Lecture 8 Tracers for Gas Exchange

Examples for calibration of gas exchange using:222Rn – short term14C - long term

E&H Sections 5.2 and 10.2

Rates of Gas ExchangeStagnant Boundary Layer Model.

Depth (Z)

ATMOCN

Cg = KH Pgas = equil. with atm

CSW

ZFilm

Stagnant BoundaryLayer – 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, p181Broecker and Peng (1974) Tellus, 26, p21Liss (1973) Deep-Sea Research, 20, p221

Expression of Air -Sea CO2 Flux

k = piston velocity = D/Zfilm

From wind speed

From CMDLCCGG network

S – SolubilityFrom Temperature & Salinity

From measurementsat sea

F = k s (pCO2w- pCO2a) = K ∆ pCO2

pCO2apCO2w

Need to calibrate!

Gas Exchange and Environmental Forcing: Wind

Liss and Merlivat,1986from wind tunnel exp.

Wanninkhof, 1992from 14C

Example conversion:20 cm hr-1 = 20 x 24 / 102 = 4.8 m d-1

~ 5 m d-1

U-Th Series Tracers

Analytical Method for 222Rn and 226RaAnalyze for 222Rn immediately, then 226Ra later (after 20 days)

charcoal

liquid N2

SW

226Ra

222Rn

Apply the principle of secular equilibrium!

5 half-lives

Activity is what is measured. Notconcentration!

226Ra profiles in Atlantic and Pacific

Q. What controls the ocean distributions of 226Ra?

226Ra – Si correlation – Pacific DataQ. Why is there a hookat the end?You can calculate 226Ra from Si!

226Ra source from the sediments

Edmond et al (1979) JGR 84, 7809-7826

222Rn Example Profile from North Atlantic

226Ra

222Rn

Does Secular Equilibrium Apply?t1/2 222Rn << t1/2 226Ra

(3.8 d) (1600 yrs)

YES! Then..A226Ra = A222Rn

Why 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 (Zml mixed layer).

Thus, there is a net evasion (gas flux) of 222Rn out of the ocean.

The simple 1-D 222Rn balance for the mixed layer, with thickness Zml, ignoring horizontal advection and vertical exchange with deeper water, is:

Zml l222Rn [222Rn]/t = Z ml l226Ra [226Ra] – Zml l 222Rn [222RnML] - D/Zfilm { [222Rnatm] – [222RnML]}

Knowns: l222Rn, l226Ra, DRn

Measure: Zml, A226Ra, A222Rn, d[222Rn]/dt

Solve for Zfilm

222Rn/dt = sources – sinks = decay of 226Ra – decay of 222Rn - gas exchange to atmosphere

Zml λ222Rn d[222Rn]/dt = Z ml λ226Ra [226Ra] – Zml λ222Rn [222Rn] - D/Zfilm { [222Rnatm] – [222RnML]}

Zml δA222Rn/ δt = Zml (A226Ra – A222Rn) + D/Z (CRn, atm – CRn,ML)

for SS = 0 atm Rn = 0

Then

-D/Z ( – CRn,ml) = Zml (A226Ra – A222Rn)

+D/Z (ARn,ml/λRn) = Zml (A226Ra – A222Rn)

+D/Z (ARn,ml) = Zml λRn (A226Ra – A222Rn)

ZFILM = D (A222Rn,ml) / Zml λRn (A226Ra – A222Rn)

ZFILM = (D / Zml λRn) ( )226

222

1

1Ra

Rn

AA

Note: diffusion isexpressed in terms ofconcentrations notactivities

Z = DRn / Zfilm l 222Rn (1/A226Ra/A222Rn) ) - 1

Average Zfilm = 28 mm

Stagnant Boundary Layer Film Thickness

Histogram showing results of film thicknesscalculations from many stations.

Organized by ocean and by latitude

Q. What are limitations of this approach?1. unrealistic physical model2. steady state assumption3. short time scale

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.

Isotope Half-life Global inventory (pre-nuclear)3H 12.3 yr 3.5 kg14C 5730 yr 54 ton10Be 1.4 x 106 yr 430 ton7Be 54 d 32 g26Al 7.4 x 105 yr 1.7 ton32Si 276 yr 1.4 kg

Carbon-14 is produced in the upper atmosphere as follows:

Cosmic Ray Flux Fast Neutrons Slow Neutrons + 14N* 14C(protons) (thermal)

The overall reaction is written:

14N + n 14C + p(7n, 7p) (8n, 6p)

(5730 yrs)

From galactic cosmic rays fromsupernova, which are more energetic thansolar 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, 341-346

Bomb Fallout Produced TracersNuclear 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:

Isotope Half Life Decay3H 12.3 yrs beta14C 5730 yrs beta90Sr 28 yrs beta238Pu 86 yrs alpha239+240Pu 2.44 x 104 yrs alpha

6.6 x 103 yrs alpha137Cs 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).

Atmospheric 14CO2 in the second half of the 20th century.

The figure shows the 14C / 12C ratio relative to the natural level in the atmospheric CO2 as a function of time in the second half of the 20th century.

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

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

Comparison of 14C in surface ocean

Pre-nuclear (1950s) and nuclear (1970s)

Atlantic

Indian

Pacific

Example – Use 14C to calculate ZFILM using the Stagnant Boundary Layer

Use Pre-bomb 14C – assume steady state

source = sink14C from gas exchange = 14C lost by decay

14Catm

14C decay

Assume [CO2]top = [CO2]bottom = [CO2]surface ocean (e.g. no CO2 gradient, only a 14C gradient)

[14C]

1-box model

AssumeD = 3 x 10-2 m2 y-1

h = 3800ml1 = 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

Example – 14C Deep Ocean Residence Time

substitute for Bvmix in cm yr-1; vC in cm yr-1 x mol cm-3

Rearrange andSolve 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 areafor 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?

PO2 = 0.20 atmKH,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 O2 ml > O2 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

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

ExampleThe activity of 222Rn is less than that of 226Ra in the surface water of theNorth Atlantic at TTO Station 24 (western North Atlantic). Calculate the thickness of the stagnant boundary layer (ZFILM).

A226Ra = 8.7 dpm 100 L-1

A222Rn = 6.9 dpm 100 L-1

Assume:λ222Rn = 2.1 x 10-6 s-1

D222Rn = 1.4 x 10-9 m2 s-1

Zml = 40m

Answer: ZFILM = 40 x 10-6 m

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-1970s

Tritium in rain (historical record)

Tritium (3H) 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 concentrationsand tritium in North Atlantic surface waters

Atmospheric Record of Thermocline Ventilation TracersConservative, non-radioactive tracers (CFC-11, CFC-12, CFC13, SF6)

Example 226Ra ProfileSouth Atlantic at 15°S ; 29.5°W

226Ra Distributions

222Rn as a tracer for gas exchange

222Rn/t = sources – sinks = decay of 226Ra – decay of 222Rn - gas exchange to atmosphere