Excursions of the Pleistocene geomagnetic field recorded in Gulf of Mexico sediments

The detrital remanent magnetism of a series of deep-sea sediment cores from the Gulf of Mexico has been measured. Together with microfaunal analysis, the data show that excursions of the geomagnetic field occurred at 17,000 ± 1500years B.P. and32,000 ± 1500 years B.P. It is suggested that the former may be the Laschamp excursion and that the latter may be the Lake Mungo excursion. No similar geomagnetic behavior is detected for the past 50,000 years. Sedimentation rates as high as 19 cm per 1000 years are indicated.Susceptibility (χ) maxima in the cores are due to tephra layers. Correlation between the intensity of magnetization (J) and χ shows that variations of intensity are more a function of ferrimagnetic mineral concentrations than geomagnetic field intensity variations.     

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.     

Palaeomagnetism and K-Ar ages of triassic igneous rocks from the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation,Argentina

The palaeomagnetism of Middle Triassic (224 ± 5 m.y.) igneous rocks from the Ischigualasto-Ischichuca Basin (67°40′W, 30°20′S) was investigated through 86 oriented hand samples from 11 sites. At least one reversal of the geomagnetic field has been found in these rocks. Nine sites yield a palaeomagnetic pole at 239°E, 79°S (α₉₅ = 15°, k = 13).The K-Ar age determinations of five igneous units of the Puesto Viejo Formation give a mean age of 232 ± 4 m.y. (Early Triassic). The palaeomagnetism of six igneous units of the Puesto Viejo Formation (68°W, 35°S) was investigated through 60 oriented samples. These units, two reversed relative to the present magnetic field of the Earth and four normal, yield a pole at 236°E, 76°S (α₉₅ = 18°, k = 14).Data from the Puesto Viejo Formation indicate, for the first time on the basis of palaeomagnetic and radiometric data, that the Illawarra Zone, which defines the end of the Kiaman Magnetic Interval, extends at least down to 232 ± 4 m.y. within the Early Triassic. The palaeomagnetic poles for the igneous rocks of the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation form an “age group” with the South American Triassic palaeomagnetic poles (mean pole position: 239°E, 77°S; α₉₅ = 6.6°, k = 190). The Middle and Upper Permian, Triassic and Middle Jurassic palaeomagnetic poles for South America would define a “time group” reflecting a quasi-static interval (mean pole position: 232°E, 81°S; α₉₅ = 4°, k = 131).     

East Pacific Rise at latitude 21°N: isotopic composition and origin of the hydrothermal sulphur

The sulphur isotope composition of 16 pyrite and chalcopyrite samples from recent sulphide deposits (“Cyana”—project RITA) and active sulphide mineralisation (“Alvin”—project RISE) associated with hydrothermal sources at 380±30°C on the East Pacific Rise at latitude 21°N have been measured. The³⁴S/³²S ratios are relatively uniform and essentially identical for both sites: δ³⁴S=+1.4to3.0%. (CDT), mean +2.1‰. The sulphides were analysed after the majority of the very numerous micro-inclusions of anhydrite had been removed.Two independent physico-chemical analyses of the data demonstrate that about 90% of the sulphur was leached from the basaltic host rocks by the circulating seawater-hydrothermal fluids.     

Afternoon Pc5 geomagnetic pulsations on the Earth’s surface and in the ionosphere (STARE radars)

Two cases when Pc5 geomagnetic pulsations were registered at the IMAGE Scandinavian network of stations and with STARE radars in the afternoon sector (1700–1800 MLT) during the recovery phase of the moderate magnetic storm are analyzed in detail. Using the ground-based observations, it has been indicated that classical quasimonochromatic resonance Pc5 pulsations were observed in the first case (on October 12, 1999; Kp = 5); in this case the maximal amplitude of the spectral maximum at a frequency of 2.5 mHz was registered at Φ ～ 65°. Two maximums were observed in the spectrum in the second case (on October 13, 1999; Kp = 4): ～2.5 mHz (the same maximum) and 2.9 mHz; in this case the maximal oscillation amplitude (2.5 mHz) shifted to Φ > 67°. These results were compared with the echo signal intensity simultaneously registered with the STARE Finland radar on a beam oriented along the 105° geomagnetic meridian. The spatial-temporal maps of the Pc5 pulsation amplitude latitudinal distribution (“keograms”), constructed based on the radar measurements in the wide range of geomagnetic latitudes (63°–70°) where the resolution was substantially higher than that of the ground-based observations, made it possible to detect two regions spaced in latitude (Φ ～ 65° and Φ ～ 67°–68°) with the simultaneous excitation of oscillations (double resonance?), between which the plasmapause projection was supposedly located.     

A paleomagnetic record from Pyramid Lake,Nevada, and its implications for proposed geomagnetic excursions

A well-dated, 3.3-m section of deep-lake clays from pluvial Lake Lahontan, now exposed along the shore of Pyramid Lake, Nevada, has provided a paleomagnetic record of secular variation during the time interval 25,000–36,000 years B.P. The measured ranges of inclination and declination are 40° and 75°, respectively, and represent values which are comparable to observed secular variation at northern mid-latitudes. These results and those from a previously reported study from Clear Lake, California, together provide strong evidence for the conclusion that northern California and western Nevada were not affected by any geomagnetic excursion during the time interval 21,000–36,000 years B.P. Such a conclusion puts severe constraints on the nature and timing of the proposed Mono Lake, Laschamp and Lake Mungo geomagnetic excursions.     

Absolute geomagnetic paleointensity as recorded by ∼1.09 Ga Lake Shore Traps (Keweenaw Peninsula, Michigan)

Absolute geomagnetic paleointensity measurements were made on 255 samples from 38 lava flows of the ～1.09 Ga Lake Shore Traps exposed on the Keweenaw Peninsula (Michigan, USA). Samples from the lava flows yield a well-defined characteristic remanent magnetization (ChRM) component within a ～375°C–590°C unblocking temperature range. Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo-single-domain magnetite and/or low-Ti titanomagnetite. Scanning electron microscopy reveals that the (titano)magnetite is present in the form of fine intergrowths with ilmenite, formed by oxyexsolution during initial cooling. Paleointensity values were determined using the Thellier double-heating method supplemented by low-temperature demagnetization in order to reduce the effect of magnetic remanence carried by large pseudosingle-domain and multidomain grains. One hundred and two samples from twenty independent cooling units meet our paleointensity reliability criteria and yield consistent paleofield values with a mean value of 26.3 ± 4.7μT, which corresponds to a virtual dipole moment of 5.9 ± 1.1×10²² Am². The mean and range of paleofield values are similar to those of the recent Earth’s magnetic field and incompatible with a “Proterozoic dipole low”. These results are consistent with a stable compositionally-driven geodynamo operating by the end of Mesoproterozoic.     

Paleomagnetic results from the Skaergaard intrusion,East Greenland

The natural remanent magnetization of 22 out of a total of 31 oriented cores from the layered series of the Skaergaard gabbroic intrusion (age: 55 m.y.) in East Greenland shows good stability in thermal and AF testing. The average direction of 22 AF and 9 thermally treated specimens isD = 170°,I = ?59°,α₉₅ = 4.2 before correction for tilt. The mean directions after rotation around strike to horizontal and after rotation to original attitudes suggested by others yields poorer population statistics. It is therefore concluded that flexuring took place between solidification and acquisition of remanent magnetization, a range in temperature of about 500°C which may represent an interval of somewhat less than 250,000 years. No evidence for secular variation is observed which may also suggest slow cooling through the blocking temperature range. The polarity is reversed and the pole position without “tilt correction” is 165°E, 61°N,d_m = 6.2,d_p = 4.6, which is similar to pole positions reported by others for the overlying slightly older basalt.     

Manifestation of the gothenburg geomagnetic field excursion in sediments on the northwestern Central Russian Upland

A paleomagnetic study of sediments at the Baranova Gora and Podol III/1 archaeological sites, located near Lake Volgo on the northwestern Central Russian Upland (56.9°N, 33.2°E), was performed. The paleomagnetic studies at both sites for the first time revealed the development of the Gothenburg geomagnetic excursion (dated 13000-12350 BP) in this region. This made it possible to specify the time interval when the Alleroed climatic phase started developing on the Central Russian Upland.     

Palaeomagnetism of the Early Carboniferous Deer Lake Group,western Newfoundland: no evidence for mid-Carboniferous displacement of “Acadia”

Early Carboniferous (Viséan to possibly earliest Namurian) sedimentary rocks of the Deer Lake Group of western Newfoundland rest unconformably on Grenvillian basement rocks of the Canadian Shield which form the western border of the Early Palaeozoic Appalachian orogen. In addition to magnetically soft magnetizations directed along the present field, three families of magnetization directions are found. Two of them (referred to as N (north) and S (south)) are antiparallel and prefolding, and were probably acquired during the depositional process. N and S are roughly of equal frequency. They have a mean direction irrespective of sign of 0.7°, ?35.2°, k = 40, α₉₅ = 8.9°, and a palaeopole 21.5°N, 121.8°E (10.3°, 6.0°) corresponding to a palaeolatitude of 20 ± 6°S. This agrees with the palaeolatitude (17 ± 5°S) determined from the somewhat older Early Carboniferous (Tournaisian) Terenceville Formation of the Avalon Platform on the eastern side of the Appalachian orogen in Newfoundland. The third magnetization, referred to as H (roughly horizontal), has a mean direction 156.8°, ?13.3°, k = 37, α₉₅ = 10.1°, and a palaeopole 45.4°N, 140.3°E (10.3°, 5.3°) corresponding to a palaeolatitude of 7 ± 4°N; we interpret this to be an early Kiaman (latest Carboniferous to early Permian) overprint probably acquired chemically. The palaeolatitude determined from the H overprint agrees with that determined from Early Carboniferous rocks of cratonic North America west of the Appalachians. Therefore, we argue, Early Carboniferous palaeofield determinations for cratonic North America have been strongly biased by unremoved Kiaman overprints. Because of this, and because of the good agreement between Early Carboniferous palaeolatitudes obtained from opposite margins of the Appalachian orogen, we suggest that there is, at present, no palaeomagnetic evidence for the previously proposed 1500 km displacement from the south of an eastern portion of the Appalachians (“Acadia”) relative to cratonic North America during the Carboniferous.     

The absence of the Mono Lake geomagnetic excursion from the paleomagnetic record of Clear Lake,California

Paleomagnetic measurements have been made on a continuously sampled, 5-m section of a core from Clear Lake, California. The sediments studied span an 8000-year interval centered at 25,000 years B.P., the approximate date of the large-scale, counterclockwise loop of the magnetic vector recorded in sediments from Mono Lake, California. The data from Mono Lake have been interpreted as a geomagnetic excursion with a duration of 600–1000 years. Because Clear Lake is only 320 km from Mono Lake and since each sample from Clear Lake represents 26 years of sedimentation, the magnetic signature of the Mono Lake geomagnetic excursion should be recorded in detail in the Clear Lake samples. Aside from a generally uniform shallowing of inclinations due to compaction, the paleomagnetic record from Clear Lake contains no anomalous features which would correspond to the Mono Lake excursion. Thus it has yet to be shown that the Mono Lake excursion was recorded anywhere besides Mono Lake. Even if the existence of the excursion is ultimately confirmed, its usefulness as a magnetostratigraphic horizon is limited.     

The variation of cosmogenic Kr and Xe in a core from Estherville mesosiderite: direct evidence that the lunar131Xe anomaly is a depth effect

Kr and Xe were measured by a stepwise heating technique in three samples of a drill core in the “Minnesota” fragment of the Estherville mesosiderite. The cosmogenic⁷⁸Kr/⁸³Kr decreased from the “top” sample to the “bottom” sample(“top” = 0.163 ± 0.005, “bottom” = 0.151 ± 0.005) while the cosmogenic¹³¹Xe/¹²⁶Xe ratio increased(“top” = 5.58 ± 0.35, “bottom” = 6.92 ± 0.17). Cosmic-ray track studies have shown that the “top” sample was indeed closer to the preatmospheric surface than the “bottom” sample by ～ 10 cm. This is the first direct evidence, in a sample of known geometry, that the cosmogenic¹³¹Xe/¹²⁶Xe ratio increases as a function of depth, and as such, confirms the hypothesis that the lunar¹³¹Xe anomaly is a bona fide depth effect due to resonance neutron capture in¹³⁰Ba.     

Paleomagnetic poles for the Carboniferous Brush Creek limestone and Buffalo siltstone from southwestern Pennsylvania

We report partial results of a larger project being undertaken in the Appalachian Basin to determine the character of the geomagnetic field in the Carboniferous. The Brush Creek limestone, which contains abundant terrigenous matter, yields reliable results from three different sites, while the overlying Buffalo siltstone appears at one locality. AF demagnetization curves, IRM acquisition curves, thermomagnetic analysis, and optical reflection microscopy indicate that the remanence carriers are magnetites or titanomagnetites, which are probably primary in origin. Stepwise AF demagnetization reveals that the best demagnetizing fields are 15–20 and 20–30 mT for the limestone and siltstone units, respectively. A total of 93 specimens from 20 samples (oriented hand samples and field-drilled cores) yields a paleopole at S 36.1, W 55.7, with an alpha-95 of 4.2° and a k of 13.1 for the Brush Creek limestone. The Buffalo siltstone paleopole is located at S 27.4, W 57.0, with alpha-95 = 6.1° and k = 13.0 using 45 specimens from 31 oriented hand samples. These results agree fairly well with the mean reversed Carboniferous paleopole and Noltimier's coal pole, but are sufficiently different to raise the possibility of doing magnetostratigraphic correlations.     

Seasonal,longitudinal, and latitudinal differences in air pressure over Russia during the solar activity maximum and minimum years

Using a large set of observational data (for ～50–100 years) obtained at 333 meteorological stations in Russia, we have shown that there is a clearly expressed differential pressure DP in the years of maximum and minimum solar activity by months and seasons, and by latitude and longitude. The values of the parameter DP were observed to be especially large at high latitudes U = 62.5°–67.5° in the longitude range D = 30°–50° in March (DP = ?4.45 ± 0.5 mbar) and in September (DP = 2.49 ± 0.21 mbar). We have concluded that the solar and geomagnetic activities can control the development of internal instabilities of the atmosphere and thus affect the climate.     

Paleomagnetism of Lower Devonian volcanics and Devonian dykes from northcentral New Brunswick,Canada

Some 50 oriented samples (120 specimens) have been collected on eight sites of volcanic rocks from the Lower Devonian Dalhousie Group of northern New Brunswick and Devonian andesitic to basic dykes from central New Brunswick. Univectorial and occasional multivectorial components were extracted from the various samples. Results after AF and thermal demagnetization compare relatively well. In the volcanics and tuffs, two components of magnetization have been isolated: A (D = 33°, I = ?58°, α₉₅ = 7.3°, K = 236) for four sites and B (D = 66°, I = +53°) for three sites. The grouping of component A is improved after tilt correction but the fold test is not significantly positive at the 95% confidence level. Component A is interpreted as being primary while component B is unresolved and appears to be the resultant magnetization of a Late Paleozoic and a recent component. The pole position obtained for tilt corrected component A is 268°E, 1°S, d_p = 6.5°, d_m = 8.8°. The paleolatitude calculated for component A is 39°S. The paleopole of in situ component A is located close to those of the Early-Middle Devonian formations from Quebec, New Brunswick and New England states while the paleopole of tilt-corrected component A is similar to Lower Devonian poles of rock units from the Canadian Arctic Archipelago. If component A is primary (as we believe it to be), then the western half of the northern Appalachians had already docked onto the North American Craton by Early Devonian time. Alternatively, if component A is secondary the same conclusion applies but the juxtaposition took place in Middle Devonian time.     

Mélanges magmatiques à la Montagne Pelée (Martinique) Origine des eruptions de type Saint-Vincent

During the last 40,000 years B.P. the eruptive activity of Mont Pelée (Martinique) has been exclusively pyroclastic, including mainly pumice flow deposits, Pelean-type and Merapi-type nuée ardente deposits, characterized by an andesitic to dacitic magma composition. In addition, a few Saint-Vincent-type nuée ardente deposits are present. Their products are compositionally more basic (basalt, basaltic andesite) and show some characteristic magma mixing features. Two well defined Saint-Vincent type eruptions, named SV1 and SV2 have been studied here. They have been dated by the C¹⁴ method respectively at 25,700±1,200 and 22,300±1,200 years B.P. Both follow a similar eruptive pattern, evolving from an andesitic to a more basic magma composition, through an intermediate stage of magma mixing. The volume of ejected products is extensive (1 km³ or more), compared with other deposits such as the Pelean-type nuée ardente. The moderate and progressive variations of magma composition (3 to 6 % SiO₂), mineralogy and crystallization pressure-temperature conditions (T: 920°–930° to 950°970°C, using Fe-Ti oxides geothermometer) demonstrate the cogenetic nature of these various magmas. These results, as well as the study of the recent activity of Mont Pelée suggest that during a former period (about 40,000 to 20,000 years B.P.), two magmatic chambers existed rather close to one another. The triggering of these Saint-Vincent type nuée ardente eruptions might involve injections of less-differentiated magma from a lower to a shallower reservoir, followed by the emptying of both reservoirs. During the recent period (less than 13,500 years B.P.), the cyclic eruptive activity of Mont Pelée Volcano has been controlled mainly by a relatively shallow and permanent magmatic chamber. The triggering of eruptions has depended on two processes: volatile overpressure and periodic replenishment of this superficial reservoir by deeper and less-differentiated magma injections. This change in eruptive character results perhaps from succession of SV1 and SV2 Saint-Vincent type eruptions; the volcano deep-structure might have changed, as a consequence of the extensive volume of ejected products.     

Blake polarity episode in two cores from the Greater Antilles Outer Ridge

Two large-diameter cores from the Greater Antilles Outer Ridge have confirmed the Blake episode 0.1 m.y. B.P. as a genuine paleomagnetic reversed interval at least in that region. The feature is clearly defined in more than 125 stably magnetized specimens of Last Interglacial abyssal brown clay, but its precise age and duration cannot yet be estimated reliably from these or other available data. During the reversed period, positions of the virtual geomagnetic pole lay about 20° from the south geographic pole, with polarity transitions traversing paths in the eastern hemisphere. Each core also recorded a normally polarized excursion within the Blake feature. Because the most acceptable Blake episode data presently span only a small geographic area, they are insufficient for distinguishing between global and local geomagnetic models for the feature.     

Geophysical effects of the interplanetary magnetic cloud on October 18–19, 1995, as deduced from observations at Irkutsk

During an interaction of the Earth’s magnetosphere with the interplanetary magnetic cloud on October 18–19, 1995, a great magnetic storm took place. Extremely intense disturbances of the geomagnetic field and ionosphere were recorded at the midlatitude observatory at Irkutsk (Φ′≈45°, Λ′≈177°, L≈2) in the course of the storm. The most important storm features in the ionosphere and magnetic field are: a significant decrease in the geomagnetic field Z component during the storm main phase; unusually large amplitudes of geomagnetic pulsations in the Pi1 frequency band; extremely low values of critical frequencies of the ionospheric F2-layer; an appearance of intense E_s-layers similar to auroral sporadic layers at the end of the recovery phase. These magnetic storm manifestations are typical for auroral and subauroral latitudes but are extremely rare in middle latitudes. We analyze the storm-time midlatitude phenomena and attempt to explore the magnetospheric storm processes using the data of ground observations of geomagnetic pulsations. It is concluded that the dominant mechanism responsible for the development of the October 18–19, 1995 storm is the quasi-stationary transport of plasma sheet particles up to L≈2 shells rather than multiple substorm injections of plasma clouds into the inner magnetosphere.     

Comparison of ionospheric equivalent slab thickness with bottomside digisonde profile over Wuhan

A comparison of the diurnal and seasonal variations in the ionospheric equivalent slab thickness (τ) and bottomside slab thickness (B0) is presented based on the observation during high solar activities at a mid-latitude station—Wuhan (114.4°E, 30.6°N). The investigated data include foF2, hmF2, B0, B1, and TEC, and are derived from the measured ionogram and GPS receiver over Wuhan from April 1999 to March 2000. The results show that τ and B0 are highly/weakly correlated during the day/night, respectively. Furthermore, a comprehensive discussion of the relation between τ, B0, and hmF2 for geomagnetic storm events is provided in this paper.     

87Rb-87Sr chronology of enstatite meteorites

A⁸⁷Rb-⁸⁷Sr analysis of some enstatite meteorites has been made. Whole rocks plot on an isochron of age 4.508 ± 0.037b.y. and strontium initial ratio 0.69880 ± 0.00044 (2σ errors; λ⁸⁷Rb= 1.42 × 10^?11yr^?1) . If the Norton County results are joined, we get an age of 4.516 ± 0.029b.y. and initial ratio of 0.69874 ± 0.00022. This result is indistinguishable from the whole rock isochron for H chondrites. It is interpreted as the age of condensation from the solar nebula. The identity of the⁸⁷Sr/⁸⁶Sr initial ratio with the ones for Allende white inclusions shows that this ratio was homogeneous in the solar nebula, and that the Rb-Sr fractionations observed between the different chondrite groups appeared only shortly before or during condensation accretion.Internal studies of the type-I enstatite chondrites Abee and Indarch and the intermediate-type Saint Mark's and Saint Sauveur have been done.Abee data scatter in the⁸⁷Rb-⁸⁷Sr diagram. For Indarch, Saint Mark's and Saint Sauveur, we obtained well-defined straight lines of “age” (T) and “initial ratio” (I): Indarch,T = 4.393 ± 0.043b.y.I = 0.7005 ± 0.0009; Saint Mark's,T = 4.335 ± 0.050b.y.I = 0.69979 ± 0.00022; Saint Sauveur,T = 4.457 ± 0.047b.y.I = 0.6993 ± 0.0014. Our result on Indarch agrees with the former result of Gopalan and Wetherill [5].A careful examination of the data shows that these straight lines are neither due to leaching effects by heavy liquids, nor result from terrestrial weathering. The “isochrons” for Indarch and Saint Sauveur can be mixing lines between enstatite and feldspar. The results are interpreted in terms of cosmochemical secondary effects: type-I and intermediate-type enstatite chondrites have been shocked 60–200 m.y. after their formation. This agrees with the idea of an early generalized bombardment of the inner solar system; this also indicates that type-I enstatite chondrites were rather situated in the outershells of their parent body and might be at the origin of the scatter of I-Xe ages of enstatite meteorites.Whole rock and enstatite from Bishopville, Cumberland Falls and Mayo Belwa have also been analysed. In these three aubrites, the⁸⁷Rb-⁸⁷Sr system is perturbed. Our Bishopsville sample might not be fresh and this makes the significance of our results uncertain. Cumberland Falls and Mayo Belwa probably suffered relatively recent shocks and open-system redistribution of Rb and Sr.