U-series Disequilibrium Part 2
9/11/12
Lecture outline:
photo of box coring device w/ core
1)
210Pb
2)
231Pa
excess and residence time
- fraternal twin to 230Th
a. 231Pa/230Th concordia
b. oceanic and sedimentary
Pa and Th
210Pb
Excess
226Ra
222Rn
210Pb
210Bi
t1/2=22.26y
238U-series
Intermediate
t1/2=1600y
Over what age range is
210Pb dating useful?
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:
ln(
210
PbEx )  ln(
210
d
Pb )  (210 )
S
0
Ex
What happens if sediment contains 226Ra?
210Pb
Excess and residence time
Phenomenon:
Explanation?
210Pb
~ 0.5(226Ra) in bottom waters, always
dN2
= l1 N1 - l2 N2 From previous lecture
dt
d
210
Pb
 226 * 226 Ra  210 * 210 Pb  k 210 Pb
dt
Activity
parent
Activity
daughter
physical
removal from
system
d 210 Pb
0
dt
steady state
assumption
If we assume no change in the amount of 210Pb, then we can calculate k
( A226  A210 )
k  210
= 0.032 yr-1
210
A
And mean residence time for 210Pb in deep ocean (Rt) equals 1/k, or ~30y
Similar calculations can be applied to calculate Rt’s for many other parent-daugther
systems, assuming your measurements represent steady state.
231Pa
- fraternal twin of 230Th
Parent half-life Rt
complication
230Th
234U
234U excess
75,000y ~30y
231Pa235U
32,000y ~110y
none
So wherever you apply 230Th, you can apply 231Pa:
1. sedimentation rates
2. carbonate dating
Who should we point
the finger at? and why?
From Cutler et al., 2003
These two geo-chronometers better agree (yield concordant ages), otherwise….
what has happened?
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:

 U  


234



U / 238U a  
  1 *1000
234
238
U / U a ,equil  
234

What is the 234U 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 δ234U of added U,
discrete vs. continuous alteration
What would a discordia for
constant U gain look like?
231Pa/230Th
ratios in the ocean: back of the envelope
Given:
-
ThA 234U A (1  e230 t )
230
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:
 230Th   234U   1  e  230 t
 231    235  
 231 t
Pa
U
1

e

A 
A
- fixed U isotopic ratios in seawater
(235U/238U)atomic = 137.88
(234U/238U)activity = 1.15
We calculate that the (230Th/231Pa)A = 10.8
or (231Pa/230Th)A = 0.093



231Pa/230Th
ratios in modern sediments
What’s 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
231Pa/230Th
as a tracer of scavenging and
water mass circulation
Kerr, 1988
231Pa/230Th
as a tracer of scavenging and
water mass circulation
Schmitz, 1996
231Pa/230Th
as a tracer of scavenging and
water mass circulation
Boyle, 1996
231Pa/230Th
ratios in modern vs LGM sediments
LGM
MODERN
How can we interpret
these panels?
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)
- so increase in 231Pa/230Th
would indicate a slowdown in
deep ocean circulation, given
constant scavenging
(cannot go greater than 0.093)
From Henderson, 2002
231Pa/230Th
ratios in a North Atlantic core (33°N, 57°W)
NADW strong
note: axis
inverted
NADW weak
From McManus et al., 2004
key points:
-LGM values 0.013 higher than Holocene  30% reduction in MOC?
-deglacial characterized by large, abrupt changes in ratio 
implies ocean circulation key to abrupt climate changes in ice cores
- reaches magic number 0.093 during Heinrich  complete collapse of MOC?
oxygen isotopes of ice for paleo-temperature
oxygen isotopes in calcite for paleo-temperature
231Pa/230Th
14C
for paleo-circulation
dating for age control
230Th-normalized
238U-234U-230Th
paleo-sea level
sedimentation rates
dated coral terraces for