Systematic Shift in Plume Bending Direction at Grotto Vent, Main
Endeavour Field, Juan de Fuca Implies Systematic Change in Venting
Output along the Endeavour Segment
Abstract
Analysis of the time-dependent behavior of the buoyant plume rising
above Grotto Vent (Main Endeavour Field, Juan de Fuca Ridge) as imaged
by the Cabled Observatory Vent Imaging Sonar (COVIS) between September
2010 and October of 2015 captures long-term time-dependent changes in
the direction of background bottom currents independent of broader
oceanographic processes, indicating a systematic evolution in vent
output along the Endeavour Segment of the Juan de Fuca Ridge. The
behavior of buoyant plumes is a convolved expression of hydrothermal
flux from the seafloor and ocean bottom currents in the vicinity of the
hydrothermal vent. Plume behavior can be quantified by describing the
volume, velocity and orientation of the effluent relative to the
seafloor. Using three-dimensional acoustic images by the COVIS system,
we looked at the azimuth and inclination of the Grotto plume in 3 hour
intervals and identified a bimodal shift in their bending from NW and SW
to SE in 2010, 2011, and 2012 to single mode NW in 2013 and 2014.
Modeling of the distribution of azimuths for each year with a bimodal
Guassian indicates that the prominence of southward bottom currents
decreased systematically between 2010 and 2014. Spectral analysis of the
azimuthal data showed a strong semi-diurnal peak, a weak or missing
diurnal peak, and some energy in the sub-inertial and weather bands.
This suggests the dominant current generating processes are either not
periodic (such as the entrainment fields generated by the hydrothermal
plumes themselves) or are related to tidal processes. The surface wind
patterns in buoy data at 2 sites in the Northeast Pacific and the
incidence of sea-surface height changes related to mesoscale eddies show
little systematic change over this time period. Given the limited bottom
current data for the Main Endeavour Field and other parts of the
Endeavour segment, we hypothesize that changes in venting either within
the Main Endeavour Field or along the Endeavour Segment have resulted in
the changes in background currents. Previous numerical simulations
(Thomsen et al 2009) showed that background bottom currents were more
likely to be controlled by the local (segment-scale) venting than by
outside ocean circulation or atmospheric patterns.