Cold seeps release methane along most coastal margins, either as gas dissolved in water or as bubbles which rise through the water column. Bubbles of methane released from cold seeps have been observed rising hundreds of meters through the water column in the Guaymas Basin in the Gulf of California, on the continental rise off the coast of the Carolinas, along the Cascadia Margin off the coast of British Colombia and the northwestern US, in the Okinawa Trough in the East China Sea, in the Black Sea, and in the Gulf of Mexico, as well as at an experimental station in Monterey Bay. When emitted at depths of less than a hundred metres, methane bubbles typically reach the surface and release their gas into the atmosphere, while bubbles released at greater depths the methane in them is lost into the water column as they rise. However, in areas where stable gas hydrates exist at deeper depths, then the bubbles may have a hydrate coating, which slows dissolution of their gas content, allowing bubbles from greater depths to reach the surface.
This gas is thought to be of minor significance compared to other sources of carbon entering the atmosphere, but the amount gas produced, and how this varies both geographically and over time, has been the subject of very little investigation. The amount of gas released at different sites is known to vary considerably, which is thought to be related to factors such as the subsurface structure and the source of the methane. External factors are also thought to affect the amount of methane being released, such as variations in hydrostatic pressure caused by the action of tides, swell, or storms, changes in sea temperature linked to seasons or longer term changes, seismic tremors, and even to isostatic rebound.
Hydrate Ridge forms part of the Cascadia Subduction Zone, an anticlinal ridge on the accretionary wedge with a north-south orientation. This ridge has two distinct areas of cold seeps, known as the Northern and Southern Hydrate Ridges, both of which have extremely large methane hydrate deposits close to the sediment surface, which lead to persistent gas emissions in the form of bubble plumes, as well as extensive chemosynthetic ecosystems and authigenic carbonates. Seismic profiling of the Southern Hydrate Ridge has found a reflective surface, thought to be the base of the gas hydrate zone, at 125 m beneath the sediment surface. This hydrate zone overlies a structure called Seismic Horizon A, along which methane-rich fluids appear to migrate from the underlying accretionary wedge. Bubble plumes are extremely visible here, and have not been shown to fluctuate with tidal cycles, as is the case on the Northern Hydrate Ridge. Instead, the rate of bubbling appears to follow a cycle unrelated to the tides. It has been suggested that this might be caused by the formation of gas hydrate seals close to the surface, which remain in place until the amount of gas accumulated in the sediment beneath them overcomes their resistance, then reform when gas emissions tail off sufficiently, a cycle which can take several years to complete. More recent surveys have established that while bubbling is not constant on the Southern Hydrate Ridge, it does appear to occur consitently at the same sites on the ridge, and can vary on an hourly timescale. This unexpected behaviour has led to a need for a more organised long-term monitoring of this system in order to understand it.
The systematic monitoring of activity has been carried out at several cold seeps, including a study on Clayoquot Slope, off the coast of Canada on the northern Cascadia Subduction Zone which lasted over a year, although the equipment used in that study was only able detect gas emissions when bottom currents were flowing in certain directions, hampering understanding of the system.
In a paper published in the journal Geochemistry, Geophysics, Geosystems on 21 September 2021, Yann Marcon of the Center for Marine Environmental Sciences and Department of Geosciences at the University of Bremen, Deborah Kelley of the School of Oceanography at the University of Washington, Blair Thornton of the Centre for In Situ and Remote Intelligent Sensing at the University of Southampton, and the Institute of Industrial Science at the University of Tokyo, and Gerhard Bohrmann, also of the Center for Marine Environmental Sciences and Department of Geosciences at the University of Bremen, present the results of a project that carried out systematic monitoring of the Southern Hydrate Ridge between July and November 2018.
Marcon et al. monitored gas emissions on the Southern Hydrate Ridge with the Southern Hydrate Ridge Rotating Sonar, which consists of a multibeam echosounder mounted on a rotator, with a rotating range of 360°. In addition they used a single-beam scanning-sonar connected to the Ocean Observatories Initiative's Regional Cabled Array to provide a finer detail monitoring gas releases at Einstein’s Grotto, one of the methane vents on the Southern Hydrate Ridge, and photo cameras also connected to the Regional Cabled Array to provide visual ground-truthing information about the dynamics and strength of bubble release, at Einstein’s Grotto and the Summit-A vent area.
The bathymetry (depth of water) of the area was established in 2008 by the Autonomous Underwater Vehicle Sentry, operating from the Research Vessel Thomas G. Thompson, during preparation for the laying of the Ocean Observatories Initiative's Regional Cabled Array. A 3D photomosaic of the area was developed from imagery gathered by the University of Tokyo's Autonomous Underwater Vehicle AE2000f during the Schmidt Ocean Institute's FK180731 #Adaptive Robotics expedition.
A conductivity, temperature, and pressure (CTP) probe equipped with a dissolved oxygen optode sensor was used to monitor the conductivity, temperature, pressure, and dissolved oxygen concentration of the bottom water at one minute intervals. High-frequency tidal seafloor pressure was recorded with a tsunami pressure sensor. Current velocities were monitored with an upward-looking acoustic Doppler current profiler. Seismic data was collected by three ocean-bottom seismometers connected to the Regional Cabled Array. Wave height data was obtained from the National Data Buoy Center of the National Oceanographic and Atmospheric Administration.
Seafloor pressure at the Southern Hydrate Ridge was found to vary considerably with the tide, and was sensitive to the neep/spring tide cycle, with pressure varying by 1.4 dbar during neap tides and up to 3.8 dbar during spring tides. The seafloor temperature varied between 3.8°C and 4.5°C over the course of the study (June-November 2018) and between 3.7°C and 4.7°C over the longer period June 2018-June 2020, showing no long-term warming or cooling trend. Seafloor salinity varied between 34.26 to 34.37 psu over the period June-November 2018, while dissolved oxygen varied between 0.24 and 0.30 ml/L.
The Southern Hydrate Ridge Rotating Sonar carried out scans every two hours during the four month period of the experiment (with some gaps due to technical problems), producing a total of 888 readings. Of these, 886 (99.8%) detected gas bubbles, suggesting that bubbling was more-or-less constant. However, the amount of bubbling varied continuously, with peaks and troughs on a twice daily basis. These peaks and troughs appear to become larger at the spring tides and weaker at the neap tides, although the short duration of the experiment combined with the gaps in the data make it difficult to be completely confident about this.
Bubbling appears to be possible at any recorded pressure on the Southern Hydrate Ridge, although it was more pronounced when the pressure was low or falling. Two thirds of all peaks in activity occurred at times when the pressure was below average. The majority (65-70%) of the remaining third of peaks happened when the tide was decreasing, or changing (18-20%), with only a small proportion (15%) occurring while the tide was rising. However, there were some anomalies with this, most notably a large bubble release in July 2018m which occurred during a rising tide when the water pressure was above average.
Eight different centres of bubbling were identified on the Southern Hydrate Ridge, with an average of four active at any given time. Five main clusters of activity were identified; each of these being 10-30 m across, and capable of producing more than one stream of bubbles at a time. These were named Smokey Tavern, Einstein's Grotto, Summit-A, Summit-B, and Summit-C. Bubbling at all of these sites was frequent, but not continuous, with sites remaining dormant for days at a time and all sites being active at the same time being a rare occurrence. As well as these main sites of activity around the summit of the Southern Hydrate Ridge, Marcon et al. detected at least six smaller venting sites, further away from the summit and much less active, which they named Smokey Tavern West, Summit-D, Far NE, Far S, Summit South, and Summit SW. Unlike the main sites, these sites were only sporadically active, and would often remain dormant for months between bouts of venting.
The seafloor around the main vents is highly uneven, and covered by white microbial mats. These areas have numerous up-domed mounds, which extend laterally by as much as 35 m, with hummocky, jagged-edged depressions eaten into them, which are probably caused by the more-or-less constant venting of gas from these areas. The venting of gas does appear to be strongly tied to these areas, with venting restricted to them at all sites except Smokey Tavern, where it occurs across the entire microbial mat-covered dome structure. In the peripheral venting area this kink appears to be broken, with dark sediments and white areas of microbial matting visible, but venting not connected to these.
The sites closest to directly underneath the Southern Hydrate Ridge Rotating Sonar (Summit A and Summit B) appeared to be the most active, with sites moving away from this area being progressively less active. This might indicate that the sonar was better act detecting gas emissions directly beneath it and less good at detecting them further away. Alternatively, it might be a reflection the fact that the sonar was placed directly over the highest point of the Southern Hydrate Ridge, so that the most active points were those in the shallowest water, with sites becoming progressively less active as the water grows deeper.
None of the vents was active all the time, although venting over the wider Southern Hydrate Ridge area was continuous. During the period 6-22 July 2018, The Summit A vent was active 75% of the time, Einstein's Grotto was active 69% of the time, Smokey Tavern was active 65% of the time, Summit B and Summit C were active 54% of the time, Summit D 22% of the time, Far NE 9%, Far S 2%, and Smokey Tavern West was active 1% of the time, while Summit South and Summit SW were completely inactive. During the period 19 October-8 November 2018, the Smokey Tavern vent was active 44% of the time, Summit B 30% of the time, Summit C 70%, Summit D 23%, and Smokey Tavern West was active 6% of the time. A sudden decrease in activity at one site was usually accompanied by an equally sudden increase at another. For example, a pause in venting at Summit A on 10 July 2018 was marked by a sudden increase at Summit C. On 12 July activity at Summit C fell off again, and activity resumed at Summit A and Summit B. During the period 19 October-8 November 2018, Summit B was only active when Summit C was inactive, and vice versa. These synchronous changes in activity were frequent enough that they are unlikely to be coincident. Their timing also appears to be linked to the daily tide cycles.
A closer examination three vents at Einstein's Grotto over a 24 hour period found that, while they were all bubbling continuously throughout the day, the amount of outflow from them varied considerably over the day, with the bubbles undergoing sudden peaks in activity, followed by slower tailing off, several times per day. Furthermore, these peaks in activity always occured when the tide was high or waning.
Cameras directed at the Summit A and Einstein's Grotto vents were able to confirm that the events detected by sonar were in fact bubble releases. The one directed at Einstein's Grotto further revealed that the depression from which the bubbles were escaping was growing slightly wider with each bubble-release event, probably due to the loss of gas hydrates within the sediment, and that during major bubble releases this was accompanied by slumping of the walls of the depression and the collapse of overhanging sections. In some cases, hydrates were exposed by sediment loss, and then rapidly lost. Currents on the seafloor also appeared to contribute to the rate of erosion, by slowly breaking up blocks of sediment. The video films also revealed that events recorded as coming from a single vent could in fact come from different locations within the vent at different times, and that sometimes the location of bubble releases shifted in response to sediment movements. On occasions bubbles were seen emerging from as many as four different locations within a vent.
It was only possible to measure the velocity of bubble streams when they were visible to a camera and immediately adjacent to a measuring scale, but those that could be measured rose at speeds of between 18 and 34 cm per second, with an average speed of 25 cm per second.
The camera directed at the Einstein's Grotto vent detected a significant gas blowout between 0.46 and 1.16 am on 23 July 2018, with a sudden release of a significant volume of gas, along with ejected sediment, which led to a change in the seafloor topography. The event happened about midway through a rising tide, with gas release being week, but active, both before and after the event.
Turbulence in the upper part of the water column was relatively high throughout the study, with currents having an average velocity of 50 cm per second. The depth of this layer varied with season and time of day, between about 300 and about 400 m. Below this the water was much calmer, with velocities below 15 m per second, although here too there were daily and seasonal variations. Currents on the seafloor generally run from the northwest. Upwelling was also a significant component of bottom currents, with upwelling currents of up to 5 cm per second rising for about 300 m (i.e. to a depth of 470 m), where they generally became undetectable, although some reached as high as 400-350 m below the surface, by which time they had lost much of their initial velocity. These upwelling flows were intermittent, generally appearing when the prevailing northwest current was either weak or reversed, and appeared to be linked to episodes of bubbling, although this could be an indicator that the northwest current was deflecting bubbles away from the sonar beam.
Contrary to expectations, no connection was found between bubble outbursts and seismic events. A number of such events were measured during the study period, none of which were linked to conspicuous changes in bubble activity.
Wave height was found to be strongly seasonal in the study area, with the monthly average wave height between October and April exceeding 3 m and individual waves reaching close to 10 m, whereas between May and September the monthly average remained below 2 m, and the highest recorded waves little over 3 m. No correlation was found between wave height and bubble activity.
Macron et al.'s study shows that the Southern Hydrate Ridge has a number of vents (where 'vent' is defined as 'a distinct area of the seafloor where gas ebullition recurs') and that each vent has a number of bubble outlets, with vents capable of producing bubbles from several bubble outlets at once, often at different rates of activity.
Five main vents, and six peripheral vents were detected, a significant increase on the number found by previous, ship-based surveys, although it is possible that others exist and were missed by the study. Macron et al. note that bubble events have been recorded at two other sites on the Southern Hydrate Ridge (named Central and Pinnacle), that were outside the scope of their survey.
The discovery that there are multiple bubble-producing sites within each vent, which may be active at different times and at different rates at the same time, is new. This potentially presents additional challenges for ship-based surveys, as snapshots of bubbling from different locations might not be recognised as the same vent, particularly if the origin of bubbles is further complicated by varying currents.
The main vents all appear to be up-domed and hummocky areas covered by microbial mats. Such areas have previously been linked to the presence of gas hydrates at shallow depths within the sediment, something which was confirmed by Macron et al.'s study. The hummocky topography seen at these locations appears to be directly linked to bubble-venting, which causes changes the seafloor morphology quite rapidly at vent sites.
The loss of gas hydrate deposits is likely to be largely due to direct disolution of gas into non-methane saturated water, without the formation of bubbles. The vent zone of the Southern Hydrate Ridge lies within the gas hydrate stability zone (approximately between 500 and 900 m below sea level), so the excess loss of gas hydrates through bubble formation is unlikely in shallow sediments here. The depressions and hummocky surfaces observed at the site could as easily be formed by hydrate dissolution as by bubble formation, particularly at a site being regularly scoured by currents of non-methane saturated water. Based upon this, Macron et al. suggest that the bubbles observed venting on the Southern Hydrate Ridge are not being formed by the disassociation of gas hydrates, but rather are bubbles of trapped within the sedimenr that are released into the water.
Previous studies of the Southern Hydrate Ridge have captured only snapshots of bubbling activity there, suggesting that bubbling was either 'off' or 'on' at different times. This is consistent with the idea that methane was being delivered to the system from a reservoir more deeply buried gas at Horizon A, which was escaping intermittently through a system of cracks in the overlying rock. Macron et al.'s study contradicts this model, showing that bubbling is more-or-less constant on the Southern Hydrate Ridge, although different parts of the system are active at different times, and that this activity is strongly linked to bottom pressure, and thereby to the tides. However, Macron et al. cannot rule out the possibility that the vents are connected in some way, or that there may be some form of interaction between them.
Tidal modulation of methane vent systems on accretionary wedges has previously been suggested, but until now there has not been sufficient evidence to support this. Notably, video evidence from the Northern Hydrate Ridge appeared to show a correlation, but was not sufficient on its own to prove the link. Therefore the long-term sonar study carried out by Macron et al. at the Southern Hydrate Ridge clearly establishes a relationship between tides and bubbling for the first time. A relationship has also been suggested between tides and bubbling at cold seep sites, which appears to have been supported by observations off the coast of Canada.
Macron et al. suggest that this link is related to the effect that pressure has upon pore spaces within sediments, which are pushed shut at higher pressures (i.e. when the tide is high) and pulled open at lower pressures, allowing gas bubbles trapped within the sediment (where the water is methane-saturated) to escape. At spring tides, the pressure loading on the seafloor would be higher than at neep tides, causing the dilation of pore-spaces deeper into the sediments, which would in turn result in more bubbles being released, something line with Macron et al.'s observations.
However, this does not appear to be the only factor at play; Macron et al.'s theory can explain why vents tail of activity after outbursts, when they might be expected to have reduced numbers of bubbles within their sediments, but not what causes the reactivation of individual vents after periods of inactivity. Furthermore, a number of significant bubble outbursts were recorded at times which appeared to show no connection to tidal cycles. Another problem with this hypothesis is the absence of any connection with seismic activity. Earthquakes would be expected to cause changes in pore spaces, which should have resulted in bubble releases, but despite several Earthquakes being recorded during the study period, none of them was linked to any change in bubble-activity.
One possibility previously suggested is that gas hydrates accumulating in pore spaces trap a layer of gas below the surface sediments, in Horizon A, which is released when the gas pressure builds up to a point where it overcomes the resilience of the hydrates. This would explain how vents become dormant and then reactivate after a period of inactivity, but the time scale predicted for this model does not fit Macron et al.'s data. Under this model the vents should be inactive for years at a time, whereas the observed quiet periods lasted from a few hours to a few months. Macron et al. suggest that possibly the basic premise of this hypothesis is correct, but that the gas is being trapped much closer to the surface by a less substantial hydrate lid, enabling smaller, more frequent bubble-outbursts from beneath a lid more easily overcome.
This would allow bubble release to be controlled at a local level, will small pockets of trapped gas building up within sediments in the gas hydrate solubility zone, beneath shallow blockages in the sediment pore-system. Overcoming these shallow blockages would require much less pressure to build up, with the increased pressure from tidal forces often being sufficient to trigger outbursts.
The perpheral sites, where venting occurs but is far less frequent, may be receiving lower levels of gas from the underlying feeder sediments, or may be acting as pressure-release valves for a larger gas system beneath the gas hydrate solubility zone, venting less frequently as is predicted by the deep-reserve, large hydrate-lid model.
The strong bottom currents observed at the Southern Hydrate Ridge also ought to play a role in the gas hydrate dissolution cycle, since these currents constantly bring in new water unsaturated in methane, leading to more rapid hydrate dissolution.
Macron et al. note that strong upwelling currents are often observed at the Southern Hydrate Ridge, and that these tend to coincide with outbursts of bubble activity. They suggest that the upwelling currents may in fact be driven by the gas bubbles dragging water along with them as they rise. Curiously, however, the upwelling component of the system appears to be more strongly tied to the tidal system than the bubble activity is, with upwellings tending to stop during rising tides, while bubble activity frequently continues. Both upwelling currents and bubbles appear to reach depths slightly above the gas hydrate stability zone, which is consistent with the two phenomena being linked and the bubbles being protected by a hydrate-skin. The height to which both bubbles and upwelling currents rise varies seasonally, which is likely to be linked to seasonal water stratification.
Methane bubbling from the Southern Hydrate Ridge is a more-or-less constant phenomenon, although the location of the bubbling does vary over time. A number of vents are present, with the controls on bubbling at each of these apparently being independent. The continuous venting of bubbles appears is linked to the slow erosion of sedimentary structures on the seafloor, with occasional more substantial outbursts provoking more substantial overturn. The timing of bubble venting appears to be linked to tidal cycles, and is probably driven by hydrostatic pressure, although some bubble releases appear to be random and unconnected to tidal cycles.
Bubbling is more prevalent at low and falling tides, when pressure is low or falling, and this is more notable around spring tides, when pressure changes are more marked. At these times pore spaces in the sediment are more open, which could conceivably be related to outbursts of bubbling. However, bubbling also occurs at other times, which may indicate that pressure within the sediment is also a significant influencing factor.
The main vents on the Southern Hydrate Ridge were active for between 50 and 75% of the time, although the level of activity at 'active' vents varied considerably in intensity, and could come from multiple locations within the vents. This makes it hard to estimate the amount of methane being released by the Southern Hydrate Ridge, and Macron et al. were unable to estimate this, despite having undertaken the most detailed study of the system to date. Based upon their findings they do not feel it would be reasonable to extrapolate data from a single vent across the whole system, instead any attempt to estimate the amount of methane being released would require long-term monitoring of the entire system.
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