I worked through all 530 AGU abstracts mentioning methane and found two more of interest. The underwater geology of Arctic Ocean continental shelf has a rich and complicated history mostly unfamiliar to mid-latitude climate scientists, leading to poorly-grounded objections to ESAS methane release that have to be tediously explained over and over.
The four N Shakhova papers from 2017 are all open access. The abstracts really don't capture what the best of what's in the articles; often the internal discussion provides much better background and explanations of the significance. Comments and questions of peer-reviewers are also available and instructive.
The April 2017 interview with Shakhova briefly summarizes responses to many asinine objections raised up over the years by CO2-oriented scientists who perceive methane as mainly a threat to the primacy of their preferred narrative. It's probably worth pulling together more detailed responses from these recent articles as well as excellent material scattered up-forum.
https://www.the-cryosphere.net/11/1333/2017/tc-11-1333-2017.pdf open access
https://www.biogeosciences.net/14/2283/2017/bg-14-2283-2017.pdf open access
https://www.nature.com/articles/ncomms15872 open access
https://www.the-cryosphere.net/11/2305/2017/tc-11-2305-2017.pdf open
http://envisionation.co.uk/index.php/nick-breeze/203-subsea-permafrost-on-east-siberian-arctic-shelf-now-in-accelerated-decline OS43B-02: Relict thermokarst carbon source kept stable within gas hydrate stability zone of the South Kara Sea
A Portnov et al
Substantial shallow sources of carbon can exist in the South Kara Sea shelf, extending offshore from the permafrost areas of Yamal Peninsula and the Polar Ural coast. Our study presents new evidence for >250 buried relict thermokarst units. These amalgamated thawing wedges formed in the uppermost permafrost of the past and are still recognizable in today’s non-permafrost areas. Part of these potential carbon reservoirs are kept stable within the South Kara Sea gas hydrate stability zone (GHSZ).
We utilize an extensive 2D high-resolution seismic dataset, collected in the South Kara Sea in 2005-2006 by Marine Arctic Geological Expedition (MAGE), to map distinctive U-shaped units that are acoustically transparent. These units appear all over the study area in water depths 50-250 m. Created by thermal erosion into Cretaceous-Paleogene bedrock, they are buried under the younger glacio-marine deposits and reach hundreds of meters wide and up to 100 meters thick.
They show the characteristics of relict thermokarst, generated during ancient episodes of sea level regression of the South Kara Sea. These thermokarst units are generally limited by the Upper Regional Unconformity, which is an erosional horizon created by several glaciation events during the Pleistocene.
On land, permafrost is known to sequester large volumes of carbon, half of which is concentrated within thermokarst structures. Based on modern thermokarst analogues we demonstrate with our study that a significant amount of organic carbon can be stored under the Kara Sea.
To assess the stability of these shallow carbon reservoirs we carried out GHSZ modeling, constrained by geochemical analyses, temperature measurements and precise bathymetry. This revealed a significant potential for a GHSZ in water depths >225 m. The relict thermokarst carbon storage system is stable under today’s extremely low bottom water temperatures ~ -1.7 °C that allows for buried GHSZ, located tens of meters below the seabed.
Noteworthy, vast parts of GHSZ do not expose on the seafloor, since both upper and lower GHSZ boundaries occur clearly sub-seafloor. Our findings show that under the deepest regions of the South Kara Sea, large areas of relict thermokarst may presently exist within the GHSZ of unique configuration, and therefore provide substantial methane source for gas hydrate.
B21C-1973: Methane fluxes from intense bubbling seep sites: Mapping and Quantification from the seafloor up to the atmosphere
J Greinert
Despite the ever increasing number of seep sites being discovered in shelf and continental slope areas, sites where dissolved or free gas fluxes at the seafloor fuel a significant sea surface gas flux into the atmosphere are rare. Here, we report on multi-year studies from a very active seep site in the Dutch North Sea that has been revisited several times since 2009, with large-scale surveys including multi-beam based bubble mapping, CTD water column sampling, direct ROV observations, sub-seafloor free gas mapping and CRDS-based sea surface flux and atmospheric measurements.
More than 800 individual flares in five main clusters were recorded and first approximations yield 280L of CH4 per minute being released from the seafloor in the entire area. These fluxes created sea surface anomalies even in the strongly stratified water column during the summer period.
Atmospheric concentrations increased by almost 1ppm above the strongest flare cluster in 42m water depth. Currently ongoing studies that aim at merging single-beam and multi-beam echosounder data on a meter scale will verify if the previously calculated seafloor gas flux estimates are correct, or if even higher fluxes occurred that explain the significant increase in the atmosphere. Spatial bubble dissolution modeling will be applied to calculate if the newly determined fluxes can support the measured sea surface concentrations and if ocean-atmosphere equilibration supports the observed atmospheric increase.
In any case, the clear spatial correlation between seafloor gas release, sea surface and atmospheric anomalies prove that the methane emanating from the seafloor is the source of the increased atmospheric CH4 concentration. Optical studies show that massive and constant gas release is needed to have such an effect. This study can be used as an ideal case study for comparison to other high intensity seeps and their potential for having local effects on CH4 budgets.