Talk:Mono Lake/chemistry

Carbon and oxygen isotopes
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.

A temperature-dependent fractionation also exists 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 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
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 (read below).

150-year record
The covariation between δ18O in lake water and lake level in Mono Lake have been recorded over a 150-year time interval in Mono Lake. The δ18O record was compared to historic lake levels recorded by the USGS. The lake level and δ18O record were observed to have a strong correlation with minor offsets. Changes in δ18O of lake water were inversely correlated with lake level. This revealed six stages in lake level in the past 150 years: high stands at 1845, 1880, and 1915 as well as low stands at 1860, 1900, and 1933. The δ18O record compared well to the recorded precipitation and streamflow of Nevada City in California. Decreases in δ18O correlated well with increases in precipitation as well as increases in streamflow and vice versa.

10,000-year record
A sediment core from Mono Lake reveals a 10,000 year record of carbonates (dated through ash beds). Here δ18O and δ13C did covary when observed through long time intervals of>5,000 years, whereas the correlation was not present during shorter time scales. It was found that the record revealed 5 periods of distinct lake conditions:

9.7 - 8.7 ka: Rising lake level. Decreasing δ18O and δ13C reflected an increased lake level. In fact, the lake level reached the Holocene High Stand. This high stand corresponded to a period of maximum effective moisture in the Great Basin.

8.7 - 6.5 ka: Dropping lake level. A sudden increase in δ18O and δ13C suggested that lake levels dropped. Following, weak correlation between δ18O and δ13C suggested that lake levels stabilized.

6.5 - 5.9 ka: Rising lake level. An increase in δ18O and δ13C correlated with a decrease in lake level. The lake level drop continued until the Holocene Low Stand at 5.9 ka, which corresponded to a period of minimum effective moisture in the Great Basin.

2 - 0.6 ka: Unconformity. The gap between 6 - 2 ka could be attributed to shallow lake conditions. In addition, sediment types observed in the core between 2 - 0.6 ka largely reflected shallow water conditions. During the Medieval Warm Period, which occurred from 0.9 - 0.7 ka, the lake level was around the same as today. In general, the period was dominated by a shallow, stable lake level with low covariance between δ18O and δ13C.

490 – 360 years ago: High, fluctuating lake levels. This period corresponded to the Little Ice Age. The isotopic record had very high annual resolution. The lake levels were generally high but fluctuated a little resulting in low correlation between δ18O and δ13C. At the end of this period, δ18O and δ13C evolved towards a trend of decreasing lake level.

Overall the lake levels of Mono Lake appeared to have corresponded to known climatic events such as periods of maximum or minimum effective moisture, the Medieval Warm Period, and the Little Ice Age.

35,000-year record
Lake levels of Mono Lake during the Pleistocene have also been reconstructed using stratigraphic inspection of paleoshorelines, radio carbon dating, and δ18O records from sediments. These analyses helped reconstruct lake levels of the past 35,000 years.

36 - 35 ka: Rising lake level. Decreasing δ18O revealed that lake level began to rise at about this time from a lake level altitude of 2015 m.

35 - 21 ka: High stable lake level. Little fluctuation in δ18O suggested a stable lake level. This stable lake level corresponded to two beds of silt that would have been deposited in a deep lake.

20 - 15 ka: Dropping lake level. There was a sudden fall in lake level at the beginning of this period. Sand delta terraces from this time period indicated a lake-surface altitude of 2035 m. Recorded δ18O increased over this time period, reflecting falling lake level.

5 - 13 ka: Rising lake level. During this period, Mono Lake rose to its highest lake-surface altitude of 2155 m. This corresponded to a decrease in δ18O.

13+ ka: Dropping lake level. Following peak lake level, the lake level decreased to 1965 m at ~ 10 ka as evidenced by an increase in δ18O and paleoshorelines.

This lake-level record has been correlated with significant climatic events including polar jet stream movement, Heinrich, and Dansgaard-Oeschger events.

Paleoclimate reconstruction
Mono Lake reveals the climate variation on 3 different time scales: Dansgaard-Oeschger (repeats every 1,000 years), Heinrich (varying repetition), and Milankovich (repeats every 10,000 years).

Dansgaard-Oeschger
From the compilation of δ18O data over the past 51,000 years from lakes throughout the Great Basin, including Pyramid Lake, Summer Lake, Owens Lake, and Mono Lake, it has been observed that changes in lake level can be correlated to Dansgaard-Oeschger events. The δ18O records from these lakes showed oscillations in δ18O compositions of carbonates from these four lakes. Oscillation between high δ18O and low δ18O reflected cold/dry (low lake level with low precipitation) and warm/wet (high lake level with high precipitation) respectively. Read more about these isotopic effects in the Lake Level History section. Furthermore, total organic carbon (TOC) of the sediments from Pyramid Lake and Owens Lake were inversely correlated with δ18O and showed the same oscillations. TOC is often an indicator of the degree of biological productivity in a lake. This would suggest that high productivity correlated with a warm/wet climate at Mono Lake, while low productivity correlated with a cold/dry climate at Mono Lake. The timing of these oscillations matched the timing of Dansgaard-Oeschger events of the GISP2 core from 46 - 27 ka. Minima in δ18O and maxima in TOC correlated with 11 different Dansgaard-Oeschger events. Dansgaard-Oeschger events are fluctuations in δ18O records from ice cores that repeat every 1,000 years. They are thought to be related to global climatic events. However, the exact causes for these fluctuations are still unresolved.

Heinrich events
The lake level history of 36,000 years was investigated through correlations between δ18O records, radiocarbon dating, and paleomagnetic secular variation from Mono Lake sediments as described in the Lake Level History section. Peaks in the δ18O records of Mono Lake sediments correlated with 3 Heinrich events known from the sea cores of the North Atlantic. Heinrich events occurred when massive numbers of icebergs broke off from the ice sheets and fell into the North Atlantic. These Heinrich events have been observed to correlate with peaks in δ18O compositions of carbonates globally. This pattern would generally indicate a global drop in temperature and a rising ice-volume. As water vapor moves from the equator to the poles, 18O is preferentially precipitated compared to 16O. When the water precipitates at the poles, it has a very depleted δ18O composition. Hence, ice sheets are vast reservoirs of 16O and have a very depleted δ18O composition. If the temperature dropped and 16O-containing ice-volumes grew, the remaining water bodies would experience a corresponding increase in δ18O composition. Three peaks in the δ18O records of Mono Lake may reflect 3 episodes of vast growth of the Pleistocene ice sheets that resulted in massive iceberg break-off at the ice-water interface.

Milankovich-scale events
The δ18O records from Mono Lake sediments also exhibit trends over longer timescales of ~10,000 years. From 35 to 18 ka, the δ18O composition of Mono Lake sediments were gradually decreasing. This decreasing trend was inversely correlated with an increase in δ18O of sediments from a North Atlantic sea core. This trend in δ18O suggested correlation with the southward movement of the polar jet stream from 35 to 18 ka. As the polar jet stream moved southward, it caused increased precipitation of isotopically depleted rain water. In turn, this caused Southern water bodies like Mono Lake to become isotopically depleted, while Northern oceans became isotopically enriched. This movement of the polar jet stream was presumably caused by an increase in the Northern American ice sheet.  Two δ18O minima at 18 ka and 13.1 ka in Mono Lake sediments reflected two lake level high-stands of Mono Lake. These lake level high-stands presumably corresponded to two passages of the polar jet stream over Mono Lake that precipitated large amounts of rainwater with a depleted δ18O composition. Following, the polar jet stream was forced south of Mono Lake. Furthermore, the sudden reduction in Total Inorganic Carbon (TIC) during 26 - 14 ka could be attributed to the Tioga glaciation. The Tioga glaciation would have caused a high in-flux of detrital materials to Mono Lake. As a result, the TIC in Mono Lake sediments would be lowered during this time period.