Paleointensity of the earth's magnetic field during the Laschamp excursion and its geomagnetic implications

The reversed paleomagnetic direction of the Laschamp and Olby flows represents a specific feature of the geomagnetic field. This is supported by paleomagnetic evidence, showing that the same anomalous direction was recorded at several distinct sites, including scoria of the Laschamp volcano. To examine this anomalous geomagnetic fluctuation, we studied the paleointensity of the Laschamp and Olby flows, using the Thellier method. Twenty-five samples were selected for the paleointensity experiments, and from seven we obtained reliable results. Because the paleointensity results of the Olby and Laschamp flows as well as Laschamp scoria are very similar, they can be represented by a single mean paleointensity,F = 7.7 μT. Considering that this low paleointensity is less than 1/6 of the present geomagnetic field and is more characteristic of transitional behavior, our results suggest that the paleomagnetic directions of the Laschamp and Olby flows were not acquired during a stable reversed polarity interval. A more likely explanation is that the Laschamp excursion represents an unsuccessful or aborted reversal.     

Complete and partial self-reversal of natural remanent magnetization of basaltic rocks from Lower Silesia,Poland

Summary Experiments of heating-cooling cycles in zero magnetic field were performed in order to study self-reversal of NRM in basaltic rocks from Lower Silesia. Complete self-reversal occurred in one sample containing titanomagnetite withT _c of 170°C and a small amount of a phase with higher Curie point. During consecutive heat treatments the phenomenon became less conspicuous. In three samples of higher oxidation level, containing several magnetic phases, only partial self-reversal of NRM occurred. For the most oxidized sample no changes of direction of NRM were observed. We suggest that the investigated phenomenon of self-reversal of NRM is due to a negative magnetic interaction between primary titanomagnetite and products of its oxidation. It seems that complete self-reversal can take place in a restricted state of oxidation.     

Age of the Laschamp paleomagnetic excursion revisited

The reverse paleomagnetism of the lava flows of Laschamp and Olby, already discovered by Bonhommet and Babkine, is confirmed. The ages of these flows, measured by¹⁴C, thermoluminescence and K-Ar dating are respectively36,000 ± 4000 and42,000 ± 5000years. VGP's comparison of the “Laschamp event” with those of the 30,000-year Lake Mungo excursion does not show evidence of coincidence between these two events.     

Composite titanomagnetite-ferrian ilmenite grains and correlative magnetic components in a dacite with self-reversed TRM

Electron microprobe and reflected light microscopic examinations confirm the presence of composite grains of ferrian ilmenite with X_ilm = 0.53 and titanomagnetite with X_usp = 0.13 in a dacite with self-reversed TRM. A parallel TRM component associated with titanomagnetite and a reversed component associated with self-reversing ferrian ilmenite are the principal NRM components. A subordinate, parallel component is associated with ferrian ilmenite which is not magnetically coupled to an “χ-phase”. The natural self-reversing properties are mainly a consequence of the dacite's high quenching temperature, calculated at 862–864°C using the Fe—Ti oxide geothermometer, and are most consistent with the self-reversal mechanism proposed by Lawson et al. [9].The conduction of thermal demagnetization and TRM induction tests in air had a much greater effect on the Fe—Ti oxides than did natural cooling, and resulted in significant oxidation with the consequent modification of some magnetic properties and the formation of another reversed TRM component. The subdivision of titanomagnetite grains by oxidation along fractures decreased its effective grain size and caused an apparent increase in its magnetic intensity, in addition to a slight increase in its resistance to alternating field demagnetization. The χ-phase associated with the reversed NRM component, with 0.42 > X_ilm 0.31, became Fe-enriched during the earlier stages of heat treatment. It is suggested that after heating at 600°C for two hours or more, this χ-phase exsolves as titanohematite with X_ilm < 0.33. The ferrian ilmenite host is consequently enriched in Ti, and another χ-phase much closer in composition to the host generates a reversed TRM component with T_b < 200°C.     

Origin of magnetic minerals and natural remanent magnetization of basaltic rocks from Lower Silesia,Poland

Summary Two groups of basaltic rocks with normal and reversed NRM were examined. It was found that in both groups the primary NRM, synchronous with the period of rock formation, is preserved. Relations of the primary NRM to the origin and the degree of oxidation of magnetic minerals were considered in detail.     

Physical and numerical models of the magnetization self-reversal in rocks

The paper addresses the physical mechanism of the magnetization self-reversal in rocks. The self-reversal is the phenomenon of magnetization of a rock in the direction antiparallel to the magnetizing field. Experimental data on the self-reversal of thermal and thermoremanent magnetizations in natural analogues of hemoilmenites and synthetic hemoilmenites are presented. It is shown that the most probable mechanism of self-reversal is the single-phase Néel mechanism of the N-type. The N-type mechanism underlies the physical mechanism and numerical model of self-reversal developed in the paper. The numerical modeling results are compared with experimental data.     

Palaeomagnetic Reversals in Mississippian Sediments of Missouri

--Normal to reversed polarity transitions have been found in Mississippian limestones of Missouri at longitude 269.7 and latitude 38.6 in an area of approximately 2,000 sq.km centered on the city of St. Louis. The transitions suggest a reversal of the geomagnetic field during the time these limestones were deposited. The mean normal NRM direction of declination 323.2, inclination 14.8, alpha-95 10.0 and corresponding reversed direction declination 144.8, inclination of 11.7 with alpha-95 of 5.2, are each tightly grouped, while in between these two groups the circles of confidence reveal a large scatter. Six normally magnetized groups of sites and corresponding VGP positions were found while seven groups are reversed. The mean normal VGP was at longitude 146.2 and latitude 44.6 and the reversed one at longitude 313.9 and latitude ⪶.8. The large scatter in between these two groups is interpreted as being due to rapid changes in the direction of the geomagnetic field when compared with the rate of deposition of the sediments. Alternating field (AF) and thermal demagnetization (TH) techniques were used to remove secondary components of the NRM.     

Palaeomagnetic investigations on the Deccan Traps from Chincholi,Mysore State,India

Summary 38 oriented samples of Deccan Traps have been collected from the neighbourhood of Chincholi, Mysore State, India. The Natural Remanent Magnetisation of these rocks has been studied using an astatic magnetometer. It has been found that these rocks are magnetically reversed, the mean magnetic direction being N154°E in declination and 61° down in inclination. Thermoremanance studies conducted on four specimens showed that two specimens with weak NRM and a high secondary magnetisation have Curie temperatures around 560°C for the NRM and exhibited partial reversal of TRM at room temperature, while two specimens with high NRM and with little secondary magnetisation have Curie temperatures much lower than 560°C for the NRM.     

An examination of the magnetic properties of basalts from Argentina and Iceland and of dolerite from the Great Whin Sill

Summary The natural remanent magnetization (NRM) of basalts from Argentina and Iceland and dolerite from the Great Whin Sill exhibit reversed magnetization. In order to test whether this was due to a self-reversing property of the rocks, samples from these three suites have been examined byx-ray and thermomagnetic techniques. No correlation between the properties of rocks and the sense of the NRM was found. The thermal stability of the magnetic extracts from these rocks was related to their lattice parameters and Curie points.     

Magnetic properties of the Olivenza meteorite—possible implications for its evolution and an early solar system magnetic field

The magnetic properties of samples of the Olivenza chondrite (LL5) obtained from four collections have been investigated. The natural remanent magnetization (NRM) consists of a very stable primary component, which is randomly scattered in direction on a scale of 1 mm³ or less within the samples, and a secondary magnetization widely varying in intensity, and probably also in direction. The origin of the secondary NRM is not clear, and may be of terrestrial origin. It is concluded that the NRM is carried by the ordered nickel-iron mineral, tetrataenite. The origin of the primary NRM could be a magnetic field associated with the solar nebula, out of which the metal grains condensed and acquired a thermo-remanent magnetization (TRM), or Olivenza could be a fine-grained breccia, the constituent fragments possessing randomly directed magnetization. The implications for the origin and evolution of Olivenza and its parent body if the former magnetizing process has occurred are discussed.     

Magnetic fields of the solar nebula as recorded in chondrules from the Allende meteorite

The natural remanent magnetization (NRM) in individual chondrules from the Allende meteorite was measured. These had previously been oriented relative to each other. The NRM directions of the chondrules are not initially random, but they become scattered after either alternating field (AF) or thermal demagnetization. The NRM is less stable than anhysteretic remanent magnetization (ARM) against AF-demagnetization.

The bulk of the NRM in the matrix is erased by 300°C. For the larger chondrules it is erased by 550°C, but for the smaller chondrules and the white inclusion a substantial decrease in NRM occurs by 350°C leaving about 20% up to 600°C. The behavior of the laboratory-induced ARM and the NRM under alternating field demagnetization suggest that the NRM of the chondrules consists of at least two components of TRM. One is a high-temperature component which was acquired when the individual chondrules were cooled through the Curie temperature and before they were assembled into the Allende meteorite. The other is a low-temperature component which was probably acquired in a field of about 1 Oe when the meteorite experienced thermal metamorphism or during the assembly of the meteorite.     

Does sea-level rise have an impact on saltwater intrusion?

Climate change effects are expected to substantially raise the average sea level. It is widely assumed that this raise will have a severe adverse impact on saltwater intrusion processes in coastal aquifers. In this study we hypothesize that a natural mechanism, identified here as the “lifting process,” has the potential to mitigate, or in some cases completely reverse, the adverse intrusion effects induced by sea-level rise. A detailed numerical study using the MODFLOW-family computer code SEAWAT was completed to test this hypothesis and to understand the effects of this lifting process in both confined and unconfined systems. Our conceptual simulation results show that if the ambient recharge remains constant, the sea-level rise will have no long-term impact (i.e., it will not affect the steady-state salt wedge) on confined aquifers. Our transient confined-flow simulations show a self-reversal mechanism where the wedge which will initially intrude into the formation due to the sea-level rise would be naturally driven back to the original position. In unconfined systems, the lifting process would have a lesser influence due to changes in the value of effective transmissivity. A detailed sensitivity analysis was also completed to understand the sensitivity of this self-reversal effect to various aquifer parameters.     

Remanent magnetization of andesitic and dacitic pumice from the 1985 eruption of Nevado del Ruiz (Colombia) reversed due to self-reversal

The natural remanent magnetization of andesitic pumice emitted during the 1985 eruption of the Nevado del Ruiz volcano (Colombia) has a direction opposite to the present geomagnetic field. The self-reversing mechanism can be re-activated in the laboratory during cycles of heating and subsequent cooling in air and zero magnetic field. Laboratory-produced thermoremanent magnetization is dominated by the same self-reversal process in fields up to several mT. Microchemical, optical and Curie temperature analyses indicate that the ferromagnetic minerals are members of the magnetite-ulvöspinel and hematite-ilmenite series with average compositions of Fe_2.73Ti_0.27O₄ and Fe_1.38Ti_0.62O₃, respectively. In analogy with the magnetic behaviour of synthetically grown antiferromagnetic-ferromagnetic FeMn-FeNi films, the self-reversal can probably be interpreted in terms of an exchange field acting between a Ti-poor canted antiferromagnetic and a Ti-rich ferrimagnetic phase in the hemoilmenite grains.     

Palaeomagnetic investigations on the East African rift in Northern Tanzania

Near Lake Manyara and north of Monduli, Northern Tanzania, Precambrian gneisses and Neogene basalts on the East African rift were sampled. Rock magnetic and microscope investigations established coarse and fine grained magnetite as carrier of the NRM in the basalts, and hematite in the gneisses. In all three sections only reversed magnetization directions were found. The Precambrian gneisses show very hard magnetizations which is known to be prefolding and possibly of Proterozoic age.     

Magnetic properties of some tertiary basalts from West Greenland

Summary Measurements of bulk magnetic properties, including the natural remanent magnetization (NRM), susceptibility and the Königsberger ratio, on over 250 samples of Tertiary basalts from Disko and Nûgssuaq, West Greenland are reported.The NRM intensities in basalts (geometric mean value 3.3 A/m in SI units) were on average three to four times as large as the induced magnetization intensities. The susceptibilities (geometric mean value 2.1×10^–2 SI units) were much more uniform than the NRM intensities. In the majority of samples, the NRM was predominantly of reverse (R) polarity, but samples from a few sites showed a remanence of normal (N) polarity.The NRM of both polarity classes (N, R) was very stable against alternating field (AF) demagnetization with median destructive fields of the order of 20,000–30,000 A/m (250–350 Oe), comparable to those for many stable continental and oceanic basalts. The viscous remanence intensity, as studied by storage tests on some specimens, was found to be an insignificant fraction of the original NRM, except in few cases.The low field hysteresis loops (Rayleigh loops) were studied for some specimens. A qualitative association was noted between wide hysteresis loop and relatively low AF stability, but no correlation was apparent between the loop type and the Königsberger ratio (Q _n) of a specimen.Contribution no. 6 Institute of Geophysics, University of Copenhagen.     

Palaeomagnetic and mineralogical studies of Devonian lacustrine sediments from Caithness,Scotland

A variety of lacustrine sediments from the Devonian Caithness Flagstone Group in northern Scotland has been subjected to palaeomagnetic and mineralogical analysis. In general, the sediments show little evidence of a Devonian magnetization although this is partly seen in the Spital Flagstones. The magnetization seems to be dominated by Mesozoic overprints of normal and reversed polarity. The different sub-components of the magnetizations cannot be resolved thermally because of the difficulty of applying thermal demagnetization techniques above 400°C. At and above this temperature, sulphide oxidation results in laboratory magnetizations which obscure the NRM.Mineralogical studies show that the overprint is carried by Fe hydroxides in association with pyrite and marcasite grains (dolostones) or hematite in association with Cr-spinels (Achanarras Limestone). In the Spital Flagstones the relict Devonian magnetization and the normal Mesozoic overprint appears to be carried by magnetite. Our results show the importance of mineralogical studies in making realistic interpretation of the origin of magnetizations in ancient sediments.     

Paleomagnetism of tertiary and recent lavas of Israel

Magnetizations in 24 flows of Tertiary age in Israel indicate two stable directions, each of which has both normal and reversed polarities. AF demagnetization decreases the scatter of the NRM results. Typical Tertiary poles are near 70°N 110°W and another set of anomalous poles are near 34°N 50°W. These are similar to other reported Tertiary and Cretaceous poles.     

Origin of the magnetization of Permo-Carboniferous sediments of Spitsbergen, Svalbard Archipelago

Rock magnetic investigations of Permo-Carboniferous carbonate sediments from two areas on Spitsbergen are described, conducted to identify the carriers of the NRM in these rocks. Since microscopic and magnetic separation techniques could not profitably be applied, the nature of magnetic minerals was investigated by thermal demagnetization of the NRM and decay of saturation isothermal remanence (I_rs) during heating to 600°C, as well as by the distribution of the median destructive fields of the NRM and observation of magnetic susceptibility after subsequent heatings. The results show that the NRM of these limestones resides mainly in magnetite, but creation of magnetic pyrrhotite and of fresh magnetite is observed during heating to 600°C. Presence of sulphides indicates that magnetite is an oxidation product of pyrite or of non-magnetic pyrrhotite. Examination of rock magnetic properties of limestones leads to the conclusion that most of the magnetite in the rocks of the Bellsund area is of detrital origin, whereas the rocks at Festningen contain magnetite derived from pyrite probably during an early stage of the diagenetic process.     

Thermal remagnetization and the paleointensity record of metamorphic rocks

We present a quantitative relationship between blocking temperature and time that, in principle, provides a calibration of thermal remagnetization in nature. For a given metamorphic temperature-time regime, one can decide whether a given laboratory blocking temperature (or for paleointensity work, a range of blocking temperatures) is consistent with primary natural remanence (NRM) or with a metamorphic overprint. Independent of the domain structure or the chemical composition of the magnetic minerals, two general types of behaviour are predicted. If the primary NRM possesses laboratory (or primary cooling) blocking temperatures within 100°C or so of the Curie temperature, thermal remagnetization at lower temperatures, even over times as long as 10⁶ years, is improbable. If the blocking temperatures are lower, viscous remagnetization is pronounced at temperatures well below those indicated by laboratory thermal demagnetization. An approximate scale of the “survival potential” of primary NRM in rocks of different metamorphic grades indicates that primary paleointensities are unlikely to be recovered from rocks metamorphosed above high-greenschist facies if the predominant magnetic mineral is nearly pure magnetite, or above middle-amphibolite facies if nearly pure hematite is predominant. Evidence from laboratory experiments and paleomagnetic field studies in metamorphic regions suggests, however, that these survival estimates are unduly optimistic. Chemical remagnetization through the destruction of primary magnetic minerals, and not thermal remagnetization, probably sets an effective upper temperature for the survival of primary NRM.