Radiocarbon91007

1
Radiocarbon 9/10/07

• Lecture outline
• radiocarbon dating principles
• Atmospheric ocean radiocarbon variability
• The Calibration Curve
• Radiocarbon as biogeochemical tracer

The Shroud of Turin
2
Radiocarbon (14C) formation and decay
-formed by interaction of cosmic ray spallation
products with stable N gas
-radiocarbon subsequently decays by ?- decay back
to 14N with a half-life of 5730y
Radiocarbon dating was first explored by W.R.
Libby (1946), who later won the Nobel Prize. Most
published dates still use the Libby half-life
of 5568y to enable comparison of 14C dates.
The activity of radiocarbon in the atmosphere
represents a balance of its production, its
decay, and its uptake by the biosphere,
weathering, etc.
Which of these three things might change through
time, and why?
3
Radiocarbon Dating
1) As plants uptake C through photosynthesis,
they take on the 14C activity of the
atmosphere. 2) Anything that derives from this C
will also have atmospheric 14C activity
(including you and I). 3) If something stops
actively exchanging C (it dies, is buried, etc),
that 14C begins to decay.
where present-day, pre-bomb, 14C activity
13.56dpm/g C

• So all you need to know to calculate
• an age is A0, which to first order
• is 13.56dpm/g, BUT
• small variations (several percent)
• in atmospheric 14C in the past
• lead to dating errors of up to 20!
• Sources of variability
• geomagnetic field strength
• solar activity
• carbon cycle changes

4
Radiocarbon Measurements and Reporting
1) Radiocarbon dates are determined by measuring
the ratio of 14C to 12C in a sample, relative
to a standard, usually in an accelerator mass
spectrometer.
standard oxalic acid that represents activity
of 1890 wood 14C ages are reported as 14C years
BP, where BP is 1950
2) Fact Most living things do not uptake C in
atmospheric ratios i.e. they fractionate
carbon, (lighter 12C preferentially used),
must correct for this fractionation because it
affects the 14C/12C ratio
Researchers collect the 13C/12C ratio, use it to
correct for missing 14C So the less 13C a
sample has, the less 14C it has, and so the
uncorrected 14C age will be _______ than the
calendar age? Samples are normalized to a
d13CPDB value of -25
3) The final step is to obtain a calibrated 14C
age using the atmospheric radiocarbon content
when the sample grew.
5
Atmospheric radiocarbon variability through time
Convention The atmospheric radiocarbon anomaly
with respect to a standard is defined as D14C

• solar activity changes

Note the D14C is 0 during 1890, b/c thats the
activity of the oxalic acid standard

• addition of isotopically light
• fossil fuel C to atmosphere

time
But how did somebody construct this curve?
6
Reconstructing atmospheric radiocarbon
variability through time
What you need absolute age radiocarbon
age What you get history of 14Catmos
tree cut in 1999A.D.
radiocarbon sampling transect
1821A.D. by ring-counting
Most of the Holocene 14Catmos variability derives
from changes in the geomagnetic field
7
Over longer timescales, whats controlling
atmospheric D14C?
Earths Carbon Reservoirs 14C content
(in Gigatons C) atmosphere 748 highest (site
of production) terrestrial 2,000 high (some
old organic matter) oceanic 38,000 high
(surface) and low (deep) geologic 4,000 zero
(isolated from atmos. for long time)

• Climate changes perturb the Es carbon
• cycle (and therefore 14Catmos) by
• changing the amount of C stored on land
• (ex ice sheets of LGM)
• -changing the ability of the deep
• ocean to sequester old carbon from
• atmosphere (ex increased stratification)

8
The ocean conveyor belt and radiocarbon
Seawater radiocarbon measurements set a
time-scale for whole-ocean mixing (1500y), and
identified the locations where deep-mixing occur
The 14C ages of waters along a N-S transect in
the Atlantic.
the youngest waters are at the surface and in the
deep North Atlantic
note non-zero age at surface
the bomb spike
the oldest waters are in the mid-depth pacifc
(age1500y)
14C measurements from vertical profiles taken in
worlds oceans
4) If dating a marine sample, you will need to
correct its calibrated 14C age with a reservoir
age, which ranges from lt100yrs in the stagnant
subtropical gyres to gt1000yrs in the mid-depth
Pacific.
9
Paleo-ventilation ages from 14C in marine
carbonates
1. compare 14C ages of benthic and
planktonic forams in same core horizon
2. compare U-series and 14C ages for deep-sea
corals
Atlantic
Pacific, LGMlarge symbols GEOSECSsmall dots
100y of growth (study modern corals)
diff. in 14C ages670y!
Adkins et al., 1998
Broecker et al., 2004
10
The Radiocarbon Calibration Curve (atmospheric
14C history)
Principle compare radiocarbon dates with
independent dates examples of independent
dating tree-ring counting, coral U-Th dates,
varve counting, correlation of climate signals
in varves with ice core
Observation radiocarbon dates are consistently
younger than calendar ages
data from corals (bright red) lake varves
(green) marine varves (blue) speleothems
(orange) tree rings (black)
So was atmospheric 14C larger or smaller at 20k
(LGM) than today?
equiline
time
Hughen et al., 2004
11
But what caused these large changes in
atmospheric 14C?
Use a carbon cycle model that includes
radiocarbon, play with different scenarios,
check fit with reality.
geomagnetic field from paleomag studies only
geomagnetic field from paleomag
magnetic anomaly at 44k
geomagnetic field mag. anomaly
reduced sedimentation during glacial change in
overturning circulation
geomagnetic field mag. anomaly
reduced sedimentation during glacial
stop transferring radiocarbon into deep ocean
Beck et al., 2001
12
So what is the average geochemist to do?
Trust the experts!
INTCAL98 established one curve to use for 14C
calibration Stuiver, M., Reimer, P.J., Bard, E.,
Beck, J.W., Burr, G.S., Hughen, K.A., Kromer,
B., McCormac, G., van der Plicht, J., Spurk, M.
1998. INTCAL98 Radiocarbon Age Calibration,
24,000-0 cal BP. Radiocarbon 40(3)1041-1084.
Use their calibration program (current version
CALIB 4.4)
http//radiocarbon.pa.qub.ac.uk/calib
M. Stuiver, P.J. Reimer, and R. Reimer
Also, avoid contamination with post-bomb/tracer
carbon at all costs! Ex diagenesis may replace
original C with post-bomb (modern) C or
contamination with tracer (super-enriched) 14C
used by biologists (next lecture)
13
The timing and structure of the bomb spike
Bomb-produced radionuclides (in 1018 Bq (1Bq1dps)




14
The radiocarbon bomb spike atmosphere vs. other
reservoirs
1000 14C doubles
Trumbore, 2000
Source of bomb 14C stratosphere, Northern
Hemisphere Incorporation of bomb 14C into
various C reservoirs depends on the residence
time of C in that reservoir Ex short
residence time quick, high-amplitude response
long residence time delayed,
low-amplitude response
Why?
15
Seawater radiocarbon observations

• GEOSECS Geochemical Ocean
• Section Study
• 1972-1978
• transects through all major oceans
• WOCE World Ocean Circulation
• Experiment
• 1990-2002
• extensive coverage of the ocean

16
Ocean models of bomb 14C
Can ocean general circulation models accurately
capture the spatial and temporal evolution of the
bomb radiocarbon spike? Our understanding of
ocean mixing (esp. vertical mixing) is extremely
limited, 14C can help.
Rodgers et al., 1999
17
Model D14C-coral D14C comparisons
French Frigate Shoals
Fiji
Rodgers et al., 1999
Why are some records smoother than others (ie
Galapagos vs. Fiji)?
Nauru
Galapagos
18
Radiocarbon in the Biosphere
Approach Use bomb 14C as a tracer of carbon
cycling in complex environments. Or use natural
level 14C to date carbon in various pools.
Gaudinski et al., 2000
19
Radiocarbon measurements of carbon cycling in the
ocean
dissolved inorganic carbon contains bomb 14C at
surface, not at deep
dissolved organic carbon old highly refractory
sinking particulate organic matter contains
bomb carbon
Figure 1. Radiocarbon in different reservoirs of
carbon at Station M in the North Eastern Pacific
Ocean. Red circles represent values for HMW DOM
(Aluwihare, 1999) blue bars at 0 m and 1500 m
represent the range of values observed for
individual monosaccharides isolated from HMW
sugars (Aluwihare, 1999). DIC, total DOC, and POC
data are from Druffel et al., 1996 and Bauer et
al., 1998.
20
Atmosphere-Ocean-Biosphere models of bomb 14C
evolution
The contributions of ocean, terrestrial
biosphere, fossil fuel combustion and bomb
testing to atmospheric D14C was investigated with
an Atmospheric General Circulation Model (GCM)
coupled to a carbon cycle model.
Atmosphere
Terrestrial biosphere w/ three different carbon
residence times
Why does the terrestrial biosphere lag the
atmosphere?
The detrended atmos D14C, showing large seasonal
cycle
Randerson et al., Global Biogeochemical Cycles,
2002
Why do Southern and Northern Hemisphere D14C
values differ in 1970?
21
Radiocarbon measurements of net primary production

• Approach
• collect seawater sample
• inject 14C-labelled bicarbonate,
• incubate for several hours
• filter out organisms of interest
• measure activity with scintillation counter

Conceptually
Quantitatively
A scintillation counter measures a samples
activity by quantifying flourescence derived
from the interaction of a ?- particle with the
scintillation cocktail. Need to calibrate with
known 14C solutions. Need high concentrations of
14C!
Ex. Scintillation Counter Output
32P
14C
3H
Count rate
Energy of Particle