Mono Lake, California

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Mono Lake, California. Successive strandlines associated with lake shrinkage are visible in the Landsat image, as are the volcanic centres of Paoha Island and the adjacent lake plain to the northwest. High relief drainage source to the west in the Sierra Nevada is clearly seen in the high proportion of creeks entering the western side of the lake(Landsat image courtesy of NASA). Geological interpretation of the type of sediment is only published for the eastern half of the lake - extent indicated by dashed line (in part after Benson et al., 2003).

Mono Lake, an endorheic meromictic alkaline saline lake with an average depth of 17m, in a volcanically active area at the edge of the Sierra Nevadas, California (Figure a), which has been accumulating laminated lacustral sediments since 760 ka. Current free surface brine volume is ≈3.2 km3, surface area is 184 km2, annual total inflow is around 283.7 x 106 km3 and current salinity is around 90,000 ppm. It lies in a Csb Köppen climate zone. Major anion content of lake brines is divided almost evenly between chloride, sulphate, and bicarbonate. The high pH (≈9.8) and alkalinity of lake waters leads to high concentrations of volcanic elements that ordinarily have short residence times in lakes (i.e. lanthanides and actinides, including naturally high levels of arsenic in bottom brines; Tomascak et al., 2002).

Calcium carbonate (calcite and aragonite) precipitates spontaneously from the lake waters and makes up 3-10 % of the laminite sediment on deeper tufa-free bottoms, the remainder is detrital. Some of the lake bottom laminites are slumped, possibly initiated by ongoing volcanic disturbance (Figure b; Benson et al., 2003). Calcite accumulates, sometimes with an ikaite precursor, where resurging groundwaters degas on the lake floor and can form spectacular biogenically-bound towers of tufa.

This saline lake has an unusually productive ecosystem based on brine shrimp that thrive in its waters, and provides critical habitat for two million annual migratory birds that feed on the shrimp and alkali flies (Ephydra hians).\] Historically, the Native American  Kutzadika'a people derived nutrition from 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 has forced the City of Los Angeles to partially replenish the lake level.

Changing lake levels and stratification

From 1850 until the middle of last century the lake water level rose some 5 metres, then the water needs of the City of Los Angeles caused a drastic water fall. In 1941 the State Resources Board diverted the distributaries that fed Mono Lake into the Los Angeles aqueduct. In the 50 years following the diversion, Mono Lake water level dropped more than 10 metres, the lake water volume was cut in half, while the alkalinity and salinity of the lake waters doubled (Figure). Water levels were as high as 1958 metres above sealevel, before the diversion of Mono’s tributary streams. Subsequent lower lake levels exposed numerous calcitic tufa mounds and pinnacles on the lake floor and enlarged the portion of the lake floor made up of subaerial saline mudflats.

To prevent further desiccation of the lake, some water from the former distributaries has been rediverted back into Mono Lake, which is now slowly refilling after reaching an all time low of 1942.2 m asl in 1982 (Figure a). The current lake level is 1945.2m asl (as of January, 2020). The projected stabilized lake elevation is 1948.3 m, but reference to the water level changes over time argues this may not be attained in the next few years. Interestingly, the anthropogenic lowering of lake water level in the last half-century and the associated increase in lake salinity is still well within the range of natural fluctuations experienced by Mono Lake in the last 2,000 years (Figure). Historic salinity and lake levels measurements can be related, exponentially, Zimmerman et al. (2011) document this relationship (Figure b). It clearly shows that as lake levels fall the lake salinity increases. It implies that lake waters were fresh in the Pleistocene at times the various high lake level terraces were formed.

In years with large fresh water inputs the lake may become meromictic. The more usual state of the lake is monomictic (annual mixing). In 1994, an unintentional consequence of lessening drainage water diversions from the Mono basin into the LA aqueduct was the onset of a period of anthropogenically induced “meromixis” (Jellison et al., 1998). Earlier, in 1982 and 1983, the lake had also become meromictic when exceptionally large volumes of freshwater runoff led to a rapid 2.6 m rise in surface elevation of the lake (Jellison and Melack, 1993). In 1984 this chemical stratification accounted for a density difference of 1.2-1.5 x 10-2 gm/cc between the 2 and 28 m water depths. At the same time the midsummer density difference between 2 and 28 m brine depths that was due to temperature was approximately 0.5 x 10-2 gm/cc. Salinities ranged from 77 to 98 gm/l across the interface and sodium, chloride, sulphate, and carbonate were the major ions in both the upper and lower water mass.

The lake’s second transition from monomixis to meromixis began in 1994 to 1995 and had ended by 2004. A comparison of density differences between the 2 and 28 m water depths indicate thermal stratification predominated in 1993 and 1994 (monomictic years), while chemical stratification dominated the density differences in 1995, 1996 and 1997 (meromictic years) (Jellison and Melack, 1998).When monomictic, the lake has typically thermally stratified in March and mixed to the bottom by December. Since 2004, the lake has mixed annually with the loss of seasonal thermal stratification as it had in 1993 and early 1994, prior to 8 years of meromixis. During the meromictic conditions in the late 1990s, the absence of holomixis during winter created persistent anoxic conditions beneath the chemocline. This led to the accumulation of ammonium in the monimolimnion, its depletion in the mixolimnion, and low mixolimnetic chlorophyll concentrations in the spring and autumn (lessened photosynthetic activity).

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Mono Lake levels A) Historic levels from 1850 - 2010 (replotted from online data of Mono Lake Preservation Society last accessed on June 3, 2013 at http://www.monolake.org/live/lakelevel/yearly.htm). B) Exponential relationship between Mono lake level and salinity is extrapolated to show very low salinities in the ancient lake levels associated with the Pleistocene terraces that surround the current lake (after Zimmerman et al., 2011)

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Mono Lake, California. A) Hydrological conditions for the last 4,000 years (after Stine, 1990). B) Map outlining different lake areas related to changing water elevations through time (after Rogers and Driese, 1995a, b).

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Tufe pinnacles exposed by lowered water levels in in Mono Lake

Tufa Mounds and pinnacles

The most visually striking geological features of Mono Lake are its metres-tall tufa columns, which protrude from lake waters and line portions of the lake edge. These mounds and needle-like pinnacles of calcium carbonate frequently define linear trends that are the seepage expression of underlying faults and joints. Soft bottom sediments surround columns and many columns are rooted in massive domal buildups of calcite. Dominant growth orientation is vertical and defines the rapid upward rise of less saline CO2-rich waters. Branching of lithoid columns is rare and “pseudo-branches” easily explained by the collapse of one or two adjacent columns imply much of the precipitation mechanism is via inorganic degassing of groundwater on reaching the lake floor. Microbial biofilms coat active tufa surfaces beneath the lake waters and create characteristic knobbly grumous microtextures in the reef mounds and pinnacles as they capture the precipitated carbonate (Figure b).

In today’s lake waters calcite and aragonite are the carbonate minerals with long-term stability and they dominate the non-siliciclastic fraction accumulating in the lake (Benson et al., 2003). But during the colder Tioga glacial period of the Late Wisconsian (12 to 9 ka), and during the present day winter season, ikaite (CaCO3.6H2O), was the dominant carbonate precipitate (Whiticar and Suess, 1998). Ikaite is a metastable cryogenic carbonate and the low temperatures needed for ikaite stability are obvious in winter in Mono Lake. In the winter of 1985 ikaite crystals were visible as coatings on the subaqueous surfaces of active spring seep tufas about the shallow lake edge (pers obs.). If these small ikaite crystals were removed from the icy cold winter waters of the lake and held in an ungloved hand, the increase in ambient temperature caused the ikaite to deliquesce, so that in a few minutes it disappeared leaving behind a pool of milky water. Council and Bennett (1993) and Bischoff et al. (1993) argued that during winter, calcite precipitation in Mono Lake is inhibited by low temperatures and high concentrations of phosphates and organic carbon. The same factors allow metastable ikaite to form. Ikaite decomposes into calcite during the spring warming, occasionally leaving pseudomorphs of the primary precipitate in the calcitic (thinolitic) tufa

At a broader scale Council and Bennett (1993) demonstrated a strong hydrological control on the position of many of the larger lithoid tufa mounds. The unconfined aquifer that dominates the western side of the basin is composed of fluviodeltaic sediment and it controls the modern discharge at many points about the current lake shoreline (Figure a, c). Currently active diffuse low-flow-rate seeps are dispersed across the strandzone, and extend from ≈ 200m above the present lakeshore to around 1 metre below the present lake water surface (Figure). Diffuse outflows are geologically defined by flat beachrock-like zones of encrusting tufa and tufa flowers, and are most active about the present lake strandline.

Further offshore the aquifer is largely confined by lake muds, so CO2-saturated groundwaters discharge onto the lake floor via fault and fracture focused conduits. The associated tufa (spring)mounds precipitate along linear trends, tied to positions where faults transect the confining clays. Other older, now exposed and mostly inactive, linear tufa mounds and encrustations define prior times of higher lake water levels and watertables. Modern springs mostly occur along the western side of Mono Lake with waters originating along the Sierra front. They discharge as Ca-HCO3 waters, which are neutral to slightly acidic and their degassing drives present day tufa precipitation. In contrast, springs along the eastern shore typically discharge Na-HCO3 waters, have lower hydraulic heads and are not generally associated with the formation of modern tufa sheets. 

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Mono Lake, California. A) Tufa pinnacles forming linear growths on the saline linear ridges in the lake floor in Nov. 1985. B) Close up of algal-bound crenulated pinnacle surface and subhorizontal layering (scale bar is 15 cm long). C) Schematic cross section showing aquifer control on the development of modern tufa mounds on the margin of Mono Lake California (after Council and Bennett, 1993).

The identification of modern ikaite (pseudo-gaylussite) in icy spash zones in some towers suggests that portions of many Pleistocene and Holocene calcite tufas in the Mono Basin and other cold saline lakes in the Great Basin first precipitated as ikaite (Whiticar and Suess, 1998; Shearman et al., 1989). These palaeo-ikaites have long since recrystallised to form their calcitic pseudomorphs (thinolites). Ikaite pseudomorphs are also found in recent and ancient sediments of Polar Regions where they are known as glendonites, or colloquially as “devil’s horns”. The environmental occurrence of ikaites (as in the modern tufa towers in Ikka Fjiord in Greenland) and their pseudomorphs  in glacial seiments make then potential palaeoclimatic indicators of cold environments, including cryogenic saline lakes. Larger ikaite (glendolite) crystals are typically restricted to colder, deeper organic-rich marine sediments or groundwater outflow zones in cold moderately saline evaporitic basins (Buchardt et al., 2001).

Aside from Mono Lake and its spectacular tufa mounds, the other obvious geological feature in the immediate area of the lake is the Mono Craters. These are the remnants of 24 domes of explosive rhyolite, which erupted over the last 40,000 years, with the last eruption occurring less than 700 years ago to form Panum Crater (actually a plug-dome volcano) located just above the south shore of Mono Lake. Black Point, Negit Island, and Paoha Island are all recent volcanic features of the lake geomorphology.

Paoha Island emerged from the lake waters within the last 350 years. This recent volcanism drives hydrothermal circulation and is still active in the surface and subsurface hydrochemistry of the Mono Lake waters. It supplies elevated levels of sodium and bicarbonate ions compared to chlorine. High concentrations of inorganic arsenic oxyanions (200 μM), rare earths and sulphate (130 mM) have also been reported (Tomascak et al., 2003); Oremland et al., 2000; Hoeft et al., 2004). Mono Lake water further contains very high levels of dissolved organic carbon (7 mM), although much of it is refractory to microbial mineralization and pulses of labile organic matter are released in spring and fall to the bottom from the breakdown of single- celled algae that inhabit the shallow lake water (Hoeft et al., 2004). Sulphate reduction accounts for 41% of the mineralization of annual primary production in Mono Lake (Hoeft et al., 2004)

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Mono craters, view looking south (Courtesy Wikipedia).

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Negrit Island in Mono Lake, one of the Mono Craters (Courtesy Wikipedia)

Hydrological and hydrogeochemical evolution

Up to 2 km of lacustral and volcanogenic sediments underlie Mono Lake, and saline groundwaters (>18,000 ppm) extend to the bottom of the basin’s sediment fill (Rogers and Driese, 1995a, b). The position of the saline-fresh groundwater interface within the lake basin reflects a balance between a dense saline brine plume or curtain beneath the lake and the force of inflowing fresh groundwaters fed from the Sierras and discharge near the lake’s western shore. The rain shadow of the Sierra Nevada to the immediate west of the lake has had a dramatic effect on the asymmetry of surface and groundwater inflow to the lake depression. More than 114 cm of precipitation falls at the Sierra crest, which then supplies waters to the western side of the lake’s drainage basin, mostly by surface drainage. Lee Vining Creek, Mill Creek, Rush Creek and Wilson Creek together supply ≈ 1.75 x 1011 l/a to the lake (Tomascak et al., 2003). Less than 13 cm of precipitation falls on the eastern shore of Mono Lake. Altogether surface inflow supplies around 75% of the lake’s water and springs supply the remainder. The much higher recharge rates for fresh surface and groundwater seepage along the western or Sierra Nevada side of the lake have pushed the saline-fresh water interface basinward of the shorezone beneath that side of the lake. This creates the artesian head that facilitates the formation of the more spectacular linear-trending tufa mounds.

In contrast, the brine plume-freshwater interface lies near or directly below the shoreline around much of the rest of the lake. There the saline groundwater discharge zone is just below the playa surface and it contributes to development of saline mudflats and flat sandy beachrock/rubble atop recently exposed former lake beds in an area that lacks significant tufa pinnacles. Simulations by Rogers and Driese (op cit) confirm that extensive faulting controls local permeability in the basin fill beneath the lake waters. Faulting increases vertical permeability and leads to focused or channelled regions of outflow and seepage in the lake floor. High permeability zones in the lake substrate on the western side means resurging less-saline groundwater can overcome the opposing force of the saline groundwater density plume and so can issue onto the lake floor where its degassing outflow is marked by linear belts of spectacular tufa precipitation, (Council and Bennett, 1993).

In the longer time frame of the Mono Lake’s Quaternary hydrology, the redistribution of the basin’s solutes between the lake and underlying saline groundwater body is today still driven by late Quaternary lake level changes (Rogers and Driese, 1995b). At times of low lake levels in the Late Pleistocene, the higher concentration and density of lake waters caused solute loss via free convection (brine reflux). This probably occurred more rapidly and penetrated deeper into the sediment column along open faults or fractures. At higher lake levels, the shoreline discharge zone moved closer to the basin edge (e.g. the Lake Russell strandline). The now-unrestrained saline groundwater mass subsided, drawing solutes from the lake waters into the basin sediments. Subsequent falling lake levels again constricted the saline groundwater zone beneath the lake, driving saline water and their solutes back into the lake waters and once again increasing solute content of surface waters. Sediment permeability below the lake is the major control on the solute transfer rate between the lake and its groundwater reservoir. Only the larger, longer-term lake level changes drive saline groundwater movement in the lake margin. Depending on permeability of the basin fill aquifer, the equilibration of the saline groundwater and lake solute content to lake water level requires hundreds to thousands of years. Rogers and Driese (1993a, b) suggest that current conditions, where a more saline Mono Lake brine pool (50,000-90,000 ppm) overlies less concentrated groundwater (≈18,000 ppm), reflects the still active impact of late Pleistocene lake high stands. In support of this they note that Mono Lake’s historical salinity data have a large scatter, but suggest there was a 5% decrease in the lake’s solute content over the last 50 years of drawdown; they suggest this indicates a diffusive solute flux into lacustrine sediments for this period. Their conclusions are the opposite of the commonly accepted view of Mono Lake drawdown automatically increasing the salinity of the lake waters.

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