1. Dating models using man-made radionuclides Part 1: 137 Cs flux, vertical profiles 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

2. 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. 2

3. Why are we interested in sediments? Sediment core Alboran Sea. W-Med after the last deglaciation. 20,000 y B.P. to present days. 14 C gives the time scale Cacho et al., 2002 3

4. 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 define a time scale for the events 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. 4

5. 5 Outline Fluxes of anthropogenic radionuclides ( 137 Cs) Fluxes of anthropogenic radionuclides ( 137 Cs) Vertical profiles in sediments Vertical profiles in sediments Factors affecting them: Factors affecting them: input input bioturbation bioturbation grain size/porosity grain size/porosity compaction compaction Inventories Inventories

6. Global fallout Hamilton, 2004 6

7. Input function of of Antrhropogenic Radionuclides 137 Cs fallout in N-Italy, 1959 - 2000 239,240 Pu 239,240 Pu : same input function, no Chernobyl peak Integrated deposition density(2010) 30- 40°N: 80 Bq m -2 7 137 Cs cumulative fallout deposition (2010) 30- 40°N: 2 kBq m -2 + Chernobyl

8. Theoretical vertical profile of 137 Cs in a sediment core If sediment acumulation rate is relatively fast (cm/y) the radionuclide vertical profile should reflect its input function. 8

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

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

11. 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. 11 Factors affecting radionuclide profiles: Bioturbation

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

13. The sediment structure: grain size, porosity Porosity Φ = Volume of water / Volume of total sediment Porosity of clay: 0.7 – 0.9 Porosity of sand: 0.3 – 0.5 13 Boudreau, 1997

14. The sediment vertical structure: compaction Compaction: loss of water from a layer of sediment, compression due to compression arising from the deposition of overlaying sediment. NEW No compaction NEW Compaction 14

15. The sediment structure: compaction The behaviour during compaction of sands and clays is different: fine-grained clays undergo continual compaction even on a cm-by-cm basis, while for sand the decrease in porosity with depth is minimal. 15

16. Porosity For sediment cores, we can plot porosity versus depth. Porosity in the surface layers is higher (lower compaction, bioturbation). Exponential decreaseHomogeneous grain size High porosityFine grained sediment 16

17. Porosity vs depth Porosity vs depth Barents Sea, CABANERA core 10, 2004 silty, homogeneous sediment. 17

18. coarser sediment, layers with different grain-size. Porosity vs depth Porosity vs depth Barents Sea, CABANERA core 10, 2004 18

19. Compaction and RN profiles constant sed. accum. rate 2002 2003 2004 2005 2006 No compaction 2002 2003 2004 2005 2006 Compaction The dry weight of the sediment is the same in every layer, water content. what changes is the water content. 19

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) 20

21. Compaction and RN profiles 21

22. Inventory Integrated radionuclide activity per unit surface (Bq m -2 )  x=0 RN conc. (Bq/g) * layer dry weight (g) I = --------------------------------------------------------- Core surface area (m 2 ) x=z 22

23. 23 Inventories of 137 Cs in different areas of the Med Sea 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. Inventories of 137 Cs in the Mediterranean Sea 24 89 Data from: Arnaud et al., 1995; Delfanti et al., 1997 Livingston, 1978 Barsanti et al., submitted. 72 155 70-150 Cumulative Fallout deposition (2010): 1600 Bq m -2 Chernobyl: 1000-15000 Bq m -2 Prodelta mud:2500-64000 Shelf mud: 700-6000 Sand: 400-1200 Rhone mouth: 1200-30000 194 190 90

25. Inventories of 137 Cs in the Mediterranean Sea 25 7 >180 90 57 47 50-34020-130 Data from: Delfanti et al., 1995; Anton et al., 1995 Delfanti e Papucci, 1989; Fowler et al., Jennings et al., 1985; Livingston, 1978. 180 3 7 2 3 Cumulative fallout deposition: 80 Bq m -2