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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
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Advisor(s)
Abstract(s)
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
Description
Keywords
Thorium-234/uranium-238 disequilibria Sediment dynamics Bottom boundary layer Topography Seamounts Internal tides Northeast Atlantic Eastern Mediterranean . Faculdade de Ciências da Vida
Pedagogical Context
Citation
Peine, F., Turnewitsch, R., Mohn, C., Reichelt, T., Springer, B., & Kaufmann, M. (2009). 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. Deep Sea Research Part I: Oceanographic Research Papers, 56(7), 1182-1202. https://doi.org/10.1016/j.dsr.2009.03.009
Publisher
Elsevier