ground ice chronometry with 36 cl/ cl

Ground ice chronometry with 36Cl/Cl

Alexander BlinovSt. Petersburg State Polytechnic University,

Saint Petersburg, Russia

AMS Workshop, Bucharest, November 1, 2012

Content and plan of the talkWe present the application of 36Cl/Cl dating method to

ground ice from the northeast Siberia and we discuss:

1.36Cl as a potential chronometer2.36Cl sources in ground ice3.Ground ice collection for age determination4.Measurements and results5.Conclusions


Motivation of the research

• Ground ice represents a unique natural archive containing ecological and climatic records

• The most ancient permafrost from northeast Siberia could date back to 2 million years BP

• In general the age of ice does not coincide with the age of surrounding sediments

• Chronological methods beyond radiocarbon range reveal the age of sediments or organic remains and never the age of the ice


Constructing a radiochronologyThree obligatory conditions of dating an object:

1. The input function Q(t) is well defined2. The system stays closed during the dated time3. Additional internal sources I(t) are well defined


Q(t) I(t)

)(),(),(),( 00 tItQtNtNfttt

N(t)

Origin of 36Cl in the environment

Cosmic ray induced nuclearcascade Object Nuclear reaction

Atmosphere 40Ar(N, 5X)36Cl

Lithosphere 40Сa(n, 5X)36Cl39K(n, 4X)36Cl35Сl(n,γ)36Cl40Ca(μ,α) 36Cl Primordial radioactivity

232Th, 238U, 235U

Atmospheric production

In-situ production

36Cl → T1/2 = 301 thousand years

Penetrating particles

Cosmic rays


Man-made contamination?

36Cl atmospheric transport Cosmogenic 36Cl produced in the atmosphere is involved in natural chlorine geochemical cycle

mobile (HCl, CH3Cl)

mobile (HCl, NaCl)

36Cl/Cl abundance is delivered to the ground level by atmospheric precipitation and then it is not changed by chemical reactions


36Cl in modern precipitation across Europe

From: Johnston, V. E., and F. McDermott (2008), The distribution of meteoric Cl-36 in precipitation across Europe in spring 2007, Earth Planet. Sci. Lett., 275, 154 – 164, doi:10.1016/j.epsl.2008.08.021


36Cl concentration in ground ice: a function of depth beneath the surface


Constructing the relative chronology

The age of a dated sample relative to a reference sample can be calculated based on the law of radioactive decay

where τ36 = 434 thousand years is the lifetime of 36Cl.

Three obligatory conditions of the dating method:

1.The 36Cl/Cl ratio is constant in surface precipitation2.After 36Cl/Cl is fixed in ice the system stays closed3.In situ production of 36Cl is negligible for moderate age


Location of sampling sites in northeast Siberia

A – Cape Svyatoy Nos

B – Bol’shoy Lyakhovsky Island

C – Oyogos Yar coast

D – Khaptashinsky Yar

E – Allaikha River

F – Bol’shoy Khomus Yuryakh River


Overview schemes of the studied exposures

Peat lenses and ice wedges of the Yukargirsky Suite covered by deposits of the Kuchchugui Suite (B, exposure L7-01)

Ice wedge of the Yukagirsky Suite covered by deposits of the Kuchchugui Suite (B, subprofile L7-02)


AMS measurements of 36Cl/Cl


Field Data, Analytical Results and Calculated Ages


Field Data, Analytical Results, and Calculated Ages

Local permafrost chronologies

A – Cape Svyatoy Nos

B – Bol’shoy Lyakhovsky Island

C – Oyogos Yar coast

D – Khaptashinsky Yar

E – Allaikha River

F – Bol’shoy Khomus Yuryakh River


Problems

•Holocene samples:reduced 36Cl/Cl ratio for most samples

younger than 11 thousand years

•Dating of small wedges could fail: possibility of boundary effects

•No universal dating protocol


Conclusions36Cl/Cl relative abundance in ground ice allowed revealing the previously unknown permafrost age. Existence of ground ice with age over 500 thousand years was demonstrated

36Cl/Cl ratios, which formed from 12–15 to 50 thousand years ago, may serve a base level for relative dating of ice sequences formed in the late and middle Pleistocene under similar climatic conditions

Several local chronologies were constructed for syngenetic late and middle Pleistocene ground ice from northeast Siberia


Acknowledgments


This work was carried out by several research teams and supported by different funds. The role of the team leaders is acknowledged.

Prof. J. BeerSwiss Federal Institute of Aquatic Science and Technology, Duebendorf, Switzerland Prof. D. GilichinskyInstitute of Physicochemical and Biological Problems in Soil Science, Russian Academy of Sciences, Pushchino, Russia Dr. L. SchirrmeisterAlfred Wegener Institute for Polar and Marine Research, Potsdam, Germany

Thank you for your attention!

Bomb-peak in 36Cl ice-core records

From: Heikkila U. et al., 36Cl bomb peak: comparison of modeled and measured data. Atmos. Chem. Phys., 9, 4145–4156, 2009


36Cl–Cl correlation in ground ice

0 10 20 30 40 50 60 70 80 900

2

4

6

8

10

12

14

16

18

Cl (10-6 g∙g-1)

36

Cl

(10

4 a

t∙g

-1)


36Cl in rain water

0 100 200 300 400 500 6000

1000

2000

3000

Precipitation rate, mm y-1

36Cl flux, at m-2 s-1


-1 -0.5 0 0.5 1

1

10

100

1000

1

10

100

1000

Sin(Latitude, deg)

36Cl flux, at m-2 s-1

Dependence on precipitation rate

Dependence on geographic position

ground ice chronometry with 36 cl/ cl

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