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Yet another unknown “carbon sink” revealed

Hercules ROV shown above deep ocean floor basalt (NOAA)

Hercules ROV shown above deep ocean floor basalt (NOAA)

Recently, NASA released the results of a study of the temperatures of the waters of Earth’s deep ocean have not warmed measurably since 2005, leaving unsolved the mystery of why global warming appears to have slowed in recent years.

Owing to a ‘global pause‘ in rising temperatures, climate scientists have increasingly having to resort to other processes in nature to account for the flattening of warming temperatures over the last 17 years. One of the more popular theories is the sea has been absorbing that CO2. Although satellites seems to confirm that the upper ocean water (0-700m) temperatures are rising, the recent NASA study indicates the same cannot be said for the ocean at depth. Given the ocean has an average depth of 4,267m (NOAA), and can go down as deep as 10,911 m in the Mariana Trench. That is a lot of water that is not warming.

However, in the recent NASA article, they professed to not know why the deep ocean water was not warming.

The blue/green areas correspond to ocean floor, most of which is ocean floor basalt (NOAA)

The blue/green areas correspond to ocean floor, most of which is ocean floor basalt (NOAA)

And then I read an interview in New Scientist (19 July, 2014) on geologist, Juerg Matter. Juerg was inspired by basaltic rocks in he saw in Oman:

When I was working in Oman, I saw these really blue, alkaline rock pools with white deposits at the bottom. The rocks were mantle peridotite, which reacts with CO2 to form white carbonates.

When he went onto work at Colombia University’s Lamont-Doherty Earth Observatory in New York, he met Taro Takahashi and Dave Goldberg who wanted to look at a way of locking up CO2 using mineral carbonation. He informed them he’d seen that in Oman.

In Iceland, they began testing trapping CO2 in the rocks:

The standard method of carbon storage is injecting pure CO2 deep into the Earth’s crust. But the risk with that approach is that the gas could leak back out. So in our pilot project in Iceland, called CarbFix, we take CO2 and wastewater from the same geothermal power plant and inject them together. The CO2 dissolves and, like in a bottle of sparkling water, it stays dissolved as long as it’s sealed. It then reacts with calcium and magnesium silicates in rocks to form carbonates.

Ironically, the results shows the CO2 reacted far quicker than predicted, meaning at present, they are trying to find out why – they don’t want the CO2 precipitating out so quickly that it blocks the porous holes in the basalt.

However, any new ‘discovery’ (in inverted commas there because nature discovered it first), the process does come with some risks and costs:

Anything you inject into the subsurface – CO2, liquid chemical waste – could produce tremors. When CO2 reacts with silicate it makes a less dense carbonate that fills more space. If porosity is limited in the subsurface you could raise the ground. We have a lot of CO2 to put under the Earth’s surface, and if you inject vast amounts of it, the subsurface could rise. In Algeria, the surface lifted at a conventional carbon storage project called In Salah, but by millimetres, not metres. It could happen with this approach too.

For the injection and storage process at the CarbFix site in Iceland it’s about $17 to $30 per tonne. (For context, a car produces roughly a tonne of CO2 every 5000 kilometres.) At $17, it’s about twice as much as direct CO2 injection, but these costs don’t include monitoring or capture.

When asked if by targeting the basalt as a means to lock up man-made CO2, would we use up the resource, the response was:

Dave Goldberg found that just one ocean ridge site – admittedly covering tens of thousands of square kilometres – could potentially store the total amount of CO2 we’ve already emitted into the atmosphere. The storage potential of all the ocean ridges is around 10 times larger than the CO2 emissions we’d get from burning all of Earth’s fossil fuel. There is no way we will use up this resource. Although how much can be practically used may depend more on politics and economics than science.

I have no idea whether New Scientists just didn’t ask him the question or not – but I want to ask – is it also not possible that the deep ocean floor basalts are already locking up the CO2 via this process? And given the ocean floor basalts cover more than 55% of the planet (oceans are more than 70% but 11% are continental shelf), is it not possible that is one awfully large carbon sink currently not included in climate models?!

However, curiosity aside, I will say that I’ve seen calcrete-like rock form (massive, white rock)  over komatiites in the Western Australia outback, but only rarely seen infilling some air bubbles in basalt.. The difference? Komatiite, like the peridotite that is referred in the interview with Juerg Matter, are rocks which are low in silicon, aluminium and potassium, but very high in magnesium. Komatiitic lavas used to erupt regularly on the Earth, but as the Earth has cooled over geologic time, it largely stopped appearing at surface about 500 million years ago. As the basalts of the ocean floor are no older than 180-200 million years old, there is likely very little magnesium-enriched komatiites on the ocean floor itself.

So as long as the processes being involved can be made to occur on basaltic rocks without the addition of massive amounts of magnesium, finger’s crossed this process will work for everyone who wants an ‘invisible’ way to store man-made CO2!

About Sally G

Sally is an Earth Scientist with experience in geology and geomorphology. She has always had an interest in general science.