Transient tracers studies of the ocean circulation
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Transient tracers studies
of the ocean circulation
Roberta DelfantiENEA, Marine Environment Research Centre
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1. The transient tracer concept
2. The input functions
3. Tracer selection
4. Examples
“Transient tracers' studies of the ocean circulation ”
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Knowing: ◊ input function (time and space) ◊ chemical/biological behaviour◊ evolution of their distributions within the
sea
The tracer concept
Information on oceanographic processes. Patterns and rates of
◊ circulation◊ ventilation◊ sediment transport◊ particle (carbon) fluxes…..
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The most important tracers
◊ Anthropogenic substances
CFC’s
3H – 3He, 14C
137Cs, 90Sr, 99Tc, 129I, 125Sb
◊ Primordial radionuclides
(mainly: U-Th series)
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The tracer conceptThe tracer concept
Radionuclides as tracers of marine processes.
Swimmingradionuclides
Ra-226, Ra-228, H-3 (C-14)
Running radionuclidesU-Th series Diving radionuclides
Th-234, Th-228, Th-230,Pb-210
Sediments: C-14, Pb-210, Th-224
Ra-226, Ra-228, Ra-222
Flying radionuclides
(Radon Rn-222)
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The tracer concept
Café Thorium web page: http://www.whoi.edu/science/MCG/cafethorium/website
Ex. Th-234 as tracer of particle dynamics in the upper ocean
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The tracer concept
14C as tracer of sediment accumulation rates in deep-sea environments
20
15
10
5
00,00 0,04 0,08 0,12
)
Dep
th (
cm)
Bq g-1 C
Vertical profile of 14Cin a sediment core ofthe deep N Atlantic.
Const rate of supply andcostant concentration atsed-water interface.
Decrease of concentrationalong core due to physical decay,clock of the process.
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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 scaleof the process studied. Most used:
CFC’s, SF63H – 3He
14C137Cs, 90Sr, 125Sb, 99Tc, 129I
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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
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The tracers of ventilation - CFCsThe tracers of ventilation - CFCs
CFCs are anthropogenic substances
Their atmospheric concentrations have increased with time
CFCs enter the ocean at the sea surface through gas exchangeEach year the imprinted concentrations in the mixed layer increase with the atmospheric concentrations
Gas exchange with atmosphere at surface
Ocean
Atmosphere
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Tritium input function
Time history of Tritium input fuction at 50°N and 50°S (Roether and Rhein, 1989)
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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.
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Selected features of transient tracers
TracerTracer ChemicChemical formal form
UnitsUnits MeasuremenMeasurement t
techinquetechinque
PrecisionPrecision Eff. dyn. Eff. dyn. rangerange
F 11F 11
F 12F 12
F 113F 113
CClCCl44
CClCCl33FF
CClCCl22FF22
CClCCl22FCClFCClFF22
CClCCl44
pmol/pmol/kgkg
or pptvor pptv
EDC gas EDC gas
chromatogr.chromatogr.1%1% 500500
bomb bomb tritiumtritium
HTOHTO TUTUgas countinggas counting
------------------------------------33He ingrowthHe ingrowth
5%5%
----------------------------------
2%2%
30…330…3
----------------------------------
300..30300..30
bomb bomb 1414CC DissolveDissolvedd
C speciesC species1414CC
gas countinggas counting
------------------------------------
AMSAMS
0.2%0.2%
----------------------------------
0.5%0.5%1515
tritiugenic tritiugenic 33HeHe
HeHe TUTU mass. spectr.mass. spectr. 0.2%0.2% 30…330…3
Roether, 1994
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THE OCEAN CONVEYORTHE OCEAN CONVEYOR
Carries warm surface waters from the tropics northward. At high Carries warm surface waters from the tropics northward. At high latitudes, the waters cool, releasing heat to the atmosphere and latitudes, the waters cool, releasing heat to the atmosphere and moderating wintertime climate in the North Atlantic region. The moderating wintertime climate in the North Atlantic region. The colder (and denser) waters sink and flow southward in the deep colder (and denser) waters sink and flow southward in the deep
ocean to keep the conveyor moving. ocean to keep the conveyor moving.
(Illustration by Jack Cook, WHOI)
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Information from tracer distributions
CFC-11 invasion in N-Atlantic along a section at 19.5°W, 1955-83http://puddle.mit.edu/~mick/cfcall.html
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Information from tracer distributions
CFC-11 invading deep N-Atlantic at 1975 m depth, 1955-83http://puddle.mit.edu/~mick/cfcall.html
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Information from tracer distributions
GEOSECS - Geochemical Ocean Sections 1972-78
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Information from tracer distributions
TRANSIENT TRACERS IN THE OCEANS
TTO
1980-83
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Information from tracer distributions
WOCE – World Ocean Circulation Experiment 1990-98
Cruise tracks in theAtlanticOcean
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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
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Information from tracer distributions –spreading and pathways
GEOSECS ATLAS – Salinity, 1972
EQ 40°N
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Information from tracer distributions –spreading and pathways
GEOSECS ATLAS – Tritium (TU), 1972
EQ 30° N
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Information from tracer distributions –spreading and pathways
GEOSECS, 1972
TTO, 1981
Western North Atlantic
10°S 80°N
Östlund and Rooth, 1990
30° N
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Information from tracer distributions –spreading and pathways
Jenkins and Smethie, 1996
BERMUDA TIME SERIES STATION 32°N – Tritium
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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.
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The tritium/helium age
3H 3He + β t1/2= 12.5 y λ= 0.055 y-1
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.
The time elapsed since the water left the surface, τ, can then be calculated from the concentrations of 3H and excess 3He.
T =
3He
3H
1
λ
1 +loge( )
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The tracer age
Jenkins and Smethie, 1996
Tritium and tritium-helim agein the core of the Deep Water Boundary Current vs. distance downstreamfrom its origin.
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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 twotracers concentrations at the surface was equal to the interior ratio.
R(t) = R0 (t – τconc)
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CFC age distribution along the isopycnal sigth=27.8, characterising the Labrador Sea Water, LSW (depth 2000m). (Rhein et al., 2001).
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CFC age distribution along the patway of DSOW. (Rhein et al., 2001).
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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.
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The tracer ages
Differences in ages calculated using differenttracers.
Ages reflect the character of the tracers as wellas chronometric time.
Differences can be quantified and used to infermixing rates.
See. Waugh et al., JGR 2003Doney et al., DSR 1997.
Doney et al., 1997, North-Central Atlantic
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The Caribbean SeaThe Caribbean Sea
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Coral records of water characteristicsCoral records of water characteristicsin the Caribbean Seain the Caribbean Sea
Massive coral Montastrraea faviolata
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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
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1963-70: Subtropical origin
1977-83: Equatorial origin
Florida Current water Florida Current water
Kilbourne et al. 2007
Red: Puerto RicoGrey: South AtlanticBlack: Subtropical Gyre
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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
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Example: transit time of water mass in N-Atlantic - Arctic
Sellafield
Point source: Sellafield reprocessing plant
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Example: transit time of water mass in N-Atlantic - Arctic
Circulation of surface waters of the North,Norwegian,Greenland andBarents Seas.
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Example: transport time of radionuclidesin N-Atlantic and Arctic
AMAP, 1997
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Example: transit time of water mass and transfer factors in N-Atlantic - Arctic
1y
3y
3-4y
4-6y6-8y
Transport timeof 137Csin surface water
6-8y
Dahlgaard et al.,1995
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Example: transit time of water mass and transfer factors in N-Atlantic - Arctic
Nies et al., 1998
Consequence of dumping ofnuclear wastes in the Arctic Ocean:transport and dispersion of 137Cs in the Arctic.
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Example: transit time of water mass and transfer factors in N-Atlantic - Arctic
Nies et al., 1998
Consequence of dumping ofnuclear wastes in the Arctic Ocean:transport and dispersion of 137Cs in the Arctic.Sellafield hindcast scenario1965-1995
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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)
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More rapid transport to North Sea (6-9 months)More rapid transport to North Sea (6-9 months)
Slow northward transport with NwCC and WSC.Slow northward transport with NwCC and WSC.
Correspondence Correspondence
with variations with variations
in NAO in NAO
winter index.winter index.
Example: transit time of water mass and transfer factors in N-Atlantic - Arctic
Kershaw et al., 2004
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Smith et al.,1998
Vertical sectionof 129Iin the Arctic Ocean
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Transient tracers help to constrain Transient tracers help to constrain circulation and mixing in the oceancirculation and mixing in the ocean
and give information that cannot beand give information that cannot be
obtained by hydrographic data alone.obtained by hydrographic data alone.
• estimate deep water formation/renewal estimate deep water formation/renewal rates rates
• detect and follow the paths of water detect and follow the paths of water masses masses
• estimate time scales of spreading estimate time scales of spreading
• validate modelsvalidate models
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Thanks for your attention !!
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Alfimov et al., 2004
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Gascard et al, 2004
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Gascard et al, 2004