User:Evalinghan/sandbox

I plan for this to be three subsections. Tufa will follow right after the Geology subsection. It will then be followed by lake levels.

Someone has made a subsection called Climate showing a table of modern values. I plan to make a subheading under this subsection called paleoclimate reconstruction.

Tufa towers
Among the most iconic features of Mono Lake are the columns of limestone that tower over the water surface. These limestone towers consist primarily of calcium carbonate minerals such as calcite, CaCO3. This type of limestone rock is referred to as tufa, which is a term used for limestone that form in low to moderate temperatures.

Tufa formation
Mono Lake is a highly alkaline lake/soda lake. Alkalinity is a measure of how many bases are in the solution and how well the water can neutralize acids. Carbonate, CO32- and bicarbonate, HCO3-, are both bases. Hence, Mono Lake has a very high content of dissolved carbon. Through supply of calcium ions, Ca2+, the water will precipitate carbonate-minerals such as calcite, CaCO3. When ground waters filled with dissolved calcium ions run into the lake water through small springs in the bottom of the lake, huge amounts of calcite precipitate around the spring orifice and create the well-known tufa towers. The tufa towers were originally created at the bottom of the lake. However, when lake levels fell, the tufa towers came to rise above the water surface and stand as the majestic pillars that they are today. The sediments found in cores of Holocene lake sediments also contain high concentrations of carbonate (5-50%).

Tufa morphology
Description of the Mono Lake tufa dates back to the 1880's, when Edward S. Dana and Israel C. Russell made the first systematic descriptions of the Mono Lake tufa These pioneering works in tufa morphology are still referred to by researchers today and were confirmed by James R. Dunn in 1953. The tufa types can roughly be divided into three main categories based on morphology :


 * Lithoid tufa - massive and porous with a rock-like appearance
 * Dendritic tufa - branching structures that look similar to small shrubs
 * Thinolitic tufa - large well-formed crystals of several cm

These tufa types vary interchangeably within the tufa towers themselves.

Through time, there were many hypotheses regarding the formation of the large thinolite crystals (also referred to as glendonite) in thinolitic tufa. It was relatively clear that the thinolites represented a calcite pseudomorph after some unknown original crystal. However, the original crystal was only determined, when the mineral ikaite was discovered in 1963. Ikaite, hexahydrated CaCO3, is metastable and only crystallizes at near-freezing temperatures. It is also believed that calcite crystallization inhibitors such as phosphate, magnesium, and organic carbon may aid in the stabilization of ikaite. With heating conditions, ikaite would break-down and become replaced by smaller crystals of calcite. In the Ikka Fjord of Greenland, ikaite was also observed to grow in columns similar to the tufa towers of Mono Lake. This has led scientists to believe that thinolitic tufa is an indicator of past climates in Mono Lake because they reflect very cold temperatures.

Tufa chemistry
Rusell (1883) studied the chemical composition of the different tufa types in Lake Lahontan, a large pleistocene system of multiple lakes in California, Nevada, and Oregon. Not surprisingly, it was found that the tufas consisted primarily of CaO and CO2. However, they also contain minor constituents of MgO (~2 wt%), Fe/Al-oxides (.25-1.29 wt%), and PO5 (0.3 wt%).

Carbon and oxygen isotopes
Mono lake water has a δ13C composition of 2 ‰ from DIC, and a δ18O of -0.1 ‰ (relative to SMOW), while surrounding river freshwaters have a δ13C composition of -14 ‰ from DIC and a δ18O of -14 to -17.5 ‰ (relative to SMOW). Because the surrounding streams of Mono Lake have a depleted δ13C and δ18O compared to the lake water, a resulting water mixture will be depleted comparative to lake water. The figure to the right shows how δ18O of a water mixture changes with the fraction of water consisting of lake water. As the fraction of lake water is lower, the δ18O is lower. The total CO2 concentration (ΣCO2) is naturally much higher in the lake than in surrounding streams. Hence, this isotopic dilution effect is less significant for δ13C, and water mixtures will be dominantly composed of δ13C with lake water signatures. In conclusion, 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.

There is a temperature- and salinity-dependent fractionation between Mono Lake water and precipitating carbonates. A study evaluated the clumped isotope composition of Mono Lake tufa. From their Δ47 values (0.734-0.735 ‰), the researchers could calculate temperature, and δ18O composition of the corresponding water from which the tufa formed. The results showed 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 ‰

However, these fractionation effects do not account for salinity-dependency.

Mono lake tufa has been studied using carbon isotopes, oxygen isotopes, and clumped isotopes. Mono lake tufa appears to have relatively heavy δ13C (‰) values of 5.06- 7.99 ‰ and 7.77-8.84 ‰ from two studies. Hence, they seem to be enriched compared to modern lake water DIC, which is unexplained by the calcite-DIC fractionation. The δ18O values are 28-32.5‰ (relative to SMOW) from the same two studies, which reflects water mixture compositions of -2‰ to 2‰ (relative to SMOW).

Lake level history
An important characteristic of Mono Lake is that it is a closed lake. This means that water does not flow out of the lake on the surface. 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 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 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.

150 year record
The covariation between δ18O in lake water and lake level were tested through a 150 year record in Mono Lake. The δ18O record was compared to historic lake levels recorded by the USGS. The two were observed to have a strong correlation with minor offsets. Here, increases and decreases in δ18O in lake water correlated with decreasing and increasing lake level correspondingly revealing 6 stages in lake level in the past 150 years: high stands at 1845, 1880, and 1915 as well as low stand at 1860, 1900, and 1933. The study compared the δ18O record to the recorded precipitation and streamflow of Nevada City in California. It was observed that decreases in δ18O correlated well with increases in precipitation as well as increases in streamflow and vice versa.

10.000 year record
A study investigating a 10000 year record through carbonates in sediment cores (dated through ash beds). Here δ18O and δ13C did covary when observed through long time intervals of >5000 years, whereas the correlation was not present during shorter time scales. The study reconstructed lake levels and changes in alkalinity through the sediment cores. It was suggested that the record revealed 5 periods of distinct lake conditions:

9.7-8.7 ka: A rise in δ18O and δ13C (R=0.97) reflects and increase lake level.In fact, the lake level reaches the Holocene High Stand. This high stand corresponds to a period of maximum effective moisture in the Great Basin.

8.7-6.5 ka: A sudden drop in δ18O and δ13C suggests that lake levels dropped. Following, weak correlation between δ18O and δ13C (R=0.46) suggests the stabilization of lake levels.

6.5-5.9 ka: A decrease in δ18O and δ13C (R=0.83) correlates with a decrease in lake volume.The decrease continued until the Holocene Low Stand at 5.9 ka, which corresponds to minimum effect moisture in the Great basin.

2-0.6 ka: The gap between 6-2 ka can be attributed to shallow lake conditions. The sediments between 2-0.6 ka are dominated by deposition in shallow water. The Midieval Warm Period at 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 (R=0.56) between δ18O and δ13C.

490-360 a: This period corresponds to the Little Ice Age. The isotopic record has very high annual resolution. An increase in δ18O and δ13C (R=0.71) corresponds to increasing lake level during 490-430 a. 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 appeared to correspond to known climatic events such as maximum/minimum period of effective moisture, Midieval 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. Lajoie (1968) mapped out terraces that corresponded to paleoshorelines. Through these now-exposed ancient shorelines, he was able to estimate past lake levels. Later, these terraces were dated with radio carbon dating. It was found that they extended over a period of 35.000 years. δ18O of the Wilson Creek Formation that represent a succession of Mono Lake sediments extends over the same time period. Hence, the lake levels over 35.000 years have been reconstructed.

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

35-21 ka: High/intermediate stable lake level. These lake levels corresponded to two beds of silt that would have been deposited in a deep lake. This stable high lake level corresponds to little fluctuation in δ18O.

20-15 ka: There was a fall in lake level prior to this period, sand delta terraces from this time period indicate a lake-surface altitude of 2035 m according to mapping by Lajoie (1968). δ18O records shows a fluctuating increase in δ18O over this time period, which would reflect falling lake level.

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

13+ ka: Following peak lake level, lake level decreased to a paleoshoreline of 1965 m at ~ 10 ka corresponding to an increase in δ18O.

This lake level record has been correlated with significant climatic events including polar jet stream movement, Heinrich, and Dansgaard-Oeschger events. You can read about these in the section on Paleoclimate resonstruction.

Paleoclimate reconstruction
The reconstruction of lake levels through carbon and oxygen isotopes have revealed interesting results that can be correlated to dramatic changes in climate in the North Atlantic. In the recent past, the Earth experienced periods of increased glaciation known as ice ages. This ice age period is known as the pleistocene, which lasted until ~11 ka. Lake levels in Mono Lake can reveal how the climate fluctuated. For example, during the cold climate of the pleistocene the lake level was higher because there was less evaporation and more precipitation. Following the pleistocene, the lake level was lower because of more evaporation and less precipitation associated with a warmer climate. Mono Lake reveals the climate variation on 3 different time scales: Milankovich (~10.000 years), Heinrich (varying), and Dansgaard-Oeschger (~1000 years).

Jet stream event
On the 10.000 year time-scale a trend in δ18O of the Wilson Creek sediments suggested correlation with a change in position of the polar jet stream caused by an increase in the Northern American ice sheet from 35-18 ka. Between 18-13 ka during a low level, the polar jet stream is presumed to have been forced to the south of Mono Lake. Furthermore, the Tioga glaciation could be correlated to a reduction in Total Inorganic Carbon content of the sediments during 26-14 ka.

Heinrich events
Through the Wilson Creek Formation, a section of well-preserved lake sediments, the lake level history of 36.000 years can be investigated. As described above, researchers used the δ18O of the Wilson Creek sediments as a proxy for lake level. They found that 3 low lake levels correlated with 3 Heinrich events. Heinrich events are events, where icebergs melted broke off from the main ice sheets in the North Atlantic. The age correlation was based on radiocarbon dating and paleomagnetic secular variation.

Dansgaard-Oeschger
From the compilation of δ18O data 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. Dansgaard-Oeschger events are rapid fluctuations in the climate on a 1000-year time scale. The causes for these fluctuations are still unresolved. Oscillation between cold/dry (low lake level with low precipitation) and warm/wet (high lake level with high precipitation) were correlated with Dansgaard-Oeschger and Heinrich events of the GISP2 core from 46-27 ka.