Variability of (234U/238U) in surface water and tufa deposits: A study in the Mono Basin, California, USA

Uranium concentrations and 234U/238U activity ratios (δ234U) of Earth’s surface waters can provide independent and complementary information on changes in weathering regime and hydroclimate. The response of δ234U variation in surface waters in US Great Basin to climate change however remains unclear, which brings ambiguities in interpreting δ234U in aquatic carbonate deposits. Here, we analysed U concentration and δ234U in a suite of surface waters (creeks, springs and lake) as well as tufa deposits from the last glacial lake highstands in the Mono Basin, California, USA to assess the modern uranium budget in the lake water and the controlling factors on its δ234U. We find that U concentrations in groundwater springs are about one order of magnitude higher than those of creek waters. Hence, even though springs only deliver about 15% of annual inflow to the lake, they contribute 70% of U in the lake water. The residence time of U in lake water is calculated to be approximately 15,000 years, on the same order as those of Li, Na, and Cl, but significantly longer than those of alkaline earth elements. The δ234U in Mono Lake water is 180‰, same as in modern-day tufa deposits. The δ234U in lake highstand tufas is ~ 220‰, suggesting much more enhanced physical weathering associated with mountain glacial activities in the basin, even though chemical weathering was also stronger due to increased precipitation. On the other hand, the higher δ234U values (~ 250‰) in modern creeks and springs is consistent with the overall dry environment and stronger physical weathering in the basin. The 40‰ decrease in δ234U of lake water however cannot be explained by radiative decay. We hypothesis that lake water was more frequently stratified in the past, during the last glacial in particular, and the resulted anoxic environment in deep lake water has probably facilitated precipitations more enriched in 234U.