Lecture 13 Tracers for Gas Exchange

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Lecture 13 Tracers for Gas Exchange
Examples for gas exchange using 222Rn 14C
EH Sections 5.2 and 10.2
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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
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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 CMDL CCGG network
From measurements
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U-Th Series Tracers
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Analytical Method for 222Rn and 226Ra
charcoal
liquid N2
SW
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226Ra in Atlantic and Pacific
Q. What controls the ocean distributions of 226Ra?
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226Ra Si correlation Pacific Data
Q. Why is there a hook at the end?
Calculate 226Ra from Si!
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226Ra source from the sediments
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222Rn Example Profile from North Atlantic
Does Secular Equilibrium Apply? t1/2 222Rn ltlt
t1/2 226Ra (3.8 d) (1600
yrs) YES! A226Ra A222Rn
222Rn
226Ra
Why is 222Rn activity less than 226Ra?
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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
?ml l222Rn d222Rn/dt ? ml l226Ra 226Ra ?
l 222Rn 222RnML
D/Zfilm 222Rnatm
222RnML
Knowns l222Rn, l226Ra, DRn Measure ? ml,
A226Ra, A222Rn, d222Rn/dt Solve for Zfilm
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?ml l222Rn d222Rn/dt ? ml l226Ra 226Ra
?ml l222Rn 222Rn
D/Zfilm 222Rnatm
222RnML ?ml dA222Rn/ dt ?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) ?ml lRn (A226Ra
A222Rn) ZFILM D (A222Rn,ml) / ?ml lRn (A226Ra
A222Rn) ZFILM (D / ?ml lRn) (
)
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Stagnant Boundary Layer Film Thickness
Z DRn / ? l 222Rn
(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

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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 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
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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)
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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
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Tritium in rain (historical record)
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Tritium (3H) in rain and surface SW
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Tritium is a conservative tracer for water (as
HTO) thermocline penetration
Eq
Meridional Section in the Pacific
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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
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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 Isotope Half Life Decay 90Sr 28
yrs beta 238Pu 86 yrs alpha 239240Pu 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).
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The bomb spike surface ocean and atmospheric
?14C 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
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Comparison of 14C in surface ocean Pre-nuclear
(1950s) and nuclear (1970s)
Atlantic
Indian
Pacific
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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 CO2top CO2bottom CO2surface
ocean (e.g. no CO2 gradient, only a
14C gradient)
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Assume D 3 x 10-2 m2 y-1 h 3800m l-1 8200
y CO2surf 0.01 moles m-3 DICocean 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)de
ep 0.84 (14C/C)atm
Then Zfilm 1.7 x 10-5 m 17 mm
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Example 14C Deep Ocean Residence Time
substitute for B
vmix in cm yr-1 vC in cm yr-1 x mol cm-3
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Rearrange and Solve for Vmix Use pre-nuclear 14C
data when surface 14C gt 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
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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 O2 ml gt 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
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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
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226Ra Distributions
Example 226Ra Profile South Atlantic at 15S
29.5W
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222Rn as a tracer for gas exchange
?
d222Rn/dt sources sinks decay of 226Ra
decay of 222Rn - gas exchange to atmosphere