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Internal tides and sediment dynamics in the deep sea: Evidence from radioactive 234Th/238U disequilibria
Stockholm University, Faculty of Science, Department of Meteorology.
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2008 (English)In: Deep-Sea Research. Part I: Oceanographic Research Papers, ISSN 0967-0637, Vol. 55, no 12, 1727-1747 p.Article in journal (Refereed) Published
Abstract [en]

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 234Th relative to its conservative (non-particle-reactive) and very long-lived parent nuclide 238U. 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. Between about 200 m and 1000 m above the seafloor at the sampling sites recurrent ‘fossil’ disequilibria were detected. ‘Fossil’ disequilibria are defined by clearly detectable 234Th/238U disequilibria (total 234Th radioactivity < 238U radioactivity, indicating a history of intense particulate 234Th scavenging and particulate-matter settling from the sampled parcel of water) and conspicuously low particle-associated 234Th activities. ‘Fossil’ disequilibria were centered around levels in the water column which correspond to the average height of the short seamounts near the sampling sites. This suggests the ‘fossil’ disequilibria are formed on the seamount slopes. Moreover, the magnitude of the ‘fossil’ disequilibria suggests that the slopes of the short seamounts in the study region are characterized by particularly vigorous fluid dynamics. Since ‘fossil’ disequilibria already occurred at ~ O(1 – 10 km) away from the seamount slopes it is likely that these vigorous fluid dynamics rapidly decay away from the slopes on scales of O(1 ? 10 km). These conclusions are supported by the horizontal distribution and magnitude of the modeled total (barotropic + baroclinic) tidal current velocities of the predominating tidal M2 constituent: On (near?)critical seamount slopes baroclinic tides lead to localized [~ O(1 km)] increases of the overall tidal current velocity by a factor of ~ 2, thereby pushing the total current velocity well above the threshold for sediment erosion. The results of this and a previous study (Turnewitsch et al. 2004, Earth and Planetary Science Letters 222, 1023-1036) show that kilometer-scale flow / topography interactions leave a marine geochemical imprint which may help to develop sediment proxies for the reconstruction of past changes of fluid dynamics in the deep sea, particularly the internal wave field. Sites with (near?)critical seafloor slopes are the most promising sites to study paleo-changes of internal tides. To reconstruct the paleo-flow of residual currents and barotropic tides, sediments covering isolated topography of comparable breadth and width, and with sub-critical slopes, seem to be more appropriate. For the sediment-based reconstruction of paleo-parameters other than physical oceanographic ones kilometer-scale topography should be avoided altogether.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 234Th relative to its conservative (non-particle-reactive) and very long-lived parent nuclide 238U. 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. Between about 200 m and 1000 m above the seafloor at the sampling sites recurrent ‘fossil’ disequilibria were detected. ‘Fossil’ disequilibria are defined by clearly detectable 234Th/238U disequilibria (total 234Th radioactivity < 238U radioactivity, indicating a history of intense particulate 234Th scavenging and particulate-matter settling from the sampled parcel of water) and conspicuously low particle-associated 234Th activities. ‘Fossil’ disequilibria were centered around levels in the water column which correspond to the average height of the short seamounts near the sampling sites. This suggests the ‘fossil’ disequilibria are formed on the seamount slopes. Moreover, the magnitude of the ‘fossil’ disequilibria suggests that the slopes of the short seamounts in the study region are characterized by particularly vigorous fluid dynamics. Since ‘fossil’ disequilibria already occurred at ~ O(1 – 10 km) away from the seamount slopes it is likely that these vigorous fluid dynamics rapidly decay away from the slopes on scales of O(1 ? 10 km). These conclusions are supported by the horizontal distribution and magnitude of the modeled total (barotropic + baroclinic) tidal current velocities of the predominating tidal M2 constituent: On (near?)critical seamount slopes baroclinic tides lead to localized [~ O(1 km)] increases of the overall tidal current velocity by a factor of ~ 2, thereby pushing the total current velocity well above the threshold for sediment erosion. The results of this and a previous study (Turnewitsch et al. 2004, Earth and Planetary Science Letters 222, 1023-1036) show that kilometer-scale flow / topography interactions leave a marine geochemical imprint which may help to develop sediment proxies for the reconstruction of past changes of fluid dynamics in the deep sea, particularly the internal wave field. Sites with (near?)critical seafloor slopes are the most promising sites to study paleo-changes of internal tides. To reconstruct the paleo-flow of residual currents and barotropic tides, sediments covering isolated topography of comparable breadth and width, and with sub-critical slopes, seem to be more appropriate. For the sediment-based reconstruction of paleo-parameters other than physical oceanographic ones kilometer-scale topography should be avoided altogether.

Place, publisher, year, edition, pages
2008. Vol. 55, no 12, 1727-1747 p.
Keyword [en]
Thorium 234, particle dynamics, sediment synamics, topography, seamounts, internal tides
Identifiers
URN: urn:nbn:se:su:diva-16013DOI: doi:10.1016/j.dsr.2008.07.008ISI: 000261725600008OAI: oai:DiVA.org:su-16013DiVA: diva2:182533
Available from: 2009-01-14 Created: 2009-01-14 Last updated: 2011-01-10Bibliographically approved

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