reversal process is essentially free of long-term memory. However, if the mean chron duration is an indicator of stability of the reversal process, it appears that long lasting episodes of stable behavior may be followed by abrupt change to another stable regime with a markedly different reversal frequency. There is no significant change of the gamma index from one regime to another although the mean polarity chron length changes by more than a factor of three. This would imply that the probability of a reversal may be constant within each regime but varies inversely with mean interval length from regime to regime

On the magnetic susceptibility anisotropy of deep-sea sediment

Susceptibility anisotropies in the form of vertically prolate ellipsoids have been reported in many deep-sea sediment cores. The results of the present investigation suggest that these anisotropies may not describe the original magnetic fabric of deep-sea sediment, but are more likely due to either a measurement effect or to deformation of the sediment during coring. Anisotropy measurements made on a spinner magnetometer sometimes were found to be greatly affected by the shape of the sample. This apparent "sample-shape effect" was not observed on a low-field torque meter. The anisotropy of samples taken near the base or the top of some piston cores often reflects sediment disturbance during the coring operation. Most samples of deep-sea sediment examined had weak anisotropies that could be interpreted as due to normal depositional processes, including bioturbation. The best-fitting susceptibility ellipsoids were usually oblate with near vertical minimum susceptibility axes

Stratigrafia magnetica ad alta risoluzione del limite Eocene-Oligocene nella successione Umbro-Marchigiana

High-resolution magnetostratigraphy across the Eocene-Oligocene boundary has been employed in a detailed investigation of the nature of low-amplitude, short-wavelength oceanic magnetic anomalies. A core, 39.4mlong and 10 cm in diameter, was drilled through the Eocene-Oligocene boundary near to the Massignano Quarry stratotype section near Ancona, Italy. The stratigraphy of the core, which traverses the Scaglia Variegata and Scaglia Cinerea formations, was correlated precisely to the quarry section by linear regression of the depths of identifiable biotite-rich layers. The good recovery of intact material allowed an average sampling interval of about 12 cm, which is closer than in preceding magnetostratigraphic studies of Umbrian-Marche sequences. The characteristic remanent magnetization was obtained by both progressive alternating field and thermal demagnetizations. The stable component of the natural remanent magnetization could be isolated by thermal demagnetization at temperatures of 300-540°C or by alternating field demagnetization in fields higher than 20 mT. It is probably carried by magnetite in the Scaglia Cinerea marls, while some amount of hematite is present in the underlying Scaglia Variegata. A stratigraphic plot of the ChRM directions shows well-defined magnetozones and the resulting polarity sequence correlates well with polarity chrons C12r to C16n-2. A few single-sample normal magnetozones that do not correspond to the geomagnetic polarity timescale are found within chron 16n.1-r. The magnetozones corresponding to chrons C12r or C13r do not exhibit short subchrons that might account for the low-amplitude and short-wavelength magnetic anomalies reported in this part of the marine magnetic record. In investigation of relative paleointensity fluctuations has been carried out in this part of the core, which embraces the Scaglia Cinerea formation. Anhysteretic remanent magnetization (ARM) has been used to normalize the natural remanent magnetization (NRM), compensating variations in sedimentary input. The ensuing NRM/ARM ratio is taken to be a proxy for relative variation of paleomagnetic field intensity. The paleointensity fluctuates systematically and has minimum values close to the reported positions of low-amplitude, short-wavelength magnetic anomalies in the marine recor

Stability of anhysteretic remanent magnetization in fine and coarse magnetite and maghemite particles

Further experiments have been performed to investigate the biasing-field dependency of alternating field demagnetization curves of anhysteretic remanent magnetization as a simple test for the domain state of magnetite and maghemite particles. The biasing-field dependency in fine-grained particles was opposite to that in coarse-grained particles. The experiments were conducted on well sized synthetic specimens in the single domain, pseudo-single domain and multi-domain grain size ranges. A single domain-like biasing-field dependency was observed in equidimensional particles up to 0.2µ mean grain size and up to 0.4µ elongated grains. Either the single domain/pseudo-single domain boundary lies above at least 0.2µ grain size or this field dependency test does not distinguish between single domain and pseudo-single domain states. A multidomainlike trend was observed in very coarse magnetite. The test may possibly distinguish the change from pseudo-single domain to multi-domain states. If both fine and coarse fractions are present a confusing overlap of the demagnetization curves occurs

Details of magnetic polarity transitions recorded in a high deposition rate deep-sea core

Measurements of the NRM of a 26 m long deep-sea core from the southern Indian Ocean indicated the presence of three transitions of magnetic polarity which have been identified as the upper and lower Jaramillo and the upper Olduvai on the basis of micropaleontological criteria. Detailed studies of the magnetic reversals were made in view of the high deposition rates (~9 cm/10^3 yr) present over sections of the core. The NRM was found stable against alternating fields. Magnetic mineralogy studies indicated the presence of titanomagnetite and magnetite which probably have not undergone any significant low-temperature oxidation. The three polarity changes had the following features in common: (1) presence of intermediate directions of magnetization; (2) a pronounced drop in the intensity of magnetization; (3) the drop in intensity of magnetization was coincident with the large directional fluctuations. Measurements of saturation isothermal and anhysteretic remanence, and bulk susceptibility, show that the decrease in NRM intensity associated with each polarity change is not due to a low concentration of the magnetic minerals. The best estimate for the duration of a polarity transition is approximately 4600 yr. There is evidence for both eastward and westward drift of the non-dipole field, which appears to be dominant during the polarity transition interval. The data presented here support a model of a reversing field in which the main dipole field decays to a low value and then builds up in the opposite direction

The relative stabilities of the reverse and normal polarity states of the earth's magnetic field

Recent analyses of the geomagnetic reversal sequence have led to different conclusions regarding the important question of whether there is a discernible difference between the properties of the two polarity states. The main differences between the two most recent studies are the statistical analyses and the possibility of an additional 57 reversal events in the Cenozoic. These additional events occur predominantly during reverse polarity time, but it is unlikely that all of them represent true reversal events. Nevertheless the question of the relative stabilities of the polarity states is examined in detail, both for the case when all 57 "events" are included in the reversal chronology and when they are all excluded. It is found that there is not a discernible difference between the stabilities of the two polarity states in either case. Inclusion of these short events does, however, change the structure of the non-stationarity in reversal rate, but still allows a smooth non-stationarity. Only 7 of the 57 short events are pre-38 Ma, but the evidence suggests that this is a real geomagnetic phenomenon rather than degradation of the magnetic recording or a bias in observation. This could be tested by detailed magnetostratigraphic and oceanic magnetic surveys of the Paleogene and Late Cretaceous. Overall it would appear that the present geomagnetic polarity timescale for 0–160 Ma is probably a very good representation of the actual history, and that different timescales and additional events now represent only changes in detail