Dating models Dating models using man-made using man-made radionuclides radionuclides Part 1: Part 1: 137 137 Cs flux, vertical profiles Cs flux, vertical profiles and inventories and inventories Roberta Delfanti ENEA –La Spezia, Italy 1 IAEA Regional Training Course on Sediment Core Dating Techniques. RAF7/008 Project CNESTEN, Rabat, 5 – 9 July 2010
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Dating models using man-made radionuclides Part 1: 137 Cs flux, vertical profiles and inventories Roberta Delfanti ENEA –La Spezia, Italy 1 IAEA Regional.
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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:
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
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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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Data from: Arnaud et al., 1995; Delfanti et al., 1997Livingston, 1978Barsanti et al., submitted.