210 pb dating of environmental records stored in natural archives peter g. appleby department of...
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210Pb dating of environmental records stored in natural archives
Peter G. ApplebyDepartment of Mathematical SciencesUniversity of Liverpool, Liverpool, L69 3BX, UK.
Third International Conference on Po and Radioactive Pb Isotopes (INCO-PoPb-2015)October 11-14, 2015, Kusadasi, Turkey
Origins of 210Pb dating
The basic methodology 210Pb dating of was established in a ground-breaking paper by Goldberg (1963).
226Ra atom
222Rn 210Pb
222Rn atom particle
The method was first applied by Goldberg to dating Greenland glacier cores. He suggested two possible assumptions for interpreting the 210Pb records:
(1) A constant rate of accumulation of 210Pb, leading to the equation
relating the cumulative activity A above a layer to the age t of that layer.
(2) A constant rate of accumulation of both 210Pb and water, leading to the equation
relating the activity Bx in a layer of depth x and age t to the activity B0 in the surface layer.
t
A
e1
tx BB e0
Other applications quickly followed, initially to the measurement of accumulation rates in ice sheets (Crozaz et al. 1964, Crozaz & Langway 1966) and glaciers (Piciotto et al. 1967).
Further applications were made during the next few years to the dating of lake sediments (Krishnaswami et al. 1971), marine sediments (Koide et al. 1972), salt marshes (Armentano & Woodwell, 1975) and peat bog sequences (Aaby et al. 1979).
Most of these early applications were concerned with using the technique to determine the mean accumulation rate, essentially using Goldberg’s second assumption.
In the case of a glacier, assuming a constant (water equivalent) accumulation rate v, ice of depth x will have age t = x/v. Goldberg’s second equation then becomes:
vBBB xvxx / whereee 0
/0
The mean accumulation rate can then be calculated by measuring the gradient of a best exponential fit to the data.
The plot shows data from a Greenland ice core (Crozaz & Langway 1966).
0.1
1
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100
0 10 20 30 40 50
Mean accumulation rate 0.32 m y-1
Mean gradient = -0.097 m-1
Depth (m) water equivalent
21
0P
b a
ctiv
ity (
Bq
m-3
)
Coupled with a growing confidence in the technique and the fidelity of environmental records stored in natural archives
this led to the development and testing of methods for dating cores where accumulation rates (of ice, sediment or peat) may have varied through time.
This had been essentially foreshadowed in Goldberg’s original paper.
An increasing number of cases arose where the 210Pb activity versus depth relationship was clearly non-exponential.
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Depth (cm)
Un
sup
po
rte
d 21
0 Pb
Act
ivity
(B
q k
g-1
)
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0 2 4 6 8 10 12 14
Depth (cm) 13
7 Cs
Act
ivity
(B
q k
g-1
)
the well-defined 137Cs peak suggests that the sediments have preserved a good record of fallout radionuclides.
Example of a non-exponential 210Pb record
Although the 210Pb record in this lake sediment core (from Pirunkuru, Finland) is clearly non-exponential
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1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 20000
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19861988198919962007
Varve date (years AD)
13
7C
s a
ctiv
ity in
po
st-C
he
rno
byl
co
res
(Bq
kg
-1)
13
7C
s a
ctiv
ity in
pre
-Ch
ern
ob
yl c
ore
(B
q k
g-1
)
The potential reliability of environmental records stored in lake sediments is illustrated by these records of pre- and post-Chernobyl 137Cs in cores from Nylandssjon (Sweden).
The cores were dated by counting annual laminae. Twenty years after the events the weapons test fallout peak was retained in the 1963/4 varve and the Chernobyl fallout peak in the 1986 varve.
Basic equations
Accumulating sediments, ice or peat samples acquire an initial 210Pb activity through direct or indirect exposure to the natural atmospheric fallout, and via in situ decay from 226Ra.
In most circumstances it can be assumed that the intermediate short-lived decay products of 226Ra are in equilibrium with 226Ra and that this initial activity decays with time in accordance with the radioactive decay law:
)e1(e)0()( tRa
tPbPb CCtC
By measuring the present day 210Pb and 226Ra concentrations CPb(t) and CRa these equations can be used to determine the sediment age t provided reliable estimates can be made of the initial 210Pb activity CPb(0).
Writing Cuns = CPb – CRa for the unsupported activity this equation can be rewritten
te)0()( unsuns CtC
Writing P(t) for the 210Pb supply rate delivered to the sediments the initial unsupported 210Pb activity they acquire can be written
where r(t) is the mass accumulation rate (dry mass in the case of sediments or peat, water equivalent in the case of ice) at that time.
2
2
m kg
m Bq
)(
)()0(
tr
tPCuns
The principal source of unsupported 210Pb activity is normally
assumed to be atmospheric fallout P. This can reasonably be assumed constant on time scales of a year or more.
P(t) will be driven by, but not necessarily equal to, the atmospheric
flux P.
Simple ModelsThere are two standard simple models used in 210Pb dating:
CRS model –assumes a constant rate of supply of 210Pb to the core site regardless of variations in the mass accumulation rate. Dates are calculated using the equation
)t(e)0()( zAzA where A(z) is the residual 210Pb inventory beneath the layer of depth z and age t(z). Foreseen in Goldberg’s paper it was developed and tested more extensively in the late 1970s by Appleby & Oldfield (1978) and Robbins (1978).
CIC model –assumes a constant initial 210Pb concentration. Dates are calculated using the equation
)t(e)0()( zCzC where C(z) is the present 210Pb concentration in the layer of depth z.
Theoretical JustificationThe CRS model would appear to be relatively well justified in the case of peat cores where there is limited scope for spatial redistribution of the direct atmospheric flux P.
,1 PF PbPbfP
(c.f. Appleby 2001) where Pb is a catchment/lake transport parameter, the catchment lake area ratio, FPb the fraction of 210Pb entering the water column transferred to the sediment record, and f a sediment redistribution factor.
In lakes transport processes governing the supply of fallout to the core site are more complicated and can be represented by the equation
The supply rate P may be reasonably constant if the various transport parameters are stable, or their impact small.
Distribution of the 210Pb supply rate over the bed of Blelham Tarn, Cumbria (Appleby et al. 2003)
The atmospheric flux was estimated to be 147 Bq m-2 y-1. High supply rates at the SW end of the lake adjacent to a stream entering the lake are largely due to inputs from the catchment. Other parts are dominated by direct fallout with some focussing into the NE basin.
Fallout 210Pb entering a lake can be distributed quite unevenly.
The CIC model is most likely to be valid for ice cores where initial 210Pb concentrations will be mainly governed by the constant mean annual 210Pb concentration in precipitation. Variations in the accumulation rate caused by snow drift may lead to non-exponential concentration versus depth records.
This model may also be valid for lake sediment cores where the production of sediment has been relative stable but sedimentation rates at particular sites have varied due to changes in the pattern of sediment accumulation.
It is unlikely to be valid for peat cores due to the effect of organic decay.
CFCS model Sites where 210Pb supply rates and mass accumulation rates are both stable will be characterised by exponential concentration versus depth records. At such sites the CRS and CIC models will yield similar results. The mean accumulation rate is calculated from the gradient of a best exponential fit to the data.
210Pb records from Øvre Neadalsvatn (Norway) and Braya Sø (Greenland)
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0 0.2 0.4 0.6 0.8 1.0
From regression line
C(0) = 426 Bq kg-1
r = 0.0057 g cm-2 y-1
Depth (g cm-2
)
Uns
uppo
rted
210 P
b A
ctiv
ity
(Bq
kg-1
)
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0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
From regression line
C(0) =1103 Bq kg-1
r = 0.0098 g cm-2 y-1
Depth (g cm-2
)
Uns
uppo
rted
210 P
b A
ctiv
ity
(Bq
kg-1
)
A priori application of either of the simple models to estuarine or marine cores with non-exponential concentration versus depth records is highly problematic without independent validation of the results.
As we will see below, even simple exponential records cannot always be trusted.
Model Validation
In most of our work on dating lake sediment cores although the CRS model has proved to be generally the more reliable, our experience has shown that neither of the two simple models is universally valid.
Model validation is an essential part of the dating process
This is most commonly achieved using chronostratigraphic dates e.g. from 137Cs records
137Cs dating is becoming increasingly important as it covers an increasing part of the 210Pb dating time-span
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Depth (cm)
Uns
uppo
rted
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0P
b A
ctiv
ity (
Bq
kg-1
)
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0 2 4 6 8 10 12 14
Depth (cm) 1
37C
s A
ctiv
ity (
Bq
kg-1
)
In the above example from Pirunkuru, Finland, the irregular 210Pb record precluded use of the CIC or CFCS models. The 1963 depth was however independently determined by a well-defined 137Cs peak.
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137
Cs datesRaw CRS model
210Pb dates
Sedimentation rates
1963
Age (y)
De
pth
(cm
)
Se
dim
enta
tion
rat
e (
g cm-2
y-1)
The validity of the CRS model dates was supported by an excellent agreement between the 210Pb and 137Cs dates
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0 2 4 6 8
Depth (cm)
Uns
uppo
rted
210 Pb
Act
ivity
(B
q kg
-1)
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0 2 4 6 8
Supported 210Pb
Depth (cm)
Tot
al 21
0 Pb A
ctiv
ity (
Bq
kg-1
)
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137Cs241Am
Depth (cm)
137 C
s A
ctiv
ity (
Bq
kg-1
)
241 A
m A
ctiv
ity
(Bq
kg-1
)
The 1963 and 1986 depths were however independently determined from the 137Cs and 241Am records.
In this example (from Karipaajarvi, Finland), the 210Pb dates were apparently unequivocal, the CRS and CIC models giving similar results.
They showed that in this case neither 210Pb model was correct,
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137Cs DatesCIC 210Pb DatesCRS 210Pb Dates
1963
1986
Age (y)
De
pth
(cm
)
demonstrating that even the simplest records can’t always be trusted.
Resolution of Dating Discrepancies
Although the CRS model has proved to be generally reliable, corrections do need to be made in those cases where there are significant discrepancies between the 210Pb and 137Cs dates.
Because of the complicated nature of the transport processes governing the supply of 210Pb to the sediment record, it is unlikely that any more general and widely applicable process based model can be found.
Discrepancies between 210Pb dates (determined by either of the simple models), and independently determined chronostratigraphic dates, necessarily imply a departure from the assumptions of the simple models.
Correction Procedures
Any correction procedure must be simple, practicable, and evidence based.
The approach we have taken is to apply the simple models in a piecewise way to different sections of the core, using 137Cs or other chronostratigraphic dates as reference points.
Area A(z1,z2)
Depth
210Pb Concentration C
z2 z1
Given two reference depths z1,z2 in the core with independently determined ages t1,t2 the mean 210Pb supply rate during the period of time spanned by this section of the core can be calculated using the formula
PA
t
e e
,t1 2
where A is the unsupported 210Pb inventory within that section of the core.
Area A(z,z2)
Depth
210Pb Concentration C
z2 z1 z
If we assume that the calculated 210Pb supply rate P is constant during that time, the age t(z) of sediments of depth z (z1 ≤ z ≤ z2) is given by the equation
)t(e)0()( znomnom AzA
where
P
Anom )0(
and A(z,z2) is the 210Pb inventory in sediments between depths z and z2.
This equation dates sediments at depths z1, z2 to times t1, t2.
,),(e)( 22 zzA
PzA t
nom
Using this approach the concept of a constant 210Pb supply rate for the entire period is in effect replaced by that of a variable 210Pb supply rate, approximated by a series of constant steps.
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Age (y)
210 P
b su
pply
rat
e (P
b m
-2 y-1
)
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supp
ort
ed
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0P
b A
ctiv
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Bq
kg-1
)
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137Cs
241Am
Depth (cm)
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7C
s A
ctiv
ity (
Bq
kg
-1)
241 A
m A
ctiv
ity (
Bq
kg-1
)
In this example (from Ulmener Maar Lake, Germany), although the 210Pb record is highly irregular the 137Cs record has two peaks clearly identifying the 1963 and 1986 depths.
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137
Cs datesRaw CRS model datesCorrected CRS model
210Pb dates
210Pb supply rate
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1986
Age (y)
De
pth
(cm
)
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0P
b s
up
ply
rat
e (
Bq m
-2 y-1
)
Discrepancies between the raw CRS model 210Pb dates and the 1963 and 1986 137Cs dates indicated small but significant variations in the 210Pb supply rate.
The corrected chronology was calculated by applying the CRS model in a piecewise way.
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137Cs239+40Pu241
Am
Depth (cm)
137
Cs
Act
ivity
(B
q kg
-1)
241 A
m &
239+
40P
u A
ctiv
ity (
Bq
kg-1
)
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Depth (cm)
Un
sup
port
ed 21
0 Pb
Act
ivity
(B
q k
g-1
)
In another example of an irregular 210Pb record, in this case from Blelham Tarn, Cumbria, UK, the 137Cs, 241Am, and 239+40Pu records clearly identify the 1986 and 1963 depths.
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137Cs/241Am datesRaw CRS 210Pb datesCorrected 210Pb datesSedimentation rates
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1986
Age (y)
Dep
th (
cm)
Sed
imen
tati
on r
ate
(g cm
-2y-1
)
A small but significant discrepancy between the raw CRS model 210Pb dates and the 137Cs/241Am/239+40Pu dates was due to an increase in the 210Pb supply rate at this core site associated with a recent increase in sedimentation rates (though not in the same proportion).
Corrected 210Pb dates were calculated by applying the CRS model piecewise to the pre- and post-1963 sediments.
Quality and Reliability of Sediment Records
The potential reliability of sediment records can be tested by repeat coring at selected sites over a period of years
In view of their well-defined origin, radionuclide records can also be regarded as indicators of the quality of sediment records.
Good quality 210Pb and 137Cs records suggest that records of other environmental indicators should also be trusted.
as demonstrated by the 137Cs records in cores from Nylandssjon (Sweden) collected during the period 1986 to 2007 (Klaminder et al. 2012).
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s a
ctiv
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st-C
he
rno
byl
co
res
(Bq
kg
-1)
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7C
s a
ctiv
ity in
pre
-Ch
ern
ob
yl c
ore
(B
q k
g-1
)
Concluding Remarks
• 210Pb dating used in conjunction with 137Cs records has proved to be a highly flexible and very reliable means for dating environmental records stored in a range of different natural archives.
Using just the above simple models applied as a whole or in part our centre has over the past 30 years successfully dating several hundred cores
with sediment accumulation rates ranging from the extremely slow (0.033 cm -1) to the extremely fast (4.4 cm y-1).
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ivity
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q k
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Depth (cm)
137 C
s A
ctiv
ity (
Bq
kg
-1)
241 A
m A
ctiv
ity (
Bq
kg-1
)
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FWLB
FWL1
Depth (cm)
Uns
uppo
rted
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0P
b A
ctiv
ity (
Bq
kg-1
)
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FWLB
FWL1
Depth (cm) 1
37C
s A
ctiv
ity (
Bq
kg-1
)
241 A
m A
ctiv
ity (
Bq
kg-1
)
Radiometric records in a core from Lake B (Greenland).
210Pb/226Ra equilibrium is reached at a depth of just 3 cm.
In this core from Freshwater Lake (Dominican Republic) equilibrium is reached at a depth of more than 200 cm.
Slow Cores
Fast Cores
Sites have ranged from the Arctic to the Antarctic
210Pb, 226Ra and 137Cs records in a lake sediment core from Tenndammen (Svalbard).
Fallout 210Pb and 137Cs records in a sediment core from Heywood Lake, Signy Island (South Orkney Islands)
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Depth (cm)
Uns
upp
orte
d 21
0 Pb
Act
ivity
(B
q k
g-1)
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0 2 4 6 8 10 12 14 16
Depth (cm) 13
7 Cs
Act
ivity
(B
q k
g-1)
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100
1000
0 2 4 6 8 10
Supported 210Pb
Depth (cm)T
ota
l 210P
b A
ctiv
ity (
Bq
kg-1
)
0
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100
120
140
160
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240
0 2 4 6 8 10
Depth (cm)
137C
s A
ctiv
ity (
Bq
kg-1
)
Arctic cores
Antarctic cores
from lakes ranging from the extremely small to the extremely large
Area 41,471 km2 Area 0.012 km2
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Depth (cm)
Svartatjonn (Norway)
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0 5 10 15
Depth (cm)
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0P
b A
ctiv
ity (
Bq
kg
-1)
Baikal (Siberia)
Large and Small Lakes
and from desert to high-rainfall environments.
210Pb, 226Ra and 137Cs records in a sediment core from Lake Qarun, Egypt (mean annual rainfall ~10 mm y-1).
210Pb, 226Ra and 137Cs records in a sediment core from Lac du Speke, Uganda(mean annual rainfall ~2500 mm y-1).
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Supported 210Pb
Depth (cm)
Tot
al 21
0 Pb A
ctiv
ity (
Bq
kg-1
)
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0 5 10 15 20
Depth (cm) 13
7 Cs
Act
ivit
y (B
q kg
-1)
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100
0 5 10 15 20 25 30 35 40
Supported 210
Pb
Depth (cm)
Tot
al 2
10P
b A
ctiv
ity (
Bq
kg-1
)
0
2
4
6
8
10
12
14
16
0 5 10 15 20 25 30 35 40
Depth (cm)
137
Cs
Act
ivity
(B
q kg
-1)
Desert Regions
High-Rainfall Regions
• 210Pb dates that have not been validated e.g. by 137Cs must always be regarded with some caution.
137Cs dates are becoming increasingly important now that they span up to two or more 210Pb half-lives.
Records of the 1963 fallout maximum from the atmospheric testing of nuclear weapons can be used to validate the recent chronology and improve the reliability of the early part of the record.
Records of fallout from the 1986 Chernobyl accident (where they exist) can provide further checks on any recent changes.
• 210Pb and 137Cs records can also provide significant information about the process of sediment accumulation.
Comparing mean 210Pb supply rates at specific sites in the lake with estimates of the atmospheric flux can provide information on the extent and nature of sediment focussing, and the importance of indirect inputs from the catchment.
Irregularities in the 210Pb record may be linked to specific events such as a sediment slump, or a major disturbance in the catchment.
• 210Pb is an ideal tracer for studying transport processes within catchment lake systems.
Transport models validated by 210Pb can be used to reconstruct quantitative histories of atmospheric pollution (trace metals, POPs) from their sediment records.
And finally:
Thank you for your attention