UC Santa Barbara

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 δ3He values (+68%) occurring within the pycnocline. The slope of the regression between 3He and 4He 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.