Palaeomagnetic evidence for stationary sources of geomagnetic secular variation

Asymmetrical curvilinear patterns of secular variation with approximately superposed “outward” and “return” trajectories can be attributed to stationary magnetic sources in the core. Some appropriate palaeomagnetic examples are presented here.     

The palaeoposition of Madagascar: Palaeomagnetic evidence from the Isalo group

Palaeomagnetic results are reported from the continental facies of the Triassic-Jurassic Isalo Group of Madagascar. Stability of the magnetic remanence was tested using the alternating field and progressive thermal demagnetization techniques. Results from 8 sites, 6 located in northwestern Madagascar and 2 from the southwestern region, yield a palaeomagnetic pole at 74.2°S, 97.1°E (N = 8, A₉₅ = 6.3°). Three models previously proposed to describe the drift history of Madagascar relative to Africa are considered and the relevant geological and geophysical information is reviewed. The palaeomagnetic data are only consistent with the pre-drift model which places Madagascar off the east coast of Africa adjacent to Kenya and Tanzania. This is also the continental drift fit favoured on geological grounds.     

Palaeomagnetic evidence from the Ordovician and Silurian of northwest Peninsular Malaysia

The Ordovician and Silurian Setul Limestone of the Langkawi Islands, northwest Peninsular Malaysia, has a mean magnetic vector ofD = 338°,I = 62° after cleaning and correction for tilt. This is equivalent to a palaeolatitude of 43°, and a palaeomagnetic pole at 46°N, 76°E. The Silurian part of the Setul limestone also shows a similar direction. The Ordovician results are equivalent to a palaeolatitude of 43°, N or S. Recent reconstructions, based on palaeontology, place Indochina and China in the northern hemisphere in the Ordovician; if this is correct, a palaeolatitide of 43° for Langkawi would imply that Malaya-Indochina was the most northerly continental fragment at that time.     

Palaeomagnetic evidence for large-scale dextral movement along the Trollfjord-Komagelv Fault,Finnmark, north Norway

Palaeomagnetic data from Late Precambrian dykes from the northern part of Varanger peninsula, north Norway, suggest a two-axis magnetization structure. The dominant component is considered to be syn- to late-tectonic and probably acquired at around 640 m.y. B.P. Superposed on this magnetization is a minor component which is compatible with the relative Lower-Middle Palaeozoic field; i.e. it was most likely imposed during the climax of the Caledonian orogenic movements in north Norway. The estimated relative Late Precambrian palaeopole cannot easily be reconciled with the European Late Precambrian polar path. This disagreement can be accounted for by assuming a post-magnetization dextral megashear, of the order of 500–1000 km, along the Trollfjord-Komagelv fracture zone. This type of displacement is in line with geological evidence and the palaeomagnetic reconstruction supports the long-held view of there having been continuity between the depositional environments of the Varanger Peninsula Barents Sea Group, the Eleonore Bay Group of east Greenland and the Hecla Hoek Formation of east Spitsbergen. The character and age of the horizontal displacement, post-640 to pre-500 m.y. B.P., is seen in conjunction with the opening up of the lapetus Ocean and reactivation of ancient deep-seated fractures during both the spreading and the contraction phases of ocean development.     

Further evidence in support of the mixing model for howardite origin

Four diogenites, four howardites and seven eucrites have been analysed for major, minor and a number of trace elements by X-ray fluorescence spectrometry. Inter-element relationships between various elements in howardites, particularly refractory elements, are interpreted in the light of the recently proposed mixing model, whereby howardites are considered to be mixtures of eucritic and diogenitic material. Differences in composition observed between two samples of Frankfort analysed in this work lend strong support to this hypothesis. The nature of the proposed end-member composition in howardites is shown to be variable.     

Oxygen isotope evidence for the origin of enriched mantle beneath the mid-Atlantic ridge

Geochemical variations in mid-ocean ridge basalts have been attributed to differing proportions of compositionally distinct mantle components in their sources, some of which may be recycled crust. Oxygen isotopes are strongly fractionated by near-surface interactions of rocks with the hydrosphere, and thus provide a tracer of near-surface materials that have been recycled into the mantle. We present here oxygen isotope analyses of basaltic glasses from the mid-Atlantic ridge south of and across the Azores platform. Variations in δ¹⁸O in these samples are subtle (range of 0.47‰) and may partly reflect shallow fractional crystallization; we present a method to correct for these effects. Relatively high fractionation-corrected δ¹⁸O in these samples is associated with geochemical indices of enrichment, including high La/Sm, Ce/Pb, and ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd. Our results suggest two first-order conclusions about these enriched materials: (1) they are derived (directly or indirectly) from recycled upper oceanic crustal rocks and/or sediments; and (2) these materials are present in the north Atlantic MORB sources in abundances of less than 10% (average 2–5%). Modeling of variations of δ¹⁸O with other geochemical variables further indicates that the enriched component is not derived from incorporation of sediment or bulk altered oceanic crust, from metasomatism of the mantle by hydrous or carbonate-rich fluids, or from partial melting of subducted sediment. Instead, the data appear to require a model in which the enriched component is depleted mantle that has been metasomatized by small-degree partial melts of subducted, dehydrated, altered oceanic crust. The age of this partial melting is broadly constrained to 250 Ma. Reconstructed plate motions suggest that the enriched component in the north Atlantic mantle may have originated by subduction along the western margin of Pangea.     

Empirical evidence for a celestial origin of the climate oscillations and its implications

We investigate whether or not the decadal and multi-decadal climate oscillations have an astronomical origin. Several global surface temperature records since 1850 and records deduced from the orbits of the planets present very similar power spectra. Eleven frequencies with period between 5 and 100 years closely correspond in the two records. Among them, large climate oscillations with peak-to-trough amplitude of about 0.1 and 0.25°C, and periods of about 20 and 60 years, respectively, are synchronized to the orbital periods of Jupiter and Saturn. Schwabe and Hale solar cycles are also visible in the temperature records. A 9.1-year cycle is synchronized to the Moon's orbital cycles. A phenomenological model based on these astronomical cycles can be used to well reconstruct the temperature oscillations since 1850 and to make partial forecasts for the 21st century. It is found that at least 60% of the global warming observed since 1970 has been induced by the combined effect of the above natural climate oscillations. The partial forecast indicates that climate may stabilize or cool until 2030–2040. Possible physical mechanisms are qualitatively discussed with an emphasis on the phenomenon of collective synchronization of coupled oscillators.     

New field and laboratory evidence for the origin of hyaloclastite flows on seamount summits

New field observations with the submersible ALVIN and photographic evidence from a study of the summits of seamounts near the East Pacific Rise show that hyaloclastite deposits occur commonly. Hyaloclastite outcrops were found on six volcanoes at depths from 1240 to 2500 m. These new observations plus laboratory study of new hyaloclastite specimens extend the results of previous studies. Most of the hyaloclastite samples are of hydrovolcanic eruptive origin, but a few show evidence of a predominantly sedimentary origin. Primarily from morphology, we identify several vent areas from which hyaloclastite presumably erupted. The surface appearance of the hyaloclastite deposits varies with distance to these vents, leading us to propose a facies model for deep-sea hyaloclastites on seamount summits. Hyaloclastites of hydromagmatic origin exhibit weak normal grading and bedding-parallel alignment of platy shards. They consist of blocky, sliver and fluidal basalt glass shards and lithics in a matrix that contains pelagic sediment. The shards themselves are remarkably free of even the tiniest crystals and are usually chemically homogeneous. We propose that the shards form mainly by cooling-contraction granulation, but cannot rule out the possibility of limited steam explosivity. Hyaloclastites are closely associated with submarine pahoehoe and we propose that a very rapid eruption rate, promoting clastic-dominated versus flow-dominated eruptive behavior, is the dominant control on hyaloclastite formation. We propose that shard formation occurs during submarine lava fountaining. Gravitational instability of the resulting slurry of shards, sea water and possibly steam causes gravity flow that carries the shards outward from the vent. Further field and modelling studies are needed to test these ideas and more quantitatively constrain the ascent mechanism, eruption dynamics and deposition of deep-sea hyaloclastites.     

Palaeomagnetic studies of the Kapiti phonolite of Kenya

Specimens of Kapiti phonolite from sixteen sites were treated in an alternating field up to 900 Oe peak. Four sites were discarded on the basis of Watson's Criterion for randomness. Variations in various magnetic parameters along a vertical section of the rock indicate that a hard secondary component of magnetization of high coercive force is present in the surface samples of the rock — a possible cause for the poor grouping of directions of the remaining sites. Weathering may have introduced this component in the surface samples.     

New evidence of microbe origin for ferromanganese nodules from the East Pacific deep sea floor

Using a fluorescence microscope and EPMA, abundant microbe ”bodies“ and clear microbic fluorescent microstructure are determined in the ferromanganese nodules recently collected from the East Pacific deep sea floor. The microbic fluorescent structure shows a close relation to the formation of the ferromanganese nodules. According to their morphological features, the microbes are classified into two types: one is named clumpy microbe, which takes a bar-shaped manganese mineral as a pillar and grows like fasciculate coral, resulting in irregular cauliflorate nodules with rough surfaces; the other is called filamentous microbe, which grows in very thin arcuate and/or concentric circular laminae composed of a microbe layer and a metal (manganese and iron)-rich layer, leading to potato-shaped nodules with relatively smooth surfaces. It also can be seen that the two types of microbes are intergrown together, resulting in nodules complicated in compositions and shapes.     

Palaeomagnetic study of some charnockites of the Palni Hills

The intensity of natural remanence magnetisation (NRM) is measured along the three mutually-perpendicular directions using an astatic magnetometer. The intensity of induced magnetisation is measured using an apparatus fabricated in the laboratory. The Koenigsberger ratio has been calculated for all the samples and the ratio has been used to test the stability of NRM in the samples. The mean direction of magnetisation is determined from the three components of the NRM intensity. The palaeomagnetic pole position of the samples is determined using the direction of magnetisation and the site location. An attempt has been made to fix the geological age of the charnockites using palaeomagnetic methods.     

Palaeomagnetic timing of the rotation and translation of Cyprus

The geological evolution of the Mesozoic Troodos Ophiolite Complex in Cyprus, and the tectonic nature and timing of the palaeomagnetically indicated anticlockwise rotation of Cyprus of some 80° and ca. 15° northward translation, have been open for debate for some time. New palaeomagnetic data from 18 sites ( 180samples) in the post-ophiolite sediments, ranging in age from Upper Cretaceous to Upper Miocene, are presented. Most of the sites are of normal geomagnetic polarity, but indications of reversed polarity have been found in an older group of sediments (the Lefkara Formation of Upper Palaeocene age).Six sites from the older group of sediments (Upper Cretaceous to Eocene in age) give a site mean direction of the AF cleaned sediments of (D, I) = (323°, 29°) with α₉₅ = 18°, while 5 sites from a younger group of sediments (Oligocene to Miocene in age) give a cleaned site mean direction of (D, I) = (334°, 58°) with α₉₅ = 9°. These and published data suggest that an anticlockwise rotation of Cyprus of 60 ± 10° occurred early during the post-igneous evolution of the Cyprus oceanic crust between 90 and 50Ma, leaving only a minor anticlockwise rotation of 20 ± 10° to occur during the last 50 Ma. It is furthermore concluded that the northward translation of Cyprus of 15° mostly took place during the last 30Ma.It thus appears that a fairly rapid rotation of the Cyprus microplate first took place in the Late Cretaceous and Early Tertiary time with an average angular velocity of 1–2°/Ma, during which the northward translation was minor or negligible. In the latter half of the Tertiary, the sense of movement appears to have radically changed, the northward translation now being dominant with an average velocity of 5–6cm/yr. This temporal evolution is found to be in good agreement with the Mesozoic and Tertiary movements of the African lithospheric plate relative to Europe, as evidenced from the Atlantic sea-floor magnetic anomaly spreading history.     

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.     

Mitochondrial DNA provides evidence of a double origin for the stone crayfish Austropotamobius torrentium in the Elbe basin

The stone crayfish Austropotamobius torrentium is the smallest native European crayfish, restricted to central and southeast Europe. Northeast boundary of its range is located within the Elbe basin but considerable uncertainties existed regarding its status in this area. Until recently, known stone crayfish populations in the Elbe basin were very scattered and human translocations have been implicated in such distribution pattern. Discoveries of additional populations in the Czech Republic and Saxony (east Germany) nevertheless suggest that the species may have been more widespread there. We provide data on genetic variation (based on 181 sequences of the mitochondrial COI gene) of 20 representative populations from the Elbe basin (both from the Czech Republic and Saxony). We tested whether the haplotype variation is consistent with a scenario of natural dispersal or whether long-range transport has been involved. All analysed individuals from the easternmost, geographically isolated Czech stone crayfish population carried a haplotype previously recorded only in Slovenia and its vicinity; it is therefore likely that such population has been introduced by humans. In contrast, all remaining studied populations were dominated by a haplotype widespread in adjacent regions of species’ distribution in Germany, and additional haplotypes differing by point mutations were occasionally detected. This is consistent with a scenario of a postglacial colonization from Bavaria (southeast Germany). Our study provides evidence for a double origin of stone crayfish populations in the upper Elbe basin, with both natural and anthropogenic factors likely affecting the present diversity and distribution of this species.     

Isotopic and other geochemical evidence for the origin of the Loch Uisg Granophyre,Isle of Mull,Scotland

Full chemical analyses, including some trace elements and both oxygen and strontium isotope abundance data are presented for samples collected from a traverse across the outcrop of the early Tertiary Loch Uisg Granophyre. Chemically, the body is rhyodacite with very uniform major and trace element composition. In contrast, depleted δ¹⁸O values vary widely from +1.5‰ in the south to ?3.7‰ in the north (a distance of about 21/2 km), a range comparable to that for the intrusive rocks of Mull as a whole. This indicates more extensive groundwater interaction (i.e. higher water/rock ratios and/or higher temperatures of isotope exchange) towards the focus of the central intrusive complex. There is some degree of correlation between δ¹⁸O and iron oxidation ratios but no other evidence that the primary igneous geochemistry of these rocks has been significantly modified by hydrothermal alteration after emplacement of the pluton. Initial⁸⁷Sr/⁸⁶Sr ratios range from 0.71350 ± 9 to 0.71624 ± 6 and correlate with both Rb content and Rb/Sr ratio, the latter correlation yielding a pseudo-isochron of 260 ± 54 Ma at the time of emplacement. These results confirm a major contribution from an old crustal source region, ruling out formation of the granophyre solely from a basic parent magma. However, Rb-Sr data are presented for the Moine schists exposed in Mull and Morvern which also appear to rule out their involvement in the petrogenesis of the granophyre, either as a source region for melting, or as a bulk contaminant for a mantle-derived magma. The only viable hypotheses are assimilation at depth of? Lewisian into a basaltic fractionation sequence or partial melting of a Proterozoic basement such as that involved in the production of Caledonian granites in the Scottish Highlands.     

Rotation of the Iberian arc: Palaeomagnetic results from North Spain

NRM directions measured from 32 sites in Middle Cambrian, Upper Silurian/Lower Devonian and Lower Carboniferous redbeds follow the trend of the Variscan arc in North Spain. Thermal demagnetisation does not significantly alter this pattern. Fold tests show that the NRM is earlier than the ?₁ folds which form the arc; consistency of angle between bedding and the tilt-corrected NRM inclination (22–28°), similarity of the corresponding palaeolatitudes to Carboniferous values and microscopic evidence of Variscan redistribution of hematite indicates that the magnetisation is post-Lower Carboniferous. A statistical plot of the orientation of ?₁ fold traces against angle between ?₁ fold traces and declination of NRM shows that where these folds curve through 165° the NRM has been rotated through 110°: the arc is an orocline. Restoration of this rotation, and that needed to close the Bay of Biscay, brings the calculated mean palaeomagnetic pole reasonably close to the Upper Carboniferous part of the apparent polar wander path for Europe.     

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.     

Isotopic composition of carbonaceous-chondrite kerogen: evidence for an interstellar origin of organic matter in meteorites

Stepwise combustion has revealed systematic patterns of isotopic heterogeneity for C, H and N in the insoluble organic fraction (m-kerogen) from the Orgueil and Murray carbonaceous chondrites. Those patterns are essentially identical for both meteorites, indicating a common source of m-kerogen. The data cannot be reconciled with a single mass-fractionation process acting upon a single precursor composition. This indicates either a multi-path history of mass-dependent processing or a significant nucleogenetic contribution, or both. If mass-fractionation were the dominant process, the magnitude of the observed isotopic variability strongly suggests that ion-molecule reactions at very low temperatures, probably in interstellar clouds, were responsible. In any case, an interstellar, rather than solar nebular, origin for at least some of the meteoritic organic matter is indicated. This has interesting implications for the origin of prebiotic molecules, temperatures in the early solar system, and the isotopic compositions of volatiles accreted by the terrestrial planets.