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Browsing by Author "Mohn, Christian"

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  • Ocean currents and acoustic backscatter data from shipboard ADCP measurements at three North Atlantic seamounts between 2004 and 2015
    Publication . Mohn, Christian; Denda, Anneke; Christiansen, Svenja; Kaufmann, Manfred; Peine, Florian; Springer, Barbara; Turnewitsch, Robert; Christiansen, Bernd
    Seamounts are amongst the most common physiographic struc tures of the deep-ocean landscape, but remoteness and geographic complexity have limited the systematic collection of integrated and multidisciplinary data in the past. Consequently, important aspects of seamount ecology and dynamics remain poorly studied. We present a data collection of ocean currents and raw acoustic backscatter from shipboard Acoustic Doppler Current Profiler (ADCP) measurements during six cruises between 2004 and 2015 in the tropical and subtropical Northeast Atlantic to narrow this gap. Measurements were conducted at seamount locations between the island of Madeira and the Portuguese mainland (Ampère, Seine Seamount), as well as east of the Cape Verde archipelago (Senghor Seamount). The dataset includes two-minute ensemble averaged continuous velocity and backscatter profiles, supplemented by spatially gridded maps for each velocity com ponent, error velocity and local bathymetry. The dataset is freely available from the digital data library PANGAEA at https://doi.pan gaea.de/10.1594/PANGAEA.883193.
    2018Journal article Open access Show more
  • The importance of tides for sediment dynamics in the deep sea—Evidence from the particulate-matter tracer 234Th in deep-sea environments with different tidal forcing
    Publication . Peine, Florian; Turnewitsch, Robert; Mohn, Christian; Reichelt, Theresa; Springer, Barbara; Kaufmann, Manfred
    Key aspects of deep-ocean fluid dynamics such as basin-scale (residual) and tidal flow are believed to have changed over glacial/interglacial cycles, with potential relevance for climatic change. To constrain the mechanistic links, magnitudes and temporal succession of events analyses of sedimentary paleo-records are of great importance. Efforts have been underway for some time to reconstruct residual-flow patterns from sedimentary records. Attempts to reconstruct tidal flow characteristics from deep-sea sediment deposits, however, are at a very early stage and first require a better understanding of the reflection of modern tides in sediment dynamics. In this context internal (baroclinic) tides, which are formed by the surface (barotropic) tide interacting with seafloor obstacles, are believed to play a particularly important role. Here we compare two modern deep-sea environments with respect to the effect of tides on sediment dynamics. Both environments are influenced by kilometre-scale topographic features but with vastly different tidal forcing: (1) two sites in the Northeast Atlantic (NEA) being surrounded by, or located downstream of, fields of short seamounts (maximum barotropic tidal current velocities 5 cm s 1 ); and (2) a site next to the Anaximenes seamount in the Eastern Mediterranean (EMed) (maximum barotropic tidal current velocities 0.5 cm s 1 ). With respect to other key fluid-dynamical parameters both environments are very similar. Signals of sedimentary particle dynamics, as influenced by processes taking place in the bottom boundary layer, were traced by the vertical water-column distribution of radioactive disequilibria (daughter/parent activity ratiosa1) between the naturally occurring, short-lived (half-life: 24.1 d) particulate matter tracer 234Th relative to its very long-lived and non-particle-reactive parent nuclide 238U. Activity ratios of 234Th/238Uo1 in water samples collected near the seafloor indicate fast 234Th scavenging onto particles followed by fast settling of these particles from the sampled parcel of water and, therefore, imply active sediment resuspension and dynamics on time scales of up to several weeks. In the Northeast Atlantic study region tides (in particular internal tides) are very likely to locally push total current velocities near the seafloor across the critical current velocity threshold for sediment erosion or resuspension whereas in the Eastern Mediterranean the tides aremuch too weak for this to happen. This difference in tidal forcing is reflected in a difference of the frequency of the occurrence of radioactive disequilibria o1 between total 234Th and 238U: In the near-bottom water column of the Northeast Atlantic region 59% of samples had detectable 234Th/238U disequilibria whereas at the Eastern Mediterranean site this fraction was only 7% (including a few disequilibria 41). The results of this study, therefore, add to the evidence suggesting that tides in the deep sea of the open oceans are more important for sediment dynamics than previously thought. It is hypothesised that (a) tide/seamount interactions in the deep open ocean control the local distribution of erosivity proxies (e.g., distributions of sediment grain sizes, heavy minerals and particle-reactive radionuclides) in sedimentary deposits and (b) the aforementioned topographically controlled sedimentary imprints of (internal) tides are useful in the reconstruction of past changes of tidal forcing in the deep sea
    2009Journal article Open access Show more