D. R. Bohnenstiehl

Seismic identification of along-axis hydrothermal flow on the East Pacific Rise

Hydrothermal circulation at the axis of mid-ocean ridges affects the chemistry of the lithosphere and overlying ocean, supports chemosynthetic biological communities and is responsible for significant heat transfer from the lithosphere to the ocean. It is commonly thought that flow in these systems is oriented across the ridge axis, with recharge occurring along off-axis faults, but the structure and scale of hydrothermal systems are usually inferred from thermal and geochemical models constrained by the geophysical setting, rather than direct observations. The presence of microearthquakes may shed light on hydrothermal pathways by revealing zones of thermal cracking where cold sea water extracts heat from hot crustal rocks, as well as regions where magmatic and tectonic stresses create fractures that increase porosity and permeability. Here we show that hypocentres beneath a well-studied hydrothermal vent field on the East Pacific Rise cluster in a vertical pipe-like zone near a small axial discontinuity, and in a band that lies directly above the axial magma chamber. The location of the shallow pipe-like cluster relative to the distribution and temperature of hydrothermal vents along this section of the ridge suggests that hydrothermal recharge may be concentrated there as a consequence of the permeability generated by tectonic fracturing. Furthermore, we interpret the band of seismicity above the magma chamber as a zone of hydrothermal cracking, which suggests that hydrothermal circulation may be strongly aligned along the ridge axis. We conclude that models that suggest that hydrothermal cells are oriented across-axis, with diffuse off-axis recharge zones, may not apply to the fast-spreading East Pacific Rise.

Rapid dike emplacement leads to eruptions and hydrothermal plume release during seafloor spreading events

The creation of ocean crust by rapid injection of magma at mid-ocean ridges can lead to eruptions of lava onto the seafloor and release of “event plumes,” which are huge volumes of anomalously warm water enriched in reduced chemicals that rise up to 1 km above the seafloor. Here, we use seismic data to show that seafloor eruptions and the release of hydrothermal event plumes correspond to diking episodes with high injection velocities and rapid onset of magma emplacement within the rift zone. These attributes result from high excess magma pressure at the dike source, likely due to a new influx of melt from the mantle. These dynamic magmatic conditions can be detected remotely and may predict the likelihood of event plume release during future seafloor spreading events.

Long-term explosive degassing and debris flow activity at West Mata submarine volcano

West Mata is a 1200 m deep submarine volcano where explosive boninite eruptions were observed in 2009. The acoustic signatures from the volcanos summit eruptive vents Hades and Prometheus were recorded with an in situ (~25 m range) hydrophone during ROV dives in May 2009 and with local (~5 km range) moored hydrophones between December 2009 and August 2011. The sensors recorded low frequency (1–40 Hz), short duration explosions consistent with magma bubble bursts from Hades, and broadband, 1–5 min duration signals associated with episodes of fragmentation degassing from Prometheus. Long-term eruptive degassing signals, recorded through May 2010, preceded a several month period of declining activity. Degassing episodes were not recorded acoustically after early 2011, although quieter effusive eruption activity may have continued. Synchronous optical measurements of turbidity made between December 2009 and April 2010 indicate that turbidity maxima resulted from occasional south flank slope failures triggered by the collapse of accumulated debris during eruption intervals.

Hydroacoustic investigation of submarine landslides at West Mata volcano, Lau Basin

Submarine landslides are an important process in volcano growth yet are rarely observed and poorly understood. We show that landslides occur frequently in association with the eruption of West Mata volcano in the NE Lau Basin. These events are identifiable in hydroacoustic data recorded between ~5 and 20 km from the volcano and may be recognized in spectrograms by the weak and strong powers at specific frequencies generated by multipathing of sound waves. The summation of direct and surface-reflected arrivals causes interference patterns in the spectrum that change with time as the landslide propagates. Observed frequencies are consistent with propagation down the volcanos north flank in an area known to have experienced mass wasting in the past. These data allow us to estimate the distance traveled by West Mata landslides and show that they travel at average speeds of ~10–25 m/s.

The sounds of submarine volcanoes

When submarine volcanoes erupt, several processes can create sounds in the ocean, mostly at low frequencies <100 Hz. Explosions may occur directly in the water column, while earthquakes and other seismicity may produce seismic waves that convert into hydroacoustic waves or boundary (Scholte) waves. Volcanic sounds can propagate large distances through the SOFAR channel. During its 2014 eruption, Ahyi seamount, Northern Mariana Islands produced repetitive signals for approximately 2 weeks at a high rate. These likely explosions were widely recorded on seismometers throughout the region and on hydrophone arrays as far as Chile, ∼12,000 km distant. Bogoslof volcano, a shallow submarine volcano in the Aleutian Islands, Alaska, began erupting in December 2016. Many of the detected earthquakes associated with this eruption have large amplitude hydroacoustic phases, likely Scholte waves. A few earthquake swarms were recorded as converted hydroacoustic waves by seismometers on Tanaga volcano, ∼700 km away. A few ...

Contribution of iceberg sounds to the ambient noise budget in the South Pacific Ocean

On May 2002, C19, a 5500 km2 iceberg calved from the Ross Ice Shelf, eventually drifting eastward into the open Pacific Ocean by 2008. As it sailed into warmer waters, thermal and wind stresses caused the iceberg to crack and break apart. These resulting “icequakes” projected wideband acoustic energy into the water column, influencing the regional ambient noise environment. Icequake noise was persistent and strong enough to be observed by NOAA’s eastern equatorial Pacific moored hydrophone (EEP-NW at 8N, 110W) as well as the hydroacoustic station of International Monitoring System (IMS) on Juan Fernandez Island (H03N at 33.44S, 78.91W). Elevated noise levels (maximum of ~+3 dB at NOAA’s EEP and ~+7 dB at IMS H03N hydrophones) were observed by both stations from early 2008 when C19a first appeared in the Pacific until it drifted into the Atlantic Ocean in early 2009. C19a’s icequake and calving activity was also most frequent during this same period. Seasonal changes and long-term trends in ambient noise levels at NOAA’s EEP-NW acoustic mooring (1996–2009) and IMS Juan Fernandez (2003–2010 years) and the unique acoustic role icebergs play in the Southern Ocean will be presented.

Larval settlement in response to estuarine soundscapes

Ambient underwater sound has the potential to be an important orientation and settlement cue for marine invertebrate larvae, yet larval responses to relevant sound patterns are largely unknown. In estuaries of the Southeastern United States, oyster reefs are patchy productive habitats that harbor many soniferous fish and invertebrates, creating distinct sound characteristics. This habitat-related sound could provide a useful cue for the planktonic larvae of obligate reef dwellers and facilitate encounter with suitable settlement substrate. To investigate sound as a settlement cue in this system, larval settlement responses to oyster reef and soft-bottom sounds, as well as a no-sound control were tested for the Eastern oyster, Crassostrea virginica. Laboratory and field experiments suggest that sound has a significant effect on oyster settlement rates: higher numbers of larvae settled in the presence of oyster reef sounds than in soft-bottom sound or silent control treatments. Improved understanding of the...

Low-frequency acoustic observations with cabled observatories on the Juan de Fuca Ridge

In the Northeast Pacific Ocean, the NSF Ocean Observatories Initiative (OOI) and Ocean Networks Canada (ONC) operate regional cabled observatories that include a network of seismometers and hydrophones enclosing the northern half of the Juan de Fuca plate, and local observatories at two sites on the Juan de Fuca Ridge: Axial Seamount (OOI) and the Endeavour segment (ONC). At each ridge site, local networks of seismometers and hydrophones provide a tool to monitor in real time the seismic and acoustic signals associated with volcanic, tectonic, and hydrothermal processes. In 2015, Axial Seamount erupted a few months after the installation of the OOI. Following the eruption, regionally recorded T-phases were used to track a dike propagating along the Axial North Rift. Acoustic signals from impulsive sources associated with lava flows, detected eruptions in the caldera and on the North Rift. Diffuse acoustic signals were also associated with the later stages of the eruption. The low-frequency acoustic capabilities of the OOI and ONC cabled observatories are underutilized and future applications could include the reestablishment of regional acoustic earthquake monitoring, tracking eruptions in real time to guide rapid autonomous and ship-based response efforts, and enhanced studies of fin and blue whales.In the Northeast Pacific Ocean, the NSF Ocean Observatories Initiative (OOI) and Ocean Networks Canada (ONC) operate regional cabled observatories that include a network of seismometers and hydrophones enclosing the northern half of the Juan de Fuca plate, and local observatories at two sites on the Juan de Fuca Ridge: Axial Seamount (OOI) and the Endeavour segment (ONC). At each ridge site, local networks of seismometers and hydrophones provide a tool to monitor in real time the seismic and acoustic signals associated with volcanic, tectonic, and hydrothermal processes. In 2015, Axial Seamount erupted a few months after the installation of the OOI. Following the eruption, regionally recorded T-phases were used to track a dike propagating along the Axial North Rift. Acoustic signals from impulsive sources associated with lava flows, detected eruptions in the caldera and on the North Rift. Diffuse acoustic signals were also associated with the later stages of the eruption. The low-frequency acoustic capabi...

Explosive processes during the 2015 eruption of Axial Seamount, as recorded by seafloor hydrophones: EXPLOSIVE ACTIVITY AT AXIAL SEAMOUNT

Following the installation of the Ocean Observatories Initiative cabled array, the 2015 eruption of Axial Seamount, Juan de Fuca ridge, became the first submarine eruption to be captured in real time by seafloor seismic and acoustic instruments. This eruption also marked the first instance where the entire eruption cycle of a submarine volcano, from the previous eruption in 2011 to the end of the month-long 2015 event, was monitored continuously using autonomous ocean bottom hydrophones. Impulsive sounds associated with explosive lava-water interactions are identified within hydrophone records during both eruptions. Explosions within the caldera are acoustically distinguishable from those occurring in association with north rift lava flows erupting in 2015. Acoustic data also record a series of broadband diffuse events, occurring in the waning phase of the eruption, and are interpreted as submarine Hawaiian explosions. This transition from gas-poor to gas-rich eruptive activity coincides with an increase in water temperature within the caldera and with a decrease in the rate of deflation. The last recorded diffuse events coincide with the end of the eruption, represented by the onset of inflation. All the observed explosion signals couple strongly into the water column, and only weakly into the solid Earth, demonstrating the importance of hydroacoustic observations as a complement to seismic and geodetic studies of submarine eruptions. Plain Language Summary Axial Seamount, a submarine volcano on the Juan de Fuca ridge, erupted in 2015. This eruption was recorded in real-time by an array of seafloor seismometers and hydrophones located on the volcano, and connected to shore by a power and data cable. Hydrophones recording the sounds generated by the eruption reveal several different types of explosions, including short bursts interpreted as lava-water interactions, and prolonged signals thought to be due to explosive ejection of gas and ash. These signals provide a window into the dynamics of the undersea eruption and are an excellent complement to other types of data including earthquakes and ground deformation.