Uranium-series dating of antarctic ice
Uranium-series dating of antarctic ice
The uranium-series method can cover the age range from 10,000 to 600,000 years. The ages of near-surface ice in the main Allan Hills icefield may be in this range. This icefield, which covers an area of approximately 100 square kilometers, is shown in the figure. More than a thousand meteorites with terrestrial ages that range from 11,000 (Fireman 1980) to 950,000 years (Nishiizumi et al. 1986) have been recovered from this field. All reside on the surface; the terrestrial age of a meteorite is the time since it fell. If the meteorite fell on snow, became encased in the ice formed from the snow, traveled with the ice to Allan Hills, became exposed when the ice ablated, and then resided on the surface for a long time, its terrestrial age would be much older than the age of the underlying ice. The meteorite ages, however, tend to increase as their locations approach Allan Hills suggesting a stratigraphic sequence for the underlying ice. According to Cassidy, dust bands imbedded in ice are easier to find than meteorites. The Cul de Sac region, shown in the figure, is near the southern corner of Allan Hills; the Cul de Sac 150 site is approximately 50 meters upstream from the Cut de Sac 100 site. The measurements of radium-226 (22oRa), thorium-230 2307h), uranium-234 234 U), and uranium-238 2 U) dissolved in ice samples, from these two sites [Fireman 1986(a), 1986(b)] are given in table 1, together with results for a clear ice sample from stake 12. The first two rows in table 1 show that the two Cut de Sac 100 samples with different dust concentrations have the same activities but that the sample with the higher dust concentration has higher 234 U, 23°Th, and 226 R activities. The third and fourth
E.L. FIREMAN Smithsonian Astrophysical Observatory Cambridge, Massachusetts 02138
It is very interesting to date polar ice radiometrically. Bands of dust imbedded in ice are frequently observed in antarctic ice fields. Our work focuses on dating ice samples with high dust contents by the uranium-series method. We obtained uraniumseries ages [Fireman 1986(a), 1986(b)] of 325 thousand (± 75) and 100 thousand (± 20) years for dusty ice samples from two sites in the main Allan Hills icefield. W.A. Cassidy collected the dust-banded ice from 50- to 100-centimeter depth at two sites, called Cul de Sac 100 and Cul de Sac 150. U. Marvin examined the particles in these samples with an optical microscope and found them to consist essentially (more than 95 percent of the particulates) of fine volcanic glass shards full of vesicles and microvesicles. Evidently the fine volcanic glass shards were deposited on snow, became incorporated in the ice, and moved with the ice to the Allan Hills sites. Ice samples with other types of particulates, such as terrestrial morraine, may also be amenable to uranium-series dating; however, it is difficult to date ice with less than 0.03 gram of fine particulates per kilogram of ice with our present technique.
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Map of Allan Hills region. A numbered stake network is shown with lines; the locations of some of the collected meteorites are shown by dots; the Cul de Sac site is marked with a circle. ("km" denotes "kilometer.")
118
ANTARCTIC JOURNAL
Table 1. Uranium-series activities dissolved in ice and the ratios of the
226 Ra, 230Th
and 234 U excesses
Sample Dust (g) 238U (226Ra excess)a (226Ra excess)a 234U 23 (weight in 0Th 226 Ra kilograms) Ice (kg) (dpm/kg) (dpm/kg) (dpm/kg) (dpm/kg) (230Th excess) (234U excess) Gui de Sac 100 0.30 0.007 ± 0.002 0.063 ± 0.003 0.141 ± 0.020 0.198 ± 0.007 1.43 ± 0.22 3.41 ± 0.28 (1.00) Guide Sac 100 0.14 0.009 ± 0.002 0.045 ± 0.003 0.081 ± 0.006 0.110 ± 0.007 1.41 ± 0.18 2.81 ± 0.39 (0.90) Gui de Sac 150 0.070 0.020 ± 0.001 0.038 ± 0.002 0.066 ± 0.004 0.129 ± 0.005 2.20 ± 0.23 6.06 ± 0.80 (1.80) Gui de Sac 150 0.008 0.019 ± 0.001 0.020 ± 0.001 0.022 ± 0.002 0.031 ± 0.005 - (3.60)
-
A.H. Stake 12 0.003 0.009 ± 0.001 0.007 ± 0.001 0.006 ± 0.002 0.008 ± 0.002 - (9.90)
-
° 226 R excess =
226 R - 238 U; 230 Th
excess =
230 Th - 238 U; 234 U
excess = -
Table 2. Age of the ice and the dissolved activity ratios (26 N26)
(26 N26)a
Age (30 N30) (34 N34) (103 yrs) Calculated (Measured) Calculated (Measured) 0 20 40 60 80 100 120 140 160 200 240 280 320 360 400 440 480 520 560 600
44.5 6.44 3.75 2.85 2.41 2.14 (2.20 ± 0.23)b150 1.97 1.84 1.74 1.63 1.55 1.47 1.40 (1.42 ± 0.15)'100 1.34 1.30 1.28 1.26 1.25 1.24 1.23
147.7 20.79 11.75 8.77 7.10 6.16 (6.06 ± 0.80)b150 5.47 4.98 4.62 4.09 3.75 3.45 3.17 (3.21 ± 0.23)100 3.01 2.87 2.77 2.68 2.60 2.54 2.49
a N26, N30, and N34 are the concentrations of dissolved 226 Ra, 230 Th, and 234 J minus the concentration of dissolved 238U. b Allan Hills Location GuI de Sac 150. Allan Hills Location Cul de Sac 100.
rows show that the two Cu! de Sac 150 samples also have the same 238U activity even though the 3.60-kilogram sample, which is visibly clear, contains very little dust. The 234U, 230Th, and 226Ra activities in the dusty Cu! de Sac 150 sample are higher than the 238U activity, but in the clear Cu! de Sac 150 sample the activities are in radioactive equilibrium. The activities in the clear ice from
1986 REVIEW
the stake 12 location are also in equilibrium. The last two columns of table 1 give the ratio of the 226 R excess to the 230Th excess and the ratio of the 226 R to the 234U excess. In dusty samples, the 226 R activity is larger than the 230Th activity, which in turn is larger than the 234U activity. These data are consistent with the idea that 226Ra, 230Th, and 234w recoiled out of the dust during the alpha decays. The ratios of the nuclides that recoil into ice from dust change with time in a calculable manner [Fireman 1986(a)]. The calculations show that these ratios are relatively insensitive to the sizes and shapes of the particulates. Table 2 gives the calculated ratios versus age. The 226Ra/230Th and 226Ra/234U ratios measured in the Cu! de Sac 150 sample both correspond to an age of (100 ± 20) thousand years. The average of the 226Ra/230Th ratios measured in the Cu! de Sac 100 samples corresponds to the age of (310 + 150, - 50 thousand years and the average 236Ra/2 U ratio corresponds to an age of (320 ± 40) thousand years. The experimental procedures involved in these measurements have been described [Fireman 1986(a)]. During the 1985 field season, members of Cassidy's group collected dust-banded ice samples from the Lewis Cliff region of the Beardmore Glacier, from the far western icefield (76°54'S 157°01 'E) near Allan Hills, and from three other sites at the main Allan Hills icefield for uranium-series dating. We thank J. DeFelice for his assistance. This work is supported by National Science Foundation grant DPP 82-17831.
References Fireman, E.L. 1980. Carbon-14 and argon-39 in ALHA meteorites. Geochimica et Cosmochimica Acta, Supplement 14, 2, 1215-1221. Fireman, E.L. 1986(a). Uranium-series dating of Allan Hills ice. Journal of Geophysical Research, 91(B4), D539-D544. Fireman, E.L. 1986(b). Uranium-series dates for ice from two Allan Hills locations. Lunar and Planetary Science XVII, (Part 1), 226-227. Nishiizumi, K., D. Elmore, P.W. Kubek, C. Bonani, M. Suter, W. Wolfli, and J. Arnold. 1986. Age of Antarctic meteorites and ice. Lunar and Planetary Science XVII, (Part 2), 621-622.
119
The uranium-series method can cover the age range from 10,000 to 600,000 years. The ages of near-surface ice in the main Allan Hills icefield may be in this range. This icefield, which covers an area of approximately 100 square kilometers, is shown in the figure. More than a thousand meteorites with terrestrial ages that range from 11,000 (Fireman 1980) to 950,000 years (Nishiizumi et al. 1986) have been recovered from this field. All reside on the surface; the terrestrial age of a meteorite is the time since it fell. If the meteorite fell on snow, became encased in the ice formed from the snow, traveled with the ice to Allan Hills, became exposed when the ice ablated, and then resided on the surface for a long time, its terrestrial age would be much older than the age of the underlying ice. The meteorite ages, however, tend to increase as their locations approach Allan Hills suggesting a stratigraphic sequence for the underlying ice. According to Cassidy, dust bands imbedded in ice are easier to find than meteorites. The Cul de Sac region, shown in the figure, is near the southern corner of Allan Hills; the Cul de Sac 150 site is approximately 50 meters upstream from the Cut de Sac 100 site. The measurements of radium-226 (22oRa), thorium-230 2307h), uranium-234 234 U), and uranium-238 2 U) dissolved in ice samples, from these two sites [Fireman 1986(a), 1986(b)] are given in table 1, together with results for a clear ice sample from stake 12. The first two rows in table 1 show that the two Cut de Sac 100 samples with different dust concentrations have the same activities but that the sample with the higher dust concentration has higher 234 U, 23°Th, and 226 R activities. The third and fourth
E.L. FIREMAN Smithsonian Astrophysical Observatory Cambridge, Massachusetts 02138
It is very interesting to date polar ice radiometrically. Bands of dust imbedded in ice are frequently observed in antarctic ice fields. Our work focuses on dating ice samples with high dust contents by the uranium-series method. We obtained uraniumseries ages [Fireman 1986(a), 1986(b)] of 325 thousand (± 75) and 100 thousand (± 20) years for dusty ice samples from two sites in the main Allan Hills icefield. W.A. Cassidy collected the dust-banded ice from 50- to 100-centimeter depth at two sites, called Cul de Sac 100 and Cul de Sac 150. U. Marvin examined the particles in these samples with an optical microscope and found them to consist essentially (more than 95 percent of the particulates) of fine volcanic glass shards full of vesicles and microvesicles. Evidently the fine volcanic glass shards were deposited on snow, became incorporated in the ice, and moved with the ice to the Allan Hills sites. Ice samples with other types of particulates, such as terrestrial morraine, may also be amenable to uranium-series dating; however, it is difficult to date ice with less than 0.03 gram of fine particulates per kilogram of ice with our present technique.
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159°E
/ •'
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5-- -
Map of Allan Hills region. A numbered stake network is shown with lines; the locations of some of the collected meteorites are shown by dots; the Cul de Sac site is marked with a circle. ("km" denotes "kilometer.")
118
ANTARCTIC JOURNAL
Table 1. Uranium-series activities dissolved in ice and the ratios of the
226 Ra, 230Th
and 234 U excesses
Sample Dust (g) 238U (226Ra excess)a (226Ra excess)a 234U 23 (weight in 0Th 226 Ra kilograms) Ice (kg) (dpm/kg) (dpm/kg) (dpm/kg) (dpm/kg) (230Th excess) (234U excess) Gui de Sac 100 0.30 0.007 ± 0.002 0.063 ± 0.003 0.141 ± 0.020 0.198 ± 0.007 1.43 ± 0.22 3.41 ± 0.28 (1.00) Guide Sac 100 0.14 0.009 ± 0.002 0.045 ± 0.003 0.081 ± 0.006 0.110 ± 0.007 1.41 ± 0.18 2.81 ± 0.39 (0.90) Gui de Sac 150 0.070 0.020 ± 0.001 0.038 ± 0.002 0.066 ± 0.004 0.129 ± 0.005 2.20 ± 0.23 6.06 ± 0.80 (1.80) Gui de Sac 150 0.008 0.019 ± 0.001 0.020 ± 0.001 0.022 ± 0.002 0.031 ± 0.005 - (3.60)
-
A.H. Stake 12 0.003 0.009 ± 0.001 0.007 ± 0.001 0.006 ± 0.002 0.008 ± 0.002 - (9.90)
-
° 226 R excess =
226 R - 238 U; 230 Th
excess =
230 Th - 238 U; 234 U
excess = -
Table 2. Age of the ice and the dissolved activity ratios (26 N26)
(26 N26)a
Age (30 N30) (34 N34) (103 yrs) Calculated (Measured) Calculated (Measured) 0 20 40 60 80 100 120 140 160 200 240 280 320 360 400 440 480 520 560 600
44.5 6.44 3.75 2.85 2.41 2.14 (2.20 ± 0.23)b150 1.97 1.84 1.74 1.63 1.55 1.47 1.40 (1.42 ± 0.15)'100 1.34 1.30 1.28 1.26 1.25 1.24 1.23
147.7 20.79 11.75 8.77 7.10 6.16 (6.06 ± 0.80)b150 5.47 4.98 4.62 4.09 3.75 3.45 3.17 (3.21 ± 0.23)100 3.01 2.87 2.77 2.68 2.60 2.54 2.49
a N26, N30, and N34 are the concentrations of dissolved 226 Ra, 230 Th, and 234 J minus the concentration of dissolved 238U. b Allan Hills Location GuI de Sac 150. Allan Hills Location Cul de Sac 100.
rows show that the two Cu! de Sac 150 samples also have the same 238U activity even though the 3.60-kilogram sample, which is visibly clear, contains very little dust. The 234U, 230Th, and 226Ra activities in the dusty Cu! de Sac 150 sample are higher than the 238U activity, but in the clear Cu! de Sac 150 sample the activities are in radioactive equilibrium. The activities in the clear ice from
1986 REVIEW
the stake 12 location are also in equilibrium. The last two columns of table 1 give the ratio of the 226 R excess to the 230Th excess and the ratio of the 226 R to the 234U excess. In dusty samples, the 226 R activity is larger than the 230Th activity, which in turn is larger than the 234U activity. These data are consistent with the idea that 226Ra, 230Th, and 234w recoiled out of the dust during the alpha decays. The ratios of the nuclides that recoil into ice from dust change with time in a calculable manner [Fireman 1986(a)]. The calculations show that these ratios are relatively insensitive to the sizes and shapes of the particulates. Table 2 gives the calculated ratios versus age. The 226Ra/230Th and 226Ra/234U ratios measured in the Cu! de Sac 150 sample both correspond to an age of (100 ± 20) thousand years. The average of the 226Ra/230Th ratios measured in the Cu! de Sac 100 samples corresponds to the age of (310 + 150, - 50 thousand years and the average 236Ra/2 U ratio corresponds to an age of (320 ± 40) thousand years. The experimental procedures involved in these measurements have been described [Fireman 1986(a)]. During the 1985 field season, members of Cassidy's group collected dust-banded ice samples from the Lewis Cliff region of the Beardmore Glacier, from the far western icefield (76°54'S 157°01 'E) near Allan Hills, and from three other sites at the main Allan Hills icefield for uranium-series dating. We thank J. DeFelice for his assistance. This work is supported by National Science Foundation grant DPP 82-17831.
References Fireman, E.L. 1980. Carbon-14 and argon-39 in ALHA meteorites. Geochimica et Cosmochimica Acta, Supplement 14, 2, 1215-1221. Fireman, E.L. 1986(a). Uranium-series dating of Allan Hills ice. Journal of Geophysical Research, 91(B4), D539-D544. Fireman, E.L. 1986(b). Uranium-series dates for ice from two Allan Hills locations. Lunar and Planetary Science XVII, (Part 1), 226-227. Nishiizumi, K., D. Elmore, P.W. Kubek, C. Bonani, M. Suter, W. Wolfli, and J. Arnold. 1986. Age of Antarctic meteorites and ice. Lunar and Planetary Science XVII, (Part 2), 621-622.
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