Continued: The story of the deep carbon cycle…

… and the big black bear

Last summer, I was fighting my way through the boreal forests of Newfoundland in Canada, a place well renowned for its wildlife. Hence I was heavily armoured with big hammers, a huge can of strong pepper spray and some bear banger cartridges in my pocket, always smelling of mosquito repellent and making a lot of noise, in order not to surprise a sleeping black bear in the bushes. In the end, I didn’t need the bear spray or the cartridges, but every now and then signs of bears in our field area reminded us of their presence. And I took these safety measures much more seriously after we encountered a black bear close to a local landfill: you just feel small and vulnerable in front of such a huge, beautiful and elegant animal that is only 20 meters away from you, even if your car is just two meters behind you.

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Fig1. Some of the wild life a geologist might need to be worried about.

But why do I need to a go to a remote peninsula in Canada, full of bears, moose and mosquitos? Couldn’t I just study the rocks outcropping in Spain, close to where I am living at the moment?

As I wrote in a previous post, I am studying processes of the deep carbon cycle, and I try to understand how carbon is transported from oceanic plates via subduction into the Earth’s mantle, and how much of it eventually comes back to the surface. It is of high importance for the climate of our planet and its habitability! Of course this is a huge topic that many people are investigating, so I focus on the role of oceanic mantle rocks in the carbon cycle. One initial question in this field is: how and how much carbon is being incorporated into mantle rocks below the oceans before they get subducted? In order to find answers to these questions, we need to look at examples where oceanic mantle rocks partially reacted with CO2, which is dissolved in seawater that hence can circulate within these rocks. The best is to benefit from examples where different stages of such a reaction are recorded in the rock, for example because at some point fluid circulation was inhibited and hence the supply of CO2 necessary for the reaction stopped, or because the reaction did not reach completion for multiple other reasons. Oceanic mantle rocks are commonly below 7 – 8 km of oceanic crustal rocks, so it is not that easy to reach them. We could try by scientific ocean drilling, but even if we are very lucky and find mantle rocks that reacted with CO2, we would not be able through a very small number of drilled holes to see the geometry and the larger context. Luckily, tectonic processes during the formation of mountain ranges sometimes carry such rocks onto the land surface (then called ophiolites), where we can study them more easily and get a larger picture of the processes. Nevertheless, oceanic mantle rocks in ophiolites are not that common, and examples where we can study their behaviour when a large amount of CO2 was flushed through are even less common. That is one of the reasons why I found myself in the forests of Newfoundland in Canada, which is, compared to other places geologists might go to work, still quite easily accessible.

Besides the academic motivation to study these rocks, there is also a more societal relevant reason why many people are currently interested in them: Maybe we can use what we learn from the natural reaction of CO2 with mantle rocks to get rid of excess CO2 from our atmosphere (like it is done for example in this pilot project).

But before we come to that, we need to understand the details of how carbon is removed from seawater or mantle fluids and fixed within rocks – a mechanism that is named “carbon sequestration by mineral carbonation” or “mineral storage”. The basic principle is that some silicate minerals readily react with CO2 to form carbonate minerals, and that this reaction can proceed very fast (it may take nature between 10 – 10000 years to completely convert a large volume of mantle rock into carbonates, if the reaction is not enhanced artificially – for a geological process this is really quick). Some of those minerals that show the fastest reaction rates are Mg-rich silicates like olivine and pyroxene, which are the most important components of rocks from the uppermost mantle. In a very simplified manner, the carbonation reaction can be written like this:

MgSiO4    +      Mg2Si2O6    +       3 CO2          =       3 MgCO3    +      3 SiO2

(olivine)       (pyroxene)         (from fluid)              (magnesite)            (quartz)

This describes a full carbonation reaction, where all non-silicon cations of the precursor minerals react with CO2 from the fluid circulating within the mantle rocks to form magnesium carbonate minerals like magnesite. The resulting rock then consists mainly of magnesite and quartz and is named listvenite. Don’t worry if you have never heard this rock name before, it is a very rare rock type.

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Fig2. Left: found some listvenite rocks! Right: picture of a bear-banger cartridge in front of mantle rocks that have been carbonated along fluid pathways.

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Nature is more complicated than the easy chemical reaction listed above of course. For example, olivine and pyroxene also easily react with water, so there are intermediate reaction steps. One of them produces a talc-magnesite rock, soapstone (soapstone is maybe better known than listvenite, because of its historical use for stone pottery and carvings). The key point of a carbonation reaction is that mobile carbon from a fluid gets fixed in the mineral structure of carbonates, where it will remain for millions of years. The described carbonation reaction is an exothermic reaction, which means that the breakdown of minerals like olivine and pyroxene in the presence of CO2 is energetically favourable at temperatures which are typical of hydrothermal fluids circulating along Mid Ocean Ridges (100 – 300 °C; read some more on hydrothermal activity here) ; the reaction may even produce heat. Therefore, in principle the reaction could run by itself in such a context, as long as enough CO2 is provided. But here is one of the problems: besides forming through the here described replacement reactions, carbonates can also precipitate directly from the fluid (calcium-bearing seawater in our case favoring the precipitation of calcium carbonate CaCO3). And when that happens, any pathways used by  the fluid are quickly clogged by the precipitates, preventing the CO2 to further come into contact with the Mg-rich silicate minerals. In addition, there are other natural processes that cause problems, like nanometer-thin silica-rich films that can form around the minerals when silicates dissolve and protect them from any further interactions with CO2. And when microbes are living in those rocks, things get even more complex and unpredictable, as they turn part of the CO2 into biomass. These are some of the reasons that hampered the idea to just pump CO2 into mantle rocks in ophiolites to turn it into solid carbonates and hence help mitigating CO2 emissions into the atmosphere. But the natural examples of listvenite rocks and soapstones show that, sometimes, the carbonation reactions can run to completion, fixing very large amounts of carbon in oceanic mantle rocks. So we may just need to better understand the process in nature, before we find a way to use it for our purpose – the purpose in this case being nothing less than to try to keep the global temperature on an acceptable level.

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