"Our work represents a breakthrough in the current understanding of the global carbon cycle," team member Valier Galy of the Woods Hole Oceanographic Institution, Massachusetts, told nanotechweb.org. "Our findings are very interesting for the long-term modelling of the Earth's climate, which actually relies on our understanding of the carbon cycle."
Galy and colleagues have shown that a significant proportion (up to 50%) of the carbon contained in rocks survives erosion (a natural process) and returns to the Earth's sedimentary reservoir. More importantly, most of this re-buried carbon is in the form of graphite. Graphitization occurring during metamorphic reactions (when mountain ranges form) produces an extremely stable form of carbon that remains trapped in the geological reservoir over very long periods.
"This mechanism is a long-term sequestration of atmospheric carbon dioxide and a long-term source of oxygen," explained Galy. "At geological time scales it partly controls the atmosphere's global chemistry and hence global climate and the evolution of life itself."
The team, which includes researchers from the Ecole Normale Supérieure in Paris and Nancy University, obtained their results by analysing crystallised carbon particles (ranging from a few nanometres up to around 20 µm) in river and marine sediments in the Himalayan range using Raman microspectroscopy and high-resolution transmission electron microscopy. This is the first time that carbon derived from rocks in these kinds of sediments has been directly studied.
The researchers then used radio-carbon dating of river sediments in the region to calculate the proportion of organic carbon derived from rocks and the proportion from the biosphere.
They are currently applying their method to rocks in other mountain regions around the world, such as the Andes. "The next crucial question is to estimate how much of the recent biosynthetic organic carbon is transformed into graphite (and other crystalline forms of carbon) and how much returns to the atmosphere as carbon dioxide during metamorphic reactions," added Galy.
Graphite is found in rocks rich in carbon but its exact role in carbon sequestration had not been studied until now. Most of the world's carbon lies in the Earth's crust: 15,000,000 PgC compared with 38,000 PgC in the oceans, 1600 PgC in soils and 750 PgC in the atmosphere. (1 PgC = 1015 g of carbon).
The work was published in Science.