Several Thousands Of Kilometres Research Articles

Late Paleozoic tectonics of the Junggar-Altai–Sayan Foldbelt

This paper presents new tectonic model for the Late Palaeozoic evolution of the Jungar-Altai–Sayan Fold Belt in the context of large-amplitude (hundreds to several thousands of kilometres) strike-slip movement along regional NW-trending fault zones. These include the following regional fault zones: Chara, Irtysh, Northeast, Charysh–Terekta and Kurai-Teletsk, as well as the thrusts associated with them kinematically:Tangbale-Kolameili,Mayile, Barleik–Hongguleleng, Kaim, North Sayan thrust fault and Kurtushuba.

Highly restricted gene flow between disjunct populations of the skunk clownfish (Amphiprion akallopisos) in the Indian Ocean

AbstractThe skunk clownfish (Amphiprion akallopisos) has a disjunct distribution, occurring in the Eastern Indian Ocean (EIO) and the Western Indian Ocean (WIO), separated by several thousands of kilometres. Information on connectivity of marine species is very important for the correct spacing of marine protected areas, a powerful instrument for the protection of coral reefs. The population genetic structure of A. akallopisos was analysed in order to investigate connectivity amongst populations and to explain the disjunct distribution of the species. A fragment of the mitochondrial control region was used to investigate the genetic population structure. Fin clips were collected from 263 individuals at 14 sites in the WIO and three sites in the EIO. The obtained DNA sequences were used to calculate genetic diversity, evaluate demographic history and to construct a haplotype network. An analysis of molecular variance (AMOVA) was conducted to evaluate the significance of the observed genetic population structure. None of the identified 69 haplotypes was shared between the WIO and EIO. Haplotype as well as nucleotide diversity was considerably higher in the EIO than in the WIO. Significant genetic population structure was revealed by an AMOVA with an overall φst‐value of 0.28 (P < 0.001) in the Indian Ocean. The overall AMOVA (φst = −0.00652) was not significant in the EIO, but was significant in the WIO (φst = 0.016; P < 0.01). Demographic analysis indicated population expansion in the EIO and WIO. Population genetic analysis revealed highly restricted gene flow between the EIO and WIO. Genetic diversity was much higher in the EIO than in the WIO, suggesting that the EIO is the geographical origin of the species. Given the large distance between the disjunct populations and the short pelagic larval duration, long‐distance dispersal is rather unlikely. A stepping stone model involving islands in the Central Indian Ocean is a more likely scenario for colonization of the WIO.

Neo-Tethys geodynamics and mantle convection: from extension to compression in Africa and a conceptual model for obduction

Since the Mesozoic, Africa has been under extension with shorter periods of compression associated with obduction of ophiolites on its northern margin. Less frequent than “normal” subduction, obduction is a first order process that remains enigmatic. The closure of the Neo-Tethys Ocean, by the Upper Cretaceous, is characterized by a major obduction event, from the Mediterranean region to the Himalayas, best represented around the Arabian Plate, from Cyprus to Oman. These ophiolites were all emplaced in a short time window in the Late Cretaceous, from ∼100 to 75 Ma, on the northern margin of Africa, in a context of compression over large parts of Africa and Europe, across the convergence zone. The scale of this process requires an explanation at the scale of several thousands of kilometres along strike, thus probably involving a large part of the convecting mantle. We suggest that alternating extension and compression in Africa could be explained by switching convection regimes. The extensional situation would correspond to steady-state whole-mantle convection, Africa being carried northward by a large-scale conveyor belt, while compression and obduction would occur when the African slab penetrates the upper–lower mantle transition zone and the African plate accelerates due to increasing plume activity, until full penetration of the Tethys slab in the lower mantle across the 660 km transition zone during a 25 Myr long period. The long-term geological archives on which such scenarios are founded can provide independent time constraints for testing numerical models of mantle convection and slab–plume interactions.

Ocean currents generate large footprints in marine palaeoclimate proxies.

Fossils of marine microorganisms such as planktic foraminifera are among the cornerstones of palaeoclimatological studies. It is often assumed that the proxies derived from their shells represent ocean conditions above the location where they were deposited. Planktic foraminifera, however, are carried by ocean currents and, depending on the life traits of the species, potentially incorporate distant ocean conditions. Here we use high-resolution ocean models to assess the footprint of planktic foraminifera and validate our method with proxy analyses from two locations. Results show that foraminifera, and thus recorded palaeoclimatic conditions, may originate from areas up to several thousands of kilometres away, reflecting an ocean state significantly different from the core site. In the eastern equatorial regions and the western boundary current extensions, the offset may reach 1.5 °C for species living for a month and 3.0 °C for longer-living species. Oceanic transport hence appears to be a crucial aspect in the interpretation of proxy signals.

Focusing frequency and significance of multi‐frequency very low‐frequency electromagnetic measurement in delineating near‐surface conducting structures

ABSTRACTThe very low‐frequency (VLF) electromagnetic method utilizes primary signals (field) transmitted from worldwide distant transmitters located in coastal areas. These transmitters are meant for long distance marine communication. VLF transmitters operate at a low communication frequency band (between 5–30 kHz) and the transmitted signal travels a long distance. Transmitted signals penetrate the Earth’s subsurface and produce electromagnetic induction in the subsurface even several thousands of kilometres away from the transmitters. The VLF method is quite simple and frequently used in the delineation of near‐surface conducting structures of various practical applications. Several conducting structures lying along a measured profile with different conductivities can be properly induced at distinct frequencies that yield the maximum response. Therefore, such conductors may not be identified or resolved well using single frequency VLF measurement.A 2D numerical modelling study was carried out over a wide frequency range (1–500 kHz) to find the frequency that produces the maximum response for a given conductor. Results show that a particular frequency (focusing frequency) produces the maximum (peak) response for a conductor. When the measuring frequency either increases or decreases with respect to the focusing frequency, then the peak response always decreases. The focusing frequency remains almost similar with an increase in target depth and host resistivity. An increase in the overburden conductivity shows a decline in the focusing frequency. Two or more targets of different conductivity present in the subsurface yield peak responses at corresponding focusing frequencies. This shows that they will be resolved well at corresponding focusing frequencies. In such circumstances, inversion using single frequency VLF data yields inaccurate results. However, the use of multi‐frequency VLF data yields better results. Inversion of multi‐frequency VLF data is presented to show the efficacy of the approach. A field measurement is also presented and the effectiveness of multi‐frequency VLF measurement is highlighted. Since the numerical modelling study is performed over a broad frequency range covering the VLF and radiomagnetotelluric signal, the focusing study is valid for radiomagnetotelluric applications as well.

Prospects for accurate distance measurements of pulsars with the Square Kilometre Array: Enabling fundamental physics

Parallax measurements of pulsars allow for accurate measurements of the interstellar electron density and contribute to accurate tests of general relativity using binary systems. The Square Kilometre Array (SKA) will be an ideal instrument for measuring the parallax of pulsars, because it has a very high sensitivity, as well as baselines extending up to several thousands of kilometres. We performed simulations to estimate the number of pulsars for which the parallax can be measured with the SKA and the distance to which a parallax can be measured. We compare two different methods. The first method measures the parallax directly by utilising the long baselines of the SKA to form high angular resolution images. The second method uses the arrival times of the radio signals of pulsars to fit a transformation between time coordinates in the terrestrial frame and the comoving pulsar frame directly yielding the parallax. We find that with the first method a parallax with an accuracy of 20% or less can be measured up to a maximum distance of 13 kpc, which would include 9,000 pulsars. By timing pulsars with the most stable arrival times for the radio emission, parallaxes can be measured for about 3,600 millisecond pulsars up to a distance of 9 kpc with an accuracy of 20%.

Sound fixing and ranging beneath sea-ice

Sound fixing and ranging techniques, the so-called SOFAR technique, is widely used in the field of oceanography for tracking neutrally buoyant floats underwater over large distances (several thousands of kilometres) and for long periods of time (several years). The range propagation depends largely on the existence or not of the SOFAR sound channel. Twenty years ago we investigated SOFAR acoustic propagation under sea-ice over large distances and at various frequencies from 80 Hz up to 1560 Hz. We discovered the range for acoustic propagation was much reduced (100 kms to 150 kms) no matter the frequency used. This was due to large scattering induced by under sea-ice topography since there is no way to avoid acoustic rays to bounce back to the surface towards sea-ice and create multiple reflections. More recently during the Tara Damocles transpolar drift across the Arctic Ocean, we reinvestigated the SOFAR acoustic range propagation under sea-ice for longer periods of time in various conditions (summer and winter conditions). Our results confirmed those obtained 20 years ago and indicated a high sensitivity to the ambient conditions. We are presenting the results obtained during the late fall- early winter months of the Tara transpolar drift across the Arctic Ocean.

E/I corrected paleolatitudes for the sedimentary rocks of the Baja British Columbia hypothesis

Paleomagnetic inclinations from sediments of the western terranes of Canada are consistently too shallow for their reconstructed paleogeographic positions. Two contradicting explanations for these discrepancies are: (1) terranes have been displaced northward with respect to the stable American craton by several thousands of kilometres between the Late Cretaceous (~75 Ma) and the Eocene (~50 Ma) and (2) sedimentary inclination error has caused a shallow bias in the paleomagnetic directions. Here, we apply the elongation/inclination (E/I) method to paleomagnetic data sets from sedimentary rocks of supposedly allochtonous terranes of western North America to correct for inclination flattening. Our results indicate that the paleomagnetic directions from the continental Silverquick sediments (95–92 Ma) of southern British Colombia are not seriously affected by inclination error, because the magnetic signal most likely concerns a chemical remanent magnetisation (CRM). In contrast, the marine sediments of the Nanaimo Group (84–72 Ma) of Vancouver Island region appear seriously affected by inclination flattening (f =0.7) and the E/I corrected mean inclinations are about 98 steeper than the original data. We arrive at corrected inclinations/paleolatitudes of I**=578/k=388N for the Silverquick and I**=558/k=368N for the Nanaimo sediments. Our corrected paleolatitudes indicate that the Canadian terranes were indeed located adjacent to the Baja Californian margin during the Late Cretaceous, thus supporting the Baja BC hypothesis. D 2005 Elsevier B.V. All rights reserved.

Orientation in the wandering albatross: interfering with magnetic perception does not affect orientation performance

After making foraging flights of several thousands of kilometers, wandering albatrosses (Diomedea exulans) are able to pinpoint a specific remote island where their nests are located. This impressive navigation ability is highly precise but its nature is mysterious. Here we examined whether albatrosses rely on the perception of the Earth's magnetic field to accomplish this task. We disturbed the perception of the magnetic field using mobile magnets glued to the head of nine albatrosses and compared their performances with those of 11 control birds. We then used satellite telemetry to monitor their behavior. We found that the ability of birds to home specific nest sites was unimpaired by this manipulation. In particular, experimental and control birds did not show significant differences with respect to either foraging trip duration, or length, or with respect to homing straightness index. Our data suggest that wandering albatrosses do not require magnetic cues to navigate back to their nesting birds.

The effect of overlying absorbing aerosol layers on remote sensing retrievals of cloud effective radius and cloud optical depth

AbstractTwo types of partially absorbing aerosol are included in calculations that are based on intensive aircraft observations: biomass burning aerosol characterized during the Southern AFricAn Regional science Initiative (SAFARI 2000) and mineral dust aerosol characterized during the SaHAran Dust Experiment (SHADE). Measurements during SAFARI 2000 reveal that the biomass burning aerosol layer is advected over the South Atlantic ocean at elevated altitudes above the marine boundary layer which is capped by semi‐permanent stratocumulus cloud sheets. Similarly, the mineral dust is measured at elevated altitudes during SHADE resulting in transport above cloud for distances of several thousands of kilometres. We perform theoretical calculations of the effect of these partially absorbing aerosol layers on satellite retrievals of cloud effective radius and cloud optical depth, and show that, in these cases, retrievals of cloud optical depth or liquid water path are likely to be subject to systematic low biases. The theoretical calculations suggest that the cloud effective radius may be subject to a significant low bias for Moderate resolution Imaging Spectrometer (MODIS) retrievals that rely on the 0.86 and 1.63 µm radiance pair for an overlying aerosol layer of either biomass burning aerosol or mineral dust. Conversely, the cloud effective radius may be subject to a significant high bias for Advanced Very High Resolution Radiometer or MODIS retrievals that rely on the 0.63 and 3.7 µm radiance pair for an overlying aerosol layer of mineral dust. Analysis of 1 km resolution MODIS data for the SAFARI 2000 period suggests that the effective radius derived from the 0.86 and 1.63 µm radiance pair is, indeed, subject to a low bias in the presence of overlying biomass burning aerosol. These results show the difficulties associated with remote sensing retrievals, which must be kept in mind when attempting to assess any potential indirect effect. © Crown copyright 2004.

The Cambrian-Silurian tectonic evolution of the northern Appalachians and British Caledonides: history of a complex, west and southwest Pacific-type segment of Iapetus

This paper is included in the Special Publication entitled 'Lyell: the past is the key to the present', edited by D.J. Blundell and A.C. Scott. This paper presents new ideas on the Early Palaeozoic geography and tectonic history of the Iapetus Ocean involved in the formation of the northern Appalachian-British Caledonide Orogen. Based on an extensive compilation of data along the length of the orogen, particularly using well-preserved relationships in Newfoundland as a template, we show that this orogen may have experienced a very complicated tectonic evolution that resembles parts of the present west and southwest Pacific Ocean in its tectonic complexities. Closure of the west and southwest Pacific Ocean by forward modelling of the oblique collision between Australia and Asia shows that transpressional flattening and non-coaxial strain during terminal collision may impose a deceptively simple linearity and zonation to the resultant orogen and, hence, may produce a linear orogen like the Appalachian-Caledonian Belt. Oceanic elements may preserve along-strike coherency for up to several thousands of kilometres, but excision and strike-slip duplication, as a result of oblique convergence and terminal collisional processes, is expected to obscure elucidation of the intricacies of their accretion and collisional processes. Applying these lessons to the northern Appalachian-Caledonian belt, we rely principally on critical relationships preserved in different parts of the orogen to constrain tectonic models of kinematically-related rock assemblages. The rift-drift transition, and opening of the Iapetus Ocean took place between c.590-550 Ma. Opening of Iapetus was temporally and spatially related to final closure of the Brazilide Ocean and amalgamation of Gondwanaland. During the Early Ordovician, the Laurentian margin experienced obduction of young, supra-subduction-zone oceanic lithosphere along the length of the northern Appalachian-British Caledonian Belt. Remnants of this lithosphere are best preserved in western Newfoundland and are referred to as the Baie Verte Oceanic Tract. Convergence between Laurentia and the Baie Verte Oceanic Tract was probably dextrally oblique. Slab break-off and a subsequent subduction polarity reversal produced a continental magmatic arc, the Notre Dame Arc, on the edge of the composite Laurentian margin. The Notre Dame Arc was mainly active during the late Tremadoc-Caradoc interval and was flanked by a southeast- or south-facing accretionary complex, the Annieopsquotch Accretionary Tract. Southerly drift of Laurentia to intermediate latitudes of c.20-25°S was associated with the compressive (Andean) nature of the arc and the accompanying backthrusting of the already-accreted Baie Verte Oceanic Tract further onto the Laurentian foreland. Equivalents of the Notre Dame Arc and its forearc elements in the British Isles have been preserved as independent slices in the Midland Valley and possibly the Northern Belt of the Southern Uplands. During the late Tremadoc (c.485 Ma), the passive margin on the eastern side of Iapetus also experienced obduction of primitive oceanic arc lithosphere. This arc is referred to as the Penobscot Arc. The eastern passive margin was built upon a Gondwanan fragment (Ganderia) that rifted off Amazonia during the Early Ordovician and probably travelled together with the Avalonian terranes as one microcontinent. The departure of Ganderia and Avalonia from Gondwana opened the Rheic Ocean. Equivalents of the Penobscot Arc may be preserved in New Brunswick and Maine, Leinster in eastern Ireland, and Anglesey in Wales. An arc-polarity reversal along the Ganderian margin after the soft Penobscot collision produced a new arc: the west-facing Popelogan-Victoria Arc, which probably formed a continuous arc system with the Bronson Hill Arc in New England. The Popelogan-Victoria Arc transgressed from a continental to an oceanic substrate from southern to northeastern Newfoundland. Rapid roll-back rifted the Popelogan-Victoria Arc away from Ganderia during the late Arenig (c.473 Ma) and opened a wide back-arc basin; the Tetagouche-Exploits back-arc basin. The Popelogan-Victoria Arc was accreted sinistrally oblique to the Notre Dame Arc and, by implication, Laurentia during the Late Ordovician. After accretion, the northwestward-dipping subduction zone stepped eastwards into the Tetagouche-Exploits back-arc basin.

Wave propagation in a multiple-scattering upper mantle-observations and modelling

SUMMARY A striking feature of Russian long-range seismic refraction data from Peaceful Nuclear Explosions is the observation of a high-frequency teleseismic P, phase, which travels with a group velocity of 8.0 km s-l out to distances of several thousands of kilometres. Modelling using the reflectivity method shows that this phase can be understood as the response of an upper mantle that contains random RMS velocity fluctuations of about 44 per cent superimposed on a positive velocity gradient. This class of model explains the existence of the teleseismic P,, its high-frequency content and its coda length. A teleseismic P, can only be generated if velocity fluctuations are strong enough to cause multiple scattering and occur on a subwavelength scale. Cross-correlation properties of P- and S-wave velocity fluctuations exert a substantial influence on the wavefield. A completely unexpected phase can be observed if the fluctuations are imposed on a negative gradient

Pb, Sr, and Nd isotopic systematics of rocks from the Galapagos Microplate

Pb, Sr, and Nd isotopic analyses were performed on a suite of compositionally variable rock samples representing an approximately 2500 m thickness of oceanic crustal lithologies from the deep rift valleys and spreading centers along the periphery of the Galapagos Microplate at the junction of the Pacific, Cocos, and Nazca plates. Samples were obtained from lower peridotite and gabbro layers, Fe–Ti basalts, very recent neovolcanic flows, and the thin sedimentary cover. The samples were dredged, cored, or video-grabbed during the research cruise Sonne 60. For unaltered igneous rocks, 206Pb/204Pb ratios range from 17.70 to 18.90, 87Sr/86Sr ratios from 0.7024 to 0.7035, clustering around 0.7027, while 143Nd/144Nd ratios range from 0.51307 to 0.51324. Based on these data, little systematic isotopic variation can be associated with any specific area or lithology of the microplate. One exception is a series of very thin volcanic flows from the Summit Graben area, where Pb and Nd isotope ratios change gradually, with each successive flow. The extent of the observed Pb isotopic variation covers almost the entire range of ratios for the East Pacific Rise mid-oceanic ridge basalts published to date, suggesting that the part of the mantle responsible for formation of the Galapagos Microplate rocks is no more isotopically homogeneous than the East Pacific Rise mantle as a whole, taken over a length of several thousands of kilometres.

Implantable data logging system for heart rate and body temperature: its application to the estimation of field metabolic rates in Antarctic predators

The metabolic rate of free-ranging animals is difficult to measure, but of great importance in understanding the interactions of a species with its environment. Heart rate can, if correctly validated and calibrated, give an estimate of metabolic rate, with both a fine time resolution and over long periods. The telemetry of heart rate is well documented, but is not appropriate over long ranges (possibly several thousands of kilometres) or for diving species. An implantable data logger has therefore been developed for the long term recording of heart rate and body temperature. The logger is built using hybrid and ASIC construction techniques, weighs 20 g and measures 55 x 24 x 6 mm. The device is programmable, and its solid-state memory holds over 70 days of data if, for example, heart rate is counted and stored every minute. Current consumption is 155 microA while logging, 50 microA during a programmable initial delay period, and less than 1 microA when the logger closes down after filling its memory. These loggers have been deployed for two field seasons in gentoo penguins, black-browed albatross and fur seals.

Rostock zooplankton studies off West Africa

Since the beginning of the seventies, upwelling research has become increasingly popular in the path of the Canary and Benguela Current, because of economic consideration, particularly in the relation to fisheries and marine geology. Many expeditions were carried out between 1970 and 1977, including 8 cruises of the German R. V. “A. v. Humboldt” operating from Rostock. Measurements covered scales ranging in time from minutes to several years and in space from hundreds of metres to several thousands of kilometres. Zooplankton studies focussed on quantitative, metabolic, taxonomic, and parasitological aspects. Plankton was collected with a WP-2-UNESCO standard net to a maximum depth of 200 m. The epipelagic mesozooplankton consists mainly of copepods, especially calanoids with developmental times of about 20 to 23 days. After an upwelling event, zooplankton is able to double its biomass. This typical biomass increase is independent of coastal distance and depth. The upwelling response lasts about 3 weeks in nearsurface waters, and 6 to 8 weeks in depths below 75m. A relationship was observed between the duration of seasonal upwelling (that means the numbers of single upwelling events) and the cumulative increase of biomass. This net growth rate of zooplankton biomass is most pronounced at the shelf break, the area with the highest fish biomass, and in the upper 25 m. Differences between the expected and the real rate values in conjunction with the known amount of nutritive demands of fishes allow the estimation of the fish biomass in a given area. The near coastal Ekman upwelling, which is an event in the time scale of about two weeks, also shows seasonality in some areas. Off Northwest Africa the largest expansion was recorded in the first half of the year, extending from 10° N to 24° N, more than 400 km offshore and at least down to 200 m. It contracts in the second half of the year to an area between 20°N and 22°N, 100 to 200 km off the coast and in an average depth of 25 m. These zooplankton biomass patterns are superimposed by mesoscale phenomena, originated by other than Ekman upwelling events. Those are, for example, long coastal parallel waves, producing cells of intensified upwelling and downwelling, and eddies, caused by instabilities in a frontal zone parallel to the coast. Different water masses can be distinguished by indicator species, species combinations or the significant absence of species. This was demonstrated for chaetognaths. The calanoidCalanus helgolandicus (Claus, 1863), a typical species of the North Atlantic, indicates North Atlantic Central Water, whereasCalanoides carinatus (Krøyer, 1849) is an indicator of South Atlantic Central Water. Finally, comparisons of near coastal current regimes, transport velocities, and developmental rates of calanoids allow one to conclude that a suitable mechanism is present to maintain plankton in the coastal environment.

Rainfall over the Arabian Sea during the onset of the 1979 monsoon

One path to understanding the summer monsoon of south-east Asia lies through its regimes of rain. There the main scales of interest range, on the small side, down at least to component rainstorms: that is, extending over hundreds of kilometres and lasting about a day. On the large side, interest might be extended to include the whole quasi-planetary monsoon system, although here, for practical reasons, the limits are taken to be 1 week and several thousands of kilometres. The main branch of the summer monsoon forms over the Arabian Sea. The rainfall of this sea, roughly 3,000 km in its dimensions, is known to us from about 40 coastal stations, mostly in India, plus six instrumented islands and the reports of seagoing ships. The ships stay mainly in a few lanes and in any case only record the frequency of rain, not the amount. Thus, even taken all together, these reports are hardly adequate for building a picture of the monsoon rains. We report here a new technique for inferring daily rainfall from visible or IR images obtained by a geostationary satellite, applied in a simplified form to the Arabian Sea during the onset of the 1979 summer monsoon. The rain area estimated increased spectacularly (from 1 to 14%) over a 1-week period coincident with the appearance of the Somali jet and development of the onset vortex.

Ammonia gas concentrations over the Southern Ocean

Ammonia is the only common, soluble, basic gas in the atmosphere and as such plays an important neutralising, sometimes rate-determining, role in the chemistry of acid gases such as SO2, H2SO4 and HNO3. It also constitutes a reservoir of labile nitrogen in the atmosphere and so must be included in any attempt to describe the cycling of nitrogen through the global environment. We report here values of ammonia concentration measured in Southern Ocean air which are considerably lower than any previously reported1–5. For our analyses ammonia was collected on oxalic acid-impregnated filters and determined colormetrically. In air apparently devoid of contact with land for several thousands of kilometres of travel the mean concentration of ammonia gas was 0.06 μg m−3 (STP). Concentrations in air apparently affected by the Australian continent were several times higher than in the maritime air.