Abstract:
The water column imprint of the hydrothermal plume observed at the Nibelungen field (8°18' S 13°30' W) is highly variable in space and time. The off-axis location of the site, along the southern boundary of a non-transform ridge offset at the joint between two segments of the southern Mid-Atlantic
Ridge, is characterized by complex, rugged topography, and thus favorable
for the generation of internal tides, subsequent internal wave breaking, and
associated vertical mixing in the water column. We have used towed transects
and vertical profiles of stratification, turbidity, and direct current measurements
to investigate the strength of turbulent mixing in the vicinity of
the vent site and the adjacent rift valley, and its temporal and spatial variability
in relation to the plume dispersal. Turbulent diffusivities Kp were calculated from temperature inversions via Thorpe scales. Heightened mixing
(compared to open ocean values) was observed in the whole rift valley
within an order of Kp around 10-3 m2 s-1. The mixing close to the vent
site was even more elevated, with an average of Kp = 4 x 10-2 m2 s-1. The
mixing, as well as the flow field, exhibited a strong tidal cycle, with strong
currents and mixing at the non-buoyant plume level during ebb
flow. Periods
of strong mixing were associated with increased internal wave activity and
frequent occurrence of turbulent overturns. Additional effects of mixing on
plume dispersal include bifurcation of the particle plume, likely as a result
of the interplay between the modulated mixing strength and current speed,
as well as high frequency internal waves in the effluent plume layer, possibly
triggered by the buoyant plume via nonlinear interaction with the elevated
background turbulence or penetrative convection.
Description:
Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 57 (2010): 931-945, doi:10.1016/j.dsr.2010.04.010.
