Chasing carbon deep into the Earth
Although the Earth and Venus are very similar planets in terms of mass and proximity to the sun, one very significant difference between them is that Venus has no plate tectonics. You might imagine that would mean that the continents on Venus don’t drift in the way those on Earth do, and you’d be absolutely right. But plate tectonics has much wider impact that just moving land masses around.
As tectonic plates collide one tends to slide beneath the other sinking deep into the planet’s mantle – a process geologists call subduction. Rocks on the surface of the subducting plate are carried deep into the mantle and undergo many chemical and physical changes under the extreme temperatures and pressures there. Where plate boundaries lie under the sea, the subducting rocks carry with them vast amounts of carbon dioxide that becomes chemically incorporated into the upper 500m or so of ocean floor basalt. Added to this is the carbon fixed by marine organisms, whose skeletal remains sink to the ocean floor creating vast carbon rich sediments.
This process, known as the Deep Carbon Cycle doesn’t happen quickly, it takes millions and even billions of years. But what it does do is help provide the Earth with a carbon cleaning mechanism that keeps CO2 levels in the atmosphere low enough for life to thrive and stops the Earth becoming another Venus.
The Deep Carbon Cycle isn’t something that can solve the rising CO2 created by human activities, at least not on a useful timescale, but it can help us to learn more about how the Earth deals with carbon. This in turn enables scientists to put climate data from the very distant past into context and refine the atmospheric models that are vital for dealing with our current problems. It may also help us to understand how we can better assist nature to undo some of the damage we’re currently inflicting on our planet.
This process is considered so important to science that in 2009 the Deep Carbon Observatory (DCO) was created at the Geophysical Laboratory of the Carnegie Institution of Washington. Although based in the US, this is a truly international effort involving many universities around the globe.
Dr Greg Yaxley from the ANU Research School of Earth Sciences, is leading a DCO research team based in Australia. “Although the basic principle of the deep carbon cycle is largely understood, a great deal remains to be learnt about the detailed chemical processes that the subducting rocks undergo.” He says.
Some of the subducted rocks are returned via volcano directly above subduction zones in what forms the fast part of the cycle, though fast in this case is millions of years. However a significant amount of subducted carbonate material is carried hundreds of kilometres deep into the mantle and stored there for billions of years, until it reappears at the Earth’s surface as carbon dioxide bubbling out of lava erupted from many volcanoes. Some of these volcanic rocks called kimberlites sometimes contain diamonds which are a high pressure form of carbon, and this carbon may have been recycled over billions of years via subduction, as described above. Some diamonds occasionally contain included minerals such as a type of garnet known as majorite which can only have formed hundreds of km deep in the Earth’s mantle. This shows that recycling of carbon via subduction is often ultradeep.
To understand the details of these processes, scientists like Dr Yaxley, must develop laboratory equipment that can simulate the conditions of the deep mantle in the lab. The difficulty is that when you get 500km below the surface of the Earth, the pressure becomes enormous. Imagine piling a hundred Mt Everests one on top of the other – that’s the weight of 500km of rock pressing down on every part of the deep mantle.
To achieve those pressures, the team use a press fabricated from massive steel plates salvaged from a World War Two battleship. The press is capable of exerting a force of 1200 tons and when that force is applied to a sample container about one mm across the local pressure becomes 250,000 atmospheres – almost 4 million psi. The sample container can be electrically heated to over 1200°C and differing oxygen levels applied creating a very similar environment to that deep in the Earth.
It would be possible to perform these experiments on actual rocks, but imperfections and impurities would make the data more difficult to interpret. So instead, the team actually make their own rocks. “We have all the chemical constituents of various minerals available in the lab so we can create samples that are structurally and chemically identical to those found in nature but have far higher purity.” Dr Yaxley explains, “This technique also allows us to introduce or eliminate some of the elements in a particular rock which makes it easier to investigate each of several specific processes in isolation. Seeing various chemical pathways work individually greatly helps in understanding how they all work together in real rocks.”