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Title: Quantification of Himalayan metamorphic CO₂ fluxes : impact on global carbon budgets
Author: Becker, John Andrew
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2006
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This thesis evaluates potential pathways to constrain the evolution of metamorphic CO2, using the observation that significant CO2 fluxes have been noted from hot springs above active metamorphic belts. Theoretical constraints on CO2 behaviour are assessed using available observations and experimental data literature. Metamorphic decarbonation, fluid immiscibility, groundwater mixing, boiling, and phase separation (CO2(aq) = CO2(g)) are modelled theoretically, chiefly using water chemistry and isotopic constrains on CO2 evolution. Measured carbon isotope compositions for spring fluids in the Marsyandi and Trisuli Valley in Nepal reach +13‰, while coexisting free gas-phase compositions are close to -4‰. The considerable variation between gas and liquid isotopic compositions constrains the possible fractionation pathways by the necessity to satisfy mass balance. The large isotopic variation between liquid and complementary gas phase suggests that degassing from some springs is quantitative and requires up to 99% CO2 loss from solution. Noble gas systematics from effervescing springs in the Marsyandi provide evidence consistent with quantitative degassing, although a lack of data precludes a more detailed appraisal. No mantle noble gas signature is detected in Marsyandi or Trisuli springs attesting to a dominant metamorphic origin of CO2 (atmospheric and organic CO2 excepted). Existing petrological methods for the orogen-wide determination of metamorphic CO2 liberation from the Himalaya are examined and advanced using the study of calc-silicates. Extrapolation of geochemically derived hot spring fluxes, assuming a minimum of ~ 99% degassing of CO2 from hot spring waters, implies a CO2 flux between 0.6 and 1.5 x 1012 mol yr-1, consistent with estimations made on the basis of petrology. This represents a significant contribution to the global carbon cycle and may potentially force climate. A paradox still remains, however, between climatic cooling during the Cenozoic and the large solid-Earth CO2 degassing fluxes (which would warm climate).
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available