page_title: Mono Lake
section_title: Carbon and oxygen isotopes
hierarchical_section_title: Mono Lake / Tufa towers / Tufa chemistry / Carbon and oxygen isotopes
caption_attribution_description: English: The d18O of a mixture between lake water and riverine freshwater would fall on the red line. The d18O composition of a mixture would depend on the fraction of lake water to riverine freshwater.
context_page_description: Mono Lake is a saline soda lake in Mono County, California, formed at least 760,000 years ago as a terminal lake in an endorheic basin. The lack of an outlet causes high levels of salts to accumulate in the lake and make its water alkaline.
  The desert lake has an unusually productive ecosystem based on brine shrimp, which thrive in its waters, and provides critical habitat for two million annual migratory birds that feed on the shrimp and alkali flies. Historically, the native Kutzadika'a people ate the alkali flies' pupae, which live in the shallow waters around the edge of the lake.
  When the city of Los Angeles diverted water from the freshwater streams flowing into the lake, it lowered the lake level, which imperiled the migratory birds. The Mono Lake Committee formed in response and won a legal battle that forced Los Angeles to partially replenish the lake level.
context_section_description: Carbon and oxygen isotopic compositions of Mono Lake tufas have the potential to reveal many interesting things about how water bodies mix in Mono Lake, how the climate changed throughout time in the Mono Basin, and how biology may or may not play a role in tufa formation. Steps have been made to understand the isotopic compositions of "modern" tufa and water isotopic composition in Mono Lake.
  Mono lake water DIC has a δ¹³C composition of 2 ‰ (relative to PDB), and a δ¹⁸O of -0.1 ‰ (relative to SMOW). Surrounding rivers feeding into Mono Lake have a δ¹⁸O of -14 to -17.5 ‰ and contains DIC with a δ¹³C composition of -14 ‰ . We observe that both the δ¹³C and δ¹⁸O compositions of Mono Lake are enriched compared to surrounding water. One explanation for the enrichment of δ¹⁸O in 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 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 comp
caption: The red line represents the δ18O of a water mixture between lake water and riverine freshwater that depends on the fraction of lake water to riverine freshwater.
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