Methane gas plumes have been discovered to issue from the seafloor in the Puget Sound estuary. These gas emission sites are co‐located over traces of three major fault zones that fracture the entire ...forearc crust of the Cascadia Subduction Zone. Multibeam and single‐beam sonar data from cruises conducted in years 2011, 2018, 2019, 2020, and 2021 identified the acoustic signature of 349 individual bubble plumes. Dissolved CH4 gas from the plumes combines to elevate seawater methane concentrations of the entire Puget Sound estuary. Fluid samples from adjacent terrestrial hot springs and deep‐water wells surrounding the estuary contain a helium‐3 isotope signature, suggesting a deep fluid source located near the underlying Cascadia Subduction Zone plate interface. However, limited data from this pilot study suggest that Puget Sound seawater emission sites lack both similar chemical isotope signatures and elevated thermal anomalies that would be expected from association with a deep plate‐interface reservoir. A shallow reservoir within the Holocene sediments that cover the older Puget Sound basement with horizontal transfer to the thinly covered Alki Point and Kingston Arch anticlines is also a possibility, as has been suggested for other methane seep areas. The existence of vigorous marine methane plumes arising from areas of thin sediment cover associated with deeply penetrating forearc fault zones but presenting no thermal or chemical anomalies found in other similar forearc environments, remains an unresolved paradox.
Plain Language Summary
Puget Sound is a large inland sea located in western Washington State where seawater circulation is dominated by vigorous tidal forcing from the North Pacific Ocean. The deep Puget Sound is the largest estuary in North America measured by contained water volume and the second largest estuary after Chesapeake Bay in terms of area. Shipboard sonar images have revealed approximately 349 bubble plumes of methane gas and fluid rising from the seafloor of the estuary. Large clusters of bubble plume sites are concentrated over the major regional fault zones that penetrate the western North American plate beneath Puget Sound, including the South Whidbey Island Fault, the Seattle Fault, and the Tacoma Fault Zones. Although the forearc Puget Basin is surrounded by terrestrial hot springs and water wells that show a clear chemical isotope signature of fluid arising from the underlying Cascadia Subduction Zone plate interface, based on our limited sampling there is currently no evidence for similar chemical or temperature anomalies in the Puget Sound plumes and the source of the methane bubble plumes is still unidentified.
Key Points
Extensive methane bubble plumes have been discovered on the Puget Sound seafloor
The emission sites of these plumes are associated with major fault zones that penetrate the Cascadia forearc
Dissolved methane arising from the plumes is mixed throughout the estuary by tides and local mixing
Ahyi seamount, a shallow submarine volcano in the Northern Mariana Islands, began erupting on 23 April 2014. Hydroacoustic eruption signals were observed on the regional Mariana seismic network and ...on distant hydrophones, and National Oceanic and Atmospheric Administration (NOAA) scuba divers working in the area soon after the eruption began heard and felt underwater explosion sounds. The NOAA crew observed yellow‐orange bubble mats along the shore of neighboring Farallon de Pájaros Island, but no other surface manifestations of the eruption were reported by the crew or observed in satellite data. Here, we detail the eruption chronology and its morphologic impacts through analysis of seismic and hydroacoustic recordings and repeat bathymetric mapping. Throughout the 2‐week‐long eruption, Ahyi produced several thousand short, impulsive hydroacoustic signals that we interpret as underwater explosions as well as tremor near the beginning and end of the sequence. The initial tremor, which occurred for 2 hr, is interpreted as small phreatomagmatic explosions. This tremor was followed by a 90‐min pause before the characteristic impulsive signals began. Occasional tremor (lasting up to a few minutes) during the last 1.5 days of the eruption is interpreted as more sustained eruptive activity. Bathymetric changes show that a new crater, about 150 m deep, formed near the former summit and a large landslide chute formed on the southeastern flank. Comparing to other geophysically detected submarine eruptions, we find that the signals from the 2014 Ahyi eruption were more similar to those from other shallow or at‐surface submarine eruptions than those at deep (>500 m) eruptions.
Plain Language Summary
Ahyi seamount, a shallow submarine volcano in the Commonwealth of the Northern Mariana Islands (CNMI), began erupting on 23 April 2014. The U.S. Geological Survey first noticed signs of the eruption during a routine data check on 24 April, while National Oceanic and Atmospheric Administration scuba divers working in the area heard and felt underwater explosion sounds. We analyze recordings of the eruption on the CNMI seismic network and on hydrophones located near Wake Island to detail how the eruption unfolded. The eruption started with about 2 hr of tremor from magma explosively interacting with water. After a 90‐min pause, short (up to a few seconds) explosions began and continued for 2 weeks. During the last 1.5 days of the eruption, longer tremor signals (up to a few minutes) from more sustained degassing eruptions occurred along with the short explosions. A comparison of bathymetric maps made before and after the eruption shows that the explosions formed a new crater 150 m deep near the summit and that a landslide chute formed on the southeastern flank. The seismic and hydroacoustic signals from the Ahyi eruption are more similar to those from eruptions at other shallow or at‐surface seamounts than to those from deep (>500 m) eruptions.
Key Points
The submarine volcano Ahyi erupted for 2 weeks in April–May 2014 and was recorded by regional seismometers and distant hydrophones
The eruption was characterized by several thousand explosions and occasional tremor at the beginning and end of the eruptive period
Repeat bathymetry reveals a new summit crater and a new, large landslide chute on the south flank
As previously summarized by Hammond et al. (2015), from 1983 to 2013, the NOAA Vents program conducted systematic and multidisciplinary exploration, discovery, and research related to hydrothermal ...vents, submarine volcanic eruptions, and associated ocean physical, chemical, and biological processes. In 2014, Vents divided into two programs, Earth-Ocean Interactions (EOI) and Acoustics, and considered a broader range of questions about how seafloor and subseafloor processes contribute to ocean health, biogeochemical cycles, ecosystem diversity, and climate change. Here, we highlight major accomplishments since 2014, including deep-sea technologies that EOI, Vents, and Pacific Marine Environmental Laboratory (PMEL) Engineering have developed to advance marine science. EOI research is driven by a need for better observational data on issues of global importance, including the role of continental margin seeps in the global methane/carbon cycle, benthic ecology, and fisheries habitat; the role of hydrothermal systems in global biogeochemical cycles, including carbon dioxide removal; the potential impact of deep-sea mining of metal sulfides on ecosystem services provided by hydrothermal vents; and how hydrothermal iron functions as an essential nutrient. NOAA Ocean Exploration, the Schmidt Ocean Institute, the Ocean Exploration Trust, and the National Science Foundation have supported and collaborated in this work. Global exploration of the deep sea with the purpose of understanding global ocean processes remains a cornerstone of EOI science.
During E/V Nautilus NA072 expedition, multibeam sonar surveys located over 800 individual bubble streams rising from the Cascadia Margin between the Strait of Juan de Fuca and Cape Mendocino at ...depths between 104 and 2,073 m. Gas bubbles were collected directly at the seafloor using gastight sampling bottles. These bubbles were consistently composed of over 99% methane with traces of carbon dioxide, oxygen, nitrogen, noble gases, and more rarely higher hydrocarbons. A common previous view was that a biogenic source was responsible for seeps from within the gas hydrate stability zone (upper limit near 500‐m isobath) and a thermogenic source was responsible for seeps from the upper slope and the shelf. Higher hydrocarbons in deep seeps with a biogenic methane signature, as well as the lack of higher hydrocarbons in some shallower seeps with a thermogenic methane signature, show that the origin of the gas cannot simply be attributed to seep location on the margin. Instead, mixing and oxidation processes play an integral role. 3He/4He ratios at Coquille SW point to a contribution of 30% mantle helium, whereas all the other investigated sites are characterized by a crustal helium signature. Hence, the Coquille SW seeps are directly or indirectly connected to the mantle or to very young oceanic crust. The detection of mantle helium in these seeps can thus be used as a tracer for deep‐reaching fracture systems and their changing pathways.
Key Points
Mantle‐derived helium detected in cold methane seeps at the Cascadia Margin can be used as tracer for deep fracture systems
Multiple methane sources as well as mixing and oxidation processes are present at the Cascadia Margin cold seeps
The Cascadia Margin seeps are unequivocally dominated by methane
West Mata is a submarine volcano located in the SW Pacific Ocean between Fiji and Samoa in the NE Lau Basin. West Mata was discovered to be actively erupting at its summit in September 2008 and May ...2009. Water-column chemistry and hydrophone data suggest it was probably continuously active until early 2011. Subsequent repeated bathymetric surveys of West Mata have shown that it changed to a style of frequent but intermittent eruptions away from the summit since then. We present new data from ship-based bathymetric surveys, high-resolution bathymetry from an autonomous underwater vehicle, and observations from remotely operated vehicle dives that document four additional eruptions between 2012-2018. Three of those eruptions occurred between September 2012 and March 2016; one near the summit on the upper ENE rift, a second on the NE flank away from any rift zone, and a third at the NE base of the volcano. The latter intruded a sill into a basin with thick sediments, uplifted them, and then extruded lava onto the seafloor around them. The most recent of the four eruptions occurred between March 2016 and November 2017 along the middle ENE rift zone and produced pillow lava flows with a shingled morphology and tephra as well as clastic debris that mantled the SE slope. ROV dive observations show that the shallower recent eruptions at West Mata include a substantial pyroclastic component, based on thick (>1m) tephra deposits near eruptive vents. The deepest eruption sites lack these near-vent tephra deposits, suggesting that pyroclastic activity is minimal below ~2500 mbsl. The multibeam sonar re-surveys constrain the timing, thickness, area, morphology, and volume of the new eruptions. The cumulative erupted volume since 1996 suggests that eruptions at West Mata are volume-predictable with an average eruption rate of 7.8 x 106 m3/yr. This relatively low magma supply rate and the high frequency of eruptions (every 1-2 years) suggests that the magma reservoir at West Mata is relatively small. With its frequent activity, West Mata continues to be an ideal natural laboratory for the study of submarine volcanic eruptions.
Back‐arc spreading centers (BASCs) form a distinct class of ocean spreading ridges distinguished by steep along‐axis gradients in spreading rate and by additional magma supplied through subduction. ...These characteristics can affect the population and distribution of hydrothermal activity on BASCs compared to mid‐ocean ridges (MORs). To investigate this hypothesis, we comprehensively explored 600 km of the southern half of the Mariana BASC. We used water column mapping and seafloor imaging to identify 19 active vent sites, an increase of 13 over the current listing in the InterRidge Database (IRDB), on the bathymetric highs of 7 of the 11 segments. We identified both high and low (i.e., characterized by a weak or negligible particle plume) temperature discharge occurring on segment types spanning dominantly magmatic to dominantly tectonic. Active sites are concentrated on the two southernmost segments, where distance to the adjacent arc is shortest (<40 km), spreading rate is highest (>48 mm/yr), and tectonic extension is pervasive. Re‐examination of hydrothermal data from other BASCs supports the generalization that hydrothermal site density increases on segments <90 km from an adjacent arc. Although exploration quality varies greatly among BASCs, present data suggest that, for a given spreading rate, the mean spatial density of hydrothermal activity varies little between MORs and BASCs. The present global database, however, may be misleading. On both BASCs and MORs, the spatial density of hydrothermal sites mapped by high‐quality water‐column surveys is 2–7 times greater than predicted by the existing IRDB trend of site density versus spreading rate.
Key Points
Exploration of 600 km of the southern Mariana back arc found 19 active vent sites, 13 more than in the authoritative InterRidge Database
Back‐arc ridges with high‐quality hydrothermal surveys and an adjacent volcanic arc exhibit the highest spatial density of vent sites
Vent site spatial density is highest where arc proximity (<∼90 km) results in enhanced magma supply to the back‐arc ridge
NW Rota‐1 is a submarine volcano in the Mariana volcanic arc located ∼100 km north of Guam. Underwater explosive eruptions driven by magmatic gases were first witnessed there in 2004 and continued ...until at least 2010. During a March 2010 expedition, visual observations documented continuous but variable eruptive activity at multiple vents at ∼560 m depth. Some vents released CO2 bubbles passively and continuously, while others released CO2 during stronger but intermittent explosive bursts. Plumes of CO2 bubbles in the water column over the volcano were imaged by an EM122 (12 kHz) multibeam sonar system. Throughout the 2010 expedition numerous passes were made over the eruptive vents with the ship to document the temporal variability of the bubble plumes and relate them to the eruptive activity on the seafloor, as recorded by an in situ hydrophone and visual observations. Analysis of the EM122 midwater data set shows: (1) bubble plumes were present on every pass over the summit and they rose 200–400 m above the vents but dissolved before they reached the ocean surface, (2) bubble plume deflection direction and distance correlate well with ocean current direction and velocity determined from the ship's acoustic doppler current profiler, (3) bubble plume heights and volumes were variable over time and correlate with eruptive intensity as measured by the in situ hydrophone. This study shows that midwater multibeam sonar data can be used to characterize the level of eruptive activity and its temporal variability at a shallow submarine volcano with robust CO2 output.
Key Points
CO2 bubble plumes were imaged by multibeam sonar at actively erupting NW Rota‐1
The bubble plumes reflected the variable style and vigor of eruptive activity
The height and shape of the bubble plumes varied in concert with ocean currents
The relationships between tectonic processes, magmatism, and hydrothermal venting along ∼600 km of the slow‐spreading Mariana back‐arc between 12.7°N and 18.3°N reveal a number of similarities and ...differences compared to slow‐spreading mid‐ocean ridges. Analysis of the volcanic geomorphology and structure highlights the complexity of the back‐arc spreading center. Here, ridge segmentation is controlled by large‐scale basement structures that appear to predate back‐arc rifting. These structures also control the orientation of the chains of cross‐arc volcanoes that characterize this region. Segment‐scale faulting is oriented perpendicular to the spreading direction, allowing precise spreading directions to be determined. Four morphologically distinct segment types are identified: dominantly magmatic segments (Type I); magmatic segments currently undergoing tectonic extension (Type II); dominantly tectonic segments (Type III); and tectonic segments currently undergoing magmatic extension (Type IV). Variations in axial morphology (including eruption styles, neovolcanic eruption volumes, and faulting) reflect magma supply, which is locally enhanced by cross‐arc volcanism associated with N‐S compression along the 16.5°N and 17.0°N segments. In contrast, cross‐arc seismicity is associated with N‐S extension and increased faulting along the 14.5°N segment, with structures that are interpreted to be oceanic core complexes—the first with high‐resolution bathymetry described in an active back‐arc basin. Hydrothermal venting associated with recent magmatism has been discovered along all segment types.
Key Points
Ridge segmentation is controlled by preexisting basement structures
Spreading is perpendicular to the back‐arc axis, allowing precise spreading directions to be determined
Segment morphology reflects magma supply, which is locally enhanced by proximity to cross‐arc volcanoes
PDF
