new 23°th/u and t4c ages from lake lahontan carbonates ... · group 2 - intermediate stand tufa...

Geochimica et Cosmochimica Acta, Vol. 60, No. 15, pp. 2817-2832, 1996 Copyright © 1996 Elsevier Science Ltd

Pergamon Printed in the USA. All rights reserved 0016-7037/96 $15.00 + .00

PII S0016-7037(96) 00136-6

New 23°Th/U and t4c ages from Lake Lahontan carbonates, Nevada, USA, and a discussion of the origin of initial thorium

J. C. LIN, ~* W. S. BROECKER, 1 R. F. ANDERSON, 1 S. HEMM1NG, 1 j. L. RUBENSTONE, i and G. BONANI 2

~Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA 2Laboratorium fur Kemphysik, ETH Honggerberg, 8093 Zurich, Switzerland

(Received March 10, 1995; accepted in revised form April 15, 1996)

Abstract--Five sets of coeval lacustrine carbonate samples from Pleistocene Lake Lahontan in western Nevada were dated by both the AMS J4C and 23°Th/U isochron methods. All five groups of samples were analyzed for U-Th isotopes by alpha spectrometry and one of the groups was additionally measured by thermal and secondary ionization mass spectrometry (TIMS and SIMS) for comparison. The 14C ages were corrected to calendar years using the calibration curve recommended by Bard et al. (1992). Without local reservoir correction on the HC ages, m e a n 23°Th/U isochron ages of some sets are apparently older than their calendar-corrected ~4C ages by up to 2300 years. Modern carbon contamination of these carbonate samples through recrystallization or deposition of secondary calcite is likely to be responsible for part of the age discrepancies. We explored additional biases associated with the isochron ages, maybe produced by the presence of initial Th coprecipitated from the lake water.

It can be shown that if dissolved (hydrogenous) Th is directly incorporated into the pure carbonates, then the three-component mixing among (1) detrital Th, (2) hydrogenous Th adsorbed on detritus, and (3) hydrogenous Th incorporated by the carbonate can introduce a positive age bias. We have developed an approach to estimate the magnitude of this bias of the Lake Lahontan carbonates. The preliminary estimates suggest a positive age bias of 1000 to 2000 years for two sets of the samples.

1. INTRODUCTION

Since the investigation of Russell (1885), the dramatic lake level fluctuations of Lake Lahontan during the late Pleisto- cene have been intensely studied (e.g., Broecker and Orr, 1958; Broecker and Kaufman, 1965; Lao and Benson, 1988; Benson and Paillet, 1989: Benson et al., 1990). The chronology of the lake-level history is based mainly on radiocarbon dating. It is well known that the assumption of a constant atmospheric ~4C/~2C ratio is not correct; thus, ~4C years must be corrected to calendar years using some other absolute dating methods. For this reason, the calibration of ~4C time scale to calendar years remains a major effort of a large group of scientists (e.g., Stuiver and Polach, 1977; Stuiver and Kra, 1986; Mazaud et al., 1991; Stuiver et al., 1991; Kromer and Becket, 1993; Hajdas et al., 1993). Beyond about 11,000 years B, P.. U-Th dating has become the domi- nant tool (Bard et al., 1990, 1992; Edwards et al., 1993). The calibration of the radiocarbon timescale is not our main emphasis here, but it is an important concern when applying both radiocarbon and U-Th dating methods to lacustrine carbonates for reconstructing the climate history of closed basin lakes.

The millennial scale Dansgaard-Oeschger cycles observed in the late Pleistocene climate records in Greenland ice cores and the iceberg discharge events (Heinrich Events) in North Atlantic Ocean have led to renewed interest in climate forc- ing theory (Dansgaard et al., 1984; Johnsen et al., 1992; Bond et al., 1992; Taylor et al., 1993; Broecker, 1994).

* P r e s e n t a d d r e s s : Department of Geography, University of Cali- fornia, Berkeley, CA 94720, USA.

2817

Abrupt lake-level fluctuations in western Great Basin of sim- ilar duration and magnitude to those changes in the polar areas may also be linked to global climate (Broecker, 1994; Phillips et al., 1994). To better understand the correlation of the abrupt changes in Great Basin climate with Greenland ice core and North Atlantic deep sea records requires precise and absolute lake-level chronology. However, accurate and precise dates on lacustrine deposits remains elusive.

Here we explore key problems that may be associated with both radiocarbon and U-Th ages of lacustrine carbonates lrom the Great Basin. For radiocarbon, two problems are known to exist: ( I ) local reservoir correction and (2) contamination with secondary calcite. U-Th isochron dating requires that all the coeval samples have the same isotopic compositions of their initial uranium and thorium.

In this paper, we compare the U-Th and 14C ages of five sets of lacustrine carbonate samples collected from the western subbasins of Lake Lahontan (Pyramid Lake and Black Rock Desert areas, see Fig. 1). The U-Th isochron age is obtained from the slope of a 23°Th/232Th vs. 234U/a32Th isochron plot and, for a better graphic demonstration, the intercept of a 234U/238U vs. ~32Th/23~U plot of each sample set

(Ku and Liang, 1984; Kaufman, 1993). The measured ~4C ages are directly converted to calendar ages using the coral- based calibration curve of Bard et al. (1990, 1992). The result is that the normal U-Th isochron ages are generally older than the calendar corrected ~4C ages. No local reservoir age correction is applied to the measured J4C ages, due to the difficulty in assessing the reservoir effect of the Pleisto- cene lake water. This correction would increase the discrep- ancy between the two dating methods. Potential sources of age bias on both dating methods are described. We conclude that in addition to contamination with younger carbon, a

2818

121 °

42" L

41'

40 °

39 °

38 ~

120 ° !

OREOON

J. C. Lin et al.

119 ° 118 ° I t I

NEV~A

I I

BLACK-RI PLAYA

I

117 °

11_ IDAHO

LAKE

Marble

SINK

~ , , , , , 100 KM !

LAKE TAHOE

LEGEND

• LAKE \ %

% \

• Present-day L a k e s

~ Pleistocene Lahontan Lake Area

- - - Major Sills 1 Astor Pass 2 Emerson 3 Darwin 4 Adrian

I I I I

FI(;. I. Geography of Lahontan Lake (revised from Benson et al., 1990).

116 ° I

nonzero initial s lope for >"Th/-~3eTh vs. 234U/232Th plot due

to the mineralogical incorporat ion of dissolved thor ium into

pure carbonate phase may also produce an age bias.

2. SAMPLES

In this study, samples grouped with the same HC age or from elevation-correlated shorelines (see Table 1) were assembled to increase the spread of isotopic ratios for isochron calculations. The drawbacks of this approach are ( I ) a possible range of ages for samples in the same group and (2) possibly in homogeneous initial 23°Th/Z~:Th ratios for samples of the same age but from different locations. Except for one group (Group 3), the samples employed in this study are lithoid tufas, a dense tk)rm of calcium carbonate precipitated, possibly by algae, on rocky portions of the shorelines. Tufa samples in Group 1 were offered and radiocarbon dated by Dr. L. Benson. Group 2 combines tufa samples, for which both '4C and U-Th isotope results were published in Lao and Benson ( 1988 ) with samples (PL17) from the same locality processed as part of this study. Group 3 consists of gastropods, chara (a tubular form of green algae), and ostracods lbr which both ~4C and U-Th isotopic measurements were published by Kaufman and Broecker (1965). Samples in Group 4 and Group 5 were collected tk)r this study with the help of L. Benson in 1990 and 1991.

Unfortunately, no currently forming carbonates have been located in the Pyramid Lake basin upon which initial uranium and thorium isotopic compositions could be determined. Instead, two late Holo- cene shell samples collected from recent beaches with uncorrected radiocarbon ages of about 2,500 years were analyzed for U-Th isotopes. Beach sands from Pyramid Lake area were also analyzed.

2 . U - T h I s o c h r o n P l o t s

Total sample dissolution (TSD) was employed for the U-Th isochron dating in this study. This technique avoids any nonreproduc- ibility of data due to differential readsorption of thorium and uranium isotopes onto residue during dissolution processes (Luo and Ku, 1991: Bischoff and Fitzpatrick, 1991). The isochrons were constructed by plotting 2~°Wh / 232Th vs. 234 W / 2) XTh activity ratios. Ideally, the slope of the plot corresponds to the radiogenic 23°Th/2~4U ratio of the sample and hence, the age. The y-intercept approximates the decay-modified initial 23°Th/232Th ratio for samples younger than approximately 40,000 years (so the decay of excess 234U from detrital phases can be neglected). In this case, the isochron equation can be written as:

- \ ~ / [ ~,,'+ ~ , (I) 232Th U ]~ 232Th

where subscript " i " and "a'" represent '~initial" and ;'authigenic", respectively: superscript "'*'" means "radiogenic" component; ;% is the decay constant of 23°Th, and t is the sample age. A complete isochron equation derived in Luo and Ku ( 1991 ) contains an addi-

'~OTh. 2-~0Th * tional [ ( ~ ) d ( 2 . ~ - - ) , d term in the intercept, Eqn. 4 of Luo

and Ku ( 1991 ), where the subscript "'d" stands for detrital phase. However, this term is negligible for young samples, due to the insignificant decay of 2:~4U. Our study uses an alternative normaliza- tion scheme to obtain the authigenic 234U/:3sU, i.e., normalized to e~SU instead of 232Th. The y-intercept of 2~4U/23gW vs. 232Th/23SU plot is the authigenic >4U/2~SU value. The isochron age can be

Dating of lacustrine carbonates

Table 1. Sample descriptions and radiocarbon a~es

Sample ID Location Material Altitude (rn)

14C age* (years)

Pleistocene samples Group 1 - Intermediate Stand Tufa (17K)

BR84-7 B lack Rock tufa 1245 Desert

PL18 Terrace Hill 1 tufa 1267

Group 2 - Intermediate Stand Tufa (19K) PL17 Terrace Hill I ' tufa 1260

PL44alx: Terrace Hill tufa 1260 PL44d Terrace Hill tufa 1260

Group 3 - Astor Pass and Truckee Canyon Marl

17,360+-260

16,980+_250

19,040i-_320 20,650-2_390 19,980-2_360

L-364CQA(1) Astor Pass I gastropod 1210 16,500-2_300 L-364CQB(1) Astor Pass chara 1210 16,800!-_400 L-364CQB(2) Astor Pass chara 1210 16,800i-_400

L-772Q Truckee Canyon ostraeodes 1219 16,800L-_500

Group 4 - Marble Bluff Tufa 1 JL90-1 Marble Bluff tufa 1245 13,920-2_220 JL90-2 Marble Bluff tufa 1245 14,500-2-420 JL90-3 Marble Bluff tufa 1245 15,540+--200

Group 5 - Highstand Tufa JL90-5 Marble Bluff tufa 1330 13,060!-_100 JL90-6 Marble Bluff tufa 1330 13,15ffL-_100

JL90-15 Terrace Hill tufa 1330 12,200-2_100 JL90-16 Terrace Hill tufa 1329 12,850~_110

JL90-17-1 Terrace Hill tufa 1328 12,320-2-_100 JL90-17-2 Terrace Hill tufa 1328 12,200-2:100 JL90-17-3 Terrace Hill tufa 1328 12,980i-_100 JL90-18-1 Terrace Hill t u f a 1326-1328 13,150i-_100 JL90-18-2 Terrace Hill t u f a 1326-1328 12,690-2_100 JL90-18-3 Terrace Hill tufa 1326-1328 12,600-2_ 95 JL90-18-4 Terrace Hill t u f a 1326-1328 12,790"2-110 JL90-18-5 Terrace Hill tufa 1326-1328 12,260-2-_100

Holocene beach samples JL052501-2 Fisheries shells =11592 2490 + 70

Resourses Center JL052502 Twin Tetons 1 shells =1164 2540 + 65 JL052601 Twin Tetons &ands =1159 - - - - -

* : ~4C ages before reservoir correction with 1 o errors l : Pyramid Lake area 2 : present day lake level a : Kaufman and Broecker (1965) b : collected and 14C dated by L. Benson c : Lao and Benson (1988) d : collected with L. Benson in 1990 and 14C dated as part of this study

Reference

2819

calculated from the slope using the equation of Kaufman and Broecker ( 1965 ) :

23(~rh * 1 234U - 234U/238 U (1 - e xo,)

1 X0 e~4, xo,) '

where ko and X4 are the decay constants of 23°Wh and 234U, respectively. The values of )to and k4 used in this study are 9.2174 × 10 6/ y and 2.7949 × 10 6/y (Ivanovich and Harmon, 1982).

3. A N A L Y T I C A L M E T H O D S

The new radiocarbon dates presented here were measured by ac- celerator mass spectrometry (AMS) at ETH in Ztirich, Switzerland. Samples were sawed into pieces of about 1 cm on one side and leached in 10% HC1 for about 5 min to remove any surficial contamination. The weight loss from this acid leaching was between 10 and 20%. About 20 mg dry weight of carbonate sample was converted to CO2 for AMS '4C dating at ETH.

Isochrons were constructed from alpha-counting U-Th results for the four groups of samples 1-4. Samples from the last highstand lake (Group 5) were later analyzed by mass spectrometry in an attempt to reduce the age uncertainty due to the smaller analytical

uncertainties. The isochron of Group 5 was then constructed using both alpha-counting and mass spectrometry results. Acid-insoluble residue contents of the samples were measured separately from ~5 g aliquots after dissolving carbonates in - 4 N HNO3. About 10 g of samples were analyzed for U-Th isotopes by alpha spectrometry. 236U and 229Th were added as spikes for measuring uranium and thorium concentrations by isotope dilution. About 10 mg of Fe, as FeCls solution, was added as a carrier for later coprecipitating the actinides with NH3OH. The sample was totally dissolved in a mixture of HNO3, HF, and HC104 solution. Extreme caution was needed in order to add just enough of HF to dissolve the silicate detritus, so that no Ca fluoride would precipitate out with the extra HF. When a significant amount of insoluble residue did exist after the coprecipitation, repeating the digestion in fuming HF and HC104 could always remove the insoluble residue effectively, but repetition of the coprecipitation procedure was then needed before the column chemistry. The analytical procedures for separating and purifying U and Th are described in Lao (1991). Purified uranium and thorium were electroplated onto Ag discs and counted using Ortec or Tennelec silicon surface barrier alpha detectors. The blanks for uranium and thorium were not detectable above counting background.

Eleven tufa samples from the last lake highstand (Group 5 ) were processed for analysis by mass spectrometry. About 0.5 g of each sample was totally dissolved in the same fashion as described above. A mixed 233U-2~gTh spike was added before the sample digestion. About 5 mg of FeC13 was added as actinides carrier. The chemical

2820 J . C . L i n e t al.

Table 2. Alpha spectrometry results (isotopic ratios are activity ratios and the uncertainties quoted are 10 errors)

Sample ID Residue U Th ThRN • 2340]238 U 230Th/234U 230Th/232T h 234Uf232Th 232Th/238U

% ppm ppm ppm

S a m p l e s o f 5 i s o c h r o n s

Group 1 BR84-7a 2.53 1.20 0.60 BR84-Tb -- 1.62 0.57 BR84-7c - - 1.53 0.50

PLISa 0.70 1.24 0.37 PLlgb 0.55 1.36 0.34 PL18c 0.45 1.30 0.31

Group 2 PLI7a - - 1.11 0.26 PLI7b 0.36 1.05 0.27 PL17c 0.31 1.11 0.26

PL44d- 11 0.4 1.54 0.24 PL44d-21 0.3 1.51 0.24 PL44abcl t -- 1.21 1.27 PI, A4abcI I 1 3.0 1.22 1.28 PLA4abclH I 1.2 1.15 0.94

Group 3

L-364CQA(1) 2 -- 1.63 0.07

I.,-364CQB(1) 2 -- 2.65 0.65 L-364CQB(2) 2 - - 2.51 0.46

L-772Q 2 - - 3.95 5.32

Group 4 JLgO-I-I 3.22 1.13 0.89 JL90-1-2 5.00 1.40 0.87 JL90-1-3 5.28 1.42 0.81 JLg0-1-4 4.81 0.94 0.61 JL90-1-5 2.08 1.18 0.57 JL90-1--6 1.14 1.06 0.26 JL90-2- la 2.87 4.07 0.94 JL90-2- lb - - 4.10 1.02 JL90-2-2 7.20 3.40 1.05 JL90-2-3 6.08 3.94 1.05 JL90-2--4 4.33 2.74 1.00 JLgO-3-1a 4.80 9.45 0.76 JL90-3-1b -- 9.63 0.93 JL90-3-2 3.56 6.59 1.08 JL90-3-3 3.91 8.20 1.11 JL90-3-4 4.93 8.95 0.92 JL90-3-5 2.24 6.67 0.97 JL90-3-6 - - 9.06 1.02

Group 5 JL90-5-1 0.90 0.97 0.20 JL90-5-2 0.72 0.88 0.21 JL90-6 1.68 0.90 0.47

JLgO-15-1 0.92 0.94 0.26 JL90-15-2 0.86 0.98 0.38 JL90-16 0.37 0.84 0.22

JL90-17-1 0.86 0.95 0.29 JL90-17-2 2.46 1.12 0.36 JL90-17-3 2.22 0.94 0.34 JLg0-18-1 1.98 1.03 0.32 JLgo-Ig-2 0.82 0.97 0.30 JL90-18-3 1.25 0.98 0.30 JL90-18-4 0.99 0.96 0.35 JLg0-18-5 1.58 0.97 0.34

24 1.40-20.03 0.334i-0.009 2.85:£-0.06 8.53:£-0.25 0.164i-0.005 - - 1.40-+0.02 0.297-+0.008 3.54+--0. I 1 11.9+-'0.4 0. I 17i-0.004 - - 1.43::t:0.03 0.286:~.008 3.77i-0.09 13.1:£-0.4 0.109-1-0,004 53 1.41-+0.02 0.276-+0.007 3.91:£-0.10 14.1:£-0.5 0.100-1"0.003 62 1.471-0.02 0.254-+0.006 4.57i-0.13 18.0i-0.6 0.082__+0.003 69 1.46-+0.03 0.259~0.007 4.89-i"0.15 18.9i-0.7 0.077:L-0.003

- - 1.41:~.02 0.254:t:0.008 4.57: i-0.20 18.0i-0.8 0.078i-0.004 75 1.44-+0.04 0.262.+.0.009 4.49-3:0.16 17 .21-0 .7 0.0g4i-0.01M 84 1.46+0.04 0.234-+0.008 4.34:£-0.16 I 8.5i-0.8 0.079i-0.004 60 1.43-+0.03 0.221:-.-.~.009 6.36-+0.71 28 .6 -+3 .3 0.050i-0,006 80 1.46:~0.05 0.215:£-0.012 6 .09-+0.83 27.8+3.8 0.053-+0.007

- - 1.35i-0.05 0.561-+0,031 2.27:£-0.20 4.05i-0.31 0.334-+0.026 43 1.40-+0.07 0.532+0.027 2.29+0.15 4.17-+0.26 0.337:t:0.021 78 1.45-+0.12 0.455+0.049 2.47L-0.45 5.52__+0.88 0.263:N).042

Pxy t'

0.51 0.67 0.56 0.63 0.66 0.66

0.76 0.66 0.65 0.94 0.92 0.78 0.69 0.80

1.43i-0.03 0.187:~0.015 18.7:.~.8 100-/30 0.014i-0.01M 0.97 1.40-3:0.04 0.25 li-0.010 4.33: t :0 .25 17.2+--0.9 0.081+0.004 0.75 1.45+0.05 0.241:t:0.010 5.74-+0.43 23.8:t:1.7 0.06 li-O.01M 0.84 1,35+0.03 0,630-t:0.020 1.91i-0.12 3.03-+0.18 0.446:t:0.027 0.87

28 1.44_+0.05 0.368+0.015 2.04i'0.06 5.55.-.-:-0.24 0.259+--0.011 17 1.41i-0.05 0.304::1:0.012 2.10-2:0.06 6.90-2:0.29 0.204-+0.009 15 1.46+0.04 0.292_+0.010 2.27i'0.07 7.78+0.31 0.188i-0.008 13 1.46i-0.05 0.343+0,013 2.34+0.08 6.82._+0.30 0.214-+0.010 27 1.43i-0.05 0.278-+0.010 2.48:t:0.05 8.94+--0.34 0.159-+0.006 23 1.40"2:0.05 0,229-2:0.009 3.98+0. l I 17.4i-0.7 0.080"20.004 33 1.48+0.03 0.217:t:0.007 4.21+0.10 19.4_+0.7 0.0765.-0.003

- - 1.44+-0.02 0.200-+0.006 4.11+0.08 18.6_-4-0.6 0.078i"0.003 15 1.42_+0.03 0.244+0.008 3.723:0.09 15.2-+0.5 0.0945.-0.003 17 1.52i-0.02 0.223+0,006 3.83+0.08 17.2_+0.5 0.088:£-0,003 23 1.49:t:0.03 0.261-+0.007 3 .23-20.05 12.4!-0.3 0,121-+0.003 16 1.38+0.03 0.177i'0.006 9.20"2-0.22 52 .0"22 .1 0,027:L'0.001

- - 1.43+0.01 0.185+0.005 8 .63-+0.22 46.7+1.7 0.032_+0.001 30 1.47i"0.03 0.212i-0.007 5.77+0.12 27 .3-+1.1 0.054:£-'0.002 28 1.44:£-0.02 0.196:£'0.005 6 .43-+0.14 32 .9-+1.1 0.044-+0.002 19 1.41-+0.02 0.197-+0.007 8.12, t :0 .27 41 .2 -+1 .9 0.034-+0.002 43 1.46-+0.02 0.214-+0.007 6.44+0.22 30.1-1-1.3 0.048i--0.002

1.41-+0.01 0.201__+0.006 7.63__+0.23 37 .9 -+1 .5 0.037!-0.002

22 1.48+0.03 0.202+0,007 4.30-+0.23 21.3+1.2 0.071-+'0.004 29 1.56!-0.03 0.206+0.006 4.12_+0.14 20.04--0.8 0.078_'2-0.003 28 1.46+0.03 0.319+_0.010 2.70-2:0.10 8.48i-0.33 0 . 1 7 ~ . 0 0 7 28 1.49-1:0.04 0.232+0.007 3.82+0.12 16.4+--0.6 0.094-+0.004 44 1.44-I-0.03 0.264+0.008 2.98+0.10 11.3i'0.4 0.128i-0.005 60 1.52:t:0.04 0.223i-0.006 4.08"1-0.13 18 .3-+0.7 0.086-+0.003 34 1.51-2-0.02 0.244:~.006 3.61:t :0.12 14.8i-0.5 0.101_+0.004 15 1.51+0.03 0.239-1:0.007 3.37::~.12 14.1i-0.5 0.107i'0.004 15 1.52.~.03 0.254::~.008 3.21+0.12 12.6i-0.5 0.121:~0.005 16 1.47i'0.lM 0.244i'0.008 3.49-2-0.10 14.31"O.6 0.103:'20.004 37 1.48-+'0.03 0.254i-0,008 3.66i-0.12 14.4i-0.6 0.103i-0.004 24 1.53:£'0.02 0.2421-0.006 3.57i'0.10 14.8:i-0.5 0.104:£-0.004 35 1.51+--0.02 0.262i'0.007 3.31+-'0.10 12 .6-+0.4 0.120-1-0.004 22 1.48+0.03 0.264!'0.008 3.43i'0.10 13.0-1-0.5 0.114~N3.004

0.45 0.43 0.52 0.54 0.33 0.43 0.51 0.55 0.53 0.53 0.34 0.54 0.63 0.48 0.58 0.64 0.69 0.67

0.79 0.69 0.64 0.66 0.65 0.68 0.71 0.68 0.68 0.56 0.66 0.66 0.64 0.61

• Residue-normalized Th content: concentration of common thorium in acid-insoluble residue, assuming that all of the common thorium resides ia the residue. 1" Error correlation of .~4UflnTh vs. ~°'I'lu'2Xq'h isochron plot, see definition in Appendix B. 1 Data from Lao and Benson (1988). 2 Data from Kaufman and Broecker (1965).

(Table 2 continued)

Sample ID Residue U Th ThI~N" % ppm ppm p p m

234U/23s U 230Thf234U 230Th/232Th 234Uf132Th 232Thf138U Pxy ?

H o l o c e n e s a m p l e s JL052501-2 - - 1.35 0.24 - - 1.45+0.02 JL052502 - - 2.20 0.17 - - 1.46+0.02 JL052601 - - 2.80 7.44 - - 1.01_-+0.03

0.083::1:0.003 2.09-~0.08 25.2+1.0 0.058:t0.002 0.039-1:0.001 2.2?+0.08 56.7+9.1 0.026"£'0.001 1.011:N3.038 1.16:f0.03 1.15-+0.04 0.878-+0.032

procedure for U and Th separation is s imilar to that of Edwards et al. ( 1 9 8 7 ) . Af ter coprecipitation, Fe hydroxide precipitate was redissolved in 7 N HNO3 and loaded on a 400 # L anion-exchange co lumn ( A G 1 × 8 res in) . Thor ium was eluted with 6 N HCI and

U with 1 N HBr. Thor ium and uran ium fractions each went through a second anion-exchange column ( 100 # L ) with an identical elution scheme as for the first column. A blank was processed with each batch of samples, and was usually less than I ng for u ran ium and

Dating of lacustrine carbonates 282 l

Table 3. Mass spectrometry results of the last highstand samples (Group 5). The isotopic ratios are activity ratios and the uncertainties quoted are 10 errors.

Sample ID U Th 234U/238U 230Th/232Th 234U1232Th 232Th./238U (ppm) (ppm)

JL90-5 0.942 0 . 2 0 2 1.457+0.011 4.58+0.018 20.63_+0.37 0.071+0.0014 JL90-5P 0.932 0 . 1 9 3 1.493+0.004 4.70-L-_0.084 21.87_+0.22 0.068+0.0007 .1L90-6 0.914 0 . 4 7 8 1.449+0.008 2 .77+0.01 8.40-+0.06 0.179+0.0016

JL90-6-1 0.996 0.444 1.445+0.005 3.00-+0.011 9.82,+.0.11 0.147+0.0018 JL90-6-2 1.382 0 . 5 7 7 1.394+0.004 2.71+0.014 10.11-+0.32 0.138+0.0043 JL90-6P 0.875 0.461 1.457+0.005 2.61-+0.01 8.39+0.06 0.174+0.0014 JL90-15 0.985 0 . 3 5 5 1.496+0.004 3.222-+0.009 12.59-+0.07 0.119-+0.0008 IL90-16 0.919 0 . 1 9 9 1.493+0.011 4.514+0.013 20.95-+0.18 0.071+0.0008

JL90-16S 0.899 0 . 1 9 9 1.507+0.006 4.181+0.016 20.62_-4-0.16 0.073-+0.0006 JL90-17 1.094 0 . 3 9 3 1.523+0.004 3.498+0.01 12.86-+0.08 0.118-+0.0008 JL90-18 0.977 0.285 1.542-+0.005 3.806+0.011 16.03+0.10 0.096+0.0006

about 10 2 ng for thorium. These values were subtracted from the sample measurements, but did not significantly change the measured values.

Mass spectrometry measurements of uranium and thorium isotopes were made with VG lsolab 54 at Lamont. Detailed descriptions of this equipment and of the calibrations for measuring thorium and uranium isotopes can be found in England et al. (1992) and Bourdon (1994). Appendix A consists of a brief review of important features for measuring the U-Th isotopes on the VG Isolab 54 mass spectrometry by thermal and secondary ionization techniques. Uranium isotopes were measured by thermal ionization on a single Re filament with colloidal graphite. Th isotopes were loaded onto graphite rods to minimize isobaric interference and measured by SIMS (Secondary Ionization Mass Spectrometry) with an Ar ~ primary beam, which yields the high ionization efficiency needed to measure the relatively lower abundance of 23()Th compared to 232Th in tufa samples (23{)'rh/ 232Th atomic ratios for tufa samples are typically on the order of 10 5). The multicollection programs for uranium and thorium measurements were written so that the low abundance isotopes, 229Th, 23°Th, 234U and 235U are collected on the Daly detector while the high abundance isotopes, 23~-Th and 2~8U are collected on Faraday cups. The data acquisition for each of thorium and uranium runs is normally 2 3 hours. In-house standards for thorium (LaThl) and uranium (LaU03) were measured frequently and the measured values are comparable to those given by Bourdon (1994). The external reproducibility for thorium isotopes is about 0.5% and about 0.4% for uranium isotopes.

In order to evaluate the contribution of detrital thorium, bulk aluminum and iron contents were measured on selected tufa samples by Direct Current Plasma Emission Spectrometry (DCP) in the La- mont petrology lab using procedures of Klein et al. ( 1991 ). About 30 mg of each tufa sample was totally dissolved for the analysis. Three external standards of well-characterized sediments (NBS-Ic, JDO-I, mixture of NBS-lc and JDO-1), one blank and one high concentration standard were repeatedly analyzed in every batch of samples for drift correction and construction of calibration curves. Aluminum and other trace elements measured by DCP show Ca interference or enhancement (matrix effect), so a pure CaO was measured in each run in order to permit corrections. Descriptions of the DCP measurement technique for sediments and of the correction for Ca matrix effect are given by Plank (1993).

4. RESULTS

The uncor rec ted rad iocarbon ages for all the samples are l is ted in Table 1. Uran ium and tho r ium alpha coun t ing results are s h o w n in Table 2. Mass spec t romet ry U-Th results o f the last h ighs tand samples ( G r o u p 5) are l isted in Table 3. All i sochron plots are s h o w n in Fig. 3. The leas t -squares l inear r egress ion was done on the vers ion 2.71 o f I soplo t p r og r am ( L udw ig , 1994) . The part ial ly corre la ted errors o f 234U/232Th and 23°Th/Z32Th values are because a large source o f analyt ical errors for 234U/232Th and 2:~°Th/232Th ratios nor-

mal ly c o m e s f rom m e a s u r e m e n t o f the long half- l i fe 232Th by a lpha-count ing . Er ror -cor re la t ions (p~,. in Table 2; Ludwig , 1994) were ca lcula ted and appl ied in the line fitting rout ine for cons t ruc t ing 234U/232Th vs. 23°Th/232Th isochron. Princi-

p les o f de te rmin ing d i f ferent mode l s o f line fitting results o f fe red by Isoplot p rog ram are l is ted in A p p e n d i x B.

Authigenic 234U/238U values ob ta ined f rom the y- in te r - cep ts o f 234U/238U vs. 232Th/238U plots are s h o w n in Fig. 2.

A mix ing t rend b e t w e e n a ca rbona te e n d m e m b e r (au th igen ic phase; 234U/238U ~ 1.5, 232Th/238U = 0) and a detr i tus end- m e m b e r (23~U/23su ~-, 1, 232Th/23sU ~ 1.3) may exist , but

the scat ter is large. The data poin ts c lus ter near the auth igenic e n d m e m b e r (Fig. 2) due to the very high carbona te con ten t o f the samples .

l sochrons in Fig. 3 all s h o w good l inear correlat ion. Ex- cep t for Group 5, the M S W D ( m e a n square we igh ted devia- t ion) o f all a - c o u n t i n g results are lower than 2 (Fig. 3) . W e regard this as ev i dence that any d iagenet ic loss or gain o f uran ium or thor ium is wi th in the m e a s u r e m e n t l imits o f a lpha spec t romet ry . The two sets o f data for Group 5 samples over lap wi th in analyt ical errors (Fig. 3, the l ower panel on the r igh t ) . H ighe r value o f M.S .W,D. o f the Group 5 isochron main ly s t ems f rom the h igher scat ter o f mass spec- t romet ry data. M o r e p r o n o u n c e d scat ter for mass spec t rome-

1.7 • G(1),I=l.S2.-t:0.04 1.6 ~ .x n G(2),I=1.44:t'0.02

l- ~ • c,(3),I=1.43~0.02 1.5 ~ , a l • c,(4),I=1.49+0.01

I= 1.56:t:0.04 1.4

1.3

1.2

1.1

1 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40

232 T h / 2 3 s U

FIG. 2. 234U/238U vs, 232Th]238U plot for all five groups of samples. The authigenic 234U/t238U value of each group is noted in the legend. The dashed line is the expected trend for mixtures of detritus, 234U/ 23~U --~ 1 and 232Th/238U ,~ 1.3, with a pure carbonate phase con- taining 234U]238U ,~ 1.5 and 232Th/238U = 0.

2822 J.C. Linet al.

[-

[-

6 • " ' ' I " ' ' ' I ' " ' ' I ' ' ' ' I ' ' ' '

45 Group 1 " ~

3 MSWD = 0.39

2

l Isoehron age:22,500+l,200 years BP ',alendar Cor.14C age:20,200+200 years B] | | 0 0 I | | | | | '

0 0 5 ll0 . . . . 15i . . . . 20a, , '25 234 U/232 Th

10

8

#6

2

0

. . . . , . . . . , . . . . , / J MSWD = 1.76

f lsochron age:19,300+_400 years BP 2alendar Cor.14C age:17,600_+820 years BI

. ..I ... .I. ... I.. ..I 'I' .I . . . .

10 20 30 40 50 60 234 U/232 Th

8

7

¢.6 5

[ - 4

3

2

1

0

25

20

10

' '' I" ' '' ,' ''' I ''" 'I ' '' 'I ' '' ' I' ' ''

: Isochron age:19,100_+l,00O years BP 3 ? 14 ~alendar Cot I. 4C age:23,000_+760 years BP~

• ..I .... l . . . . l . . . . l .... I,,,,I.,,, 1 5 10 15 20 25 30 35

~4 U/z32 Th

' ' 1 . . . . . . I ' ' ' i ' ' ' i f ' ' ' I ~J I ¢

• c ~ m I I /

* /,-g

Group 3 / l w o

~ e - 2 0 , 9 0 0 + l , l l ~ ) years BP

---0.37

.qal.e~d.ar .qor.pC, a.g.e:29,00%+~sp, y.epr.s. 0 20 40 60 80 100 120

234 U/2a2 Th

#

6

5

4

3

2

1

0 0

• MS data I

• isoehron age:16~400_+700 years Bp . ~ 1 Calendar Cor.14C age:14,900+350 years Bill]

5 10 15 20 25 2a4 U/232 Th

140

FK;. 3. U-Th isochrons constructed from the data in Tables 2 and 3. The last panel ol] the right shows both c~ counting and mass spectrometry results of Group 5 samples. The choice of different model results from the line fitting program used is detailed in Appendix B. Thc calendar-corrected HC age (see definition in Table 5) is based on the 23~Fh-~4C age comparisons on Barbados corals (Bard et al., 1992). MSWD is the mean square weighted deviation of the data with respect to the regression line.

try data than alpha-counting data may not be surprising, since the much larger sample size and higher analytical uncertainty of the latter tend to smooth out the existing geologic scatter of the samples.

Before local reservoir age correction for ~4C ages, for four of the five isochrons constructed, the mean 23°Th-isochron ages exceed the HC-based calendar ages (Table 4) . A Age, defined as the difference between 23°Th age and the calendar corrected uC age, for three groups are significantly above zero (Groups 1, 4, and 5: Table 4) . In contrast, the Z~°Th- isochron age of Group 2 is significantly younger than the

calendar corrected 14C age ( A A g e 3,900 ± 1,800 years: Table 4) . With a local reservoir correction, the 2~°Th-~4C age differences would move towards positive sign by several hundred years.

5. DISCUSSION

5.1. Possible uC Age Biases

As demonstrated by Broecker and Walton (1959) through radiocarbon measurements on prenuclear lake water, A ~4C in Pyramid Lake was about --70%~, corresponding to a reser-

Dating of lacustrine carbonates 2823

25000

~Q 21000

L .

19000

,,ooo + / -->< / / ~ , , i Corals based cal. curve ,

13000

t 1 ooo 11000 13000 15000 17000 19000 21000 23000 ZSO00

t4 C A g e (years BP)

FIG. 4. Comparison of the U-Th isochron ages and uncorrected 14C ages. The dashed line is the 14C calibration curve obtained from measurements on Barbados corals (Bard et al., 1992). The error bars shown are lcr uncertainties.

voir correction of 600 years. This age cannot be entirely accounted for by the low 14C/C ratio in river H C O 3 ; in addition, a sizable component of radiocarbon-free water from springs must be entering the lake from beneath (calcu- lation of this phenomenon is shown by Lin et al., unpubl. data). There is no assurance that this effect was the same during higher lake levels, leading to difficulties in assessing the correction for the geological past. At this stage we do not, therefore, apply the reservoir age correction to radiocarbon ages. Uncorrected radiocarbon ages are converted to calendar ages according to the calibration curve constructed from the Barbados corals (Bard et al., 1990, 1992).

Lacustrine carbonates in Lahontan Basin have been generally found to be vulnerable to contamination by secondary carbon resulting in anomalously young radiocarbon ages (Benson, 1993). Less than 15% contamination by modem carbon is required to explain the positive age difference obtained for Groups 1, 4, and 5. Less than 5% modern carbon is required if the contamination was added recently (heavy lines in Fig. 5 ), or 10 to 15% if the contamination was added continuously since the samples were formed (light lines in Fig. 5, defined by Eqn. C5 in Appendix C) . So far we have not found an effective analytical procedure to ensure elimination of contamination.

Most of the carbonate samples analyzed for age comparisons in this study are calcareous tufas, which were usually precipitated as calcite in Lahontan Basin; thus, identification of diagenetic alteration can not be easily achieved by X-ray diffraction analysis, as is the case for aragon±tic corals. Acid leaching of the carbonate samples is the only way which might remove the recrystallized calcite. This approach was successfully applied on corals (Burr et al., 1992). We have attempted this approach with limited success (see Table 5). The increase in ~4C ages produced by sequential leaching is only on the order of a few hundred years.

Although there is no direct evidence for radiocarbon con- tam±nation, the timing of the last highstand of Lake Lahontan indirectly implies this error on 14C ages. Radiocarbon ages of the last highstand lake samples (Group 5 ) span more than 900 years (12,200-13,150 years B.P., Fig. 6 and Table 1), yet the duration of this pluvial event has been previously estimated to be on the order of a century or less according to the accumulation rate of tufas (Benson, 1991 ). Further- more, the timing of this pluvial event determined by Benson (1993) is 13.8 ___ 0.2 ka, based on radiocarbon ages of tufa samples from Walker Lake subbasin of Lake Lahontan. His reasoning was that since the radiocarbon ages are subject to modem carbon contamination, the oldest 14C ages should

Table 4. Comparison of 14C and U-Th isochron ages (1~ uncertainties quoted)

Uncorrected Calendar 230Th A Age Group Altitude 14C Cor. 14C Isochron (yrs) 2

(m) Age (yrs BP) A~e (~rs BP) 1 Age (yrs BP)

1 1250-1267 17,2005:270 20,200-~70 22,500+1,200 2,300±1,470 2 1260 19,900-3:800 23,000~:800 19,100±1,000 - 3,900~1,800 3 1210-1219 16,650+_550 20,00(050 20,900-~ 1,100 900 ±1,650 4 1245 14,650"2820 17,600~820 19,300-J: 400 1,700±1,220

5(or+MS) 1326-1330 12,690~350 14,900-~50 16,400"3:700 1,500 ±1,050 I Sample 14C ages are converted to calendar ages using the relationship between 14C age and

calendar age in Bard et a1.(1992). 2 Defined as 2~"fh - Calendar Cor. 14C which presents the difference between the mean 2~Th-

isochron age and the calendar-corrected 14C age (see foot notes 1 and 2 above).

2824 J.C. Lin et al.

2000

"~ 1500

,., 1000 ' ~

500

0 0 0.05 0.1 0.15 0.2 0.2

Fraction of Secondary CaCO a

FIG. 5. Age reduction (true age minus calendar-corrected I4C age) as a function of fraction of secondary CaCO3. Two cases are shown, one when the contamination is recent and the other where it accumu- lated continuously.

be the most reliable. Organic matter beneath rock varnish collected from the highstand shoreline at the northern end of Pyramid Lake yields 14C ages as old as 13.9 ka (Fig. 6; T. Liu and R. Dorn pers. commun.). Because the rock varnish could only have formed after the lake level dropped, this places a minimum age of this pluvial event and suggests that 14C ages of Group 5 samples are all too young by at least 800 years (Fig. 6) .

The case of Group 2 where 14C age is significantly higher than 23°Th age is explained here as much too old 14C ages of the samples. As will be discussed below that the U-Th isochron age has a tendency to become older, the addition of older carbonates may be the only explanation.

We may conclude, therefore, that contamination of the radiocarbon ages with secondary carbon is responsible for at least pan of the discrepancies between 23°Th and ~4C ages in this study, but it has not been possible to estimate the exact magnitudes of this error.

5.2. Possible Errors in U-Th Isochron Ages

In theory, initial 23°Th in a coeval set of impure carbonates can be accounted with isochron technique, but this assumes that the samples all have the same initial 23°Th/232Th ratio (Luo and Ku, 1991 ). Concern arises when more than one component of initial thorium exists in the carbonate samples, each with its own 23°Th/232Th ratio. As outlined below, there may be two sources of initial thorium in the Pleistocene

carbonate samples, one detrital and the other from the dissolved pool in the lake water. We will hereafter refer to the dissolved thorium as "hydrogenous" .

Major-element measurements from selected tufa samples show that A1 and Fe oxides are present in the bulk samples at the level of several tenths of a percent (Table 6). The detritus contents in the carbonate samples was estimated by assuming that they contain the same A1 and Fe contents as average shales (15.47% A1203 and 5.14% Fe203 by weight; Garrels and Mackenzie, 1971 ). Detritus contents in the tufa samples estimated using A1 and Fe contents are in reasonable agreement with estimates assuming that the acid insoluble residue is dominated by detrital silicates.

Two lines of evidence are consistent with the speculation that a significant portion of the initial thorium in these carbonate samples had a hydrogenous origin. While either of these observations by itself could be argued to be representa- tive of the detritus, the combination would only be found in unusual rock types. First, based on the three independent estimates (residue-, AI-, and Fe-based), if all the 232Th resides in the detritus, then the 232Th content in the detritus must average about 21 ppm (Table 6). Because the residue contains organic matter and opal as well as silicate detritus and part of the Fe and A1 could also be hydrogenous, this is a minimum estimate of the thorium content of the silicate detritus. If the detritus is assumed to have a 232Th content of 10-11 ppm, typical of average upper crust (Taylor and McLennan, 1985), then the remainder of the measured 232Th could reasonably be inferred to be hydrogenous. Accord- ingly, the hydrogenous component of initial thorium could be as large as the detrital component. However, 21 ppm thorium is not outside the range reported for all types of sediments ( 1 - 3 0 ppm; Gascoyne, 1982).

The high initial 23°Th/232Th ratios derived from the isochrons is also consistent with a significant hydrogenous component of initial Th. According to Eqn. 1, the y-intercept of the isochron approximates the decay-modified 23°Th/Z32Th ratio of the initial thorium. In conventional U-Th isochron dating it is assumed that this value is the 23°Th/232Th ratio of the aluminosilicate detritus contained in the carbonates. If this initial ratio is independently known, then a 23°Th growth age can be obtained from a single U-Th isotopic measurement. The values obtained in this study averaged 1.65 _+ 0.20, consistent with the 1.7 value obtained in Kauf- man and Broecker (1965) Table 7). This value is significantly higher than the 23°Th/232Th activity ratios expected for aluminosilicate detritus with average crustal and sedimentary T h / U ratios. The average upper continental crust T h / U ratios

Table 5. Effect of leaching on ~4C ages

Sample ID Material Sample Location

JLg0-18-3 tufa Pyramid Lake

C2 shell Searles Lake

t4C Age Leaching Experiment (yrs B.P.) (by 10% HCI)

12,600-k-95 10% leached 12,720-k95 20% leached 12,830-L-95 80% leached 11,360t--:-100 leachate (~33%) 12,070-L-_ 100 residue (-67%)


i l:I 6

d z 2

m l~ramid Lalte htl~tand tufu w m , = l . ~ hlShu~ mt'u (Bmson, 1993)

Age for Pyramid Lake t hlghstand tusge.st~l tufu

bY U-TIt imchron age

Or$~aic tmtt~ bemath rock vm'nl~ (Dora et M. 1990 and peas. com.)

0 1 12 13 14 15

Radiocarbon Age (Ka)

FIG. 6. Summary of the available radiocarbon ages obtained for the last highstand of Lake Lahontan (includes data from Benson, 1993, Dorn et al., 1990 and 1994 pers. comun.). The U-Th isochron age for the high shoreline samples has been converted to the radiocarbon timescale using the results of Bard et al. (1992). As the organic matter beneath rock varnish began to accumulate after the pluvial event, these ages are supposed to be the lower limits.

were estimated to range from 3.4 to 3.8 (Taylor and McLen- nan, 1985) and for average shales are 2 . 7 - 7 (Gascoyne, 1982). Assuming equilibrium between 23°Th and 238U, the 23°Th/232Th activity ratios for average upper continental crust are 0 .80-0 .89 and for average shales are 0 .43-1.12. The T h / U ratios in igneous rocks conmaonly fall between 3 to 5, which correspond to 23°Th/232Th activity ratios of 0 . 6 1 - 1.01 (Harmon and Rosholt, 1982). The sources and mineral compositions of the detritus in the Lake Lahontan carbonates are poorly known, but unless they are composed of some unusual rock types, their 23°Th/232Th activity ratios are expected to fall in the whole range estimated above from several rock types, i.e., 0 .43-1.12. The 23°l"h/232Th ratio observed for the sediments in Pyramid Lake is about 1.17 (Table 7) , which is in agreement, although high, with the ranges expected from the two categories of rocks discussed above. A beach sand sample from Pyramid Lake has a 23°Th/ 232Th activity ratio of 1.16, although it is less likely the type of material incorporated in the carbonates (more likely clays). Note that these measured 23°Th/232Th ratios of lake sediments could only be the maximum values of original detrital sediments, since adsorption of hydrogenous thorium after the sediments entered the lake would have raised these ratios. Unless the actual sources of the detritus in the carbonate samples had abnormally high thorium contents as well

as low T h / U ratios (e.g., black shales), the initial 23°Th/ 232Th ratios (average 1.65) observed from the isochrons of Lake Lahontan carbonates imply an additional and nondetri- tal source of initial thorium.

We can not rule out the possibility of black shales being the source of detritus incorporated in the carbonates, nor could we prove if carbonate rich detritus was not actually the material incorporated (this type of detritus could have high 23°Th/232Th ratios). However, the following observation also suggests that dissolved thorium from the alkaline lake is likely to have played an important role in this issue.

The alkaline lake waters in Great Basin were observed to contain both high contents of thorium (and uranium) and high 23°Th/232Th ratios of the dissolved thorium (Table 8; Simpson et al., 1984). Thorium content in modem Pyramid Lake was more than two orders of magnitude higher than the thorium contents in seawater (Table 8; Chen et al., 1986). Higher contents of dissolved thorium and uranium in alkaline lakes were believed to be the results of carbonate ion complexing (Simpson et al., 1984; LaFlamme and Mur- ray, 1987). We anticipate that the dissolved thorium could possibly be incorporated into carbonates by the sanae mechanism as dissolved uranium. Likewise, it could be adsorbed onto particles before they were incorporated into the lacustrine carbonates. Because of the poorly known distribution coefficients of dissolved thorium in the alkaline environments, it is not possible at this point to quantitatively estimate the amount of hydrogenous thorium in the alkaline lake carbonates.

In addition to the abundant dissolved thorium in the alkaline lake waters, high 23°Th/232Th ratios in the lake, due to in situ decay of dissolved uranium, makes hydrogenous thorium an ideal candidate for the additional source of initial thorium in Lake Lahontan carbonates. The 23°Th/232Th activity ratio in Pyramid Lake water today is only 1.3 (Simpson et al., 1984), but much higher 23°Th/232Th ratios of dissolved thorium are not uncommon in the alkaline lakes of Great Basin (see Table 8). We attempted to determine the initial 23°Th/232Th ratio in present day carbonates in Pyramid Lake by analyzing late Holocene gastropod shells for 14C ages and U-Th isotopes. Initial 23°Th/232Th in the shells obtained by correcting for the ingrown 23~1"h (using local reservoir corrected 14C ages with a reservoir age of 600 years, Broecker and Walton, 1959) are 1.65 and 1.25, respectively (JL052501-2 and JL052502 in Table 7, U-Th results shown

Table 6. The residue %, aluminum, iron and thorium contents of selected tufa samples. The last three columns are the detritus-normalized thorium concentrations based on residue, A1 and Fe methods described in the texL

Sample Residue AI20 3 Fe203 Th Detritus-normalized Th (ppm)* Residue AI Fe

ID (%) (%) (%) (ppm) Method Method Method JL90-2-I 2.87 0.727._+0.009 0.296.£-0.0001 0.94£-0.02 33 20 16 JL90-2-2 7.20 0.724:L-0.012 0.292£-0.001 0.96d:0.03 13 21 17 JL90-2-3 6.08 0.825:L-0.003 0.327i-0.0009 1.05x',-0.02 17 20 17 JL90-3-1 4.80 0.837£-'0.002 0.344i--0.001 0.76t:0.02 16 14 l l JL90-3-2 3.56 0.615~-0.014 0.266:t,-0.002 1.08_+0.03 30 27 21 JL90-3-3 3.91 0.826i-0.008 0.356£-0.001 1.11£-0.03 28 21 16 JLg0-4- l 3.14 0.241:1:0.002 0.088-+0.0002 0.47x'-0.02 15 30 27

Average 22 22 18 *Assuming that all the thorium resides in the residue. If as does the average shale, the detritus actually contains about 10 ppm thorium then half or more of the thorium in these tufa must be hydrogenous.


Table 7. ~I'h/732Th activity ratios of the isochron samples, Holocene lake deposits and sediments from the three alkaline lakes.

I. Isochron samples Isochron Age Observed Initial l

Group (yr BP) 230Th/232Th 230Th/232Th

1 22,600+700 1.24+0.07 1.54_+0.15 2 19,000-L-900 1.57+0.09 1.86i-0.14 3 20,900-i--400 1.37+0.03 1.66:L-0.13 4 20,100+500 1.03+0.06 1.34_+0.10

5(ct + MS) 16,400+700 1.48+0.03 1.72i-0.10 Average 1.62_-)-0.20

I1. Holoeene samples Res. Cor. 14C Measured Initial Age (yr BP) 23°Th/232Th 23°Th/232Th

JL052501-2 1890 2.09+0.08 1.65+0.082 JL052502 1940 2.22+0.08 1.25+0.082

III. Lake sediments Measured

23OTh/'232Th Mono Lake 3 1.58+0.09 Walker Lake 3 1.11 +0.05 Pyramid Lake 3 1.17+0.06

JL052601 (Pyramid L.) 4 1.16+0.03 1 230Th ;init - 230]'h',obs ett, and

232Th, - 232Th., 230Th ",obs = intercept of isochron 232Th

230Th )iv.it are obtained as discussed in the text. 2 232Th

3 Lake bottom sediments, data from Simpson et al. (1984) 4 beach sands

in Table 2) . The reason for the difference of these ratios is not known, but these data indicate that the lake water did have excess 23°Th compared to the expected 23°Th/23ZTh ra-

tios for aluminosi l icate detritus. If the detrital Th is assumed to have a 23°Th/232Th activity ratio of 1.2, a simple calcula- tion shows that the 23°Th/232Th ratio of the Pleistocene hydrogenous thor ium might have ranged from 2.06 to 2.38 (us ing the average detr i tus-normalized thor ium content of 21 ppm from Table 6 and hypothesized detritus with 1 0 - 1 3 ppm of thorium.

We freely admit that there are still uncertainties in the above argument about the presence of hydrogenous thor ium in the Lake Lahontan carbonates. However, the goal is to improve our unders tanding of the system in order to improve the accuracy of the method. Accordingly, it is necessary to discuss this issue because it can significant influence the isochron age which will become important as improved precision is obtained through more precise isotopic measure-

ment or some device. Isochron age biases will be the main focus of the fol lowing section.

5.3. Evaluat ion of Isochron Age Biases

It is critical to the isochron method that the initial Z3°Th/ 232Th ratios in the impure carbonates are uniform, rather than variable due to mixing of the detrital and hydrogenous thor ium components , as variable mixtures of initial components in certain ways may produce a bias in the isochron slope. Two mechanisms can be envis ioned by which carbonates take up dissolved thorium: (1 ) dissolved thorium first adsorbs to detrital particles, which are then incorporated into the carbonates, or (2 ) dissolved thor ium is incorporated directly into the carbonates. If hydrogenous thorium is adsorbed by detrital particles with a constant partit ion coeffi- cient ( lq ) between water and particles, and if detritus all has the same thorium content, then the ratio of hydrogenous thor ium to detrital thorium will be the same among coeval samples. Mixing of the two Th components in this way results in an uniform initial 23°Th/232Th ratio that lies between the isotopic ratios of hydrogenous and detrital Th (Fig. 7a) . The initial 23°Th/23ZTh ratio in this case is determined by the mixing ratio " k " of the two kinds of thorium carried by detritus (k ~d . . . . d~ = Th hydro./Thaetr, where Th hydro, is the hydrogenous thorium adsorbed by detritus, Fig. 7a) . In contrast, if the uptake of hydrogenous thor ium is by direct inclusion of dissolved thor ium into the carbonate crystal structure, hence, correlated with that of hydrogenous uranium, the zero-age isochron will have a posit ive slope (Fig. 7b) . This is due to the increasing importance of hydrogenous thorium (with a larger 23°Th/232Th ratio) towards larger 234U/232Th values. In this case, the initial Z3°Th/232Th ratio is equal to the detrital ratio (Fig. 7b) . Isochrons constructed from real samples can only produce initial 23°Th/232Th ratios (age-corrected y- intercept) greater than detrital 23°Th/232Th ratios if at least part of the hydrogenous thorium was incorporated by adsorption onto detritus, as demonstra ted in Fig. 7a. If the argument in the previous section about the hydrogenous origin of a significant fraction of initial thorium in the Lake Lahontan carbonates is true, then the scenarios demonstra ted in Fig. 7a,b suggest that part of the hydrogenous thorium must be in the form of adsorbed component on the included detritus (initial 23°Th/232Th activity ratios of ~ 1.65 are greater than detrital ratio of 1.17).

Table 8. U and Th concentrations and isotopic compositions (activity ratios) in the present-day waters of three Great Basin alkaline lakes (Simpson et al., 1984). U and Th contents in a seawater sample are given for comparison.

Water 238U Z32T h Column 234U/238U 230Th/232Th

Depth (m) (dpm/l) (dpmfl)

Mono Lake 1 360+44 1.66+0.16 1.51+0.19 3.50+0.71 30 404+ 13 1.50+0.13 1.16+0.05 2.26+0.23

Walker Lake 1 126 + 4 .......... 1.34 +__ 0.06 ............ 25 115+4 0.0115+0.0004 1.30+0.06 1.56+0.08

Pyramid Lake 1.3 13.5 + 0.7 0.0071 + 0.0004 1.97 + 0.13 1.28 + 0.10 75 1.7 + 0.2 0.0049 + 0.0004 1.40 + 0.17 1.36 + 0.08

Seawater* 690 2.39-!0.005 3.57_+0.10 xl0 -5 . . . . . . . . . . . . . . . . . . . . . . . . * Unfiltered seawater from Atlantic Ocean (Chen et al., 1986)


(*) (b) 1:1 mixing (k =1) ~ I (k = ~

_ _ { . . . .

234U/~"l'h ~.3,tO/232'l'h

(c) (2) (3) pure carbonates dettitus adsorbed with hydrogenous

drogenous Th ~ ) ~ . ~ = a =~ . . . . . . r-~ a~a. ~

l/a ~U/XUTh

Flc. 7. Schematic diagrams showing the impacts of hydrogenous thorium uptake on the slope of the age-zero isochron plot ( solid lines ): (a) hydrogenous thorium added only by adsorption to detritus particles, and ( b ) hydrogenous thorium added only by direct incorporation of dissolved thorium into carbonates in constant proportion to the amount of authigenic uranium. Panel (c) shows a zero-age isochron where hydrogenous thorium is both adsorbed to silicate detritus and incorporated directly into the carbonate matrix in constant proportion to authigenic uranium, k is the mixing ratio between detritus-adsorbed hydrogenous thorium and detrital thorium. For age biases less than 3000 years, the slope of the zero-zage isochron can be approximated by the following relationship:

/~230Th/234U ~ ((23°Th/Z32Th)hydro. - (23°Th/232Th)detr.)*a/(k + 1),

where " a " is defined as the 232Th/234U ratio in pure carbonates.

To summarize the above, initial thorium may be viewed as a three component mixture: (1) detrital thorium, (2) hydrogenous thorium adsorbed to detritus, and (3) hydrogenous thorium directly incorporated by carbonates (Fig. 7c). The hydrogenous thorium adsorbed on detritus and the hydrogenous thorium directly incorporated in carbonates have the same 23°Th/232Th ratio. If a significant amount of hydrogenous thorium was incorporated directly into carbonates, or, if the " a " value (defined as the 232Th/234U activity ratio in the pure carbonate phase, see Fig. 7c) is significantly greater than zero, there will be a positive slope associated with the zero-age isochron. The apparent slope ( A 230Fh/234 U ) of the zero-age isochron can be approximated by a simple function when A23°Th/234U is small ( A t < ~3000 years):

A 23°Th / 234 U

(23°Th/232Thhydro. - 23°Th/232Thdetr.)*a/(k + 1), (3)

Where " a " = 232Th/234U in the pure carbonate phase and " k " ads. = ThhydroJThd ..... Three unknowns, 23°Th/232Thhyd .... " a " , and " k " (assuming known 23°Th/232Thdetr,), are in- volved in calculating the A23°Th/234U, hence At of a set of samples. A sensitivity analysis (Fig. 8) shows values of " a " and " k " required to produce age biases ( A t ) of up to several thousand years. Using a simple algorithm, we can device a means to determine whether a set of samples is subject to an age bias from the available data.

The three required parameters (a , k, and 23°Th/232Thhydro. ) may be evaluated by the mass balance equation of total Th in each sample that contains these three components of thorium (in dpm/g) :

232Thtotal = 232Thd~tr. + 232Thhydro.

= 232Thdetr. -}- k*232Thden.. + a*234Utotal, (4)

assuming that hydrogenous thorium consists of two components, one adsorbed to detritus with a constant partition coef- ficient (k* 232Thd~t,.), and the other incorporated directly into carbonates in constant proportion to authigenic 234U by a factor " a " ; i.e.:

,234 232Thhydro. = k*232Thdetr. + a Ucarbonate

k* 232Thd~tr. + a * 234Utotal.

The approximation is drawn because the majority of total uranium resides in the carbonates. Detritus content of each sample in theory can be estimated by carefully removing both carbonate and organic matter. If the thorium concentra-

2500 • ' ' ' I ' • ' ' I * " ' ' | ' ' ' '

2000 23°T~rhhY~' "2~rn/~Tn~" ' = ~

~ 1500

"-" 1000 / . . -

500 i . - -

o , i

0 0 0 .005 0.01 0 .015 1.02 a

FIG. 8. An example shows the sensitivity of age bias (At) to the " a " and " k " values, assuming the difference between hydrogenous 23°Th/232Th and detrital 23°Th/232Th to be 1; where " a " is the 232Th/ 234U ratio in pure carbonate phase and " k " is the ratio of adsorbed hydrogenous thorium to detrital Th.


tion of the detritus is known, the detrital thorium term (232Thdetr.) can be fixed. Solving the remaining two unknowns in Eqn. 4, " k " and " a " , becomes a nonunique solution system provided more than two samples (data of each sample offers one equation). Normalizing Eqn. 4 by 232Thd,~., we may transform this system to a linear regression problem:

232Tht°ta' -- (1 + k) + a* ( 234Ut°ta' 23ZThdCt ~ - \ 232Thd~t-- j . (5)

Plotting 232Thtotal/E32Thd~tr. vs. 234UtotJ232Tl~¢~. for all samples, the slope of the best-fit line will yield " a " and "1 + k" from the intercept. Furthermore, given the detrital 23°Th / 232Th activity ratio ( ( 23°Th/232Th )det~. ; e.g., 1.1 7 from Pyramid Lake sediments), the hydrogenous 23°Th/232Th ((23°Th/232Th)hyd~o.) can be calculated from the " k " value and the initial 23°Th/232Th ratio ((23°Th/232Th)ini. ; derived from the isochron plots), using the following relationship (based on the definition of " k " ) :

23°Th/232Thi~i.

1 k = 23°Th/232Thdetr'* k + 1 + 23°Th/232Thhydr°'* k + 1 (6)

We are now ready to compute the A 23°Th/234U value from Eqn. 3, inserting all the parameters needed. The age bias (At) is then calculated from the A23°Th/234U value by Eqn. 2.

Derivation of age biases following the algorithm outlined above is illustrated in Fig. 9 for two groups of samples with positive 23°Th-14C age differences (Groups 1 and 4). Assuming a detrital 232Th content of 10 ppm, a plot of 232Thtotal/232Thdetr. vs. 234Utotal/232Thdetr" yields the parameters " a " ((232Th/234U)hydro. in pure carbonate phase) and " k " (232Thhydro/232Thdetr. in detrital phase) from the slope and intercept, respectively (Eqn. 5 ); Fig. 9a,b). With these parameters, (23°Th/232Th)hydro. ratio, A 23°Th/234U using Eqn. 3, and finally At were computed sequentially. A substantial difference in At exists between these two groups, with Group 1 having a much larger error (At = 2204 years) than that of Group 4 (At = 450 years). The larger error associated with Group 1 is related to: ( 1 ) the larger apparent value of (23°Th/ 232Th)hydro. for Group 1 (2.03 vs. 1.62) and (2) the larger value of " a " for Group 1 (0.041 vs. 0.015; i.e., the component of hydrogenous thorium directly incorporated by carbonates is larger in Group 1 samples). Larger values of 23°Th/232Thhydro. and of " a " will each contribute to a larger slope of the zero-age isochron (Eqn. 3) and, therefore, to larger errors in the isochron age.

Derived parameters are sensitive to the value assumed for the 232Th content of detritus. The calculations described above are repeated using a detrital 232Th concentration of 7 ppm (Fig. 9c,d). The lower value for detrital 2a2Th has little effect on " a " , whereas values of " k " increase substantially for both groups and the derived values for 23°Th/232Thhydro. decrease. Computed age biases are smaller using the lower values of 2 3 2 T h d e t r " . At is sensitive to 232Thdet~. used because the derivation of 23°Th/232Thhydro. (via Eqns. 5 and 6) depends on the " k " value determined by 232Thdeu., and because of the further dependence of At on 23°Th/232Thhydro. (via Eqns. 3 and 2). These relationships are illustrated in Fig. 10, which

shows the relationship between 23°Th/232Thhydro. and 232Thdetr , along with computed At. If we assume the 23°Th/232Thhydro. for the two groups of samples were the same, we may use this and other criteria to constrain the best estimated range of age errors for both groups. For Group 1, the 232Thdetr. applied can not be greater than - 1 0 ppm, since then the computed At is larger than the 14C-23°Th age difference observed (using the mean value 2300 years, Table 4). Along with the 23°Th/232Thhydro. values evaluated earlier, the best estimated range of 23°Th/232Thhydro. is around 2 to 2.1 and the best estimated age errors of Groups 1 and 4 are roughly 2000 years and 1000 years, respectively (Fig. 10).

Computed At is positively correlated with the difference between hydrogenous and detrital 23°Th/232Th ratios. While detrital 23°Th/Ea2Th ratios may reasonably be assumed to vary little over time, our results suggest significant variability of the hydrogenous 23°Th/232Th ratio, ranging from the estimated Pleistocene values of between 2 and 2.1 to the modem value of 1.3 in Pyramid Lake water. Conditions responsible for variability through time in the hydrogenous 23°Th/232Th ratios are unknown, although changes in lake water alkalinity may be a contributing factor because thorium is stabilized in solution by carbonate and hydroxyl-carbonate complexing (LaFlamme and Murray, 1987; 0sthols et al., 1994).

We are unaware of any published studies documenting a significant contribution by hydrogenous thorium to the initial thorium content of impure carbonates. For Group 1 samples, with a derived value of 0.041 for the 232Th/234U activity ratio in the pure carbonate phase, the implied amount of hydrogenous 232Th is 55% of the total 232Th in these samples (Table 9). Assuming that modem 232Th/234U activity ratios of Pyramid Lake water (0.0003-0.002; Table 8) are repre- sentative of Pleistocene conditions, the 232Th/234U ratio of the pure carbonate phases of Group 1 samples (0.041) implies an enrichment of the Z32Th/234U ratio in the carbonate phase, with respect to the lake-water source, by more than an order of magnitude, and possibly by as much as two orders of magnitude.

The impact on U-Th isochron ages by the mechanism modeled above may be only on the order of one to two thousand years, which in many cases is well within the uncertainties associated with the U-Th isochron dating method. The main purpose of the above discussion is to trigger further studies of thorium geochemistry in hydrological system, both in mineral and aqueous phases. This is especially important for isotope geochemists, who are eager to improve the accuracy and precision of the U-Th dating method and to understand the isotopic systematics.

6. CONCLUSIONS

Three out of five isochrons constructed from lacustrine carbonate samples collected from Lahontan Basin yield U- Th isochron ages significantly older than their calendar-corrected 14C ages. Contamination by modem carbon is partially responsible for these age discrepancies. Due to the direct incorporation of hydrogenous thorium into the pure carbonate phase, biases on the U-Th isochron ages offer an additional explanation for 14C-23°yh age differences. Evidence for a hydrogenous component of common thorium in the


(a)

8

7

5

~" 4

~ 3

10 ' ' i , ' " I ' " " I " " " I " ~ i l ' ' " I" " G r o u p 1 Detr• 10 ppm

8 a = 0.04123 At = 2204 years k = 0.758

6 ( . = '

2

y = 1.758 + 0.04123x R= 0.99 i i I . • l I . • , I . • • I , , • I • . • • •

20 40 60 80 100 120 234 1232 .

Utot . T h d e t r

• ' I l ' ' l ' ' ' I " = ' I " ' ' I " ' " ' ' ' I " •

G r o u p 4 Detr.Th =' 10 ppm

a = 0.01472 At = 450 years k -- 0.61

2 3 0 3 2 ( Tl~ Th)~o. = 1.62

• o - 1

0 0

. • 1 . . , i , . o i • , o 1 • • • 1 . • • i , . • 1 o |

20 40 60 80 100 120 140 z34 Utet" /232 Thdetr"

140

(c) 15

l O

o

(d) 8

7

6 A~

5

2

o 16o o

• • • ' I ' • • • I ' • ' " I ' • " '

G r o u p 1 Detr. Th - 7 ppm

a = 0.04101 At = 1066 years k = 1.509

• ( 2 ~ r ~

y = 2.51 + 0.04101x R= 0.99 • • • • I • • • • I • . • • I • . • •

50 100 150 200

234 Utot" ]23;7, T h d e t r '

I 'Grollp 4 . . . . ' " D'etr,T'h ~ "7 ppm"

a=001504 At = 216 years k = 1129 •

(23°Th/232Th)hy<~o. = 1.42

y = 2.29 + 0.01504x R= 0.67 I , . , • • • I . • . • I , • | • I • • , •

50 lOO 15o 200 234 /232

Utot. T h d e t r .

FIo. 9. Graphic illustration of the procedure used, according to Eqn. 5, to evaluate the parameters " a " ((232Th/ 234U)hydro. in pure carbonate phase) and " k " (232Thhydro./232Thdetr. in the detrital phase) from the slope and intercept, respectively, of a plot of 232ThtotJ232Thde~. against 23+UtotJ232Thde~ for samples from Group 1 (a) and Group 4 (b) , assuming an initial concentration of 232Th in the detritus of 10 ppm. Upon derivation of the parameters " a " and " 'k," the hydrogenous 23°Th/Z32Th ratio as well as the age bias (At ) derived from the computed slope of the zero- age isochron are also derived, as described in the text. Derivations are repeated in (c) and (d) using a detrital 232Th concentration of 7 ppm.

Lake L a hon t a n ca rbona tes is twofo ld : ( 1 ) h igh thor ium con- cen t ra t ions are e s t ima ted for the detr i tus in these lacust r ine ca rbona tes and ( 2 ) an init ial 23°Th/Z32Th act ivi ty ratio aver-

aging 1.65 is ob ta ined for these samples f r o m their U - T h

i sochrons , m u c h grea ter than typical detr i tal rat ios o f 0,8 +_ 0.2. B a s e d on a rgument s p re sen ted in this paper , the 2 3 ° T h / 2 3 2 T h ratio o f the h y d r o g e n o u s tho r ium in the Pleis to-

cene Lake Lahon tan was poss ib ly in the range o f 2 to 2.1,

(a) 2.4

2.3

2.2

2.1

2

1.9 I

1.8

1.7 6

Co) , . , , , i , , , , i . , , , i , . , , i , w , , i , , , , i , ,

G r o u p 1 At U

At = 1384 years

. . . , . . . . , . . . . , . . . . , . .

7 8 9 10 11 12 13

D e t r i t u s za2 Th C o n t e n t ( p p m )

2 . 2 ~ , . . . I , , ' ' 1 " , ' ' 1 ' ' , , I ' ' ' , l , ' ' ' 1 . . . . I ' . .

2.1~" G r o u p 4 At= l158yearsp

:z:I I . V ' " ; . . . . . . . . ; . . . . . . . .

Detritus 2a2 T h Content ( p p m )

FIG. 10. Relationship between derived values for hydrogenous 2 3 ( ~ F h / 2 3 2 T h ratios and assumed values for the 232Th content in the pure detrital phase: (a) using parameters " a , " " k , " and initial 23°Th/232Th ratio representing Group 1 samples, and (b) using the corresponding parameters for Group 4 samples. Errors in isochron ages (At ) corresponding to selected points along the trend of each relationship are shown to illustrate the sensitivity of estimated age bias to the assumed value for the detrital 232Th concentration.

2830 J .C . Lin et al.

Table 9. Distribution of the three Th components in an average sample of Group 1, assuming detrital Z3OTh/232Th activit 7 ratio = I. 17.

% of three Th components Detrital At hydrogenous k a in an average sample # Th cone.

(ppm) (years) Z30Th/232Th Thdetr. k*Thdetr, a*234U

7 1096 1.79 1.509 0.04101 18 27 55

8 1384 1.85 1.197 0.04101 21 24 55

10 2204 2.03 0.758 0.04123 26 18 56

12 3576 2.33 0.468 0.04108 31 14 55 # The average sample has a Th concentration of 0.10 dptrdg or 0.405 ppm (average from the four samples with ~cid-insoluble residue measurements, see Table 2). Thdetr" is the fraction of total 232Th associated with the pure detrital phase; k*Thdetr, represents hydrogenous Th adsorbed to detritus prior to incorporation of detritus into the carbonates; a*234U represents hydrogenous 232Th incorporated into the carbonates direcdy from solution, assumed to be in constant proportion to 234U.

c o m p a r e d to the value in the m o d e m Pyramid Lake water o f about 1.3.

Initial 23°Th/232Th ratios der ived f rom i sochron plots exceed normal detri tal 23°Th/232Th ratios, indica t ing that s o m e o f the thor ium wi th h igh 23°Th/232Th was assoc ia ted wi th

fine detri tal part icles . Incorpora t ion o f h y d r o g e n o u s thor ium by this m e c h a n i s m wou ld in t roduce no age bias to an isochron. H o w e v e r , i f a s ignif icant amoun t o f h y d r o g e n o u s tho- r ium was incorpora ted direct ly into the pure ca rbona te phase a long wi th uran ium, it w o u l d in t roduce a pos i t ive s lope for the ze ro -age i sochron, and hence , a pos i t ive apparen t initial age. P rov ided that the detr i tus con ten t o f each sample is know n , a t h r e e - c o m p o n e n t mix ing d iag ram can be used to es t imate the magn i tude o f this i sochron age bias. Our analysis sugges ts that b iases o f up to two thousand years for the i sochrons in this s tudy are poss ib le .

Tradi t ional a s sumpt ions for U - T h i sochron dat ing are that the pure ca rbona te phase is free o f tho r ium and all the c o m - m o n thor ium is assoc ia ted wi th detri tal impuri t ies . W e sugges t that these a s sumpt ions are not a lways valid. S tudies o f impure ca rbona tes requir ing accuracy o f U - T h ages bet ter than 1000 to 2000 years still need a careful evalua t ion o f potent ia l age b iases assoc ia ted wi th initial uptake o f hydrogenous thor ium.

Acknowledgments--We would like to express many thanks to Dr. L. Benson, for helping us collecting samples and kindly offering some tufa samples from his collection. We thank Ms. M. Klas and Dr. I. Hajdas for preparing samples for 14C AMS measurements. Constructive comments were given by Dr. J. L. Bischoff, Dr. H. P. Schwarcz, Dr. K. Ludwig, Dr. L. Benson and another reviewer in their reviews on different versions of this paper. The first author also appreciates Dr./Mr. Y. Lao, J. Clark, G. Henderson, M. Fleisher, A. Zindler, B. Bourdon, J. Severinghouse, T. Ku, R. Dorn, and T. Liu for their helpful discussion on this study and/or the lab technique, and Ms./Dr. G. Boitnott, T. Plank, and N. Kumar for helping with the measurements on Direct Current Plasma Emission Spec- trometry. Dr. K. Ludwig kindly offered the Version 2.71 Isoplot program. This work was supported by a grant from NASA NAGW- 895.

Editorial handling: H. P. Schwarcz

REFERENCES

Bard E., Hamelin B., Fairbanks R. G., and Zindler A. (1990) Cali- bration of the 14C time-scale over the past 30,000 years using

mass spectrometric U-Th ages from Barbados corals. Nature 345, 405-410.

Bard E., Fairbanks R. G., Arnold M., and Hamelin B. (1992) 23~JTh/ 234U and ~4C ages obtained by mass spectrometry on corals from Barbados (West Indies), Isabela (Galapagos) and Muruoa (French Polynesia). In The Last Deglaciation: Absolute and Ra- diocarbon Chronologies (ed. E. Bard and W. S. Broecker); NA TO ASI Series I: Global Environmental Change, 2, 103-110.

Benninghoven A., Rtidenauer F. G., and Wemer H. W. (1987) Sec- ondary Ion Mass Spectrometry. Wiley.

Benson L. V. ( 1991 ) Timing of the last highstand of Lake Lahontan. J. Paleoclim. 5, 115-126.

Benson L. V. ( 1993 ) Factors affecting ~4C ages of Lacustrine carbonates: Timing and duration of the last highstand Lake in the Lahon- tan Basin. Quat. Res. 39, 163-174.

Benson L. V. (1994) Carbonate deposition, Pyramid Lake subbasin, Nevada: 1. Sequence of formation and elevational distribution of carbonate deposits (tufas). Palaeogeogr. Palaeoclimatol. Palaeo- ecol. 109, 55-87.

Benson L. V. and Paillet F. L. (1989) The use of total lake-surface are as an indicator of climatic change: Examples from the Lahon- tan Basin. Quat. Res. 32, 262-275.

Benson L. V. et al. ( t990) Chronology of expansion and contraction of four Great Basin lake systems during the pat 35,000 years. Palaeogeogr. Palaeoclimatol. Palaeoecol. 78, 241-286.

Bischoff J. L. and Fritzpatrick J. A. ( 1991 ) U-series dating of impure carbonates: An isochron technique using total-sample dissolution. Geochim. Cosmochim. Acta 55, 543-554.

Bond G. et al. (1992) Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period. Nature 360, 245-249.

Bourdon B. (1994) Mass spectrometric measurements of U-Th disequilibrium in young volcanics: implications for magmatic processes. Ph.D. dissertation, Columbia Univ.

Broecker W. S. (1994) Massive iceberg discharges as triggers for global climate change. Nature 372, 421-424.

Broecker W. S. and Kaufman A. (1965) Radiocarbon chronology of Lake Lahontan and Lake Bonneville lI, Great Basin. GSA Bull. 76, 537-566.

Broecker W. S. and Orr P. C. (1958) Radiocarbon chronology of Lake Lahontan and Lake Bonneville. GSA Bull. 69, 1009 1032.

Broecker W. S. and Walton A. (1959) The geochemistry of '4C in freshwater systems. Geochim. Cosmochim. Acta 16, 15-38.

Burr G. S., Edwards R. L., Donahue D. J., Druffel E. R. M., and Taylor F. W. (1992) Mass spectrometric J4C and U-Th measurements in corals. Radiocarbon 34, 611 618.

Chen J. H., Edwards R. L., and Wasserburg G. J. (1986) 23SU, 2~4U and 232Th in seawater. Earth and Planet. Sci. Lett. 80, 241-251.

Dansgaard W., Johnsen S. J., Clausen H. B., Dahl-Jensen D., Gun- destrup N., and Hammer C. U. (1984) North Atlantic oscillations revealed by deep Greenland ice cores. In Climate Processes and Climate Sensitivi~" (ed. J. E. Hansen and T. Takahashi ) ; Geophys. Monogr. 29, pp. 288-298.


Dorn R. I., Jull A. J. T., Donahue D. J,, Linick T. W., and Toolin L. J. (1990) Latest Pleistocene lake shorelines and glacial chronology in the Western Basin and Range Province, U.S.A.: insights from AMS radiocarbon dating of rock varnish and paleoclimatic implications. Palaeogeogr. Palaeoclimatol. Palaeoecol. 78, 3 1 5 - 331.

Edwards R. L., Chen J. H., and Wasserburg G. J. (1987) 238U-234U- 23eTh-232Th systematics and the precise measurement of time over the past 500,000 years. Earth Planet. Sci. Lett. 81, 175-192.

Edwards R. L. et al. (1993) A large drop in atmospheric ~4C/t2C and reduced melting in the Younger Dryas, documented with 23°Th ages of corals. Science 260, 962-968.

England J. G. et al. (1992) The Lamont-Doherty Geological Obser- vatory Isolab 54 isotope ratio mass spectrometer. Int. J. Mass Spect. and Ion Processes 121, 201-240.

Garrels R. M. and Mackenzie F. T. ( 1971 ) Evolution of sedimentary rocks. Norton.

Gascoyne M. (1982) Geochemistry of the actinides and their daugh- ters. In Uranium Series Disequilibrium Applications to Environ- mental Problems (ed. M. Ivanovich and R. S. Harmon) , pp. 3 3 - 52. Clarendon Press.

Hajdas I. et al. (1993) AMS radiocarbon dating and varve chronology of lake Soppensee: 6000 to 12000 ~4C years BP. Climate Dynamic 9, 107-116.

Harmon R. S. and Rosholt J. N. (1982) Igneous Rocks. In Uranium Series Disequilibrium Applications to Environmental Problems (ed. M. Ivanovich and R. S. Harmon) , pp. 145-164. Clarendon Press.

Ivanovich M. and Harmon R. S. eds. (1982) Uranium Series Dis- equilibrium: Applications to Environmental Problems. Clarendon Press.

Johnsen S. J. et al. (1992) Irregular glacial interstadials recorded in a new Greenland ice core. Nature 358, 311-313.

Kaufman A. ( 1993 ) An evaluation of several methods for determin- ing 23°Th/U ages in impure carbonates. Geochim. Cosmochim. Acta 57, 2303-2317.

Kaufman A. and Broecker W. S. (1965) Comparison of ~3°Th and ~4C ages for carbonate materials from Lake Lahontan and Bonneville. J. Geophys. Res. 70, 4039-4054.

Klein E. M., Langmuir C. H., and Staudigel H. ( 1991 ) Geochemistry of basalts from the Southeast Indian Rifge, 115°E- 138°E. J. Geo- phys. Res. 96, 2089-2107.

Kromer B. and Becker B. (1993) German oak and pine r4C calibration, 7200 to 9400 BC. Radiocarbon 35, 125-135.

Ku T. L. and Liang Z. C. (1984) The dating of impure carbonates with decay-series isotopes. Nucl. Instrum. Meth. Phys. Res. 223, 563-571.

LaFlamme B. D. and Murray J. W. (1987) Solid/solution interac- tion: The effect of carbonate alkalinity on adsorbed thorium. Geo- chim. Cosmochim. Acta 51, 243-250.

Lao Y. (1991) Transport and burial rates of ~°Be and :3'pa in the Pacific Ocean. Ph.D. dissertation, Columbia Univ.

Lao Y. and Benson L. (1988) Uranium-series age estimation and paleoclimatic significance of Pleistocene tufas from the Lake La- hontan Basin, California and Nevada. Quat. Res. 30, 165-176.

Ludwig K. R. (1994) I S O P L O T - - a plotting and regression program for radiogenic-isotope data. USGS Open-File Rept. 91-445.

Luo S. and Ku T. L. ( 1991 ) U-series isochron dating : a generalized method employing total sample dissolution. Geochim. Cos- mochim. Acta 55, 555-564.

Mazaud A., Laj C., Bard E., Arnold M., and Tric E. ( 1991 ) Geomag- netic control of ~4C production over the last 80 KY: implications for the radiocarbon time-scale. Geophys. Res. Lett. 18, 1885- 1888.

Osthols E., Bruno J., and Ingmar G. (1994) On the influence of carbonate on mineral dissolution: III. The solubility of microcrys- talline ThO2 in CO2-H,O media. Geochim. Cosmochim. Acta 58, 613-623.

Phillips F. M., Campbell A. R., Smith G. I., and Bischoff J. L. (1994) Intersdadial climatic cycles: A link between western North America and Greenland? Geology 22, 1115-1118.

Plank A. T. (1993) Mantle melting and crustal recycling in subduc- tion zones. Ph.D. dissertation, Columbia Univ.

Russel I. C. ( 1885 ) Quaternary History of Lake Lahontan, a Quater- nary Lake of Northern Nevada; USGS Monogr. 11.

Simpson H. J., Trier R. M., Li Y.-H., Anderson R. F., and Herczeg A. L. (1983) Field Experiment Determinations of Distribution Coefficients of Actinide Elements in Alkaline Lake Environments'; NUREG/CR-3940, U. S. Nuclear Regulatory Commission.

Stuiver M. and Kra R, (1986) Radiocarbon calibration volume. Ra- diocarbon 28, 805-1030.

Stuiver M. and Polach H. A. ( 1977 ) Reporting of ~4C data. Radiocar- bon 19, 355-363.

Stuiver M., Braziunas T. F., Becker B., and Kromer B. (1991) Climatic, solar, oceanic, and geomagnetic influences on Late-Gla- cial and Holocene atmospheric ~4C/~2C change. Quat. Res. 35, 1 - 24.

Taylor K. C. et al. (1993) The 'flickering switch of late Pleistocene climate change. Nature 361, 432-436 .

Taylor S. R. and McLennan S. M. (1985) The Continental Crust: Its Composition and Evolution (An Examination of the Geochemical Record Preserved in Sedimentary Rocks). Blackwell.

APPENDIX A

Mass spectrometry measurements of U-Th isotopes by VG Isolab 54 (also see more details in England et al., 1992; Bourdon, 1994)

Ionization Efficiency of Thorium by SIMS

The best ionization efficiency for thorium measurement achieved by the VG Isolab 54 at Lamont is approximately 1% in a high resolution mode (resolving power ~ 1200). But the ion yield of the thorium obtained by SIMS depends critically on the spread of the loading solution on the small platform of the graphite rod (with a diameter of ~300 #m). The ionization efficiency may vary if the sample solution is not well focused in the smallest possible area on the substrate.

Faraday-Daly Gain Calibration

The Daly detector of Isolab 54 is of the traditional ion-counting device with a polished and aluminized stainless steel knob, a fast plastic scintillator, and a photomultiplier tube. Measurements of 23°Th/232Th and 234U/23sU ratios involve simultaneous measurement of a high intensity beam on Faraday cup and a low intensity beam on Daly detector. Accordingly, the relative gain between Faraday and Daly detectors must be calibrated and monitored throughout the runs. This is achieved by a peak jumping routine with a uranium or thorium beam of 105-106 cps intensity, switching between Faraday bucket and Daly detector. Standard dead-time correction (nt,.,c = n ......... a/( 1 - n . . . . . . d*7-; where ~- is the dead-time correction) for the counting system was determined and applied to the ion yield of Daly detector. For individual runs, Faraday-Daly gain calibration is measured before and after each run. The isotopic measurements of each run are then recalculated by interpolating the isotopic ratios between two measured gain values. Normally the gain is stable to within 0.5% before starting isotopic measurements for the samples and between beginning and end of each run. An intensity dependence of Daly gain was also calibrated and corrected for beam intensities between 10 and 105 cps. Results are reported in Bourdon (1994).

SIMS and TIMS Inter ferences

Although the isobaric interference is the most serious problem for running thorium by SIMS, the impediment is largely eliminated by several designs of the VG Isolab 54 at Lamont. The interference is minimized by loading the sample solution on the pyrolitically coated ultra-high purity graphite rods, according to an experiment using various substrates. A metal source housing operating in 10 -~- 10 m mbar and a wien filter fitted to the primary beam stack to clean up the impure gases in the A r + primary beam (such as O +, N f , Ar~ etc.) both substantially reduce the isobaric interference to least extent. Higher resolving power of up to 1200, achieved by adding a source slit and a slit in the focal plane of the axial collector with selectable apertures and widths, was applied to decrease the

2832 J .C. Lin et al.

interference at mass 230. The adjustable a-baffles in front of the first ESA also has the effect of reducing the ion beam image when use the smaller slits.

The major source of secondary ion interference for mass 230 was found to be Zr~O3 and Zr2C4 for basalts (Bourdon, 1994), but for impure carbonates the level of interference is much lower and insignificant. For some samples the interference was easily eliminated by decreasing the acceleration voltage by 5 -10 V or the so-called energy filtering. Energy filtering applies the theory that the mon- atomic ion has a higher energy distribution than that of a large molecular ion which is determined to be the type of interference at mass 230.

For uranium measurements on TIMS, the major interference is the hydrocarbon at mass 233 and 234. As the hydrocarbon burnt off, the measured 233U/238U ratio will decrease to its true value. Regular clean up of the mass spectrometry source reduced the build up of hydrocarbons.

D a t a c o l l e c t i o n

Uranium was loaded with a graphite slurry on a single Re filament and ionized thermally. Usually with a 238U signal of 1-3 x 10 ~2 amps, 234U/238U ratio of the samples can be measured to an internal precision of 0.1% ( 2 a ) .

The 235U/238U ratio in the sample is used to correct for mass fractionation. This correction normally amounts to about 0.1% for per amu. The spike to normal isotopic ratio is calculated by the following equation:

/ (23~U~ \

k P

Standard isotope dilution calculations were used to estimate the uranium concentration of samples, taking into account the contribution of isotopes form both the spike and the sample.

The mass fractionation was not corrected for thorium, but the mass fractionation of thorium was estimated using an empirical expression given by Benninghoven et al. (1987):

f = (Mheavy/Mlight) 2m, (A2)

where m is an empirical constant and its value is less than 0.25 for the primary ion energy range used for SIMS. When m = 0.2, the fractionation for 230Th and 232Th is about 3.5%0. Isotopic measurements of thorium appear to be reproducible but a limit on the external precision for running thorium by SIMS is thus set by the uncorrected mass fractionation.

Uranium and thorium isotope standards were measured regularly every two to three samples. The reproducibility of thorium and uranium isotopic measurements are about 5 and 4%0 (2a) , respectively. The data reported in Table 3 have taken into account the external reproducibility of both uranium and thorium.

A P P E N D I X B

Line fitting of the isochrons (also see more details in Ludwig, 1994)

The Version 2.71 Isoplot program of Ludwig (1994) normally attempts to first fit the points assuming that the only cause for scatter from a straight line is the assigned errors and results in the Model 1 fit. Model 1 fit provides two lcr uncertainties of the slope and

intercept, i.e., uncertainties propagated from assigned errors (a pri- ori) and uncertainties calculated from observed scatter from the line.

When the probability of the assumption (that the scatter is caused only by assigned errors) in the model is low, the program will offer a Model 2 fit where the program weights each point equally with zero-error correlation. Model 2 fit only produces the error calculated from observed scatter from the straight line.

The principles used in this paper for choosing these various regression results are:

1 ) If the program offers a choice of model results, the Model 2 result is chosen (eg., Group 4 and mass spectrometry data of Group 5 ).

2) All alpha-counting results choose the l a errors propagated from the assigned errors of the points. Since for most of the groups analysed by alpha-counting, the analytical uncertainties are obvi- ously the major source of the scatter (except Group 4).

A P P E N D I X C

Radiocarbon age bias model with continuous addition of modern carbon

d N - k N + a , ( C 1 )

dt

where

N = total no. of ~4C atom in the sample; a = no. of ~4C atom added/y; k = decay constant of 14C ( 1/y) ; t = in years.

Solve Eqn. AI by multiplying both sides with an integrating factor, e xt, and integrate. The solution for Eqn. A1 is:

N - Noe -~'t a = k (C2)

l - - e k t ,

where No = N a t t = 0. Assume

R = 14C/12C atomic ratio in the atmosphere of anytime: Co = total no. of tzc atom in the sample at t = 0; C = total no. of ~2C atom in the sample at time t; x = a / R , no. of ~2C atom added/y; x -- = molar fraction of secondary carbon; C

t , t ' = true age, apparent age (year); At = t - t ' , age bias (year);

Eqn. A2 can be rewritten as the following:

x R k R C e ~" - R C o e x~ - ( C 3 )

C C 1 - e -x~

Canceling R on both sides and rearranging yields:

e ~ , Co x C - - = X - ( C 4 ) C e ~'' - 1

Further rearranging Eqn. C2 by replacing Co with C - xt , will lead to a solution of x / C :

X e x ~ t - 1 - - = X ( C 5 ) C e ~' - l - kt"

new 23°th/u and t4c ages from lake lahontan carbonates ... · group 2 - intermediate stand tufa...

Documents