At some submarine volcanoes, the influx and output of magma vary over time producing years-to-decades-long cycles of inflation and deflation, which in turn cause pronounced physical changes in the overlying oceanic crust and the hydrothermal circulation hosted within. Permeability within the oceanic crust exerts primary control on seafloor fluid circulation and hence has important influences on the heat and chemical exchange between the earth's lithosphere and oceanic hydrosphere, as well as surface and subsurface biological communities. Despite its importance, permeability is one of the most poorly constrained hydrologic properties for most of the mid-ocean ridge system. In this study, harmonic analysis of a high-resolution, long-term time series of effluent temperature measured at a high-temperature hydrothermal vent on Axial Seamount yields time-varying estimates of the effective permeability within the hydrothermal upflow zone. Comparing the records of permeability and volcanic deformation during and after the April-May 2015 eruption at Axial suggests a decrease in upflow-zone permeability during co-eruptive deflation and an increase in permeability during post-eruption re-inflation from July 2015 to June 2019. Modeling of the three-dimensional strain field suggests that the temporal variations in effective upflow-zone permeability can be explained by closing and opening of hydrothermal pathways that accompany crustal compression and extension in relation to the volcano's deformation cycle.

Previous work using multibeam sonar to map diffuse hydrothermal flows is extended to estimate the heat output of diffuse flows. In the first step toward inversion, temperature statistics are obtained from sonar data and compared to thermistor data in order to set the value of an empirical constant. Finally, a simple model is used to obtain heat-flux density from sonar-derived temperature statistics. The method is applied to data from the Cabled Observatory Vent Imaging Sonar (COVIS) deployed on the Ocean Observatories Initiative’s Regional Cabled Array at the ASHES vent field on Axial Seamount. Inversion results are presented as maps of heat-flux density in MW/m2 and as time series of heat-flux density averaged over COVIS’ field of view.

The Cabled Observatory Vent Imaging Sonar (COVIS) was initially installed on the Ocean Observatories Initiative’s Cabled Array (OOI-CA) observatory at ASHES hydrothermal vent field on Axial Seamount in July 2018. COVIS recorded the acoustic backscatter from the water-column plumes formed above hydrothermal sources and the seafloor within the sonar’s field-of-view until Oct 2018, when an instrument malfunction suspended regular data-collection procedures. In July 2019, COVIS was redeployed after repairs and has since been collecting data at full capacity. Here, we present a comprehensive analysis of the acoustic backscatter data recorded by COVIS along with the in-situ temperature measurements in 2018 and 2019. The results demonstrate significant influences of ocean tides and bottom currents on diffuse hydrothermal discharge within ASHES. In addition, comparison with local seismicity shows a positive correlation between diffuse hydrothermal venting and the seismic activity in the vicinity of the vent field, which provides evidence for an intimate connection between hydrothermal activity and geological processes during the dynamic period leading up to the next eruption of Axial Seamount. Overall, our results showcase the capabilities of underwater acoustic techniques as remote-sensing tools for long-term, quantitative monitoring of seafloor hydrothermal discharge.

The Cabled Observatory Vent Imaging Sonar (COVIS) was installed on the Ocean Observatories Initiative’s Cabled Array observatory at ASHES hydrothermal vent field on Axial Seamount in July 2018. The acoustic backscatter data recorded by COVIS, in conjunction with in-situ temperature measurements, are used to investigate the temporal and spatial variations of hydrothermal discharge within ASHES. Specifically, sonar data processing generates three-dimensional backscatter images of the buoyant plumes above major sulfide structures and two-dimensional maps of diffuse hydrothermal sources within COVIS’s field-of-view. The backscatter images show drastic changes of plume appearance and behavior that potentially reflect episodic variabilities of vent fluid composition and/or outflow fluxes. The diffuse-flow maps show that the areal extent of hydrothermal discharge on the seafloor varies significantly with time, which is largely driven by bottom currents and potentially tidal loading. These findings demonstrate COVIS’s ability to quantitatively monitor hydrothermal discharge with sufficient spatial and temporal coverage to provide the research community with key observational data for studying the linkage of hydrothermal activity with oceanic and geological processes during the dynamic period leading up to the next eruption of Axial Seamount.