The story of the oceanic crust and the deep carbon cycle

Where does a story start? Or should we better ask: Where does it end? Some stories even run in circles, happening all the time. One of those stories is the global cycle of chemical elements, a topic many geoscientists are investigating. In the ABYSS project, we all study phenomena that are more or less related to the formation of new oceanic crust. That is the start of a story: magma from the Earth’s mantle rises underneath mid-ocean ridges, where plates are moving apart below kilometers of water; the magma crystallizes and the resulting rocks form these new plates. We can already tell a lot of stories about the processes that happen while new crust is formed, in this dynamic zone of interactions between water, rocks and magma. But in this post, I would like to put those into a larger context: the story does not end after the oceanic crust cools down and is carried away from the mid ocean ridges, where it formed.

MM1_fig_1.png
Sketch of the deep carbon cycle – how carbon is transported through Earth`s interior.


The fate of most oceanic crust is subduction underneath another oceanic or continental plate (first image), after being transported for times that range from  ~10-20 million years (as is the case for the Cocos plate, Mid-America) to ~160 million years (that is the age of the Pacific oceanic crust before subducting underneath Japan). Only a very small percentage of oceanic crust escapes this fate and is eventually transported onto continents – forming what we call ophiolites.

But is that how the story ends? The oceanic crust is either being recycled into the mantle or, more rarely, being accreted to continents? In principle, we could say yes – the rocks of the ocean crust are going back down into the mantle, where they started before as melts. However, what goes down has been significantly modified since the rocks crystallized from the primary melt at a mid ocean ridge. For a start, a lot of water has been added to rocks along their journey at the seafloor by seawater circulation within fractures and faults. The water stored in pore spaces and within the crystal structure of the rock minerals below the seafloor sums up to 20 – 30% of the oceans above. Along with this water, a bunch of elements is added and carried by the oceanic plates into subduction zones and eventually into the mantle. There, 20 – 80 km below the surface, the subducting oceanic rocks slowly heat up, which causes water-bearing minerals to break down and release their water. Since water has a much lower density than the surrounding rocks, it rises into the overlying warm mantle rocks as soon as it is liberated from the minerals – which can partially melt in response to presence of water (just like adding salt to ice is reducing the melting point of ice, water lowers the melting point of rocks). Broadly speaking, released water triggers the formation of magma, which then rises through all the rock layers above because of its lower density. Water can dissolve in the magma, is transported back to the surface in it and exits the Earth’s inner parts via volcanoes like those found in the Andes. But as water makes its way down to the mantle and back to the Earth surface, it carries a lot of other elements and compounds. In fact, so many compounds and gas phases can be dissolved in water at high temperatures and pressures that its properties are quite different than pure H2O, which is why normally geologists speak of fluids (gas-water mixtures) instead of water. Probably the most important compound that is carried together with the water is carbon, a key element for energy, climate and life.

We know now that the carbon content in the atmosphere (as CO2) has a great effect on Earth’s surface temperature and climate, and we know that the atmospheric carbon content is intimately linked to the CO2-content in the oceans and within biological matter. This is what we call the short-term carbon cycle, because a carbon-bearing molecule can pass through all of these three main reservoirs (atmosphere, oceans and biosphere) quite rapidly – equilibrating within a timescale of a few thousands of years. That is why we can already measure an increasing CO2-content in the ocean water, caused by our increased emissions of CO2 through burning of fossil fuels. What is not known so much is that a very large amount of carbon undergoes a similar deep cycle as I described above for water. That is the long-term (or deep-) carbon cycle as shown in the sketch in the first image, working in a timescale of millions to billions of years.

Carbon is removed from the short superficial cycle as CO2 dissolved in seawater infiltrates into rocks of the ocean crust. There, it can precipitate and can even replace some rock types completely, forming carbonate minerals (see second image). Additionally, carbon is removed from the oceans via sediments: carbonate shells of (micro)organisms and other biogenic matter can accumulate on the seafloor and are partly carried deep down into subduction zones.

From there on, we are still not entirely sure what happens to the carbon. We know that quite large amounts of CO2 follow pathways similar to water, being released back into the atmosphere through active volcanoes. However, we also know that carbon is not uncommon in the deeper parts of the Earth’s mantle, where it is present as diamonds (the high-pressure phase of pure carbon) or graphite (the low pressure one). But how much carbon is going down that deep, and how much of it is rising up to the surface with magmas or supercritical fluids? Is there more going down with subducting plates than is coming up in volcanoes? Or is it the other way around, or equal?

manuel 2
Carbon stored in “sea-rocks”: White carbonate minerals precipitate from seawater that flows through cracks (left); and carbon-rich oceanic sediments (right); the white layers are carbonates while the dark layers contain a lot of clay and organic carbon.

You may ask now why we care, since a carbon atom needs millions of years to go through that deep cycle once, apparently not affecting us very much. But over the course of geological times, things are different. If we want to know how the climate was hundreds of millions years ago or how and why it changed (we know it did change), if we want to know how diamonds form and how life has emerged on our planet, these questions matter a lot. Hence, we as geologists are interested in the conditions at different tectonic settings at different depths and temperatures. In order to correctly interpret our observations in rocks and to deduce what reactions and processes occurred, we also need to have an idea of what elements are present in what amounts, how much oxygen is available in the system (this is called the redox-state), and what the pH of a fluid is. All these properties depend a lot on how much carbon is present, so we should always be concerned about that. And in fact, quite a lot of scientists are investigating the processes and fluxes involved in the deep carbon cycle – about 80 of us met in July this year for a workshop organized by the Deep Carbon Observatory (funded by the Sloan Fundation) in Berkeley (USA), just to talk about these matters. Have a look at their website if you are interested – over 700 scientists worldwide are associated with this thematic network. And of course, keep in mind that carbon is just one element of many from the whole periodic table that undergoes a deep recycling through Earth`s mantle.

In case you are still not convinced, just think about this: during the last 2-4 billion years, statistically, a lot of the carbon atoms in your body (if not all) have gone through the described cycle one or several times – brought to depths of tens of kilometres below the surface and shot out in violent eruptions from volcanoes, before being incorporated in plants and eventually in your cells. Wouldn’t it be nice to know what all that carbon in our bodies experienced throughout the geological history? And wouldn’t it be a good idea to artificially put some of the CO2, which we produce in too high amounts, back into rocks of the ocean crust, so that it stays fixed in the deep carbon cycle for millions of years instead of being discharged in the atmosphere and changing our climate? This is called mineral storage; but that is a story worth telling another time.

Next time, I will show you what strategies geologists use to explore the deep carbon cycle, and why I needed to care about bears and other wildlife while doing so.

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