Lecture 8 Tracers for Gas Exchange Examples for calibration of gas exchange using: 222 Rn – short term 14 C - long term E&H Sections 5.2 and 10.2

Rates of Gas Exchange Stagnant Boundary Layer Model. Depth (Z) ATM OCN C g = K H P gas = equil. with atm C SW Z Film Stagnant Boundary Layer – transport by molecular diffusion well mixed surface SW well mixed atmosphere 0 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

Expression of Air -Sea CO 2 Flux k = piston velocity = D/Z film From wind speed From CMDL CCGG network S – Solubility From Temperature & Salinity From measurements at sea F = k s (pCO 2w - pCO 2a ) = K ∆ pCO 2 pCO 2a pCO 2w Need to calibrate!

Gas Exchange and Environmental Forcing: Wind Liss and Merlivat,1986 from wind tunnel exp. Wanninkhof, 1992 from 14 C Example conversion: 20 cm hr -1 = 20 x 24 / 10 2 = 4.8 m d -1 ~ 5 m d -1

U-Th Series Tracers

Analytical Method for 222 Rn and 226 Ra Analyze for 222 Rn immediately, then 226 Ra later (after 20 days) charcoal liquid N 2 SW 226 Ra 222 Rn Apply the principle of secular equilibrium! 5 half-lives Activity is what is measured. Not concentration!

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

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

226 Ra source from the sediments Edmond et al (1979) JGR 84,

222 Rn Example Profile from North Atlantic 226 Ra 222 Rn Does Secular Equilibrium Apply? t 1/2 222Rn << t 1/2 226Ra (3.8 d) (1600 yrs) YES! Then.. A 226Ra = A 222Rn Why is 222 Rn activity less than 226 Ra?

222 Rn is a gas and the 222 Rn concentration in the atmosphere is much less than in the ocean mixed layer (Z ml mixed layer). Thus, there is a net evasion (gas flux) of 222 Rn out of the ocean. The simple 1-D 222 Rn balance for the mixed layer, with thickness Z ml, ignoring horizontal advection and vertical exchange with deeper water, is: Z ml 222Rn  [ 222 Rn]/  t = Z ml 226Ra [ 226 Ra] – Z ml 222Rn [ 222 Rn ML ] - D/Z film { [ 222 Rn atm ] – [ 222 Rn ML ]} Knowns: 222Rn,  226Ra, D Rn Measure: Z ml, A 226Ra, A 222Rn, d[ 222 Rn]/dt Solve for Z film  222 Rn/dt = sources – sinks = decay of 226 Ra – decay of 222 Rn - gas exchange to atmosphere

Z ml λ 222Rn d[ 222 Rn]/dt = Z ml λ 226Ra [ 226 Ra] – Z ml λ 222Rn [ 222 Rn] - D/Z film { [ 222 Rn atm ] – [ 222 Rn ML ]} Z ml δA 222Rn / δt = Z ml (A 226 Ra – A 222 Rn) + D/Z (C Rn, atm – C Rn,ML ) for SS = 0 atm Rn = 0 Then -D/Z ( – C Rn,ml ) = Z ml (A 226Ra – A 222Rn ) +D/Z (A Rn,ml /λ Rn ) = Z ml (A 226Ra – A 222Rn ) +D/Z (A Rn,ml ) = Z ml λ Rn (A 226Ra – A 222Rn ) Z FILM = D (A 222Rn,ml ) / Z ml λ Rn (A 226Ra – A 222Rn ) Z FILM = (D / Z ml λ Rn ) ( ) Note: diffusion is expressed in terms of concentrations not activities

Z = D Rn / Z film  222Rn (1/A 226Ra /A 222Rn) ) - 1 Average Z film = 28  m Stagnant Boundary Layer Film Thickness Histogram showing results of film thickness calculations from many stations. Organized by ocean and by latitude Q. What are limitations of this approach? 1.unrealistic physical model 2.steady state assumption 3.short time scale

Cosmic Ray Produced Tracers – including 14 C 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 (pre-nuclear) 3 H12.3 yr3.5 kg 14 C5730 yr54 ton 10 Be1.4 x 10 6 yr430 ton 7 Be54 d32 g 26 Al7.4 x 10 5 yr1.7 ton 32 Si276 yr1.4 kg

Carbon-14 is produced in the upper atmosphere as follows: Cosmic Ray Flux  Fast Neutrons  Slow Neutrons + 14 N*  14 C (protons) (thermal) The overall reaction is written: 14 N + n  14 C + p (7n, 7p) (8n, 6p) (5730 yrs) From galactic cosmic rays from supernova, which are more energetic than solar wind. So these are not from the sun. So the production rate from cosmic rays can be calculated For more detail see: von Blanckenburg and Willenbring (2014) Elements, 10,

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. The main bomb produced isotopes have been: IsotopeHalf LifeDecay 3 H12.3 yrsbeta 14 C5730 yrsbeta 90 Sr28 yrsbeta 238 Pu86 yrsalpha Pu2.44 x 10 4 yrsalpha 6.6 x 10 3 yrsalpha 137 Cs30 yrsbeta, gamma Nuclear weapons testing has been the overwhelmingly predominant source of 3 H, 14 C, 90 Sr and 137 Cs to the ocean. Nuclear weapons testing peaked in 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).

Atmospheric 14 CO 2 in the second half of the 20th century. The figure shows the 14 C / 12 C ratio relative to the natural level in the atmospheric CO 2 as a function of time in the second half of the 20th century.

The bomb spike: surface ocean and atmospheric Δ 14 C since 1950 Massive production in nuclear tests ca ( “ bomb 14 C ” ) Through air-sea gas exchange, the ocean took up ~half of the bomb 14 C by the 1980s bomb spike in 1963 data: Levin & Kromer 2004; Manning et al 1990; Druffel 1987; Druffel 1989; Druffel & Griffin 1995

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

Example – Use 14 C to calculate Z FILM using the Stagnant Boundary Layer Use Pre-bomb 14 C – assume steady state source = sink 14 C from gas exchange = 14 C lost by decay 14 C atm 14 C decay Assume [CO 2 ] top = [CO 2 ] bottom = [CO 2 ] surface ocean (e.g. no CO 2 gradient, only a 14 C gradient) [ 14 C] 1-box model

Assume D = 3 x m 2 y -1 h = 3800m  1 = 8200 y [CO 2 ]surf = 0.01 moles m -3 [DIC]ocean = 2.4 moles m -3  14 CO 2 /  CO 2 = ( 14 C-CO 2 is more soluble than CO 2 )(  equals solubility constant) ( 14 C/C) surf = 0.96 ( 14 C/C)atm ( 14 C/C)deep = 0.84 ( 14 C/C)atm Then: Z film = 1.7 x m = 17  m

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

Rearrange and Solve for V mix Use pre-nuclear 14 C data when surface 14 C > deep 14 C ( 14 C/C)deep = 0.81 ( 14 C/C)surf V mix = (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

Example: What is the direction and flux of oxygen across the air-sea interface given? P O2 = 0.20 atm K H,O2 = 1.03 x mol kg -1 atm -1 O 2 in mixed layer = 250 x mol l -1 (assume 1L = 1 kg) The wind speed (U 10 ) = 10 m s -1 Answer: O 2 in seawater at the top of the stagnant boundary layer = K H P O2 = 1.03 x x 0.20 = 206 x mol l -1 So O 2 ml > O 2 atm 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 D C = (250 – 206) x = 44 x mol l -1 Flux = 5 m d -1 x 44 x mol l -1 x 10 3 l m -3 = 5 x 44 x x 10 3 = 220 x mol m -2 d -1

Example The activity of 222 Rn is less than that of 226 Ra in the surface water of the North Atlantic at TTO Station 24 (western North Atlantic). Calculate the thickness of the stagnant boundary layer (Z FILM ). A 226Ra = 8.7 dpm 100 L -1 A 222Rn = 6.9 dpm 100 L -1 Assume: λ 222Rn = 2.1 x s -1 D 222Rn = 1.4 x m 2 s -1 Z ml = 40m Answer: Z FILM = 40 x m

Tritium ( 3 H) is produced from cosmic ray interactions with N and O. After production it exists as tritiated water ( H - O - 3 H ), thus it is an ideal tracer for water. Tritium concentrations are TU (tritium units) where 1 TU = ( 3 H / H) Thus tritium has a well defined atmospheric input via rain and H 2 O 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 3 H in the ocean at that time. The inventory increased by 200x and was at a maximum in the mid-1970s

Tritium in rain (historical record)

Tritium ( 3 H) in rain and surface SW

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

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

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

222 Rn as a tracer for gas exchange   Rn/  t = sources – sinks = decay of 226 Ra – decay of 222 Rn - gas exchange to atmosphere 