Transient tracers studies

of the ocean circulation

Roberta DelfantiENEA, Marine Environment Research Centre

1. The transient tracer concept

2. The input functions

3. Tracer selection

4. Examples

“Transient tracers' studies of the ocean circulation ”

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…..

The most important tracers

◊ Anthropogenic substances

CFC’s

3H – 3He, 14C

137Cs, 90Sr, 99Tc, 129I, 125Sb

◊ Primordial radionuclides

(mainly: U-Th series)

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)

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

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.

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

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 - 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

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

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

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)

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

Information from tracer distributions

CFC-11 invading deep N-Atlantic at 1975 m depth, 1955-83http://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 theAtlanticOcean

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

Information from tracer distributions –spreading and pathways

GEOSECS, 1972

TTO, 1981

Western North Atlantic

10°S 80°N

Östlund and Rooth, 1990

30° N

Information from tracer distributions –spreading and pathways

Jenkins and Smethie, 1996

BERMUDA TIME SERIES STATION 32°N – Tritium

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

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( )

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.

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)

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 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

The Caribbean SeaThe Caribbean Sea

Coral records of water characteristicsCoral records of water characteristicsin the Caribbean Seain 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

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

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 andBarents Seas.

Example: transport time of radionuclidesin N-Atlantic and Arctic

AMAP, 1997

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

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.

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

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)

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

Smith et al.,1998

Vertical sectionof 129Iin the Arctic Ocean

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

Thanks for your attention !!

Alfimov et al., 2004

Gascard et al, 2004

Gascard et al, 2004