New 23°Th/U and t4c ages from Lake Lahontan carbonates, Nevada

Geochimica et Cosmochimica Acta, Vol. 60, No. 15, pp. 2817-2832, 1996
Copyright © 1996 Elsevier Science Ltd
Printed in the USA. All rights reserved
0016-7037/96 $15.00 + .00
Pergamon
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. BONANI2
~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 A M S 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. W e 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. W e 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.
Abrupt lake-level fluctuations in western Great Basin of similar 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 coralbased 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 Pleistocene lake water. This correction would increase the discrepancy 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
1. INTRODUCTION
Since the investigation of Russell (1885), the dramatic lake
level fluctuations of Lake Lahontan during the late Pleistocene 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 dominant 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 forcing 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 California, Berkeley, CA 94720, USA.
2817
J. C. Lin et al.
2818
121°
120°
119°
!
I
OREOON
42" L
118°
t
116°
117 °
I
I
11_
IDAHO
NEV~A
I
I
BLACK-RI
PLAYA
41'
I
SINK
40°
LAKE
~,,,,,
Marble
39°
100 KM
!
LAKE
TAHOE
LEGEND
•
\
38~
•
LAKE
Present-day L a k e s
~
%
%
\
Pleistocene Lahontan
Lake Area
- - - Major Sills
1
2
3
4
I
I
Astor Pass
Emerson
Darwin
Adrian
I
I
FI(;. I. Geography of Lahontan Lake (revised from Benson et al., 1990).
n o n z e r o initial slope for >"Th/-~3eTh vs. 234U/232Th plot due
to the m i n e r a l o g i c a l i n c o r p o r a t i o n o f dissolved t h o r i u m into
p u r e c a r b o n a t e p h a s e m a y also p r o d u c e 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 Holocene 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-Th
Isochron
Plots
Total sample dissolution (TSD) was employed for the U-Th isochron dating in this study. This technique avoids any nonreproducibility 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. 234W/ 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 234Ufrom detrital
phases can be neglected). In this case, the isochron equation can be
written as:
232Th
- \ ~ / [
~,,'+
~
U ]~ 232Th
,
(I)
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 normalization 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
2819
Table 1. Sample descriptions and radiocarbon a~es
Sample ID
Location
Material
Pleistocene samples
Group 1 - Intermediate Stand Tufa (17K)
BR84-7
B lack Rock
tufa
Desert
PL18
Terrace Hill 1
tufa
Altitude
(rn)
14C age*
(years)
1245
17,360+-260
1267
16,980+_250
Group 2 - Intermediate Stand Tufa (19K)
PL17
Terrace HillI '
tufa
PL44alx:
Terrace Hill
tufa
PL44d
Terrace Hill
tufa
1260
1260
1260
19,040i-_320
20,650-2_390
19,980-2_360
Group 3 - Astor Pass and Truckee Canyon Marl
L-364CQA(1)
Astor Pass I
gastropod
L-364CQB(1)
Astor Pass
chara
L-364CQB(2)
Astor Pass
chara
L-772Q
Truckee Canyon
ostraeodes
1210
1210
1210
1219
16,500-2_300
16,800!-_400
16,800i-_400
16,800L-_500
Group 4 - Marble Bluff Tufa 1
JL90-1
Marble Bluff
JL90-2
Marble Bluff
JL90-3
Marble Bluff
tufa
tufa
tufa
1245
1245
1245
13,920-2_220
14,500-2-420
15,540+--200
Group 5 - Highstand Tufa
JL90-5
Marble Bluff
JL90-6
Marble Bluff
JL90-15
Terrace Hill
JL90-16
Terrace Hill
JL90-17-1
Terrace Hill
JL90-17-2
Terrace Hill
JL90-17-3
Terrace Hill
JL90-18-1
Terrace Hill
JL90-18-2
Terrace Hill
JL90-18-3
Terrace Hill
JL90-18-4
Terrace Hill
JL90-18-5
Terrace Hill
tufa
tufa
tufa
tufa
tufa
tufa
tufa
tufa
tufa
tufa
tufa
tufa
1330
1330
1330
1329
1328
1328
1328
1326-1328
1326-1328
1326-1328
1326-1328
1326-1328
13,060!-_100
13,15ffL-_100
12,200-2_100
12,850~_110
12,320-2-_100
12,200-2:100
12,980i-_100
13,150i-_100
12,690-2_100
12,600-2_ 95
12,790"2-110
12,260-2-_100
Reference
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
----* : ~4Cages 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
calculated from the slope using the equation of Kaufman and
Broecker ( 1965 ) :
23(~rh *
234U -
1
234U/238 U ( 1 -
xo,)
e
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 METHODS
The new radiocarbon dates presented here were measured by accelerator mass spectrometry ( A M S ) 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.
T a b l e 2. A l p h a spectrometry results (isotopic ratios are activity ratios and the uncertainties quoted are 10 errors)
Sample ID
Residue
%
Samples
2.53
0.60
0.57
0.50
0.37
0.34
0.31
3.0
1.11
1.05
1.11
1.54
1.51
1.21
1.22
0.26
0.27
0.26
0.24
0.24
1.27
1.28
1.2
1.15
0.94
-0.70
0.55
0.45
-0.36
0.31
0.4
0.3
--
I
2
I.,-364CQB(1)2
L-364CQB(2)2
L-772Q 2
Group 4
JLgO-I-I
JL90-1-2
JL90-1-3
JLg0-1-4
JL90-1-5
JL90-1--6
JL90-2- la
JL90-2- lb
JL90-2-2
JL90-2-3
JL90-2--4
JLgO-3-1a
JL90-3-1b
JL90-3-2
JL90-3-3
JL90-3-4
JL90-3-5
JL90-3-6
1.20
1.62
1.53
1.24
1.36
1.30
--
Group 2
PLI7a
PLI7b
PL17c
PL44d- 11
PL44d-21
PL44abcl t
PI,A4abcII1
Group 3
L-364CQA(1)
Th
ThRN
•
2340]238 U
230Th/234U
230Th/232T h
234Uf232Th
232Th/238U
Pxy t'
ppm
of 5 isochrons
Group 1
BR84-7a
BR84-Tb
BR84-7c
PLISa
PLlgb
PL18c
PLA4abclH
U
ppm ppm
---
--3.22
5.00
5.28
4.81
2.08
1.14
2.87
-7.20
6.08
4.33
4.80
--
3.56
3.91
4.93
2.24
--
1.63
0.07
2.65
2.51
3.95
0.65
0.46
5.32
1.13
1.40
1.42
0.94
1.18
1.06
4.07
4.10
3.40
3.94
2.74
9.45
9.63
6.59
8.20
8.95
6.67
9.06
0.89
0.87
0.81
0.61
0.57
0.26
0.94
1.02
1.05
1.05
1.00
0.76
0.93
1.08
1.11
0.92
0.97
1.02
24
--53
62
69
- -
75
84
60
80
-43
78
28
17
15
13
27
23
33
-15
17
23
16
- -
30
28
19
43
1.40-20.03
1.40-+0.02
1.43::t:0.03
1.41-+0.02
1.471-0.02
1.46-+0.03
0.334i-0.009
0.297-+0.008
0.286:~.008
0.276-+0.007
0.254-+0.006
0.259~0.007
2.85:£-0.06
3.54+--0.I 1
3.77i-0.09
3.91:£-0.10
4.57i-0.13
4.89-i"0.15
8.53:£-0.25
11.9+-'0.4
13.1:£-0.4
14.1:£-0.5
18.0i-0.6
18.9i-0.7
0.164i-0.005
0. I 17i-0.004
0.109-1-0,004
0.100-1"0.003
0.082__+0.003
0.077:L-0.003
0.51
0.67
0.56
0.63
0.66
0.66
1.41:~.02
1.44-+0.04
1.46+0.04
1.43-+0.03
1.46:~0.05
1.35i-0.05
1.40-+0.07
1.45-+0.12
0.254:t:0.008
0.262.+.0.009
0.234-+0.008
0.221:-.-.~.009
0.215:£-0.012
0.561-+0,031
0.532+0.027
0.455+0.049
4.57:i-0.20
4.49-3:0.16
4.34:£-0.16
6.36-+0.71
6.09-+0.83
2.27:£-0.20
2.29+0.15
2.47L-0.45
18.0i-0.8
17.21-0.7
I 8.5i-0.8
28.6-+3.3
27.8+3.8
4.05i-0.31
4.17-+0.26
5.52__+0.88
0.078i-0.004
0.0g4i-0.01M
0.079i-0.004
0.050i-0,006
0.053-+0.007
0.334-+0.026
0.337:t:0.021
0.263:N).042
0.76
0.66
0.65
0.94
0.92
0.78
0.69
0.80
1.43i-0.03
1.40-3:0.04
1.45+0.05
1,35+0.03
0.187:~0.015
0.25 li-0.010
0.241:t:0.010
0,630-t:0.020
18.7:.~.8
4.33:t:0.25
5.74-+0.43
1.91i-0.12
100-/30
0.014i-0.01M
17.2+--0.9 0.081+0.004
23.8:t:1.7
0.06 li-O.01M
3.03-+0.18 0.446:t:0.027
0.97
0.75
0.84
0.87
1.44_+0.05
1.41i-0.05
1.46+0.04
1.46i-0.05
1.43i-0.05
1.40"2:0.05
1.48+0.03
1.44+-0.02
1.42_+0.03
1.52i-0.02
1.49:t:0.03
1.38+0.03
1.43+0.01
1.47i"0.03
1.44:£-0.02
1.41-+0.02
1.46-+0.02
1.41-+0.01
0.368+0.015
0.304::1:0.012
0.292_+0.010
0.343+0,013
0.278-+0.010
0,229-2:0.009
0.217:t:0.007
0.200-+0.006
0.244+0.008
0.223+0,006
0.261-+0.007
0.177i'0.006
0.185+0.005
0.212i-0.007
0.196:£'0.005
0.197-+0.007
0.214-+0.007
0.201__+0.006
2.04i'0.06
2.10-2:0.06
2.27i'0.07
2.34+0.08
2.48:t:0.05
3.98+0. l I
4.21+0.10
4.11+0.08
3.723:0.09
3.83+0.08
3.23-20.05
9.20"2-0.22
8.63-+0.22
5.77+0.12
6.43-+0.14
8.12,t:0.27
6.44+0.22
7.63__+0.23
5.55.-.-:-0.24
6.90-2:0.29
7.78+0.31
6.82._+0.30
8.94+--0.34
17.4i-0.7
19.4_+0.7
18.6_-4-0.6
15.2-+0.5
17.2_+0.5
12.4!-0.3
52.0"22.1
46.7+1.7
27.3-+1.1
32.9-+1.1
41.2-+1.9
30.1-1-1.3
37.9-+1.5
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
0.259+--0.011
0.204-+0.009
0.188i-0.008
0.214-+0.010
0.159-+0.006
0.080"20.004
0.0765.-0.003
0.078i"0.003
0.0945.-0.003
0.088:£-0,003
0,121-+0.003
0,027:L'0.001
0.032_+0.001
0.054:£-'0.002
0.044-+0.002
0.034-+0.002
0.048i--0.002
0.037!-0.002
Group 5
JL90-5-1
0.90
0.97
0.20
22
1.48+0.03
0.202+0,007
4.30-+0.23
21.3+1.2
0.071-+'0.004 0.79
JL90-5-2
0.72
0.88
0.21
29
1.56!-0.03
0.206+0.006
4.12_+0.14
20.04--0.8 0.078_'2-0.003 0.69
JL90-6
1.68
0.90
0.47
28
1.46+0.03
0.319+_0.010 2.70-2:0.10 8.48i-0.33
0.17~.007
0.64
JLgO-15-1
0.92
0.94
0.26
28
1.49-1:0.04 0.232+0.007
3.82+0.12
16.4+--0.6 0.094-+0.004 0.66
JL90-15-2
0.86
0.98
0.38
44
1.44-I-0.03 0.264+0.008
2.98+0.10
11.3i'0.4
0.128i-0.005
0.65
JL90-16
0.37
0.84
0.22
60
1.52:t:0.04 0.223i-0.006
4.08"1-0.13
1 8 . 3 - + 0 . 7 0.086-+0.003 0.68
JL90-17-1
0.86
0.95
0.29
34
1.51-2-0.02 0.244:~.006
3.61:t:0.12
14.8i-0.5
0.101_+0.004 0.71
JL90-17-2
2.46
1.12 0.36
15
1.51+0.03
0.239-1:0.007 3.37::~.12
14.1i-0.5
0.107i'0.004
0.68
JL90-17-3
2.22
0.94
0.34
15
1.52.~.03
0.254::~.008
3.21+0.12
12.6i-0.5
0.121:~0.005 0.68
JLg0-18-1
1.98
1.03 0.32
16
1.47i'0.lM
0.244i'0.008
3.49-2-0.10
14.31"O.6 0.103:'20.004 0.56
JLgo-Ig-2
0.82
0.97
0.30
37
1.48-+'0.03 0.254i-0,008
3.66i-0.12
14.4i-0.6
0.103i-0.004
0.66
JL90-18-3
1.25
0.98
0.30
24
1.53:£'0.02 0.2421-0.006
3.57i'0.10
14.8:i-0.5 0.104:£-0.004 0.66
JL90-18-4
0.99
0.96
0.35
35
1.51+--0.02 0.262i'0.007
3.31+-'0.10
1 2 . 6 - + 0 . 4 0.120-1-0.004 0.64
JLg0-18-5
1.58
0.97
0.34
22
1.48+0.03
0.264!'0.008
3.43i'0.10
13.0-1-0.5 0.114~N3.004 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
H o l o c e n e
JL052501-2
JL052502
JL052601
Residue
U
%
ppm
samples
----
1.35
2.20
2.80
Th
ppm
0.24
0.17
7.44
ThI~N"
ppm
----
234U/23s U
1.45+0.02
1.46+0.02
1.01_-+0.03
procedure for U and Th separation is similar to that of E d w a r d s
et al. ( 1 9 8 7 ) . A f t e r coprecipitation, Fe hydroxide precipitate was
redissolved in 7 N HNO3 and loaded on a 400 # L a n i o n - e x c h a n g e
c o l u m n ( A G 1 × 8 r e s i n ) . T h o r i u m was eluted with 6 N HCI and
230Thf234U
0.083::1:0.003
0.039-1:0.001
1.011:N3.038
230Th/232Th 234Uf132Th
2.09-~0.08
2.2?+0.08
1.16:f0.03
232Thf138U
Pxy ?
25.2+1.0
0.058:t0.002
56.7+9.1
0.026"£'0.001
1.15-+0.04 0.878-+0.032
U with 1 N HBr. T h o r i u m and u r a n i u m fractions each went through
a second a n i o n - e x c h a n g e c o l u m n ( 100 # L ) with an identical elution
s c h e m e as for the first column. A blank w a s processed with each
batch o f samples, and was usually less than I ng for u r a n i u m 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
JL90-5
JL90-5P
.1L90-6
JL90-6-1
JL90-6-2
JL90-6P
JL90-15
IL90-16
JL90-16S
JL90-17
JL90-18
U
(ppm)
0.942
0.932
0.914
0.996
1.382
0.875
0.985
0.919
0.899
1.094
0.977
Th
(ppm)
0.202
0.193
0.478
0.444
0.577
0.461
0.355
0.199
0.199
0.393
0.285
234U/238U
1.457+0.011
1.493+0.004
1.449+0.008
1.445+0.005
1.394+0.004
1.457+0.005
1.496+0.004
1.493+0.011
1.507+0.006
1.523+0.004
1.542-+0.005
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 Lamont 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).
230Th/232Th
234U1232Th
4.58+0.018
4.70-L-_0.084
2.77+0.01
3.00-+0.011
2.71+0.014
2.61-+0.01
3.222-+0.009
4.514+0.013
4.181+0.016
3.498+0.01
3.806+0.011
20.63_+0.37
21.87_+0.22
8.40-+0.06
9.82,+.0.11
10.11-+0.32
8.39+0.06
12.59-+0.07
20.95-+0.18
20.62_-4-0.16
12.86-+0.08
16.03+0.10
232Th./238U
0.071+0.0014
0.068+0.0007
0.179+0.0016
0.147+0.0018
0.138+0.0043
0.174+0.0014
0.119-+0.0008
0.071+0.0008
0.073-+0.0006
0.118-+0.0008
0.096+0.0006
mally c o m e s f r o m m e a s u r e m e n t o f the long half-life 232Th by
a l p h a - c o u n t i n g . E r r o r - c o r r e l a t i o n s (p~,. in Table 2; L u d w i g ,
1994) w e r e calculated and applied in the line fitting routine
for c o n s t r u c t i n g 234U/232Th vs. 23°Th/232Th isochron. Principles o f d e t e r m i n i n g different m o d e l s o f line fitting results
o f f e r e d by Isoplot p r o g r a m are listed in A p p e n d i x B.
Authigenic 234U/238U values o b t a i n e d f r o m the y - i n t e r c e p t s o f 234U/238U vs. 232Th/238U plots are s h o w n in Fig. 2.
A m i x i n g trend b e t w e e n a c a r b o n a t e e n d m e m b e r ( a u t h i g e n i c
phase; 234U/238U ~ 1.5, 232Th/238U = 0 ) and a detritus endm e m b e r (23~U/23su ~-, 1, 232Th/23sU ~ 1.3) m a y exist, but
the scatter is large. T h e data points cluster near the authigenic
e n d m e m b e r (Fig. 2) due to the very high c a r b o n a t e c o n t e n t
o f the samples.
l s o c h r o n s in Fig. 3 all s h o w g o o d linear correlation. Exc e p t for G r o u p 5, the M S W D ( m e a n square w e i g h t e d deviation) o f all a - c o u n t i n g results are l o w e r than 2 (Fig. 3). W e
regard this as e v i d e n c e that any diagenetic loss or gain o f
u r a n i u m or t h o r i u m is within the m e a s u r e m e n t limits o f alpha
s p e c t r o m e t r y . T h e t w o sets o f data for G r o u p 5 s a m p l e s
o v e r l a p within analytical errors (Fig. 3, the l o w e r panel on
the r i g h t ) . H i g h e r value o f M . S . W , D . o f the G r o u p 5 isoc h r o n m a i n l y s t e m s f r o m the h i g h e r scatter o f m a s s spect r o m e t r y data. M o r e p r o n o u n c e d scatter for m a s s s p e c t r o m e -
1.7
1.6 ~ .x
•
G(1),I=l.S2.-t:0.04
l- ~
n
•
G(2),I=1.44:t'0.02
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
4. RESULTS
1.2
T h e u n c o r r e c t e d r a d i o c a r b o n ages for all the s a m p l e s are
listed in Table 1. U r a n i u m and t h o r i u m alpha c o u n t i n g results
are s h o w n in Table 2. M a s s s p e c t r o m e t r y U - T h results o f
the last h i g h s t a n d s a m p l e s ( G r o u p 5) are listed in Table 3.
All i s o c h r o n plots are s h o w n in Fig. 3. T h e least-squares
linear r e g r e s s i o n was d o n e on the v e r s i o n 2.71 o f I s o p l o t
p r o g r a m ( L u d w i g , 1994). T h e partially correlated errors o f
234U/232Th and 23°Th/Z32Th values are b e c a u s e a large source
o f analytical errors for 234U/232Th and 2:~°Th/232Th ratios nor-
1.1
1
0.00
0.20
0.40
0.60
0.80
232 T h / 2 3 s
1.00
1.20
1.40
U
FIG. 2. 234U/238U vs, 232Th]238Uplot 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 containing 234U]238U ,~ 1.5 and 232Th/238U = 0.
2822
J.C. Linet al.
6
•
"
45
'
'
I
"
'
'
'
Group 1
I
'
"
'
'
"
I
'
'
'
'
I
'
'
'
~
. . . .
8
#6
[3
[-
/
10
'
MSWD = 0.39
,
. . . .
,
J
. . . .
,
MSWD = 1.76
2
l
2
Isoehronage:22,500+l,200 years BP
',alendar Cor.14C age:20,200+200 years B]
0 0
8
| | 00 I | | | |
5
'
''
I"
'
15i . . . .
234 U/232 Th
''
,'
ll0 . . . .
'''
I
''"
'I
'
''
20a, ,
'I
'
''
'
I'
| '
.
0
'25
'
f
lsochron age:19,300+_400 years BP
2alendar Cor.14C age:17,600_+820 years BI
..I
...
10
.I.
20
...
I..
..I
'I'
.I
30
40
234 U/232 Th
50
. . . .
60
6
''
7
•
5
¢.6
#
5
MSdata
I
4
3
[-4
3
2
2
:
Isochronage:19,100_+l,00Oyears BP
3
1 ?
~alendar Cot I.14
4C age:23,000_+760 years BP~
•
0
..I
....
5
25
' ' 1
•
20
l . . . . l . . . . l
10
. . . . . .
I
c~m
I
'
''
i
'
'
/
'if'
30
'
'
I
1
35
0
0
5
10
15
2a4 U/232 Th
20
25
~J I¢
I /
*
Group 3
I,,,,I.,,,
....
15
20
25
~4 U/z32 Th
isoehron~ age:16~400_+700years Bp 1
1 •.
Calendar Cor.14C age:14,900+350 years Bill]
l
w
/,-g
o
10
---0.37
~e-20,900+l,ll~)
years BP
.qal.e~d.ar .qor.pC, a.g.e:29,00%+~sp,y.epr.s.
0
20
40
60
80
100
234 U/2a2 Th
120
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 m u c h 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~°Thisochron 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 ( 1 9 5 9 ) 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 ( y e a r s B P )
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 (calculation 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 m o d e m
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 contam±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 ). Furthermore, 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
m o d e m carbon contamination, the oldest 14C ages should
Table 4. Comparison of 14C and U-Th isochron ages (1~ uncertainties quoted)
Group
1
2
3
4
5(or+MS)
Altitude
(m)
Uncorrected
14C
Age (yrs BP)
Calendar
Cor. 14C
A~e (~rs BP)1
230Th
Isochron
Age (yrsBP)
A Age
(yrs)2
1250-1267
1260
1210-1219
1245
17,2005:270 20,200-~70 22,500+1,200 2,300±1,470
19,900-3:800 23,000~:800 19,100±1,000 - 3,900~1,800
16,650+_550 20,00(050
20,900-~1,100
900 ±1,650
14,650"2820 17,600~820 19,300-J:400
1,700±1,220
1326-1330
12,690~350
14,900-~50
16,400"3:700
1,500 ±1,050
I Sample 14Cages are convertedto calendarages using the relationshipbetween 14Cage and
calendarage in Bard et a1.(1992).
2 Defined as 2~"fh - CalendarCor. 14Cwhich presents the differencebetween the mean 2~Thisochron age and the calendar-corrected14Cage (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
Fraction of
Secondary CaCO a
0.2
0.2
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 accumulated 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
Table 5. Effect of leaching on ~4C ages
Sample ID Material
Sample
Location
JLg0-18-3
tufa
PyramidLake
C2
shell
Searles Lake
carbonate samples, one detrital and the other from the dissolved pool in the lake water. We will hereafter refer to the
dissolved thorium as " h y d r o g e n o u s " .
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 representative 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. Accordingly, 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 Kaufman 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
t4C Age
(yrs B.P.)
12,600-k-95
12,720-k95
12,830-L-95
11,360t--:-100
12,070-L-_100
Leaching Experiment
(by 10% HCI)
10% leached
20% leached
80% leached
leachate (~33%)
residue (-67%)
Dating of lacustrine carbonates
i l:I
m
w m , = l . ~ hlShu~
mt'u (Bmson, 1993)
6
t
d
z
Age for Pyramid Lake
hlghstandtufu tusge.st~l
bYU-TItimchronage
Or$~aictmtt~ bemath
rockvm'nl~(Dora et M.
1990and peas. com.)
2
0
12
l~ramidLalte
htl~tand tufu
13
14
1
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 McLennan, 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 . 8 0 - 0 . 8 9 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 nondetrital source of initial thorium.
W e 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/232Thratios). 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/232Thratios of the dissolved thorium (Table 8;
Simpson et al., 1984). Thorium content in m o d e m 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 Murray, 1987). W e 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/232Thactivity 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 - T h 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
2825
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
ll
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.
2826
J.C. Lin et al.
Table 7. ~I'h/732Th activity ratios of the isochron samples, Holocene
lake deposits and sediments from the three alkaline lakes.
ment or some device. Isochron age biases will be the main
focus of the following section.
I. Isochron samples
Group
Isochron Age
(yr BP)
1
2
3
4
5(ct + MS)
22,600+700
19,000-L-900
20,900-i--400
20,100+500
16,400+700
Observed
Initial l
230Th/232Th 230Th/232Th
1.24+0.07
1.57+0.09
1.37+0.03
1.03+0.06
1.48+0.03
Average
1.54_+0.15
1.86i-0.14
1.66:L-0.13
1.34_+0.10
1.72i-0.10
1.62_-)-0.20
I1. Holoeene samples
JL052501-2
JL052502
Res. Cor. 14C
Age (yr BP)
1890
1940
Measured
Initial
23°Th/232Th 23°Th/232Th
2.09+0.08
1.65+0.082
2.22+0.08
1.25+0.082
III. Lake sediments
MonoLake3
WalkerLake3
PyramidLake3
JL052601 (PyramidL.)4
1 230Th;init - 230]'h',obsett, and
232Th, - 232Th.,
230Th",obs = interceptof isochron
232Th
Measured
23OTh/'232Th
1.58+0.09
1.11 +0.05
1.17+0.06
1.16+0.03
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 ratios for aluminosilicate detritus. If the detrital Th is assumed
to have a 23°Th/232Th activity ratio of 1.2, a simple calculation shows that the 23°Th/232Th ratio of the Pleistocene hydrogenous thorium m i g h t have ranged from 2.06 to 2.38
(using the average detritus-normalized thorium content of
21 p p m from Table 6 and hypothesized detritus with 1 0 - 1 3
p p m of thorium.
W e freely admit that there are still uncertainties in the
above argument about the presence of hydrogenous thorium
in the Lake Lahontan carbonates. However, the goal is to
improve our understanding 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 b e c o m e important as improved precision is obtained through more precise isotopic measure-
5.3. Evaluation 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
thorium components, as variable mixtures of initial components in certain ways may produce a bias in the isochron
slope. Two m e c h a n i s m s can be envisioned 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 thorium is incorporated directly into the carbonates. If hydrogenous thorium is adsorbed by detrital particles with a constant partition coefficient ( l q ) between water and particles, and if detritus all has
the same thorium content, then the ratio of hydrogenous
thorium to detrital thorium will be the same a m o n g 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 = T h ~d
.
.
. where
.
d~ is the hydrogehydro./Thaetr,
Th hydro,
nous thorium adsorbed by detritus, Fig. 7a). In contrast, if
the uptake of hydrogenous thorium is by direct inclusion of
dissolved thorium into the carbonate crystal structure, hence,
correlated with that of hydrogenous uranium, the zero-age
isochron will have a positive 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. 7 b ) . Isochrons constructed from real samples
can only produce initial 23°Th/232Th ratios (age-corrected yintercept) greater than detrital 23°Th/232Th ratios if at least
part of the hydrogenous thorium was incorporated by adsorption onto detritus, as demonstrated 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 demonstrated in Fig.
7a,b suggest that part of the hydrogenous thorium must be
in the form of adsorbed c o m p o n e n t 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
Z32Th
Column
234U/238U 230Th/232Th
Depth (m)
(dpm/l)
(dpmfl)
Mono Lake
1
360+44
1.66+0.16
30
404+ 13
1.50+0.13
Walker Lake
1
126 + 4
..........
25
115+4
0.0115+0.0004
Pyramid Lake
1.3
13.5 + 0.7 0.0071 + 0.0004
75
1.7 + 0.2 0.0049 + 0.0004
Seawater*
690
2.39-!0.005 3.57_+0.10xl0 -5
* Unfiltered seawater from Atlantic Ocean (Chen et al., 1986)
1.51+0.19
3.50+0.71
1.16+0.05
2.26+0.23
1.34 +__0.06
............
1.30+0.06
1.56+0.08
1.97+ 0.13
1.28 + 0.10
1.40+ 0.17
1.36 + 0.08
........................
Dating of lacustrine carbonates
(*)
(b)
1:1 mixing(k =1)
~
_ _
I
(k
{
.
2827
= ~
.
.
234U/~"l'h
(c)
=~
~.3,tO/232'l'h
(2)
dettitus adsorbed
drogenous Th
.
.
.
.
.
~ ) ~ .
.
.
r-~a~a.
(3) pure carbonates
with hydrogenous
~
=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 < ~ 3 0 0 0 years):
assuming that hydrogenous thorium consists of two components, one adsorbed to detritus with a constant partition coefficient (k* 232Thd~t,.), and the other incorporated directly into
carbonates in constant proportion to authigenic 234U by a
factor " a " ; i.e.:
232Thhydro. = k*232Thdetr. +
,234
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-
A 23°Th / 234U
(23°Th/232Thhydro. - 23°Th/232Thdetr.)*a/(k + 1),
(3)
Where " a " = 232Th/234U in the pure carbonate phase and
" k " = ThhydroJThd
ads.
..... Three unknowns, 23°Th/232Thhyd....
" a " , and " k " (assuming known 23°Th/232Thdetr,), are involved in calculating the A23°Th/234U, hence A t 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 d p m / g ) :
232Thtotal = 232Thd~tr. + 232Thhydro.
= 232Thdetr. -}- k*232Thden.. + a*234Utotal, (4)
2500
2000
•
'
'
'
I
'
•
'
'
I
*
"
'
'
|
23°T~rhhY~' "2~rn/~Tn~" '
'
=
'
'
'
,
i
~
~ 1500
"-" 1000
/
500
i
..-
.-o
00
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.
2828
J.C. Lin et al.
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., 1.17 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/234Uvalue 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 232Thdetr" . 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°Thage 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/232Thratios. 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 232This 55% of the total 232Thin these samples
(Table 9). Assuming that modem 232Th/234U activity ratios
of Pyramid Lake water (0.0003-0.002; Table 8) are representative of Pleistocene conditions, the 232Th/234U ratio of
the pure carbonate phases of Group 1 samples (0.041) implies an enrichment of the Z32Th/234Uratio 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 UTh 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
Dating of lacustrine carbonates
(a)
10
8
' ' i , ' "
Group 1
a = 0.04123
(c) 15
I ' " " I " " " I " ~ i l ' ' " I" "
Detr•
10 ppm
At = 2204 years
•
•
2829
•
'
I
Group
'
•
•
•
I
1
'
a = 0.04101
k = 0.758
•
'
"
I
'
•
"
'
Detr. Th - 7 ppm
At = 1066 years
k = 1.509
lO
6
•(
.='
(
2
~
r
~
2
y = 2.51 + 0.04101x
y = 1.758 + 0.04123x R= 0.99
i
i
I
.
•
l
I
20
.
•
40
,
I
.
•
60
234
•
I
•
'I
l ' ' l
Group
7
•
I
•
.
100
•
140
' ' I
5
" ' "
''
(d)
' I "
A~
6
5
4
2
•o 1
. . , i
20
0
, . o
40
i•
, o 1 •
I
• • 1
. • •
i , .
60
80 100 120
z34 Utet" /232 Thdetr"
•
•
•
•
I
•
.
•
R= 0.99
•
100
I
•
.
•
•
150
200
8
7
32
. • 1
•
50
•1o
140
'Grollp 4 . . . .
' " D'etr,T'h ~ "7 ppm"
a=001504
At = 216 years
k = 1129
•
(23°Th/232Th)hy<~o.= 1.42
•
~ 3
0
•
234 Utot" ]23;7, T h d e t r '
Detr.Th =' 10 ppm
4
•
.
Thdet r
= ' I "
•
o
•
120
1232
I"
•
a = 0.01472
At = 450 years
k -- 0.61
( Tl~ Th)~o. = 1.62
230
~"
' ' '
,
80
Utot.
8
,
o
|
16o
I
I,.
,
•
o
•
y = 2.29 + 0.01504x R= 0.67
•
I
.
•
50
.
•
I
,
•
|
lOO
234
•
I
•
•
,
•
200
15o
/232
Utot.
Thdetr.
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 ( A t ) derived from the computed slope of the zeroage isochron are also derived, as described in the text. Derivations are repeated in (c) and (d) using a detrital 232Th
concentration of 7 ppm.
L a k e L a h o n t a n c a r b o n a t e s is t w o f o l d : ( 1 ) h i g h t h o r i u m conc e n t r a t i o n s are e s t i m a t e d for the detritus in t h e s e lacustrine
c a r b o n a t e s a n d ( 2 ) an initial 23°Th/Z32Th activity ratio averaging 1.65 is o b t a i n e d for t h e s e s a m p l e s f r o m their U - T h
(a) 2.4
i s o c h r o n s , m u c h greater than typical detrital ratios o f 0,8
+_ 0.2. B a s e d on a r g u m e n t s p r e s e n t e d in this paper, the
ratio o f the h y d r o g e n o u s t h o r i u m in the Pleistoc e n e L a k e L a h o n t a n was p o s s i b l y in the r a n g e o f 2 to 2.1,
2 3 ° T h / 2 3 2 T h
Co)
, . , , ,
2.3
i , , , , i
Group
. , , ,
i , . , ,
1
i,
w,,
i , , , ,
2.2~,...
i , ,
I , , ' ' 1 " ,
' ' 1 ' ' , ,
2.1~" G r o u p 4
At U
I ' ' ' , l , '
''1
....
I ' . .
At= l 1 5 8 y e a r s p
2.2
:z:I
2.1
2
1.9 I
At = 1384 years
1.8
1.7
. . . ,
6
. . . .
7
8
Detritus
9
, . . . .
10
,
. . . .
11
,..
12
13
I.V'";
....
.... ; ....
Detritus 2a2 T h Content
za2 T h C o n t e n t ( p p m )
. . . .
(ppm)
FIG. 10. Relationship between derived values for hydrogenous
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 ( A t ) 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.
23(~Fh/232Th
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-insolubleresidue 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 P y r a m i d L a k e water
o f about 1.3.
Initial 23°Th/232Th ratios d e r i v e d f r o m i s o c h r o n plots exc e e d n o r m a l detrital 23°Th/232Th ratios, indicating that s o m e
o f the t h o r i u m with high 23°Th/232Th was a s s o c i a t e d with
fine detrital particles. I n c o r p o r a t i o n o f h y d r o g e n o u s t h o r i u m
by this m e c h a n i s m w o u l d i n t r o d u c e n o age bias to an isochron. H o w e v e r , if a significant a m o u n t o f h y d r o g e n o u s thorium w a s i n c o r p o r a t e d directly into the pure c a r b o n a t e p h a s e
a l o n g with u r a n i u m , it w o u l d i n t r o d u c e a positive slope for
the z e r o - a g e isochron, and h e n c e , a p o s i t i v e a p p a r e n t initial
age. P r o v i d e d that the detritus c o n t e n t o f each s a m p l e is
k n o w n , a t h r e e - c o m p o n e n t m i x i n g d i a g r a m can b e used to
e s t i m a t e the m a g n i t u d e o f this i s o c h r o n age bias. O u r analysis s u g g e s t s that biases o f up to t w o t h o u s a n d years for the
i s o c h r o n s in this study are possible.
Traditional a s s u m p t i o n s for U - T h i s o c h r o n dating are that
the pure c a r b o n a t e p h a s e is free o f t h o r i u m and all the c o m m o n t h o r i u m is a s s o c i a t e d with detrital impurities. W e suggest that t h e s e a s s u m p t i o n s are not a l w a y s valid. Studies o f
i m p u r e c a r b o n a t e s requiring a c c u r a c y o f U - T h ages better
than 1000 to 2000 years still n e e d a careful evaluation o f
potential age b i a s e s a s s o c i a t e d with initial uptake o f h y d r o g e n o u s thorium.
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 Spectrometry. Dr. K. Ludwig kindly offered the Version 2.71 Isoplot
program. This work was supported by a grant from NASA NAGW895.
Editorial handling: H. P. Schwarcz
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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 ~ 3 0 0 #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 T I M S Interferences
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 - 1 0 V or the so-called
energy filtering. Energy filtering applies the theory that the monatomic 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.
intercept, i.e., uncertainties propagated from assigned errors (a priori) 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 obviously the major source of the scatter (except Group 4).
APPENDIX
Data
collection
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~
\
C
Radiocarbon age bias model with continuous addition of modern
carbon
dN
-
kN
+ a,
(C1)
dt
where
N
a
k
t
=
=
=
=
total no. of ~4C atom in the sample;
no. of ~4C atom added/y;
decay constant of 14C ( 1/y) ;
in years.
Solve Eqn. AI by multiplying both sides with an integrating factor,
e xt, and integrate. The solution for Eqn. A1 is:
k
P
a = k
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 ( 2 a ) , respectively. The data reported in Table 3 have taken into account the
external reproducibility of both uranium and thorium.
N l
N o e -~'t
--
e
kt
(C2)
,
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:
xR
C
-
k RCe
~" - R C o e
C
1 - e -x~
x~
(C3)
Canceling R on both sides and rearranging yields:
e~,
APPENDIX
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
x
-C
= X
Co
C
e ~'' - 1
(C4)
Further rearranging Eqn. C2 by replacing Co with C - x t , will lead
to a solution of x / C :
X
C
-- = X
ex~t- 1
e ~' - l - kt"
(C5)