Ikaite precipitation by mixing of shoreline springs and lake water, Mono Lake, California, USA

Geochknicaet Cwtiia Copy+! 63 1993 Peqmon

&a Vol. 57, pp. 3855-3865 Rest Ltd. Printed in US.A.

Ikaite pr~ipi~tion by mixing of shoreline springs and lake water, Mono Lake, California, USA JAMES

L. BISCHOFF,I SCOTTSTINE,* ROBERT J. ROSENBAUER,’ JOHN A. FITZPATRICK,’ and THOMAS W. STAFFORD JR.~ ‘US Geological Survey, 345 Mi~~e~eld Road, Menlo Park, CA 94025, USA 2Department of Geography and Environmental Studies, California State University, Hayward, CA 94542, USA ‘INSTAAR, University of Colorado,Boulder, CO 80309, USA (Received September 2 1, 1992; accepted in revised&m April 1, 1993)

Abstract-Metastable ikaite (C&O,* 6H2O) forms abundantly duriug winter months along the south shoreline of Mono Lake where shoreline springs mix with lake water. Ikaite precipitates because of its decreased solubility at low temperature and because of orthophosphate-ion inhibition of calcite and aragonite. During the spring some of the ikaite is transformed to anhydrous CaCO,, and is incorporated into tufa, but most is dispersed by wave action into the lake where it reacts to form gayhrssite ( NarCa(CO~f2 - 5H20). Spring waters have low pH values, are dominancy Ca-Na-HCOs , have low radiocarbon activities, and are mixtures of deepseated geothermal and cold groundwaters. Chemical modeling reveals that precipitation of CaCO3 can occur over a broad range of mixtures of spring and lake water with a maximum production occurring at 96% spring water and 4% lake water. Under these eonditions all the Ca and a significant fraction of the CO, of the precipitate is spring supplied. A radiocarbon age of 19,580 years obtained on a natural ikaite sample supports this conchrsion. With the springs supplying a large and probably variable portion of the carbonate, and with apparent “C age of the carbonate varying from spring to spring, tufa of similar actual antiquity may yield significantly different “C dates, making tufa at this location unsuitable for absolute age dating by the radiocarbon method. I~ODU~ON

this first known nonmarine natural occurrence of ikaite. Although ikaite at this locality is an interesting mineralogic curiosity in itself, it paragenetically relates to the widespread occurrence of gaylussite ( Na2Ca( COsh 5Hz0) in the lake and also to the mechanics of tufa formation. We conclude that ikaite at Mono Lake forms as a consequence of mixing spring and lake water during cold winter conditions. Massbalance considerations show that all of the Ca and about half of the carbonate for ikaite formation are provided by the spring water, a conclusion of importance for radiocarbon dating of tufa.

THE HEXAHYDRATEOF CaCOs has long been known as a laboratory precipitate, one among the several forms of CaCOs (JOHNSTON et al., 19 16), but its natural occurrence and its designation as a new mineral was not established until 1962 when it was first observed forming in the bottom of Bca Fjord in Greenland (PAULY, 1963). Only four other natural occurrences have been reported, all in marine sediments. SUESS et al. ( 1982) discovered ikaite crystals growing at -1.6’C within anoxic marine sediments of the Bransfield Strait off Antarctica. STEIN and SMITH ( 1985) reported ikaite from anoxic sediments of the Nankai Trough of Japan, JANSENet al. ( 1987 ) identified ikaite in anoxic marine sediments of the Zaire deep-sea fan, and KENNEDY et al. ( 1987) found ikaite in anoxic estuarine sediments at Barrow, Alaska, USA. All known Occurrences appear to be under anoxic conditions having water temperatures of 3°C or lower. These investigators found that samples of natural ikaite disintegrate, within minutes to hours, into ‘a mush of water and small whitish crystals of vaterite and/or calcite when exposed to atmospheric conditions. This explains why the natural occurrence of this mineral has remained unknown until so recently. These discoveries may resolve the parentage of certain peculiar calcite pseudomorphs in sedimentary rocks ranging in age from Permian to Quatema~ ( SUESS et al., 1982; SHEARMANand SMITH, 1985; SHAIKH and SHEARMAN, 1986). RecentIy SHEARMANet al. ( 1989) suggested that ikaite was the precursor of the enigmatic thinolite tufas that were formed in the Lahontan and Mono Basins of eastern California and western Nevada in Pleistocene time. We recently discovered ikaite forming along the shoreline of Mono Lake, California, at several isolated areas where springs discharge into Mono Lake. In what follows we describe

l

MONO LAKE

Mono Lake occupies a hydrographically closed volcani&Iy active basin at the foot of the eastern escarpment of the Sierra Nevada (Fig. I ) . The lake has shrunk dramatically since 1947 due to diversion of tributary streams by the Los Angeles Department of Water and Power (LADWP). This shrinkage has caused a significant increase in the concentration of dissolved salts. The salinity of the lake has increased since 194 1 from 50960to about 97%0 today (STINE, 1988), resulting in the precipitation of gaylussite ( BISCHOFFet al. 199 1) . While only recently discovered, gaylussite has apparently been precipitating since about AD 1970 when the salinity 8rst exceeded 80%. Gaylussite crystals encrust hard substrates under much of the lake, and its abundance has raised questions about the supply of Ca which, at 3-S ppm, is a trace element in the lake water and the component limiting gaylussite formation. The lake is in an advanced state of brine evolution: highly alkaline and dominated by Na-CO&Zl-SO4 (Table 1). isolated groupings of shoreline springs are most apparent along the northwestern, western, and southwestern shores of the lake. These springs are recharged by runoff from the Sier3855

J. L. Bischoff et al.

3856

-- .-

I

--- .-- _-_ i-. -_- --- -

i

NEVADA

i

i.. ‘*.

MONO LAKE

N

-.\_ ‘\’

t 0

t

0

200km 100

I miles

‘Y/

1

O?km

u

Parking lots

300m

To Highway 120

FIG. I. Location map of ikaite occurrence and springs 1 through 4 and Navy Beach Hot Spring (NBHS) at South Tufa Grove, Mono Lake, California.

Some of the springs occur as diffuse seepage from littoral sands while others are artesian with clearly defined orifices at or below lake level. Individual springs are commonly surrounded by calcareous tufa (LEE, 1969; BASHAM, 1988 ) _“Groves” of stranded tufa towers, composed of aragonite and calcite, are common and grew at orifices of sublacustrine springs at times when the lake level was higher than today. Stranded by the retreat of the lake, the towers are highly picturesque and constitute one ofthe major scenic features of the basin (Fig. 2 ) . The distribution of tufa towers, shoreline tufa and CaC03-cemented beachrock are largely associated with the regions of discharge of these springs. These features are absent along much of the eastern lake shore where ran escarpment.

spring discharge is minimal. RUSSELL ( 1889) was the first to suggest that tower growth resulted from simple combination of Ca-bearing spring water with CO+earing lake water. IKAITE OCCURRENCE

Ikaite was first observed on 31 December 1991 in the vicinity of South Tufa (Fig. 1 ), a concentration of tufa towers along the southern shore of the lake. Here springs can be seen emanating along the shore immediately above and below the lake surface. The springs enter the lake as diffuse seepage from littoral sands or, more conspicuously, as low but steady flow from individual orifices or linear fractures. These springs

bite

3857

in Mono Lake

Table 1 C&m&at Analyses of Mono Lake Water and shoreline springs at South Tufa Grove

NavyBeach lake watep thermal spring

Springs

I

1

68.5

2

3

4

18 6.4

15 6.5

15 6.6

35 6.6

5 10.0

!za

629

648

132 12

620 39

466 44

Ca Mg

135 25

111 23

134 37

169 47

125 83

alkafinityt

574

566

645

1280

1891

36201

%I,

26 18

22 17

:ff

367 296

136 34

18368 9990

Sio;! Fe f&l 5

77 4.6 1.1 1.1

81 7 0.6 6.4 1.0

81 2.5 !:“6

TN*

1

933

094

1326

80 7 2.3 10.8 2936

137 CO.1 ::55

2945

30612 1782 343

14 0.8 0.07 490 97494

akalitity as rug/LHCC3

2 total dissolved solids 3 un~blished analysis by Los Angeles Dept. of Water & Power, 1982 (salinity =95.6 mg/L) concentrated to represent 1992 salinity of 97.5 g/L

arc included within BASHAM’S( 1988) “Southern Sub-Rcgion” in her study of the shoreline springs of Mono Lake. This region is located immediately north of the Mono Craters, which last erupted around AD 1345 ( SIEHand BURSIK,1986). The area of active volcanism extends northward from the Mono Craters into the lake, and cinder cones and lava flows are present on the lake’s two major islands. Precipitates of ikaite ofcur along the near-shore zone from a few centimeters above lake level and extending no deeper than about 20 cm below (Fig. 3). They were observed at no fewer than fifty individual sites along some 1500 m of shoreline, locally as a rn~~rne~r-hick coating on littoral sands at sites of diffuse spring seepage, and as thicker, centimeter-thick crusts around the orifices of springs immediately below lake level. Mixing of spring and lake water is neither rapid nor homogeneous in the vicinity of shallow sublacustrine orifices of springs. Rather, ~mrne~ng ~~~mtifi~tion and in~~n~~~ of the waters is observed extending many meters away from the orifices. The encrustation was thickest (2-3 cm) on pieces of submerged tufa that lay near the spring orifices (Fig. 3). The ikaite differed from the more typical tufa deposits observed forming over the past decade ( STINE,1987) in that it has a bright wbite color and displays an intricate, reticulate surface pattern ( Fig. 3 ) . The precipitate is exceptionally soft and was collected by lightly scraping with a knife. The fragile nature of the deposit suggeststhat it could be removed readily by even small waves. It may be significant that wind condi-

tions, and therefore wave activity, on the lake had been minimal for the 6 days pmceeding discovery, according to records kept by the University of California’s Sierra Nevada Aquatic Research Laboratory (SNARL). These!same records indicate the water temperatures at a depth of 10 cm on the open lake ranged from 3 to 6°C during this period (G. Dana, pers. ~ommun., 1992). The exact temperature of ikaite precipitation at the spring orifices cannot be precisely defined, however, because of the local micro-stratification of the waters. Samples in plastic bags were packed in ice for shipment back to the laboratory, where the ikaite was unambiguously identified by X-ray diffraction and microscopy. The ikaite occurs as 0. 1- 1 mm crystals with a blocky chevron habit suggesting oscillation of forms (Fig. 4)) in overall morphology similar to the ikaite recovered by SUESSet al. ( 1982) from Antarctic marine sediments. Ikaite crystab do not persist into the summer months. A visit to the spring sites in August revealed the lake level had dropped some 20 cm and all the ikaite was gone from newly emerged and still submerged springs. Although the springs were continuing to discharge, albeit at’a reduced rate, there was no visible precipitation where spring and lake waters mix. Exactly at lake level and within the wave zone we observed occasional patches (up to 20 cm across) of coarsely crystalline gayltite ( 1- 10 millimeter-sized crystals) adhering to tufa towers at some of the sites where we had earlier observed ikaite. The abundance of gaylussite, however, is con-

3858

J. L. Bischoff et al.

FIG. 2. Tufa towers at South Tufa Grove, Mono Lake, view toward northwest. Now stranded, the towers grew around the orifices of sublacustrine springs at a time when the lake level was higher than present.

siderably less than the original ikaite, raising the question about the ultimate fate of the bulk of the ikaite. Some of it apparently transforms to anhydrous CaCOs and is incorporated into shoreline tufa, but a larger portion must be mechanically dispersed from the shoreline area to the deeper parts of the lake by wave action. SPRING CHEMISTRY We sampled four lake-shore springs all associated with the active precipitation of ikaite, and the Navy Beach Hot Spring (NBHS, a series of three interconnected warm springs at 35°C) which issue at several points along a 150 m trend oriented obliquely to the shoreline at Navy Beach (Fii 1). Ikaite is observed forming along the shoreline where waters of NBHS enter the lake. Water samples were taken via plastic tubing using a peristaltic pump and filtered through 0. I pm membrane into polyethylene bottles. The bottles were completely filled, tightly capped, and returned to the laboratory refrigerated. Temperature and pH were measured in the field, and other components were analyzed in the laboratory. Cations were analyxed by atomic absorption spectroscopy, alkalinity via infrared CO* analyzer and pH, Cl by AgNOs titration, SiO* and B by ICP, and SO1 by ion chromatography (Table 1). Our springs 2, 3, and NBHS are from the same general locality of BASHAM’S (1988) springs 301,302, and 305, which she sampled in 1987. The analytical results of the two studies are similar, indicating that spring chemistry is relatively stable over time. In addition, NBHS has been sampled and analyzed by several other investigators beginning in 1967 as reported by LEE (1%9), LADWP (unpubl. data) in 1968, by MARINERet al. ( 1977 ), and by the Los Alamos National Laboratory (unpubl. data) in 1984.

Analytical results of the present study are shown in Table I and the time series for NBHS in Table 2. Temperatures of the four springs ranged from 8 to 18°C and NBHS was 35°C. All are sliahtlv acidic with pH values ranging from 6.4 to 6.8 (in contrast to 9.5 fo; Mono Lake water). Total dissolved solids (TDS) ranged From 933-2945 mg/ L. All the springs (Table I ) are Ca-Na-HCOs waters with high levels of SiOz (77-137 mg/L), Fe and Mn (0.3-7 mg/L), and B ( I- 11 mg/L). NBHS has a particuIarIy high alkalinity, and CO1 is actively evolving from the spring. a3He of this spring is a very high 5.9, similar to much hotter thermal springs (>2OO”C) in Long Valley CaIdera immediately to the south of Mono Lake, and indicates a significant input of magmatic He ( WELHANet al., 1988). Chemical variability of the springs is shown by a Piper plot of the data (Fig. 5a), which results in a pattern very similar to that plotted by BASHAM ( 1988 ) for the south-shore springs. Springs 1,2,3, and NBHS display a linear mixing trend with NBHS and Spring 1asendmembers, while Spring 4 plots away from this trend. The latter is o&et from the others in the direction of Mono Lake water (Fig. 5a), suggesting significant contamination of this particular spring by lake water. Of the springs sampled, Spring 4 discharges closest to the shoreline, but otherwise has no special geometry that might explain the contamination by lake water. BASHAM( 1988) suggested there were two different sources for the springs in general of this Southern subregion; a fresher, cooler and Ca-rich water recharged through the nearby Rush Creek delta and a warmer, Na-rich hydrothermal water coming from the volcanic complex to the south. Our results point to the same conclusion with NBHS representing the hydrothermal endmember. That admixed lake water does not contribute a significant proportion of the spring chemistry (except for Spring 4) is indicated by the small fraction of Cl in the anionic charge. Chloride is a major anion of Mono Lake water, accounting for 43% of the anion charge,

Bcaitein Mono Lake much higher than in any of the springs If WCassume, as an upper limi~thatautheQinthespringwateriaduetoLPkaMaadmmmrQ then other components are present in pmportion and the lake con~~~~a~rn~ ~bution~~~~~ of 32%of lake mixture forsprings 1,2,3, and NBHS, and up to 56% of TDS of spring 4. Such admixture, however, can contribute only neg&ible amounts of Ca, Mg, and alkalinity. For example, none of ~e~~~of~~~~o~ya~murnof6to 14%of the measured alkahnity of springa 1,2,3, and NBHS can be from lake water admixture, although Spring 4 can have as much as 46% derived~~.Becauscthc~probeblycontainsomeptimary Cl, the actual amounts of lake admixture are likely smaller. Except for Spring 4, the buik of the obaerwd d&sob& components of the springs must be primary. This conclusion is also supportedby the small variation of NBHS over time, the lack of ‘“Gactivity, and the high content of mantiederived He. The temperatureof NBHS has remainedat 35’ -C6% the pH 6.6 f 0.3, and TDS 2730 f 300 mgf L over tbe past 25 years.The Piper plot of the NBHS analyses over time (Fig. 5b) shows very small scatter. The relatively stable composition since 1967, in spite of sign&ant variation in lake salinity ov~~~~~(~ 1988),~~y~~~~ load of the spring is essentiai~yuna&cted by lake water.

IUDIOCAR3ONANALYSES Radiocarbon analyses were performed by accelerator-mass-spectrometer on dissolved carbonates from springs 1,2,3, and NBHS sampled in July of 1992, and on ikaite sampled in December I99 1 (colRcted from the spring orifice of Fw 3, located just below faice Ievelbetween springs f and 2), and of secondarygayhtssitecollected in August of 1992 (from near spring 2). Results(Table 3) show that the apparentage of the carbonatein spring I is 5360 years, 45,200 years for Spring 2, and about 20,000 yearsfor NBHS. The apparent age of the ikaite is tikewise about 20.080 vears while the __ aavlussim _ i;l190 years. The age of Spring2 is essentiallyat the limitof m&carbon dating

witha ‘Y activitylessthan0.5%that of me&m, and shouldprobably be considered a minimum. The finite ages of the other springs can be explained by varyingadmixturesof lake watercarbonate;60%for spri~1,IoWforsping3,and6%~996forNBHs.~~forSpring 1, these amounts are slightly less than, but similar to, the limits of lake-supplied~~~ cakxdatedabove from the Cl contents on the springs sampIed earlier in the year. Likewise the age of the ikaite suggeststhat only 11%of its CO, is lake supplied,and that 89%must be from the spring. The apparentradiacarbonage of the gaylussite 1988). is essentially that of pn%ent&y lake water( BROECKER etal., MIXING OF LARJZAND SPRING WATER Ilcnite is forming as a consequence of the mixing of spring and lake waters, both by diffuse mixing in the littoral zone and by direct mixing in open lake water. The lake water itself is supersaturated approximately tenfold with respect to calcite and aragonite, but pupation of these phases and/or vaterite is strongly inhibited by the utmsually high concentmtion of orthophosphate (60 mg/ L, MASON, 1967 ) in Mono Lake, which poisons the surface sites on the anhydrous forms of CaCOs, but which does not affect the precipitation of ikaitc (BISCHOFF etal., 1993, and references therein). Ikaite is more soluble than any of the anhydrous forms at all Earth’s surface tempemtunq so its oaxunma in nature is due to suppression of the anhydrous forms and its relative ease of crystallization from highly supersaturated waters. Its precipitation is also favored at near-freezing temperatures because its solubility is lower at low temperature, opposite in tmnd to that of the anhydrous forms which have retrograde solubility. Thus, ideal conditions for the metastable occurrence of ikaite are nearfreezing temperatures in waters with high ~n~~~ons of orthophosphate. These conditions are met at Mono Lake

3859

during the winter. With information on chemical composition oftw~tisp&@~itispossibletomodelthemixing process which gives rise to the production of ikaite. At issue is the temperature and relative fractions of spring and lake components which maximize ikaite supematuration and production. The results apply as well to the genesis of common tufa in the South Tuba area. The calculations are made possible by the newly determined solubility and Standard State parameters of ikaite ( BISCHOFFet al., 1993) . In that study, the solubility product (log K of dissolution) was pmcisely determined from the reversible solubility of synthetic ikaite carried out between 0’ and 2J’C at b&red PCO,to yield the expression logKiUe = 0.15981 -2011,1/T,

(1)

where T = Kelvin, and O’C < f z~ 25°C. Values of log K were reproduced within M.01 log units in experiments Ram undersaturation and supersaturation, and propagation of errors indicates accuracy of log K is within ti.02 log units. WetakeSpring3astyp$Gngthecompositewaterdischarging at South T&I and mix it at 6°C with Mono Lake water of 1992 salinity as shown in Table 1, using the computer code PHRQPITZ for chemical speciation via Pitzer ion-interaction model (PLUMMER et al., 1988). The lotions (Fii 6) indicate that pure Mono Lake water is slightly undetsaturated withrespecttoikaite(logsI = -0.27) but is about thirteenfold supersaturated with respect to calcite (log SI = 1.13) at a calculated Ccz, partial pressure (log PCs = -3.0) near atmospheric (log Pm = -3.5). The spring endmember, on the other hand, is initially undersaturated with both calcite ~di~~~d~~a~~~~~~~of CO, (lo8 Pa2 = -0.74) which cannot be sustained very long after spring discharge. The consequence of Co2 outgassing is dramatic; the spring becomes highly supersaturated with respect to calcite and ikaite (Fig. 6)) calcite by one hundredfold (log SI = 2.0) and ikaite by twentyfold (log SI = 1.3). Thus, spring discharge alone is sufficient to precipitate CaC!OJ as a consequence of outgassing. The field evidence however, shows that CaCO~ is not forming where springs dischaqe above lake level but rather where spring water mix with lake water. Mixing is the key to sign&ant CaCQr production. Calculations of the effects of mixing were next carried out allowing the CO2 of the mixture to equilibrate with the atmosphere. The results (Fig 6) show that the mixed lakespring water solution is supersaturated with respect to calcite at all mixtures and with respect to ikaite at all mixtures up to about 95 wt% lake water. The point of maximum ikaite supersaturation, however, occurs at the far right of the diagram where the mixture is only 4% lake water: 96% spring water, at which point supersaturation for calcite is twohundred fiftyfold (log SI = 2.4) and for ikaite twelvefold (log SI = 1.I ) . The spring water is undoes with respect to gaylussite and remains so for mixtures of up to 20% lake water. Above 20% lake water the mixtutu becomes supersaturated reaching a maximum tenfold sumon at about 70 wt% lake water and declining to threefold snperanturation in pure lake water. Although Mono Lake water itself is significantly supersaturated with respect to calcite, only tiny amounts (CO.01 g/ L) can actually form because of the limited amounts of Ca

3860

J. L. Bischoff et al

3861

Ikaite in Mono Lake

RG. 4. Photomicrograph of ikaite crystals, transmitted light.

available (ca. 4 rn& L) . In terms of grams of CaCOs that can be precipitated per liter of mixture (Fig. 7)) the maximum is a prominent peak at the same 4 wt% lake water mixture, at which 0.62 g of ikaite and 0.33 g of calcite are precipitated from each liter of mixture. The precipitate from this mixture takes fully 99.9% of the Ca from the spring water. The proportion of spring-supplied COs in the precipitate is 53%, an important conclusion from the modeling. The proportion of spring carbonate would have been even larger if we had used NBHS instead of Spring 3 for the mixing model because NBHS contains more than twice the alkalinity of Spring 3. In the field, precipitation can and will occur at various mixtures, some more and some less enriched in lake water than the maximum shown in the calculations. The important conclusion to be drawn, however, is that significant, although variable, amounts of the precipitated carbonate is spring sup plied. This conclusion is supported by the apparent radiocarbon age of 19,580 years for the modem ikaite, and this has major implications for the application of radiocarbon dating to fossil tufa, as considered in the following text. Calculations of the amounts of precipitate from the optimum mixture at different temperatures show that the amount of calcite produced is a rather constant 0.32 g/L over the entire range from 0-25”C, whereas the amount of ikaite decreases over the same interval from about 0.62-0.52 g/L. RELATION TO GAYLUSSITE FORMATION The bulk of the ikaite is apparently dispersed into the deeper parts of the lake by wave action. This dispersed ikaite is likely the Ca supply for the gaylussite found forming on solid surfaces throughout the lake floor (BISCHOFFet al., 199 1). The Ca source for the relatively abundant gaylussite has been an enigma. Although present-day Mono Lake water is slightly supersaturated with respect to gaylussite, dissolved Ca (4 ppm) in the lake is insufficient to produce any significant accumulations of the mineral. Calcium must be passing

through the lake water over time to produce the amount of gaylussite observed, but anhydrous CaCO, cannot be the source material. Calcite is supersaturated approximately thirteenfold throughout the lake, so neither it nor other anhydrous forms of CaC03 can be buffering Ca in the lake. However, calculations outlined above show that Mono Lake water is slightly undersaturated with ikaite, so the dispersed ikaite produced at the shore springs will eventually dissolve. In so doing it may supply the Ca to form gaylussite. Kinetic experiments carried out in the course of the solubility experiments suggest that ikaite is the source of Ca in gaylussite. These experiments were performed to assess the me&stable persistence of ikaite ( BISCHOFFet al., 1993). In these experiments 1 g of synthetic ikaite was slurried with 7 mL of Mono Lake water, placed in a test tube thermostated at 6”C, and aerated for the first 6 h to establish atmospheric equilibration. Daily sampling revealed that separate crystals of gaylussite form at the expense of about 30% of the ikaite during the first week of the experiment, but that the mixture remains unchanged thereafter (> 12 months at present writing), suggesting that the solution had quickly reached simultaneous equilibrium with both minerals. Calculations using PHRQPITZ reveal that Mono Lake water can equilibrate with both ikaite and gaylussite via the reaction CaC03 - 6H20 + 2Na+ + CO; ikaite = NaZCa(C03)z.2H20

+ 4Hz0).

gaylussite Equilibration with both phases, however, requires the concentration of dissolved Na to decrease by 33% and alkalinity to decrease by 72%, a significant modification of the bulk composition of lake water. Analysis of the lake water in the experiment showed that both Na and alkalinity changed in

3862

J. L. Bischoff et al. Table 2 Chemical analyses of thermal spring(NBHS) at Navy Beach-South Tufa Grove A

B

C

0

E

present study

collection date T% PH

11167

2l86

10174

iOl84

42 6.6

31 7.0

33 6.4

34 6.6

p !a

510 160 43

472 246 38

410 1;:

515 150 40

452 131 40

466 125 44

38

10

61

80

94

83

1830 85 48

1757 180 20

1560 105 28

1623 128 32

1710 185 40

1891 136 34

Mel alkalinity1 Cl so4 SiO2 Fe Mn B F Li AS TDS

97 0.1

0.1

4.2 1.1

6.1 1.3

2836

2715

130 1.4

143 0.06

3.6 0.4 2.8

;:86

2456

3.7 1 .o 2724

9187 36 6.3

2l92 35 6.6

137 co.1 :::

0.18 2694

2945

1 aikaWy as rn~/L HC03 dash (-) indicatesnot analyzed A. Lee (1969) B. unpubfffad analysis, Los Angeles Dept. of Water 8 Power C. MafIner et al., (1977) D. unpubfiattedanalysis. Los Alamos National Laboratory E. Basham (1988)

the manner predicted. In the natural environment, however, the mass of ikaite is insignificant compared to the mass of lake water and the above xeaction will continue until all ikaite is converted to gaylussite without significantly modifying the lake chemistry. The.resulting solution is slightly supersaturated with respect to gaylussite, but undersaturated with respect to ikaite. The lake remains supersaturated, however, with respect to the anhydrous forms of CaCOJ because of the kinetic control by orthophosphate. Thus, ikaite provides a quasi-steady state control on the concentration of lakewater Ca which provides for the production of gaylussite throughout the lake. Prior to diversion of the source waters to Mono Lake, the salinity was signticantly lower and gayltite was not a factor in carbonate deposition from the lake. At these lower salinities it is possible that calcite and/or aragonite took the place of ikaite as the primary precipitate from the mixing of spring and lake water. IMPLICATIONS FOR ABSOLUTE RADIOCARBON DATING

The results of the present study show that the springs can supply a surprisingly large and variable fraction of the carbonate taken up in tufa deposited in the vicinity of the south

shore springs, and that this carbonate can have low 14C activity. For example, the CaCOs precipitating from the hypothetical optimum 96:4 ratio of lake water to water of Spring 3 will have an apparent age of 6307 radiocarbon years. The radiocarbon age of 19,580 observed for the natural ikaite (Table 3 ) indicates almost 90% of this sample’s CO, is spring supplied. With the springs supplying a large and probably variable portion of the carbonate, and with the apparent 14C age of the carbonate varying from spring to spring tufa of similar actual age may yield significantly -rent “C dates, making tufa at this location unsuitable for radiocarbon dating. These conclusions apply to the dating of tufa mounds and towers whose growth is the result of the mixing of spring and lake waters. Conversely, tufa that forms away from areas of spring discharge, such as the dense laminar forms that appear related to algal growth ( STINE, 1987), probably have radiocarbon activities that reflect the current lake values, as evidenced by the secondary gaylussite in the present study. BROECKER et al. ( 1988) reconstructed the radiocarbon budget of the lake from 550 to 1140 y BP by using stumps encrusted by such laminar tufa collected from the western and northwestern shorelines (see also STINE, 1990). They found that the tufa 14Cage ranged from 1280 to 2690 years older than the wood. The modem (pre-1957) 14Cage of the lake carbonate is about 1700 years ( F%NG and BROECKER,

kite

4-

Ca

e

Ca

in Mono Lake

3863

Cl -

~G.5.piperd.@~~~~(Piper, 1944)ofMonoLalcewaterandqningsatSouthTufaGrove.(a)Chcmicalrelationships of springs 1-4, NBHS and Mono Lake water. Note that springs I, 2,3, and NBHS show linear trends in cation, anion, and combined portions of the diagram indicative of a mixing series from hvo source waters, both independent of Mono Lake water (ML). Spring 4 plots away but on a trend toward Mono Lake water from Spring I, indicating that spring 4 has some admixture of lake water. (b) Composition of Navy Reach Hot Spring as it varied in time from 1967 to the present. Points A through E refer to earlier analyses and P represents analysis from present study as listed in Table 2. Composition of the spring has remained relatively constant over time indicating the dissolved load is primary and is essentially unaI&ted by lake water.

3864

J. L. Bischoffet al. Table 3 Radiocarbon dates of dissolved carbonate from springs and from storyline ikaite and gaylussite pre~pi~tesi material Spring 1

1-Wdate, yrs b.p.

lab. number

5,360 rt 70

CAMS 4666

Spring 2

45,200 f 1210

CAMS 4924

Spring 3

19,710 f 90

CAMS 4669

20,970 f 150 24,100 rt 402

CAMS 4668

19,580 f 90

CAMS 4670

1,190 f 70

CAMS 4676

NBYS (theryal

spring)

ikaite3 gaylussite4

1 Graphite targets made from SrCO3 precipitates of the spring waters and of ikaite and gaylussite were analyzed at the Center for Acceteratory Mass Spectroscopy (CAMS) of the Lawrence Livermore Laboratory by John Southon. 2 Data reported in Oremland et at. (1987) 3 Ikaite was collected in December 1991 from oriface of sublacustrine spring shown in Fig. 2. 4 Gaylussite was collected in August 1991 from IO cm above lake level from side of small tufa tower near Spring 1. 1980). Noting a particularly significant divergence at around 800 years ago, they concluded that large amounts of radiocarbon-free carbon had entered episodically from beneath the lake. In addition, BROECKER et al. ( 1988) showed that

Mono Lake had undergone a rapid increase in 14Cactivity during the years since 1957. They showed that the rate of this increase could not bc explained by any natural process such as gas exchange. The present results o&r no explanation for this rapid increase in lake radiocarbon and the cause remains a mystery. SUMMARY AND CONCLUSIONS The field observations of the present study, chemical analyses of the springs, and the~~ynam~c modeling have ap-

0.6 0.5 0.4 0.3

-8

t

-10

0.2

100

80

60 40 20 lake water wt%

0

FIG. 6. Log saturation index (SI ) vs. percent Mono lake waterin

mixture with spring water at South Tufa Grove for ikaite, calcite, and gaylussite. Calculations were made using compositions of Mono Lake water and !?+priag3 given in Table I, carried out at 6°C using Pitxer ion-in&action model with computer code PHRQPITZ by PLUMMERet al. ( 1988). Log K for ikaite solubility (-7.04) taken from BI!ICHOFF et al. ( 1993). Spring 3 endmember is shown at far right before and after dqasaing of Coz to e&librium with atmosphere. Maximum in tog SEfor ikaite occurs at %% spring water and 4% lake water.

0.1 0 100

80

60

40

20

0

WI% lake water f%G. 7, Massesof ikaite and calcite precipitated per liter of mixture of spring water and Mono Lake water at 6°C. Calculations were made taking initial degree of saturation from Fig. 6 and allowing mixture to come to saturation with sespect to ikaite and calcite. Note that maximum mass of ikaite precipitate occurs at 96% spring water and 4% lake water.

Ikaite in Mono Lake plication to the paragenesis of carbonate minerals at Mono Lake as follows.

1) fkaite precipitates along the south shore as a consequence of mixing of spring and lake waters during winter.

2) South shore spring-waters are composed of Ca-Na-HCO3,

3)

4)

5)

6)

have low radiocarbon activity, and have salinities ranging from 933-2947 mg/L, only a small fraction of which is admixed lake water. Chemical variabiity indicates two sources of water issuing from the springs~ a chemi~y evoIved ground water and a hydrothermal water, both of which are rich in HCQ. Modeling of spring and lake water mixtures reveals that the hypothetical maximum precipitation ofCaC0~ occurs at a mixture of 96 wt% spring water: 4 wt% lake water. Calcium in the precipitate is 100% from the spring water while CO, is 53% from spring and 47% from lake water. The fate of the metastable ikaite is twofold: some fraction converts in situ either to gaylussite or to anhydrous forms of CaCO~ and is incorporated into shoreline tufa. Another .&action is mechanically dispersed by waves to the lake bottom where it eventually dissolves to supply the Ca for the widespread growth of gaylussite throughout the lake. The significant fraction of tufa carbonate apparently sup plied by spring waters suggests that tufa at the south shore may not be suitable for radiocarbon dating.

Ac~~~le~enfs-John Cain of Lee Vining and AndrewCohen of Berkeleyhelped in the ikaite collection. Larry Ford of the U.S. Forest Service aided in the sampling of the springs. Radiocarbon samples were analyzed by accelerator-mass-spectrometer at Lawrence Livermore Laboratov by John Southon. Help&ddiscussions and can&l reviews of an early version of the manuscript were expertly provided by Larry Benson, Larry Miller, Douglas Shearman, and Geoqe Smith. Janet Herman, Carol Stein, and an anonymous reviewer provided critical readings which improved and chuified the manuscript. Editorial handling: T. S. Bowers REFERENCES BASHAMS. L. ( 1988) Analysesof shoreline springs in the Mono Basin California: With applications to the groundwater system. M.S. thesis, Univ. California Santa Cruz. BISCHOFF J. L., HERBSTD. B., and ROSENBAUER R. J. ( 1991) Gaylussite formation at Mono Lake, California. Geochim. Cosmochim. Acfa 55, 1743-1747. BIXZH~FF 1. L., ~WZPATRKK J. A., and ROSENBAU~R R. J. f 1993) The solubility and stabilization of ikaite (CaCO~*6H,O) tirn 0 to 25°C: Environmental and paleoclimatic implications for thinolite tufa. J. Geol. 101,21-33. BROECKER W., WANN~NKHOF R., MATHIEWG., &NG T-H., STINE S., ROBINSON S., HERCZEG A., and STLWERM. ( 1988) The radiocarbon budget for Mono Lake: An unsolved mystery. Earth Planet. Sci. Lett. 88, 16-26. JANS~ZN J. H. F., WOENSDREGT C. F., KOOISTRAM. J., and VAN

3865

DER GAASTS. J. ( 1987) Ikaite pseudomorphs in the Zaire deep sea fan: An intermediate between calcite and porous calcite. Geology 15,245-248. JOHNSTONJ., MET~IN H. E., and WILLIAMSON E. D. ( 1916f The several forms of calcium carbonate. Amer. J. Sci. 41,473-5 12. KENNEDY G. L., HOPKINS D. M., and PICKTHORN W. J. (1987) Ikaite, the glendonite precursor, in estuarine sediments at Barrow, Arctic Alaska. G. S. A. Ann. h&g. 1987 Abstr. Prog. 9,725. LEEK. ( 1969) Infrared exploration for shoreline springs at Mono Lake, California. Ph.D. thesis, Stanford Univ. h&RlNE,R R. H., PRFSsER T. S., and EVANSW. C. ( 1977) Hot sprlnk of the central Sierra Nevada, California. LrSGSOpett-File Report 77-559. MAXIND. T. ( 1967) Limnology of Mono Lake, Califmia; Univ. California Publ. Zoology 83. University of California Press. OREMLANDR. S., MILLERL. G., and WHITKAR M. J. ( 1987) Sourcea and flux of natural gases from Mono Lake, California. Geochim. Cosmochim. Acta 51,29 15-2929. PAULYH. ( 1963) “Ikaite”, A New Mineral from Greenland. Arctic 16,263-264. PENGT.-H. and BR~CKER W. ( 1980) Gas exchange rates for three closed-basin lakes. Limnol. Oceanogr. 25,789-7%. PIPERA. M. ( 1944) A graphic procedure in the geochemical interpretation of water-analyses.Amer. Geophys. Union Trans. 25,9 M923. PLUMMER L. N., PARKHUR?X, D. L., FLEMING, G. W., and DUNKI& S. A. ( 1988) A computer program incorporating Pitzer’sequations for calculation of geochemical reactions in brines. USGS WaterResources invest. Rep. 88-4153. RUSSELL I. C. ( 1889) ~a~~ History of Mono Valley, California. Eighth Annuai Report of the USGS 267-394. SHAIKHA. and SHF,ARMAN G. J. ( 1986) On ikaite and the morphology of its pseudomorphs. In Proceedingsof the International Meeting on the G~h~istry of the Earth Su&ce and Processes ofMineral Formation, Granada. Spain, pp. 791-803. SHEARMAN D. J. and SMITHA. J. ( 1985) Ikaite, the parent mineral of jamnvite-type pseudomorphs. In Proceedingsof the Geologists’ Ass~iation ofLo~on 96, pp. 305-3 14. SHEARMAN D. J., MCGUGANA., STEINC., and SMITHA. J. (1989) Ikaite, CaCOa*6Hz0, precursor of the thinolites in the Quatemary tufas and tufa mounds of the Iahontan and Mono Lake basins, western United States. GSA Bull. 101,9 i3-9 17. SIEXK. and BUR~IKM. ( 1986) Most recent eruption of the Mono Craters, eastern central California. J. Geophys. Res. 91, 12,53912,571. SIINES. ( 1987) Mono LakezThe past 40@ years. Ph.D. dissertation, Univ. California. STINES. ( 1988) The Future of Mono Lake, Appendix D. Community Organization and ResearchInstitute, University of California, Santa

Barbara.

STINES. ( 1990) Late Holocene flu~uations of Mono Lake, eastern California. Palaeoeeoar. Palaeoclim. Palaeoecol.78.333-38 1. STEINC. L. and SMI%H~. J. ( 1985) Authigenic carbdnate nodules in the Nankai Trough, Site 583. Initial Reps. of the Deep Sea grilling Project 87, pp. 659-668. U.S. Gov. Printing Office. SUESSE., BALZERW., HESSEK. F., MULLERP. J., and WEFERG. ( 1982) Calcium carbonate hexahydrate from organic-rich sediments of the Antarctic Shelf: Precursor of Glendonites. Science X216, 1128-I 131. WELHANJ. A., POREDA,R. J., RISONW., and CRAIGH. f 1988) Helium isotopes in geothermal and volcanic gases of the western United States: II. Long Valley Caldem. J. Volcano/Geotherm. Res. 34,201-209.