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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 Department of Applied Physics (I); University of Seville (Spain). Lecture 3:Clasical dating models using 210 Pb 210 Pb ex fluxes Radionuclide profiles and inventories Radiometric dating models

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J.M. Abril Department of Applied Physics (I); University of Seville (Spain)

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  1. IAEA Regional Training Course on Sediment Core Dating Techniques. RAF7/008 Project J.M. Abril Department of Applied Physics (I); University of Seville (Spain) • Lecture 3:Clasical dating models using 210Pb • 210Pbex fluxes • Radionuclide profiles and inventories • Radiometric dating models • CIC CF-CSR, CRS, CMZ-CSR , CD-CSR • IMZ (*)-CSR J.M. Abril, University of Seville

  2. 2 J.M. Abril, University of Seville

  3. aw z 137Cs 222Rn 210Pb J.M. Abril, University of Seville

  4. 222Rn exhalation depends, among other factors, on 226Ra content in soil, soil texture and structure, water content, and the forcing factors… Abril et al. (JENVRAD, 2009) 4 J.M. Abril, University of Seville

  5. J.M. Abril, University of Seville Author: Israel López, Univ. Huelva (Spain)

  6. J.M. Abril, University of Seville

  7. Some global patterns for 210Pbex fallout • Predominant west-east movement of air masses  210Pbex fallout is low in the western areas of the continents • 210Pbex fallout is higher in the North hemisphere • 210Pbex fallout is positively correlated with rainfall Figures from P.G. Appleby, STUK-A145 J.M. Abril, University of Seville

  8. Some reference values for annual fallout of excess 210Pb (Bq m-2 y-1) Global scale , F ~ 23-367 Bq m-2 y-1 (Robbins, 1978) Tropical Australia , F ~ 50 Bq m-2 y-1 (Brunskill and Pfitzner,2000) Inputs and Inventories (Bq m-2 ) in sediments Catchment concentration factor (normalization or focusing factor) : Z Input (*) = ZF Steady State Inventories Σ = ZF/λ For 210Pb = ln2/T1/2 with T1/2 = 22.26 y. J.M. Abril, University of Seville

  9. 210Pb [Bq/kg] total 210Pb (unsupported) 226Ra Z [cm] Radiometric dating with 210Pb: Basic aspects Supported fraction If we assume that there is no Rn exhalation from the sediment, then the total activity of 210Pbtotal will be 210Pbtotal = 210Pbsupported + 210Pbunsupported and 210Pbsupported = 226Ra activity J.M. Abril, University of Seville

  10. aw z Basic Concepts and definitions J.M. Abril, University of Seville

  11. V mw ms z Bulk density Compaction and bulk density As depth increases in the sediment core, water pores are replaced by solids Saturated porous media J.M. Abril, University of Seville

  12. mw m w ms s Drying and gravimetric method Practical measurement of bulk densities J.M. Abril, University of Seville

  13. Practical measurement of bulk densities. Refinement mw w m ms,o s,0 ms,i s,i Drying and gravimetric method and loss by ignition J.M. Abril, University of Seville

  14. 0.7 0.6 0.5 0.4  (g/cm3) 0.3 0.2 0.1 0 0 5 10 15 20 Depth [cm] Bulk density versus depth profiles in sediment cores J.M. Abril, University of Seville

  15. [ g dry weight cm-2] z Mass thickness, Δm , and mass depth:, m Δz J.M. Abril, University of Seville

  16. [ g dry weight cm-2 y-1] (Mass) Sedimentation rate : w ≈ A (Zi-1, t) w (Zi-1, t) Time versus m for constant w (*) A (Zi, t) Zi w (Zi+1, t) A (Zi+1, t) Z J.M. Abril, University of Seville

  17. Basic processes ≈ A (Zi-1, t) w (Zi-1, t) A (Zi, t) Zi w (Zi+1, t) A (Zi+1, t) Z J.M. Abril, University of Seville

  18. Fundamental equations Mass conservation for a particle-associated radiotracer Mass conservation for solids BOUNDARY CONDITIONS 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 J.M. Abril, University of Seville

  19. Constant Flux and Constant Sedimentation rate (CF-CSR) F incoming flux [Bq L-2 T-1] w sedimentation rate Activity concentration at interface (non post-depositional mixing)  Constant A0 J.M. Abril, University of Seville

  20. The sediment-water interface displaces upwards z=z(t) Specific activity A0 Layer at time t=0 time = 0 time = t =m/w m=m(t) (non post-depositional mixing) J.M. Abril, University of Seville

  21. Ln(A) m Curve-fitting model , free parameters : Ao , w Validation: Goldberg first validated the 210Pb dating method in varved sediments Think about: Any implicit assumption concerning compaction? J.M. Abril, University of Seville

  22. EXAMPLE from a case study ZF = 172 Bq m-2 y-1 J.M. Abril, University of Seville

  23. w , (mass) sedimentation rate Dates or chronology: Year of sampling – Age Age : T(m) or T(z) , from m(z)/w Don't forget: Estimated sedimentation rates, ages and dates have to be provided with the corresponding uncertainties. W = 0.115 ± 0.014 g cm-2 y-1 J.M. Abril, University of Seville

  24. Associated uncertainties in 210Pb chronology General formulae for error propagation m G.F. J.M. Abril, University of Seville

  25. w Lest squares fitting a, b, R2 easily produced with excel or other shifts Associated uncertainties in 210Pb chronology t J.M. Abril, University of Seville

  26. Time resolution . Each sectioned layer in the core corresponds to a time interval Δt = dm/w Remember: As the analytical method is homogenizing the material from each layer, it is not possible to solve other time marks within such an interval (e.g. two 137-Cs peaks). • Note for advanced students: • Apply lineal regression taking into account the associated uncertainties in measurements J.M. Abril, University of Seville

  27. CAUTION ! • Estimation of the supported fraction is not a trivial task ! • 226Ra may be non uniform in depth and being different from the 210Pb baseline • Settling particles can be depleted in 226Ra in the water column while enriched in 210Pb Data fromAxelsson and El-Daoushy, 1989 J.M. Abril, University of Seville

  28. 10000 Redó Gossenkollesee 1000 210Pb (Bq/kg) 100 10 0 0.2 0.4 0.6 0.8 1 Mass depth (g cm-2) PROBLEMS: 1.- Many unsupported 210Pb profiles do not follow a simple exponential decay pattern  More complex models are required J.M. Abril, University of Seville

  29. CIC model (Constant Initial Concentration) F incoming flux sedimentation rate w Activity concentration at interface (no post-depositional mixing) • CIC modelassumes constant Ao; Thus, changes in F must be compensated with changes in w. • Also , it assumes non post-depositional mixing • Reasonable when F is associated with inputs of solids J.M. Abril, University of Seville

  30. CIC model can equally be formulated in terms of actual depth (z) or mass depth (m) A A0 A(m) Chronology (one date per data point) m Alternative estimation of sedimentation rates (one per data point) – only for cores with high spatial resolution- • Unknowns for CIC: Ao and wi (N+1; N= number of sections in the core) • It is a “mapping” model • CAUTION ! • Estimation of the initial concentration, Ao, is not a trivial task ! J.M. Abril, University of Seville

  31. EXAMPLE from a case study ZF (recent) = 76 Bq m-2 y-1 CF-CSR CIC J.M. Abril, University of Seville

  32. CRS model (Constant Rate of Supply) F incoming flux w Initial concentration CRS modelassumes constant F, independently of w. Ao can vary. Also assumes non post-depositional mixing. -Reasonable when F is not coupled with inputs of matter J.M. Abril, University of Seville

  33. After a time t, the horizon now at z=0 will be located at depth z(t), and because of the radioactive decay. At “geological” timescale the inventory is steady state; thus, CRS model Inventory under the horizon z z Z J.M. Abril, University of Seville

  34. Once the chronology is established, sedimentation rates can be obtained for each two adjacent layers: CRS model CRS Chronology: Alternatively, from the mass balance in the steady state inventory below depth z z • Unknowns for CRS: F, wi (N+1; N= number of sections in the core) • It is a “mapping” model J.M. Abril, University of Seville

  35. CAUTION • Check for completeness of inventories (sometimes it will be necessary to estimate the “missing” part of the total inventory) J.M. Abril, University of Seville

  36. EXAMPLE from a case study J.M. Abril, University of Seville

  37. ZF = 170 Bq m-2 y-1 from CF-CSR w = 0.115 ± 0.014 g cm-2 y-1 J.M. Abril, University of Seville

  38. Steady-state mass balance F Aama Radioactive decay wAa Sediment growth Complete mixing zone model with constant sedimentation rate and constant flux. F w ma Mixing Curve-fitting model , free parameters : Aa, w, ma J.M. Abril, University of Seville

  39. Example CMZ-1 mixing ma=9.5 g cm-2; w=0,374 g cm-2 y-1 J.M. Abril, University of Seville

  40. 10000 Redó Gossenkollesee 1000 210Pb (Bq/kg) 100 10 0 0.2 0.4 0.6 0.8 1 Mass depth (g cm-2) PROBLEMS: 2.- Many times unsupported 210Pb profiles can be equally explained by different models 210Pb chronologies must be validated against an independent dating method Acceleration or mixing? J.M. Abril, University of Seville

  41. Think about: What other hypothesis are implicitly assumed in all the previous models ? J.M. Abril, University of Seville

  42. Constant flux, CSR and constant difussion Model Demonstration will be provided within lecture 6 Curve-fitting model , free parameters : ZF, km , w Data: CF-CS-C Diffusion Fit : CF-CSR Model J.M. Abril, University of Seville

  43. J.M. Abril, University of Seville

  44. J. N. Smith proposed a protocol for research journals for the acceptance of papers that rely on 210Pb dating to establish a sediment core geochronology: ‘‘The 210Pb geochronology must be validated using at least one independent tracer which separately provides an unambiguous time-stratigraphic horizon’’. J.M. Abril, University of Seville

  45. Examples generated with numerical solutions Constat aceleration, constant diffusion or CF-CSR? ZFo=10 mBq/(cm^2 y) , w=0.1+0.1 t/150 g/(cm^2 y) D=0 J.M. Abril, University of Seville

  46. Examples generated with numerical solutions Effect of “episodic” changes in sedimentation rates? J.M. Abril, University of Seville

  47. λ=0 Ts =150 y T= - 50 y sgt= 5 y Numerical algorithm: MSOU J.M. Abril, University of Seville

  48. λ=0 Ts =150 y T= - 20 y sgt= 2 y Numerical algorithm: MSOU J.M. Abril, University of Seville

  49. Examples generated with numerical solutions When data are smooth enough to apply CSR models? Periodic changes in w with T=7 y J.M. Abril, University of Seville

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