Dating models Dating models using man-made radionuclidesusing man-made radionuclides

Part 1:Part 1:137137Cs flux, vertical profiles and inventoriesCs flux, vertical profiles and inventories

Roberta DelfantiENEA –La Spezia, Italy

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IAEA Regional Training Course on Sediment Core Dating Techniques. RAF7/008 Project

CNESTEN, Rabat, 5 – 9 July 2010

Why are we interested in sediments?Why are we interested in sediments?

Sediments are environmental archives where the events that have taken place in the sea are recorded.

Changes in particle supply from catchement basins, pollution, harmful algal blooms, changes in temperature, etc.

All events are characterised by “markers” stored in the sediment.

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Why are we interested in sediments?Why are we interested in sediments?

Sediment coreAlboran Sea.

W-Med after thelast deglaciation.

20,000 y B.P.to present days.

14C givesthe time scale

Cacho et al., 20023

Why are we interested in sediments?Why are we interested in sediments?

Sediments in the coastal areas concentrate most heavy metals, POPs and radionuclides.

More, they contain the whole history of recent pollution.

Radionuclides allow us to Radionuclides allow us to define a time scale for the events define a time scale for the events

registered in sediments.registered in sediments.

The knowledge of how and how fast sediments are accumulated in a coastal area is one of the basic parameters for understanding its functioning and hence for its management.

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OutlineOutline

• Fluxes of anthropogenic radionuclides (Fluxes of anthropogenic radionuclides (137137Cs)Cs)• Vertical profiles in sediments Vertical profiles in sediments • Factors affecting them:Factors affecting them:

inputinput bioturbationbioturbation grain size/porositygrain size/porosity compactioncompaction

• InventoriesInventories

Global falloutGlobal fallout

Hamilton, 2004 6

Input function of Input function of of Antrhropogenic Radionuclides of Antrhropogenic Radionuclides

60 65 70 75 80 85 90 950

200

400

600

800

1000

1200

1400

1600

1800

2000

Flu

x of 1

37 C

s (B

q m

-2 y

-1)

Year

137Cs fallout in N-Italy, 1959 - 2000

239,240239,240PuPu : same input function,no Chernobyl peak

Integrated deposition density(2010)

30- 40°N: 80 Bq m-2

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137Cs cumulative fallout deposition (2010) 30- 40°N: 2 kBq m-2 + Chernobyl

60

65

70

75

80

85

90

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Conc. 137Cs

Ann

o

Theoretical vertical profile of Theoretical vertical profile of 137137Cs Cs in a sediment corein a sediment core

If sediment acumulation rateis relatively fast (cm/y)the radionuclide vertical profile should reflectits input function.

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Factors affecting radionuclide profilesFactors affecting radionuclide profiles

Real profilesare influenced by several factors:

0

5

10

15

20

25

30

0 2 4 6 8 10 12 14 16 18 20

Cs-137 (Bq kg-1)

Dep

th (

cm)

NW Med

differences in athmospheric input

river inputs

sedimentary regime

bioturbation

grain size 9

The presence/magnitudeof the Chernobyl and fallout peaks depends ondeposition in the area.

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35

Activity, Bq/kg

Dept

h, cm

NE Med

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Factors affecting radionuclide profiles:Factors affecting radionuclide profiles:inputinput

Construction of borrows and constant irrigation due to biological activity results in a higher water content of the surface sediment layers

Particle mixing due to biological activity modifies radionuclides profiles.

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Factors affecting radionuclide profiles:Factors affecting radionuclide profiles:BioturbationBioturbation

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Factors affecting radionuclide profiles:Factors affecting radionuclide profiles:BioturbationBioturbation

137Cs vertical profile,NW Med, 2009Depth: 15 m

The sediment structure: The sediment structure: grain size, porositygrain size, porosity

Porosity Φ = Volume of water / Volume of total sediment

Porosity of clay: 0.7 – 0.9Porosity of sand: 0.3 – 0.5

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Boudreau, 1997

The sediment vertical structure: The sediment vertical structure: compactioncompaction

Compaction: loss of water Compaction: loss of water from a layer of sediment, due to compressioncompression arising from the deposition of overlaying sediment.

NEW

No compaction

NEW

Compaction

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The sediment structure: The sediment structure: compactioncompaction

The The behaviour during compaction behaviour during compaction of sands and claysof sands and claysis different: is different: finefine-grained clays undergo -grained clays undergo continual continual compaction compaction even on a cm-by-cm basis, while for even on a cm-by-cm basis, while for sandsandthe decrease in porosity with depth is the decrease in porosity with depth is minimal.minimal.

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PorosityPorosity

For sediment cores, we can plot porosity versus depth.Porosity in the surface layers is higher(lower compaction, bioturbation).

Exponential decrease Homogeneous grain size

High porosity Fine grained sediment

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Porosity Porosity vs depthvs depth

Barents Sea,CABANERA

core 10,2004

0

5

10

15

20

25

30

0 20 40 60 80

Porosity * 100D

ep

th (

cm

)

silty, homogeneoussediment.

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0

5

10

15

20

25

30

0 20 40 60 80

Porosity *100D

epth

(cm

)

coarser sediment,layers with differentgrain-size.

Porosity Porosity vs depthvs depth

Barents Sea,CABANERA

core 10,2004

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Compaction and RN profilesCompaction and RN profilesconstant sed. accum. rate constant sed. accum. rate

2002

2003

2004

2005

2006

No compaction

20022003

2004

2005

2006

Compaction

The dry weight of the sediment is the same in every layer,what changes is the water content.water content.

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Compaction and RN profiles

How can we correct our vertical profilesfor the effect of compaction?

An easy way is to calculate the integrated sedimentmass per unit area and re-plot theradionuclide vertical profile versus mass depth.

mass depth (g cm-2)

weight of dry sediment at a given depth (g)= --------------------------------------------------------- core surface (cm-2)

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Compaction and RN profilesCompaction and RN profiles

30

25

20

15

10

5

00 2 4 6 8 10 12 14 16 18 20

Cs-137 (Bq kg-1)

Mas

s D

epth

(g

cm

-2)

0

5

10

15

20

25

30

0 2 4 6 8 10 12 14 16 18 20

Cs-137 (Bq kg-1)

Dep

th (

cm)

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InventoryInventory

Integrated radionuclideactivityper unit surface(Bq m-2)

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35

Activity, Bq/kg

Mas

s De

pth,

g/c

m2

x=0 RN conc. (Bq/g) * layer dry weight (g)

I = --------------------------------------------------------- Core surface area (m2)

x=z

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Inventories of Inventories of 137137Cs Cs in different areas of the Med Sea in different areas of the Med Sea

Algerian Basin, 2007Depth: 2500 m

Inventory: 0.2 kBq m-2

30

25

20

15

10

5

00 2 4 6 8 10 12 14 16 18 20

Cs-137 (Bq kg-1)

Mass D

ep

th (g

cm

-2)

Ligurian Sea, 2000Depth: 20 m

Inventory: 1.2 kBq m-2

Inventories of Inventories of 137137Cs Cs in the Mediterranean Seain the Mediterranean Sea

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89

Data from: Arnaud et al., 1995; Delfanti et al., 1997Livingston, 1978Barsanti et al., submitted.

72155

70-150

Cumulative Fallout deposition (2010): 1600 Bq m-2

Chernobyl: 1000-15000 Bq m-2

Prodelta mud:2500-64000Shelf mud: 700-6000Sand: 400-1200

Rhone mouth:1200-30000

194

19090

Inventories of Inventories of 137137Cs Cs in the Mediterranean Seain the Mediterranean Sea

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7

>180

9057

4750-34020-130

Data from: Delfanti et al., 1995; Anton et al., 1995Delfanti e Papucci, 1989; Fowler et al., Jennings et al., 1985; Livingston, 1978.

180

37

2

3

Cumulative fallout deposition: 80 Bq m-2