The Boron Concentrations and Isotopic Compositions of Waters of the Mono Basin, Californi

Mono Lake is a hydrologically closed lake in the center of Mono Basin. Evaporative concentration of the lake waters results in extremely high salinity and alkalinity. Mono Lake derives its water predominantly from creeks that drain from the Sierra Nevada mountains (~ 85% of the total input), with the remainder supplied by springs and a very small input from direct precipitation. While the hydrology and geochemistry are well studied, little is known about the boron (B) isotopic compositions of the lake and inputs. This study of the modern boron budget of Mono Basin provides a foundation for future paleoenvironmental studies. Forty water samples were collected from Mono Lake and its sources, including creeks, cold springs, and warm springs, and were analyzed for boron and other element concentrations (Li, Na, Sr) and boron isotopic compositions. Creeks are volumetrically the most important influx of water to Mono Lake, but they also have the lowest boron concentrations (~ 2 ppb), and are thus unlikely to exert primary control over the average boron isotopic composition of the Mono Lake. The delta11B offset (at least -- 9‰) of the combined sources to Mono Lake from the lake water itself suggests that, like seawater, the boron isotopic composition of the lake may be dominated by preferential removal of light boron by coprecipitation in carbonates. The extreme difference in concentration and measured isotopic composition of the sources suggests that boron fractionation likely occurs prior to the water sources entering the lake. Future work might consider the importance of a mechanism---"chemical delta" for controlling the isotope composition of the lake, focusing on tufa deposition along the shoreline. Based on measured concentrations and assumptions of volume from sources to Mono Lake, the residence time of boron in Mono Lake is estimated to be between 25000 and 80000 yr.