A potential major lithium source from the Devonian Marcellus Shale of Pennsylvania.

Lithium for use in batteries is a major requirement for a transition to electric vehicles, and as such a potential limiting factor in such a transition. In the US, the Infrastructure Investment and Jobs Act requires that all of the materials used in the production of batteries for electric vehicles be sourced within the country by 2030, making the identification of viable sources of lithium a priority for the industry. Recent research has shown that the saline fluids produced as a biproduct of gas and petroleum extraction are often rich in lithium, presenting a potential solution to this problem. 

This appears to be particularly true for Palaeozoic groundwaters from the Appalachian region. One such promising source is the Middle Devonian Marcellus Shale Formation. The Marcellus Shale is currently worked as an 'unconventional' natural gas field, in which methane is extracted from the shale by means of hydraulic fracturing (fracking), a process by which water, sand and chemicals are forced into buried sediments in order to shock them into releasing trapped hydrocarbons, which can then be extracted for commercial use.

This process has also resulted in the extraction of large amounts of hypersaline groundwater being brought to the surface, for which there is little current use, with the effect that 95% of this is pumped back into the ground in further fracking operations. The total dissolved solids present in the Marcellus Shale groundwater exceed 100 000 mg/L, leading to a requirement for it to be treated before re-injection of the water into the ground can be carried out. A significant proportion of this dissolved material is lithium, derived from layers of volcanic ash within the shale. This high proportion of lithium makes the Marcellus Shale groundwater a potential target for lithium extraction. 

Current extraction infrastructure targeting the Marcellus Shale is concentrated around two hotspots for methane production, one in the northeast of Pennsylvania and one in the southwest of the state. The Marcellus Shale is known to vary in composition over its extent, leading to quite different groundwater chemistry in the two areas. Several previous studies have highlighted the possibility of extracting lithium from Palaeozoic brines, but to date none have considered geochemical variation within these sources, which could have a significant impact on the viability of such projects.

In a paper published in the journal Scientific Reports on 16 April 2024, Justin Mackey of the National Energy Technology Laboratory and the University of Pittsburgh, Daniel Bain, also of the University of Pittsburgh, and Greg Lackey, James Gardiner, Djuna Gulliver, and Barbara Kutchko, also of the National Energy Technology Laboratory, present the results of a study which used chemical and production compliance data reported to the Pennsylvania Department of Environmental Protection to predict the potential yields of lithium from Marcellus Shale groundwaters in the two areas of Pennsylvania where the Shale is currently accessed.

Map of study area showing the Marcellus shale extent, well locations using in decline curve analysis, Water samples used in this study, and previous USGS sample locations. Lithium concentration data was calculated using new data from the study and existing data from the USGS National Produced Waters Database. Mackey et al. (2024).

The proportions of lithium and magnesium in groundwater vary between locations within the Marcellus Shale, leading to differences in the maximum likely estimation of lithium yields in different areas. The proportion of lithium in the water is more variable at wells in the northeastern sector of the Shale, varying between 139 and 267 mg/L. In the southwestern area the proportion of lithium is more constant, varying between 112 and 140 mg/L. Although the lithium is more concentrated in the northeast, more water is produced in the southwest, leading to a higher overall lithium yield. Mackay et al. estimate that a well in the southwest portion of the field could produce between 2.80 and 2,99 tonnes of lithium over ten years, while in the same time a well in the northeast could produce between 1.86 and 2.07 tonnes.

Histogram plot of Monte Carlo simulation results (25 000 simulations) of estimated ultimate lithium mass yield from a single Marcellus Shale gas well over 10-years of assumed continuous production. Regional estimates on lithium yields from a southwest Pennsylvania (SW) well is marginally more (approximately 33%) than its northeast Pennsylvania (NE) counterpart. Mackey et al. (2024).

The proportion of magnesium in the water is far higher in the southwest, averaging 2300 mg/L, compared to an average of 1000 mg/L in the northeast. Thus wells in the southwest will produce between 14.3 and 20.7 times as much magnesium as they do lithium, while wells in the northeast will produce between 2.66 and 7.26 times as much magnesium as lithium. 

Lithium extracted from pore waters from the Marcellus Shale in Pennsylvania could go a significant way towards meeting the US's need for the metal, although the extraction of the lithium would presumably have an impact on the way the industry works (currently salt rich water is pumped back into wells to produce more methane, with more water produced as a biproduct, which is then re-injected). 

The US currently uses about 3000 metric tonnes of lithium per year, and the Marcellus Shale Pore Waters could potentially provide 38-40% of this need, if all the dissolved lithium in the extracted water was recovered. However, about 95% of the water currently extracted from wells targeting the Marcellus Shale in Pennsylvania is reinjected, so if the pore water was diverted for lithium extraction, water from a different source would need to be used for this purpose. This would also involve considerable reorganization of the infrastructure surrounding the wells, and reconsideration of the associated the associated environmental and social impacts. Pore water is currently shipped from wells via a pipeline, then redistributed to other wells for re-injection with minimal treatment. Adding lithium-extraction to this process would require a substantial increase in the infrastructure needed, and subsequently the environmental footprint of the operation, although the value of the lithium would probably more than offset the financial cost of this.

The regional variation in the proportion of lithium in the water in the different areas is likely to have an impact on both the method used to extract lithium from the water, and the ultimate lithium yield. The southwestern portion of the Shale appears likely to produce more lithium overall, as well as having a slower decline in productivity. However, this difference is not huge, with southwestern wells probably producing only 26-38% more lithium than northeastern wells over their lifetime. Mackey et al. also note that it is less efficient and more expensive to extract lithium from water with higher magnesium contents, as is the case in southwestern Pennsylvania. The pore water extracted in the northeast of Pennsylvania has a magnesium/lithium ratio similar to that found in the salar Atacama brines of Chile, from which lithium is currently extracted via a combination of evaporation and distillation, demonstrating the economic viability of the process. Extracting lithium from pore water obtained from the southwestern part of the Marcellus Shale may prove to be a more expensive process, due to the need for additional treatment.

The rate at which lithium production from the Marcellus Shale will decline also needs to be taken into consideration. A typical well in the are suffers an 80% decline in water production over two years. This means that to achieve sustainable production of lithium, new wells would have to be brought into production more-or-less continuously, to replace wells where production had fallen below the point of viability. It is possible that improvements in technology will lead to the lives of wells being extended if the main target of the operation becomes lithium rather than shale gas; Mackey et al.'s production estimates are based entirely upon current extraction methods.

Mackey et al.'s study demonstrates that the Marcellus Shale has the potential to make a significant contribution towards the US's lithium needs, based upon a reasonably conservative set of assumptions. Even allowing for significant over-estimation of potential yields, it is likely that the Shale could produce 30% of curent US lithium demands. Current estimates suggest that the Marcellus Shale hosts about 2.7 billion cubic metres of undiscovered methane gas, suggesting that well-drilling activity here is likely to persist for several decades, and expand into other parts of the deposits, such as north-central Pennsylvania. Thus, even if lithium remains a biproduct of shale gas production, there is a potential for this to continue for decades to come.

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Explosion at Turkish coal mine kills 41.

Forty one miners have now been confirmed dead following an explosion at the Amasra Muessese Mudurlugu Coal Mine in Bartin Province, on the north coast of Anatolian Turkey, on Friday 14 October 2022. All of the 110 workers who were below ground at the mine have now been accounted for, with 28 suffering injuries, 11 of whom required hospital treatment, one having subsequently been discharged. The explosion is reported to have occurred at a depth of about 300 m, 49 of them working in what is considered to be the mines 'danger zone' between 300 and 350 m below the surface, where the coal is particularly rich in methane gas.

Smoke billowing from the Amasra Muessese Mudurlugu Coal Mine in Bartin Province, Turkey, on Friday 14 October 2022. BBC.

Coal is formed when buried organic material, principally wood, in heated and pressurised, forcing off hydrogen and oxygen (i.e. water) and leaving more-or-less pure carbon. Methane is formed by the decay of organic material within the coal. There is typically little pore-space within coal, but the methane can be trapped in a liquid form under pressure. Some countries have started to extract this gas as a fuel in its own right. When this pressure is released suddenly, as by mining activity, then the methane turns back to a gas, expanding rapidly causing, an explosion. This is a bit like the pressure being released on a carbonated drink; the term 'explosion' does not necessarily imply fire in this context, although as methane is flammable this is quite likely.

Fire is much feared in coal mines due to this combination of flammable gas and solids, with methane and coal dust both potentially explosive when they come into contact with naked flames. To make matters worse, the limited oxygen supply in mines often means that such fires will involve incomplete combustion, in which all the oxygen is used up, but instead of forming carbon dioxide forms the much more deadly carbon dioxide, with potentially lethal consequences for anyone in the mine.

As coal is comprised more-or-less of pure carbon, and therefore reacts freely with oxygen (particularly when in dust form), to create carbon dioxide and (more-deadly) carbon monoxide, while at the same time depleting the supply of oxygen. This means that subterranean coal mines need good ventilation systems, and that fatalities can occur if these break down. 

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Investigating the drivers of methane release from cold seeps on the Southern Hydrate Ridge off the coast of Oregon.

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. 

 

Top left: Location of Southern Hydrate Ridge. Top right: Map of the primary infrastructure of the Ocean Observatories Initiative's Regional Cabled Array observatory. Bottom: Overview map of the Southern Hydrate Ridge summit with the location of the Southern Hydrate Ridge Regional Cabled Array fibre optic cables, junction boxes and monitoring instruments. Shaded areas show the location of the main known vents. Marcon et al. (2021).

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.

 

Temporal variations of the Southern Hydrate Ridge Rotating Sonar backscatter magnitude and bottom pressure between 6 and 22 July 2018 (top plots) and between 19 October and 8 November 2018 (bottom plots). The backscatter magnitude non-linearly reflects the strength of the gas bubble emissions. The bottom pressure plot shows the local mixed tidal regime with diurnal and semi-diurnal constituents, as well as the fortnightly neap/spring tidal cycles. Bubble release is commonly stronger during ebb tide, and possibly also during spring tidal phases. However, some ebullition events do not correlate with the tide and may be triggered by local accumulation of pressurized free gas in the subsurface; the prominent peak observed on 18 July 2018 corresponded to the reactivation of the Summit-A vent after a very short venting interruption of about 4 hours; it did not affect the other vents at the Southern Hydrate Ridge summit and happened during flood tide within a neap tidal phase, hinting at shallow, local changes in the sediments. Marcon et al. (2021).

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.

 

(a) Location of flare base points recorded with the Southern Hydrate Ridge Rotating Sonar between 8 July and 8 November 2018; the base points are grouped into clusters marking the location of the different Southern Hydrate Ridge vent sites. (b) Location of the main and periphery vents overlain on the photomosaic; the main vent sites are all located on areas covered with microbial mats. (c) Close-up view of the Southern Hydrate Ridge Rotating Sonar location and the Summit-A vent, with the 3D photomosaic in the background; a depression on the seafloor from Ocean Drilling Program drill site 1949 (ODP Leg 204) can be seen in the top-left corner as well as in the bathymetric data. (d) Close-up view of the 3D photomosaic at the Smokey Tavern vent showing the distribution of the microbial mats and the domed, collapsed and hummocky areas. Marcon et al. (2021).

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. 

Southern Hydrate Ridge Rotating Sonar magnitude data from 19 October to 8 November 2018 for individual plume clusters. Einstein's Grotto and Summit-A are not shown because of the restricted 245° scanning sector of the Southern Hydrate Ridge Rotating Sonar after 10 October 2018. Far NE, Far S, Summit South, and Summit SW are not shown because they were inactive over this period. The vertical axis is logarithmic to facilitate visualisation of low magnitude variations. Absolute magnitude values cannot be compared between the clusters due to a distance bias and are not shown. Marcon et al. (2021).

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. 

 

A pressure outburst was documented by the CAMDSB103 camera on 23 July 2018. The camera images show sediment resuspension shortly (0–30 min) after the outburst. Images taken after visibility improved show significant seabed changes including the presence of a large well-lithified sediment block into the collapsed area subsequent to the blow out. Scanning-sonar scans recorded before and after the event show that the seabed morphology at the location of the outburst changed over an area of at least 3 m². The hummocky area east of the sonar is part of the Einstein's Grotto vent. The range of the sonar scans is 20 m. The laser pointers on the camera images are 10 cm apart. In the difference plot, blue and red colours show negative and positive differences respectively. Marcon et al. (2021).

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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Thirteen killed in two separate incidents at coal mines in Balochistan Province, Pakistan.

Eleven miners and two rescue workers have been killed in two separate incidents at coal mines in Balochistan Province, Pakistan, within the last week. On Thursday 11 March 2021, eight miners working 300 m below ground at a mine in the town of Marwār, close to the border with Afghanistan, were buried following a methane explosion. Two of these miners were later dug out alive, but the remaining six died. On Monday 15 March five miners entered a mine near Tor Ghar in Harnai District, to carry out repair work following a fire, also thought to have been caused by a methane build-up. All five miners were overcome by fumes within the mine and died, as did two rescue workers who entered the mine after them.

 

Rescue workers at a mine in Harnai District, Balochistan, where seven people died this week. Al Jazeera.

Coal is formed when buried organic material, principally wood, in heated and pressurised, forcing off hydrogen and oxygen (i.e. water) and leaving more-or-less pure carbon. Methane is formed by the decay of organic material within the coal. There is typically little pore-space within coal, but the methane can be trapped in a liquid form under pressure. Some countries have started to extract this gas as a fuel in its own right. When this pressure is released suddenly, as by mining activity, then the methane turns back to a gas, expanding rapidly causing, an explosion. This is a bit like the pressure being released on a carbonated drink; the term 'explosion' does not necessarily imply fire in this context, although as methane is flammable this is quite likely.

Fire is much feared in coal mines due to this combination of flammable gas and solids, with methane and coal dust both potentially explosive when they come into contact with naked flames. To make matters worse, the limited oxygen supply in mines often means that such fires will involve incomplete combustion, in which all the oxygen is used up, but instead of forming carbon dioxide forms the much more deadly carbon dioxide, with potentially lethal consequences for anyone in the mine.

Coal is also comprised more or less of pure carbon, and therefore reacts freely with oxygen (particularly when in dust form), to create carbon dioxide and (more-deadly) carbon monoxide, while at the same time depleting the supply of oxygen. This means that subterranean coal mines need good ventilation systems, and that fatalities can occur if these break down.  

The coal industry in Balochistan is notoriously dangerous, with extensive deposits of coal, much of which is rich in methane, and a large coal industry, where safety standards are generally very low, with miners having little access to specialist training or equipment. Four miners were killed in a methane explosion in Harnai District in February 2021, and the Balochistan Coal Mines Workers Federation has recorded 102 fatalities in mines in the province in the past year. The situation is made more complicated by an insurgency being waged by Balochistan nationalists, who wish to unite Balochistan Province with Sistan and Baluchestan Province in Iran and the Balochistan region of southern Afghanistan into a new, independent, state, and who sometimes carry out attacks on projects such as mines, which they see as exploiting the region's wealth for the benefit of outside interests.

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Understanding the formation of the Yamal Peninsula Gas-Emission Crater.

Methane is a powerful greenhouse gas, and the recent abrupt events of degassing and crater formation on the Yamal and Gydan Peninsulas have caused major concern that a warming Arctic may lead to increased thawing of permafrost and gas emissions. In addition, exploration and development of oil and gas fields in northern West Siberia, with construction of production, transportation and support facilities upon permafrost, face problems due to the harsh Arctic climate, low negative temperatures of air and ground and complex periglacial processes like frost heaving, thermokarst, thermal erosion. Processes in shallow permafrostmaylead to the formation of gas-emission craters, a phenomenon discovered during exploration for the past decade in the Arctic West Siberia. The first methane-leaking crater (Yamal Crater) was found in the Yamal Peninsula 42 km from the Bovanenkovo gas field and that discovery was followed by several more from the Yamal and Gydan Peninsulas. The Yamal Crater has been better documented than the others, which are only imaged in few photographs, but the available data are insucient to concede about the conditions and causes of its origin. The crater origin has been unanimously attributed to an explosive gas emission event, but the origin of the gas remains a subject of discussions.

In a paper published in the journal Geosciences on 8 May 2020, Evgeny Chuvilin of the Center for Hydrocarbon Recovery at the Skolkovo Institute of Science and Technology, Julia Stanilovskaya of Total, Aleksey Titovsky of the Department of Science and Innovation of the Yamal-Nenets Autonomous District, Anton Sinitsky of the Arctic Research Center of the Yamal-Nenets Autonomous District, Natalia Sokolova, Boris Bukhanov, Mikhail Spasennykh, Alexey Cheremisin, Sergey Grebenkin, and Dinara Davletshina, also of the Center for Hydrocarbon Recovery at the Skolkovo Institute of Science and Technology, and Christian Badetz, also of Total, present a summary of the proposed hypotheses for the formation of the Yamal Crater.

 

Location map of the Erkuta Crater and oil and gas fields in the Yamal Peninsula. Chuvilin et al. (2020).

The proposed hypotheses explaining the formation of the Yamal Crater, and other craters in the Yamal and Gydan Peninsulas, can be divided into two main groups that invoke either deep-seated or shallow causes. The deep-seated causes of crater formation include increased deep heat flux, upward migration of deep gaseous fluids through fault zones and fault intersections to shallow permafrost and dissociation of intrapermafrost gas hydrates driven by ascending heat and gas flows. These processes can produce reservoirs of pressurized gas in shallow permafrost which can release explosively, break up the frozen cap and form a crater.

 

Helicopter view of the Erkuta crater and adjacent territory in summer 2017. Chuvilin et al. (2020).

The crater was discovered in June 2017 by people from the Erkuta Polar Station on the floodplain of the Erkuta River, 30 km east of the station. The area is of interest to biologists for its proximity to the nesting-place of falcons. Two years before, the terrain was absolutely flat, as witnessed by Alexander Sokolov in a TV broadcast (Vesti Yamal) of 30 June 2017.

The team of biologists led by Sokolov observed heaving of the previously flat surface, as well as cracks in soil, during field works in July 2016, a year before the crater was first discovered in June 2017. The newly formed crater had a cylindrical shape, 10 to 12 m in diameter, with smooth walls was 20 m deep. The original heave mound was not fully eliminated: the crater was encircled by a 2–3 m high parapet-like ridge, with its slopes covered with silt and clay silt ejected during the explosive gas emission.

In December 2017, a field trip to the Erkuta Crater was organised jointly by the Government of the Yamal–Nenets Autonomous District, Total SA and the Skolkovo Institute of Science and Technology. The field work included sampling of soil, ice and water from the crater rim. By that time, the southern wall of the crater had collapsed and the diameter increased to 17.5 m. The crater was partly filled with water which made an up to 8-m-deep lake covered by about 1-m-thick ice under approximately 0.8 m of snow. A part of the parapet-like ridge remained around the crater next to a lake.

 

(Top) Panoramic view of the Erkuta cCrater in December 2017. (Bottom left) Lake next to the remnant parapet-like ridge around the Erkuta Crater in December 2017. (Bottom right) Panoramic view of the Erkuta Crater in summer 2018. Julia Stanilovskaya, Evgeny Chuvilin, and Alexander Sokolov in Chuvilin et al. (2020).

In June 2018, the crater was imaged by a drone survey, which showed further degradation of its wall. The deep crater of a year before became almost fully filled with water and transformed into a lake semi-circled by a remnant 2–3 m parapet. The crater wall was deformed by thermal erosion and slumping in its outer part but remained vertical in the inner part.

Field studies in December 2017 revealed a layer of massive ground ice, 3–4 m of visible thickness, in the inner wall of the crater.

 

Sampling points (red circles) in the inner wall of the Erkuta crater, December 2017. (left) Fragment of the sampling sight, (right). Red dashed line in left panal delineates the ice line. Evgeny Chuvilin in Chuvilin et al. (2020).

Ice at the sampling site was generally transparent and pure, but locally enclosed 1-2 cm thick layers of fine grained soil and a minor amount of intricately shaped 1-3 mm gas bubbles. The clear massive ice was topped by a dirty gray ice layer of a few centimtres thick with abundant soil inclusions.

 

Massive ground ice in the crater wall, December 2017. (left) The surface of the ice, (right) fresh cut of ground ice with soil inclusions. Evgeny Chuvilin in Chuvilin et al. (2020).

The ground ice was overlain by frozen silt with organic inclusions (plant remnants). The sediments had cross and wavy stratification common to aluvial facies. Some organic layers on lenses were a few centimeters thick and a few cm to tens of centimetres long.

The sandy crater wall was locally cut by discontinuous branching fractures partly filled with ice, often within zones of iron impregnation. The fractures possibly formed under stress produced by fluid (water or gas) pressure from underlying sediments.

 

Interbedded sandy and silty soils with inclusions of organic matter in the crater wall (December 2017). (left) Discontinuous branching fractures, (right) wavy stratification of organic material. Evgeny Chuvilin in Chuvilin et al. (2020).

The ejected clay silt on the outer crater wall was originally wet and was deposited in a floodplain environment; it diered markedly from the overlying organic-rich alluvial sand.

Shrubs on the remnant slope facing the lake do not dier much from the surrounding lowland vegetation. Therefore, heaving shortly preceded the gas emission event and caused substantial changes to the vegetation. Otherwise, at years-long heaving, vegetation would have adapted to new conditions. Shrubs on Arctic tundra are usually lower on topographic highs than on the plainland and are absent in many areas where only grass can grow.

The samples collected in the December 2017 field trip to the crater included frozen soil from the northern wall, fine-grained material ejected from the crater and ice.

The particle size distribution of soil samples and soil inclusions in ground ice was analyzed in the laboratory of Fundamentproekt. The particles sizes were determined by the pipette method in silt and clay and by sieve analysis in sand.

The particle sizes of soil over the ground ice mainly correspond to silty sand while the ejected material is mainly light clay silt and silt, with particle sizes similar to those of soil inclusions in visible ground ice.

The mineralogy of soil samples was analyzed at the Geological Faculty of the Lomonosov Moscow State University on a Rigaku ULTIMA-IV X-ray diractometer. The samples consist mainly of quartz and feldspar minerals (microcline and albite), about 80%–90% in total. Quartz percentages reach 61.8% and 74.3% in silty sand samples 8 and 9, respectively, notably more than in clay silt sample 2 (about 45.2%). The feldspar minerals are 32.3% in sample 2, 23.9% in sample 8 and about 15.4% in sample 9. Clay minerals are mainly mixed-layer illite–smectite, from 5.8% in sample 9% to 10.2% in sample 2. Other minerals occur in minor amounts (1% or less). Samples 2 and 8 share similarity in mineralogy, with similar amounts of microcline, illite, smectite and kaolinite, and thus were originally involved in similar deposition processes.

The soil and ice samples were analysed for the contents of soluble salts by using soil–water extracts prepared from 100 g of dry substance. Soil-free massive ground ice was analysed in the molten state. The total percentage of salts did not exceed 0.1% in soil samples and was in a range of 40–173 mg per litre in ice samples. The predominant major ions were alkaline metals sodium, potasium and magnesium as cations and sulphate and chlorine as anoins. 

The contents of unfrozen pore water in samples of erupted material (sample 2) and sand from the crater wall (sample 9) were determined from pore water activity. The amount of liquid water in the sand samples decreased abruptly at temperatures below -1°C and was less than 1% at -4°C. The clay silt sample (2) contained more unfrozen water than the sand sample (9): about 4% at -4°C and around 3% at -10°C.

Chuvilin et al. compared major-ion chemistry and stable isotopes of water in ice samples from the crater wall and in crater lake water. The ice samples (11 and 13) were recovered from the middle part of the ice lens free from visible soil inclusions. Prior to analyses, ice samples were melted, and the obtained water was collected without filtering.

The salinity of lake water is higher than that of ground ice: contents of some major ions are 7 to 11 times higher, especially sulphate, magnesium and calcium.

The oxygen and hydrogen isotope compositions of water from the lake and ice are also different: those of ground ice are more depleted than in surface water from the crater. These compositions indicate that the mean annual temperature was 7 to 10°C lower than now when the ice was forming. 

The structure and texture of soil and ice samples were analyzed at Skoltech. The microstructure of frozen soil was studied in replica samples (imprints of fresh fracture planes on a plexiglass film) under an optical microscope (at the Skolkovo Institute of Science and Technology) and a scanning electron microscope (at Moscow University). The techniques for preparing replica samples and their optical and electron microscopy were reported in a number of publications. Ice structure and texture were studied in thin sections between crossed Polaroids; the thin sections were prepared following the standard procedures.

Soil microstructure was studied in sand from the crater wall (sample 9) and in ejected clay silt (sample 2). According to reflected light optical microscopy, the sand sample (9) mostly consists of 0.1–0.25 mm subrounded isometric quartz particles, with lesser amounts of fine-grained material, lenses and layers of more or less strongly degraded organic remnants, distinct 1–2 mm brown organic inclusions, as well as black organic–mineral concretions of silty sand and decayed organics.

Scanning electron microscope images highlight the morphology of quartz grains, with signatures of brittle fracture and dissolution and with organic inclusions of dierent sizes, shapes and decay degrees. Finer silt or clay particles make continuous or discontinuous films and clusters on the surface of sand particles.

 

Scanning electron microscope images of sand sample 9 at dierent magnification factors, with signatures of fracture and dissolution (left) and organic inclusions (right). Chuvilin et al. (2020).

The microstructure of clay silt ejected from the crater (sample 2) was also examined under the optical and electron microscopes. Reflected light optical microscopy revealed quite uniform silt and clay particles with rare sand grains and fuzzy dark brown organic inclusions. Scanning electron microscope images of dierent magnifications resolve fine sand and coarse silt particles (and their replica imprints) cemented by clay silt at a magnification of x 500 and 5 μm to 20 μm particles at x 2000. The x 2000 images reveal orientations of mineral matrix particles delineated by sericite flakes, as well as organic inclusions easily spotted due to their particular shapes.

 

Scanning electron microscope images of clay silt sample 2 at different magnification factors: general view (left) and detail image (right). Chuvilin et al. (2020).

The ice macrostructure was studied in samples 6, 8 and 11 of ground ice and sample 22 from the top of the ice lens, at the ice-soil contact. Samples 6 and 11 are generally similar: massive transparent ice with chains of rare 0.3-0.5 cm air bubbles and thin flaky layers of silt and clay particles. Clusters of mineral particles occur at ice crystal boundaries. Mineral inclusions more numerous than air bubbles. Ice crystals in samples 6 and 11 appear as exceeding 5–7 cm in size, but the actual size is difficult to estimate because only crystal fragments fit into the thin sections.

 

Ice sample 6: general view (left) and structure under polarised light (right). Chuvilin et al. (2020).

Unlike samples 6 and 11, ice sample 8 encloses a layer of silty sand in pure ice almost free from soil particles and air bubbles. The ice crystals are fine (few mm) along the soil layer and coarser (2–3 cm) away from it. Although the true size of the ice crystals remains unknown because of the limited thin section size, they may be commensurate with those in samples 6 and 11. The soil layer, in its turn, encloses numerous small 3–5-mm-long, and up to 2-mm-thick lenses of ice with fine crystals (fractions of mm).

 

Ice sample 8: general view (left) and structure under polarised light (right). Chuvilin et al. (2020).

Ice sample 22 from the top of the ice lens differs markedly in colour from the other ice samples. It encloses numerous scattered small soil particles and air bubbles which make it look like dirty opaque ice. The ice crystals are 2-4 mm and generally isometric. The sand-silt material occurs both along the boundaries of ice crystals and inside them. The ice crystals in this sample are finer than in the three other samples possibly because they nucleated and grew in the presence of mineral components in the medium which provide numerous centres of crystallisation but is unfavorable for the formation of large ice crystals.

 

Ice sample 22: general view (left) and structure under polarized light (right). Chuvilin et al. (2020).

The gas component was analysed in sample 9 of sandy permafrost samples 6, 21, 22, and 23 of ground ice. Intrapermafrost gas from sample 9 was extracted by 150 ml syringes from thawing, roughly 50 g specimens in a concentrated salt solution following a standard technique, using pure nitrogen or helium as carrier gas.

Methane was present in all samples: mostly a few cm³ per 1 kg of soil or ice. Its content reached 94 cm³/kg at the top of ground ice (sample 22) but was about 10 times less in sample 21 from the middle of the ground ice lens. All samples except for 21 contained much more carbon dioxide than methane, which may be evidence of cryogenic concentration in frozen sand, e.g. during freezing of a talik. The contents of ethane and propane (methane homologs) varied from fractions to 2–3 cm³/kg. The ratios of methane to its homologs in ice samples were generally from 2 to 40 and indicated the presence of both biogenic methane and a component associated with sediment maturation (deep gas). The carbon isotope composition of methane in ground ice, analysed at the Hydroisotope Laboratory (Germany) likewise suggests biogenic origin of the gas.

The obtained results have implications for the formation mechanism of the Erkuta Crater, which formed on the site of the palaeo-channel of the Erkuta–Yakha River. The contours of the dried riverbed can be seen in a photograph of 2017, as well as in a satellite image of 2013. As a result of evolution, the palaeo-channel gradually turned into an oxbow lake, which continued to degrade and split up into several small drying lakes. Then a heaving mound began to form within one of these dried-up lakes. Contour of a mound is a bright spot (probably slightly elevated and drained soils) against the background of a dark thawed water-saturated soils, which can be distinguished in a satellite image of the beginning of summer 2013. 

 

Satellite image of the Erkuta crater area at the beginning of summer 2013. Chuvilin et al. (2020).

The thermal eect of such lakes often produces a zone of unfrozen rocks (a talik) underneath. The lake sediments within the taliks contain organic matter recycled by microorganisms with release of biogenic methane. Additionally, gases can penetrate into the lake sediments from deep subsurface through permeable deformed zones. The talik beneath the Erkuta crater was most likely closed, given that the permafrost thickness in the area is about 200 m deep and the lake was small. The lake was gradually shoaling and shrinking whereby the talik was freezing from below and from the sides, which caused stress buildup inside the remaining confined talik. talik [19,20]. The stress released explosively by eruption of the gas–water–soil mixture from the freezing talik and the ensuing formation of the crater in its place.

Based on our results and available information, Chuvilin et al. propose the following conceptual model for formation of the Erkuta Crater.

 

Formation of the Erkuta crater in several stages: I: a lake and a talik underneath; II: onset of talik freezing after the lake has dried out; III: confined freezing of the talik and buildup of cryogenic pressure. Arrows show the expulsion direction of gas (blue dots); IV stratification of gas, water, and soil in the freezing talk and frost heaving; V: collapse of the frozen cap above the talik by pressurised gas (cryovolcanism); VI: active eruption of the gas-water-soil mass and onset of crater formation; VII: end of eruption and crater formation; VIII: lake formation as crater becomes filled with water. Chuvilin et al. (2020).

Stage I: A lake is underlain by a talik, with periodic inputs of organic matter into the lake in summer seasons. The organic matter in the lake sediments is recycled microbially with generation of biogenic methane which is accumulated in winter and emitted into the air in spring. The gaseous component of the sediments in the talik increases additionally due to migration of deeper thermogenic gases along faults and fractures in the crust. Emission of deep-seated gases from Arctic lakes is known from several other areas.

Stage II: Onset of talik freezing takes place after the lake had dried out. The talik undergoes confined freezing and becomes saturated with biogenic and deep-seated thermogenic gas.

Stage III: Confined freezing of the talik and buildup of cryogenic pressure takes place. Freezing of gas-saturated pore moisture under gas pressure at this stage has been studied previously by thermodynamic modeling in laboratory experiments.

As the lake is drying, the sub-lake unfrozen sediments are freezing from the top and from the sides, which leads to cryogenic gas concentration and stress buildup in the freezing closed talik. Gas-bearing sediments in the latter are confined by the surrounding ice-rich sediments, which increases gas pore pressure in the talik. The pressure may lead to ductile deformation of the permafrost cap above the talik if it exceeds the overburden pressure. At this stage, gas, water and soil in the residual talik can start to stratify.

Stage IV: Stratification of gas, water and soil in the residual talik and heaving. This process occurs in fairly homogeneous alluvial deposits. represented by sandy and silty sediments. As a result of stratification, heavier and denser soil stays on the bottom, while the light volatile gas component rises to the top; liquid water is in the middle. The layers of predominant soil, water and gas components are separated by dash lines in the figure, which are drawn tentatively because each layer contains some amounts of other components. If the pressure buildup is slow, the frozen cap can deform ductily, producing long-lasting heaves on the surface, pingo-like structures. However, if pressure increases rapidly, no slow loss heaving occurs before the stress release.

Stage V: Pressure increase in the talik saturated with water and gas and explosive pressure release breaking through the frozen cap. This phenomenon is known as cryovolcanism; eruption of water, fluids and liquefied soil triggered by overpressure in a freezing confined or open water-bearing system. The collapse of the frozen cap may be accompanied by outpouring of water or mud. No explosion occurs if the amount of gas in the talik is small, but the gas–water–soil mass from the talik erupts explosively and becomes dispersed, together with the frozen cap debris, in the presence of a gas cap.

Stages VI and VII: Progress of cryovolcanism. At stage VI, the gas–water–soil mixture erupts vigorously, and a crater starts to form. The explosive gas emission breaks up the meters thick frozen cap and disperses its material around the cryovolcano vent. A part of the ejected material falls near the crater and produces a parapet-like ridge rising above the surface around the crater, while some other part obviously falls back into the crater and gradually sinks to the bottom. As the eruption continues, the ejected unfrozen soil falls over the debris of the frozen cap. The soil, water and gas components of the talik, which were previously stratified during the confined freezing, mix again when erupting. (Stage VI). The level of the gas–water–soil mixture in the crater gradually decreases and the crater walls emerge. At stage VII, the eruption stops and leaves a crater, with its diameter commensurate with that of the residual closed talik prior to the emission. The ice-rich debris of the cap and the ejected talik sediments are scattered around the crater and cover its bottom.

Note that stages V, VI and VII follow one after another in a few hours to days.

Stage VIII: Stable evolution of the crater and its gradual transformation into a dierent landform. As the ejected material becomes involved into seasonal freezing–thawing cycles, the crater becomes filled with water in a few months and transforms into a circular lake surrounded by ejected material.

The suggested model explains the crater formation as a result of gas generation and accumulation in a sub-lake talik and the evolution of the talik exposed to confined freezing as the lake is drying out. Gas accumulation in the talik is additionally maintained by ascend of deep fluids migrating upwards through permeable faulted and fractured bedrock.

The data on the structure and composition of soil samples from the crater wall, as well as the proposed conceptual model for formation of the Erkuta crater, characterize it as a feature of explosive gas emission. Apparently, the crater formed in the place of a freezing closed talik under a dried lake by explosion of pressurized gas in the confined unfrozen sediments. The formation of the crater was preceded by rapid heaving within one or two years, judging by remnants of a mound detected in the first helicopter view of the area. The crater had a shape of a vertical cylinder with smooth walls, possibly because the explosion broke the frozen cap above the talik and mobilized the unfrozen soil–water–gas mass from the talik. A similar process of cryovolcanism has been proposed for formation of the Yamal Crater. Chuvilin et al. also consider the talik zone freezing. However, the essential role in the gas accumulation process (unlike the model for the Yamal crater) is played not by the biogenic gas generated in the bottom sediments of the thermokarst lake, but by the deep gas entering through the permeable zones.

The presence of fractures partly filled with ice on the crater walls also indicates that the freezing talik underwent buildup and partial release of stress. Our results indicate that the gas accumulated in and emitted from the talik came from two sources: it was biogenic gas resulting from microbially mediated decay of organic matter in lake sediments and thermogenic gas that migrated from deeper hydrocarbon reservoirs through permeable deformed bedrock. The proposed formation model of the Erkuta crater, unlike that of cryovolcanism suggested for the Yamal Crater, includes the contributions of thermogenic gas that had migrated along faults and fractures from deeper hydrocarbon reservoirs in addition to biogenic gas that formed within the talik.

In general, proposed formation model for the Erkuta crater associated with the emission of gas, which is accumulated in shallow permafrost. Its main feature is the consideration of the combined influence of deep-seated (deep gas migration) and shallow (oxbow lake evolution and closed talik freezing) causes in the process of Erkuta gas-emission crater formation. This vision is fundamentally dierent from the models of other authors, where only one prerequisites type of crater formation is considered: either deep-seated causes or only shallow ones.

The study presents exceptional data on the Erkuta gas-emission crater which was discovered in the summer of 2017 in the floodplain of the Erkuta–Yakha River on the Yamal Peninsula, south of all other craters of this kind found in the North of West Siberia for the past decade.

The main value of the research was the timely organized field trip to the crater in December 2017, which allowed collecting field data and sampling soil, ice and water before the crater became fully filled with water. The lifetime of these features being very short (less than 2 years), the soil and ground ice collected in December 2017 are the only samples suitable for laboratory analyses.

The study provides field data on the crater evolution in 2017–2018 and laboratory results for samples of frozen soil and ground ice from the crater walls. The crater formation was preceded by rapid heaving (within 1–2 years) detectable in aerial photographs. The presence of fractures partly filled with ice on the crater walls records buildup and partial release of stress in the freezing talik.

The carbon isotope composition of the gas component in ground ice proves the biogenic origin of methane in the surrounding permafrost. The presence of ethane and propane indicates that deep-seated gases generated during sediment maturation processes may have been involved into the gas-emission event. The higher contents of carbon dioxide compared to methane in several samples confirms the assumption of cryogenic concentration, which usually occurs during freezing of taliks.

The results are used to model the formation of the Erkuta gas-emission crater in shallow permafrost caused by the evolution of a talik under a dry lake, assuming a deep gas flow into the unfrozen zone. The model describes the crater evolution in several stages from geological prerequisites to the formation of a new lake-like landform.

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