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Bremerhaven, Germany

Geibert W.,University of Edinburgh | Assmy P.,Alfred Wegener Institute for Polar and Marine Research | Bakker D.C.E.,University of East Anglia | Hanfland C.,Alfred Wegener Institute for Polar and Marine Research | And 7 more authors.
Global Biogeochemical Cycles

The Southern Ocean (SO) plays a key role in modulating atmospheric CO 2 via physical and biological processes. However, over much of the SO, biological activity is iron-limited. New in situ data from the Antarctic zone south of Africa in a region centered at ∼20°E-25°E reveal a previously overlooked region of high primary production, comparable in size to the northwest African upwelling region. Here, sea ice together with enclosed icebergs is channeled by prevailing winds to the eastern boundary of the Weddell Gyre, where a sharp transition to warmer waters causes melting. This cumulative melting provides a steady source of iron, fuelling an intense phytoplankton bloom that is not fully captured by monthly satellite production estimates. These findings imply that future changes in sea-ice cover and dynamics could have a significant effect on carbon sequestration in the SO. Copyright 2010 by the American Geophysical Union. Source

Laderach C.,Alfred Wegener Institute for Polar and Marine Research | Korger E.I.M.,Alfred Wegener Institute for Polar and Marine Research | Schlindwein V.,Alfred Wegener Institute for Polar and Marine Research | Muller C.,Alfred Wegener Institute for Polar and Marine Research | And 2 more authors.
Geophysical Journal International

The ultraslow spreading Southwest Indian Ridge (SWIR) is a prominent end-member of the global mid-ocean ridge system. It spreads with a full-rate of 14-16mmy -1 and shows several segments of various obliquities. The western SWIR consists of the Oblique and Orthogonal Supersegments lying at an epicentral distance of ~21° to the VNA2 seismic array operated by the German Neumayer station in East Antarctica. The array monitors backazimuth, apparent velocity and signal-to-noise ratio of arriving waves and provides a data set of seismicity from the western SWIR over several years. Compared to the global seismological network, its detection threshold for earthquakes occurring at the western SWIR is more than 0.5 m b lower enabling a more comprehensive study of mid-ocean ridge processes than the teleseismic earthquake catalogues. We identified a total number of 743 earthquakes occurring at the western part of the SWIR and calculated the body-wave magnitudes (m b) from P-wave amplitude picks on the VNA2 broad-band sensor obtaining a magnitude range from m b 3.18 to m b 5.34. In the years of 2001, 2004, 2005 and 2008, significantly increased event rates indicated four earthquake swarms with up to 164 events lasting for several days. All swarms had strong events registered in the International Seismological Centre catalogue. The relocalization of these events confirmed that all swarms occurred in the same region on the Orthogonal Supersegment. We analysed event and moment release rate histories, b-values and aftershock decay rates (Modified Omori Law) finding that the swarms of 2001, 2004 and 2005 have similarities in the temporal distribution of seismic moment and event numbers. The swarm of 2008 is smaller with high magnitude events at the swarm's onset which represent shear failure on normal faults. The application of the Modified Omori Law and the b-value show that the earthquakes of the swarms do not follow the classical main shock-aftershock pattern of purely tectonic earthquake sequences. At the Orthogonal Supersegment, a continuous positive magnetic anomaly along the rift axis, a negative mantle Bouguer anomaly, basalts at the seafloor and potentially volcanic edifices indicate robust magmatic crustal accretion. The high-resolution bathymetry shows ubiquitous rift-parallel ridges with steep flanks towards the rift axis indicating high-angle normal faults. The high-magnitude earthquakes detected teleseismically during the swarms are generated there. We interpret that the swarms are caused by magmatic accretion episodes at a suggested volcanic centre of the Orthogonal Supersegment and that possible magma injection activates the steeply dipping fault planes. © 2012 Alfred Wegener Institute for Polar and Marine Research Geophysical Journal International © 2012 RAS. Source

Lampert A.,Alfred Wegener Institute for Polar and Marine Research | Lampert A.,TU Braunschweig | Maturilli M.,Alfred Wegener Institute for Polar and Marine Research | Ritter C.,Alfred Wegener Institute for Polar and Marine Research | And 14 more authors.

The Arctic atmospheric boundary layer (AABL) in the central Arctic was characterized by dropsonde, lidar, ice thickness and airborne in situ measurements during the international Polar Airborne Measurements and Arctic Regional Climate Model Simulation Project (PAMARCMiP) in April 2009. We discuss AABL observations in the lowermost 500 m above (A) open water, (B) sea ice with many open/refrozen leads (C) sea ice with few leads, and (D) closed sea ice with a front modifying the AABL. Above water, the AABL had near-neutral stratification and contained a high water vapor concentration. Above sea ice, a low AABL top, low near-surface temperatures, strong surface-based temperature inversions and an increase of moisture with altitude were observed. AABL properties and particle concentrations were modified by a frontal system, allowing vertical mixing with the free atmosphere. Above areas with many leads, the potential temperature decreased with height in the lowest 50 m and was nearly constant above, up to an altitude of 100-200 m, indicating vertical mixing. The increase of the backscatter coefficient towards the surface was high. Above sea ice with few refrozen leads, the stably stratified boundary layer extended up to 200-300 m altitude. It was characterized by low specific humidity and a smaller increase of the backscatter coefficient towards the surface. © 2012 by the authors. Source

Gutt J.,Alfred Wegener Institute for Polar and Marine Research | Barratt I.,Queens University of Belfast | Domack E.,Hamilton College | d'Udekem d'Acoz C.,Royal Belgian Institute Of Natural Sciences | And 27 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography

The marine ecosystem on the eastern shelf of the Antarctic Peninsula was surveyed 5 and 12 years after the climate-induced collapse of the Larsen A and B ice shelves. An impoverished benthic fauna was discovered, that included deep-sea species presumed to be remnants from ice-covered conditions. The current structure of various ecosystem components appears to result from extremely different response rates to the change from an oligotrophic sub-ice-shelf ecosystem to a productive shelf ecosystem. Meiobenthic communities remained impoverished only inside the embayments. On local scales, macro- and mega-epibenthic diversity was generally low, with pioneer species and typical Antarctic megabenthic shelf species interspersed. Antarctic Minke whales and seals utilised the Larsen A/B area to feed on presumably newly established krill and pelagic fish biomass. Ecosystem impacts also extended well beyond the zone of ice-shelf collapse, with areas of high benthic disturbance resulting from scour by icebergs discharged from the Larsen embayments. © 2010 Elsevier Ltd. Source

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