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Triple oxygen isotope signatures of evaporation in lake waters and carbonates: A case study from the western United States
【发布时间:2019-05-30 】 【 】【打印】【关闭
Benjamin H. Passey, Haoyuan Ji.
Evaporation can increase the δ18O values of lake waters and carbonates by several per mil. If not accounted for in geological studies, this can lead to substantial misinterpretation of δ18O values in terms of paleoclimate and paleoelevation. Evaporation also leads to a lowering in residual waters of Δ17O, a measure of the departure of δ′17O from a characteristic relationship with δ′18O. 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 Δ17O n closed basin lakes and freshwater lakes relative to their source rivers. The evaporation slope in triple oxygen isotope space (λlake) 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 Δ17O, meaning that the slope can be estimated on the basis of the measured Δ17O value of the carbonate. We show how triple oxygen isotopes in lake waters and carbonates and ‘clumped isotopes’ (Δ47) in carbonates can be combined to reconstruct the δ18O values of primary (unevaporated) catchment precipitation (δ18Orucp). We use our lacustrine carbonate data as a test case for this approach, and find that δ18Orucp values closely approximate independently-measured δ18O values of catchment precipitation. However, the δ18Orucp values are consistently higher than δ18O of catchment precipitation by ~2‰, 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 λlake, and the Δ17O values of unevaporated meteoric waters. In conclusion, triple oxygen isotope analysis of lake waters and lacustrine carbonates is a promising new method for studying evaporation in fossil lake systems, but will benefit from additional research into triple oxygen isotope systematics relevant to meteoric waters, lake systems, and carbonate-water fractionation.
(来源:Earth and Planetary Science Letters, 2019, 518:1-12)
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