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Lecture 13 Tracers for Gas Exchange

Lecture 13 Tracers for Gas Exchange. Examples for gas exchange using: 222 Rn 14 C. E&H Sections 5.2 and 10.2. Rates of Gas Exchange Stagnant Boundary Layer Model. well mixed atmosphere. C g = K H P gas = equil. with atm. ATM. 0. OCN. Stagnant Boundary Layer – transport by

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Lecture 13 Tracers for Gas Exchange

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  1. Lecture 13 Tracers for Gas Exchange Examples for gas exchange using: 222Rn 14C E&H Sections 5.2 and 10.2

  2. Rates of Gas Exchange Stagnant Boundary Layer Model. well mixed atmosphere Cg = KH Pgas = equil. with atm ATM 0 OCN Stagnant Boundary Layer – transport by molecular diffusion ZFilm Depth (Z) CSW well mixed surface SW Z is positive downward C/ Z = ­ F = + (flux into ocean) see: Liss and Slater (1974) Nature, 247, p181 Broecker and Peng (1974) Tellus, 26, p21 Liss (1973) Deep-Sea Research, 20, p221

  3. Expression of Air -Sea CO2 Flux Need to calibrate! S – Solubility From Temperature & Salinity k = piston velocity = D/Zfilm From wind speed F = k s (pCO2w- pCO2a) = K ∆ pCO2 pCO2a pCO2w From measurements From CMDL CCGG network

  4. U-Th Series Tracers

  5. Analytical Method for 222Rn and 226Ra charcoal liquid N2 SW

  6. 226Ra in Atlantic and Pacific Q. What controls the ocean distributions of 226Ra?

  7. 226Ra – Si correlation – Pacific Data Q. Why is there a hook at the end? Calculate 226Ra from Si!

  8. 226Ra source from the sediments

  9. 222Rn Example Profile from North Atlantic Does Secular Equilibrium Apply? t1/2222Rn << t1/2 226Ra (3.8 d) (1600 yrs) YES! A226Ra = A222Rn 222Rn 226Ra Why is 222Rn activity less than 226Ra?

  10. 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: d222Rn/dt = sources – sinks = decay of 226Ra – decay of 222Rn - gas exchange to atmosphere mll222Rnd[222Rn]/dt = mll226Ra [226Ra] – l222Rn [222RnML] + D/Zfilm { [222Rnatm] – [222RnML]} Knowns: l222Rn, l226Ra, DRn Measure: ml,A226Ra, A222Rn, d[222Rn]/dt Solve for Zfilm

  11. ml l222Rn d[222Rn]/dt = mll226Ra [226Ra] – mll222Rn [222Rn] + D/Zfilm { [222Rnatm] – [222RnML]} mlδA222Rn/ δt = ml(A226Ra – A222Rn) + D/Z (CRn, atm – CRn,ML) atm Rn = 0 for SS = 0 Then -D/Z ( – CRn,ml) = ml(A226Ra – A222Rn) +D/Z (ARn,ml/lRn) = ml(A226Ra – A222Rn) +D/Z (ARn,ml) =mllRn(A226Ra – A222Rn) ZFILM = D (A222Rn,ml) / mllRn(A226Ra – A222Rn) ZFILM = (D /mllRn) ()

  12. Stagnant Boundary Layer Film Thickness Z = DRn /  l222Rn (1/A226Ra/A222Rn) ) - 1 Histogram showing results of film thickness calculations from many stations. Organized by Ocean and by Latitude Average Zfilm = 28 mm • Q. What are limitations of • this approach? • unrealistic physical model • steady state assumption

  13. Cosmic Ray Produced Tracers – including 14C Cosmic ray interactions produce a wide range of nuclides in terrestrial matter, particularly in the atmosphere, and in extraterrestrial material accreted by the earth. IsotopeHalf-lifeGlobal inventory 3H 12.3 yr 3.5 kg 14C 5730 yr 54 ton 10Be 1.5 x 106 yr 430 ton 7Be 54 d 32 g 26Al 7.4 x 105 yr 1.7 ton 32Si 276 yr 1.4 kg

  14. Carbon-14 is produced in the upper atmosphere as follows: Cosmic Ray Flux  Fast Neutrons  Slow Neutrons + 14N* 14C (thermal) (5730 yrs) From galactic cosmic rays which are more energetic than solar wind. So these are not from the sun. The reaction is written: 14N + n 14C + p (7n, 7p) (8n, 6p)

  15. 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) where 1 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-1970s

  16. Tritium in rain (historical record)

  17. Tritium (3H) in rain and surface SW

  18. Tritium is a conservative tracer for water (as HTO) – thermocline penetration Eq Meridional Section in the Pacific

  19. Atmospheric Record of Thermocline Ventilation Tracers Conservative, non-radioactive tracers (CFC-11, CFC-12, CFC13, SF6) Time series of northern hemisphere atmospheric concentrations and tritium in North Atlantic surface waters

  20. Bomb Fallout Produced Tracers Nuclear 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: IsotopeHalf LifeDecay 90Sr 28 yrs beta 238Pu 86 yrs alpha 239+240Pu 2.44 x 104 yrs alpha 6.6 x 103 yrs alpha 137Cs 30 yrs beta, gamma Nuclear 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).

  21. 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 1980s data: Levin & Kromer 2004; Manning et al 1990; Druffel 1987; Druffel 1989; Druffel & Griffin 1995 bomb spike in 1963

  22. Comparison of 14C in surface ocean Pre-nuclear (1950s) and nuclear (1970s) Atlantic Indian Pacific

  23. Example – Use 14C to calculate ZFILM using the Stagnant Boundary Layer 14Catm Use Pre-bomb 14C – assume steady state 1-box model source = sink 14C from gas exchange = 14C lost by decay [14C] 14C decay Assume [CO2]top = [CO2]bottom = [CO2]surface ocean (e.g. no CO2 gradient, only a 14C gradient)

  24. Assume D = 3 x 10-2 m2 y-1 h = 3800m l-1 = 8200 y [CO2]surf = 0.01 moles m-3 [DIC]ocean = 2.4 moles m-3 a14CO2/aCO2 = 1.015 (14C-CO2 is more soluble than CO2)(a equals solubility constant) (14C/C) surf = 0.96 (14C/C)atm (14C/C)deep = 0.84 (14C/C)atm Then: Zfilm = 1.7 x 10-5 m = 17 mm

  25. Example – 14C Deep Ocean Residence Time substitute for B vmix in cm yr-1; vC in cm yr-1 x mol cm-3

  26. Rearrange and Solve for Vmix Use pre-nuclear 14C data when surface 14C > deep 14C (14C/C)deep = 0.81 (14C/C)surf Vmix = (200 cm y-1) A A = ocean area for h = 3200m thus age of deep ocean box (t) t = 3200m / 2 my-1 = 1600 years

  27. Example: What is the direction and flux of oxygen across the air-sea interface given? PO2 = 0.20 atm KH,O2 = 1.03 x 10-3 mol kg-1 atm-1 O2 in mixed layer = 250 x 10-6 mol l-1 (assume 1L = 1 kg) The wind speed (U10) = 10 m s-1 Answer: O2 in seawater at the top of the stagnant boundary layer = KH PO2 = 1.03 x 10-3 x 0.20 = 206 x 10-6 mol l-1 So O2ml > O2atm and the flux is out of the ocean. What is the flux? With a wind speed = 10 m s-1, the piston velocity (k) = 5 m d-1 DC = (250 – 206) x 10-6 = 44 x 10-5 mol l-1 Flux = 5 m d-1 x 44 x 10-6 mol l-1 x 103 l m-3 = 5 x 44 x 10-6 x 103 = 220 x 10-3 mol m-2 d-1

  28. Example The activity of 222Rn is less than that of 226Ra in the surface water of the North Atlantic at TTO Station 24 (western North Atlantic). Calculate the thickness of the stagnant boundary layer (DZFILM). A226Ra = 8.7 dpm 100l-1 A222Rn = 6.9 dpm 100L-1 Assume: l222Rn = 2.1 x 10-6 s-1 D222Rn = 1.4 x 10-9 m2 s-1 ml = 40m Answer: ZFILM = 40 x 10-6 m

  29. 226Ra Distributions Example 226Ra Profile South Atlantic at 15°S ; 29.5°W

  30. 222Rn as a tracer for gas exchange  d222Rn/dt = sources – sinks = decay of 226Ra – decay of 222Rn - gas exchange to atmosphere

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