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A review of atmospheric 10Bein Dry Valley soils
Warren Dickinson, Martin Schiller Ian Graham, Bob Ditchburn, and Albert Zondervan
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Beryllium
Metal ion: soluble in low pH; insoluble in hi pH
9-Be Stable isotope (ug/g)Trace element in rx (weathering)
10-Be Cosmogenic isotope (atoms/g)AtmosphericHalf life = 1.5 Ma (15 Ma max age)Attaches to atm. particles and falls out Wet (snow/ice) in low & mid latitudesDry (dust) in high latitudes
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Dating Soils with 10-Be
Open System:Total inventory of 10-BeAssume no loss of 10-Be through erosion Must know/assume input & output rate orresidence time
Closed System:10-Be becomes fixed in authigenic soil mineralUse of 10Be/9Be avoids knowing input/output rates
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Dating with Atm. – Derieved 10Be(closed system)
1) 10Be produced in upper atm.
2) Falls via dust & moisture
3) Accum. in salts & particles
4) 9Be (stable) from silicate weathering
5) Assume:10Be/9Be = fixed at surface & locked into alteration minerals
6) Age of mineral related to 10Be decay in authigenic mineral
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Table Mtn.Roberts Massif
Wright Vly
Beacon Heights
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0 0.1 0.3 0.5 0.7 0.90
0.5
11.
52
2.5
3
De
pth
(m
etr
es)
9Be (ppm)
>62 microns
<62 microns
Example fromSirius Group, Table Mt
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10-1
1
10-1
0
10-0
9
10-8
10-7
00.
51
1.5
22.
53
Dep
th (m
etre
s)
10Be/9Be
>62 microns
<62 microns
Example fromSirius Group, Table Mt
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105
106
107
108
109
00.
51
1.5
22.
53
De
pth
(m
etr
es)
>62 microns
<62 microns
10Be (atoms/g)Example fromSirius Group, Table Mt
11.5 Ma depending on ‘background’ 10-Be
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Conclusions from closed system model of datingAntarctic soils:
1) Reasonable ages obtained
2) Nagging problems:- Migration of 10Be in hi pH soils- 9Be and 10Be from different sources and probably not mixed esp. in dry alkaline soils- Little diff. in 10Be/ 9Be ratio compared to 10Be
• Needed an independent test of age
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For test: Used a soil on the Hart Ash (3.9 Ma)Wright Valley
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Hart Ash Profile H5
Spls (cm)1,4,5,7,9,10,11,15,15,20,30,50,70.
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Salts: 1) Conc. in fine grained, porous and perm. ash2) Mostly Na-Cl from sea (sw dilution line)3) No salt in paleosol (may have migrated upward
in to ash
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9Be:1) Conc in volcanic ash rather than doloritic soil 2) Corr. to dissolution of material that contains Be3) 9Be cannot be used to normalize 10Be
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10Be: 1) None in ash 2) No migration of Be from surface thru ash3) 10Be mobility nil in alkaline soil
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Why so little 10-Be in the upper part of the soil?
1) Erosion? Not likely with in situin situ ash.
Alternatives:2) 10-Be input rate is very low3) 10-Be not incorporated into soil (same effect as #2)
Using age of ash and amount of 10-Be in paleosol:Input rate 3.9Ma was higher than present
Possible that most 10-Be blows away under currentdry conditions.
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Where we think we are now
1) Atm 10-Be cannot be used to date Dry Valleysoil surfaces by closed or open system models
2) Still not clear how 10-Be gets into Dry Valley soils,but need for wetter, vegetated conditions may be necessary
3) Be may help in understanding past environmental conditions.
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10-Be Surface Concentrations in the Dry Valleys
Taylor Dome Input
Bulk Input
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Dating with Atm. – Derieved 10Be(closed system)
Advantages:1) Independent of 10Be deposition rate2) Seemed to give good ages
Problems:1) Be somehow transported downward2) 10/9 must be totally mixed but they come from different sources3) 9Be increases with silicate weathering
Needed to find an independent test of age
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