F4 : MYSTHIC – MobilitY versus STorage of HImalayan Carbon

The long-term carbon cycling is intimately related to the appearance and evolution of life on Earth and to the chemical exchanges between Earth’s surface (oceans and atmosphere) and deep interior (crust and mantle). Over geological times, the equilibriums between these reservoirs have been intensely influenced by plate tectonic initiation and evolution during oceanic crust subduction, continent formation or collision. However, the processes that control carbon storage and mobilization at convergent plate boundaries remain an active research frontier. The current limitations of our observations and our understanding can be summarized as follows:

1. Balance between carbon sources and sinks during orogeny. Except for human activity, the main Earth’s carbon dioxide (CO₂) emissions are believed to be controlled by active and inactive subarerial arc volcanism and rift volcanism, and by the metamorphism of crustal carbonated rocks associated with mountain belt formation. The main CO₂ sinks are associated with silicates weathering and carbonation of the oceanic crust. Although the ratio emission/sink is believed to be close to one, it is difficult to argue whether a steady state is reached or not, since some sources are still unconstrained. Indeed, although the emissions of CO₂ associated with active volcanism are well studied, there is a lack of observations and measurements of carbon emissions associated with mountain belt formation, hence hampering proper evaluation of the global carbon budget.
2. Sources of carbon emissions during mountain belt formation. During subduction, numerous studies have shown that solid carbonates are soluble in aqueous fluids at relatively low temperatures, suggesting that slab decarbonation should be highly efficient at shallow depths. The migration of slab-derived fluids through the slab/mantle wedge interface will therefore allow the hydration and the storage of carbon in the overlying shallow forearc mantle wedge. Although they did not report carbon concentration, several studies described carbonate minerals, hydrocarbon-bearing fluid inclusions and/or graphite in forearc mantle wedge peridotites. However, almost all the carbon data of peridotites recording subduction zone metamorphism are inferred to represent subducted seafloor peridotites that were serpentinized near mid-oceanic ridges. If these interpretations are correct, then these samples provide little information on the storage of carbon in the forearc mantle wedge. Similarly, the fate of this carbon reservoir during mountain belt formation remains unexplored.
3. Diversity and abundance of abiotic carbon and energy sources sustaining the development of subsurface microbial ecosystems. In the oceanic lithosphere, the formation of reduced carbon compounds is related to complex redox reactions associated with the formation of serpentinites. Serpentinization reactions release dihydrogen (H₂), but also promote the production of abiotic organic matter including methane, formate, polycyclic aromatic hydrocarbons, condensed carbonaceous matter and amino acids. While likely impacting fluid-rock interactions and thermodynamic equilibriums, all these compounds may sustain dense and active microbial communities at depth, by providing both carbon sources for metabolic strategies disconnected from photosynthesis and an efficient energy source in these active H₂-producing geochemical systems. These subsurface environments are hence considered as parts of the largest habitat on Earth. In addition, active geological settings through intense seismic activity that release H₂ fluxes may also promote the formation of endoterrestrial microbial communities, potentially capable to regulate geochemical exchanges between the lithosphere and the hydrosphere, notably carbon fluxes.

The MYSTHIC project aims at evaluating the storage and mobility of carbon during orogenic processes. It is designed to take advantage of the Himalayas, a place that allows analysing the fate of carbon from the subduction to the collision, and from the upper mantle to the critical zone in the lithosphere, hydrosphere and biosphere compartments.

The MYSTHIC objectives are:

1. To identify and quantify under which form carbon is stored into the serpentinized forearc mantle wedge during Tethysian subduction, and then mobilised in fluids during the India-Eurasia collision.
2. 2. To evaluate the impact of subsurface microorganisms on the remobilisation of carbon and dihydrogen in the Himalayan orogen.
3. To assess the along-strike heterogeneity of the current CO₂ mobility in the Nepal Himalayas.