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Roberta Delfanti ENEA –La Spezia, Italy

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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Roberta Delfanti ENEA –La Spezia, Italy

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  1. 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

  2. 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.

  3. 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

  4. 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.

  5. Outline • Fluxesofanthropogenicradionuclides (137Cs) • Vertical profiles in sediments • Factorsaffectingthem: • input • bioturbation • grainsize/porosity • compaction • Inventories

  6. Global fallout Hamilton, 2004

  7. 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

  8. Theoreticalverticalprofileof137Cs in a sedimentcore If sediment acumulation rate is relatively fast (cm/y) the radionuclide vertical profile should reflect its input function.

  9. Factorsaffecting radionuclide profiles Real profiles are influenced by several factors: • differences in • athmospheric input • river inputs • sedimentary regime • bioturbation • grain size NW Med

  10. Factorsaffecting radionuclide profiles: input The presence/magnitude of the Chernobyl and fallout peaks depends on deposition in the area. NE Med

  11. 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.

  12. Factorsaffecting radionuclide profiles: Bioturbation 137Cs vertical profile, NW Med, 2009 Depth: 15 m

  13. 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

  14. No compaction NEW Compaction NEW The sedimentverticalstructure: compaction Compaction: loss of water from a layerofsediment, due tocompression arisingfrom the depositionof overlayingsediment.

  15. The sedimentstructure: compaction The behaviourduringcompactionofsands and clays isdifferent: fine-grainedclaysundergocontinual compactioneven on a cm-by-cmbasis, whileforsand the decrease in porositywithdepthisminimal.

  16. 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

  17. Porosityvs depthBarents Sea,CABANERAcore10,2004 silty, homogeneous sediment.

  18. Porosityvs depthBarents Sea,CABANERAcore 10,2004 coarser sediment, layers with different grain-size.

  19. 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.

  20. 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)

  21. Compaction and RN profiles

  22. 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)

  23. 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

  24. 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.

  25. 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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