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  • 1
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    In:  [Talk] In: DFG SPP SAMPLE Colloquium 2011, 07.06.-10.06. 2011, Potsdam .
    Publication Date: 2013-01-21
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 2
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should ...
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  • 3
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Most models of melt generation beneath mid-ocean ridges predict significant reduction of melt production at ultraslow spreading rates (full spreading rates 〈20 mm yr-1) and consequently they predict thinned oceanic crust. The 1,800-km-long Arctic Gakkel mid-ocean ...
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  • 4
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    In:  [Poster] In: XVIII INQUA Congress, 21.-27.07.2011, Bern, Switzerland .
    Publication Date: 2012-02-23
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  • 5
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    In:  [Poster] In: EGU General Assembly 2011, 03.-08.04.2011, Vienna, Austria .
    Publication Date: 2012-02-23
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  • 6
    Publication Date: 2015-08-03
    Description: During the Pleistocene glaciations, Arctic ice sheets on western Eurasia, Greenland and North America terminated at their continental margins. In contrast, the exposed continental shelves in the Beringian region of Siberia are thought to have been covered by a tundra landscape. Evidence of grounded ice on seafloor ridges and plateaux off the coast of the Beringian margin, at depths of up to 1,000 m, have generally been attributed to ice shelves or giant icebergs that spread oceanwards during glacial maxima. Here we identify marine glaciogenic landforms visible in seismic profiles and detailed bathymetric maps along the East Siberian continental margin. We interpret these features, which occur in present water depths of up to 1,200 m, as traces from grounding events of ice sheets and ice shelves. We conclude that the Siberian Shelf edge and parts of the Arctic Ocean were covered by ice sheets of about 1 km in thickness during several Pleistocene glaciations before the most recent glacial period, which must have had a significant influence on albedo and oceanic and atmospheric circulation.
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  • 7
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    In:  [Talk] In: AGU 2015, 14. – 18.12.15, San Francisco, USA .
    Publication Date: 2016-01-08
    Description: Tristan da Cunha is a volcanic island in the South Atlantic located very close to the Mid-Atlantic Ridge. Generally, it is accepted to be the location of a mantle plume, which has been active at least since the breakup of Gondwana at 130 Ma, the time when the Paraná/Etendeka flood basalts were emplaced. Furthermore, it is associated with the formation of the Walvis Ridge and the Rio Grande Rise, and therefore it’s one of the key examples of a hot spot track linking a flood basalt province to an active ocean island volcano. However, global tomography models are contradicting about the origin of Tristan da Cunha: Whether it is a deep mantle plume or caused by shallow plate tectonics. To gain a better understanding, we deployed 24 broadband ocean-bottom seismometers, 26 ocean-bottom electromagnetic stations and 2 seismological land stations in January 2012 with the German research vessel Maria S. Merian. We acquired continuous seismological data for one year and recovered the instruments in January 2013. We use cross-correlated travel time residuals of teleseismic earthquakes to perform a finite-frequency tomography to resolve the P wave velocity upper mantle structure beneath the island. Here we show our preliminary results of the 3-D velocity perturbations in the upper mantle: We do not image a plume-like structure directly beneath the island. Instead we observe a low velocity region in the southwest of our array that might be related to a local mantle upwelling (mantle plume). Additionally we show the local seismicity in the Tristan da Cunha region. Chen et al. and Baba et al. will present the first results on the magnetotelluric experiment and Ryberg et al. will present the crustal structure around the Tristan da Cunha hotspot.
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  • 8
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    In:  [Talk] In: EGU General Assembly 2016, 17.-22.04.2016, Vienna, Austria .
    Publication Date: 2016-02-04
    Description: According to classical plume theory, the Tristan da Cunha hotspot is thought to have played a major role in the rifting of the South Atlantic margins and the creation of the aseismic Walvis Ridge during and after the breakup of the South Atlantic. Between February 2012 and January 2013 a network of 24 broadband ocean-bottom seismometers was in operation around the volcanic archipelago of Tristan da Cunha. Ambient noise data from the OBS and a seismic station on Nightingale Island were used to constrain the crustal and uppermost structure around the island. From the vertical and hydrophone recordings of more than 300 days we could reconstruct the ambient noise Green's functions by cross-correlation. The dispersion curves of Rayleigh/Scholte waves could be derived from the cross-correlations in the period range from 2 to 32 seconds. Group velocity maps were determined for each individual period using travel time tomography. These group velocity maps were converted to depth by dispersion curve inversion to construct a 3D S-wave velocity model of the crust and uppermost mantle in the region. This model shows a strong velocity anomaly beneath the Tristan da Cunha archipelago. The influence of the water depth on the inversion is discussed.
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  • 9
    Publication Date: 2016-07-19
    Description: A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and that hydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity is abundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, lies between abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observations demonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantle chemistry (or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests that first-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmatic activity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed.
    Type: Article , PeerReviewed
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  • 10
    ISSN: 1573-0581
    Keywords: passive continental margin ; continent-ocean transition ; crustal structure ; active ridge ; deep seismic sounding ; airgun ; explosives ; gravity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Results are presented from a deep seismic sounding experiment with the research vessel POLARSTERN in the Scoresby Sund area, East Greenland. For this continental margin study 9 seismic recording landstations were placed in Scoresby Sund and at the southeast end of Kong Oscars Fjord, and ocean bottom seismographs (OBS) were deployed at 26 positions in and out of Scoresby Sund offshore East Greenland between 70° and 72° N and on the west flank of the Kolbeinsey Ridge. The landstations were established using helicopters from RV POLARSTERN. Explosives, a 321 airgun and 81 airguns were used as seismic sources in the open sea. Gravity data were recorded in addition to the seismic measurements. A free-air gravity map is presented. The sea operations — shooting and OBS recording — were strongly influenced by varying ice conditions. Crustal structure 2-D models have been calculated from the deep seismic sounding results. Free-air gravity anomalies have been calculated from these models and compared to the observed gravity. In the inner Scoresby Sund — the Caledonian fold belt region — the crustal thickness is about 35 km, and thins seaward to 10 km. Sediments more than 10 km thick on Jameson Land are of mainly Mesozoic age. In the outer shelf region and deep sea a ‘Moho’ cannot clearly be identified by our data. There are only weak indications for the existence of a ‘Moho’ west of the Kolbeinsey Ridge. Inside and offshore Scoresby Sund there is clear evidence for a lower crust refractor characterised byp-velocities of 6.8–7.3 km s−1 at depths between 6 and 10 km. We believe these velocities are related to magmatic processes of rifting and first drifting controlled by different scale mantle updoming during Paleocene to Eocene and Late Oligocene to Miocene times: the separation of Greenland/Norway and the separation of the Jan Mayen Ridge/Greenland, respectively. A thin igneous upper crust, interpreted to be of oceanic origin, begins about 50 km seaward of the Liverpool Land Escarpment and thickens oceanward. In the escarpment zone the crustal composition is not clear. Probably it is stretched and attenuated continental crust interspersed with basaltic intrusions. The great depth of the basement (about 5000 m) points to a high subsidence rate of about 0.25 mm yr−1 due to sediment loading and cooling of the crust and upper mantle, mainly since Miocene time. The igneous upper crust thickens eastward under the Kolbeinsey Ridge to about 2.5 km; the thickening is likely caused by higher production of extrusives. The basementp-velocity of 5.8–6.0 km s−1 is rather high. Such velocities are associated with young basalts and may also be caused by a higher percentage of dykes. Tertiary to recent sediments, about 5000 m thick, form most of the shelf east of Scoresby Sund, Liverpool Land and Kong Oscars Fjord. This points to a high sedimentation rate mainly since the Miocene. The deeper sediments have a rather high meanp-velocity of 4.5 km s−1, perhaps due to pre-Cambrian to Caledonian deposits of continental origin. The upper sediments offshore Scoresby Sund are thick and have a rather low velocity. They are interpreted as eroded material transported from inside the Sund into the shelf region. Offshore Kong Oscars Fjord the upper sediments, likely Jurassic to Devonian deposits, are thin in the shelf region but thicken to more than 3000 m in the slope area. The crust and upper mantle structure in the ocean-continent transition zone is interpreted to be the result of the superposition of the activities of three rifting phases related to mantle plumes of different dimensions: 1. the ‘Greenland/Norway separation phase’ of high volcanic activity, 2. the ‘Jan Mayen Ridge/Greenland separation phase’ and 3. the ‘Kolbeinsey Ridge phase’ of ‘normal’ volcanic activity related to a more or less normal mantle temperature. During period 2 and 3 only a few masses of extrusives were produced, but large volumes of intrusives were emplaced. So the margin between Scoresby Sund and Jan Mayen Fracture Zone is interpreted to be a stretched margin with low volcanic activity.
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