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A review of atmospheric 10 Be in Dry Valley soils

A review of atmospheric 10 Be in 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)

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A review of atmospheric 10 Be in Dry Valley soils

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  1. A review of atmospheric 10Be in Dry Valley soils Warren Dickinson, Martin Schiller Ian Graham, Bob Ditchburn, and Albert Zondervan

  2. 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) Atmospheric Half life = 1.5 Ma (15 Ma max age) Attaches to atm. particles and falls out Wet (snow/ice) in low & mid latitudes Dry (dust) in high latitudes

  3. Dating Soils with 10-Be Open System: Total inventory of 10-Be Assume no loss of 10-Be through erosion Must know/assume input & output rate or residence time Closed System: 10-Be becomes fixed in authigenic soil mineral Use of 10Be/9Be avoids knowing input/output rates

  4. 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

  5. Wright Vly Beacon Heights Roberts Massif Table Mtn.

  6. 9Be (ppm) 0.1 0.3 0.5 0.7 0.9 0 0 0.5 1 Depth (metres) 1.5 2 2.5 >62 microns <62 microns 3 Example from Sirius Group, Table Mt

  7. 10Be/9Be 10-11 10-10 10-09 10-7 10-8 0 0.5 1 1.5 >62 microns Depth (metres) <62 microns 2 2.5 3 Example from Sirius Group, Table Mt

  8. 10Be (atoms/g) 105 106 107 108 109 0 0.5 1 Depth (metres) 1.5 >62 microns <62 microns 2 2.5 3 Example from Sirius Group, Table Mt 11.5 Ma depending on ‘background’ 10-Be

  9. Conclusions from closed system model of dating Antarctic soils: • Reasonable ages obtained • 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

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

  11. Hart Ash Profile H5 Spls (cm) 1,4,5,7,9,10,11,15, 15,20,30,50,70.

  12. Salts: Conc. in fine grained, porous and perm. ash Mostly Na-Cl from sea (sw dilution line) No salt in paleosol (may have migrated upward in to ash

  13. 9Be: 1) Conc in volcanic ash rather than doloritic soil Corr. to dissolution of material that contains Be 3) 9Be cannot be used to normalize 10Be

  14. 10Be: 1) None in ash No migration of Be from surface thru ash 3) 10Be mobility nil in alkaline soil

  15. Why so little 10-Be in the upper part of the soil? Erosion? Not likely with in situ ash. Alternatives: 2) 10-Be input rate is very low 3) 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 current dry conditions.

  16. Where we think we are now Atm 10-Be cannot be used to date Dry Valley soil 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.

  17. 10-Be Surface Concentrations in the Dry Valleys Taylor Dome Input Bulk Input

  18. Dating with Atm. – Derieved 10Be (closed system) Advantages: 1) Independent of 10Be deposition rate 2) Seemed to give good ages Problems: 1) Be somehow transported downward 2) 10/9 must be totally mixed but they come from different sources 3) 9Be increases with silicate weathering Needed to find an independent test of age

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