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The radiocarbon budget for Mono Lake: an unsolved mystery
Earth and Planetary Science Letters, 88 (1988) 16-26 Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
16
[6]
The radiocarbon budget for Mono Lake: an unsolved mystery Wallace S. B r o e c k e r 1, Rik W a n n i n k h o f 1, G u y M a t h i e u 1, T s u n g - H u n g Peng 2, Scott Stine 3, Steve R o b i n s o n 4, A n d y H e r c z e g s a n d M i n z e Stuiver 6 1 Lamont-Doherty Geological Observatory, Palisades, N Y 10964 (U.S.A.) : Environmental Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 (U.S.A.) 3 Department of Geography, University of California, Berkeley, CA 94720 (U.S.A.) 4 U.S. Geological Survey, Menlo Park, CA 94025 (U.S.A.) 5 Australian National University, G.P.O. Box 4, Canberra, A. C. 72 2601 (Australia) 6 Department of Geological Sciences, University of Washington, Seattle, WA 98195 (U.S.A.) Received July 21, 1987; revised version received December 31, 1987 Since 1957 the 14C/C ratio of the dissolved inorganic carbon in Mono Lake has risen by about 60%v. The magnitude of this increase is about four times larger than that expected from the invasion of bomb-produced 14C from the atmosphere. We have eliminated the following explanations: (1) measurement error, (2) an unusually high physical exchange rate for non-reactive gases, (3) inorganic enhancement of the CO 2 exchange rate, and (4) biological enhancement of the CO 2 exchange rate. Clandestine disposal of waste radiocarbon remains a dark-horse explanation. In the course of our investigations we have uncovered evidence for at least one episodic input of radiocarbon-free carbon to the lake over the last 1000 years. We speculate that this injection was related to a hydrothermal event resulting from sublacustrine volcanic activity.
1. Introduction
Mono Lake lies in the high steppe country east of the Sierra Nevada front at an elevation of 6380 feet. Before the diversion of its supply waters to the Los Angeles Water Supply commenced it had a high salinity ( = 50%o) and a high alkalinity (pH = 9.9). Since the £CO2 concentration in the lake was then about 260 m o l / m 3 (compared to about 2 for the ocean) and the mean depth of Mono Lake was then about 22 m (compared to about 4000 m for the ocean), Mono Lake had about the same amount of dissolved inorganic carbon beneath each square meter of its surface as does the ocean. Because of this high carbon inventory radioactive decay is a significant term in Mono Lake's radiocarbon budget. As with the ocean, the supply of radiocarbon to Mono Lake is almost entirely from the atmosphere (via CO 2 exchange). The wind velocity over Mono Lake averages about 3 m / s while that over the ocean averages about 8 m / s ; hence the invasion of ~4CO2 into the ocean should occur at a several times higher rate [1]. Unlike the ocean, which has large depth and geographic gradients in its 14C/C ratio, 0012-821X/88/$03.50
© 1988 Elsevier Science Publishers B.V.
Mono Lake has none. Except during periods of stratification initiated by very high runoff, Mono Lake mixes completely each winter. After thirty years of study, the radiocarbon budget for Mono Lake remains unexplained. The problem is that the measured 60%0 increase in the 14C/C ratio for the lake ECO2 is several times greater than expected from the invasion of bomb radiocarbon from the atmosphere. In this paper we summarize new measurements designed to explain this anomaly. While we have confirmed the anomaly, we still cannot explain it. However, in the course of our investigation we uncovered evidence supporting a suspicion we have held; namely that sizable amounts of radiocarbon-free carbon are being added from beneath the lake. These additions appear to be episodic. This finding complicates an already puzzling situation. Measurement of the 14C/C ratio in Mono Lake water collected in 1957 yielded 0.82 the 14C/C ratio for equilibrium with the prenuclear atmospheric CO 2. At that time the buildup of bombproduced radiocarbon in the atmosphere was as yet too small to influence the lake. Based on the assumption that the lake's radiocarbon and carbon
17
budgets were at steady state, Broecker and Walton [2] concluded from this result that the CO 2 exchange rate between the atmosphere and lake was about 4 m o l / m 2 yr. However, subsequent measurements demonstrated that the 14C content of lake carbon was increasing far faster in response to the bomb-produced ~4C content of atmospheric CO 2 than would be expected if the CO 2 exchange rate were as low as 4 m o l / m 2 yr [3]. Measurements on Mono Lake water samples collected during the late 1970's (see Fig. 1) confirmed this steep rise and suggested a CO2 exchange rate of about 30 m o l / m 2 yr [4]. As this high exchange rate was inconsistent with the low prenuclear 14C/C ratio in lake water, the suggestion was made that 14C-deficient hot spring carbon enters the lake [4]. Assuming these springs to be 14C-free, the steady-state addition rate was estimated to be 5 mol C / m 2 yr. This exceeds the input of carbon dissolved in fiver water by tenfold. The problem with this solution is that the required CO 2 invasion rate is inconsistent with the well documented value of 20 m o l / m 2 yr for the ocean [1,5]. As stated above the average wind speed over Mono Lake is only 3/8ths that over the ocean. At the higher elevation of Mono Lake (1950 m) the air pressure (and thus the CO 2 partial pressure) is only 0.8 that at sea level. Taken together these two differences would suggest a CO 2 invasion rate for Mono Lake about one -12C
l
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-180 [~ -200
I 1960
I
I I 1970 YEAR
• USGS x U. WASH. E]O LDGO I I 1980
1990
Fig. 1. Plot radiocarbon results on Mono Lake water samples as a function of collection date. Shown for comparison is model curve for a CO 2 invasion rate of 35 m o l / m 2 yr and a hot spring input of 7.4 m o l / m 2 yr. The former is the value required to yield the observed 1957-1986 increase and the latter is the value required to yield the prenuclear radiocarbon content of the lake. The open square is based on the analysis of a shoreline tufa; the others are based on the analysis of water samples (see Table 1).
quarter that for the ocean (i.e., about 5 m o l / m 2 yr). While M o n o Lake's higher p H leads to a roughly two-fold chemical enhancement of the CO 2 exchange rate, this enhancement is not great enough to explain the anomolously high of 14C buildup since 1957. During the last several years we have endeavored to get to the b o t t o m of this mystery. The following steps were taken. (1) The L a m o n t - D o h e r t y radiocarbon measurements were checked by sending two water samples collected in 1986 to the laboratory at the University of Washington for precise radiocarbon analysis. After submitting this paper for publication we learned that Steve Robinson had made a number of high-precision measurements on Mono Lake surface water. Both the University of Washington and the U S G S results are consistent with those made at L a m o n t - D o h e r t y (see Table 1, Fig. 1). (2) A strategy for checking the 1957 measurement was also conceived. Pebbles supplied to the lake from streams are transported from their point of entry around the lake margin by long-shore currents. These pebbles become coated with tufa. During periods of falling lake level the pebbles are left stranded. The C a C O 3 coatings that formed since 1950 (when the lake began to shrink in response to the diversion of its source water via the Los Angeles aqueduct) are thought to have grown during the period of months that they were exposed to wave action. If this scenario is correct, then the coatings should bear the ] 4 C / C ratio of lake water during the year the shoreline was formed. As can be seen from Table 1, tufa from pebbles on the 1958 shoreline yields a ~4C/C ratio close to that for the 1957 water sample. (3) An SF6 tracer experiment was conducted in the lake in order to determine whether the air-lake exchange rate for gases was anomalously high. The results of this experiment yielded an SF6 evasion rate consistent with expectation for the mean wind speed [7,16]. (4) Laboratory experiments were performed to determine the ratio of CO 2 to SF6 exchange rate for Mono Lake water. These experiments suggest an inorganic chemical enhancement for the exchange rate of CO 2 of 2.5 + 0.5 [7]. This enhancement is attributable to the reaction CO 2 + O H - ¢ H C O s
18 TABLE 1 Summary of radiocarbon results on water samples and a shoreline tufa from Mono Lake Collection
Material
Depth
Date 1919 1938-1947 July 1957 1958 July 1966 July 1966 Sept. 1976 July 1977 May 1978 May 1978 May 1978 June 1979 Oct. 1979 Aug. 1980 Aug. 1980 Aug. 1980 Aug. 1980 Oct. 1984 Oct. 1984 April 1986 April 1986 Feb. 1987 Feb. 1987
tufa tufa H20 tufa H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20
surface surface surface surface surface deep surface surface surface surface surface surface surface surface 8m 12 m 30 m surface 26 m surface surface surface 24m
614C
~13 C a
A14C b
(7~)
(%~)
(%o)
-322 -253 - 130 -150 - 129 -127 - 99 -88 - 92 -92 -82 - 87 -93 - 78 - 82 - 95 - 84 -74 -81 - 73 - 78 -74 -84
(2) (2) 2 (2) 2 (2) 2 -1.6 1 (2) 0.9 1.6 2.0 1.9 (2) 2.3 2.5 1.8 1.8 2.1 2.1 2.0 1.9
-358+ 4 -292+ 6 - 177 + 10 -196+ 5 -178+ 8 -174+ 8 - 148 + 8 -136+ 3 - 139-+ 8 -142-+ 8 -130-+ 4 - 134-+ 4 -141-+ 3 - 128-+ 7 - 132-+ 7 - 145-+ 7 - 134-+ 7 -124-+ 2 -131-+ 2 - 123-+ 3 - 128-+ 3 -124-+ 5 -133:1:5
Laboratory LDGO LDGO LDGO LDGO LDGO LDGO LDGO USGS LDGO LDGO USGS USGS USGS LDGO LDGO LDGO LDGO USGS USGS WASH WASH LDGO LDGO
a ( ) indicate estimated values. b ~14 C = ~14C_2(~13 +25)(1 + ~14C/1000)
occurring within the b o u n d a r y layer. The value of 9 m o l / m 2 yr for the CO 2 exchange rate o b t a i n e d from the whole lake SF6 tracer e x p e r i m e n t a n d the l a b o r a t o r y C O J S F 6 exchange rate ratio, however, falls well short of the 35 m o l / m 2 yr value required to explain the rise i n 1 4 C / C ratio for lake E C O 2 over the last thirty years. (5) R a d i o c a r b o n m e a s u r e m e n t s of CaCO3-wood pairs (i.e., tufa deposited o n the stems a n d t r u n k s of shore-zone plants killed d u r i n g rises i n lake level) suggest that the ratio of 1 4 C / C i n lake water has changed significantly over the past millenium. These changes lend strong s u p p o r t to the hypothesis that significant a m o u n t s of radiocarbon-deficient inorganic c a r b o n enter the lake.
2. Steady-state carbon and radiocarbon budgets The major terms in the inorganic c a r b o n a n d r a d i o c a r b o n budgets for M o n o Lake are shown i n Fig. 2. C a r b o n is added b y C O 2 invasion, I, b y rivers, R, a n d b y upwelling fluids entering the
lake from beneath, U. C a r b o n is lost by C O 2 evasion, E, s e d i m e n t a t i o n , S (as C a C O 3 a n d organic matter), a n d b y downwelling fluids, D. While we do n o t believe that either b u d g e t is at steady state, it is instructive to consider the situation that w o u l d exist were this the case. A n equation relating to t h e (14C/C)lake/(14C/C)atm ratio to the m a g n i t u d e of I, U a n d the a m o u n t of c a r b o n dissolved in the lake can easily be derived (see Fig. 2) if two a s s u m p t i o n s are made. First the river b i c a r b o n a t e is a s s u m e d to have a 1 4 C / C ratio a p p r o x i m a t e l y equal to that for atmospheric C O 2. Second the c a r b o n dissolved in the water upwelling b e n e a t h the lake is assumed to be nearly free of r a d i o c a r b o n . As the a m o u n t of c a r b o n a d d e d b y rivers ( = 0.5 m o l / m 2 yr) is at least 8 times smaller t h a n the a m o u n t of CO2 i n v a d i n g the lake from the air ( > 4 m o l / m 2 yr) the river term can be safely d r o p p e d from the equation. The term for r a d i o d e c a y (i.e., [Y.CO2]h~k ) has a value of 0.7 m o l / m 2 yr (h = 22 m, E C O 2 = 260 m o l / m 3, a n d X = 1 / 8 2 0 0 years).
19
3. C02 exchange rate based on the radiocarbon increase
AT STEADY STATE R m x 14c \
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14,-, \
u
t,
s..
r~
Ux0
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14c
,
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Dx --~)LAKE
~ /LAKE
CARBON BUDGET I + R+U = E+S+D
RADIODECAY
RADIOCARBON BUDGET &r (I + R) I~.~C)ATM= (E+S+D+[ZC02] hX)--C),AKE 4- \ 1 4 C \ 14C/C)LAK E OR
14C/C)AT M
I+R = I
+ R + U + [ECOz]t~X
ASSUMING R<< I 14C/C ) L A K E 14C/C)ATM
I
= I
+ u + [ECOz]~k I
Z+U+0.7 Fig. 2. The terms in the carbon budgets for M o n o Lake are as follows: gain from rivers, R, gain through CO 2 invasion, I, gain from upweUing of waters, U, loss through CO 2 evasion, E, loss through sedimentation, S, and loss through downwelling of lake water into the underlying sediment, D. If steady state is assumed and the upwelled water is radiocarbon free, then an equation relating the 1 4 C / C ratio in lake carbon to the CO 2 invasion rate and the carbon upwelling rate can be obtained. Taking the prediversion E C O 2 to be 260 m o l / m 3, and the m e a n depth of the lake, h, to be 22 m, the constant term in the equation becomes 0.7 m o l / m 2 yr.
If we assume that the increase in the 1 4 C / / C ratio for lake carbon over the period 1958 to present was the result of invasion of b o m b 14C from the atmosphere, then the radiocarbon record given in Table 1 and Fig. 1 fixes the CO 2 invasion rate. We assume the lake's carbon and radiocarbon budgets to be at steady state before 1950. Since 1950 several perturbations have occurred. The atmosphere's CO 2 content has risen due to fossil-fuel burning. The atmosphere's 14C/Cratio then rose dramatically due to nuclear testing. Finally, the diversion of waters from the lake's Sierra N e v a d a catchment has significantly reduced the lake's area. These changes are summarized in Figs. 4 and 5. The model-generated 14C/C history for Mono Lake water is constrained to pass through the mean for the 1958 shoreline tufa measurement and the 1957 water measurement. The 1957-1986 14C/C increase determines the CO 2 invasion rate and the 1957-1958 mean 14C/C ratio determines the input rate of radiocarbon-free carbon from 15
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If no dead carbon were being added to the lake from beneath, then with a CO 2 invasion rate of 35 m o l / m 2 yr, the 14C/C ratio in lake water should (after correction for air-lake carbon isotope fractionation) be 0.98 that for atmospheric CO 2 and if the CO 2 invasion rate were 9 m o l / m 2 yr it should be 0.93. As shown in Fig. 3, in order to explain the 1957-58 value for the lake's ~4C/C ratio using the CO 2 invasion rate based on the SF6 experiment it is necessary to invoke an input of radiocarbon-free CO 2 into the lake of 1.3 m o l / m 2 yr. If the 1957 to present 14C/C buildup is due to the invasion of bomb-produced radiocarbon, then an input rate of 7.4 m o l / m 2 yr is required.
//y, 0
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I0
/
20
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.
1958
14C/C
,
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CO2 INVASION RATE, I
(tool/m2yr }
Fig. 3. Dependence of steady-state ( 1 4 C / / C ) l a k e / ( 1 4 C / C ) a t m ratio (contours) on the CO 2 invasion rate and on the input rate of radiocarbon-free carbon in waters upwelling beneath the lake. The CO 2 invasion rate obtained from the SF 6 experiments and the CO 2 invasion rate obtained assuming that the 1957 to present rise in 1 4 C / C ratio for the lake is caused by the invasion of bomb-produced radiocarbon are also shown. The dotted contour represents the 1957-58 1 4 C / C ratio for lake water, the circles define the corresponding a m o u n t s of 14C-free carbon entering the lake ( = 7.4 m o l / m 2 yr if the b o m b 14C based rate is accepted and 1.3 if the SF6 based rate is accepted).
20 360
LU n" C) 03 03 Ld n" (3,..
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260
LAKE
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1860
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,o;[
200 r '<3
,
I
1800
1820
1840
I
1860
1880 1900 1920 YEAR
J
1940 1960
1980 2000
Fig. 4. Plots of 14C/C ratio in atmospheric CO 2 and of atmospheric CO 2 content as a function of time. These changes are used in the model calculations.
thermal springs beneath the lake. The best match is obtained for an input rate of radiocarbon-free carbon of about 7.4 m o l / m 2 yr and a CO 2 invasion rate about 35 m o l / m 2 yr. Given that the atmospheric pressure at the elevation of Mono Lake is 0.8 that at sea level, the corresponding sea level CO 2 invasion rate would be 44 m o l / m 2 yr. As this value is more than twice the mean oceanic CO 2 invasion rate, something is amiss. 4. C O z invasion rate based on S F 6 tracing
In order to verify this very high CO 2 invasion rate, experiments were performed which would yield an independent estimate [7,16]. During the summer and fall of 1984 a tracer gas, SF6, was injected into the surface mixed-layer of Mono Lake. For a period of two months the inventory of SF6 was intensively monitored (see Fig. 6). In this way the mean evasion rate of SF6 was obtained. As wind speed was also monitored, it was possible to obtain the wind speed dependence of the SF6
evasion rate (see Fig. 7). The results are entirely consistent with those for freshwater lakes and the ocean. Hence Mono Lake is not anomalous for chemically inert gases. Reaction of CO2 with OH in the water boundary layer can enhance CO 2 exchange with respect to other gases whose exchange are limited by molecular diffusion through the boundary layer [8-11]. The high O H concentration in Mono Lake should lead to chemical enhancement of the CO 2 exchange rate. Theoretical estimates on the magnitude of this enhancement differ [4,7] as the necessary chemical rate constants are uncertain for the chemical compositions of Mono Lake water. In order to directly determine the enhancement, a laboratory experiment was performed to measure the ratio of CO 2 and SF6 piston velocities (see Table 2). Such an experiment automatically takes into account differences in solubility and diffusivity as well as the inorganic enhancement of CO 2 exchange rate. Based on these results a curve can be constructed of average CO 2 invasion flux
21 1960
,
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u (/N S/TU) (m/s)
1910
1930
1950
1970
1990
YEAR
Fig, 5. Plots of the elevation of Mono Lake, the volume of Mono Lake and the surface area of Mono Lake over the last 80 years. The corresponding salinity change is shown by the dashed line. The long-term drop in lake level which commenced in the late 1940s is the result of diversion of about half of the freshwater supply to the lake via the Los Angeles aqueduct. The sharp recovery after 1982 reflects an unusually large amount of precipitation.
as a function of SF6 piston velocity (see Fig. 8). From this figure and the range of SF6 piston velocities observed on Mono Lake during the summer of 1984, we estimate an average CO2 invasion rate of 9 m o l / m 2 yr. This value is four times smaller than the 35 m o l / m 2 yr required to
i 40
explain the rise in the 14C//C ratio of the lake between 1957 and 1986. An additional mechanism for enhanced CO2 exchange exists. Berger and Libby [12] suggested that the enzyme carbonic anhydrase, leaking from aquatic animals, might catalyze CO 2 uptake. Although this hypothesis was subsequently placed in doubt for the oceans [13], it remained a possibility for Mono Lake which has a large brine shrimp standing crop during the summer. As carbonic anhydrase has a short chemical life span in water,
TABLE 2
I
Mono Lake
SF6 Concentration
X
zo
Fig. 7. Relationship between piston velocity, k, for gas evasion and wind speed for Mono Lake and Crowley Lake. Crowley Lake is a freshwater reservoir located 50 k m south of Mono Lake. The figure shows that the exchange rate of SF6 is not anomalous for Mono Lake. The fines are the least-square linear fits through the data points. W i n d speeds were measured at 2 m height on the shore of each lake.
X
X
Results of the laboratory experiments designed to determine the ratio between the gas exchange rates (expressed as piston velocities) for CO 2 and SF 6 [16]
X x X
,,,D
X X
X
O3
X
x x
._~ ~
x x
5
x x
8
Mode of agitation
ksF6 a (cm/hr)
k~o2 b (cm/hr)
kco: c (cm/hr)
Enhancement factor a
Stirring Stirring Waterfall Waterfall
1.8 2.9 17.6 21.0
2.2 3.6 21.8 26.0
7.0 9.4 20.0 29.0
3.2 2.6 0.9 1.1
Piston velocity for SF 6 determined experimentally. b Piston velocity of CO 2 calculated from the SF 6 results assuming that no enhancement takes place, i.e., k~02 = ksv 6 ( S c c o J S c s v r ) - l / z ; where Sc = J,/D, v = kinematic viscosity and D = diffusion coefficient. c Piston velocity for CO2 determined experimentally. d Enhancement factor = k c o J k ' c o 2.
a 2 7/22
I 7•30
I 8/7
I 8/15
I 8/23
I 8/31
Date (1984)
Fig. 6. Results of the SF6 tracer experiment conducted in Mono Lake during the s u m m e r of 1984. The decrease in the tracer concentration with time provides a measure of the evasion rate of gas from the lake.
22 5c
4c
z>_< ozc
y i
i
i
i
,5
2"
3'o
40
××
o
I
5o
SF6 PISTON V E L O C I T Y (NORMALIZED TO 20°C) (cm/hr)
Fig. 8. The solid line represents the relationship between the invasion rate of CO2 and the piston velocity of SF6 calculated using the laboratory enhancement factor data from Table 2 and boundary layer theory [9]. The flattening below 8 cm/hr is due to the fact that diffusion coupled with chemical reaction becomes the dominant transport mechanism for CO2 for conditions of low turbulence. The mean piston velocity for SF6 invasion from Mono Lake observed during the summer of 1984 was 4 cm/hr. From the range of wind speeds during this experiment the SF6 piston velocity was less than 10 cm/hr 90% of the time. Hence for Mono Lake, the CO2 invasion rate is nearly independent of wind velocity. The × 's represent results of helmet experiments for CO2 and CH 4 evasion conducted on the lake (see text). The piston velocities for CH 4 have been converted to equivalent SF6 velocities. As can be seen no evidence for biological enhancement of the CO 2 evasion rate was found.
this c o m p o u n d w o u l d n o t likely have b e e n p r e s e n t in the water used for our l a b o r a t o r y experiments. T o test the " b i o l o g i c a l e n h a n c e m e n t " hypothesis, field experiments were p e r f o r m e d b y floating a helmet on the lake a n d d e t e r m i n i n g the rate of increase of CO2 a n d C H 4 in the nitrogen-filled headspace. F r o m the initial rate of increase in the helmet, the exchange coefficients of the gases can b e d e t e r m i n e d from: k
e x p e r i m e n t s the p i s t o n velocities were 4.9 c m / h r for C O 2 evasion a n d 1.2 c m / h r for C H 4 evasion were o b s e r v e d for the first experiment, a n d 9.5 c m / h r for C O 2 invasion of 0.9 c m / h r for C H 4 evasion for the second. Since the diffusion coefficient of C O 2 a n d C H 4 a r e similar [14] these results yield a c h e m i c a l e n h a n c e m e n t of 2 in the spring a n d 4 or 10 for the s u m m e r experiments. T h e results of the e x p e r i m e n t s are p l o t t e d in Fig. 8. The p i s t o n velocity of C H 4 w a s c o n v e r t e d to a p i s t o n velocity of S F 6 a s s u m i n g that the p i s t o n velocity is p r o p o r t i o n a l to the square r o o t of the S c h m i d t n u m b e r (which is the ratio of k i n e m a t i c viscosity o f the w a t e r to the diffusion coefficient of the gas). W e o b s e r v e r e a s o n a b l e a g r e e m e n t b e t w e e n the h e l m e t results a n d r e l a t i o n s h i p derived f r o m the l a b o r a t o r y m e a s u r e m e n t s d e s c r i b e d above. If a n y t h i n g the h e l m e t results yield somew h a t lower e n h a n c e m e n t factors. Also, no signific a n t difference is seen b e t w e e n the spring experim e n t when no b r i n e s h r i m p were p r e s e n t a n d the s u m m e r e x p e r i m e n t w h e n the lake was t e e m i n g with b r i n e shrimp. If e n h a n c e m e n t g e n e r a t e d b y c a r b o n i c a n h y d r a s e occurs o n l y d u r i n g the s u m m e r months, it m u s t have a s p e c t a c u l a r i m p a c t in o r d e r to yield an average y e a r l y C O 2 flux f r o m 9 to 35 m o l / m 2 yr. T h e c a r b o n i c a n h y d r a s e e n h a n c e m e n t w o u l d have to b e at least 10-fold, driving the s u m m e r C O 2 e x c h a n g e r a t e t o - - 1 2 0 m o l / m 2 yr. Hence, we m u s t cross biological e n h a n c e m e n t off of the list as p o s s i b l e e x p l a n a t i o n s for the 60%0 increase in the 14C c o n t e n t of M o n o L a k e c a r b o n b e t w e e n 1957 a n d the present.
5. Alternate explanation for post-1957 radiocarbon increase
h Ap At Pe
where h is the height of the airspace in the helmet, At is the d e p l o y m e n t time, A p is the change in the p a r t i a l pressure of the gas in the helmet, a n d Pe is the p a r t i a l pressure of the gas at e q u i l i b r i u m with the lake water. H e l m e t s were d e p l o y e d d u r i n g the spring of 1986 when no b r i n e s h r i m p were p r e s e n t a n d in the s u m m e r of 1987 when there was an a b u n d a n c e of b r i n e shrimp. D u r i n g the spring e x p e r i m e n t p i s t o n velocities of 5.0 c m / h r for C O s a n d 2.5 c m / h r for CH4 were observed. F o r the s u m m e r
O n e p o s s i b l e w a y a r o u n d the necessity for a very high M o n o L a k e C O 2 exchange rate is to p o s t u l a t e that M o n o L a k e has b e e n used for the c l a n d e s t i n e d i s p o s a l of waste r a d i o c a r b o n . W e say c l a n d e s t i n e b e c a u s e o n l y p u r p o s e f u l d i s p o s a l of r a d i o w a s t e s could p r o v i d e the 5 curies of r a d i o c a r b o n n e e d e d to e x p l a i n the increase in the lakes 1 4 C / C r a t i o f r o m 1957 to 1986. W h i l e there is no evidence t h a t M o n o L a k e has been used for a n y sort of waste disposal, the p o s s i b i l i t y of a large i n p u t s of waste 14C d u r i n g the p e r i o d 1966-1976 remains a dark-horse explanation.
23
6. Wood-tufa radiocarbon age pairs
influx of radiocarbon-deficient carbon from beneath the lake has varied with time. Several more wood-tufa pairs were collected and analyzed at Lamont-Doherty. Results of the analyses are listed in Table 3 and graphically shown in Fig. 9. These results suggest that during the period 1200-900 years ago the a4C/C ratio in Mono Lake water was higher than the 1957-58 value and that between 800 and 550 years ago the lake value was lower than the 1957-58 value. The rather sharp drop in I 4 C / C ratio about 800 years ago is best explained by an episodic input of radiocarbon-flee carbon from beneath the lake. For example, one could envision that a volcanic event greatly increased the flow of radiocarbon-free CO 2 into the lake, driving its 14C/C ratio from about 0.86 to about 0.71 the atmospheric value. This would require an input of about 1200 moles of radiocarbon-flee carbon per square meter of lake surface. Such an input would have raised the E C O z content of the lake by about 20%. It is interesting to note that the volcanic events which gave rise to much of Negit Island, a composite of dacite cones, domes, and flows that occupies the northwestern quadrant of Mono Lake, occurred sometime between 600 and 1200 years ago [15]. Hence sublake volcanism may have been responsible for the sudden input of a large amount of radiocarbon-free carbon from beneath the lake.
On the shorelines of Mono Lake are tufas encapsolating wood. The wood is from the trunks of trees and shrubs that grew on the shorelands when the lake was low and that were killed during subsequent rises in lake level. The wood offers a substrate for the nucleation of tufa growth. These tufa-wood pairs provide a means of establishing the ratio of the 14C/C for lake water at times in the past. The difference between the ~4C/C ratio for the tufa and that for the wood establishes the difference between the 14C/C ratio in lake and atmosphere carbon at the time the pair formed. This assumes, of course, that the time between wood growth and tufa formation is no more a few decades. Were the time substantial (i.e., more than 100 years), the paleoradiocarbon content of lake carbon calculated in this way would be a maximum. The first tufa-wood pair was collected by Scott Stine and submitted by Ken Lajoie to the U S G S laboratory in Menlo Park for radiocarbon analysis. When one of us (W.S.B.) was made aware of these results, he was struck by the l~ck of agreement between the age of 1700 years established by direct measurements on 1957 M o n o Lake water and that of about 3000 years inferred the results on the wood-tufa pair. This suggested that the
TABLE
3
Radiocarbon
p a i r s f r o m t h e s h o r e l i n e s o f M o n o L a k e . T h e d i f f e r e n c e b e t w e e n the 1 4 C / C
results on wood-tufa
the encasing tufa provides an estimate of the difference between the 14C/C the wood grew Sample No.
Sample
Wood
eleva-
814C
8x3C a
tion (feet)
(%O)
(%O)
ratio for atmospheric
for the wood and for
CO 2 and lake ECO 2 at the time
Tufa
14 C / C I ~ k e
A14 C
Age
8a4 C
~13 C a
A14C
A14C o
(fits)
(years)
(%o)
(%o)
(%o)
(%o * )
14C/Catm
4/27/86/8
6388
- 66
( - 25)
- 66
550
- 289
(0)
- 324
- 276
2/9/87/1
6386
- 68
( - 25)
- 68
565
- 291
(0)
- 326
- 278
0.722
4/27/86/2 USGS1167-8
6388 6388
- 71 - 77
( - 25) - 27
- 71 - 77
570 640
- 294 - 292
(0) + 4
- 329 - 332
- 280 - 276
0.720 0.724
c
0.724
4/27/86/1
6399
- 91
( - 25)
- 91
770
- 333
(0)
- 366
- 302
0.698
5/21/86/1 5/18/86/1
6390 6390
- 102 - 110
( - 25) ( - 25)
- 102 - 110
860 940
- 188 - 193
(0) (0)
- 228 - 233
- 141 - 136
0.859 0.864
4/27/86/3
6387
- 132
( - 25)
- 132
1140
- 223
(0)
- 262
- 149
0.850
a T h e 8 ~ 3 C v a l u e s a r e all e s t i m a t e d . b Corrected to time of formation c Measurements
using radiocarbon
made at the USGS,
age of wood.
Menlo Park; others made at LDGO.
24 I.C
i
O.C
'Z-
I
~O~o_I~F LAKESURFACE
0.8
I
~
z
I ~ I INPUT OF ~ 1200 MOLES OF R A D I O C A R B O N FREE }]CO 2 PER m 2
/
I1~0' o
"
l
Ld c9
~l~
7
~-000 ~Z) Ld
LAKE H20
5000 O:: O Z
~<~
~OO0 ~ O
TOWARD E Q U ~ U a R l U M
WITH ATMOSPHERE
0]5200
I
{~
8 0
I
I
400
n< O_
I
RADIOCARBON AGE ( Y E A R S BEFORE P R E S E N T )
Fig. 9. Radiocarbon to carbon ratios in Mono Lake water as reconstructed from tufa coating radiocarbon dated wood. The dotted line shows a scenario involving an episodic input of radiocarbon-free carbon followed by gradual equilibration with the atmosphere.
A t a C O 2 invasion rate of 35 m o l / m 2 yr the 1 / e time r e q u i r e d for isotope r e - e q u i l i b r a t i o n with a t m o s p h e r i c c a r b o n after such an event is a b o u t 160 years. A t a C O 2 invasion rate of 9 m o l / m 2 yr the response time is a b o u t 630 years. If the 9 m o l / m 2 y r C O 2 invasion rate proves to b e the correct one, it is possible that over the last 800 or so years no further i m p o r t a n t i n p u t s of h y d r o t h e r mal inorganic c a r b o n have occurred. I n s t e a d the ~ 4 C / C ratio in the lake has been relaxing t o w a r d the steady-state value expected for U = 0 (i.e., (14C/C)lake = 0.93 ( a 4 c / C ) a t m ) .
2~
5g
~:
m
2o
td E w ~_
~ 5 0
YRS AGO
4 LjE 3Ore
~E
~g ~2 c~ z
I
0.6
YIRS AGO 0.7 0.8
[ 0.9
3
7. Measurements of tufa from historically dated shorelines
I000 z
RELAXAT,O. o.6
I0
ck z - -
1.0
I4C/C)LAKE/I4C/C)AT M
Fig. 10. The relationship between the rate of input of MC-free carbon from beneath Mono Lake and the steady-state 14C/C ratio in its dissolved carbon (assuming a CO2 invasion rates of 35 mol/m 2 yr and 9 mol/m 2 yr). Shown by ellipses are the steady-state dead carbon fluxes required to maintain the 14C/C ratios observed for the time periods 1150-900 years and 800-550 years ago.
H a v i n g o b t a i n e d a r e a s o n a b l e result for the tufa c o a t e d p e b b l e s f r o m the 1958 shoreline, we a t t e m p t e d to e x t e n d the lake 14C//C record b a c k in time b y r u n n i n g t u f a - c o a t e d p e b b l e s from two o t h e r shorelines; one o c c u p i e d b y the lake f r o m 1938 to 1947 a n d the o t h e r o c c u p i e d b y the lake in 1919, at the time of its historic stand. T h e results of these analyses are listed in T a b l e 1. Both are far lower in a 4 C / C r a t i o t h a n expected. If these results are i n d i c a t i v e of the lake waters, then the rise in r a d i o c a r b o n c o n t e n t b e t w e e n 1919 a n d 1958 is far m o r e r a p i d t h a n that o b s e r v e d for the last 30 years! T o p r o d u c e such a rise w o u l d require a C O 2 exchange rate of 150 m o l / m 2 yr. Since these results a p p e a r a n o m a l o u s , a few w o r d s need b e said a b o u t the m a n n e r in which the p e b b l e s a m p l e s were h a n d l e d . A s the coatings are thin ( < 1 m m ) a n d f i r m l y a t t a c h e d to the p e b b l e surface, we p l a c e d the p e b b l e s in a flask a n d dissolved the C a C O 3 coatings in acid. C a r b o n a t e rocks are rare in this area; m o s t of the p e b b l e s were silicate. T o assure that no c a r b o n a t e p e b b l e s were i n c l u d e d , a n y p e b b l e suspected to be limestone or d o l o m i t e was p i c k e d out a n d discarded. A s a check on the results f r o m the shoreline p e b b l e , we r a n tufa c o a t i n g the face of a volcanic cliff on N e g i t Island. A s the 600 y e a r old ash does n o t d r a p e this intrusion, the tufa m u s t be less than 600 years in age. T h e elevation of the tufa is the same as that of a p r o m i n e n t lake shoreline d a t e d by r a d i o c a r b o n at a b o u t 250 years. Based on this age, the initial 1 4 C / C ratio for the tufa is 0.65 that for a t m o s p h e r i c CO2. This value is consistent with that o b t a i n e d for the t u f a - c o a t e d p e b b l e f r o m the 1919 shoreline. T h u s it is quite p o s s i b l e that the low 1 4 C / C ratios s h o w n in Fig. 9 for the 800 to 550 year B.P. interval persisted to a b o u t 250 years ago a n d p e r h a p s fight u p to W o r l d W a r II. If this is correct then the post-1940 rise in 1 4 C / C ratio in the lake can be e x p l a i n e d o n l y b y c l a n d e s t i n e d i s p o s a l of r a d i o a c t i v e waste in M o n o Lake. T h e aggregate a m o u n t of r a d i o c a r b o n a d d e d to the lake m u s t be a b o u t 20 curies. M o s t of this r a d i o c a r b o n m u s t have been a d d e d b e t w e e n 1940 a n d 1957. O n e c a n d i d a t e is the g r a p h i t e f r o m the W o r l d W a r II U n i v e r s i t y of Chicago g r a p h i t e reactor.
25 8. Radiocarbon measurements on isolated deep water Abnormally high inflow in 1982 and 1983 caused the lake to stratify. While the boundary between the upper mixed water and the isolated deep water has slowly deepened, this stratification remained as of the time this paper was completed. We used this opportunity to determine whether the input of dead carbon to the lake could be detected, the idea being that the 14C/C ratio in the carbon of the isolated water would drop. The last two entries in Table 1 suggest that such a drop has occurred. While the surface water 14C/C ratio lies within the error of the trend for the last decade, the 14C/C for deep water is 15 + 7%0 lower than that for surface water. We plan to do further sampling which will enable us to confirm this result; we also intend to determine the volume of lake water and area of bottom sediment involved so that we can obtain a carbon in flux estimate from these measurements. 9. Direct evidence for the hydrothermal inputs One way to confirm that 14C-free carbon is being added to the lake from beneath is to make measurements on sediment pore waters. While we plan to do profiles of the 14C/C ratio in sediment pore waters we have not yet done so. We have, however, conducted one radiocarbon analysis which bears on this matter. The sample consisted of tufa-cemented pumice sands collected from the south shore of the lake. Because this particular sample surrounds an electrical cable strung along the lake floor (in about 3 m of water) by the N a v y in 1965, we know that the tufa formed between that date and 1974, when the cable was exposed by the receding waters of the lake. We are also quite sure that the cable was never covered by more than a few tens of centimeters of sand. Yet this cement has a a 4 C / C ratio about half that for the lake water during the 1965-1974 time interval (i.e., its apparent radiocarbon age is 6300 years). Thus we have evidence that a strong gradient in 14C/C ratio exists in the upper few tens of centimeters of the sediment pore water (at least at this particular site). If, as we propose above, ~4Cfree carbon enters the lake from beneath, then such gradients are to be expected. For example, if
this carbon were carried upward uniformly across the lake floor in waters carrying the s a m e ~ ] C O 2 content as the lake waters, then the upward advection rate would have to be on the order of 1.0 c m / y r . Coupled with an effective molecular diffusion rate of 1 x 10 -6 cm2/s within the sediment pores, this would yield a relaxation depth for the transition zone between lake water and upwelling water of about 30 cm. While the actual process is surely more complex, the calculation shows that at least in some places on the lake floor 14C-deficient pore waters m a y extend very close to the sedim e n t / w a t e r interface. 10. Conclusions New measurements confirm the validity of earlier studies and clearly demonstrate that a 60%0 increase in N4C value for the lake occurred between 1957 and 1986. If this increase is attributed to the entry of bomb-produced radiocarbon from the atmosphere, then a CO 2 invasion rate of 35 m o l / m 2 yr is required. By contrast the results of an SF6 tracer experiment conducted in the Mono Lake suggest a CO 2 invasion rate of only 9 m o l / m 2 yr. We suspect that the 9 m o l e / m 2 yr value is the correct one and that the 60%0 rise has its origin in clandestine disposal of waste radiocarbon. Regardless of which CO 2 invasion rate proves to be the correct one, there must be a substantial input of 14C-free carbon to the lake. Evidence in support of such inputs comes from measurements of wood-tufa pairs which suggest that the 14C/C ratio in lake water has changed with time. The most likely explanation for such changes is an input of dead carbon pulsed by volcanic events. Acknowledgements Gayle D a n a of the Sierra N e v a d a Aquatic Research Laboratory of UCSB aided in the lake sampling. Discussions with John Melach, Taro Takahashi, Kerry Sieh and Ron Oxburgh helped in developing some of the concepts discussed here. Financial support was provided by N S F grant OCE85-15693. Peng's work was supported jointly by the National Science Foundation's Ecosystem Studies Program under Interagency Agreement BSR-8417923 and the Carbon Dioxide Research Division. Office of Energy Research, U.S. Depart-
26
ment of Energy, under contract DE-AC05840R21400 with Martin Marietta Energy Systems, Inc., Publication No. 3091, Environmental Sciences Division, ORNL. Lamont-Doherty Geological Observatory Contribution No. 4276. References 1 W.S. Broecker, T.-H. Peng, G. Ostlund and M. Stuiver, The distribution of bomb radiocarbon in the ocean, J. Geophys. Res. 90, 6953-6970, 1985. 2 W.S. Broecker and A. Walton, The geochemistry of 14C in freshwater systems, Geochim. Cosmochim. Acta 16, 15-38, 1959. 3 D.L. Thurber and W.S. Broecker, The behavior of radiocarbon in the surface water of the Great Basin, Nobel Symp. 12, 379-400, 1972. 4 T.-H. Peng and W.S. Broecker, Gas exchange rates for three closed-basin lakes, Limnol. Oceanogr. 25, 789-796, 1980. 5 W.S. Broecker, J.R. Ledwell, T. Takahashi, Ray Weiss, L. Merlivat, L. Memery, T.-H. Peng, B. Jahne and K.O. Munnich, Isotopic versus micrometeorologic ocean CO z fluxes: a serious conflict, J. Geophys. Res. 91, 10517-10527, 1986. 6 R. Wanninkhof, J.R. Ledwell and W.S. Broecker, Gas exchange-wind speed relation measured with sulfur hexafluoride on a lake, Science 227, 1224-1226, 1985.
7 R. Wanninkhof, Gas exchange across the air-water interface determined with man made and natural tracers, Ph.D. Thesis, Columbia University, 1986. 8 B. Bolin, On the exchange of carbon dioxide between the atmosphere and the sea, Tellus 12, 274-281, 1960. 9 T.E. Hoover and D.C. Berkshire, Effects of hydration in carbon dioxide exchange across an air-water interface, J. Geophys. Res. 74, 456-464, 1969. 10 J.A. Quinn and N.C. Otto, Carbon dioxide exchange at the air-sea interface: flux augmentation by chemical reaction, J. Geophys. Res. 76, 1539-1548, 1971. 11 S. Emerson, Chemically enhanced CO 2 gas exchange in an eutrophic lake; a general model, Limnol. Oceanogr. 20, 743-753, 1975. 12 R. Berger and W.F. Libby, Equilibrium of atmospheric carbon dioxide with sea water: possible enzymatic control of the rate, Science 164, 1395-1397, 1969. 13 J.C. Goldman and M.R. Dennet, Carbon dioxide exchange between air and seawater: no evidence for rate catalysis, Science 220, 199-201, 1983. 14 D.M. Himmelblau, Diffusion of dissolved gases in liquids, Chem. Rev. 64, 527-550, 1964. 15 S. Stine, Mono Lake: the past 4000 years, Ph.D. Dissertation in Geography, University of California, Berkeley, Calif., 1987 (unpubfished). 16 R. Wanninkhof, J.R. Ledwell, W.S. Broecker and M. Hamilton, Gas exchange on Mono Lake and Crowley Lake, California. J. Geophys. Res. 92, 14,567-14,580, 1987.
16
[6]
The radiocarbon budget for Mono Lake: an unsolved mystery Wallace S. B r o e c k e r 1, Rik W a n n i n k h o f 1, G u y M a t h i e u 1, T s u n g - H u n g Peng 2, Scott Stine 3, Steve R o b i n s o n 4, A n d y H e r c z e g s a n d M i n z e Stuiver 6 1 Lamont-Doherty Geological Observatory, Palisades, N Y 10964 (U.S.A.) : Environmental Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 (U.S.A.) 3 Department of Geography, University of California, Berkeley, CA 94720 (U.S.A.) 4 U.S. Geological Survey, Menlo Park, CA 94025 (U.S.A.) 5 Australian National University, G.P.O. Box 4, Canberra, A. C. 72 2601 (Australia) 6 Department of Geological Sciences, University of Washington, Seattle, WA 98195 (U.S.A.) Received July 21, 1987; revised version received December 31, 1987 Since 1957 the 14C/C ratio of the dissolved inorganic carbon in Mono Lake has risen by about 60%v. The magnitude of this increase is about four times larger than that expected from the invasion of bomb-produced 14C from the atmosphere. We have eliminated the following explanations: (1) measurement error, (2) an unusually high physical exchange rate for non-reactive gases, (3) inorganic enhancement of the CO 2 exchange rate, and (4) biological enhancement of the CO 2 exchange rate. Clandestine disposal of waste radiocarbon remains a dark-horse explanation. In the course of our investigations we have uncovered evidence for at least one episodic input of radiocarbon-free carbon to the lake over the last 1000 years. We speculate that this injection was related to a hydrothermal event resulting from sublacustrine volcanic activity.
1. Introduction
Mono Lake lies in the high steppe country east of the Sierra Nevada front at an elevation of 6380 feet. Before the diversion of its supply waters to the Los Angeles Water Supply commenced it had a high salinity ( = 50%o) and a high alkalinity (pH = 9.9). Since the £CO2 concentration in the lake was then about 260 m o l / m 3 (compared to about 2 for the ocean) and the mean depth of Mono Lake was then about 22 m (compared to about 4000 m for the ocean), Mono Lake had about the same amount of dissolved inorganic carbon beneath each square meter of its surface as does the ocean. Because of this high carbon inventory radioactive decay is a significant term in Mono Lake's radiocarbon budget. As with the ocean, the supply of radiocarbon to Mono Lake is almost entirely from the atmosphere (via CO 2 exchange). The wind velocity over Mono Lake averages about 3 m / s while that over the ocean averages about 8 m / s ; hence the invasion of ~4CO2 into the ocean should occur at a several times higher rate [1]. Unlike the ocean, which has large depth and geographic gradients in its 14C/C ratio, 0012-821X/88/$03.50
© 1988 Elsevier Science Publishers B.V.
Mono Lake has none. Except during periods of stratification initiated by very high runoff, Mono Lake mixes completely each winter. After thirty years of study, the radiocarbon budget for Mono Lake remains unexplained. The problem is that the measured 60%0 increase in the 14C/C ratio for the lake ECO2 is several times greater than expected from the invasion of bomb radiocarbon from the atmosphere. In this paper we summarize new measurements designed to explain this anomaly. While we have confirmed the anomaly, we still cannot explain it. However, in the course of our investigation we uncovered evidence supporting a suspicion we have held; namely that sizable amounts of radiocarbon-free carbon are being added from beneath the lake. These additions appear to be episodic. This finding complicates an already puzzling situation. Measurement of the 14C/C ratio in Mono Lake water collected in 1957 yielded 0.82 the 14C/C ratio for equilibrium with the prenuclear atmospheric CO 2. At that time the buildup of bombproduced radiocarbon in the atmosphere was as yet too small to influence the lake. Based on the assumption that the lake's radiocarbon and carbon
17
budgets were at steady state, Broecker and Walton [2] concluded from this result that the CO 2 exchange rate between the atmosphere and lake was about 4 m o l / m 2 yr. However, subsequent measurements demonstrated that the 14C content of lake carbon was increasing far faster in response to the bomb-produced ~4C content of atmospheric CO 2 than would be expected if the CO 2 exchange rate were as low as 4 m o l / m 2 yr [3]. Measurements on Mono Lake water samples collected during the late 1970's (see Fig. 1) confirmed this steep rise and suggested a CO2 exchange rate of about 30 m o l / m 2 yr [4]. As this high exchange rate was inconsistent with the low prenuclear 14C/C ratio in lake water, the suggestion was made that 14C-deficient hot spring carbon enters the lake [4]. Assuming these springs to be 14C-free, the steady-state addition rate was estimated to be 5 mol C / m 2 yr. This exceeds the input of carbon dissolved in fiver water by tenfold. The problem with this solution is that the required CO 2 invasion rate is inconsistent with the well documented value of 20 m o l / m 2 yr for the ocean [1,5]. As stated above the average wind speed over Mono Lake is only 3/8ths that over the ocean. At the higher elevation of Mono Lake (1950 m) the air pressure (and thus the CO 2 partial pressure) is only 0.8 that at sea level. Taken together these two differences would suggest a CO 2 invasion rate for Mono Lake about one -12C
l
I
I
'i
T : 35m°l/m2y r S = 74 mol/m 2 -140
"
Y~
/~TT
i i ,-~T • ~ • ~ Yl'k''t---~
Y--
(~T
o
L) -160
z
<3
-180 [~ -200
I 1960
I
I I 1970 YEAR
• USGS x U. WASH. E]O LDGO I I 1980
1990
Fig. 1. Plot radiocarbon results on Mono Lake water samples as a function of collection date. Shown for comparison is model curve for a CO 2 invasion rate of 35 m o l / m 2 yr and a hot spring input of 7.4 m o l / m 2 yr. The former is the value required to yield the observed 1957-1986 increase and the latter is the value required to yield the prenuclear radiocarbon content of the lake. The open square is based on the analysis of a shoreline tufa; the others are based on the analysis of water samples (see Table 1).
quarter that for the ocean (i.e., about 5 m o l / m 2 yr). While M o n o Lake's higher p H leads to a roughly two-fold chemical enhancement of the CO 2 exchange rate, this enhancement is not great enough to explain the anomolously high of 14C buildup since 1957. During the last several years we have endeavored to get to the b o t t o m of this mystery. The following steps were taken. (1) The L a m o n t - D o h e r t y radiocarbon measurements were checked by sending two water samples collected in 1986 to the laboratory at the University of Washington for precise radiocarbon analysis. After submitting this paper for publication we learned that Steve Robinson had made a number of high-precision measurements on Mono Lake surface water. Both the University of Washington and the U S G S results are consistent with those made at L a m o n t - D o h e r t y (see Table 1, Fig. 1). (2) A strategy for checking the 1957 measurement was also conceived. Pebbles supplied to the lake from streams are transported from their point of entry around the lake margin by long-shore currents. These pebbles become coated with tufa. During periods of falling lake level the pebbles are left stranded. The C a C O 3 coatings that formed since 1950 (when the lake began to shrink in response to the diversion of its source water via the Los Angeles aqueduct) are thought to have grown during the period of months that they were exposed to wave action. If this scenario is correct, then the coatings should bear the ] 4 C / C ratio of lake water during the year the shoreline was formed. As can be seen from Table 1, tufa from pebbles on the 1958 shoreline yields a ~4C/C ratio close to that for the 1957 water sample. (3) An SF6 tracer experiment was conducted in the lake in order to determine whether the air-lake exchange rate for gases was anomalously high. The results of this experiment yielded an SF6 evasion rate consistent with expectation for the mean wind speed [7,16]. (4) Laboratory experiments were performed to determine the ratio of CO 2 to SF6 exchange rate for Mono Lake water. These experiments suggest an inorganic chemical enhancement for the exchange rate of CO 2 of 2.5 + 0.5 [7]. This enhancement is attributable to the reaction CO 2 + O H - ¢ H C O s
18 TABLE 1 Summary of radiocarbon results on water samples and a shoreline tufa from Mono Lake Collection
Material
Depth
Date 1919 1938-1947 July 1957 1958 July 1966 July 1966 Sept. 1976 July 1977 May 1978 May 1978 May 1978 June 1979 Oct. 1979 Aug. 1980 Aug. 1980 Aug. 1980 Aug. 1980 Oct. 1984 Oct. 1984 April 1986 April 1986 Feb. 1987 Feb. 1987
tufa tufa H20 tufa H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20 H20
surface surface surface surface surface deep surface surface surface surface surface surface surface surface 8m 12 m 30 m surface 26 m surface surface surface 24m
614C
~13 C a
A14C b
(7~)
(%~)
(%o)
-322 -253 - 130 -150 - 129 -127 - 99 -88 - 92 -92 -82 - 87 -93 - 78 - 82 - 95 - 84 -74 -81 - 73 - 78 -74 -84
(2) (2) 2 (2) 2 (2) 2 -1.6 1 (2) 0.9 1.6 2.0 1.9 (2) 2.3 2.5 1.8 1.8 2.1 2.1 2.0 1.9
-358+ 4 -292+ 6 - 177 + 10 -196+ 5 -178+ 8 -174+ 8 - 148 + 8 -136+ 3 - 139-+ 8 -142-+ 8 -130-+ 4 - 134-+ 4 -141-+ 3 - 128-+ 7 - 132-+ 7 - 145-+ 7 - 134-+ 7 -124-+ 2 -131-+ 2 - 123-+ 3 - 128-+ 3 -124-+ 5 -133:1:5
Laboratory LDGO LDGO LDGO LDGO LDGO LDGO LDGO USGS LDGO LDGO USGS USGS USGS LDGO LDGO LDGO LDGO USGS USGS WASH WASH LDGO LDGO
a ( ) indicate estimated values. b ~14 C = ~14C_2(~13 +25)(1 + ~14C/1000)
occurring within the b o u n d a r y layer. The value of 9 m o l / m 2 yr for the CO 2 exchange rate o b t a i n e d from the whole lake SF6 tracer e x p e r i m e n t a n d the l a b o r a t o r y C O J S F 6 exchange rate ratio, however, falls well short of the 35 m o l / m 2 yr value required to explain the rise i n 1 4 C / C ratio for lake E C O 2 over the last thirty years. (5) R a d i o c a r b o n m e a s u r e m e n t s of CaCO3-wood pairs (i.e., tufa deposited o n the stems a n d t r u n k s of shore-zone plants killed d u r i n g rises i n lake level) suggest that the ratio of 1 4 C / C i n lake water has changed significantly over the past millenium. These changes lend strong s u p p o r t to the hypothesis that significant a m o u n t s of radiocarbon-deficient inorganic c a r b o n enter the lake.
2. Steady-state carbon and radiocarbon budgets The major terms in the inorganic c a r b o n a n d r a d i o c a r b o n budgets for M o n o Lake are shown i n Fig. 2. C a r b o n is added b y C O 2 invasion, I, b y rivers, R, a n d b y upwelling fluids entering the
lake from beneath, U. C a r b o n is lost by C O 2 evasion, E, s e d i m e n t a t i o n , S (as C a C O 3 a n d organic matter), a n d b y downwelling fluids, D. While we do n o t believe that either b u d g e t is at steady state, it is instructive to consider the situation that w o u l d exist were this the case. A n equation relating to t h e (14C/C)lake/(14C/C)atm ratio to the m a g n i t u d e of I, U a n d the a m o u n t of c a r b o n dissolved in the lake can easily be derived (see Fig. 2) if two a s s u m p t i o n s are made. First the river b i c a r b o n a t e is a s s u m e d to have a 1 4 C / C ratio a p p r o x i m a t e l y equal to that for atmospheric C O 2. Second the c a r b o n dissolved in the water upwelling b e n e a t h the lake is assumed to be nearly free of r a d i o c a r b o n . As the a m o u n t of c a r b o n a d d e d b y rivers ( = 0.5 m o l / m 2 yr) is at least 8 times smaller t h a n the a m o u n t of CO2 i n v a d i n g the lake from the air ( > 4 m o l / m 2 yr) the river term can be safely d r o p p e d from the equation. The term for r a d i o d e c a y (i.e., [Y.CO2]h~k ) has a value of 0.7 m o l / m 2 yr (h = 22 m, E C O 2 = 260 m o l / m 3, a n d X = 1 / 8 2 0 0 years).
19
3. C02 exchange rate based on the radiocarbon increase
AT STEADY STATE R m x 14c \
I
E 14r \
14,-, \
u
t,
s..
r~
Ux0
Ul4r\
14c
,
S x -~F-}
Dx --~)LAKE
~ /LAKE
CARBON BUDGET I + R+U = E+S+D
RADIODECAY
RADIOCARBON BUDGET &r (I + R) I~.~C)ATM= (E+S+D+[ZC02] hX)--C),AKE 4- \ 1 4 C \ 14C/C)LAK E OR
14C/C)AT M
I+R = I
+ R + U + [ECOz]t~X
ASSUMING R<< I 14C/C ) L A K E 14C/C)ATM
I
= I
+ u + [ECOz]~k I
Z+U+0.7 Fig. 2. The terms in the carbon budgets for M o n o Lake are as follows: gain from rivers, R, gain through CO 2 invasion, I, gain from upweUing of waters, U, loss through CO 2 evasion, E, loss through sedimentation, S, and loss through downwelling of lake water into the underlying sediment, D. If steady state is assumed and the upwelled water is radiocarbon free, then an equation relating the 1 4 C / C ratio in lake carbon to the CO 2 invasion rate and the carbon upwelling rate can be obtained. Taking the prediversion E C O 2 to be 260 m o l / m 3, and the m e a n depth of the lake, h, to be 22 m, the constant term in the equation becomes 0.7 m o l / m 2 yr.
If we assume that the increase in the 1 4 C / / C ratio for lake carbon over the period 1958 to present was the result of invasion of b o m b 14C from the atmosphere, then the radiocarbon record given in Table 1 and Fig. 1 fixes the CO 2 invasion rate. We assume the lake's carbon and radiocarbon budgets to be at steady state before 1950. Since 1950 several perturbations have occurred. The atmosphere's CO 2 content has risen due to fossil-fuel burning. The atmosphere's 14C/Cratio then rose dramatically due to nuclear testing. Finally, the diversion of waters from the lake's Sierra N e v a d a catchment has significantly reduced the lake's area. These changes are summarized in Figs. 4 and 5. The model-generated 14C/C history for Mono Lake water is constrained to pass through the mean for the 1958 shoreline tufa measurement and the 1957 water measurement. The 1957-1986 14C/C increase determines the CO 2 invasion rate and the 1957-1958 mean 14C/C ratio determines the input rate of radiocarbon-free carbon from 15
I
o•iI
I
,q/° ,/ V
u_E
/
o.
7
~b
.I ~
/"
If no dead carbon were being added to the lake from beneath, then with a CO 2 invasion rate of 35 m o l / m 2 yr, the 14C/C ratio in lake water should (after correction for air-lake carbon isotope fractionation) be 0.98 that for atmospheric CO 2 and if the CO 2 invasion rate were 9 m o l / m 2 yr it should be 0.93. As shown in Fig. 3, in order to explain the 1957-58 value for the lake's ~4C/C ratio using the CO 2 invasion rate based on the SF6 experiment it is necessary to invoke an input of radiocarbon-free CO 2 into the lake of 1.3 m o l / m 2 yr. If the 1957 to present 14C/C buildup is due to the invasion of bomb-produced radiocarbon, then an input rate of 7.4 m o l / m 2 yr is required.
//y, 0
I
. / /"
I0
/
20
l
.
1958
14C/C
,
30
I
40
CO2 INVASION RATE, I
(tool/m2yr }
Fig. 3. Dependence of steady-state ( 1 4 C / / C ) l a k e / ( 1 4 C / C ) a t m ratio (contours) on the CO 2 invasion rate and on the input rate of radiocarbon-free carbon in waters upwelling beneath the lake. The CO 2 invasion rate obtained from the SF 6 experiments and the CO 2 invasion rate obtained assuming that the 1957 to present rise in 1 4 C / C ratio for the lake is caused by the invasion of bomb-produced radiocarbon are also shown. The dotted contour represents the 1957-58 1 4 C / C ratio for lake water, the circles define the corresponding a m o u n t s of 14C-free carbon entering the lake ( = 7.4 m o l / m 2 yr if the b o m b 14C based rate is accepted and 1.3 if the SF6 based rate is accepted).
20 360
LU n" C) 03 03 Ld n" (3,..
I
I
I
I
I
f
f
I
340 SEA LEVEL 320
o-E
300 oS '4) _ ' o 280
r~ LLI T 003
0
I<
MONO
260
LAKE
240 220 1800
1820
9oo I
I
I
1840
1860
I
I
I
I
I
1880 1900 1920 YEAR I
I
I
I
I
I
I
I
1940
1960
I
I
1980 2000 I
800 r (.3 or" hi 03
0 I--
700
r
600 F 500 r ~° 4001-300~
,o;[
200 r '<3
,
I
1800
1820
1840
I
1860
1880 1900 1920 YEAR
J
1940 1960
1980 2000
Fig. 4. Plots of 14C/C ratio in atmospheric CO 2 and of atmospheric CO 2 content as a function of time. These changes are used in the model calculations.
thermal springs beneath the lake. The best match is obtained for an input rate of radiocarbon-free carbon of about 7.4 m o l / m 2 yr and a CO 2 invasion rate about 35 m o l / m 2 yr. Given that the atmospheric pressure at the elevation of Mono Lake is 0.8 that at sea level, the corresponding sea level CO 2 invasion rate would be 44 m o l / m 2 yr. As this value is more than twice the mean oceanic CO 2 invasion rate, something is amiss. 4. C O z invasion rate based on S F 6 tracing
In order to verify this very high CO 2 invasion rate, experiments were performed which would yield an independent estimate [7,16]. During the summer and fall of 1984 a tracer gas, SF6, was injected into the surface mixed-layer of Mono Lake. For a period of two months the inventory of SF6 was intensively monitored (see Fig. 6). In this way the mean evasion rate of SF6 was obtained. As wind speed was also monitored, it was possible to obtain the wind speed dependence of the SF6
evasion rate (see Fig. 7). The results are entirely consistent with those for freshwater lakes and the ocean. Hence Mono Lake is not anomalous for chemically inert gases. Reaction of CO2 with OH in the water boundary layer can enhance CO 2 exchange with respect to other gases whose exchange are limited by molecular diffusion through the boundary layer [8-11]. The high O H concentration in Mono Lake should lead to chemical enhancement of the CO 2 exchange rate. Theoretical estimates on the magnitude of this enhancement differ [4,7] as the necessary chemical rate constants are uncertain for the chemical compositions of Mono Lake water. In order to directly determine the enhancement, a laboratory experiment was performed to measure the ratio of CO 2 and SF6 piston velocities (see Table 2). Such an experiment automatically takes into account differences in solubility and diffusivity as well as the inorganic enhancement of CO 2 exchange rate. Based on these results a curve can be constructed of average CO 2 invasion flux
21 1960
,
~
~
~
i
,
i
' I
I
~955
Z
- -
£ ~"
X
1950
"-C
1945
E
uJ
LU
MONO LAKE
O CROWLEY
LAKE
o
6
.
.
.
4100
j ,
.
51-
-
x -
.... ,,
~._
480 /
.....
.
_
j.-.
-"
......
50
u
I I
I
I 2
3
1
I
4
5
u (/N S/TU) (m/s)
1910
1930
1950
1970
1990
YEAR
Fig, 5. Plots of the elevation of Mono Lake, the volume of Mono Lake and the surface area of Mono Lake over the last 80 years. The corresponding salinity change is shown by the dashed line. The long-term drop in lake level which commenced in the late 1940s is the result of diversion of about half of the freshwater supply to the lake via the Los Angeles aqueduct. The sharp recovery after 1982 reflects an unusually large amount of precipitation.
as a function of SF6 piston velocity (see Fig. 8). From this figure and the range of SF6 piston velocities observed on Mono Lake during the summer of 1984, we estimate an average CO2 invasion rate of 9 m o l / m 2 yr. This value is four times smaller than the 35 m o l / m 2 yr required to
i 40
explain the rise in the 14C//C ratio of the lake between 1957 and 1986. An additional mechanism for enhanced CO2 exchange exists. Berger and Libby [12] suggested that the enzyme carbonic anhydrase, leaking from aquatic animals, might catalyze CO 2 uptake. Although this hypothesis was subsequently placed in doubt for the oceans [13], it remained a possibility for Mono Lake which has a large brine shrimp standing crop during the summer. As carbonic anhydrase has a short chemical life span in water,
TABLE 2
I
Mono Lake
SF6 Concentration
X
zo
Fig. 7. Relationship between piston velocity, k, for gas evasion and wind speed for Mono Lake and Crowley Lake. Crowley Lake is a freshwater reservoir located 50 k m south of Mono Lake. The figure shows that the exchange rate of SF6 is not anomalous for Mono Lake. The fines are the least-square linear fits through the data points. W i n d speeds were measured at 2 m height on the shore of each lake.
X
X
Results of the laboratory experiments designed to determine the ratio between the gas exchange rates (expressed as piston velocities) for CO 2 and SF 6 [16]
X x X
,,,D
X X
X
O3
X
x x
._~ ~
x x
5
x x
8
Mode of agitation
ksF6 a (cm/hr)
k~o2 b (cm/hr)
kco: c (cm/hr)
Enhancement factor a
Stirring Stirring Waterfall Waterfall
1.8 2.9 17.6 21.0
2.2 3.6 21.8 26.0
7.0 9.4 20.0 29.0
3.2 2.6 0.9 1.1
Piston velocity for SF 6 determined experimentally. b Piston velocity of CO 2 calculated from the SF 6 results assuming that no enhancement takes place, i.e., k~02 = ksv 6 ( S c c o J S c s v r ) - l / z ; where Sc = J,/D, v = kinematic viscosity and D = diffusion coefficient. c Piston velocity for CO2 determined experimentally. d Enhancement factor = k c o J k ' c o 2.
a 2 7/22
I 7•30
I 8/7
I 8/15
I 8/23
I 8/31
Date (1984)
Fig. 6. Results of the SF6 tracer experiment conducted in Mono Lake during the s u m m e r of 1984. The decrease in the tracer concentration with time provides a measure of the evasion rate of gas from the lake.
22 5c
4c
z>_< ozc
y i
i
i
i
,5
2"
3'o
40
××
o
I
5o
SF6 PISTON V E L O C I T Y (NORMALIZED TO 20°C) (cm/hr)
Fig. 8. The solid line represents the relationship between the invasion rate of CO2 and the piston velocity of SF6 calculated using the laboratory enhancement factor data from Table 2 and boundary layer theory [9]. The flattening below 8 cm/hr is due to the fact that diffusion coupled with chemical reaction becomes the dominant transport mechanism for CO2 for conditions of low turbulence. The mean piston velocity for SF6 invasion from Mono Lake observed during the summer of 1984 was 4 cm/hr. From the range of wind speeds during this experiment the SF6 piston velocity was less than 10 cm/hr 90% of the time. Hence for Mono Lake, the CO2 invasion rate is nearly independent of wind velocity. The × 's represent results of helmet experiments for CO2 and CH 4 evasion conducted on the lake (see text). The piston velocities for CH 4 have been converted to equivalent SF6 velocities. As can be seen no evidence for biological enhancement of the CO 2 evasion rate was found.
this c o m p o u n d w o u l d n o t likely have b e e n p r e s e n t in the water used for our l a b o r a t o r y experiments. T o test the " b i o l o g i c a l e n h a n c e m e n t " hypothesis, field experiments were p e r f o r m e d b y floating a helmet on the lake a n d d e t e r m i n i n g the rate of increase of CO2 a n d C H 4 in the nitrogen-filled headspace. F r o m the initial rate of increase in the helmet, the exchange coefficients of the gases can b e d e t e r m i n e d from: k
e x p e r i m e n t s the p i s t o n velocities were 4.9 c m / h r for C O 2 evasion a n d 1.2 c m / h r for C H 4 evasion were o b s e r v e d for the first experiment, a n d 9.5 c m / h r for C O 2 invasion of 0.9 c m / h r for C H 4 evasion for the second. Since the diffusion coefficient of C O 2 a n d C H 4 a r e similar [14] these results yield a c h e m i c a l e n h a n c e m e n t of 2 in the spring a n d 4 or 10 for the s u m m e r experiments. T h e results of the e x p e r i m e n t s are p l o t t e d in Fig. 8. The p i s t o n velocity of C H 4 w a s c o n v e r t e d to a p i s t o n velocity of S F 6 a s s u m i n g that the p i s t o n velocity is p r o p o r t i o n a l to the square r o o t of the S c h m i d t n u m b e r (which is the ratio of k i n e m a t i c viscosity o f the w a t e r to the diffusion coefficient of the gas). W e o b s e r v e r e a s o n a b l e a g r e e m e n t b e t w e e n the h e l m e t results a n d r e l a t i o n s h i p derived f r o m the l a b o r a t o r y m e a s u r e m e n t s d e s c r i b e d above. If a n y t h i n g the h e l m e t results yield somew h a t lower e n h a n c e m e n t factors. Also, no signific a n t difference is seen b e t w e e n the spring experim e n t when no b r i n e s h r i m p were p r e s e n t a n d the s u m m e r e x p e r i m e n t w h e n the lake was t e e m i n g with b r i n e shrimp. If e n h a n c e m e n t g e n e r a t e d b y c a r b o n i c a n h y d r a s e occurs o n l y d u r i n g the s u m m e r months, it m u s t have a s p e c t a c u l a r i m p a c t in o r d e r to yield an average y e a r l y C O 2 flux f r o m 9 to 35 m o l / m 2 yr. T h e c a r b o n i c a n h y d r a s e e n h a n c e m e n t w o u l d have to b e at least 10-fold, driving the s u m m e r C O 2 e x c h a n g e r a t e t o - - 1 2 0 m o l / m 2 yr. Hence, we m u s t cross biological e n h a n c e m e n t off of the list as p o s s i b l e e x p l a n a t i o n s for the 60%0 increase in the 14C c o n t e n t of M o n o L a k e c a r b o n b e t w e e n 1957 a n d the present.
5. Alternate explanation for post-1957 radiocarbon increase
h Ap At Pe
where h is the height of the airspace in the helmet, At is the d e p l o y m e n t time, A p is the change in the p a r t i a l pressure of the gas in the helmet, a n d Pe is the p a r t i a l pressure of the gas at e q u i l i b r i u m with the lake water. H e l m e t s were d e p l o y e d d u r i n g the spring of 1986 when no b r i n e s h r i m p were p r e s e n t a n d in the s u m m e r of 1987 when there was an a b u n d a n c e of b r i n e shrimp. D u r i n g the spring e x p e r i m e n t p i s t o n velocities of 5.0 c m / h r for C O s a n d 2.5 c m / h r for CH4 were observed. F o r the s u m m e r
O n e p o s s i b l e w a y a r o u n d the necessity for a very high M o n o L a k e C O 2 exchange rate is to p o s t u l a t e that M o n o L a k e has b e e n used for the c l a n d e s t i n e d i s p o s a l of waste r a d i o c a r b o n . W e say c l a n d e s t i n e b e c a u s e o n l y p u r p o s e f u l d i s p o s a l of r a d i o w a s t e s could p r o v i d e the 5 curies of r a d i o c a r b o n n e e d e d to e x p l a i n the increase in the lakes 1 4 C / C r a t i o f r o m 1957 to 1986. W h i l e there is no evidence t h a t M o n o L a k e has been used for a n y sort of waste disposal, the p o s s i b i l i t y of a large i n p u t s of waste 14C d u r i n g the p e r i o d 1966-1976 remains a dark-horse explanation.
23
6. Wood-tufa radiocarbon age pairs
influx of radiocarbon-deficient carbon from beneath the lake has varied with time. Several more wood-tufa pairs were collected and analyzed at Lamont-Doherty. Results of the analyses are listed in Table 3 and graphically shown in Fig. 9. These results suggest that during the period 1200-900 years ago the a4C/C ratio in Mono Lake water was higher than the 1957-58 value and that between 800 and 550 years ago the lake value was lower than the 1957-58 value. The rather sharp drop in I 4 C / C ratio about 800 years ago is best explained by an episodic input of radiocarbon-flee carbon from beneath the lake. For example, one could envision that a volcanic event greatly increased the flow of radiocarbon-free CO 2 into the lake, driving its 14C/C ratio from about 0.86 to about 0.71 the atmospheric value. This would require an input of about 1200 moles of radiocarbon-flee carbon per square meter of lake surface. Such an input would have raised the E C O z content of the lake by about 20%. It is interesting to note that the volcanic events which gave rise to much of Negit Island, a composite of dacite cones, domes, and flows that occupies the northwestern quadrant of Mono Lake, occurred sometime between 600 and 1200 years ago [15]. Hence sublake volcanism may have been responsible for the sudden input of a large amount of radiocarbon-free carbon from beneath the lake.
On the shorelines of Mono Lake are tufas encapsolating wood. The wood is from the trunks of trees and shrubs that grew on the shorelands when the lake was low and that were killed during subsequent rises in lake level. The wood offers a substrate for the nucleation of tufa growth. These tufa-wood pairs provide a means of establishing the ratio of the 14C/C for lake water at times in the past. The difference between the ~4C/C ratio for the tufa and that for the wood establishes the difference between the 14C/C ratio in lake and atmosphere carbon at the time the pair formed. This assumes, of course, that the time between wood growth and tufa formation is no more a few decades. Were the time substantial (i.e., more than 100 years), the paleoradiocarbon content of lake carbon calculated in this way would be a maximum. The first tufa-wood pair was collected by Scott Stine and submitted by Ken Lajoie to the U S G S laboratory in Menlo Park for radiocarbon analysis. When one of us (W.S.B.) was made aware of these results, he was struck by the l~ck of agreement between the age of 1700 years established by direct measurements on 1957 M o n o Lake water and that of about 3000 years inferred the results on the wood-tufa pair. This suggested that the
TABLE
3
Radiocarbon
p a i r s f r o m t h e s h o r e l i n e s o f M o n o L a k e . T h e d i f f e r e n c e b e t w e e n the 1 4 C / C
results on wood-tufa
the encasing tufa provides an estimate of the difference between the 14C/C the wood grew Sample No.
Sample
Wood
eleva-
814C
8x3C a
tion (feet)
(%O)
(%O)
ratio for atmospheric
for the wood and for
CO 2 and lake ECO 2 at the time
Tufa
14 C / C I ~ k e
A14 C
Age
8a4 C
~13 C a
A14C
A14C o
(fits)
(years)
(%o)
(%o)
(%o)
(%o * )
14C/Catm
4/27/86/8
6388
- 66
( - 25)
- 66
550
- 289
(0)
- 324
- 276
2/9/87/1
6386
- 68
( - 25)
- 68
565
- 291
(0)
- 326
- 278
0.722
4/27/86/2 USGS1167-8
6388 6388
- 71 - 77
( - 25) - 27
- 71 - 77
570 640
- 294 - 292
(0) + 4
- 329 - 332
- 280 - 276
0.720 0.724
c
0.724
4/27/86/1
6399
- 91
( - 25)
- 91
770
- 333
(0)
- 366
- 302
0.698
5/21/86/1 5/18/86/1
6390 6390
- 102 - 110
( - 25) ( - 25)
- 102 - 110
860 940
- 188 - 193
(0) (0)
- 228 - 233
- 141 - 136
0.859 0.864
4/27/86/3
6387
- 132
( - 25)
- 132
1140
- 223
(0)
- 262
- 149
0.850
a T h e 8 ~ 3 C v a l u e s a r e all e s t i m a t e d . b Corrected to time of formation c Measurements
using radiocarbon
made at the USGS,
age of wood.
Menlo Park; others made at LDGO.
24 I.C
i
O.C
'Z-
I
~O~o_I~F LAKESURFACE
0.8
I
~
z
I ~ I INPUT OF ~ 1200 MOLES OF R A D I O C A R B O N FREE }]CO 2 PER m 2
/
I1~0' o
"
l
Ld c9
~l~
7
~-000 ~Z) Ld
LAKE H20
5000 O:: O Z
~<~
~OO0 ~ O
TOWARD E Q U ~ U a R l U M
WITH ATMOSPHERE
0]5200
I
{~
8 0
I
I
400
n< O_
I
RADIOCARBON AGE ( Y E A R S BEFORE P R E S E N T )
Fig. 9. Radiocarbon to carbon ratios in Mono Lake water as reconstructed from tufa coating radiocarbon dated wood. The dotted line shows a scenario involving an episodic input of radiocarbon-free carbon followed by gradual equilibration with the atmosphere.
A t a C O 2 invasion rate of 35 m o l / m 2 yr the 1 / e time r e q u i r e d for isotope r e - e q u i l i b r a t i o n with a t m o s p h e r i c c a r b o n after such an event is a b o u t 160 years. A t a C O 2 invasion rate of 9 m o l / m 2 yr the response time is a b o u t 630 years. If the 9 m o l / m 2 y r C O 2 invasion rate proves to b e the correct one, it is possible that over the last 800 or so years no further i m p o r t a n t i n p u t s of h y d r o t h e r mal inorganic c a r b o n have occurred. I n s t e a d the ~ 4 C / C ratio in the lake has been relaxing t o w a r d the steady-state value expected for U = 0 (i.e., (14C/C)lake = 0.93 ( a 4 c / C ) a t m ) .
2~
5g
~:
m
2o
td E w ~_
~ 5 0
YRS AGO
4 LjE 3Ore
~E
~g ~2 c~ z
I
0.6
YIRS AGO 0.7 0.8
[ 0.9
3
7. Measurements of tufa from historically dated shorelines
I000 z
RELAXAT,O. o.6
I0
ck z - -
1.0
I4C/C)LAKE/I4C/C)AT M
Fig. 10. The relationship between the rate of input of MC-free carbon from beneath Mono Lake and the steady-state 14C/C ratio in its dissolved carbon (assuming a CO2 invasion rates of 35 mol/m 2 yr and 9 mol/m 2 yr). Shown by ellipses are the steady-state dead carbon fluxes required to maintain the 14C/C ratios observed for the time periods 1150-900 years and 800-550 years ago.
H a v i n g o b t a i n e d a r e a s o n a b l e result for the tufa c o a t e d p e b b l e s f r o m the 1958 shoreline, we a t t e m p t e d to e x t e n d the lake 14C//C record b a c k in time b y r u n n i n g t u f a - c o a t e d p e b b l e s from two o t h e r shorelines; one o c c u p i e d b y the lake f r o m 1938 to 1947 a n d the o t h e r o c c u p i e d b y the lake in 1919, at the time of its historic stand. T h e results of these analyses are listed in T a b l e 1. Both are far lower in a 4 C / C r a t i o t h a n expected. If these results are i n d i c a t i v e of the lake waters, then the rise in r a d i o c a r b o n c o n t e n t b e t w e e n 1919 a n d 1958 is far m o r e r a p i d t h a n that o b s e r v e d for the last 30 years! T o p r o d u c e such a rise w o u l d require a C O 2 exchange rate of 150 m o l / m 2 yr. Since these results a p p e a r a n o m a l o u s , a few w o r d s need b e said a b o u t the m a n n e r in which the p e b b l e s a m p l e s were h a n d l e d . A s the coatings are thin ( < 1 m m ) a n d f i r m l y a t t a c h e d to the p e b b l e surface, we p l a c e d the p e b b l e s in a flask a n d dissolved the C a C O 3 coatings in acid. C a r b o n a t e rocks are rare in this area; m o s t of the p e b b l e s were silicate. T o assure that no c a r b o n a t e p e b b l e s were i n c l u d e d , a n y p e b b l e suspected to be limestone or d o l o m i t e was p i c k e d out a n d discarded. A s a check on the results f r o m the shoreline p e b b l e , we r a n tufa c o a t i n g the face of a volcanic cliff on N e g i t Island. A s the 600 y e a r old ash does n o t d r a p e this intrusion, the tufa m u s t be less than 600 years in age. T h e elevation of the tufa is the same as that of a p r o m i n e n t lake shoreline d a t e d by r a d i o c a r b o n at a b o u t 250 years. Based on this age, the initial 1 4 C / C ratio for the tufa is 0.65 that for a t m o s p h e r i c CO2. This value is consistent with that o b t a i n e d for the t u f a - c o a t e d p e b b l e f r o m the 1919 shoreline. T h u s it is quite p o s s i b l e that the low 1 4 C / C ratios s h o w n in Fig. 9 for the 800 to 550 year B.P. interval persisted to a b o u t 250 years ago a n d p e r h a p s fight u p to W o r l d W a r II. If this is correct then the post-1940 rise in 1 4 C / C ratio in the lake can be e x p l a i n e d o n l y b y c l a n d e s t i n e d i s p o s a l of r a d i o a c t i v e waste in M o n o Lake. T h e aggregate a m o u n t of r a d i o c a r b o n a d d e d to the lake m u s t be a b o u t 20 curies. M o s t of this r a d i o c a r b o n m u s t have been a d d e d b e t w e e n 1940 a n d 1957. O n e c a n d i d a t e is the g r a p h i t e f r o m the W o r l d W a r II U n i v e r s i t y of Chicago g r a p h i t e reactor.
25 8. Radiocarbon measurements on isolated deep water Abnormally high inflow in 1982 and 1983 caused the lake to stratify. While the boundary between the upper mixed water and the isolated deep water has slowly deepened, this stratification remained as of the time this paper was completed. We used this opportunity to determine whether the input of dead carbon to the lake could be detected, the idea being that the 14C/C ratio in the carbon of the isolated water would drop. The last two entries in Table 1 suggest that such a drop has occurred. While the surface water 14C/C ratio lies within the error of the trend for the last decade, the 14C/C for deep water is 15 + 7%0 lower than that for surface water. We plan to do further sampling which will enable us to confirm this result; we also intend to determine the volume of lake water and area of bottom sediment involved so that we can obtain a carbon in flux estimate from these measurements. 9. Direct evidence for the hydrothermal inputs One way to confirm that 14C-free carbon is being added to the lake from beneath is to make measurements on sediment pore waters. While we plan to do profiles of the 14C/C ratio in sediment pore waters we have not yet done so. We have, however, conducted one radiocarbon analysis which bears on this matter. The sample consisted of tufa-cemented pumice sands collected from the south shore of the lake. Because this particular sample surrounds an electrical cable strung along the lake floor (in about 3 m of water) by the N a v y in 1965, we know that the tufa formed between that date and 1974, when the cable was exposed by the receding waters of the lake. We are also quite sure that the cable was never covered by more than a few tens of centimeters of sand. Yet this cement has a a 4 C / C ratio about half that for the lake water during the 1965-1974 time interval (i.e., its apparent radiocarbon age is 6300 years). Thus we have evidence that a strong gradient in 14C/C ratio exists in the upper few tens of centimeters of the sediment pore water (at least at this particular site). If, as we propose above, ~4Cfree carbon enters the lake from beneath, then such gradients are to be expected. For example, if
this carbon were carried upward uniformly across the lake floor in waters carrying the s a m e ~ ] C O 2 content as the lake waters, then the upward advection rate would have to be on the order of 1.0 c m / y r . Coupled with an effective molecular diffusion rate of 1 x 10 -6 cm2/s within the sediment pores, this would yield a relaxation depth for the transition zone between lake water and upwelling water of about 30 cm. While the actual process is surely more complex, the calculation shows that at least in some places on the lake floor 14C-deficient pore waters m a y extend very close to the sedim e n t / w a t e r interface. 10. Conclusions New measurements confirm the validity of earlier studies and clearly demonstrate that a 60%0 increase in N4C value for the lake occurred between 1957 and 1986. If this increase is attributed to the entry of bomb-produced radiocarbon from the atmosphere, then a CO 2 invasion rate of 35 m o l / m 2 yr is required. By contrast the results of an SF6 tracer experiment conducted in the Mono Lake suggest a CO 2 invasion rate of only 9 m o l / m 2 yr. We suspect that the 9 m o l e / m 2 yr value is the correct one and that the 60%0 rise has its origin in clandestine disposal of waste radiocarbon. Regardless of which CO 2 invasion rate proves to be the correct one, there must be a substantial input of 14C-free carbon to the lake. Evidence in support of such inputs comes from measurements of wood-tufa pairs which suggest that the 14C/C ratio in lake water has changed with time. The most likely explanation for such changes is an input of dead carbon pulsed by volcanic events. Acknowledgements Gayle D a n a of the Sierra N e v a d a Aquatic Research Laboratory of UCSB aided in the lake sampling. Discussions with John Melach, Taro Takahashi, Kerry Sieh and Ron Oxburgh helped in developing some of the concepts discussed here. Financial support was provided by N S F grant OCE85-15693. Peng's work was supported jointly by the National Science Foundation's Ecosystem Studies Program under Interagency Agreement BSR-8417923 and the Carbon Dioxide Research Division. Office of Energy Research, U.S. Depart-
26
ment of Energy, under contract DE-AC05840R21400 with Martin Marietta Energy Systems, Inc., Publication No. 3091, Environmental Sciences Division, ORNL. Lamont-Doherty Geological Observatory Contribution No. 4276. References 1 W.S. Broecker, T.-H. Peng, G. Ostlund and M. Stuiver, The distribution of bomb radiocarbon in the ocean, J. Geophys. Res. 90, 6953-6970, 1985. 2 W.S. Broecker and A. Walton, The geochemistry of 14C in freshwater systems, Geochim. Cosmochim. Acta 16, 15-38, 1959. 3 D.L. Thurber and W.S. Broecker, The behavior of radiocarbon in the surface water of the Great Basin, Nobel Symp. 12, 379-400, 1972. 4 T.-H. Peng and W.S. Broecker, Gas exchange rates for three closed-basin lakes, Limnol. Oceanogr. 25, 789-796, 1980. 5 W.S. Broecker, J.R. Ledwell, T. Takahashi, Ray Weiss, L. Merlivat, L. Memery, T.-H. Peng, B. Jahne and K.O. Munnich, Isotopic versus micrometeorologic ocean CO z fluxes: a serious conflict, J. Geophys. Res. 91, 10517-10527, 1986. 6 R. Wanninkhof, J.R. Ledwell and W.S. Broecker, Gas exchange-wind speed relation measured with sulfur hexafluoride on a lake, Science 227, 1224-1226, 1985.
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