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

In Mono Lake, concentrations of He-3 and He-4 are greater than atmospheric equilibrium values, indicating a subsurface helium source. This assertion i s supported by vertical concentration profiles that show maximum delta He-3 values (+68%) occurring within the pycnocline. The slope of the regression between He-3 and He-4 concentrations is near 2.8 R-a (R-a is the atmospher ic helium isotope ratio), indicating that the subsurface source has a compo nent of mantle helium. Hydrothermal springs and gas vents from Paoha Island have similar ratios, which suggests that discharge from this thermal syste m is the most important source. A 3He mass balance indicated that below the deep pycnocline at 17 m, similar to 25 cc STP of He-3 accumulates each yea r (similar to 90% from the influx of hydrothermal water and similar to 10% from the in situ decay of tritium). Although it is not retained because of gas transfer across the air-water interface, similar to 70 cc STP of He-3 i s injected each year into the upper 17 m. When He-3 is used as a tracer, th e influx of hydrothermal water below the deep pycnocline was estimated to b e 0.045 +/- 0.025 m(3) s(-1). Because the hydrothermal water is fresher tha n Mono Lake, this influx causes the salinity of the monimolimnion to decrea se by similar to0.1 salinity units per year and may play a small but import ant role in the salinity budget of this layer, which is presently denser th an and isolated from the surface water.