Isotopic and geochemical evidence of palaeoclimate changes in Salton Basin, California, during the past 20 kyr: 2. 87Sr/86Sr ratio in lake tufa as an indicator of connection between Colorado River and Salton Basin

As a part of the Colorado River drainage system, Southern California's Salton Basin has two major tributaries: Colorado River and Whitewater River. Interpretations on palaeoclimatic records from the basin strongly depend on the Colorado River inflow to the basin. In this study, we have used the 87Sr/86Sr ratio in lacustrine deposits to identify the time and duration of the Colorado River input. Our measurements of 87Sr/86Sr ratios made on water samples collected from Salton Basin indicate that the 87Sr/86Sr ratio (0.710169) of the Colorado River input is identical to that of Salton Sea (0.710105), implying little change in the lake water ratio since 1905 when Salton Sea formed and began receiving water mostly from Colorado River; and the 87Sr/86Sr ratio of Colorado River is much lower than that of Whitewater River (0.715960). These results enable us to trace past changes in the Colorado River input using 87Sr/86Sr in lake carbonates. Using a two-end-member mixing model, we discuss the major factors that control the 87Sr/86Sr variation of Lake Cahuilla, which was an ancient lake in the Salton Basin between 400 and 20,500 yr BP. Thirty measurements of 87Sr/86Sr were made on AMS–14C-dated tufa carbonates in two slabs from Lake Cahuilla. The results show relatively constant 87Sr/86Sr ratios throughout the past 20 kyr, ranging from 0.709944 to 0.710140. The average of these 87Sr/86Sr ratios, 0.710060 ± 0.000049 (n = 30), is very close to that of the Colorado River input. Our results indicate that (1) the Colorado River input was the dominant water source for Lake Cahuilla at least during 800–20,500 yr BP when the tufa grew in the basin, and (2) the provenance of run-off to the Colorado River did not change significantly during 800–20,500 yr BP. Hence, the palaeoclimatic proxies retrieved from the Lake Cahuilla tufas in the Salton Basin can decipher the discharge and flood history of the Colorado River under the influence of climate variability in the Colorado River drainage basin.