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 attempted 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 after the early research, advances in accelerator mass spectrometry permitted 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, focusing on Mono Lake in eastern California. Our data show that in general the 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 could 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 reservoirs indicates major subsurface chloride input, presumably from volcanic 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 long-term humid to arid climate in the region and may indicate that hydrological closure of the basin was triggered by this event.