J.M. Abril Department of Applied Physics (I); University of Seville (Spain) IAEA Regional Training Course on Sediment Core Dating Techniques. RAF7/008 Project J.M. Abril, University of Seville 1 Lecture 4: Dating models using Man-Made radionuclides ( 137 Cs, 241 Am…) Dating models Combined use of 137 Cs and 210 Pb. Constrains in the use of 137 Cs Dating of recent sediments: Mixing or acceleration ?

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Independent dating methods for recent sediments: 137 Cs chronology (also Pu, 241 Am) 7 Be (quality control ) J.M. Abril, University of Seville 3

Time –dependent fluxes J.M. Abril, University of Seville 4

In situations where the tracer is partially carried by pore water or in presence of selective and/or translocational bioturbation Eqs. has to be revisited Fundamental equations BOUNDARY CONDITIONS Mass conservation for a particle-associated radiotracer Mass conservation for solids J.M. Abril, University of Seville 5

Constant sedimentation rate with non post-depositional mixing The first atmospheric input took place at t=0 Z, “normalization” factor J.M. Abril, University of Seville 6

Exercise J.M. Abril, University of Seville 7

Example: Sediment from lake Zürich (Erten et al, 1985) Using peak-marks 1963 ΔtΔt J.M. Abril, University of Seville 8 Data from Erten et al. (1985)

Example: Sediment from lake Zürich (Erten et al, 1985) CRS dates 137 Cs peak J.M. Abril, University of Seville 9

CAUTION ! Pre-depositinonal processes J.M. Abril, University of Seville 10

CAUTION ! Post-depositinonal processes J.M. Abril, University of Seville 11

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CSR with a zone of complete mixing CSR with a zone of incomplete mixing J.M. Abril, University of Seville 15 w Mixing m a mama F

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Example of time-dependent fluxes : Gaussian pulse J.M. Abril, University of Seville 18

Example 1: Solution with CMZ model (λ = 0 ) J.M. Abril, University of Seville 19

Example 2: Solution with CMZ model 50-10= 40 y W ~16/40 = 0.4 g cm -2 y -1 J.M. Abril, University of Seville 20

Example 3: Solution with IMZ model J.M. Abril, University of Seville 21

CSR with constant diffusion coefficient J.M. Abril, University of Seville 22

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Total 210 Pb and 137 Cs specific activities versus mass depth profiles measured by Erten et al. (1985) in the core from Lake Zurich (sampled in 1979). J.M. Abril, University of Seville 24

Erten et al. (1985) measured constant 210 Pb activities in the top 6 cm of a sediment core from Lake Zurich. They found 7 Be only in the uppermost layer, the distinct 137 Cs maximum at 6 cm depth, and undisturbed varves. The fast mixing seemed then opposed to common sense. ! CRS model shows acceleration and it adequately matches the position of the Cs peak. Counting of annual layers showed sedimentation rates of 0.07 g cm -2 y -1 for the last 30 years J.M. Abril, University of Seville 25

The CRS model and the 137 Cs time-mark. The time-mark from the 137 Cs (1963) is in good agreement with the CRS dates J.M. Abril, University of Seville 26

The averaged CRS sedimentation rates in the top 5 cm (the last 20 years) reasonable agreed with the value found from the counting of annual layers J.M. Abril, University of Seville 27

Everything seems to adequately match; and the whole work seems to fulfill the Smith’s criteria But, …. Everything seems to adequately match; and the whole work seems to fulfill the Smith’s criteria But, …. Almost constant 210 Pb activity in the upper layers, should be the result of an acceleration in sedimentation rates J.M. Abril, University of Seville 28

1. The distinct maximum in 137 Cs activity is compatible with a fast (but incomplete) mixing, as shown in the IMZ model. 2. The presence of 7 Be only in the uppermost sediment layer is not necessarily incompatible with a fast mixing, due to its short half-live, as it will be shown further. 3. Results from the texture analysis only limit the mixing to a fraction of solids and to the mobile phase (pore water and those microstructures it can carry on). Weaknesses in qualitative arguments J.M. Abril, University of Seville 29

Time series of 137 Cs atmospheric fallout in Denmark served to estimate the atmospheric deposition in the studied area. Differences by latitude and other local factors can be treated as a global multiplicative constant included in the normalization factor (the ratio between the measured inventory in the sediment and the integrated atmospheric deposition). J.M. Abril, University of Seville 30

From the CRS sedimentation rates, the atmospheric fluxes and the normalization factor, it is possible to estimate the specific activity versus depth profile for 137 Cs: J.M. Abril, University of Seville 31

If 137 Cs can reach these sediment layers then some post-depositional effect has to take place. Let us consider diffusion Cs (dpm/g) Mass depth (g/cm^2) From CRS model Measured Diffusion CD model with km = g 2 cm -4 y -1 and w = g cm -2 y -1 J.M. Abril, University of Seville 32

Diffusion invalidates CRS model for 210 Pb, since both radionuclides are assumed to be particle-associated tracers. Postulating a so different behaviour for 210 Pb and 137 Cs requires empirical demonstration ! Constant diffusion with CSR cannot explain the plateau in the 210 Pb profile Limitations : J.M. Abril, University of Seville 33

Pb (dpm/g) Mass depth (g /cm 2 ) Model Data Cs (dpm/g) Mass depth (g /cm 2 ) Model Data Numerical solution IMZ Model ma = 1.1 g cm -2, g = 0.6 and w = g cm -2 y -1.  0 =0.7 dpm cm- 2 y -1. J.M. Abril, University of Seville 34

The hypothesis of an infinity value for k m (mixing coefficient) is useful to find out analytical solutions. Nevertheless, from a physical point of view, an infinity value of k m has to be understood as being large enough. Thus, the same figure shows the numerical solution of Eq.1 under the conditions and parameter values of the IMZ model, but with k m = 0.34 g 2 cm -4 y -1 within the mixing zone (and null value elsewhere). … the IMZ model was solved numerically, keeping the previous parameter values, but with a constant flux of 7 Be (T 1/2 = 53 days) of 0.2 dps cm -2 y -1 lasting 3 years.  The 7 Be problem… J.M. Abril, University of Seville 35

The same figure plots the solution for the same constant input of 137 Cs (T 1/2 = y) Be and 137-Cs activities (dps/g) Mass depth (g cm -2 ) 7-Be Model 7-Be data 137-Cs Model J.M. Abril, University of Seville 36

Appleby (2000) presented Pb and Cs data from several European mountain lakes. He identified some sites with recent increases in sedimentation rates through the presence of the Cs peak within the 210 Pb plateau zone. Two of such sites were the Lake Redo´, in the Spanish Pyrenees, and the Lake Gossenköllesee, in Austria Further validation… J.M. Abril, University of Seville 37

IMZ Model J.M. Abril, University of Seville 38

CONCLUSIONS It has been questioned the qualitative argument consisting in the use of the distinct 137 Cs peak as a definitive demonstration of acceleration in sedimentation rates. Furthermore, It has been questioned the use of the distinct 137 Cs peak as an unambiguous time mark to establish a chronology. The full quantitative modelling of its activity versus depth profiles is a more powerful tool… but new problems will arise with input functions. J.M. Abril, University of Seville 39

More details and other examples J.M. Abril, University of Seville 40