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

• Published in: Deep Sea Research Part I: Oceanographic Research Papers
• Main Authors: Peine, Florian, Turnewitsch, Robert, Mohn, Christian, Reichelt, Theresa, Springer, Barbara, Kaufmann, Manfred
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2009
• Subjects: Thorium-234/uranium-238 disequilibria; Sediment dynamics; Bottom boundary layer; Topography; Seamounts; Internal tides; Northeast Atlantic; Eastern Mediterranean; Faculdade de Ciências da Vida
• Online Access: http://hdl.handle.net/10400.13/3709; https://doi.org/10.1016/j.dsr.2009.03.009

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 ...