Chlorine-36 Production in the AtmospherePresentor: Jeannie Bryson

SummaryIntroductionProduction MechanismsProduction RatesCalculationsCase StudiesQuestions

Introduction:Chlorine-36 Radionuclide with half life = 301 kaConservative tracer with large range for dating old ground watersUnlike 14C which requires significant geochemical interpretation

Introduction: ReactionsPrimary-Ray Reactions40Ar+p 36Cl+aMajor reaction of atmospheric production36Ar+n 36Cl+pAlmost negligibleSecondary Ray Reaction35Cl+n 36Cl+aNuclear Reaction40Ca+- 36Cl+a

Production MechanismsCosmic RayBoth atmospheric and surface interactionsNatural RadioactivityRadioactive decayNuclear ActivityAnthropogenic, recent nuclear tests

Production MechanismsAtmospheric ProductionSource: Primary and Secondary Cosmic-Ray bombardment of heavier elements36Cl produced mainly by spallation by high energy protons and less by neutron adsorption

Production MechanismsNuclear Activity1950s-1960s: High neutron flux iradiated marine aerosols such as 40Ca, 36Ar, and especially 35Cl.Creates bomb pulseImportant near ocean and confined waters

Production RatesRate at which each reaction occurs controlled by:Intensity of the incident radiationReaction cross sectionsExample: 36Ar reaction occurs at small rates due to its cross section (Andrews, et al 1994)

Production RatesFlashback: Altitude DependencyHigh energy cosmic-ray flux low, low energy flux increases as high energy decreases and produces more low energy raysPeak occurs at altitude=20kmLow energy flux decreases exponentially

Production Rates

Closer Look at the AtmosphereSource:http://csep10.phys.utk.edu/astr161/lect/earth/atmosphere.html

Production RatesAltitude Dependency

Most production occurs in the stratosphere

Around 40% occurs in the troposphere (Bird et al. 1991)

Production RatesLatitude Dependency Rate controlled by Cosmic Flux Cosmic Flux controlled by magnetic field Rate controlled by geomagnetic latitudeDependency decreased by atmospheric mixingExample: Greenland rates are 3-5 Xs higher than predicted

Production Rate

Production RateSolar ActivityControls production inverselyHowever, 36Cl varies form 14C and 10BeRelationship not fully understood

Production RateNuclear ActivityGreenland Ice Core500 xs greater than natural 36Cl depositionFallout varies w/ latitude-must be assessed for each site

Production RateSeasonal VariationsIncreased Production in Spring (Hainsworth, et al 1994)Why??1.) tropopause rises in spring allowing 36Cl to be added into troposphere and scavenged2.) increase in cosmic-ray penetration into troposphere

Calculations36Cl/Cl Ratio

Ratio of 36Cl annual fallout to Cl- in annual precipitationUseful because it is not sensitive to ET but it is affected by ancient chloride addition (Davis, et al. 1996)

Calculations36Cl/Cl RatioR (36Cl/Cl) = 1854*F/[Clp]*PF=36Cl fall-out (atoms m-2 s-1)Clp=Cl- in precipitationP=mean annual precipitation[Cl]gw = [Cl]p*P/(P-E)E=Evapotranspiration

Calculations 36Clflux: Variation of deposition flux with time

V=Volume (liters) collected in 1 m2 area during sampling periodD=length (days) of sampling period

Source: Hainsworth, et al. 1994

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

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

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

Case Study Example:Canadian ShieldAbility to measure low concentrations of 36Cl with AMS opens door for many hydrological investigationsHowever, 36Cl interpretation is not always straight forwardBomb 36Cl and 3H peaks do not correlate. Why?Source: Milton, et al. 2003

Results WaterPrecipitation-very few measurementsIce cores record that by 1975, 36Cl -> pre- bomb levelsSeasonal variability in 36ClLongitudinal effects?Why is this important?? -INITIAL VALUE

Source: Article

the initial value problemLatitude to estimate cosmogenic productionPrecipitation-neglect anthropogenic componentShallow ground-waterDesert soil profilesGlacial ice coresIsolated water

Source: Davis et al (1998)

Source: Article

Results-WaterGroundwater Profiles (1993-1995)Predict flow velocityRecharge Conditions/Sources

Source: Article

Source: Article

Vegetation36Cl decline in vegetation < Arctic Precip.Decline also varies with speciesForested sites have larger inventory of 36Cl than previously expected

Mass BalanceBomb 36Cl < expected amountWhy?? 1.) Latitude-too large? 2.) wrong model 3.) fallout not uniform 4.) mysterious pool of 36Cl-organochloride emission from trees?

Conclusions36Cl held in surface matter more than 3H36Cl is non-conservative in highly forested environments Cannot assume steady state

Questions?

ReferencesAndrews 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: 159-171Bird JR, Davie RF, Chivas AR, Fifield LK, Ophel TR (1991) Chlorine-36 production and distribution in Australia. Palaeogeography, Palaeoclimatology, Palaeoecology 84: 299-307Davis SN, Cecil D, Zreda M, Sharma P (1996) Chlorine-36 and the initial value problem. Hydrogeology Journal 6: 104-114Milton 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

Measured CFC profile and determined a gw velocity b/w 0.2 and 0.34 m/a. Tritium matches this model using a velocity of 0.32However, there is some evidence of Tritium in organic materialCorrelation b/w ratio and concentration of chlorine-36 suggests that almost all of the chlorine-36 is atmospheric and that stable chloride is not derived from any mineral phase.