Transient tracers studies
of the ocean circulation
Roberta Delfanti
ENEA, Marine Environment Research Centre
“Transient tracers' studies of the ocean circulation ”
1. The transient tracer concept
2. The input functions
3. Tracer selection
4. Examples
The tracer concept
Knowing:
◊ input function (time and space)
◊ chemical/biological behaviour
◊ evolution of their distributions within the sea
Information on oceanographic processes.
Patterns and rates of
◊ circulation
◊ ventilation
◊ sediment transport
◊ particle (carbon) fluxes…..
The most important tracers
◊ Anthropogenic substances
CFC’s
3H
– 3He,
14C
137Cs, 90Sr, 99Tc, 129I, 125Sb
◊ Primordial radionuclides
(mainly: U-Th series)
The tracer concept
Radionuclides as tracers of marine processes.
Flying radionuclides
Running radionuclides
U-Th series
Diving radionuclides
Th-234, Th-228, Th-230,
Pb-210
(Radon Rn-222)
Ra-226, Ra-228, Ra-222
Swimming
radionuclides
Ra-226, Ra-228,
H-3 (C-14)
Sediments: C-14, Pb-210, Th-224
The tracer concept
Ex. Th-234 as tracer of particle dynamics in the upper ocean
Café Thorium web page: http://www.whoi.edu/science/MCG/cafethorium/website
The tracer concept
14C
as tracer of sediment accumulation rates
in deep-sea environments
Bq
g-1 C
)
0,00 0,04 0,08 0,12
0
Vertical profile of 14C
in a sediment core of
the deep N Atlantic.
Depth (cm)
5
10
15
20
Const rate of supply and
costant concentration at
sed-water interface.
Decrease of concentration
along core due to physical decay,
clock of the process.
The tracer concept
In physical oceanography:
Use of conservative tracers:
- “soluble” in seawater
- known input function
for radioactive tracers, select:
t1/2 compatible with time scale
of the process studied.
Most used:
CFC’s, SF6
3H – 3He
14C
137Cs, 90Sr, 125Sb, 99Tc, 129I
Input functions
Time series of
atmospheric
concentrations of
CFC-11, CFC-12,
CFC-113 and SF6.
Time series of
tritium in North
Atlantic Ocean
surface waters.
http://www.jhu.edu/~dwaugh1/ttd_tracerages.html
The tracers of ventilation - CFCs
Atmosphere
Gas exchange with
atmosphere at surface
Ocean
 CFCs are anthropogenic substances
 Their atmospheric concentrations
have increased with time
 CFCs enter the ocean at the sea surface
through gas exchange
Each year the imprinted concentrations in
the mixed layer increase with the
atmospheric concentrations
Tritium input function
Time history of Tritium input fuction at 50°N and 50°S (Roether and Rhein, 1989)
The tracer concept
Information from tracer distribution:
Water mass spreading and pathways
Deep ventilation and
water mass transformation processes
“Ages”
Rates of movements and amount of dilution
Model validation.
Selected features of transient tracers
Tracer
F 11
F 12
F 113
CCl4
Chemical
form
CCl3F
CCl2F2
CCl2FCClF2
CCl4
Units
Measurement
techinque
EDC gas
chromatogr.
1%
500
TU
gas counting
-----------------3He ingrowth
5%
----------------2%
30…3
----------------300..30
0.2%
----------------0.5%
15
0.2%
30…3
HTO
bomb
Dissolved
C species
D 14C
gas counting
-----------------AMS
He
TU
mass. spectr.
tritiugenic
3He
Roether, 1994
Eff. dyn.
range
pmol/kg
or pptv
bomb
tritium
14C
Precision
THE OCEAN CONVEYOR
Carries warm surface waters from the tropics northward. At high latitudes,
the waters cool, releasing heat to the atmosphere and moderating
wintertime climate in the North Atlantic region. The colder (and denser)
waters sink and flow southward in the deep ocean to keep the conveyor
moving.
(Illustration by Jack Cook, WHOI)
Information from tracer distributions
CFC-11 invasion in N-Atlantic along a section at 19.5°W, 1955-83
http://puddle.mit.edu/~mick/cfcall.html
Information from tracer distributions
CFC-11 invading deep N-Atlantic at 1975 m depth, 1955-83
http://puddle.mit.edu/~mick/cfcall.html
Information from tracer distributions
GEOSECS - Geochemical Ocean Sections
1972-78
Information from tracer distributions
TRANSIENT
TRACERS IN
THE OCEANS
TTO
1980-83
Information from tracer distributions
WOCE – World Ocean Circulation Experiment 1990-98
Cruise tracks
in the
Atlantic
Ocean
Information from tracer distributions
FUTURE:
GEOTRACES is an international study of the
global marine biogeochemical cycles of
trace elements and their isotopes.
Promoted by SCOR
Information from tracer distributions –
spreading and pathways
GEOSECS ATLAS – Salinity, 1972
EQ
40°N
Information from tracer distributions –
spreading and pathways
GEOSECS ATLAS – Tritium (TU), 1972
EQ
30° N
10°S
Information from tracer distributions –
80°N
spreading and pathways
30° N
Western North Atlantic
GEOSECS, 1972
TTO, 1981
Östlund and Rooth, 1990
Information from tracer distributions –
spreading and pathways
BERMUDA TIME SERIES STATION 32°N – Tritium
Jenkins and Smethie, 1996
The “age” concept
Elapsed time since a water mass was last
at the ocean surface in direct contact with
the atmosphere.
The age information can be derived from
radioactive decay as for tritium, or from the
temporally changing atmospheric
concentrations, as for the CFCs.
The tritium/helium age
3H
3He
+β
t1/2= 12.5 y
λ= 0.055 y-1
The time elapsed since the water left the surface, τ, can
then be calculated from the concentrations of 3H and
excess 3He.
In the mixed layer 3He is in equilibrium with atmospheric
He, while in the ocean interior tritium decay leads to
ingrowth of an excess 3He.
T=
1
λ
loge ( 1 +
3He
3H
)
The tracer age
Tritium and
tritium-helim age
in the core of
the Deep Water
Boundary Current
vs. distance downstream
from its origin.
Jenkins and Smethie, 1996
The CFC age
Ages can also be computed from CFC concentrations and
can be defined as
1. elapsed time since the surface concentration
was equal to the one we measure at time t.
c(t) = C0 (t – τconc)
2. elapsed time since the ratio of two
tracers concentrations at the surface
was equal to the interior ratio.
R(t) = R0 (t – τconc)
CFC age distribution along the isopycnal sigth=27.8,
characterising the Labrador Sea Water, LSW (depth 2000m).
(Rhein et al., 2001).
CFC age distribution along the patway of DSOW.
(Rhein et al., 2001).
The CFC age
The calculated ages are correct if:
◊
◊
the water mass was in equilibrium with the
atmosphere at the time of formation;
mixing occurs only with tracer-free water.
The tracer ages
Differences in ages
calculated using different
tracers.
Ages reflect the character
of the tracers as well
as chronometric time.
Differences can be
quantified and used to infer
mixing rates.
Doney et al., 1997, North-Central Atlantic
See. Waugh et al., JGR 2003
Doney et al., DSR 1997.
The Caribbean Sea
Coral records of water characteristics
in the Caribbean Sea
Massive coral Montastrraea faviolata
Source water variations in the Caribbean Sea
Radiocarbon of
seawater DIC
1981 - 1983
(Nydal and Løvseth
1996; Ostlund and
Grall 1987) and
coral radiocarbon
values at Bermuda,
Puerto Rico and Cabo
for 1983 outlined in
black.
Kilbourne et al. 2007
Florida Current water
1963-70: Subtropical origin
Kilbourne et al. 2007
Red: Puerto Rico
Grey: South Atlantic
Black: Subtropical Gyre
1977-83: Equatorial origin
Anthropogenic radionuclides
137Cs, 99Tc, 125Sb
and 129I
have mainly been used in specific areas,
where point sources are present.
Nuclear fuel
reprocessing plants
Sellafield
La Hague
Example: transit time of water mass in
N-Atlantic - Arctic
Sellafield
Point source: Sellafield reprocessing plant
Example: transit time of water mass in
N-Atlantic - Arctic
Circulation of
surface waters of the
North,Norwegian,
Greenland and
Barents Seas.
Example: transport time of radionuclides
in N-Atlantic and Arctic
AMAP, 1997
Example: transit time of water mass and
transfer factors in N-Atlantic - Arctic
6-8y
4-6y
Transport time
of 137Cs
in surface water
6-8y
3-4y
3y
1y
Dahlgaard et al.,1995
Example: transit time of water mass and
transfer factors in N-Atlantic - Arctic
Consequence of dumping of
nuclear wastes
in the Arctic Ocean:
transport and dispersion of
137Cs in the Arctic.
Nies et al., 1998
Example: transit time of water mass and
transfer factors in N-Atlantic - Arctic
Consequence of dumping of
nuclear wastes
in the Arctic Ocean:
transport and dispersion of
137Cs in the Arctic.
Sellafield hindcast scenario
1965-1995
Nies et al., 1998
Example: transit time of water mass and
transfer factors in N-Atlantic - Arctic
Discharges of 99Tc from Sellafield and La Hague (Kershaw et al., 2004)
Example: transit time of water mass and
transfer factors in N-Atlantic - Arctic
More rapid transport to North Sea (6-9 months)
Slow northward transport with NwCC and WSC.
Correspondence
with variations
in NAO
winter index.
Kershaw et al., 2004
Vertical
section
of 129I
in the
Arctic Ocean
Smith et al.,
1998
Transient tracers help to constrain
circulation and mixing in the ocean
and give information that cannot be
obtained by hydrographic data alone.
• estimate deep water formation/renewal rates
• detect and follow the paths of water masses
• estimate time scales of spreading
• validate models
Thanks for your attention !!
Alfimov et al., 2004
Gascard et al, 2004
Gascard et al, 2004