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The cycle of petrogenic organic carbon during orogeny: from metamorphism to erosion

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Organic C (OC) exported by rivers is a mix of biospheric OC and petrogenic OC derived from erosion of rocks (e.g. Galy et al. 2007, Hilton et al. 2008). Burial of petrogenic OC is a simple recycling of reduced C and has no effect on atmospheric CO2 and O2 levels. Conversely, its oxidation consumes O2 from and returns CO2 to the atmosphere. Addressing the role of continental erosion on the global C cycle thus requires assessing the fate of petrogenic OC during erosion. A structural characterization of petrogenic OC contained in source rocks, river and marine sediments from the Himalaya and Amazon erosionnal systems has been performed using Raman Microspectrometry and High-Resolution Transmitted Electron Microscopy. Dating of bulk OC by radiocarbon allows for quantifying the proportion of petrogenic and recent OC in river sediments. Altogether, it is shown that 30 to 50% of the OC contained in the Himalayan rocks appears to be preserved and recycled in the Bengal fan during the erosion cycle (Galy et al. 2008), whereas only ~10% of the petrogenic OC is preserved between Andean rivers and the Amazon floodplain (Bouchez et al., accepted). Structural investigations reveal that graphitic phases are the most resistant to oxidation. The thermal factory during orogeny thus plays a key role by transforming sedimentary OC into graphite (e.g. Beyssac et al. 2002a, 2002b) and subtracts C from the external cycle (atmosphere-biosphere-ocean) and locks it into the geological cycle (crust-mantle). At geological timescales, graphitization thus maintains an imbalance between photosynthesis and respiration by locking reduced C into the crust.

Some related papers (+ references therein!):

Beyssac O., Goffé B., Chopin, C. and Rouzaud J.N. (2002a) Raman spectra of carbonaceous material from metasediments: a new geothermometer. Journal of Metamorphic Geology, 20, 859-871.

Beyssac O., Rouzaud J.N., Goffé B., Brunet F. and Chopin C. (2002b) Graphitization in high-pressure, low-temperature metamorphic gradient: a HRTEM and Raman microspectroscopy study. Contributions to Mineralogy and Petrology, 143, 19-31.

Bouchez J., Beyssac O., Galy V., Gaillardet J. & France-Lanord C. Oxidation of fossil organic carbon in the Amazon floodplain as a source of atmospheric CO2 . Geology, accepted pending minor revisions. [PDF available on request]

Galy V., Beyssac O., France-Lanord C. & Eglinton T. (2008) Selective recycling of graphite during Himalayan erosion: a geological stabilisation of C in the crust. Science, 322, 943-945.

Galy V., France-Lanord C., Beyssac O., Kudrass H., Faure P. & Pailhol F., (2007) Efficient organic carbon burial in the Bengal Fan sustained by the Himalayan erosional system. Nature, 450, 407-410.

Hilton RG., Galy A., Hovius N., Chen MC., Horng J. & Chen H. (2008) Tropical-cyclone-driven erosion of the terrestrial biosphere from mountains. Nature Geosciences, doi 10.1038/ngeo333.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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