RESEARCH· ·1970·

SZABO, DOOLEY, TAYLOR, AND ROSHOLT
B91
TABLE 1.-Sample descriptions, uranium content, and calculated ages of fossil shells and bones
Sample
No.
Material
Uranium
(ppm)
Collection locality
Open-system age '
(years)
Closed-system ago'
(years)
California
M-1710-A 3 _ __ Tivela stultorum _______ 3. 22 ± 0. 05
PVII-S-1 3 ____ T ·ivela stultorum _____ __ 2. 14±0. 02
Dumc terrace at Point Dume_ ____ 103, 000 ± 20, 000
First terrace,
Palos Verdes Ilills.
86, 000 ± 15, 000
128, 000 ± 10, 000
104, 000±8, 000
S-2 3 ____ Tressus nuttali ________ 2. 94±0. 03 _____ do_________________________ 95, 000±15, 000 104, 000±8, 000
PVI-I -S-7_____ Olivella biplicata_ ______ 5. 13 ± 0. 08 Twelfth terrace, __________________ ' 380, 000 ± 60, 000
Palos V crdes Hills.
S-8 H aliotis cracherodii ___ _ 1. 95 ± 0. 03 _____ do _______________________ -------------------- '380, 000± 60, 000
CC-S-1 3 ___ _ Tivela stultorum ______ _
NB-S-3 ___ __ Macoma nasuta _______ _
8. 71 ± 0. 09 Terrace C at Point Dumc_ _ _ _ _ _ _ _ 115, 000± 15, 000 139, 000 ± 10, 000
7. 82±0. 12 Newport Beach ___________________________________ '74, 000±7, 000
Mexico
M-B-3 6 ______ Camel pelvis---------- 86. 5±0. 9 Valsequillo, ncar Puebla _________ 245, 000 ± 40, 000
B-4 6 ______ Horse metapodiaL ____ 58. 6±0. 6 _____ do ________________________ _
260, 000 ± 60, 000
B-5 6 ______ Proboscidean tusk _____ 78. 6±0. 8 _____ do ________________________ _
19, 000±2, 000
B-6 6 ______ Proboscidean vertebra_ 77. 3 ± 0. 8 _____ do ________________________ _
20, 000 ± 2, 000
245, 000 ± 40, 000
340, 000 ± 100, 000
19, 000±2, 000
20, 000 ± 2, 000
t Calculated from measured "'Th/"'U and 23tPa/"'U (Szabo and Rosholt, 1069).
'Calculated from measured "'Thf'" U activity ratio~ .
' Szabo and Rosholt (1960).
plates with respect to the samples, the thin sections
were marked '"ith ink or aluminum containing trace
amounts of uranium "-hich, in turn, produced a fissiontrack
duplication of the mark on the Lexan. A sufficient
number of tracks (over 100 per unit area) were counted
so that the counting error did not exceed ±10 percent.
Examination of the etched Lexan plates showed no
single uranium-rich grains even on a microscopic scale.
Images from the fossil shells revealed that some dark
layers rich in organic material have twice as much
uranium as adjacent white layers. In the fossil bones,
a thin layer at the surface and around open centers of
Haversian canals had as little as one-tenth the amount
of uranium elsewhere within the sample (fig. 1). In
general, uranium was dispersed rather uniformly in
the bulk of shell and bone samples independently of
age, uranium content, or the departure from closed
system as revealed by discordant closed- and opensystem
dates.
REFERENCES
Barnes, J. W., Lang, E. J., and Potratz, H. A., 1956, Ratio of
ionium to uranium in coral limestone: Science, v. 124, no.
3213, p. 175-176.
Blanchard, R. L., Cheng, M. H., and Potratz, H. A., 1967, Uranium
and thorium series disequilibria in recent and fossil
marine molluscan shells : Jour. Geophys. Research, v. 72,
no. 18, p. 4745-4757.
Broecker, W. S., 1963, Preliminary evaluation of uranium series
inequilibrium as a tool for absolute age measurement on
marine cari.Jonates: .Jour. Geophys. Research, v. 68, no. 9,
p. 2817-2834.
Broecker. W. S., and Thuri.Jer, D. L., 1963. Uranium-series dating
of corals and oolites from Bahaman and Florida Key limestones
: Science, Y. 149, no. 3679, p. 58-90.
F1-anale, E. P., and Schaeffer, 0. A., 196:J, Helium-uranitUll ratios
for Pleistocene a nd Tertiary fossil aragonites: Science, v.
149, no. 3681, p. 312-317.
' After Fannie and Rchaeffer (1965).
'Calculated from measured '"Pa/"'U activity ratio.
• Szabo, Malde, and Irwin-Williams (1969).
l~IGUim
Zone of uranium
depletion
0.1 mm
I.-Photomicrographs of a thin section of camel 'bone
(top) nnd of Lexan plastic (bottom) with induced fission
track~ ..\ zone with lower t rack density, and hence lower
uranium concentration, around the open center of the HaYersian
c-anal shown iu bottom photograph coincides wi th the
li ghter area around the canal in top. The top photomicrograJ)h
was taken with oblique top lighting; the bottom was taken
with transmitted light using crossed polarizers, as the replication
of the bone structure was primarily a yariation in the
I.Jirefringence of the Lexan.
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