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

Abstract

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 ratios≠1) 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/ 238U<1 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 are much 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 <1 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 >1). 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.