16. Sediment Transport Across the Continental Shelf and Lead-210 ...

210-Pb in sediments
Sediments contain a background level of 210 Pb that is
supported by the decay of 226 Ra (radium is an alkali
metal) which is easily eroded from rocks and incorporated
into sediments. As fast as this background 210 Pb is lost by
radioactive decay, new 210 Pb atoms are created by the
decay of 226 Ra.
Young sediments also include an excess or unsupported
concentration of 210 Pb. Decaying 238 U in continental rocks
generates 222 Rn (radon is a gas) some of which escapes
into the atmosphere. This 222 Rn decays to 210 Pb which is
then efficiently incorporated into new sediments. This
unsupported 210 Pb is not replaced as it decays since the
radon that produced it is in the atmosphere.
Measurements of how the excess 210 Pb decreases with
depth can be used to determine rates.
Depth, Z
(or age)
Pb-210 concentrations in sediments
A B
Pb-210 activity
Region of radioactive
decay.
Background Pb-210 levels from
decay of Radon in sediments
(supported Pb-210)
Surface mixed layer - bioturbation
Measured Pb-210 activity
Excess Pb-210 activity
(measured minus
background)
t 1
t 2
Age of
sediments, t
Excess Pb-210 concentrations
A 2 A 1
Excess Pb-210 activity
Work with data in this region
For a constant
sedimentation rate, S
(cm/yr), we can
replace the depth
axis with a time axis
z = St
t = z S
! dA
dt = " A
A 2
Solving the equation - 1
" ! dA = # dt
A
A 1
t 2
"
t 1
A
"# ! ln A$ 2
% A1
= & " # t $ t 2
% t1
The equation relating activity to the
radioactive decay constant
Integrating this with the limits of
integration set by two points
! ln A 2
+ ln A 1
= ln A 1
= "( t
A 2
! t 1 )
2
A relationship between age and activity
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