Evaluating CO2 flux and recharge source in geothermal springs, Garhwal Himalaya, India: stable isotope systematics and geochemical proxies

摘要

Evaluation of geogenic carbon fluxes between solid Earth and its atmosphere is essential to understand the global geological carbon cycle. Some of the key geogenic CO2 suppliers are the magmatic mantle and metamorphic degassing from active and quiescent volcanoes, fault zones, geothermal systems and CO2 rich groundwater. Indian Himalayan geothermal field hosts about 340 geothermal springs in natural as well as artesian condition that eject hot waters and volatiles with varied temperature and chemical composition. These sites provide an opportunity to analyse tectonically driven gas emissions and their impact on regional and global climate. Here we adopt a method for direct measurement of Dissolved Inorganic Carbon (DIC approximate to HCO3) concentration in the geothermal springs to estimate geogenic CO2 flux from an active region based on water discharge and area of geothermal system between the tectonic boundaries of the Main Central Thrust (MCT) and Main Boundary Thrust (MBT) of the Garhwal (Northwest) Himalaya. In the study area, geothermal spring water contain high delta C-13(DIC) ratio (- 8.5 parts per thousand to + 4.0 parts per thousand (VPDB)), and among the major ions, bicarbonate (HCO3-) varies by an order of magnitude from 1697 to 21,553 mu Eq/L; chloride and sodium vary from 90 to 19,171 mu Eq/L and 436 to 23181 mu Eq/L. The elevated concentration of Cl- and Na+ in geothermal spring waters suggests affinity towards their deeper origin. These geothermal springs cover a large area of nearly 10,000 km(2) of the Garhwal region showing a significant discharge of CO2 rich water with an estimated carbon dioxide degassing flux of similar to 7.2 x 10(6) mol/year to the atmosphere. Considering widespread occurrences of geothermal springs in tectonically active areas worldwide, the proposed direct measurement of DIC may be used as a reliable tool to estimate CO2 fluxes in different active orogenic settings within the Earth system. Results of stable isotopes of delta O-18 ((VSMOW)) and delta D ((VSMOW)) in these geothermal spring waters follow the Global Meteoric Water Line (GMWL), suggesting affinity of their recharge through the meteoric origin.