Abstract Stable isotope ratios (δ 13 C, δ 18 O) of authigenic calcium carbonate frequently are used to interpret past changes in lakes. However, isotopic systems in perennially ice‐covered lakes are characterized poorly, making interpretation of sediment δ 18 O and δ 13 C in those settings difficult. Here, we present a framework for interpreting isotopic variability in such lakes and then apply this understanding to Lake Fryxell, a closed‐basin lake in Antarctica. Our work suggests that meltwater source can be the large‐scale control on δ 18 O values, whereas the controls on carbon are complex, but related in many cases to stratification. Stable isotopic values, coupled with U/Th chronology, sedimentology, and a novel use of 14 C as a water‐mass and lake‐level tracer, suggest that Lake Fryxell has been in continuous existence since at least ∼60 ka. The data furthermore lend support to earlier studies indicating that a deep ice‐dammed lake existed during the last glacial maximum and that Lake Fryxell in its present configuration has persisted for the last ∼6–8 ka. Key Points Meltwater source is the large‐scale control on δ 18 O in perennially ice‐covered lakes in the Antarctic dry valleys and potentially elsewhere Depth of precipitation in a stratified water column is the primary control on carbonate δ 13 C values Stable isotope and radiocarbon values record the evolution of Lake Fryxell over the last 60 k
