1-3‰ compared to lake water, i.e. a tufa composition of 3-5‰. The reason for this small δ13C enrichment of tufa is still unclear and requires follow-up studies. It could be related to changes in DIC composition of Mono Lake, riverine water, and subsurface water compositions in the immediate past, which in turn could be related to climate or biological productivity in the lake. However, it could also be related to the fact that the isotopic composition of the subsurface waters that flow into Mono Lake is not well understood. These subsurface waters may have a very different composition compared to both Mono Lake waters and riverine surface waters. In addition, secondary crystallization (e.g. formation of calcite from aragonite) or meteoric diagenetic effects could have some control on the isotopic composition as well. Lastly, scientists may have to revisit the salinity-dependency of calcite/aragonite-DIC fractionation to understand whether this 1-3‰ fractionation may actually be larger in a lake with conditions similar to Mono Lake. Lake-level history An important characteristic of Mono Lake is that it is a closed lake, meaning it has no outflow. Water can only escape the lake if it evaporates or is lost to groundwater. This may cause closed lakes to become very saline. The lake level of closed lakes will be strongly dependent on changes in climate. Hence, studying lake levels can reveal information about climate change in the past and present. Geochemists have observed that carbonates from closed lakes appear to have δ13C and δ18O (carbon and oxygen isotopes) with covariant trends. It has been proposed that this covariation occurs because of coupled evaporation and CO2 degassing. The lighter isotopes, 12C and 16O, will preferentially go to the gas phase with increased evaporation. As a result, δ13C and δ18O in the remaining lake both become increasingly heavy. Other factors such as biology, atmospheric properties, and freshwater compositions and flow may also influence δ13C and δ18O in lakes. These factors must be stable to achieve a covariant δ13C and δ18O trend. As such, correlations between δ18O and δ13C can be used to infer developments in the lake stability and hydrological characteristics through time. It is important to note that this correlation is not directly related to the lake level itself but rather the rate of change in lake level. Three different studies with three different methods provide different resolutions to understanding the lake level history of Mono Lake