Chlorine-36 Production in the Atmosphere

1. Chlorine-36 Production in the Atmosphere Presentor: Jeannie Bryson

2. Summary • Introduction • Production Mechanisms • Production Rates • Calculations • Case Studies • Questions

3. Introduction:Chlorine-36 • Radionuclide with half life = 301 ka • “Conservative” tracer with large range for dating old ground waters • Unlike 14C which requires significant geochemical interpretation

4. Introduction: Reactions • Primary-Ray Reactions • 40Ar+p36Cl+a • Major reaction of atmospheric production • 36Ar+n 36Cl+p • Almost negligible • Secondary Ray Reaction • 35Cl+n 36Cl+a • Nuclear Reaction • 40Ca+π- 36Cl+a

5. Production Mechanisms • Cosmic Ray • Both atmospheric and surface interactions • Natural Radioactivity • Radioactive decay • Nuclear Activity • Anthropogenic, recent nuclear tests

6. Production Mechanisms • Atmospheric Production • Source: Primary and Secondary Cosmic-Ray bombardment of heavier elements • 36Cl produced mainly by spallation by high energy protons and less by neutron adsorption

7. Production Mechanisms • Nuclear Activity • 1950’s-1960’s: High neutron flux iradiated marine aerosols such as 40Ca, 36Ar, and especially 35Cl. • Creates “bomb pulse” • Important near ocean and confined waters

8. Production Rates • Rate at which each reaction occurs controlled by: • Intensity of the incident radiation • Reaction cross sections • Example: 36Ar reaction occurs at small rates due to its cross section (Andrews, et al 1994)

9. Production Rates • Flashback: Altitude Dependency • High energy cosmic-ray flux low, low energy flux increases as high energy decreases and produces more low energy rays • Peak occurs at altitude=20km • Low energy flux decreases exponentially

10. Production Rates • Closer Look at the Atmosphere Source:http://csep10.phys.utk.edu/astr161/lect/earth/atmosphere.html

11. Production Rates • Altitude Dependency • Most production occurs in the stratosphere • Around 40% occurs in the troposphere (Bird et al. 1991)

12. Production Rates • Latitude Dependency Rate controlled by Cosmic Flux Cosmic Flux controlled by magnetic field Rate controlled by geomagnetic latitude • Dependency decreased by atmospheric mixing • Example: Greenland rates are 3-5 X’s higher than predicted

13. Production Rate

14. Production Rate • Solar Activity • Controls production inversely • However, 36Cl varies form 14C and 10Be • Relationship not fully understood

15. Production Rate • Nuclear Activity • Greenland Ice Core • 500 x’s greater than natural 36Cl deposition • Fallout varies w/ latitude-must be assessed for each site

16. Production Rate • Seasonal Variations • Increased Production in Spring (Hainsworth, et al 1994) • Why?? • 1.) tropopause rises in spring allowing 36Cl to be added into troposphere and “scavenged” • 2.) increase in cosmic-ray penetration into troposphere

17. Calculations • 36Cl/Cl Ratio • Ratio of 36Cl annual fallout to Cl- in annual precipitation • Useful because it is not sensitive to ET but it is affected by “ancient” chloride addition (Davis, et al. 1996)

18. Calculations • 36Cl/Cl Ratio • R (36Cl/Cl) = 1854*F/[Clp]*P • F=36Cl fall-out (atoms m-2 s-1) • Clp=Cl- in precipitation • P=mean annual precipitation • [Cl]gw = [Cl]p*P/(P-E) • E=Evapotranspiration

19. Calculations • 36Clflux: Variation of deposition flux with time • V=Volume (liters) collected in 1 m2 area during sampling period • D=length (days) of sampling period Source: Hainsworth, et al. 1994

20. Case Study Example:Maryland Source: Hainsworth, et al. 1994

21. Case Study Example:Maryland Source: Hainsworth, et al. 1994

22. Case Study Example:Maryland Source: Hainsworth, et al. 1994

23. Case Study Example:Canadian Shield • Ability to measure low concentrations of 36Cl with AMS opens door for many hydrological investigations • However, 36Cl interpretation is not always straight forward • Bomb 36Cl and 3H peaks do not correlate. Why? Source: Milton, et al. 2003

24. Results – Water • Precipitation-very few measurements • Ice cores record that by 1975, 36Cl -> pre- bomb levels • Seasonal variability in 36Cl • Longitudinal effects? • Why is this important?? -“INITIAL VALUE” Source: Article

25. “the initial value problem” • Latitude to estimate cosmogenic production • Precipitation-neglect anthropogenic component • Shallow ground-water • Desert soil profiles • Glacial ice cores • Isolated water Source: Davis et al (1998)

26. Source: Article

27. Results-Water • Groundwater Profiles (1993-1995) • Predict flow velocity • Recharge Conditions/Sources Source: Article

28. Source: Article

32. Vegetation • 36Cl decline in vegetation < Arctic Precip. • Decline also varies with species • Forested sites have larger inventory of 36Cl than previously expected

35. Mass Balance • Bomb 36Cl < expected amount • Why?? 1.) Latitude-too large? 2.) wrong model 3.) fallout not uniform 4.) mysterious pool of 36Cl-organochloride emission from trees?

36. Conclusions • 36Cl held in surface matter more than 3H • 36Cl is non-conservative in highly forested environments • Cannot assume steady state

37. Questions?

38. References • Andrews JN, Edmunds WM, Smedley PL, Fontes J-Ch, Fifield LK, Allan GL (1994) Chlorine-36 in groundwater as a paleoclimatic indicator: the East Midlands Triassic sandstone aquifer (UK). Earth and Planetary Science Letters 122: 159-171 • Bird JR, Davie RF, Chivas AR, Fifield LK, Ophel TR (1991) Chlorine-36 production and distribution in Australia. Palaeogeography, Palaeoclimatology, Palaeoecology 84: 299- 307 • Davis SN, Cecil D, Zreda M, Sharma P (1996) Chlorine-36 and the initial value problem. Hydrogeology Journal 6: 104-114 • Milton GM, Milton JCD, Schiff S, Cook P, Kotzer TG, Cecil LD (2003) Evidence for chlorine recycling-hydrosphere, biosphere, atmosphere-in a forested wet zone on the Canadian Shield. Applied Geochemistry 18: 1027-1042