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Radionuclides (geochronometers and tracers)
Used to measure rates of processes in the ocean:
Rates of removal of reactive chemical species
• Air-sea exchange
• Particle scavenging
 Rate of sediment accumulation
 Growth rates of authigenic deposits and marine
organisms (e.g. Mn nodules, coral skeletons, shells)
 Rates of sediment mixing by benthic
organisms
Mixing rates in water & water mass tracing
 Aging of organic matter
Types of natural radionuclides in the environment
Primordial - Present since Earth’s
formation (long lived nuclides)
Cosmogenic - Formed by cosmic rays in
the atmosphere
Anthropogenic - Man made (nuclear
reactors, bombs etc.)
Less than
21 kg of
3H on the
entire
Earth –
and this
can be
measured
in a few
liters of
water!
Nuclear decay
- results in change in the neutron/proton
ratio
- decay results from thermodynamic
instability of the nucleus and is an
attempt to reach the most stable nuclear
configuration
Different modes of nuclear decay
Alpha decay () of larger nuclides - loss of a helium nucleus
(42He) to lower neutron/proton ratio. Mass and element changes.
238
92U -->
234
4 He + Q (radiation e.g. gamma rays)
Th
+
90
2
Beta decay (-) - converts a neutron to a proton with emission of a
high energy electron (e-) - for atoms with extra neutrons
14
6C
--> 147N + e-
note increase in # protons, changes element, but not mass
Electron capture - proton in nucleus grabs an electron from lowest
orbital and combines to form a neutron. To fill empty orbital,
another e- falls to lower energy level, emitting X-rays.
40
19K
Ion or
mineral
--> 4018Ar note decrease in # protons, changes element but not mass
Inert gas
I’m bored, I don’t
want to be
uranium any more
238U
t1/2 = 4.5 billion years
Parent nuclide
Alpha decay
Life is short and then you decay
4
2He +
234Th
t1/2 = 24.1 days
Daughter nuclide
α decay
β decay
All primordial series end with stable (nonradioactive) form of lead (Pb)
Characteristics of Uranium and Thorium Series - Primordial Nuclides
Element
Name
T1/2 range
U
Uranium
2.5x105 - 4.5 x 109 y
Pa
Protactinium
1.2 min - 3.2 x 104 y
Th
Thorium
26h-1.4 x 1010 y
Ac
Actinum
6h - 22 y
Ra
Radium
3.6 d - 1600 y
Rn
Radon
4 sec. - 3.8 days
Po
Polonium
Pb
Lead
10-7 sec. - 138 days
30 min. - 22 y
Chemistry in
Oceans
Soluble in seawater,
especially in oxic
waters. Insoluble in
reduced form.
Conservative with
salinity.
Particle reactive surface adsorbed
Particle reactive surface adsorbed.
Chemistry similar to
iron.
Particle reactive. Short
lived
Soluble, chemistry like
Ca.
Noble gas - unreactive,
soluble.
Nutrient element
behavior
Reactive, heavy metal
tracer-particle reactive
Each time a nucleus decays it is an “event” or
disintegration.
Detection of radioactivity
 ionization detector- energy windows (each nuclide decay
emits a characteristic energy spectrum (e.g. photons of gamma
radiation) and can be distinguished from another)
 Fission tracks
 Scintillation counting (uses chemicals to absorb radiation
energy, leading to chain reactions that produce light. Light
pulses are detected with high sensitivity. Again, different
nuclides can be distinguished based on energy of emission.
Radiation is the amount of energy emitted
Radioactivity is a measure of nuclear disintegrations per unit
time, often given as disintegrations per minute (dpm)
Common units of radioactivity
Curie = 2.22 x 1012 disintegrations per minute (dpm). A curie is
defined by the amount of radioactivity in 1 gram of Radium.
In practice we commonly work in millicuries (2.22 x 109 dpm),
or microcuries (2.22 x 106 dpm) or just plain dpm.
Becquerel - The SI unit for radioactivity
1Bq = 1 disintegration/sec (dps)
So one Curie is = 3.7 x 1010 Becquerels (dps)
Specific activity – the amount of radioactivity per mole of
substance, e.g., mCi/mmol or dpm/pmol
238U
is the most abundant radionuclide in seawater
~3 g liter-1 mainly as uranyl tricarbonate [UO2(CO3)3]-4 which has
uranium in the oxidized form U(+VI). [Uranium] is conservative
with salinity.
238U
dpm/liter = 0.07081 x salinity
At salinity = 35, 238U activity = 2.48 dpm/Liter of
seawater
Uranium (+VI) can be
reduced by microbes under
anoxic conditions, adding 2 eand producing U(+IV). This
form is insoluble and
precipitates. Iron reducing
bacteria can carry out this
reduction (much interest in this).
In seawater
238U
99.28% of total U based on the # of atoms
235U
0.72%
234U
0.0055%
For nuclides in solution, the chemical
The absolute
concentration (N/liter) is directly proportional to concentrations of many
nuclides in seawater is
radioactivity per liter (N/liter) since:
very low, and not easily
measured by chemical
means. But, their
radioactivity can be
measured!
Emerson & Hedges, 2008 Chap 5
Secular equilibrium (daughter-parent relationships)
For a parent nuclide (P) with a long half life relative to its daughter
nuclide (D), the activity of the Parent is given by:
dP/dt = P[P] and this is the production rate of the Daughter
(since daughter is short lived, its existence depends on its
production from parent).
The rate of change of the Daughter nuclide is determined by its
production and loss
dD/dt
=
P[P]
-
rate of change = Production -
D[D]
Loss (by radioactive decay)
At steady state:
dD/dt = 0 = P[P] - D[D]
P[P] = D[D]
or
Ap = AD
or
AD/ AP = 1
Thus, for nuclides with short-lived daughters and long lived parents,
one predicts that the daughter/parent activity ratio ( AD/ AP) = 1. This
situation is termed secular equilibrium. For a system starting out
with parent nuclide, but no daughter, AD will grow into the system. In
other words, it takes time to reach secular equilibrium.
2
Total activity
Activity
(parent+daughter)
Parent activity
1
In-growth of
daughter activity
0
Time
Parent activity is
constant with
time since very
few atoms decay
(because of long
half life)
It takes about
6-8 daughter
half-lives to
reach secular
equilibrium
Daughter activity
becomes constant
with time because
Production = Loss
The activity of the daughter is
supported by the parent
Changing the decay
constant for daughter
will change ND but
not λND (if λ goes up, ND
goes down, and vice versa)
The rate of flow into
the daughter tank
λNP is equal to the
flow out λND
Emerson & Hedges Chap 5
Deviations from secular equilibrium
If all nuclides were in secular equilibrium we couldn’t learn
anything from them!
The deviations from equilibrium are the basis for using the nuclides
as tracers and chronometers!
234Th
activity in the water column is often less than its
parent 238U because of scavenging which removes the
daughter
Any process that adds or removes daughter nuclide will cause
deviations from secular equilibrium.
The deviations from secular equilibrium can be used to estimate
the scavenging rate constant (particle removal rate constant) in the
ocean water column (see steady state box model calculations as
used in Coale and Bruland, 1987 L&O 32: 189).
Production of
daughter
Loss of daughter
d[D]/dt = P[P] - {D[D] + k[D]}
Loss by rad
decay
Other first order loss (e.g.
scavenging)
The “k” here would be the scavenging rate constant (the fraction of particles
exported from the surface ocean to depth per unit time. Something very
useful to know.
Which nuclide to
use?
Must use a nuclide with a
half-life close to the rate
of the process of interest.
Nuclides with short half
lifes can only be used to
study fast processes.
Long-lived nuclides
cannot be used to study
fast processes (too few
decays over short time),
and only are useful for
slow processes.
coastal
234Th
This matching of decay
rate to process rate
applies to radio-dating
(aging) as well.
(t1/2 = 24 d) is
useful for water
column particle
scavenging rates
oceanic
y axis
x axis
Coale & Bruland, L&O 1987 – Application of 234Th scavenging
Expected
234Th
is particle reactive so most is rapidly adsorbed to
particles. If particles sink quickly, then have deficit of 234Th
Mixed layer
pycnocline
Euphotic
Shaded area
is deficit of
234Th due to
scavening
• Maximum scavenging near pigment maximum
• Less scavenging in upper mixed layer (due to efficient
recycling of particles & biomass)
234Th-derived
Moran et al., 2003. Limnol. Oceanogr. 48: 1018
Radio-dating of materials with nuclides
Useful for determining the age of a particular piece of matter
(organism, fossil, rock etc).
By obtaining an age for a piece of an accreting deposit (e.g.
sediment, coral skeleton, clam shell, Mn-nodule) at some depth
into the deposit, the accretion rates of deposit can be determined
(assuming steady deposition).
Depth z
Sediment
core
If you can put an age on the sediment in this
layer, you therefore know how long it took to
build up the sediment above it. From the
depth of the layer and its age (t), you can
determine the sediment accretion rate (z/t).
How to determine ages in deposits?
Use unsupported nuclide activities
If deposits are laid down with unsupported daughter
activity, and no additional inputs (other than supported
activity) occur within the deposit, then the
unsupported (excess) activity will decay with time
(=depth) into the deposit. Sediment is a good example.
AD-excess (unsupported daughter)
0
s= z/t Thus,
Depth (cm)
Sediment-water
interface
The sedimentation or
accretion rate is given by:
20
Exponential
decay of
excess activity
with depth
t = z/s
or
t = z/s
Substitute z/s for t in decay law
ADz = ADo e-λ(z/s)
Where ADz is the unsupported activity at depth z and ADo is
the unsupported activity at the surface of the deposit. This
can be rearranged to:
ADz/ADo = e-λ(z/s)
And linearized as:
ln(ADz) – ln(ADo) = -λ(z/s) which is the same as:
ln(ADz) = ln(ADo) - λ(z/s) and the same as:
ln(ADz) = ln(ADo) – (λ/s) z
Y-intercept
slope
X-coordinate
210Pb
(t1/2 = 22.3 y) is often used to estimate sediment
accretion rates in coastal areas where sedimentation is high
210Pb
is derived from decay of gaseous 222Rn (t1/2 = 3.8
days) which originates in rocks on land but goes into
the atmosphere where it is carried over water.
210Pb
produced in the atmosphere is rapidly rained out
and it attaches to particles in the water which sink to
the sediments.
This leads to unsupported 210Pb activity at the
surface of the sediment (activity ratio of
210Pb
226Ra
/
(daughter)
(parent) > 1).
This is also referred to as excess activity because it is
in excess of what is supported by the secular
equilibrium of the sedimentary 226Ra parent.
Log scale
Concentrations of
unsupported 210Pb
in sediments – can
give estimate of
sediment accretion
rates
Linear slope (a) of the semilog plot gives the sediment
accretion rate. If slope not
linear – steady state
sedimentation model does not
apply
Fig. 10.7 in Pilson
This figure focuses on the longer lived nuclide
230Th (t
1/2 = 75,200 y). Its chemistry (i.e. particle
reactivity) is the same as 234Th, but its decay is too
slow to be useful for particle scavenging rates in
the surface waters. It is, however, useful for
sediment accretion rates in the deep ocean where
accretion rates are relatively slow.
(Bioturbated)
Excess
Supported 230 Th activity (from
234U decay)
Use of nuclides as event
markers
137Cs
Picocuries per section
Peak emissions of manmade 137Cs (t1/2 = ~30 y)
into the atmosphere
occurred in 1963.
This particle reactive
nuclide is scavenged to
sediments, where profiles
reflect time inputs. Depth
above 137Cs peak has
accreted since 1963.
Core
taken
in
1986
~10 cm
per 23 y
137Cs
first appeared in
atmosphere in ~1953
Wetland sediments
DeLaune et al 1989
Natural 14C- a cosmogenic nuclide
• Produced in the upper
atmosphere by spallation of 14N
• Becomes 14CO2 in atmosphere
• Dissolves in ocean and taken up
by plants
• Diluted by fossil fuel burning of
low 14C carbon (Suess effect)
(Illustration by Jayne Doucette, Woods Hole Oceanographic Institution)
http://www.whoi.edu/nosams/page.do?pid=40138
Man-made 14C
Produced from weapons testing – peak production in 1960’s
Increased atmospheric 14C by over 2x – slowly taken up by ocean
Applications of 14C dating
Much progress with introduction of accelerator mass
spectrometer analysis – 14C content of micro- to
milligram quantities of carbon can be determined.
• Invasion of atmospheric CO2 into ocean can be
observed
• DIC of ocean water can be aged – giving estimate of
deep residence time.
• POC and DOC in seawater have been aged – DOC
found to be old.
• Bacteria in surface ocean use a mixture of old and new
carbon 14C-content of natural materials (mg quantities) can be measured at the
National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS)
Facility at Woods Hole - http://www.whoi.edu/nosams/
14C-ages
for compounds containing
carbon
• At the time of carbon fixation (photosynthesis)
some 14C will be incorporated into organic matter,
based on the amount of 14C in the atmosphere (or
seawater) at the time of fixation.
• Once an organism dies, no replacement of 14C
occurs, therefore the 14C radioactivity can only
decrease, due to decay.
• Since the decay rate of 14C is known (1.209 x 10-4
y-1), the deficit of 14C activity can tell us how
much time has elapsed since that organic matter
was alive.
14C =
(14C/C)sample - (14C/C)std x 1000 - IF
(14C/C)std
Fractionation
factor (a small
correction)
A zero value for Δ14C represents the 14C content of preindustrial atmosphere
From Bauer & Bianchi, 2011. Dissolved organic carbon cycling and transformation. In: A treatise on Estuarine and Coastal Science. Vol 5: 7-67
WOCE = World Ocean Circulation Experiment
Deep DOC ~5900 years old
Deep DOC ~4100 years old
Belize and
Florida corals
Galapagos
corals
(upwelling
area)
Estuarine Site
Bacterioplankton use
mainly recently fixed
carbon – but in the
open ocean, some
older carbon from
DOC is utilized also.
Oceanic Site
From Cherrier et al, 2000
End
Fraction Modern
S=14C/12C Sample
B= 14C/12C Blank
M = 14C/12C Modern reference
Fm is corrected to that of -25 o/oo δ13C
Where lambda is 1/(tru mean-life) of radiocarbon = 1/8267 = 0.00012097
Yc is year of collection.
Age = -8033 ln (Fm13C corr)
Residence time =1/k
Large
uncertainty
in residence
time or k
Large
uncertainty
in residence
time or k
From Coale & Bruland, 1987
Primordial decay series (three major parent nuclides)
see Fig 10.2 in Pilson for decay chain and half lives of 238U series
Parent
238U
-> 234Th ->->234U -> 230Th -> 226Ra -> 222Rn ->….210Pb->-> 206Pb
232Th
235U
Daughters
Stable
end
product
-> 228Ra ->-> 228Th -> 224Ra -> 220Rn -> 216Po ->
-> 231Th -> 231Pa ->-> 227Th -> 223Ra -> 219Rn ->
All primordial series end with stable (nonradioactive) form of lead (Pb)
…208Pb
…-> 207Pb
In seawater
238U
99.28% of total U based on the # of atoms
235U
0.72%
234U
0.0055%
Although the atom ratio of 238U/235U is 140, the activity ratio is only
21.7 because 235U has a much shorter half-life than 238U (so a
greater fraction of the 238U atoms are undergoing decay at any time)
This figure shows two shallow north-south sections from approximately the same area of
the East Pacific depicting the C-14 concentrations, measured 20 years apart. Clearly
visible is the evolution of the "bomb" C-14 signal (yellow-red) since the end of the nuclear
bomb tests (top) to the present (top plot), especially at intermediate and high latitudes
1974
1992
http://www.nosams.whoi.edu/woce/wocegeos.html