Fluid composition or bottom currents: What drives extremes in behavior
of plumes at ASHES vent field, Axial Volcano?
Abstract
The Cabled Observatory Vent Imaging Sonar, otherwise known as COVIS, has
acquired several months of plume centerlines and strengths for several
vents in the ASHES vent field of Axial Seamount. COVIS was initially
installed at ASHES vent field in July 2018 and acquired imaging data
throughout July-September 2018 and since July 2019. COVIS uses acoustic
imaging to monitor the strength and behavior of the plumes formed above
black smokers and diffuse discharge sites in an approximately 40 m by 40
m region which includes Inferno and Mushroom vents. Preliminary
observations suggest that the plumes above Inferno are highly variable;
sometimes a distinct rising column is seen to expand with height while
other times there is little acoustic evidence for a plume at all.
Potential explanations range from variable discharge rates to variable
discharge salinity driving collapsing plumes to extremes in bottom
currents. The obvious simple explanation of bent plumes produced by
extreme bottom currents is unsatisfactory as such bent (even horizontal)
plumes should be visible in the acoustic imaging data. Initial
explorations of the impact of near seawater salinity variations suggest
this is a plausible explanation for variations in plume maximum height
independent of heat content. However, the paucity of recent or
continuous salinity and temperature sampling on Inferno limits the
certainty of interpretations suggesting variations in venting. In
contrast to the variable plumes, the sulfide mounds of the region
(Inferno, Mushroom, Hell and Phoenix) appear as consistent (stable)
silhouettes in the acoustic images. On Inferno, we can even see
indications of the thin chimneys on top of the mound from which primary
venting occurs. Preliminary work is focused on refining the
classification of acoustic returns between rock, sulfide, and water to
see if we can track the growth (and collapse) of the actively venting
chimneys.