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Keywords:

  • Lake volume change;
  • Searles Lake;
  • stable isotopes;
  • tufa facies

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological setting
  5. Tufa facies and sequences
  6. Methods
  7. Results
  8. Interpretation and discussion
  9. Conclusions
  10. Acknowledgements
  11. References

Four tufa facies have been identified in abundant tufa mounds that accumulated in Searles Lake, California, USA, during the Pleistocene. Covariance of δ18O and δ13C values within the tufa facies has been analysed and correlated with palaeo-lake history. This study demonstrates that characteristics of δ18O and δ13C values are different in each of the tufa facies and can be related to palaeo-lake stages. The first facies of tufa formation (P) has relatively low average δ18O and δ13C values (δ18O = −2·7‰ and δ13C = +1·2‰, n = 14). These low values are attributed to rapid increases in the lake volume. This increase was followed by higher average δ18O and δ13C values of both the nodular (N) facies (δ18O = +1·8‰ and δ13C = +4·1‰, n = 24) and the columnar (C) facies (δ18O = +0·6‰ and δ13C = +4·0‰, n = 7). These higher values are interpreted to record a decrease in lake volume due to high evaporation and increased biogenic productivity. Following formation of facies C, the lake was essentially dry during formation of the finely laminated (LC) facies. Facies LC formed subaerially as spring water flowed up through the central conduit of the mounds and cascaded down their periphery. The relatively high δ18O and δ13C values for this facies (δ18O = +1·4‰ and δ13C = +4·4‰, n = 12) are due to evaporation and fast CO2 degassing. Although covariant trends are well-displayed, variations in lake chemistry alter some of these trends due to periods of hyper-alkalinity. This study confirms the utility of stable isotopic analyses of lacustrine tufa facies to determine palaeo-lake history.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological setting
  5. Tufa facies and sequences
  6. Methods
  7. Results
  8. Interpretation and discussion
  9. Conclusions
  10. Acknowledgements
  11. References

Tufas and travertines are composed of crystal fabrics and compositions that are sensitive to environmental changes in hydrology, water chemistry, climate and microbial populations (Chafetz et al., 1991a; Fouke et al., 2000; Pedley, 2009). Unlike speleothems that have recorded long periods of time (>100 000 years), tufa growth generally represents short periods of time (Andrews, 2006). The rapid and continued carbonate accumulations of the tufa can provide a high-resolution palaeoclimatic record in their stable isotopic compositions. In this way, tufa records can provide important complementary information to other long-term data, which provide insights into specific palaeoclimates, palaeohydrologies and palaeogeochemistry of the waters in which the tufa formed (Chafetz et al., 1991b; Guo et al., 1996; Garnett et al., 2004; Smith et al., 2004; Andrews & Brasier, 2005; Andrews, 2006).

Despite the fact that the covariance between the δ18O and δ13C values of tufa depends on several factors, such as lake hydrology, composition of spring water, diagenesis, temperature, equilibrium versus non-equilibrium fractionation, microbiological activity, CO2 outgassing and evaporation (Talbot, 1990; Johnson et al., 1991; Benson, 1994; Li, 1995; Li & Ku, 1997), the stable isotope data from tufa are a useful tool to reconstruct the palaeo-lake history (Li, 1995; Li & Ku, 1997). A strong covariance of δ18O to δ13C values is shown by primary carbonates that formed in many ancient and modern closed-basin lakes (Talbot, 1990; Johnson et al., 1991; Lister et al., 1991; Benson, 1994; Li, 1995; Li & Ku, 1997; Utrilla et al., 1998). In a closed-basin lake, when the lake volume goes through large-scale changes (increase or decrease), both δ18O and δ13C values of the lake display similar trends to lower or higher values. In contrast, hydrologically open lakes generally lack such strong correlation between δ18O and δ13C values, and are mostly characterized either by relatively fixed oxygen and carbon isotopic values, or by relatively loose correlation between the two sets of values (Talbot, 1990; Talbot & Kelts, 1990; Utrilla et al., 1998). Thus, the covariance of the δ18O to δ13C values is a valuable indicator of a hydrologically closed lake.

In Searles Lake, California, an impressive accumulation of approximately 500 tufa mounds, some up to 45 m high, formed on the lake bottom during the Late Pleistocene (Scholl, 1960; Guo & Chafetz, 2012). These mounds are composed of both calcite and aragonite, and formed around spring orifices (Scholl, 1960; Guo & Chafetz, 2012).

The Searles Lake mounds are very porous and display a highly irregular, non-laminated structure typical of tufa deposits. However, Ford & Pedley (1996) specifically referred to the Searles Lake tufa towers as examples of travertines. In that regard, the Searles Lake towers are remarkably similar to the modern structures around Lake Bogoria, Kenya, depicted by Renaut et al. (2013, see in particular, figs. 7A, 8A, E, F and 9B, C, D, E). In addition, the small tower-like structures are referred to as travertines by Renaut et al. (2013). This ambiguity in terminology was discussed recently by Jones & Renaut (2010); these authors stated that: “there is little consensus on usage of the terms ‘tufa’ and ‘travertine’ ”. Jones & Renaut (2010) cite the definition used by Pentecost (2005) as stated in the book Travertine, that is, in essence travertines include all: “chemically precipitated continental limestone formed around ground water seepages, springs and along streams and rivers, occasionally in lakes and consisting of calcite and or aragonite, of low to moderate intercrystalline porosity and often high mouldic or framework porosity within a vadose or occasionally shallow phreatic environment”. The Searles Lake towers fall under that rubric. Additionally, the 5th edition of the Glossary of Geology by the American Geological Institute, states that tufa is: “a variety of travertine that is commonly spongy or porous due to precipitation around a variety of floral structures, such as reeds, plant roots, leaves, etc.” (Neuendorf et al., 2005). The tufa towers in Searles Lake display the internal structure of tufa; however, they do not exhibit any evidence of higher taxa of flora or fauna, although there is abundant evidence of the presence of fossil microbes (Guo & Chafetz, 2012). Given the above, the towers in Searles Lake can be referred to as either travertine or tufa, in agreement with the usage of Ford & Pedley (1996), Neuendorf et al. (2005), Pentecost (2005), Jones & Renaut (2010), and Renaut et al. (2013); in the present study the towers are referred to as ‘tufa’.

This article presents δ18O and δ13C values of the different tufa facies and indicates that these values provide a valuable data set with regard to lake volume changes of Searles Lake during tufa formation, and an indication of the influence of lake alkalinity and salinity on δ18O and δ13C values in closed-basin lakes. This use of lacustrine tufa stable isotopic analyses to determine lake hydrological history of Searles Lake helps to demonstrate the value of this technique.

Factors controlling δ18O and δ13C values in closed-basin lakes

In a closed-basin lake, oxygen and carbon isotopic composition of the carbonates that precipitated in equilibrium with the lake water and their covariant relation is considered a function of hydrological balance, lake alkalinity and salinity, and biogenic productivity (Stuiver, 1970; Peng & Broecker, 1980; McKenzie, 1985; Benson et al., 1990, 1996; Talbot, 1990; Johnson et al., 1991; Lister et al., 1991; Phillips et al., 1992; Li & Ku, 1997; Li et al., 1997, 2000, 2004; Utrilla et al., 1998; Smith, 2001; Smith et al., 2004).

Lake hydrological balance

In a closed-basin lake, water is recharged by direct rain or snowfall, surface run-off and ground water seepage, and leaves only by evaporation. The δ18O and δ13C values of water in the closed lakes are, in general, higher than those of the water coming into the basin (Li & Ku, 1997). Therefore, a large amount of new water will lower the lake δ18O and δ13C values as the lake enlarges. These values will increase when the lake shrinks because of reduced input and intense evaporation. Therefore, when there is a significant change in lake water, oxygen and carbon values of water in a closed-basin lake, and thus the carbonates precipitated from these waters, will show a strong covariant relation. Consequently, a strong covariant relation between oxygen and carbon values within the tufa facies is an indication that the lake was a hydrologically closed system during the carbonate formation (Talbot, 1990; Benson et al., 1996; Li & Ku, 1997; Li et al., 1997, 2000, 2004; Utrilla et al., 1998).

Lake alkalinity and salinity

The covariant relation between δ18O and δ13C values is commonly absent or weak when the lake water experiences high lake alkalinity and salinity, even if there is a large change in lake volume (Talbot, 1990; Li, 1995; Li & Ku, 1997). Under high lake alkalinity and salinity (Li & Ku, 1997), lake water has a high concentration of carbonate ions. When the lake is charged with a large amount of new water, δ18O and δ13C values of the lake water can be decreased due to an isotopic dilution effect. The δ13C values, however, do not decrease as much as δ18O values due to the large concentration of carbonate ions in the highly alkaline lake water. Therefore, high alkalinity and salinity can greatly dampen the sensitivity of carbon isotopic values to hydrological changes (Talbot, 1990; Li, 1995; Li & Ku, 1997). Thus, caution should be exercised when using the presence or absence of covariance of δ18O and δ13C values as an indication of lake palaeohydrology.

Evaporation and biogenic productivity effect

Carbon values can be increased greatly by high biogenic productivity (McKenzie, 1985; Benson et al., 1990; Talbot, 1990; Li, 1995). Photosynthetic organisms preferentially consume 12CO2 (McKenzie, 1985). After these organisms die and sink to the bottom of the lake, their lighter carbon also sinks and is sequestered within these sediments if no oxidation occurs. When more and more organisms with lighter carbon die and sink, the heavier isotope (13C) increases relative to the lighter one in the dissolved carbon in the lake water (McKenzie, 1985). Thus, carbonates precipitated from such lake water are rich in 13C. Meanwhile, high evaporation can favour vertical mixing across the thermocline or chemocline of the lake, supplying more nutrients from deep water to the euphotic zone where phytoplankton grow. As a consequence, biogenic productivity of the surface water becomes enhanced, which in turn elevates the δ13C value of lake water (photosynthetic removal of lighter organic carbon). Li (1995) noted that the δ13C value can be up to +6‰ in a highly productive lake. Thus, elevated δ18O and δ13C signatures in carbonate deposits can be produced under the conditions of high evaporation and biogenic productivity.

In summary, the covariant relation between the δ18O and δ13C values in lacustrine carbonates generally indicates hydrologically closed lakes during formation of these carbonates. There is no, or only weak, correlation between δ18O and δ13C values when a lake level is stable or slowly changes. High alkalinity of lake water buffers the response of carbon composition to any hydrological change within the lake. Under such conditions, there is weak or no correlation between δ18O and δ13C values even though the lake is hydrologically closed.

Geological setting

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological setting
  5. Tufa facies and sequences
  6. Methods
  7. Results
  8. Interpretation and discussion
  9. Conclusions
  10. Acknowledgements
  11. References

The area known as Searles Lake, which is now totally dry, is located within the Searles Lake basin, a north-trending graben of the Basin and Range Province in south-eastern California (Fig. 1). Searles Lake is surrounded by several mountain ranges: the Argus Range and Spangler Hill to the west, Lava Mountain to the south, and the Slate Range on the north and east sides (Smith, 1965, 2009). During the late Pleistocene, especially during the wetter periods, Searles Lake, as well as Owens, China and Panamint lakes, formed a chain of lakes (Fig. 1).

image

Figure 1. Map showing the location of Searles Lake, south-eastern California, as well as the lakes linked with Searles Lake, during the wet Tahoe stage of the late Pleistocene (modified from Rieger, 1992).

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Within the Searles Lake basin, the Christmas Canyon Formation and Searles Lake Formation comprise the predominantly lacustrine late Pleistocene and Holocene deposits (Smith, 2009). The Christmas Canyon Formation is older than 150 ka and mainly composed of lacustrine sandstones, with high CaCO3 content, and alluvial boulder conglomerates. The overlying Searles Lake Formation is composed of, from older to younger, Bottom Mud, Lower Salt, Parting Mud, Upper Salt and Overburden Mud. The age of the Bottom Mud unit ranges from 150 to 50 ka and is made up of calcareous sandstone and massive tufa deposits. The Lower Salt unit is 30 to 25 ka and is composed of siltstones and sandstones. Overlying the Lower Salt is the Parting Mud, which is 25 to 10 ka and is composed of calcareous siltstones and sandstones and abundant tufa deposits. The Upper Salt and Overburden Mud units are less than 10 ka and made up of siltstones and sandstones and lack tufa deposits.

According to strandlines along the hillsides around Searles Lake, lake sediments and core samples, two important pluvial Pleistocene lakes have been recognized, which corresponded to two glacial stages, Tahoe and Tioga, of the Sierra Nevada (Flint & Gale, 1958; Lin et al., 1998). During these two pluvial stages, the tufa mounds formed within a small bay of Searles Lake due to subaqueous precipitation around spring vents (Scholl, 1960). Radiocarbon and U-Th dating from the tufa deposits in Searles Lake indicated that the tufa mounds formed during the Tioga glacial stage (Flint & Gale, 1958; Scholl, 1960; Stuiver, 1964; Peng et al., 1978; Bischoff et al., 1985; Lin et al., 1998). The ages of these mounds are estimated to be from 24 to 10 ka, which indicates that they were formed during the same period as the deposition of the Parting Mud (Flint & Gale, 1958). Therefore, spring water that carried Ca and HCO3 and precipitated tufa mounds in Searles Lake must have been dissolved from rocks older than the Parting Mud. The Bottom Mud and Christmas Canyon Formation are both older than the Parting Mud and have a high CaCO3 content (Smith, 2009); they may be the primary source of the dissolved carbonates.

Bailey (1902) was the first to correctly discuss the hydrological relation between these lakes. Owens Lake discharged into China Lake which then flowed into Searles Lake and then to Panamint and Manly lakes when the highest water level was attained (Russell, 1885; Bailey, 1902; Scholl, 1960; Stuiver, 1964; Hutchins, 1977; Peng et al., 1978; Rieger, 1992). Water from Owens River, Owens Lake and China Lake were the main water volumes that entered into Searles Lake (Smith, 2009). Both ‘clastic’ and ‘chemical’ deltas were formed as sediment and dissolved Ca and HCO3 from upstream drainages entered into the water of Searles Lake (Smith, 2009).

Searles Lake overflowed from time to time but most of the time its volume dropped below the overflow threshold and became a closed-basin lake due to the more arid climate during the Holocene (Peng et al., 1978). The present surface area is ca 105 km2. Surface temperatures at Searles Lake are from −12·2°C to 47·8°C, and the average annual temperature is 19·1°C (Smith, 1979). Annual precipitation in the Searles Lake basin ranges from 43 to 246 mm and averages ca 100 mm (Smith, 2009). The annual evaporation rate in the Searles Lake basin would be ca 1 to 2 m annually (Benson et al., 1990). Consequently, Searles Lake basin is very hot and totally dry due to its high evaporation rate.

All of the mounds in Searles Lake are arranged roughly along the trends N.65°W, N.50°W, N.30°E, N.55°E and N.65°E (Scholl, 1960). The trends were controlled by spring orifice locations, where ground water flowed into the lake through fractures associated with the Garlock Fault. The mounds are distributed into three groups in the Searles Lake basin: northern, middle and southern, occupying ca 96 km2 of the whole basin (Scholl, 1960; Guo & Chafetz, 2012). There are more than 200 tufa mounds in the northern area, which encompasses ca 5 km2. It has a base elevation of 500 to 550 m above sea-level. The middle area has more than 100 tufa mounds and covers ca 5 km2 with a base elevation ca 600 m above sea-level. The mounds in the southern group have been significantly degraded by weathering and erosion. For this reason, the present study has focused on the mounds in the northern and middle groups.

Tufa facies and sequences

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological setting
  5. Tufa facies and sequences
  6. Methods
  7. Results
  8. Interpretation and discussion
  9. Conclusions
  10. Acknowledgements
  11. References

Over 300 tufa mounds occur in the middle and northern areas of Searles Lake. The mounds range from minor features to 45 m in height; most are 5 to 12 m high (Fig. 2). Based on the distinct characteristics, four facies were recognized from the tufa mounds: (i) porous tufa (P); composed of highly irregular very porous depositional fabrics (Fig. 3A); made up of two parts, the innermost (P1) and the outmost (P2) parts of the mounds (Fig. 4A); (ii) nodular tufa (N), which is formed after the P1 deposits (Figs 3B and 4A); (iii) columnar tufa (C) succeeded the P1 deposits (Figs 3C and 4A); and (iv) laminated crusts (LC), which is formed right after the N or C facies and before the P2 deposits (Figs 3D and 4A).

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Figure 2. Field view of tufa mounds in the northern area, Searles Lake, California. Dirt roads for scale.

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Figure 3. Four different tufa facies make up the mounds at Searles Lake, California. (A) Porous tufa deposits (P1 and P2) are composed of the highly irregular sheets with very porous fabric. (B) Nodular deposits (N) have a highly irregular surface composed of thin laminae and millimetre-size nodular grains. (C) Columnar tufa deposits (C) are arranged radially outwards from the centre of the mounds. (D) Laminated crusts (LC) are the thinnest deposits. Pen is 14 cm long.

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Figure 4. Schematic of facies distributions (A) within the mounds and field views (B) to (D) of the mounds in Searles Lake, California. (B) A large mound with the complete facies structure, from core to outside, P1 – N – LC – P2. Red lines mark the boundaries between adjacent facies. Most of the tops of these facies have been significantly eroded. Person for scale is ca 1·8 m tall. (C) A mound composed of a number of cylindrical columns (red arrows point to the white surfaces of facies LC). The P2 deposits sit on the top of the mounds. Horizontally bedded strata between the columns are lake bed deposits. (D) Cross-section view of a broken columnar mound in Searles Lake. From core to outside, facies are P1 – C – and a thin LC. Red dashed line points to the boundary between facies P1 and C. Blue dashed line refers to the boundary between facies C and LC.

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Two temporal sequences recognized from the mounds are: (i) P1 tufa overlain by nodular tufa, succeeded by finely laminated and then enveloped by P2 tufa (P1 – N – LC – P2); and (ii) P1 tufa overlain by columnar tufa, succeeded by finely laminated and then enveloped by P2 tufa (P1 – C – LC – P2; Fig. 4A; Guo & Chafetz, 2012). However, the typical tufa sequence cannot always be observed in the outcrop. In the northern area, the mounds show the first tufa sequence (P1 – N – LC – P2). However, in most cases, the mounds only demonstrate that P2 facies directly envelops P1 deposits (Fig. 4B). Most of the intervening deposits of other facies, nodular, laminated crust, and even some of the porous 1 deposits, were eroded from the mounds before facies porous 2 was deposited. This feature is evident in the blocks that make up the erosional debris aprons that essentially surround all mounds and some well-preserved mounds. In the middle area, the mounds are composed of several cylindrical columns covered by porous (P2) deposits. The cylindrical columns (part of the second tufa facies sequence: P1 – C – LC) in the mounds were also partly eroded before P2 deposition. Therefore, there is a significant erosional surface that was formed between deposition of the laminated crust and porous P2 deposits, indicating that the mounds were exposed during the period between laminated crust and P2 formation. The laminated crusts are interpreted to have formed as the lake level dropped below the elevation of the mounds. Spring flow must still have been sufficient for water to flow up through the lake bottom vents, continue up the centre of the cylindrical columns, and flow as a sheet down their exterior surface forming these crusts (Guo & Chafetz, 2012). Deposition of P2 facies indicates a flooding event following the erosion after formation of the laminated crust deposits (Guo & Chafetz, 2012).

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological setting
  5. Tufa facies and sequences
  6. Methods
  7. Results
  8. Interpretation and discussion
  9. Conclusions
  10. Acknowledgements
  11. References

About 50 tufa mounds in Searles Lake were investigated in the field. Ninety-two samples were collected from 12 tufa mounds, which covered all different facies and groups of the tufa mounds in Searles Lake. The aragonitic samples did not display any characteristics of alterations, so these are pristine carbonates. Samples were ground into very fine powder using a mortar and pestle and filtered through a standard 200-mesh sieve in the laboratory for X-ray diffraction and stable isotopic analyses. Mineralogy of the tufa deposits was determined by X-ray diffraction analyses using a Siemens D-5000 Diffractometer scanning (Bruker AXS Inc., Madison, WI, USA) from 23·5° to 30·5° 2θ at 40 kV and 30 mA with Cu-λ radiation. The relative proportions of aragonite to calcite were determined by calculation of relative heights of the respective 2θ peaks using a standard calibration curve (Tucker, 1988, fig. 7.25). The tufa samples for stable oxygen and carbon isotope analyses were reacted with 100% phosphoric acid in a vacuum at a constant 50°C for at least one night to permit complete reactions (McCrea, 1950). Finally, the CO2 gases were extracted using a gas extraction line and the isotopic ratios were determined using a Finnigan-MAT Delta S mass spectrometer (Finnigan-MAT, Bremen, Germany) in the Department of Earth and Atmospheric Sciences at the University of Houston. Isotopic data are reported in per mil (‰) versus Vienna Pee Dee Belemnite (V-PDB) standard using δ notation. The precision (±1δ) from repeated analyses of in-house calcium carbonate standards was 0·2‰ for oxygen and 0·05‰ for carbon.

In order to correctly analyse evolutionary trends of lake palaeohydrology of Searles Lake during tufa formation, mineralogical corrections were made for oxygen and carbon isotopic composition. For carbon isotopic ratio calculations, the fractionation factor between aragonite and calcite is ca +1·7‰ at 25°C (Romanek et al., 1992). The average fractionation factor in oxygen between aragonite and calcite is +0·8‰ at 25°C (Tarutani et al., 1969). In the present study, δ18Oraw represents the oxygen ratio value of a sample before its mineralogical correction, and δ18O refers to the oxygen ratio value of the sample after mineralogical correction relative to 100% calcite. In the sample, the δ18O value relative to calcite was calculated by determining the percentage of the sample composed of aragonite (XRD) and then using the fractionation between aragonite and calcite. Following the same procedure, δ13C values were obtained. Therefore, all oxygen and carbon isotope data used in the graphics and discussions in this study are corrected values relative to calcite.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological setting
  5. Tufa facies and sequences
  6. Methods
  7. Results
  8. Interpretation and discussion
  9. Conclusions
  10. Acknowledgements
  11. References

X-ray diffraction analyses of these carbonate samples showed that tufa deposits in Searles Lake contain both calcite and aragonite (Fig. 5). Aragonite comprises up to 100% of some samples. There is strong evidence for a relation displayed by δ13Craw and δ18Oraw values versus percentage of aragonite. The δ13Craw and δ18Oraw values tend to increase with increased aragonite content in the tufa deposits in Searles Lake (Fig. 5). The correlation coefficients (R) of δ18O and δ13C values relative to aragonite content are 0·88 and 0·87, respectively.

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Figure 5. Covariant plots of oxygen (A) and carbon (B) isotopes against aragonite content (in %) of the four different facies of the tufa mounds, Searles Lake. Aragonite content subtracted from 100 is calcite content in percentage. ‘n’ equals the number of analyses for each facies. δ18Oraw and δ13Craw refer to oxygen and carbon values before corrections for mineralogy.

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After mineralogical corrections, δ18O values range from −4·85 to +4·66‰ and δ13C values range from −1·35 to +4·95‰ (Fig. 6, data are listed in Table 1). In general, the data fall into two groups, group 1 and group 2, as marked in Fig. 6. Group 1 samples are from the porous tufa deposits (P), including the innermost parts (P1) and outermost parts (P2) of the mounds; whereas group 2 samples are from the other three facies in the mounds: columnar tufa deposits (C), nodular deposits (N) and crusts (LC). The δ18O and δ13C values are relatively low in group 1. In group 1, δ18O values range from −4·85 to −0·11‰ with an average of −2·67‰; whereas δ13C values are from −1·35 to +3‰ with an average of +1·19‰ (Fig. 6). The δ18O and δ13C values are higher in group 2 than in group 1. The δ18O values in group 2 range from −0·76 to +4·66‰ with an average of +1·27‰; and δ13C values are from +1·55 to +4·95‰ with an average of +4·18‰ (Fig. 6).

Table 1. Carbon and oxygen stable isotope data and mineralogical composition from the tufa mounds in Searles Lake, California
Group No.SamplesFaciesAragonite (%)δ18O (‰ PDB)δ13C (‰ PDB)Group No.SamplesFaciesAragonite (%)δ18O (‰ PDB)δ13C (‰ PDB)
Group 1TFM1AP22−2·91·2Group 2TFM1C(a)LC820·54·3
TFM1BP20−4·31·2TFM2FLC801·24·3
TFM1DP50−0·52·1TFM3A1LC731·14·6
TFM1EP23−3·61·0TFM4A1LC771·04·3
TFM1GP50−1·60·2TFN49C1LC531·44·5
TFM1HP20−4·50·5TFN49C2LC701·94·5
TFM1IP22−2·71·6TFN50B4LC531·64·7
TFM1JP50−2·2−0·1TFN50B5LC54−0·74·6
TFM1LP55−3·70·5TFN52B1LC842·34·5
TFM1MP20−3·51·0TFN52B2LC851·54·4
TFM2AP25−3·30·1TFN3ALC952·24·3
TFM2BP25−3·40·0TFN14A2LC902·24·3
TFM2CP19−4·00·0TFM1C(b)C701·94·6
TFM2DP6−4·9−0·5TFM2GC700·13·6
TFM2EP11−4·5−1·4TFM3A2C900·24·1
TFM2HP55−1·71·7TFM4A2C950·74·2
TFM3CP38−1·80·4TFN49C4C680·03·4
TFM3DP20−3·20·6TFN50B3C82−0·33·3
TFM3EP18−4·00·5TFN52B4C851·74·9
TFM3FP24−3·91·1TFM3A3N84−0·33·7
TFM3GP30−3·80·1TFM3BN830·44·4
TFM4CP5−3·10·3TFM4BN950·74·2
TFM4DP0−4·1−0·3TFN49C3N750·64·5
TFM4EP20−2·21·3TFN50B2N640·73·4
TFM4FP30−2·70·8TFN52B3N831·44·6
TFN50B1P0−2·40·6TFN1HN540·21·6
TFN 52CP0−3·40·8TFN1IN923·14·3
TFN1AP4−0·52·1TFN1GN934·74·9
TFN1BP77−0·13·0TFN1KN953·24·4
TFN1CP38−0·43·0TFN1LN1003·24·2
TFN1DP0−1·21·5TFN1MN982·54·7
TFN1EP35−0·72·8TFN1NN842·54·8
TFN1FP15−1·32·5TFN1QN651·34·0
TFN1GP30−1·50·5TFN3BN912·24·5
TFN2AP15−3·71·5TFN3CN952·83·9
TFN2BP70−1·32·2TFN3DN953·44·1
TFN2CP24−2·61·5TFN4AN940·94·1
TFN2DP0−3·31·7TFN4BN701·34·6
TFN2EP0−3·01·8TFN4CN820·04·1
TFN2FP0−2·12·7TFN14BN981·74·4
TFN3EP0−2·32·4TFN14CN982·64·3
TFN3FP12−0·72·2TFN14DN954·45·0
TFN3GP0−3·32·0TFN14D1N961·94·2
TFN3HP0−3·72·2     
TFN3IP20−2·21·9     
TFN3JP18−2·42·6     
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Figure 6. A bivariant plot of δ18O and δ13C values of tufa deposits in Searles Lake. δ18O and δ13C refer to oxygen and carbon values after corrections for mineralogy. Group 1 is classified by all sample data from facies P (P1 and P2), and Group 2 includes all data from samples in facies C, N and LC.

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For different tufa facies, the covariances between δ18O and δ13C values vary significantly. The 14 samples from P1 deposits show that δ18O values are moderately to highly correlated with δ13C values [correlation coefficient (R) is 0·73; Fig. 7A]. Data from facies C display a strong correlation (R = 0·93) between δ18O and δ13C values (Fig. 7B). There are 24 samples in facies N, which has a moderate correlation (R = 0·57; Fig. 7C). Twelve samples of facies LC were analysed and display a weak correlation (R = 0·37) between δ18O and δ13C values (Fig. 7D). There are 33 samples from P2 deposits showing that δ18O values are moderately correlated with δ13C values (R = 0·54; Fig. 7E). Despite the differences in correlation in tufa facies, there is a strong correlation between δ18O and δ13C values for all samples (R = 0·87; Fig. 6).

image

Figure 7. Correlated plots of δ18O and δ13C values for each facies in the mounds. (A) is for the innermost part of porous tufa deposits (P1) (R = 0·73); (B) is for columnar tufa deposits (C) (R = 0·93); (C) is for multiple generations of nodular deposits (N) (R = 0·57); (D) is for finely laminated crusts (LC) (R = 0·37); and (E) is for the outermost part of porous tufa deposits (P2) (R = 0·54). δ18O and δ13C refer to oxygen and carbon values after corrections for mineralogy; ‘n’ refers to numbers of samples.

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Interpretation and discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological setting
  5. Tufa facies and sequences
  6. Methods
  7. Results
  8. Interpretation and discussion
  9. Conclusions
  10. Acknowledgements
  11. References

Water source to Searles Lake

Previous studies noted that three types of water entered the Searles Lake basin during tufa formation: stream water, springs and direct precipitation (Scholl, 1960; Peng et al., 1978; Bischoff et al., 1985; Benson et al., 1990; Lin et al., 1998; Smith, 2009). The stream water which overflowed from Owens Lake through the Owens River was primarily snowmelt water from the eastern Sierra Nevada. As input of fresh water to a lake, δ18O values of stream water are usually lower than that of the lake water (Li et al., 2000).

Spring water was another important water source for tufa precipitation in Searles Lake (Scholl, 1960; Benson et al., 1990; Smith, 2009). However, there are no records pertaining to the geochemical composition of spring water in Searles Lake, and there are no active springs in the area now. Thus, the oxygen and carbon isotopic compositions of spring water in Searles Lake cannot be ascertained directly. However, the study by Mariner et al. (1977) may provide some clues about the geochemical composition of spring water in Searles Lake. The study presented the geochemical composition of hot and cold springs from 14 locations in the central Sierra Nevada. The results of Mariner et al. (1977) demonstrated that δ18O values of the spring water in the central Sierra Nevada are almost the same as those of the local meteoric water [for example, δ18O = −15‰ standard mean ocean water (SMOW)]. The variations that occur at some spring locations are due to either chemical reactions between water and host rocks, or contamination of the studied spring samples by saline lake water (Mariner et al., 1977). Li et al. (1997) demonstrated that the spring water in Mono Lake also has similar values as the local meteoric water (for example, δ18O = −15‰ SMOW). Thus, the spring water in Searles Lake probably had the local meteoric signature of the southern Sierra Nevada. Davisson et al. (1999) reported a δ18O value map of southern Sierra Nevada. The map shows that the δ18O value at Searles Lake is around −13‰ SMOW.

Phillips (2008) shows that the hydrological history of Searles Lake is similar to the Palaeo-Owens system. Therefore, δ18O and δ13C values of spring water in Searles Lake should also reflect the regional hydrological history. As a consequence, tufa deposits should record the general changes in the oxygen and carbon isotopic composition of the lake water.

δ18O to δ13C covariance indicating hydrological closure of Searles Lake

The δ18O values can be readily altered by meteoric water during diagenesis, whereas δ13C values can still retain the original, or very close to original, carbon values. Therefore, the increase or decrease in stable carbon isotope ratios can provide some information about the original carbon source values (Banner & Hanson, 1990; Chafetz et al., 2008). Data from the different facies show that the correlations of the δ18O to δ13C values are significantly different (Fig. 7). The P1 and C facies have relatively high covariance; P2 and N facies have moderate to marked covariance, whereas LC facies has low covariance.

Facies P1 deposition

Facies P1 is the innermost part of the mounds. It grew around spring orifices (Fig. 4A). P1 deposits have lower δ18O and δ13C values than the LC, N and C facies (Figs 6 and 7A). Influx of a large amount of water during P1 deposition resulted in these lower values. As previously discussed, δ18O and δ13C values of water coming into Searles Lake are commonly lower than those already in the lake. Thus, when the lake enlarged due to the influx of a large amount of fresh water, δ18O and δ13C values of the lake water were greatly lowered and, in turn, resulted in low oxygen and carbon isotopic values in the tufa deposits.

The P1 samples were collected from the outermost parts of P1 deposits, which were formed near the end of the period of P1 deposition. It is believed that the water influx into Searles Lake at that time had already decreased and the lake water had started to stabilize. Therefore, geochemical changes of lake water had started to be controlled by vapour exchange between lake water and the atmosphere, rather than solely by change in lake volume. An average δ13C (+1·68‰) value in this data set indicates that carbon composition was already at the equilibrium value (+1 to +3‰, by Li & Ku, 1997) controlled by atmospheric CO2 exchange.

Aragonite and calcite contents of carbonates formed in a closed-basin lake can reflect hydrological change, lake alkalinity and salinity, biological activities, and ratios of magnesium and calcium (Chafetz et al., 1991a; Guo & Riding, 1992; Li, 1995). Basically, relatively high aragonite content in carbonates indicates increasing lake alkalinity and salinity due to lake volume shrinkage. Low aragonite content (average 24%) in P1 deposits indicates that the lake alkalinity and salinity were relatively low when P1 was deposited. Relatively high covariance between δ18O and δ13C values in P1 deposits also indicates low lake alkalinity and salinity during P1 deposition.

Therefore, during the period of P1 deposition, Searles Lake went through a marked increase in lake volume. It was the first lake flooding event during tufa formation. The δ18O and δ13C values in the lake water were first lowered by influx of fresh water and then gradually enriched by lake evaporation and vapour exchange.

Facies C deposition

Columnar tufa (facies C) developed within the mounds after the P1 deposits (Fig. 4A). The columnar deposits mainly occur in the middle tufa mound area in Searles Lake, where the lake was shallower than it was in the northern area. The δ18O and δ13C values of facies C are higher than those of the P1 deposits (Figs 6 and 7B); this indicates that there was a change in the geochemistry of the depositional environments between P1 and C deposition. The depositional change is also indicated by the sharp contact between the tufa comprising these two facies. As stated in an above section, in a closed lake, δ18O and δ13C values can reflect the water volume change of the lake (Talbot, 1990; Benson et al., 1996; Li & Ku, 1997; Li et al., 1997, 2000, 2004; Utrilla et al., 1998), and increasing lake alkalinity and salinity can indicate lake volume shrinkage (Guo & Riding, 1992; Li, 1995); therefore, the great increase in δ18O and δ13C values, as well as the increase in aragonite content (Fig. 5) from facies P1 into facies C, may indicate a large decrease in lake volume due to high evaporation and little water influx during facies C deposition. Carbon isotopic composition of the lake during this period may also have been affected by high biogenic productivity (Guo & Chafetz, 2012).

A high aragonite percentage (average 80%) in facies C indicates relatively high lake alkalinity and salinity. However, the δ18O to δ13C covariant trend during the facies C deposition is very strong (R = 0·93; Fig. 7B); this indicates that lake alkalinity and salinity were not high enough to dampen the carbon shift.

Therefore, during this period, the lake volume decreased due to high evaporation and little fresh water input into the basin. Besides high evaporation, δ13C values of facies C deposits were probably increased by enhanced biogenic activities. During facies C deposition, lake alkalinity and salinity are believed to have been moderately high.

Facies N deposition

Nodular tufa (facies N) succeeded the P1 deposits (Fig. 4A). Facies N dominantly occurs in the northern group where the lake water was deeper than in the middle tufa mound area. Similar to facies C, facies N deposits show high δ18O and δ13C values (Figs 6 and 7C), suggesting the strong influence of evaporation and biogenic productivity. Lake alkalinity and salinity were very high during facies N deposition. Several pieces of evidence support this interpretation. Firstly, the δ18O to δ13C covariant trend during N deposition is moderate (R = 0·57; Fig. 7C). Carbon shift due to evaporation and biogenic productivity had started to be buffered by high lake alkalinity and salinity. The second piece of evidence is that the average percentage of aragonite in samples from this facies is ca 87%, with some samples composed of 100% aragonite (Fig. 5). In general, the higher the aragonite content in carbonates, the higher the lake alkalinity and salinity (Chafetz et al., 1991a; Guo & Riding, 1992; Li, 1995). Finally, the third point is that the N deposits mainly occurred in the northern area where Searles Lake water was the deepest. When the lake sharply shrank, water remained in the northern area much longer compared to other areas. The longer the lake water remains in a closed state, the higher the lake alkalinity and salinity become.

Facies LC deposition

Finely laminated crusts (facies LC) formed after either facies N or C (Fig. 4A). Facies LC was formed after the mounds were exposed to the air but spring water kept flowing up through the centre of the mounds and down the sides (see cylindrical columns in Fig. 4C). Facies LC has relatively high δ18O and δ13C values (Figs 6 and 7D); this means that during LC deposition the environment was still dominated by high evaporation and CO2 outgassing. The high aragonite content of facies LC (Fig. 5) further supports the interpretation of high evaporation and fast rate of CO2 degassing as the water flowed up through the cylindrical columns and down the sides of the subaerially exposed mounds.

Facies P2 deposition

P2 deposits envelop the mounds completely and were formed during the last depositional stage of mound growth (Fig. 4A). There is a large decrease in the δ18O and δ13C values from LC to P2 deposits (Figs 6 and 7E). The δ18O and δ13C values are weakly correlated in LC facies (R = 0·37) and become moderately correlated in P2 deposits (R = 0·54). The δ18O and δ13C values of P2 deposits are in a similar range to those of P1 deposits (Figs 7A and E). All of these observations indicate that the lake volume increased and water influx was probably high during P2 deposition. P2 deposits formed during the second lake flooding event. As stated in an earlier section, the aragonitic samples did not display any characteristics of alteration, which indicates that the abundance of aragonite was not decreased by diagenesis, i.e. its abundance represents the original amount. Therefore, low abundance of aragonite and moderate to marked correlation between δ18O and δ13C values in P2 deposits indicate that lake alkalinity and salinity were low during P2 deposition.

Correlation of all isotopic data

Despite the differences in the correlation between δ18O and δ13C values in different tufa facies, Fig. 6 shows that δ18O and δ13C values are correlated strongly and the correlation coefficient (R) between δ18O and δ13C values for all samples is 0·87. Because it is well-accepted that δ18O and δ13C values of carbonates are covariant in closed-basin lakes, whereas the covariance is commonly absent or weak in hydrological open lakes (Talbot, 1990; Li & Ku, 1997), the present authors interpret the strong correlation of δ18O and δ13C values as an indication that Searles Lake was hydrologically closed during tufa formation, which agrees with a previous study by Smith (2000).

Relation to δ13C values of other tufas

Pentecost & Viles (1994) recognized a distinction in the stable isotopic values between those deposits associated with thermally heated waters and those in which the waters are ambient in temperature and predominantly from soil horizons. These authors defined two types, meteogene travertines that: “originate in the soil and epigene atmospheres” and “range mostly from about 0 to −11‰ δ13C, reflecting the depleted 13C of soil CO2” whereas thermogene travertines are derived from hot rising waters and usually range from −4 to +8‰ δ13C (Pentecost & Viles, 1994). This distinction can be observed in data from other studies (e.g. Chafetz & Lawrence 1994; Gandin & Capezzuoli 2008, fig. 5), that is, positive δ13C values for precipitates derived from thermally heated waters and negative values for those that precipitated from ambient temperature streams.

The δ13C values of Searles Lake tufa deposits are dominantly positive, similar to those of Pyramid Lake (Benson et al., 1996). According to Pentecost & Viles (1994) the deposits from Searles Lake and Pyramid Lake fall within the thermogene range, a generalization which suggests that they originated from hot rising waters. It is highly unlikely that either the Searles Lake tufas or the Pyramid Lake deposits precipitated from thermogenic waters. As discussed above as well as in other papers (e.g. Benson et al., 1996; Tanner, 2010), the composition of lacustrine tufas is influenced by a variety of factors. Consequently, whereas the meteogene–thermogene distinction appears to hold for fluvial deposits, it does not appear to be applicable to these types of lacustrine accumulations.

Age of tufa deposits

Four major lacustrine stratigraphic units that were recognized in the Searles Lake basin from previous research are: (i) Bottom Mud; (ii) Lower Salt; (iii) Parting Mud; and (iv) Upper Salt (Smith & Pratt, 1957; Flint & Gale, 1958; Smith, 1979, 2009; Benson et al., 1990; Phillips et al., 1992; Lin et al., 1998; Smith et al., 2004). The Parting Mud was deposited between two salt layers, and represents the last major glacial stage in the Great Basin (Flint & Gale, 1958; Lin et al., 1998). Using the hydrocarbon chain length of organic components in the Parting Mud, Mankiewicz (1975) determined that this unit was formed between ca 24 and 17·1 kyr bp. Based on pollen and lithological data, Roosma (1958) determined that Searles Lake went through a maximum influx (infilling), a minimum influx (desiccation) and another small influx during the whole period of the Parting Mud deposition. Radiocarbon and U-Th dating from the tufa deposits in Searles Lake indicated that the tufa mounds are subaqueous carbonate deposits that formed during the Tioga glacial stage (Flint & Gale, 1958; Scholl, 1960; Stuiver, 1964; Peng et al., 1978; Bischoff et al., 1985; Lin et al., 1998). The ages of these mounds are estimated to be from 24 to 10 kyr bp, which indicate that the tufa mounds and the Parting Mud were both formed during the Tioga stage.

There are abundant stable isotope studies for both Owens Lake and Searles Lake in the literature (Jannik et al., 1991; Phillips et al., 1992; Benson et al., 1996, 1997; Phillips, 2008). Phillips (2008) presented a graphical comparison of stable isotope data. In order to correlate the various episodes of tufa facies deposition with the known isotopic stratigraphy in Searles Lake, the δ18O values were converted to SMOW values and then plotted against the tufa facies (Fig. 8). Although the exact sequences and the dates of the formation of tufa samples are not known, Fig. 8 shows a similar trend to that presented by Phillips (2008), i.e. δ18O values trend from low to high then to low along the time axis during the 24 to 10 kyr bp period. Therefore, following the δ18O values versus radiocarbon age plot of Phillips (2008), P1 facies probably formed before 16 ka during high lake level, P2 facies more recently than 14 ka during the other high lake level period, and N, C and LC facies formed between 16 and 14 ka when the lake level was low. Thus, the lake went through a flooding high level stage, followed by essentially total desiccation, and then another flooding stage (Fig. 9). The three palaeohydrological stages recognized by the isotope and tufa facies analyses agree with the interpretation from the Parting Mud deposits and previous studies.

image

Figure 8. δ18O values (SMOW) versus tufa facies in Searles Lake, California.

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image

Figure 9. Schematic of facies distributions within the mounds in Searles Lake, California, and hydrological illustrations of these facies interpreted from the oxygen and carbon isotopic values.

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological setting
  5. Tufa facies and sequences
  6. Methods
  7. Results
  8. Interpretation and discussion
  9. Conclusions
  10. Acknowledgements
  11. References

In summary, the δ18O and δ13C values of the tufa deposits in Searles Lake indicate that Searles Lake was hydrologically closed during tufa formation. During this period, Searles Lake went through two flooding events. The first flooding event was represented by the P1, N, and C deposits in the mounds, while the second flooding event was represented by the P2 deposits. Prior to formation of the P2 deposits, the LC deposits formed when Searles Lake attained its low lake volume and most of the mounds were subaerially exposed.

By integrating the oxygen and carbon isotope data and previous studies pertaining to the hydrological history of Searles Lake during the Pleistocene, the following conclusions can be drawn:

  1. In general, δ18O and δ13C values of tufa samples from Searles Lake are moderately to strongly correlated, which indicates that Searles Lake was hydrologically closed during tufa formation.
  2. Two flooding events during tufa formation were recognized by the analyses of tufa facies. One was during P1 deposition and the other was during P2 deposition.
  3. During the deposition of P1 facies, lake alkalinity and salinity were relatively low and δ18O and δ13C values are highly correlated; this indicates that P1 tufa formation was dominantly controlled by lake volume change.
  4. Facies C and N have high aragonite content and δ18O and δ13C values are moderately to highly correlated; this indicates that water influx and lake volume decreased, and lake evaporation and biogenic productivity were higher during the deposition of facies C and N compared to the deposition of facies P1.
  5. Lake water reached its shallowest depth before LC deposition. The mounds were subaerially exposed but spring water kept flowing up through the mounds and down the sides. High evaporation and rapid CO2 degassing promoted LC deposition.
  6. Lake alkalinity and salinity were significantly decreased during P2 deposition, which indicates that the second lake flooding event occurred during this period.

Based on oxygen and carbon isotopic data of the tufa deposits, the lake hydrological history during tufa formation in Searles Lake has been deduced. Therefore, this study has shown that the analysis of oxygen and carbon isotopic composition in lacustrine tufa deposits is a useful technique for recovering palaeohydrological and palaeoenvironmental information of a closed-basin lake. However, caution must be used when employing this type of data because, as this investigation also confirms, lake alkalinity and salinity can dampen the δ13C value changes relative to lake volume changes in a closed-basin lake and thus affect the δ18O to δ13C covariance.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological setting
  5. Tufa facies and sequences
  6. Methods
  7. Results
  8. Interpretation and discussion
  9. Conclusions
  10. Acknowledgements
  11. References

We sincerely thank David Scholl for accompanying us during our initial field study of the Searles Lake tufa deposits and are equally grateful to Corelabs, Inc. (in particular to Larry Bruno) for financial support. We sincerely appreciate the contributions made by anonymous reviewers. Their efforts have definitely resulted in an improved manuscript.

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  2. Abstract
  3. Introduction
  4. Geological setting
  5. Tufa facies and sequences
  6. Methods
  7. Results
  8. Interpretation and discussion
  9. Conclusions
  10. Acknowledgements
  11. References
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