methane in the mantle?

A recent report describes the discovery of methane seeps and unusual features called carbonate chimneys off the west coast of North America,

… “It appears that the entire coast off Washington, Oregon and California is a giant methane seep,” … a Lost City of carbonate chimneys towering 60 meters …

They don’t mention it in the article but the carbonate chimneys that were discovered are directly related to the methane seeps in that the the methane is the actual source of the carbon in the carbonate.

The carbonate chimneys are probably what is known in the trade as Methane Derived Authigenic Carbonates or MDAC’s for short. Authigenic is the term used to describe sediments that are generated in situ as opposed to being transported from elsewhere.
The carbonate is precipitated when the methane reacts with the hydrothermal mileu which includes both chemical and metabolic/biological activity.

The precipitation of authigenic carbonates at fluid seepage sites is a common phenomenon that can be triggered by the activity of a consortium of archaea and bacteria that oxidize methane close to the seafloor … or that can occur due to chemical reactions (i.e. without microbial mediation). Such carbonate deposits reveal different morphologies depending on combined internal (seepage-related) and external (setting-related) factors.

Being associated with methane, the carbonate features are widely distributed and occur in a variety of forms. The carbonates can be formed in both shallow and deep water and can manifest as both surface (chimneys) and subsurface features such as nodules or column/cone shaped features which go by the rather unwieldy name of Positive High Amplitude Anomalies (PHAA’s), a term which refers to their seismic characteristics (more consolidated than the background sludge).

… The carbonate morphologies include thin (∼1 cm) platy carbonate crusts, blocky and massive carbonate ridges up to several metres in size, and irregularly shaped carbonate deposits consisting of interconnected tubular and uneven intervals displaying high porosity. …

This carbonate formation is the opposite of the process needed to convert carbonate to methane or other hydrocarbon. Methane to carbonate is an oxidation reaction whereas converting carbonate to methane is a reduction or de-oxidation reaction. In chemical lingo it is called a redox reaction. Depending on energy and catalytic conditions, and ability to remove product, the reaction can be pushed in the opposite direction.

So there may be both chemical and/or biological pathways available via hydrothermal/catalytic (with generous amounts of finely divided metals in the vicinity the potential for catalytic activity is good) or biological activity that convert carbonate to methane/hydrocarbon. And although the presence of hydrocarbon is attributed to “cooking” of organic matter, the possibility that this reverse reaction may occur can’t in my opinion be summarily dismissed at present.

… The geologists also noticed that their [carbonate] rock samples smelled like diesel. They hypothesize that hot hydrothermal fluids migrating upward through the thick sediments of the Pescadero Basin ‘cook’ organic matter in the sediment, converting it into petroleum-like hydrocarbons — a process that has been observed at several other vents in the Pacific. Hydrocarbons may provide nutrition for the unusual microbes that thrive at these vents. …

This says nothing about the origin of the carbon in the geological column or whether the carbon in source rocks is a product of methane or vice versa, but it suggests to me at least that the idea of surface derived and subsequent burial of biological carbon needs to be kept under review.

Nor can one rule out the possibility that the methane (and its carbon component) is the product of some as yet unknown process operating in the upper portion of the mantle (perhaps the region which has access to water). There is some recent evidence that adds support to this possibility by way of the identification of methane in the inclusions of large diamonds which are thought to originate in the mantle.

… Here we report that large, exceptional gem diamonds like the Cullinan, Constellation, and Koh-i-Noor carry direct evidence of crystallization from a redox-sensitive metallic liquid phase in the deep mantle. These sublithospheric diamonds contain inclusions of solidified iron-nickel-carbon-sulfur melt, accompanied by a thin fluid layer of methane ± hydrogen, and sometimes majoritic garnet or former calcium silicate perovskite. The metal-dominated mineral assemblages and reduced volatiles in large gem diamonds indicate formation under metal-saturated conditions. We verify previous predictions that Earth has highly reducing deep mantle regions capable of precipitating a metallic iron phase that contains dissolved carbon and hydrogen.

Finally, one can’t rule out the possibility that the methane generation process includes some type of mantle metabolic activity as some lifeforms may be present even in high temperature/pressure environments.



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