Kenneth,

Most of the biogenic methane seeps, such as found on the Norwegian Continental shelf and also in submarine deltaic regions, such as the Mississippi Canyon area (Gulf of Mexico) and the Nile Delta, are 'cold-seeps' (see for example Erwin Suess's last review paper). They are normally not regarded as having excessive heat that transmits to the water column. But, I am starting to doubt this simplistic way of thinking. This is because I know there are numerous seeps, regarded to be 'biogenic' cold-seeps, that are actually associated with warmer sub-surface fluids than those in the ambient (surrounding) water column. Therefore, I may agree partly with you, in that the seawater may be warmed slightly by this type of seepage also. However, my question is to which extent we should start collecting methane that is constantly seeping out of the seafloor. Is it an interesting and viable renewable resource of the same dimension as tidal energy and wind power? In certain areas of the world, I actually think it can (and should) be utilized.

No, Kenneth, I am referring to the seeping gases from the seafloor. They stream out of the seafloor, just like streams of water on land. (Biogenic, shallow gas resembles groundwater on land...). Please see our article from 2012, attached.

Martin,

It does not matter whether you are capturing biogenic or thermogenic gas;  the problem is the same.  Steel 'tents' were erected over the Coal Oil Point seeps as far back as 1982 and the gas piped ashore.  This type of harvesting, especially at shallow sites where a lot of the methane will enter the atmosphere, ought to be eligible for green subsidies.  This type of capture also applies to onshore seepage.  Biogenic gas has been utilised on a household scale in Vendsyssel, Denmark, since at least 1942 using pits in the ground covered in polythene with a piped outlet into the house.  (I think there was only a single case of a gas exposion wrecking the property.)  More robust tents are needed for collecting gas offshore, especially at depths subject to storm surges.

Thanks, Paul,

yes, that is a spectacular onshore and offshore methane seepage region in the town of Fredrikshavn on the eastern coast of Northern Denmark. There must be many tonnes of methane entering directly into the atmosphere every year. Some of that gas could be capped and, utilized, instead of entering directly into the lower atmosphere.

The source varies.  Only a minority of described seeps are from melting clathrates. This is more common in deep water and in the Arctic.  Some seeps are of thermogenic methane and there are examples of carbon dioxide seeps that contain only a minor amount of methane.  Most shallow-water seeps are from biogenic gas reservoirs and the gas is channeled to the surface along fault lines.  For example, in the North Sea nearly all the described seeps are of shallow biogenic methane, even though there are thermogenic gas reservoirs deeper in the sedimentary column. The methane may have been generated in interglacial deposits, as in the shallow Danish seeps, or have come from more recent carbon in fast-sedimenting areas such as the Skagerak and northern Gulf of California, or be from mixed sources.

Kenneth, the rough 'rule of thumb'for marine sediments is that the delta 13C value for thermogenic methane is greater than -60 ppt whilest that for thermogenic methane is less than -60 and can be as low as -100.  You can also use deuterium ratios and amounts of higher hydrocarbon gases to help discriminate.  One problem is that aerobic bacteria can enrich the residual methane by up to 50 ppt ,making biogenic methane look like thermogenic.  The source of the biogenic methane (formation by CO2 reduction or from acetate) contributes to the wide range in isotope ratios.  Note that most biogenic methane rising to the surface is oxidized within the sediment.

I think maybe, Paul, the values of DelC13 for thermogenic methane should be between -35 to -50, whereas, the biogenic methane is reckoned to have lighter values, i.e., from -50 to -120.

The primordial methane is reckoned to be heavier than -35, i.e., generally around -12.

Well, Ken,

as long as one of the largest source of methane on this planet seems to come from the serpentinization process in the ocean, the amount of primordial methane that feeds in to the bacterial food chain must be immense. This means that methane with C13 of -12 feeds into methanotrophs and is converted to CO2, which then feeds into the methanogens and comes out as biogenic methane at C13 of -60 permil.

Serpentinization occurs when the magnesium-rich olivine in the rock peridotite reacts with seawater at temperatures around 200 C. The Lost City Vents is one of the studied sites.

Sorry it took me 4 years to find this question from you, Ken

All the best

Martin

Thanks, Ken,

good to hear from you again.

We are now getting more interested in the methane cycles in the ocean. It's so complicated that it seems like a new universe...Sorry, have to stop there, of propriatory reasons....

Thanks, Ken,

In Norway, the politicians have just decided to provide money to extract CO2 from the Cement industry near Oslo (3 billion Kroner) and after that they don't know what to do with it. The only way of getting it out of the atmosphere seems to be shipping it by boat or tanker trucks to the west coast, and to feed it into pipelines and bury it in the North Sea sediments.

We call it the Norwegian Second Moon Landing. This is because Jens Stoltenberg about 10 years ago managed to Squander about 9 billion Kroner on a similar project, which ended up in northing usable at all, i.e., the First Norwegian Moon Landing.

It seems we are doing this to get good marks in the EU, and the Paris 'Agreement''.

Thanks Ken,

we found a lot of methane gas in a shallow fjord basin near Stavanger recently. This discovery came as a spin off result of some seafloor mapping with high resolution Sub Bottom Profiler (SBP) and sidescan sonar, done last year.

We are looking for human (Viking) artefacts that are older than 100 years and up to 1200 years. There was a severe battle in the fjord in 872, so there should be some remains. So far, we have documented 2 out of 120 'anomalies', and one of these objects turned out to be a very well preserved ~150 year old wreck of a 36 foot sailing vessel. It is lying at 50 m water depth, and is near intact, even with some rope on the anchor 'winch' arrangement. The remarkably good condition of this wreck must be because there is little or no oxygen in the water and sediments it's lying in.

We have also found nearby indications of plenty of methan gas in the sediments. The gas is leaking out of the sediments in pockmarks and elsewhere and may be a reason the water is free of oxygen. This is exiting work, and I will definately do some more work on the gas (CH4) aspects. Perhaps this is the location we have been looking for to test the concept of utilizing biogenic methane leaking out of the seafloor, on a small scale.

My idea is to place a water-filled tank over the seep, open two valves in the tank, one for the gas to seep in, and one for the water to be pushed out of the tank. The concept I have called SUMECO (Submarine Methane Collection System). When the tank is full of gas, both valves can be closed and up it rises to the surface. In the tank there is combustible, renewable, sustainable, climate-friendly methane (if you oxidise it) at a pressure of ~5 bar. You place the tank in your garden, and plug it into your house's gas system and you have free energy for ever.... So, the main question remains: Is it going to be cost-effective. Or, in other words: Will the government or anyone else subsidise such a concept?????

The attached image has Norwegian caption, but will be easily understood even so.

Yes, sure, Ken,

but, before the excess methane leaves the sediments, some of it is oxidized via a consortium of bacteria/archaea, in a process called AOM (anoxic oxidation of methane), which includes SRB. So, I think these processes, together with those you mention, are still only poorly understood. I would like to take some water samples from the fjord and perhaps do some 16S rRNA gene analyses to see what is produced into the fjord water column, etc.

So far, not a single object...., I am sorry to say.

However, we have identified 118 'targets' or 'anomalies' on our geophysical data (SBP & SSS), and will be using divers and cameras to check these, over the next year or so. We hope that a couple of these will represent ballast stone heaps, remaining from the 100 or so viking ships that must have participated in the epic battle...