Useries Disequilibrium9507

1
U-series Disequilibrium 9/5/07

• Lecture outline
• 1) U-Th systematics
• 2) U-Th disequilbrium dating
• 3) Th excess and sedimentation rates
• 4) 210Pb excess and residence time
• 5) 231Pa - fraternal twin to 230Th
• a. 231Pa/230Th concordia
• b. oceanic and sedimentary
• Pa and Th

Zircon
Geological archives dated by U-series
disequilibrium speleothems (top) and fossil
corals (bottom)
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Secular equilibrium and disequilibrium
Secular equilibrium all radioactive species in
a decay chain have the same activity Disequilibri
um system is perturbed (removal/enrichment of
daughter/parent), and system decays back to
secular equilibrium And if you know how
(disequilibrium)initial, can calculate t since
disequilibrium
For U-series decay chain, what are some examples
of processes that cause disequilibrium ?
There are two types of disequilibria 1) Daughter
excess (i.e. activity of daughter gt activity of
parent) 2) Daughter deficit (Ad lt Ap)
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Decay Chain Systematics
Consider a 3-member decay chain
Evolution of this system is governed by the
coupled equations
Note that at secular equil,
As you can see, the solution of these
differential equations is quite complicated
(except for N1), so we will derive some
equations from the disequilibria of 234U and
230Th.
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NOTE everything in this lecture will be
activities (A), unless otherwise noted
234U Excess
Given (234U/238U)A of ocean 1.15 Explanation
you tell me The activity of (234U)excess
decreases with time
And excess 234U corresponds to the 234U not
supported by 238U
And dividing through by 238U activity, we obtain
So if you measure 234U/238U, and know
(234U/238U)initial can calculate age
Note that fixed analytical error (0.5) yields
larger and larger age error bars as you approach
secular equilibrium
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230Th Deficiency
Given Many U-rich minerals (such as carbonate)
precipitated with virtually no Th Explanation
you tell me So you grow in 230Th due to decay
of 238U and excess 234U (in atom number)
And converting to activity, substituting formula
for 234UEx, dividing by 238UA, and simplifying,
we obtain
So if you measure (230Th/238U), and assume
initial (234U/238U)A1.15, then you can
calculate a samples age.
So when/where is the assumption that initial
(234U/238U)A1.15 not a good one?
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230Th-234U activity growth lines
Development of mass spectrometry techniques
enable U-Th ages to be measured to 0.1-5
precisions (Edwards et al., 1987)
secular equilibrium
For most samples
secular equilibrium
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Common U-Th series applications
Cutler et al., 2003
1. Corals - sea level from fossil terraces -
climate reconstruction
Edwards et al., 1987
2. Cave Stalagmites - climate reconstruction
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230Th Excess and Deep-Sea sediments
Phenomenon excess 230Th present in ocean
sediments Explanation? The activity of
(230Th)excess decreases with time
If you assume that delivery of 230ThEx is
constant through time, can calculate age as a
function of depth - or - sedimentation rate
We can define sedimentation rate
as Sdistance/time so td/S and
and
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230Th Excess and sedimentation rate changes
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230Th Excess - interpretation
But how can you have an increase of 230Thex with
depth, given constant 230Thex delivery?
Down-core measurements in Norwegian core
show sedimentation rate changes
Increased scavenging of 230Th during
interglacials in Norway, or sediment focusing.
So 230Thex delivery is not constant. Changes in
the rain-rate of particles will lead to
increases and decreases in Th scavenging.
What could change scavenging rate?
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210Pb Excess
Over what age range is 210Pb dating useful?
226Ra 222Rn 210Pb 210Bi
t1/222.26y
238U-series Intermediate t1/21600y
gas that escapes into atmosphere
210Pb removed from atmosphere by precipitation
and/or dry deposition, becomes incorporated into
lake sediments, ice sheets, ocean surface, etc.
Like 230Thex, this unsupported, or excess
210Pb can be used to date sediments
What happens if sediment contains 226Ra?
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210Pb Excess and residence time
Phenomenon 210Pb 0.5(226Ra) in bottom waters,
always Explanation?
From previous lecture
steady state assumption
Activity parent
Activity daughter
physical removal from system
If we assume no change in the amount of 210Pb,
then we can calculate k
0.032 yr-1
And mean residence time for 210Pb in deep ocean
(Rt) equals 1/k, or 30y
Similar calculations can be applied to calculate
Rts for many other parent-daugther systems,
assuming your measurements represent steady state.
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231Pa - fraternal twin of 230Th
Parent half-life Rt complication 230Th 234U 75,00
0y 30y 234U excess 231Pa 235U 32,000y 110y none
So wherever you apply 230Th, you can apply
231Pa 1. sedimentation rates 2. carbonate
dating These two geo-chronometers better agree
(yield concordant ages), otherwise. what has
happened?
From Cutler et al., 2003
Who should we point the finger at? and why?
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231Pa/230Th concordia and open system behavior
Unlike the U/Pb concordia, 230Th activity varies
with 234U, so concordia change with
234U/238U, usually expressed in delta 234U
What is the d234U of seawater?
from Cheng et al, 1998
Because Th and Pa are both sticky, U mobility
is the most common cause of open system
behavior. upper intercept true age of
deposit lower intercept ambiguous, depends on
d234U of added U, discrete vs. continuous
alteration
What would a discordia for constant U gain look
like?
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231Pa/230Th ratios in the ocean back of the
envelope

• Given
• -
• where t is the time between production
• of 230Th and its incorporation into
  sediment
• (t for Th and Pa is 300y in the ocean)
• dividing the 230Th and 231Pa equations
• - fixed U isotopic ratios in seawater
• (235U/238U)atomic 137.88
• (234U/238U)activity 1.15

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231Pa/230Th ratios in modern sediments
Whats going on here?
Observation The sediments do not reflect
theoretical seawater values. This observation
implies that Pa and Th are scavenged
differentially. Which is scavenged more
efficiently? How can we understand values in
the Southern Ocean which are greater than 0.093?
Yu, 1996
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231Pa/230Th as a tracer of scavenging and water
mass circulation
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231Pa/230Th ratios in modern vs LGM sediments
LGM
MODERN
How can we interpret these panels?
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231Pa/230Th ratios in paleoceanography
More recently, paleoceanographers have taken
advantage of the fractionation of 231Pa and 230Th
associated with scavenging to reconstruct 1.
scavenging intensities and/or 2) deep-water
advection strength (given constant scavenging)
NADW strong
From Henderson, 2002
NADW weak
Sediment core from North Atlantic
From McManus et al., 2004