U.S. Geological Survey Water-Resources Investigations Report 00-4207
By Michael L. Sorey, Cindy Werner, Robert G. McGimsey, and William C. Evans
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Shrub mud volcano, one of three mud volcanoes of the Klawasi group in the Copper River Basin, Alaska, has been discharging warm mud and water and CO2–rich gas since 1996. A field visit to Shrub in June 1999 found the general level of hot-spring discharge to be similar, but somewhat more widespread, than in the previous two years. Evidence of recent animal and vegetation deaths from CO2 exposure were confined to localized areas around various gas and fluid vents. Maximum fluid temperatures in each of three main discharge areas, ranging from 48-54°C, were equal to or higher than those measured in the two previous years; such temperatures are significantly higher than those observed intermittently over the past 30 years. At Upper Klawasi mud volcano, measured temperatures of 23-26°C and estimated rates of gas and water discharge in the summit crater lake were also similar to those observed in the previous two years. Gas discharging at Shrub and Upper Klawasi is composed of over 98% CO2 and minor amounts of meteoric gases (N2, O2, Ar) and gases partly of deeper origin (CH4 and He). The rate of CO2 discharge from spring vents and pools at Shrub is estimated to be ~10 metric tonnes per day. This discharge, together with measured concentrations of bicarbonate, suggest that a total CO2 upflow from depth of 20-40 metric tonnes per day at Shrub.
Measurements were made of diffuse degassing rates from soil at one ~300 m2 area near the summit of Shrub that included vegetation kill suggestive of high CO2 concentrations in the root zone. Most of measured gas flow rates in this area were significantly higher than background values, and a CO2 concentration of 26 percent was measured at a depth of 10 cm where the gas flow rate was highest. Although additional measurements of diffuse gas flow were made elsewhere at Shrub, no other areas of vegetation kill related to diffuse degassing and high soil-gas CO2 concentrations could be seen from the air.
Chemical and isotopic compositions of the gas and water discharging at Shrub and Upper Klawasi indicate derivation from a combination of mantle (magmatic) and crustal (marine sedimentary rock) sources and suggest a common fluid reservoir at depth. In particular, both the total dissolved carbon and values of 13C in CO2 are similar for fluids and gas sampled at each area, and do not appear to have changed with the onset of increased spring temperatures and fluid discharge at Shrub. This suggests that the underlying cause of the recent changes in discharge rate and temperature at Shrub is not an increase in the rate of input of magmatic heat and volatiles, but rather increases in the permeability of the upflow conduits that connect the gas-rich reservoir to the surface.
Abstract
Introduction
1999 Activity
Shrub Mud Volcano
Upper Klawasi Mud Volcano
Measurement Techniques
Gas and Water Sampling
Vent Degassing
Diffuse Gas Flux from Soils
Results and Discussion
Gas and Water Compositions
Vent Gas Discharge
Diffuse Gas Discharge
Conclusions
Acknowledgements
References
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