j.m. abril department of applied physics (i); university of seville (spain)
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
J.M. AbrilDepartment 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
Lecture 3:Clasical dating models using 210Pb
210Pbex fluxes
Radionuclide profiles and inventories
Radiometric dating modelsCIC CF-CSR, CRS, CMZ-CSR , CD-CSR IMZ (*)-CSR
1
2J.M. Abril, University of Seville2
z
aw
222Rn
210Pb
137Cs
J.M. Abril, University of Seville
3
y = 9,3 1 x - 1,87
R2 = 0,689*
0
10
20
30
40
50
60
70
1 2 3 4 5 6
ETo (mm/d)
222 R
n E
xh
ala
tio
n (
Bq
h-1
m-2
)
Abril et al. (JENVRAD, 2009)
222Rn exhalation depends, among other factors, on 226Ra content in soil, soil texture and structure, water content, and the forcing factors…
4J.M. Abril, University of Seville
4
Author: Israel López, Univ. Huelva (Spain)J.M. Abril, University of Seville5
J.M. Abril, University of Seville6
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 Seville7
Some reference values for annual fallout of excess 210Pb (Bq m-2 y-1) 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)
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.
Inputs and Inventories (Bq m-2 ) in sediments Inputs and Inventories (Bq m-2 ) in sediments
J.M. Abril, University of Seville8
Z [cm]
210Pb
[Bq/kg]
226Ra
total210Pb (unsupported)
Radiometric dating with 210Pb: Basic aspects
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
Supported fraction
J.M. Abril, University of Seville9
Basic Concepts and definitionsBasic Concepts and definitions
z
aw
J.M. Abril, University of Seville10
Compaction and bulk density
As depth increases in the sediment core, water pores are replaced by solids
Saturated porous media
V
ms
Bulk density
z
Vmw
ms
J.M. Abril, University of Seville11
Practical measurement of bulk densities
w
s
s
s
w
wsw
mmVVV
w
s
s
w
s
s
s
w
w
s
mmm
m
1
mw
ms
m
Drying and gravimetric method
J.M. Abril, University of Seville12
Practical measurement of bulk densities. Refinement
mw
ms,om
Drying and gravimetric method and loss by ignition
w
s,0
ms,i s,i
is
is
os
os
w
wisosw
mmmVVVV
,
,
,
,,,
is
is
os
os
w
w
isos
mmm
mm
,
,
,
,
,,
is
is
os
os
w
w
isos
mmm
mm
,
,
,
,
,,
J.M. Abril, University of Seville13
Bulk density versus depth profiles in sediment cores
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 5 10 15 20
(g
/cm
3)
Depth [cm]
ze 1
ze 1
J.M. Abril, University of Seville14
z
Mass thickness, Δm , and mass depth:, m
zm z
dzm0
'
[ g dry weight cm-2]
Δz
J.M. Abril, University of Seville15
(Mass) Sedimentation rate : w
dt
dmw [ g dry weight cm-2 y-1]
Time versus m for constant w (*)
w
dmdt w
mt
Z
≈
ZiA (Zi, t)
A (Zi+1, t)
A (Zi-1, t)
w (Zi-1, t)
w (Zi+1, t)
J.M. Abril, University of Seville16
Basic processes
Z
≈
ZiA (Zi, t)
A (Zi+1, t)
A (Zi-1, t)
w (Zi-1, t)
w (Zi+1, t)
J.M. Abril, University of Seville17
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 equationsFundamental equations
BOUNDARY CONDITIONS
Mass conservation for a particle-associated radiotracer
Mass conservation for solids
J.M. Abril, University of Seville18
[Bq L-2 T-1]
Constant Flux and Constant Sedimentation rate (CF-CSR)
Activity concentration at interface(non post-depositional mixing) Constant A0 w
FA 0
w
F incoming flux
sedimentation rate
J.M. Abril, University of Seville19
(non post-depositional mixing)
Layer at time t=0
The sediment-water interface displaces upwards
Specific activity A0
time = 0
z=z(t)
time = t =m/w
toeAA
m=m(t)
J.M. Abril, University of Seville20
m
Ln(A)
Validation:
Goldberg first validated the 210Pb dating method in varved sediments
Curve-fitting model , free parameters : Ao , w
wmt
eAA w
m
/0
Think about: Any implicit assumption concerning compaction?
J.M. Abril, University of Seville21
Schweiz. Z. Hydrol. 49/3, 1987
ZF = 172 Bq m-2 y-1
EXAMPLE from a case study
J.M. Abril, University of Seville22
Don't forget:
Estimated sedimentation rates, ages and dates have to be provided with the corresponding uncertainties.
Don't forget:
Estimated sedimentation rates, ages and dates have to be provided with the corresponding uncertainties.
Age : T(m) or T(z) , from m(z)/w
w , (mass) sedimentation rate
Dates or chronology: Year of sampling – Age
W = 0.115 ± 0.014 g cm-2 y-1
J.M. Abril, University of Seville23
Associated uncertainties in 210Pb chronology
is
is
os
os
w
w
isos
mmm
mm
,
,
,
,
,,
is
is
os
os
w
w
isos
mmm
mm
,
,
,
,
,,
iii zm
i
imm
i
im2
2,
2, zrrii m
,r
,...),,( 321 xxxf
...,;,;, 332211 xxx
i
ji
f x
f2
General formulae for error propagation
G.F.
mm
J.M. Abril, University of Seville24
bxaAxf
mx
ln)(
11
2
/1
2
2
2
RN
a
x
a
ab
2,
2, brrw w
bw
2,
2, wrmrt
w
mt t
t
Associated uncertainties in 210Pb chronology
J.M. Abril, University of Seville25
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 Seville26
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 from Axelsson and El-Daoushy, 1989Data from Axelsson and El-Daoushy, 1989
J.M. Abril, University of Seville27
10
100
1000
10000
0 0.2 0.4 0.6 0.8 1
210 P
b (B
q/kg
)
Mass depth (g cm-2)
RedóGossenkollesee
1.- Many unsupported 210Pb profiles do not follow a simple exponential decay pattern
More complex models are required
PROBLEMS:
J.M. Abril, University of Seville28
CIC model (Constant Initial Concentration)
w
F incoming flux
Activity concentration at interface(no post-depositional mixing) w
FAo
CIC model assumes 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
sedimentation rate
J.M. Abril, University of Seville29
CIC model can equally be formulated in terms of actual depth (z) or mass depth (m)
Chronology (one date per data point)
Alternative estimation of sedimentation rates (one per data point) – only for cores with high spatial resolution-
CAUTION !•Estimation of the initial concentration, Ao, is not a trivial task !
A0
A(m)
m
A
-Unknowns for CIC: Ao and wi (N+1; N= number of sections in the core)
- It is a “mapping” model
J.M. Abril, University of Seville30
Schweiz. Z. Hydrol. 49/3, 1987
EXAMPLE from a case study
CF-CSR CIC
J.M. Abril, University of Seville31
ZF (recent) = 76 Bq m-2 y-1
CRS model (Constant Rate of Supply)
w
F incoming flux
Initial concentration w
FAo
CRS model assumes 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 Seville32
CRS model
Inventory under the horizon z
z
dzzzAz ')'()'()(
After a time t, the horizon now at z=0 will be located at depth z(t), and because of the radioactive decay.
tez 0)(tez 0)(
0
0 ')'()'()0( dzzzAz
At “geological” timescale the inventory is steady state; thus,
000
Fdt
dz 0F 0F
Z
z
J.M. Abril, University of Seville33
)(
ln1
)( 0
zzt
)(
ln1
)( 0
zzt
CRS Chronology:
Once the chronology is established, sedimentation rates can be obtained for each two adjacent layers:
t
zw
dtwdzdm
t
zw
dtwdzdm
z
Alternatively, from the mass balance in the steady state inventory below depth z
CRS model
-Unknowns for CRS: F, wi (N+1; N= number of sections in the core)- It is a “mapping” model
J.M. Abril, University of Seville34
CAUTION
•Check for completeness of inventories (sometimes it will be necessary to estimate the “missing” part of the total inventory)
0
0.5
1
1.5
2
2.5
3
0 5 10 15 20 25 30
Uns
uppo
rted
Pb-
210
(pC
i/g)
Depth (cm)
MARINE SEDIMENT- GOTEBORG-
"data2"2*exp(-0.09*(x-9))
2
J.M. Abril, University of Seville35
Schweiz. Z. Hydrol. 49/3, 1987
EXAMPLE from a case study
J.M. Abril, University of Seville36
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 Seville37
Complete mixing zone model with constant sedimentation rate and constant flux.
Mixing ma
w
F
F
Aama
Radioactive decay
wAa Sediment growth
aa mw
FA
aa mw
FA
Steady-state mass balance
w
mm
aa
a
eAmmA
)( w
mm
aa
a
eAmmA
)(
Curve-fitting model , free parameters : Aa, w, ma
J.M. Abril, University of Seville38
0
0.5
1
1.5
2
2.5
3
0 5 10 15 20 25 30
Un
sup
po
rte
d P
b-2
10
(p
Ci/g
)
Depth (cm)
MARINE SEDIMENT- GOTEBORG-
"data2""cmz"
Example CMZ-1
mixing
ma=9.5 g cm-2; w=0,374 g cm-2 y-1ma=9.5 g cm-2; w=0,374 g cm-2 y-1
J.M. Abril, University of Seville39
Acceleration or mixing?10
100
1000
10000
0 0.2 0.4 0.6 0.8 1
210 P
b (B
q/kg
)
Mass depth (g cm-2)
RedóGossenkollesee
2.- Many times unsupported 210Pb profiles can be equally explained by different models
210Pb chronologies must be validated against an independent dating method
PROBLEMS:
J.M. Abril, University of Seville40
J.M. Abril, University of Seville41
Think about:
What other hypothesis are implicitly assumed in all the previous models ?
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 DiffusionFit : CF-CSR Model
w 0,1 g cm^(-2) y^(-1)
km 6 g^2 cm^(-4) y^(-1)
ZF 200 Bq m^(-2) y^(-1)
w 0,49 g cm^(-2) y^(-1)ZF 200,6 Bq m^(-2) y^(-1)
J.M. Abril, University of Seville42
J.M. Abril, University of Seville43
J. N. Smith proposed a protocol for research journals for theacceptance 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 Seville44
ZFo=10 mBq/(cm^2 y) , w=0.1+0.1 t/150 g/(cm^2 y) D=0
Examples generated with numerical solutions
Constat aceleration, constant diffusion or CF-CSR?
J.M. Abril, University of Seville45
Effect of “episodic” changes in sedimentation rates?
J.M. Abril, University of Seville46
Examples generated with numerical solutions
Numerical algorithm: MSOU
T= - 50 y sgt= 5 y
J.M. Abril, University of Seville47
λ=0
Ts =150 y
J.M. Abril, University of Seville48
T= - 20 y sgt= 2 y
Ts =150 y
λ=0
Numerical algorithm: MSOU
Periodic changes in w with T=7 y
When data are smooth enough to apply CSR models?
J.M. Abril, University of Seville49
Examples generated with numerical solutions