Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20The bomb spike: surface ocean and atmospheric Δ14C since 1950Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Lecture 13 Tracers for Gas ExchangeExamples for gas exchange using:222Rn14CE&H Sections 5.2 and 10.2Rates of Gas ExchangeStagnant Boundary Layer Model.Depth (Z)ATMOCNCg = KH Pgas = equil. with atmCSWZFilmStagnant BoundaryLayer – transport by molecular diffusionwell mixed surface SWwell mixed atmosphere0Z is positive downwardC/ Z = F = + (flux into ocean)see:Liss and Slater (1974) Nature, 247, p181Broecker and Peng (1974) Tellus, 26, p21Liss (1973) Deep-Sea Research, 20, p221Expression of Air -Sea CO2 Fluxk = piston velocity = D/ZfilmFrom wind speedFrom CMDLCCGG networkS – SolubilityFrom Temperature & SalinityFrom measurements F = k s (pCO2w- pCO2a) = K pCO∆2pCO2apCO2wNeed to calibrate!U-Th Series TracersAnalytical Method for 222Rnand 226Racharcoalliquid N2SW226Ra in Atlantic and PacificQ. What controls the ocean distributions of 226Ra?226Ra – Si correlation – Pacific DataQ. Why is there a hookat the end?Calculate 226Ra from Si!226Ra source from the sediments222Rn Example Profile fromNorth Atlantic226Ra222RnDoes Secular Equilibrium Apply?t1/2 222Rn << t1/2 226Ra (3.8 d) (1600 yrs)YES!A226Ra = A222RnWhy is 222Rn activity less than 226Ra?222Rn is a gas and the 222Rn concentration in the atmosphere is much less than in the ocean mixed layer ( mixed layer).Thus there is a net evasion of 222Rn out of the ocean.The 222Rn balance for the mixed layer, ignoring horizontal advection and vertical exchange with deeper water, is:ml 222Rn [222Rn]/t =  ml 226Ra [226Ra] –   222Rn [222RnML] + D/Zfilm { [222Rnatm] – [222RnML]}Knowns: 222Rn, 226Ra, DRnMeasure:  ml, A226Ra, A222Rn, d[222Rn]/dtSolve for Zfilm 222Rn/dt = sources – sinks = decay of 226Ra – decay of 222Rn - gas exchange to atmosphereml 222Rn d[222Rn]/dt =  ml 226Ra [226Ra] – ml 222Rn [222Rn] + D/Zfilm { [222Rnatm] – [222RnML]}ml δA222Rn/ δt = ml (A226Ra – A222Rn) + D/Z (CRn, atm – CRn,ML)for SS = 0atm Rn = 0Then-D/Z ( – CRn,ml) = ml (A226Ra – A222Rn) +D/Z (ARn,ml/Rn) = ml (A226Ra – A222Rn) +D/Z (ARn,ml) = ml Rn (A226Ra – A222Rn)ZFILM = D (A222Rn,ml) / ml Rn (A226Ra – A222Rn) ZFILM = (D / ml Rn) ( )22622211RaRnAA-Z = DRn /  222Rn (1/A226Ra/A222Rn) ) - 1Average Zfilm = 28 mStagnant Boundary Layer Film ThicknessHistogram showing results of film thicknesscalculations from many stations.Organized by Ocean and by LatitudeQ. What are limitations of this approach?1. unrealistic physical model2. steady state assumptionCosmic Ray Produced Tracers – including 14CCosmic ray interactions produce a wide range of nuclides in terrestrial matter, particularly in the atmosphere, and in extraterrestrial material accreted by the earth. Isotope Half-life Global inventory3H 12.3 yr 3.5 kg14C 5730 yr 54 ton10Be 1.5 x 106 yr 430 ton7Be 54 d 32 g26Al 7.4 x 105 yr 1.7 ton32Si 276 yr 1.4 kgCarbon-14 is produced in the upper atmosphere as follows:Cosmic Ray Flux  Fast Neutrons  Slow Neutrons + 14N*  14C (thermal)The reaction is written:14N + n  14C + p(7n, 7p) (8n, 6p)(5730 yrs)From galactic cosmic rayswhich are more energetic thansolar wind. So these are not from the sun.Tritium (3H) is produced from cosmic ray interactions with N and O. After production it exists as tritiated water ( H - O -3H ), thus it is an ideal tracer for water. Tritium concentrations are TU (tritium units) where1 TU = 1018 (3H / H)Thus tritium has a well defined atmospheric input via rain and H2O vapor exchange. Its residence time in the atmosphere is on the order of months.In the pre-nuclear period the global inventory was only 3.5 kg which means there was very little 3H in the ocean at that time. The inventory increased by 200x and was at a maximum in the mid-1970sTritium in rain (historical record)Tritium (3H) in rain and surface SWTritium is a conservative tracer for water (as HTO) – thermocline penetrationMeridional Section in the PacificEqTime series of northern hemisphere atmospheric concentrationsand tritium in North Atlantic surface watersAtmospheric Record of Thermocline Ventilation TracersConservative, non-radioactive tracers (CFC-11, CFC-12, CFC13, SF6)Bomb Fallout Produced TracersNuclear weapons testing and nuclear reactors (e.g. Chernobyl) have been an extremely important sources of nuclides used as ocean tracers. In addition to 3H and 14C the main bomb produced isotopes have been:Isotope Half Life Decay90Sr 28 yrs beta238Pu 86 yrs alpha239+240Pu 2.44 x 104 yrs alpha6.6 x 103 yrs alpha137Cs 30 yrs beta, gammaNuclear weapons testing has been the overwhelmingly predominant source of 3H, 14C, 90Sr and 137Cs to the ocean. Nuclear weapons testing peaked in 1961-1962. Fallout nuclides act as "dyes" Another group of man-made tracers that fall in this category but are not bomb-produced and are not radioactive are the chlorofluorocarbons (CFCs).The bomb spike: surface ocean and atmospheric Δ14C since 1950•Massive production in nuclear tests ca. 1960 (“bomb 14C”)•Through air-sea gas exchange, the ocean took up ~half of the bomb 14C by the 1980sbomb spike in 1963data: Levin & Kromer 2004; Manning et al 1990; Druffel 1987; Druffel 1989; Druffel & Griffin 1995Comparison of 14C in surface oceanPre-nuclear (1950s) and nuclear (1970s)AtlanticIndianPacificExample – Use 14C to calculate ZFILM using the Stagnant Boundary LayerUse Pre-bomb 14C – assume steady statesource = sink14C from gas exchange = 14C lost by decay14Catm14C decayAssume [CO2]top = [CO2]bottom = [CO2]surface ocean (e.g. no CO2 gradient, only a 14C gradient)[14C]1-box modelAssumeD = 3 x 10-2 m2 y-1h = 3800m1 = 8200 y[CO2]surf = 0.01 moles m-3[DIC]ocean = 2.4 moles m-314CO2/CO2 = 1.015 (14C-CO2 is more soluble than CO2)( equals solubility constant)(14C/C) surf = 0.96 (14C/C)atm(14C/C)deep = 0.84 (14C/C)atmThen:Zfilm = 1.7 x 10-5 m = 17 mExample – 14C Deep Ocean Residence Timesubstitute for Bvmix in cm yr-1; vC in cm yr-1 x mol cm-3Rearrange andSolve for VmixUse pre-nuclear 14C data when surface 14C > deep 14C(14C/C)deep = 0.81