Mono Lake water is evaporation. The lighter isotope (16O) will preferentially be evaporated, leaving more of the heavier isotope (18O) behind. It is also important to note that tufa forms from a mixture between lake water and subsurface waters. Because the streams surrounding Mono Lake are depleted in δ18O compared to the lake water, combining the two sources will result in a water mixture that is more depleted than lake water. The figure above shows how δ18O of a water mixture changes with the fraction of lake water component. As the fraction of lake water decreases, the δ18O decreases. The total CO2 concentration (ΣCO2) is much higher in the lake than in surrounding streams. Hence, this isotopic dilution effect is less significant for δ13C, and water mixtures should be dominantly composed of δ13C with lake water signatures. In theory, Mono Lake tufa should have a δ13C composition reflecting the Mono lake water DIC composition, and a δ18O composition reflecting a mixture between Mono Lake and surrounding riverine water. This is only if we assume that the subsurface waters have similar compositions to riverine water. There is also a temperature-dependent fractionation between Mono Lake water and precipitating carbonates. The clumped isotope composition (Δ47, representing the amount of 13C18O16O in the carbonate) of Mono Lake tufa are 0.734-0.735 ‰. The temperature and δ18O of the corresponding water from which the tufa formed can be calculated using these Δ47-values From these values, it is calculated that Mono Lake tufa formed at a temperature of ~15 °C in water. For δ18O, the calcite-H2O fractionation is given by: ε=18.03(1000/T)-32.42 ~ -30‰ (SMOW) For δ13C, the calcite-DIC fractionation is roughly given by: ε ~ 1-2 ‰ (PDB) at 25 °C The calcite-aragonite fractionation is roughly given by: ε ~ 2.7 ‰ (PDB) at 25 °C However, these fractionation effects do not account for salinity-dependency. The δ18O values of modern tufas are 28–32.5‰, which reflects a corresponding water mixture composition of -2‰ to 2‰. This range is similar to the composition of a mixture between Mono Lake and riverine waters. However, this mixture appears to be largely dominated by Mono Lake water. Mono lake tufa have δ13C values that range from 5–8 for tufa of aragonitic composition and 7-9‰ for tufa of calcitic composition. These tufas are a little enriched compared to modern lake water DIC. As stated above, calcite/aragonite-DIC fractionation can only explain an enrichment of