A review of atmospheric 10 Be in Dry Valley soils Warren Dickinson, Martin Schiller Ian Graham, Bob...

A review of atmospheric 10Bein Dry Valley soils

Warren Dickinson, Martin Schiller Ian Graham, Bob Ditchburn, and Albert Zondervan

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

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

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

Table Mtn.Roberts Massif

Wright Vly

Beacon Heights

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

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

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

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

For test: Used a soil on the Hart Ash (3.9 Ma)Wright Valley

Hart Ash Profile H5

Spls (cm)1,4,5,7,9,10,11,15,15,20,30,50,70.

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

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

10Be: 1) None in ash 2) No migration of Be from surface thru ash3) 10Be mobility nil in alkaline soil

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.

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.

10-Be Surface Concentrations in the Dry Valleys

Taylor Dome Input

Bulk Input

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