Internal tides and sediment dynamics in the deep sea - Evidence from radioactive 234Th/238U disequilibria

Residual flow, barotropic tides and internal (baroclinic) tides interact in a number of ways with kilometer-scale seafloor topography such as abyssal hills and seamounts. Because of their likely impact on vertical mixing such interactions are potentially important for ocean circulation and the mechanisms and the geometry of these interactions are a matter of ongoing studies. In addition, very little is known about how these interactions are reflected in the sedimentary record. This multi-year study investigates if flow/topography interactions are reflected in distributional patterns of the natural short-lived (half-life: 24.1 d) particulate-matter tracer Th-234 relative to its conservative (non-particle-reactive) and very long-lived parent nuclide U-238. The sampling sites were downstream of, or surrounded by, fields of short seamounts and, therefore, very likely to be influenced by nearby flow/topography interactions. At the sampling sites between about 200 and 1000m above the seafloor recurrent 'fossil' disequilibria were detected. 'Fossil' disequilibria are defined by clearly detectable Th-234/U-238 disequilibria (total Th-234 radioactivity