Dating models using man made radionuclides part 1 137 cs flux vertical profiles and inventories
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IAEA Regional Training Course on Sediment Core Dating Techniques. RAF7/008 Project CNESTEN, Rabat, 5 – 9 July 2010. Dating models using man-made radionuclides Part 1: 137 Cs flux , vertical profiles and inventories. Roberta Delfanti ENEA –La Spezia, Italy.

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Dating models using man made radionuclides part 1 137 cs flux vertical profiles and inventories

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

CNESTEN, Rabat, 5 – 9 July 2010

Datingmodelsusingman-maderadionuclidesPart 1:137Cs flux, verticalprofiles and inventories

Roberta Delfanti

ENEA –La Spezia, Italy


Why are we interested in sediments

Why are weinterested 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.


Why are we interested in sediments1

Why are weinterested in sediments?

Sediment core

Alboran Sea.

W-Med after the

last deglaciation.

20,000 y B.P.

to present days.

14C gives

the time scale

Cacho et al., 2002


Why are we interested in sediments2

Why are weinterested in sediments?

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

More, they contain the whole history of recent pollution.

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.

Radionuclidesallowusto

define a time scale for the events

registered in sediments.


Roberta delfanti enea la spezia italy

Outline

  • Fluxesofanthropogenicradionuclides (137Cs)

  • Vertical profiles in sediments

  • Factorsaffectingthem:

    • input

    • bioturbation

    • grainsize/porosity

    • compaction

  • Inventories


Roberta delfanti enea la spezia italy

Global fallout

Hamilton, 2004


Input function of of antrhropogenic radionuclides

Input functionofofAntrhropogenicRadionuclides

239,240Pu : same input function,

no Chernobyl peak

Integrated

deposition density(2010)

30- 40°N: 80 Bq m-2

137Cs fallout in N-Italy, 1959 - 2000

137Cs cumulative fallout deposition (2010) 30- 40°N:

2 kBq m-2 + Chernobyl


Roberta delfanti enea la spezia italy

Theoreticalverticalprofileof137Cs

in a sedimentcore

If sediment acumulation rate

is relatively fast (cm/y)

the radionuclide

vertical profile

should reflect

its input function.


Factors affecting radionuclide profiles

Factorsaffecting radionuclide profiles

Real profiles

are influenced

by several factors:

  • differences in

  • athmospheric input

  • river inputs

  • sedimentary regime

  • bioturbation

  • grain size

NW Med


Roberta delfanti enea la spezia italy

Factorsaffecting radionuclide profiles:

input

The presence/magnitude

of the Chernobyl

and fallout peaks

depends on

deposition in the area.

NE Med


Roberta delfanti enea la spezia italy

Factorsaffecting radionuclide profiles:

Bioturbation

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.


Roberta delfanti enea la spezia italy

Factorsaffecting radionuclide profiles:

Bioturbation

137Cs vertical profile,

NW Med, 2009

Depth: 15 m


The sediment structure grain size porosity

The sedimentstructure: grainsize, porosity

Porosity of clay: 0.7 – 0.9

Porosity of sand: 0.3 – 0.5

Boudreau, 1997

Porosity Φ = Volume of water / Volume of total sediment


The sediment vertical structure compaction

No compaction

NEW

Compaction

NEW

The sedimentverticalstructure: compaction

Compaction: loss of water

from a layerofsediment,

due tocompression

arisingfrom the depositionof

overlayingsediment.


The sediment structure compaction

The sedimentstructure: compaction

The behaviourduringcompactionofsands and clays

isdifferent: fine-grainedclaysundergocontinual

compactioneven on a cm-by-cmbasis, whileforsand

the decrease in porositywithdepthisminimal.


Porosity

Exponential decrease

High porosity

Porosity

For sediment cores, we can plot porosity versus depth.

Porosity in the surface layers is higher

(lower compaction, bioturbation).

Homogeneous grain size

Fine grained sediment


Porosity vs depth barents sea cabanera core 10 2004

Porosityvs depthBarents Sea,CABANERAcore10,2004

silty,

homogeneous

sediment.


Roberta delfanti enea la spezia italy

Porosityvs depthBarents Sea,CABANERAcore 10,2004

coarser sediment,

layers with different

grain-size.


Compaction and rn profiles constant sed accum rate

2006

2005

2006

2004

2005

2003

2004

2002

2003

2002

Compaction and RN profilesconstantsed. accum. rate

No compaction

Compaction

The dry weightof the sediment

is the same in everylayer,

whatchangesis the water content.


Compaction and rn profiles

Compaction and RN profiles

How can we correct our vertical profiles

for the effect of compaction?

An easy way is to calculate the integrated sediment

mass per unit area and re-plot the

radionuclide vertical profile versus mass depth.

mass depth (g cm-2)

weight of dry sediment at a given depth (g)

= ---------------------------------------------------------

core surface (cm-2)


Compaction and rn profiles1

Compaction and RN profiles


Inventory

x=z

x=0RNconc. (Bq/g) * layer dry weight (g)

I = ---------------------------------------------------------

Core surface area (m2)

Inventory

Integrated radionuclide

activity

per unit surface

(Bq m-2)


Roberta delfanti enea la spezia italy

Inventoriesof137Cs

in differentareasof the MedSea

Algerian Basin, 2007

Depth: 2500 m

Inventory: 0.2 kBq m-2

Ligurian Sea, 2000

Depth: 20 m

Inventory: 1.2 kBq m-2


Inventories of 137 cs in the mediterranean sea

Prodelta mud:2500-64000

Shelf mud: 700-6000

Sand: 400-1200

Inventoriesof137Cs in the MediterraneanSea

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

Chernobyl: 1000-15000 Bq m-2

Rhone mouth:

1200-30000

190

90

70-150

194

89

72

155

Data from:

Arnaud et al., 1995; Delfanti et al., 1997

Livingston, 1978

Barsanti et al., submitted.


Inventories of 137 cs in the mediterranean sea1

180

90

57

20-130

50-340

>180

47

Inventoriesof137Cs in the MediterraneanSea

Cumulative fallout deposition: 80 Bq m-2

2

3

7

7

3

Data from:

Delfanti et al., 1995; Anton et al., 1995

Delfanti e Papucci, 1989; Fowler et al.,

Jennings et al., 1985; Livingston, 1978.


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