More Carbon Could Be Stored Deep in the Earth

Cambridge University and NTU Singapore have found that slow-motion collisions of tectonic plates drag more carbon into Earth’s interior than previously thought. Only about a third of the carbon recycled beneath volcanic chains returns to the surface via recycling, in contrast to previous theories that what goes down mostly comes back up.

Carbon rich materials could be placed where tectontic plates are going into the earth and almost two-thirds of the carbon will remain trapped for billions of years. Subduction is the process that destroys old lithosphere. An oceanic plate can descend beneath another oceanic plate – Japan, Indonesia, and the Aleutian Islands are examples of this type of subduction.

There are a number of ways for carbon to be released back to the atmosphere (as CO2) but there is only one path in which it can return to the Earth’s interior: via plate subduction. Here, surface carbon, for instance in the form of seashells and micro-organisms which have locked atmospheric CO2 into their shells, is channelled into Earth’s interior. Scientists had thought that much of this carbon was then returned to the atmosphere as CO2 via emissions from volcanoes. But the new study reveals that chemical reactions taking place in rocks swallowed up at subduction zones trap carbon and send it deeper into Earth’s interior – stopping some of it coming back to Earth’s surface.

The team conducted a series of experiments at the European Synchrotron Radiation Facility, “The ESRF has world-leading facilities and the expertise that we needed to get our results,” said co-author Simon Redfern, Dean of the College of Science at NTU Singapore, “The facility can measure very low concentrations of these metals at the high pressure and temperature conditions of interest to us.” To replicate the high pressures and temperatures of subductions zones, they used a heated ‘diamond anvil’, in which extreme pressures are achieved by pressing two tiny diamond anvils against the sample.

The work supports growing evidence that carbonate rocks, which have the same chemical makeup as chalk, become less calcium-rich and more magnesium-rich when channelled deeper into the mantle. This chemical transformation makes carbonate less soluble – meaning it doesn’t get drawn into the fluids that supply volcanoes. Instead, the majority of the carbonate sinks deeper into the mantle where it may eventually become diamond.

White headed black arrows indicate carbonate flux and blue arrows water flux. Blue shaded areas indicate water-rich regions. The melting of carbonated igneous oceanic crust is not shown as it starts at depths of 300 km22. The image is to scale, apart from the thickness of oceanic sediments that has been exaggerated.

Nature – Deep carbon cycle constrained by carbonate solubility

Earth’s deep carbon cycle affects atmospheric CO2, climate, and habitability. Owing to the extreme solubility of CaCO3, aqueous fluids released from the subducting slab could extract all carbon from the slab. However, recycling efficiency is estimated at only around 40%. Data from carbonate inclusions, petrology, and Mg isotope systematics indicate Ca2+ in carbonates is replaced by Mg2+ and other cations during subduction. Here we determined the solubility of dolomite [CaMg(CO3)2] and rhodochrosite (MnCO3), and put an upper limit on that of magnesite (MgCO3) under subduction zone conditions. Solubility decreases at least two orders of magnitude as carbonates become Mg-rich. This decreased solubility, coupled with heterogeneity of carbon and water subduction, may explain discrepancies in carbon recycling estimates. Over a range of slab settings, we find aqueous dissolution responsible for mobilizing 10 to 92% of slab carbon. Globally, aqueous fluids mobilize 35+20−17% (27+16−13 Mt/yr) of subducted carbon from subducting slabs.

SOURCES- Nature Communications, University of Cambridge
Written By Brian Wang,