CHLORINE-36 IN GREAT-BASIN WATERS - REVISITED

Inspired by recently published chloride balance chronologies of closed -basin lakes in the Great Basin, researchers of the early 1960s attemp ted to estimate residence times of chloride in these lakes using Cl-36 . Unfortunately, the analytical methods of the period were not capable of measuring the Cl-36 levels found in these waters, About 20 years a fter the early research, advances in accelerator mass spectrometry per mitted Cl-36 measurement at the required sensitivity. In this study we follow up on those pioneering efforts by remeasuring and reevaluating the Cl-36 content at several of the previously studied sites, focusin g on Mono Lake in eastern California. Our data show that in general th e streams in the region have Cl-36/Cl ratios similar to those expected in present-day atmospheric fallout, but that the terminal lakes into which the streams flow have much lower ratios. These lower ratios coul d result from either a very long (>1 million years) residence time of the chloride in the basin sinks or from subsurface influx of low-Cl-36 chloride. In the case of Mono Lake, a mass balance model based on the Cl-36 data and on independent estimates of chloride fluxes and reserv oirs indicates major subsurface chloride input, presumably from volcan ic sources, and an accumulation time in the range of 100-450 kyr. The upper bound of this range is similar to the timing of a shift from lon g-term humid to arid climate in the region and may indicate that hydro logical closure of the basin was triggered by this event.