Quantifying the flux of hydrothermal fluids into Mono Lake by use of helium isotopes

Abstract

In Mono Lake, concentrations of 3He and 4He are greater than atmospheric equilibrium values, indicating a subsurface helium source. This assertion is supported by vertical concentration profiles that show maximum d3He values (168%) occurring within the pycnocline. The slope of the regression between 3He and 4 He concentrations is near 2.8 Ra (Ra is the atmospheric helium isotope ratio), indicating that the subsurface source has a component of mantle helium. Hydrothermal springs and gas vents from Paoha Island have similar ratios, which suggests that discharge from this thermal system is the most important source. A 3He mass balance indicated that below the deep pycnocline at 17 m, ~25 cc STP of 3He accumulates each year (~90% from the influx of hydrothermal water and ~10% from the in situ decay of tritium). Although it is not retained because of gas transfer across the air‐water interface, ~70 cc STP of 3He is injected each year into the upper 17 m. When 3He is used as a tracer, the influx of hydrothermal water below the deep pycnocline was estimated to be 0.045 ± 0.025 m3 s−1. Because the hydrothermal water is fresher than Mono Lake, this influx causes the salinity of the monimolimnion to decrease by ~0.1 salinity units per year and may play a small but important role in the salinity budget of this layer, which is presently denser than and isolated from the surface water.