The deep carbon cycle
Ultra-high pressure marbles occur in subducted slices of oceanic and continental crust, providing evidence that carbonates are stable to mantle depth within subduction zones and thus represent part of the long-term burial of carbon. One key aspect of the deep carbon cycle is how CO2 is recycled in subduction zones, which will determine if the carbon is returned to the exogene cycle in (geologically) short time scales of a few million years or whether carbon is subducted to the deeper mantle and potentially sequestered for several hundreds of million years. This has great influence on the long-term concentration of greenhouse gasses in the Earth atmosphere. Therefore, for the correct interpretation of geological climate records, it is fundamental to understand the deep carbon cycle.
The Alpine-Apennine belt provides an excellent natural laboratory to study key aspects of the deep carbon cycle. Firstly, there are extensive outcrops of ophiolites, which allow studying carbon uptake during alteration of oceanic crust. Of particular importance is that there are slices of Cretaceous ophiolites. We plan to test whether the higher CO2 content of the atmosphere at that time led to a more extensive uptake of CO2 in the oceanic crust alteration. We plan to study the release of CO2 during subduction from carbonate-bearing rocks that were subducted to different depths. We are particularly interested on how CO2 bearing fluids released from subducted crustal materials interact with ultramafic rocks of the mantle wedge as this potentially represents a so far neglected major reservoir of carbon on Earth. The Alps provide a whole range of rocks that experienced conditions from lowest grade metamorphism up to 120 km of subduction depth, permitting to study the transfer and reaction of CO2-bearing fluids with crustal and mantle rock types.