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Oceanic density fronts steering bottom-current induced sedimentation deduced from a 50 ka contourite-drift record and numerical modeling (off NW Spain)
Hanebuth, T.J.J.; Zhang, W.; Hofmann, A.L.; Löwemark, L.A.; Schwenk, T. (2015). Oceanic density fronts steering bottom-current induced sedimentation deduced from a 50 ka contourite-drift record and numerical modeling (off NW Spain). Quat. Sci. Rev. 112: 207-225.
In: Quaternary Science Reviews. Pergamon Press: Oxford; New York. ISSN 0277-3791; e-ISSN 1873-457X, more
Peer reviewed article  

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Author keywords
    Contouritic deposition; Deep-sea bottom currents; Eastern Atlantic Ocean; Late Quaternary

Authors  Top | Dataset 
  • Hanebuth, T.J.J.
  • Zhang, W.
  • Hofmann, A.L.
  • Löwemark, L.A.
  • Schwenk, T.

    How the various bottom-near hydrographic and sedimentary processes control the formation of contourite drifts, i.e. of bottom-current related confined deep-sea depocenters, usually remains widely speculative. This study uses a transect of six sediment cores and a sediment echosounder profile across a whole contourite system off NW Spain to address the sediment dynamics responsible for the depositional pattern. This “mounded patch”-type contourite drift (18 km long, 20 km wide) with a 150-m deep channel (moat) has formed around an 800-m high structural obstacle. Deposition on the contourite drift in the past was characterized by alternating calm and high-energy bottom-flow conditions. Calm conditions (Last Glacial period: 27–17 cal ka BP; late Holocene times: <4 cal ka BP) led to slightly current-influenced deposition of fine-grained sediments (10 μm) in the entire basin. This regime was interrupted by periods of short-lasting waxing-and-waning high-energy conditions (D/O events during Marine Isotope Stage 3; the Deglacial/early Holocene time interval at 17–4 cal ka BP), resulting in the deposition of coarse sediments (70 μm). Process-based numerical modeling demonstrates that pulse-like oceanic density fronts traveling within the transition zone of two water masses (Labrador Sea Water, Mediterranean Outflow Water) provide a powerful mechanism for contouritic deposition, rather than the core of a water mass itself. These gravity-driven density fronts lead to local re-suspension of sands stored inside the drift's moat and to subsequent upward transport towards the drift's crest. Here, the oceanic density fronts produce additional km-scale eddies. These migrating eddies provide an efficient mechanism for further widespread sediment re-distribution. In comparison with paleoceanographic reconstructions, a downward migration or expansion of the Mediterranean Outflow Water by about 300 m led most probably to such temporary contouritic sand deposition. We finally propose a conceptual model to explain how seafloor obstacles redirect and perturbate bottom currents. This model proposes not only a sharp contact between two water masses but also the transition zone between those as an important high-energy regime, offering oceanic density fronts a travel medium. These fronts are strong enough to distribute fine sands across highly pronounced seabed topography. On the respective time scale, the moat itself seems to act as the main source for those sands, making a remote source and a long-distance sediment transport unnecessary.

  • Global contourite distribution database, version 3, more

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