Triple oxygen isotope signatures of evaporation in lake waters and carbonates: A case study from the western United States

摘要

Evaporation can increase the delta O-18 values of lake waters and carbonates by several per mil. If not accounted for in geological studies, this can lead to substantial misinterpretation of delta O-18 values in terms of paleoclimate and paleoelevation. Evaporation also leads to a lowering in residual waters of Delta O-1(7), a measure of the departure of delta'O-17 from a characteristic relationship with delta'O-18. We present new triple oxygen isotope data from waters and carbonates from lakes and their source rivers in the western United States (Bear Lake, Great Salt Lake, Lake Tahoe, Mono Lake, and Pyramid Lake). Consistent with predictions from steady-state isotopic mass balance models, the data illustrate marked lowering of Delta O-1(7) in closed basin lakes and freshwater lakes relative to their source rivers. The evaporation slope in triple oxygen isotope space (lambda(lake)) is similar for these lakes, averaging 0.5229 and ranging between 0.5219 and 0.5239. Moreover, models and data both show that the evaporation slope correlates with Delta O-1(7), meaning that the slope can be estimated on the basis of the measured Delta O-1(7) value of the carbonate. We show how triple oxygen isotopes in lake waters and carbonates and 'clumped isotopes' (Delta(47)) in carbonates can be combined to reconstruct the delta O-18 values of primary (unevaporated) catchment precipitation (delta O-18(rucp)). We use our lacustrine carbonate data as a test case for this approach, and find that delta O-1(8)rucp values closely approximate independently-measured delta O-18 values of catchment precipitation. However, the delta O-18(rucp) values are consistently higher than delta O-18 of catchment precipitation by similar to 2 parts per thousand, which may reflect present incomplete understanding of a number of triple oxygen isotope parameters used in the calculation, such as the fractionation exponent for carbonate-water equilibrium, the evaporation slope lambda(lake), and the Delta O-1(